Field Geology

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FIELD GEOLOGY

GUIDEBOOK AND NOTES

ILLINOIS STATE UNIVERSITY

2019 Version

Table of Contents

Syllabus 5

Schedule 8

Hazard Recognition Mitigation 9

Geologic Field Notes 17 Reconnaissance Notes 17 Measuring Stratigraphic Column Notes 18 Geologic Mapping Notes 19

Geologic Maps and Mapping 22 Variables affecting the appearance of a geologic map 23 Techniques to test the quality and accuracy of your map 23 Common map errors 24 Official USGS map colors 24 Rule of V’s 25

Geologic Cross Sections 26 Basic principles of cross section construction 26 Apparent dips: correct use of strike and dip data in cross sections 27 Common cross section errors 27 Steps in making a topographic profile for a geologic cross section 28 Constructing geologic cross sections using down-plunge projection 29

Phanerozoic Stratigraphy of North America 33

Tectonic History of the U.S. Cordillera 38 Regional Cross-sections through Wyoming 41 Wyoming Stratigraphic Nomenclature Chart 42 Rock Sequence in the Bighorn Basin 43 Rock Sequence in the Powder River Basin 44

Black Hills Precambrian Geology 45

Project Descriptions 47 Regional Stratigraphy 47 Amsden Creek Big Game Winter Range 49 Steerhead Ranch 50 Alkali 53 South Fork 55 Mickelson 57 Moonshine 60

Appendix 1: Essential Analysis Tools and Techniques for Field Geology 62 Field description of rocks 62 Measuring stratigraphic sections 69 Calculating layer thicknesses 76 Alignment diagram for calculating apparent dip 77 Calculating strike and dip of a surface from contacts on a map 78 Calculating outcrop patterns from field observations - constructing a map 79

Appendix 2: General Guidelines for Map Preparation 81

Appendix 3: General Guidelines for Cross Section Construction 83

Appendix 4: Standard Geologic Symbols and Patterns 86

Appendix 5: Geologic Time Scale 87

Appendix 6: Basic First Aid Advice and Treatment Procedures 88

Appendix 7: Field Guide to the Heart Mountain Detachment 107

Appendix 7: Field Guide to the Heart Mountain Detachment 139

Policy for Field Camp Dismissal

GEO 395 – Field Geology COURSE PURPOSE AND OBJECTIVES

The course is five and a half weeks long, for six credit hours, and is taught on location in northern Wyoming and western South Dakota.

COURSE DATES: May 20-June 27, 2018.

DEPARTURE: 6:15 AM from Felmley Hall, followed by 6:30 at the Baymont.

DIRECTOR: Dr. David Malone

COURSE OBJECTIVES: 1. To learn basic field techniques, particularly: using the Brunton compass, measuring geologic sections, describing rocks, taking field notes, and making field sketches. 2. To learn the latest technologies that are used in the construction of geologic maps. 3. To learn the skill of geologic mapping, a process that involves total immersion in the science and in the project at hand, and the associated skills of location on topographic maps and air photos and interpretation of features on these. 4. To learn to interpret the structure and geologic history of an area based on your field observations and geologic map. Such ability is demonstrated mainly through the construction of geologic cross-sections from the geologic maps. 5. To learn the importance of accuracy in data acquisition and placement on a geologic map. 6. To integrate aspects of prior coursework into a comprehensive package in which the student becomes aware of the interdependence of all parts of the science of geology. 7. To develop an appreciation of the scale of geologic features and of the “reality” of geologic features, as compared to their depiction in print media. 8. To develop the skills and expertise needed to make the transition from student to professional geologist. 9. To develop senses of self-confidence and professional competence.

ASSIGNMENTS AND GRADING: A variety of assignments will be given and graded, including, but not limited to, mapping projects from 1 to 4 days duration, cross-section constructions, interpretations of field features, rock and section descriptions and measurements, and a field exam on local stratigraphy. Field notebooks and written assignments also will be critiqued and graded along the way. At the end of camp, the faculty will collective determine a grade component based on professionalism, work ethic, and attitude that will be worth at least 10% of the final course grade. All assignments will be due at a specified time, and it is assumed that students will respect the deadlines. Faculty from both universities grade assignments and written comments are made on most projects. Students are expected to learn from critique comments and to improve continuously throughout the course. Most assignments will have some points allotted to “style and grace”, i.e. neatness and presentation. Correct spelling of all words, particularly geologic terms, is expected. On most projects, students will work in pairs or small groups. The reasons for this are for safety in the field and for professional discussion of observations and interpretations. However, it is fully expected that each student will acquire his/her own field data, take his/her own notes and produce his/her own map and cross-sections. It is not acceptable for partners to each do a part of the work and to then exchange information to complete the project. Failure to abide by the above policies may result in dismissal from this course.

RULES: Adherence to several rules is absolutely necessary. More are provided in your field guide.

1. There is to be absolutely NO association of alcohol or drugs with University vehicles or in the dorms at the Colleges at which we stay. Failure to abide by this rule will precipitate dismissal from the course. 2. No-smoking rules apply in university vehicles. 3. You are not to leave any garbage in the field (GIGO rules!) and not to carve your name, etc. on rocks or trees. 4. You are to leave gates as you find them, open or closed, and you are not to climb wire fences. 5. Do not walk across alfalfa or hay fields. 6. Vehicles leave for the field promptly at 8:00 AM. Be there and be ready or be left behind. 7. You are reminded of the ISU student code of conduct. Prior to your departure, you will be asked to sign an agreement as to how you will be expected to conduct yourselves while we are working an on the days off.

WORKING CONDITIONS: The working conditions in the field can be both physically and mentally taxing. You may walk ten miles in a single day, climb 2000 ft in elevation and spend all day above 2000 ft. Keep yourself in top working condition and keep a positive attitude. Wear a hat to protect yourself from the sun. Drink adequate water to prevent dehydration and eat balanced meals. You need electrolytes and carbohydrates. This is not a time to be dieting. A tentative schedule will be issued that provides for a six-day workweek. One day per week is scheduled off. However, because of unpredictable weather conditions, it is often necessary to shuffle the schedule. No scheduled day off is guaranteed. It is important to note that most evenings will be working too, as lectures and project work will occur during these times. Weather conditions over the six weeks are likely to vary between cold, wet and snowy with temperatures barely above freezing, to hot and dry, with temperatures in the lower 100’s. Particularly during the first half of the course the weather can be truly nasty. If at all possible we work rain or shine, so always be prepared with adequate clothing and rain gear. Prepare to dress in layers, as the conditions often vary widely within a single day. You will be working on steep slopes, some with loose rock. Always be cautious with your footing and be aware of people below you. If you loosen a rock on a slope, yell “ROCK!” loudly to warn those below. Some critters and plants can present dangers in the field. Rattlesnakes are common, particularly in the outer edges of the Black Hills. If you hear their warning, let them be and go around them. Warn others of the snake’s location. Do not try to kill them. Be careful stepping or reaching where you cannot see. Deer ticks may carry Lyme disease and wood ticks may carry Rocky Mountain spotted fever. You should make a “tick check” every night. Be wary of cattle, some are friendly, some are not. They are curious creatures and like to follow you around. Poison ivy is extensive in some areas, especially prevalent on limestone and dolomite. Learn to recognize it. Ask faculty to point it out until you learn. Alcohol and smoking retard your ability to function efficiently in the field. Much of the time we work on private property. Respect it and leave no trace of your presence. After the rainy season, forest fires are a very real hazard. It is best not to smoke in the field. If you must, then sit down in one place to smoke. It is not acceptable to be walking around smoking. Cigarette butts are to be ground out, then field stripped before being released. That means you must use your fingers to strip the paper and physically disaggregate the tobacco (if it is too hot to do this, it is too hot to put down), then grind the remains into the ground.

MISCELLANEOUS COMMENTS: Cell coverage varies, and internet access is available in spotty conditions at each of the places that we stay. Sturdy field boots are required. Tennis/running shoes are not acceptable. 6

You will be staying in dormitory rooms. Students from all schools will be paired, but not males and females together. Rooms are Spartan (beds, mattress, closet, desk, dresser, chair, wastebasket, four walls, ceiling, door and a window. There are electric outlets. Bedding and pillows are not provided. Bringing a small sleeping bag is recommended over linens. You may wish to have a few geology texts for reference. Don’t bring your entire collection, as weight and storage become a problem. It is better if a group of students agrees for each person to bring one or two books to share, so the spectrum is covered. If you maintain a positive attitude and an open mind, and if you try your best, you will most likely learn more in this course than in any other course you have ever taken and will enjoy doing so. Alumni say they will remember it fondly the rest of their lives. This course ties together most of the geology courses you have taken in preparation for being here and prepares you to enter the professional world. Laundry facilities are available at most of the places that we stay.

MANDATORY ITEMS TO BRING ......

Geology Equipment pocket knife heavy belt for field gear hand lens, approx. 10X geology hammer watch Water bottle calculator, with trig functions knapsack or small pack protractor-ruler (6”) pencils drawing pens colored pencils, erasers

Personal Stuff Bedding (sleeping bag) & pillow Clothes for about a week. Avoid cotton for the field clothes. medications towels, swim suit hat field boots (Vibram soles are best) & socks insect repellent and sunscreen field clothes, including gloves sunscreen rain gear cold-weather jacket or coat toiletries

Sunday Monday Tuesday Wednesday Thursday Friday Saturday 20-May 21-May 22-May 23-May 24-May 25-May 26-May Day Arrive ISU Chamberlain Badlands and Bighorns Steamboat Paleozoic Mesozoic Devils Tower Recon

Eve.

27-May 28-May 29-May 30-May 31-May 1-June 2-June Day Draft Off Amsden Amsden Amsden Steerhead Steerhead Draft PM

Eve.

3-June 4-June 5-June 6-June 7-June 8-June 9-June Day Steerhead Draft Off South Heart Heart Heart Fork Mountain Tour Recon Eve.

10-Jun 11-Jun 12-Jun 13-Jun 14-Jun 15-Jun 16-Jun Day Heart Heart Draft Off South Fork South Fork South Fork

Eve.

17-Jun 18-Jun 19-Jun 20-Jun 21-Jun 22-Jun 23-Jun Day Draft Travel to Black Hills PC Outlaw Rochford Rochford Rochford Spearfish Recon

Eve.

24-Jun 25-Jun 26-Jun 27-Jun 28-Jun 29-Jun 30-Jun Day Rochford Draft Return to Return Home Illinois

Eve.

How Friends and Family Can Contact You Below are the addresses and phone numbers at which you can be contacted. Note that while at field camp you can receive mail at any location where we will be staying for more than a week. We do not recommend that anyone try and send you mail at any of our other locations because it is highly unlikely you will receive it while we are there and it is difficult if not impossible to track down letters or packages that might arrive after we depart from one of our "base camps." For our Sheridan and Cody destinations (first month of camp), internet access and cell coverage will be limited. You should have good cell coverage as we drive to and from field areas, but the evening accommodations and field sites will have limited coverage. Please plan accordingly. Coverage is excellent in Spearfish.

Lee's Motor Inn/Best Western Chamberlain, South Dakota (605) 734-5575

Sheridan College 3059 Coffeen Avenue Sheridan, WY 82801 (307) 674-6446

A.L. Mickelson Field Station 32 Painter Road Cody, Wyoming 82414 (307)- 754-6737

Black Hills State University 1200 University Unit 9100 Spearfish, SD 57799-9100 (605)-642-6464

SOME THINGS TO REMEMBER

1. Up the mountain!

2. It is a marathon!

3. Get better every day!

4. The first thing you do when you look at the rock is look at the rock!

5. Don’t worry about what doesn’t matter!

6. Help other folks when then need it. Keep an eye on them!

7. Keep a positive attitude. Yesterday is gone. Tomorrow is the best day of your life!

8. Down the Mountain!

9. Carpe Diem!

Environmental Hazards

Considerations

 Field camp activities occur at elevations ranging from 4,000 to 10,000 ft (1,500 to 3,000 m). The reduced amount of oxygen at altitude may adversely affect pre-existing medical conditions. Symptoms such as shortness of breath and rapid pulse may develop. Blood pressure may increase transiently and some may develop swelling in their feet and ankles.  Expect a wide range of temperatures. Typical daytime temperatures range from ~50 to 90 °F (10 to 32 °C). Evenings are generally mild, but be prepared for nighttime temperatures from 30 to 50 °F. In a single day the weather can go from sunny and warm to a blizzard. BE PREPARED FOR THE UNEXPECTED!  Dehydration develops quickly at field camp because of the very low relative humidity and intense sunshine.  The thinner atmosphere at high altitude filters less UV light and thus predisposes one to sunburn. Similarly, the sun is very bright in the mountains so bring a good pair of sunglasses.  High winds can occur. Be prepared with a suitable windbreaker... wind can be exhausting and annoying for the unprepared.  Intense snow and thunderstorms are possible, again be prepared for the unexpected.  Vegetation will vary from mountain forests to desert terrain at Alkali. Be aware of your surroundings at all time.

Associated Safety Hazards

 Altitude sickness is a syndrome with potentially incapacitating symptoms. Although it generally only occurs when one sleeps at altitude above 8,000 ft (2,500 m), students may develop symptoms. Frequent symptoms are headaches, nausea, insomnia, extreme fatigue, listlessness, lack of appetite, and light-headedness. Generally, symptoms will improve with rest and fluids in 24 to 48 hours. Report any persistent symptoms to the Director. Alcohol, tranquilizers, sleep medication, and antihistamines may make altitude sickness worse.  Dehydration can cause fatigue, severe headaches, and result in heat exhaustion & sunstroke.  Exposure can result in sunburn, windburn, cold-related illness (hypothermia & frostbite), heat-related illness (heat exhaustion & sunstroke), and snow blindness (sunburn of the eyes).

Mitigation of Hazards

 To prevent altitude sickness and dehydration, always drink enough water to cause the need for urination at least every three hours while at camp. While in the field, students should drink a minimum of two to three liters of water per day. Drink small amounts of water at a time throughout the day. Hydration packs (e.g., Camelbacks) are great investments.  Sunburn and heat exhaustion are common and unnecessary camp ailments. Sun block is mandatory for those with sensitive skin and should be used by all. People who do not usually burn are more apt to get caught because they do not take necessary precautions. Wear wide-brimmed hats, pants, and light shirts to keep cool and prevent sunburn.  Wearing UV filtering sunglasses will protect your eyes from snow blindness. Contact lenses can be troublesome in the field because of dust and low humidity. If you prefer contact lenses, bring lots of lens solution and a backup pair of glasses. It is important to wear eye protection (glasses, goggles, sunglasses, etc.) in the field to protect your eyes when you (or those around you) are breaking rocks.

Health Hazards

Considerations

 Field camp is physically challenging and students must be prepared for a rigorous field experience. You will be conducting physically demanding field exercises, including considerable hiking at high elevations. Students in good physical condition are able to complete the course without difficulty. However, the performance of students in poor physical health or condition could be hindered by an inability to access portions of field project areas. Field camp is a physically demanding and may not be right for everyone.  Venomous rattlesnakes are common at lower elevations, but they are timid, avoid people, and rarely bite. Mountain lions and bears have been seen in some of the field areas in the past. Moose are commonly encountered in the alpine meadows of our field areas. These are large and sometimes dangerous animals. Do not enter an area occupied by moose. All animals should be left alone – harassing any animal could result in your dismissal. Do not feed any animal, either intentionally or through your own inaction.  Stinging insects such as hornets, wasps and bees are present. Ticks are common at lower elevations.

Associated Safety Hazards

 Poor physical conditioning may lead to overexertion and severe fatigue that can exacerbate preexisting medical conditions and contribute to accidents. Participants in poor physical health or condition and those with serious medical conditions must check with their physician prior to attending this camp.  You may have allergic reactions to insect bites or plant puncture wounds.

 Burrowed ticks can cause infection or spotted fever.

Mitigation of Hazards  Please consult your physician before considering attending camp – especially if you have a history of cardiac or pulmonary problems, physical, emotional, or mental conditions. In such cases, a physician’s permission may be required prior to acceptance. We urge you to have thorough and complete medical and dental checkups before field camp. Minor complaints will be amplified under the stresses of heat, altitude, and hard work at camp. These issues should be taken care of in advance. You should have shots for tetanus if not currently protected.  Participants should determine their ability to handle the short periods (20 - 60 min) of strenuous exertion at relatively high altitudes required to access some of the field areas. Vertical changes in elevation are generally on the order of 800 ft (250 m) or less at base elevations of 5,000 to 10,000 ft (1,500 to 3,000 m). The importance of beginning daily cardiovascular exercise at least one month prior to departure cannot be stressed enough.  Fully disclose all regular medications (prescription and OTC) in the Personal Medical Assessment form and notify the Director(s) of any special medications you may be taking before any emergency situation arises.  Fully disclose all known dangerous allergies (e.g., insect bites, foods, etc.) in the Personal Medical Assessment form. Bring any medicines or antidotes (epi-pens) that you might require. To ensure you receive appropriate care in the event of an emergency, personally bring your allergies to the attention of the Director(s) on the first day of camp (so we can associate your face with the form).  Health insurance is required. You are responsible for all medical and dental expenses while at camp – the camp has no responsibility for the medical expenses of students and does not provide students with any forms of medical insurance. Work closely with your family and your university to determine what health insurance policies are available to you and what the limits of your coverage are before coming to camp. Make sure to carry your medical insurance information with you at all times.  Use insect repellant and check yourself carefully for ticks each day.  Your chances of being bitten by a snake become remote when wearing sturdy, over-ankle boots and by not putting your hands (or any other part of your body) in places you cannot see. Leave the snakes alone – a little fear is a very healthy thing. If you should be bitten, make sure the snake was a rattlesnake before doing anything. If it was a rattlesnake: (1) Slow down circulation and be as inactive as possible. If possible, don't run or walk, and have someone assist you to a vehicle. Allow the wound to bleed freely. (2) Have someone get you to a doctor as soon as possible. Make sure the doctor checks for reaction to horse-serum, if necessary.

Activity Hazards

Considerations

 Expect daily hikes in the field areas that cover several miles and up to 1000 ft (300 m) of relief.  Hikes typically cross rough, steep, and/or unstable terrain. Good judgment and extreme care for yourself and those around you are critical. Many people have been injured (some seriously) or even killed (fortunately, not our students) wandering through the areas we work in.  Unsafe cliffs, overhangs, and steep slopes are common in field areas. Stay away from these hazards and be aware of the people above and below you. A former student broke both legs in an accident when another student unintentionally kicked loose a boulder. She also required an airlift.  Many students have limited experience working in the outdoors and with the tools of a field geologist. Misuse of field gear (e.g., hammers, bruntons, GPS units, etc.) may create unexpected hazards. When used improperly, personal gear may provide less than optimal protection from the elements (e.g., improperly laced boots, hats in backpacks instead of on head, etc.).  The most serious injuries at field camps do not occur in the field where geologists are all generally fully aware. On the contrary, most serious injuries occur when students are relaxing: playing ultimate Frisbee, soccer, mountain biking or even walking home from “an evening out”.  Reckless behavior in the field (running and jumping over gullies, wandering away from the group, etc.) or at camp (roughhousing) that compromises the safety of yourself or the others on the trip will not be tolerated and could result in dismissal from camp.

Associated Safety Hazards

 Trip and fall hazards are extremely common, especially along ledges and steep slopes. Loose rocks and overhanging rocks exist on hillsides and trails. Remember that an injury to your knees, back, etc. could seriously limit or end your career as a field geologist, certainly for the summer.  Inadequate footwear commonly results in a range of avoidable injuries, including puncture wounds (from thorny brush, cactus spines, and sharp rocks), severe ankle injuries, and slips and falls.  Improper use of hammers and/or chisels can result in serious injury to yourself or those around you. Common injuries include crushing wounds and metal shards and/or rock chips in eyes.  Reckless behavior creates a wide range of completely avoidable hazards.

Mitigation of Hazards

 While each person is primarily responsible for his or her own safe conduct, they must also contribute to the welfare of the entire group. In each field area, the leaders will brief participants on expected and potential hazardous situations and conditions.  If you are not comfortable participating in any of the particular activities for any reason, you are encouraged to notify an instructor. There are no negative implications for this decision. 13

 Sturdy, close-toed boots are required in the field at all times. Boots with Vibram-type soles, good tread and sturdy leather (or similar) uppers provide excellent protection against injury. Participants without adequate hiking footwear will not be allowed to enter field sites and will be barred from participating in some activities. For example, Newmont Mining Corp requires boots and long pants at all times. Please contact an instructor, ideally before camp begins, if you have any questions regarding the field boot policy.  Long pants are recommended in the field  When breaking rocks, move away from others and turn your back towards them. Always warn those around you when plan to use your hammer! Never use another hammer as a chisel. Always protect your eyes with some form of safety glasses (e.g., shatter resistant sunglasses). Once again – an injury to your eyes (and those of your classmates) that could seriously limit a career in geology.  Please be careful when not in the field. When relaxing, remember that you’ve been working hard and are probably more tired than you realize. You have spent a lot of time and money to get to this stage of your geological education - don't screw it up with a careless injury that prevents you from completing the course.  Reckless behavior is unacceptable. If your behavior becomes a significant problem, you may be dismissed from Field Camp and will be responsible for your own trip home.

Transportation Hazards

Considerations

 Thousands of freeway and off-road miles are put on vehicles at camp each summer. Drivers have a great responsibility and must be extremely vigilant and careful – the lives of all of passengers, members of the caravan, and the public at large are in their hands.

Associated Safety Hazards

 Driver-related hazards are significant and include fatigue, distractions, and inattention during driving.  Car-person collisions are a serious hazard during activities that require work along busy roadways.  Flat tires introduce significant hazards associated with both the operation of a moving vehicle and roadside repairs.

Mitigation of Hazards

 All participants must pay careful attention to safety briefings by the instructors. It is your responsibility to be fully informed of potentially hazardous conditions associated with use of vehicles.  Passengers must never do anything to interfere with the driver's ability to operate the vehicle safely.  Passengers riding shotgun should never sleep and should help keep drivers awake and aware. 14

 Personnel driving university vehicles must have completed ISU’s van driver training.  Before driving, drivers should take whatever time needed to familiarize themselves with their vehicle and routes.  Participants must wear high-visibility clothing (and/or safety vests if provided) when working along roadways and bike trails.  Read, understand, and follow the procedures in the Guidelines for Driving University Vehicles.

Guidelines for Driving University Vehicles

The following guidelines regarding the use of university vehicles must be read, understand, and followed at all times:

For All Vehicle Occupants  Safety restraints (seat belts, both lap and shoulder) must be worn by everyone whenever a vehicle is in motion.  Keep Vehicles Clean. Regularly remove trash. Loose bottles, rocks and materials on the floor are significant hazards to safe vehicle operation. Whenever possible, maintain the driver’s ability to see out of the rear window when packing the vehicles.  Hazardous materials (e.g. flammables, corrosives, explosives, compressed gases, etc.) must not be transported in a university vehicle.  Alcohol consumption in any university vehicle is strictly prohibited.

For Drivers  Authorized drivers are individuals cleared to drive specified vehicles by a university’s department of risk management. Drivers must immediately inform field camp and their home university if they receive a suspension, probation, cancellation, or disqualification of his/her driver's license.  Authorized drivers are subject to all traffic laws and are financially responsible for any traffic citations. In addition to posted limits, drivers must operate at speeds suitable for vehicle, road, traffic, and weather conditions. Where vision is restricted, drivers must slow to a speed that will permit the safe negotiation of curves, hills, or intersections.  Driver fatigue is a leading cause of fatal accidents. Requests for breaks or driver swaps have priority over any itinerary.  Keep to the right on highways. Do not linger in the fast lane – use it only to pass.  Do not pass the lead vehicle. Unless safety dictates otherwise, maintain your position in the line of vehicles. Do not worry about “keeping up” with the vehicle ahead of you – instead, slow your vehicle to maintain visual contact with the vehicle behind you.

 Avoid distractions. The use of cellular telephones and iPods, etc. while driving are not permitted (even with a “hands-free” device), except in immediate emergency situations.  Drivers are responsible for thoroughly inspecting their vehicle prior to initial use and regularly thereafter for unsafe conditions or damage. Drivers must immediately report vehicles that are damaged or in an unsafe condition.  If you experience a flat tire or other vehicle malfunction that requires leaving the roadway, pull to the right shoulder – never stop in the center median. Carefully pull off as far as possible, but do not go over the shoulder. If possible, park on a hard (paved) surface.  Accidents involving any vehicle (university or private) during field camp must be reported according to relevant accident reporting guidelines. The camp director must be notified immediately.  Use of university vehicles while under the influence of alcohol or drugs (including prescription drugs that may impair the ability of the driver) is prohibited.

GEOLOGICAL FIELD NOTES

Taking good field notes is an integral part of geologic mapping and field work. Your field notebook is among the most important pieces of equipment you take with you every day and it should be useful to other geologists who are not familiar with the area in which you are working. Good notes are legible, organized, detailed, and informative. The organization of your notes will determine how accessible they are to other geologists. If another geologist cannot understand what you are writing about, where you are working, how your data are recorded, what led you to interpret your data the way you did, or why you are conducting this study in the first place, then your notes are essentially useless to anyone but you. In the inside cover of your notebook you should list your name, employer, and complete contact information. A table of contents that lists each project should also be included in the first page of your notebook. Be sure to update the as you progress through this course. Remember to continually update notes as needed; NEVER expect to complete your notes after you have left the field area at the end of the day. You will record three distinctly different types of field notes during your time at Field Camp. These include: reconnaissance notes, stratagraphic column notes, and geologic mapping notes. Each of these has its own unique style but involve much of the same information. Examples of each of type of notes are provided on the final three pages of this section.

1. Reconnaissance Notes These are notes taken during local or regional reconnaissance of a field area. They are a log of all the stops you make and a record of your observations and interpretations that will be referenced later as you conduct more detailed work in that area. Good reconnaissance notes can be a great help in understanding the details of a smaller area of study in that region. Reconnaissance notes should contain information in each of the following subject areas.

Date: Record the date at the beginning of each day. Each day of vanology notes must begin on a new page. Purpose: This is a sentence or two addressing the reason for conducting this reconnaissance. Weather: Record the weather at the beginning of the day and note changes as the day progresses. Rocks look different on wet cloudy days than they do on dry sunny days. Stop Number: (these are simply numbered) Time: Record this for each stop. Location: May include road names, mile markers, distance from nearby towns, grid system coordinates or other forms of location identification. Someone else should be able to find this stop using your notes. You should be paying attention to where you are when we travel so you gain a better understanding of how one stop is geographically and geologically related to the next. Purpose: Explains the reason you are making each particular stop. It could be to get a regional overview, to describe a road cut, or some other specific reason. This can be as short as a single sentence. Example: To describe outcrop along road which is representative of Cambrian stata in the region. Observations and Data: This can take many forms; outcrop and hand sample description, structural data, stratigraphic columns, and sketches. This is the most important part of your notes and must be as detailed, organized, and legible as possible. Interpretation: This is separated from your data and is YOUR explanation of what you’re looking at, how it got there, and its significance. These are always open to debate whereas your data is not.

Reconnaissance Note Grading – scores will be bases on: organization, outcrop/hand sample descriptions, sketches, and style and grace (including penmanship). 17

2. Measuring Stratigraphic Column Notes These notes include a scaled sketch of a section with notes describing the material and may include your interpretations.

Date: Record the date at the beginning of each day. Location: This should be specific. Measured sections can vary greatly in a short distance. Partner: Record the names of all those you are directly working with. Purpose: This is a sentence or two addressing the reason for creating this column. Weather: Record the weather at the beginning of the day and note changes as the day progresses. Methodology: Document the method you will use to measure the section and any formulas you will use for calculating thicknesses. Include eye height and pace if used. Observations and Data: (See example) this will include a scaled sketched column with weathering profile, formation/unit names, geologic time labels, structural data if acquired, descriptions, and interpretations. Include published geologic patterns in sketch. When creating your own original patterns, include an explanation of that pattern.

Stratigraphic Column Grading – scores will be based on: organization, column sketch and labeling, descriptions, and style and grace.

(eg., strike &dip, traverse bearing, Obstacles or complications in

Binding of field

Example layout of your field notebook when measuring stratigraphic

3. Geologic Mapping Notes These notes document everything needed to complete a geologic map and cross section. They are a record of where you were, what you saw, and your interpretations. They include location information, traverse direction, structural data – in textual and tabular format, sketches, outcrop and hand sample descriptions, and interpretations. Although you will document data on a topographic base map, anyone should be able to recreate the geologic map using your field notes.

Part A Introduction – this part begins each mapping project and sets the stage for the entire multi-day project. The introduction will be on the first page of each mapping project and should be setup before entering the field.

The introduction includes; Title: For the project, example: Geologic Mapping of the Clear Creek Thrust near Buffalo, WY. Location: The location of the project which can be directly copied from the final map you will be given. Partner: You will be assigned a different partner for each project. We do this for safety reasons. You will be expected to collect your own data. Purpose: This is a couple of sentence addressing the reason for this project. Benchmarks: (if used) Document the benchmarks you may use for field station location in this area. Benchmarks can be a variety of things including: road intersections, UTM gridline intersections, Township and Range gridline intersections, or X’s at hill tops. You may need multiple benchmarks to cover the map area. All should be identified and documented prior to entering the field.

Part B – this part must be done for each day of mapping.

Date: Record the date at the beginning of each day. Weather: Record the weather at the beginning of each day and note changes as the day progresses. Location: This is the specific region of the mapping area you will work be working during this day. Purpose: This is the purpose specific for each day. Data: (see example) this is a record of everything you see, do, and think. This is where organization is crucial. These notes should read like a book, telling the story of everything you see, do, and think, both texturally and graphically. Data includes:

Field stations: You will assign a field station ID at each place you collect data. This is a combination of letters and sequential numbers. Example: CC01

Locations: All field stations locations will be recorded with a distance and bearing from one of your predetermined benchmarks.

Traverse: Record the direction you will travel as you leave a field station and why you are going that way. This is just a sentence letting the reader know where you are going and why. For instance: Walking ~SW from CC23 toward 6530 hill looking for change in soil color which would indicate a change in mapping units.

Descriptions: You will be describing a variety of things in the field. Outcrop description will consist of size of exposure, bedding thickness and orientation, weathering profiles, color, etc. Hand samples must be systematically described just as they are in sedimentology, mineralogy, or petrology. When approaching a new outcrop, first describe the outcrop. This gives the reader a mental image of what you are looking at and sets the stage for hand sample description. You may also be describing offset along faults, changes in soil color and texture, and change in vegetation. All measurement for a project must adhere to the same measurement system. This means, if you start with the metric system stay with it for the entire project. This applies at all scales and includes sketches.

Sketches: These are an important part of your notes and visually support your textual notes. Following are the standards you will need to follow.  Orientation – Sketches are oriented either top is toward the top of the notebook or top is to the front cover. (see examples)  Labels – each sketch must have a title indicating whether the sketch is map view or cross- sectional view. Include N, S, E, or W directional labels.  Explanation – each sketch needs a textual description indicating the significance of the drawing.  Dimensions – label both vertical and horizontal distances on cross sections and map distance on map views.  Other things – you may find it appropriate to label mapping units, structural data, or fault slip directions directly on the sketch.

Structural data: Structural data must be recorded both in your textual notes and in the structural data table.

Interpretations: Your interpretations must be separate from your observations. These will be revisited as you begin creating your finished geologic map and help you remember what you were thinking as collected data at a field station. They also will help a reader of your field notes get a better understanding of what you think occurred in an area. They might include your thoughts on the depositional environment, sense of deformation, relative age between units, or anything else you may want the reader to understand about what you see and think. Keep in mind, these are always open to debate and need not be accurate. You may refine or change your interpretations as the project progresses. Your interpretations are unlikely to be “right” the first time. Strive to correctly record how things “are,” and then develop multiple hypotheses to explain the geology you observe.

In general, you should never erase any hypotheses or interpretations you enter in your notebook. Use it as a running log of your daily ideas and struggles. Who knows, on the third day of mapping you may uncover new data that supports an interpretive cross section you made on day one, but that you thought was crazy on day two!

Taking and maintaining good field through the duration of a project is hard. Developing good field note taking habits not only will help you with the activities here at field camp, they also help you organized your observations make you a more competent scientist.

Geologic Mapping Notebook Grading – scores will be based on: organization, note quality, sketches, rock descriptions, and style and grace.

GEOLOGICAL MAPS AND MAPPING

The objective of geologic mapping is to produce a map that shows the surface distribution of rock units, with contacts drawn between them, and sufficient structural data to construct cross sections illustrating the structure. You will have two copies of the map you are making, a field map and an office map. The field map is a work copy and is done in pencil. Although you should strive to keep it neat, it is not expected to be immaculate. Instructors will periodically collect and grade your field map. The office map is your insurance policy. Every night when you return from the field, you should transfer data and contacts from your field map to your office map. Instructors will not check to make sure you have an office map, but, if it rains all over your field map on the next to last day of mapping and you lose all your data for the previous two days, you'll be sorry you didn't keep an office map. The following are some general comments, guidelines and rules regarding maps and mapping.

• Keep track of yourself on the topographic map at all times. Know the scale of the map, the contour interval and how to triangulate to locate yourself in problem areas. • All mapping is to be done in the field, including placement of contacts and plotting of structural data. This means that strikes and dips are to be plotted on the map in their correct location and orientation at the time they are measured. This is important because, 1) it allows you to recognize inconsistent data when you can still correct it, and 2) it allows you to see the structure developing, so you can plan a logical strategy for continued mapping. This is the most difficult thing the faculty have to convince students to do and why we often collect field maps unannounced. • Field stations should be accurately located and marked on the map. Your field notes for each station should be identified by the station number and should include the GPS coordinates and/or a brief geographic and geologic description of the station location. • Contacts: Solid lines for contacts located within the spacing of one contour interval, dashed lines for less accurately located contacts and dotted lines for contacts covered by other units, such as by Qal or Tg. • Contacts between sedimentary rocks do not normally meet or truncate one another, unless there is an angular unconformity. Sedimentary rock contacts usually can be projected logically as a mapping strategy. • Contacts of igneous bodies commonly are irregular and truncate other contacts. They must be “walked out” or followed carefully in the field. • Qal (Quaternary alluvium) refers to Recent stream deposits. These are flat-lying, so contacts should closely parallel contour lines. There is no Qal on hill slopes. Qal should not be used to hide contacts, instead contacts beneath Qal should be dotted or concealed. • Be aware of the “Rule of V’s”. Unless formations are vertical, contacts must obey the Rule of V’s when crossing topography. In general, a contact will "V" in the direction of younger rocks, unless beds are overturned. (See figure on the following pages.) • Be aware of “strike ridges”, those that follow the strike of bedding. Be suspicious of plotted strikes that are not parallel to strike ridges and contacts that cross strike ridges.

• Do not place contacts along the crests of ridges. Linear topographic ridges, or hogbacks, are often held up by a resistant unit, the base of which is some distance below the crest. The contact making the base of the resistant unit is commonly at the base of the ridge. Contacts can easily be projected along hogbacks. • Strikes and dips: Bedding plane surfaces commonly are irregular at a given outcrop. When taking S&D measurements, survey the outcrop and select a place where the attitude is representative of the overall trend at the outcrop. A sufficient number of S&D’s should be plotted to determine the structure of the area, no matter where you are asked to draw a cross section. • Spacing of strikes and dips: If S&D is consistent for a considerable distance, then you need not record them very close together. However, if the attitude changes rapidly, then S&D must be recorded with a closer spacing. You can’t have too many structure data. • Coloring: As you traverse an area of the map, color it in lightly for the appropriate rock unit; this gives you some perspective on how much has been accomplished and how much remains to be done. • Field sketches: As you piece together the structure, make interpretive, labeled sketches of the map (or cross sections) in your notebook. This should help you to interpret the geologic history, to figure out the strategy for completing the project and to make cross sections later.

Variables Affecting the Appearance of a Geologic Map Outcrop patterns of sedimentary and metasedimentary rocks are a function of relatively few variables. Having a firm grasp of the general influence of each of these variables will be a great help when you are mapping because it will allow you to qualitatively or quantitatively check your map for common errors and inconsistencies. The general variables affecting the outcrop pattern of any planar feature are: 1. formation thickness 2. formation strike and dip 3. surface topography Remember that just like contacts between formations, faults are also planar features which are affected by these variables. You should be able to infer the general strike and dip and thickness of a layer from its map pattern. Conversely, when you are mapping, if you know the formation thickness and strike and dip, you should be able to estimate or predict the general outcrop pattern for any planar feature. All bets are off when you are mapping igneous rocks or any feature that is not curviplanar in 3-D. These features can have highly irregular outcrop patterns that are totally unpredictable!

Techniques to Test the Quality and Accuracy of Your Map If you are unsure about the placement of contacts on your map, there are several tools you can use to ensure you placed your contact reasonably. These tools include: 1. calculate the formation strike and dip from your map pattern and compare it with the strike and dip data you collected – they should match! 2. use three locations where you are certain of a contact to construct an outcrop pattern over the topography of the problematic region 3. calculate the formation thickness from your outcrop pattern – it must be close to the known thickness of the formation!

Note that these approaches only work if the rocks are homoclinally dipping throughout the area in question. If there are abrupt, significant changes in strike and dip over the region in which you are trying to construct the outcrop pattern or calculate the strike and dip, don't waste your time making these types of checks.

Common Map Errors • violation of stratigraphic sequence without a fault or unconformity present • violation of the Rule of V's • improper use of contact types (e.g. precise, approximate, covered) • placement of contacts requires incorrect formation thicknesses • inaccurate structural data • incomplete coverage of area with structural data

Official United States Geological Survey Map Colors

Age of Rock Unit Map Color Quaternary orange Tertiary yellow ocher Cretaceous olive-green Jurassic blue-green Triassic bluish gray-green Permian blue Pennsylvanian gray Mississippian blue-violet Devonian heliotrope Silurian purple Ordovician red-violet Cambrian brick red Precambrian pink or gray-brown

The “Rule of V’s” shows the relationship between outcrop patterns and formation dip

GEOLOGICAL CROSS SECTIONS

Basic Principles of Cross Section Construction For rocks that have undergone low temperature (≤ 250˚ C) and low to moderate stress (≤ 200 MPa) deformation, a number of simplifying assumptions and principles are commonly used when constructing geologic cross sections. Interpretations that adhere to these guidelines and honor all available data are deemed “viable and admissible,” and cannot be discounted as possible interpretations of the subsurface geology. A cross section that does not honor the viability and admissibility criteria outlined below is automatically rejected as a possible interpretation of the subsurface geology because these cross sections require or imply that highly improbable or impossible physical situations existed before, during or after deformation. It is equally important to recognize, however, that viable and admissible cross sections are not automatically “correct,” and that many different viable and admissible cross sections can be drawn along any given section line.

Viability Criteria Conservation of Layer Length and Area: During all stages of deformation, the lengths and areas of rock layers can be assumed to remain constant. Under plane strain conditions, these restraints are equivalent to “volume balance,” meaning that, excluding the effects of erosion, rocks that exist prior to the deformation cannot increase or decrease in volume during the deformation. In terms of faulting, this means that although layers may be shortened or extended by slip along a fault, the sum of the lengths of a faulted layer in all fault blocks is equivalent to the original length of that layer. When the slip is removed from all the faults, all the layers must also have realistic pre-faulting geometries. In terms of intrusions, this means that if the intrusions do not assimilate significant volumes of host rock, the lengths and areas of intruded country rocks will not change during the intrusion process. Consequently, it should be possible to “deflate” an intrusion and have all the surrounding rocks return to a realistic pre-intrusion geometry; there must be no holes or overlapping layers. This principle does not apply if there is significant penetrative deformation in the layers (e.g. cleavage).

Return to Regional: In the absence of significant basement topography, undeformed units throughout a region should each lie at a unique depth known as the “regional” for that unit. Away from intrusions and areas uplifted by faulting, each unit should “return to” its regional depth.

Fault Template Criterion: Every fault hanging-wall flat and ramp must have a corresponding footwall flat and ramp. Fault blocks restored to a pre-faulting geometry should match perfectly with no gaps or overlaps along the fault.

Admissibility Criteria Honor Regional Tectonic Styles: The general structural style of a region is defined by the types and geometries of structures in that region: high angle reverse faults, intrusions, low angle thrust faults, detached extensional faults, salt domes or some combination of structures. Previous work and a general understanding of the tectonic setting of a region have typically established the kinds of structures that are present in an area. When drawing a cross section through an area you should always interpret and draw structures in a style that is compatible with the styles established by previous work or styles we expect in similar tectonic settings. This does not mean 27 that you cannot discover a new or unique structure in an area; one that is different from the usual structural style. However, if you’re going to interpret a low angle thrust fault in the Black Hills, an area where Paleozoic rocks are dominated by intrusions and high-angle reverse faulting, you better have very good evidence to support your interpretation.

Occam’s Razor —Simpler is Better: Always strive to draw the simplest possible subsurface interpretation. Simple interpretations can be supported by very basic assumptions like the viability criteria. The applicability of these assumptions has been demonstrated in a number of studies in a variety of tectonic settings. In the absence of data constraining complexities in the subsurface, it is best to rely on the strength of the viability assumptions. Subsurface complexities may be present, but should not be stylistically added to a cross section; you must have evidence that such complexities exist!

Apparent Dips: Correct Use of Strike and Dip Data in Cross Sections Cross sections must accurately and precisely reflect the orientation of formations crossed by the section line. Because section lines are rarely perpendicular to every strike and dip near the line of section, formations will exhibit apparent dips in the plane of the cross section. Correct representation of subsurface geometry requires the calculation and use of apparent dips. Plotting these dips on the cross section will insure that you honor your data during section construction and will illustrate to others, the surface constraints on your subsurface interpretation. Apparent dips are calculated and plotted as described below.

1. Project data along strike to the section line • do not project across formation boundaries • allowable projection distance is inversely proportional to the structural complexity between the data point and the section line 2. Calculate the apparent dip in the section plane using a nomogram (see Appendix 1) or trigonometric formula 3. Precisely plot the apparent dip in the section using a “tadpole” drawn on the topographic profile. The tail of the tadpole points in the dip direction and is inclined to the horizontal at an angle equal to the apparent dip angle. 4. Write the apparent dip angle above the tadpole on the section

Common Cross Section Errors • topography-induced folding • intrusion pipes and feeders don’t match between sections • contacts on cross sections must match those on the map • cross sections must be identical along their line of intersection • kinematic linkage between intrusions and faults • honor your surface dip data • no blank spaces underground • unsupported changes in formation thickness

Steps in making a topographic profile for a geologic cross section

Step 1: draw the cross section line over the topographic base map.

Step 2: Lay a thin strip of paper along the section line and mark the locations where topographic contour lines intersect the cross section line. Also mark the locations where slope changes (e.g. streams and ridge tops). Step 3: Graph the elevations of the marked lines on your strip of paper. Make sure the vertical scale is equivalent to the horizontal (i.e. map) scale if you want a true-scale cross section. Step 4: Connect your graphed dots to make a smooth topographic profile.

Constructing Geologic Cross Sections using Down-plunge Projection Down-plunge projections can be used to construct geologic cross sections in areas where the topographic relief is relatively small and the structural geometries are relatively simple. In areas with complex structure or high relief, down-plunge methods can still be used to construct geologic cross sections, but there will be increasingly greater error in the projection.

Step 1: Before You Start Determine the average orientation of the major fold axes in the map area. To do this, plot together on a stereonet, the axes of all the minor folds and intersection lineations you measured in the map area. Once these data are plotted together, visually estimate the vector mean of the lineations and plot this estimated mean as a new point on the stereoplot. Record this mean plunge and bearing at the bottom of your stereoplot and reserve it for later use. If you have access to a computer-based stereonet program, you may want to use the program to precisely and accurately calculate the mean. Remember you cannot determine the vector mean plunge and bearing by simply adding together all the bearings and finding their average, and then adding together all the plunges and finding their average.

Step 2: Preparing to Draw Your Map Projection Grid Draw across your map, a light pencil line whose orientation is the same as the bearing of the average fold axis you determined in Step 1 above. Place your map on a tabletop or light table so that the down-plunge ends of the major folds are close to you and the up-plunge ends of the major folds are away from you. (i.e. the folds should be plunging toward you as you sit in front of your map.) Rotate the map slightly so that the light pencil line representing the average bearing of the fold axes is oriented vertically. Tape your map down in this orientation. Tape a sheet of velum or tracing paper over your map so that the long axis of the paper is parallel to the average fold axis bearing you determined in Step 1 above. The tracing paper must be large enough to completely cover the map area.

Step 3: Drawing Your Map Projection Grid Once your map and overlay are taped down, complete the following steps in sequence. All of the drawing for this step will be done on the tracing paper overlay and should be done with a ruler or straightedge. (a) draw three Reference Lines that are parallel to the average fold axis bearing and that extend across the entire map area. Draw one of the lines near the center of your map, and two lines that pass through the endpoints of your section line. (b) starting at one edge of your map, draw a series of parallel, equally spaced lines that are perpendicular to the average fold axis bearing. Draw these lines so that their spacing (s) is small enough to allow many lines to cut across your map area. The spacing will be a function of the complexity of your map; accurate projection of more complex maps requires smaller line spacings whereas simple maps can be projected with wider line spacings. (c) consecutively number each of the parallel lines you just drew, beginning with the line closest to you (i.e. on the down-plunge end of the major folds)

Step 4: Calculate the Spacing of the Cross Section Grid To calculate the new spacing (s') that your map grid lines will have when you project them into your cross section plane, we use the equation:

s   s  tan( plunge )

30 where s is the spacing of your map grid lines and (plunge) is the average plunge angle of the major folds you determined in Step 1. The cross section grid spacing (s') will be larger than the map grid spacing (s) if your plunge angle is greater than 45˚, and smaller than the map grid spacing if the plunge angle is less than 45˚. If your cross section grid spacing is larger, folds in the cross section will appear "stretched out" compared to how they appear on the map. If your cross section grid spacing is smaller, folds in the cross section will appear "squished" compared to how they appear on the map. If the average plunge angle of your major folds is exactly 45˚, then your cross section grid is identical to your map grid and your cross section geometry will be identical to that of your map.

Step 5: Constructing Your Cross Section Grid Calculate the total height of your cross section by multiplying s' by the number of grid lines you have on your Map Projection Grid. This is the minimum height your cross section will be, so make sure you design your final project so that it can accommodate a cross section of this height and still have room for labels, etc. Cut a new piece of velum or tracing paper that is at least as wide as your section line and as tall as the minimum height you just calculated. Tape this piece of paper over the two pieces of paper on which you have drawn your map and map Projection Grid. Tape it so that its long axis pointing away from you. This is the paper on which you will construct your cross section, but you must first construct a Cross Section Grid to project to. Drawing with pencil on your cross section construction sheet, lightly trace over the Reference Lines you drew on your Map Projection Grid in Step 3. Extend these lines all the way to the top and bottom of the page. Beginning near the bottom of the page, draw in a line perpendicular to the Reference Lines. This is your first Cross Section Grid line, and it should extend all the way across the page. Moving up the page from the first grid line, draw in a series of Cross Section Grid lines that are perpendicular to the Reference Lines, and which are spaced a distance of s' from one another. Draw in the same number of lines that you have in your Map Projection Grid, and number each line consecutively in the same manner that you did in Step 3c. When you are finished, untape your Cross Section Grid sheet from the table.

Step 6: Projecting the Map into the Cross Section Plane Place your Cross Section Grid sheet over the map and Map Projection Grid that are still taped to the tabletop. Line up the Reference Lines and the grid line labeled "1." Choose any particular contact on your map and mark on the Cross Section Grid sheet, the intersections between that contact and grid line one. When you have marked all the intersections with grid line one, slide your Cross Section Grid along the Reference Lines so that grid line 2 of the Cross Section Grid matches up with Grid Line 2 of your Map Projection Grid and mark the intersections between Grid Line 2 and the contact you have chosen. Repeat this process for all the grid lines until you have marked the intersection of one contact with every grid line of the Cross Section Grid. When you are done, connect the dots to create the smoothly curving cross sectional view of the contact you just projected. To complete your projection, you must project each map contact in a similar fashion, consecutively aligning the Cross Section and Map Projection Grid lines, marking the contact intersections with each grid line, and then connecting all the intersection points to obtain a cross sectional view of the contact you projected. When you have projected all your map contacts onto the Cross Section Grid sheet, your Cross section Grid sheet is now your geologic cross

31 section. However, it is not a complete cross section because we do not yet know how it "attaches" to the local topography, or more specifically, how to place the vertical scale along the edge of the section. You will use your topographic profile to do that in the next two steps.

Step 7: Construct a Topographic Profile along the Line of Section Using a scrap piece of velum or graph paper, construct a topographic profile along your chosen line of section. Be sure to construct your section with no vertical exaggeration. Put a vertical scale on your section and mark the location of each geologic contact on the topographic profile.

Step 8: Placing the Topographic Profile and Vertical Scale on Your Cross Section Tape your topographic profile to a tabletop. Flip your Cross Section over (i.e. rotate it about its short axis so that Grid Line 1 is now upside down and at the top of the page and you are looking through the back side of the velum; see notes below). On the "back side" of your section, locate the numbered Cross Section Grid line that is closest to your cross section line on your map (i.e. which numbered line was closest on the Map Projection Grid?). Place the flipped- over cross section sheet over the topographic profile so that the Cross Section Grid line closest to the map cross section line runs through the center of your topographic profile. Keeping the Reference Lines at the edge of the cross section aligned with the ends of your topographic profile, slowly slide the cross section up or down until the contacts marked on the topographic profile match those on the cross section. There is likely to be a slight (1-3 mm) misalignment of some of the contacts, but if you have done the projection right, and there is not too much relief or structural complexity in your area, you should be able to find a very good match by moving the cross section no more than 5-10 mm up or down. When you have found the best match that minimizes all the contact misalignments, tape your flipped-over cross section sheet to the table over your topographic profile and trace the topographic profile and its associated scale onto the back of your cross section sheet. When you are finished tracing, remove the cross section sheet and hold it up to the light, view it on a light table, or over some white paper. You now have a projected cross section with topography and a vertical scale!

Why do we flip the cross section over and draw on its back? Because we drafted our Cross Section Grid looking in the up-plunge direction of the structures, the "bottom" of our projected cross section contains the down-plunge ends of the structures and the "top" of the cross section contains the up-plunge ends of the structures. If you think about the map and how the 3-D structures intersect the map surface and the cross section surface, you will notice two important things: 1. map points on the up-plunge side of your cross section line should project into the subsurface of your cross section, below the topographic profile, and 2. map points on the down-plunge side of your cross section line should project into the air, above the topographic profile of your cross section. If we were to draw on the "front" of our projected cross section, the down-plunge ends of our folds would appear at the top of our cross section, above ground! This is impossible. The reason we flip our cross section before adding the topography is to ensure that in our cross section, the down-plunge ends of your structures appear above ground, and that the up-plunge ends of the structures appear below ground.

PHANEROZOIC STRATIGRAPHY OF NORTH AMERICA

1 Precambrian Basement: Foundation of the Continent 1.1 Archean Rocks • Greenstone Belts • Gray Granites • High Grade Gneisses • In the Archean, few basalts and andesites are around. 1.2 Early and Middle Proterozoic Rocks • Although graywackes were still forming, light-colored, well-sorted quartz sandstones became more important. • More mature sandstones were formed many were subsequently metamorphosed to quartzite. For example: Baraboo Sequence • Stromatolites (wavy, laminated) structures formed by the accumulation of cyanobacteria. 1.3 Late Proterozoic Rocks • Keweenawan Rift • Grenville Orogeny • Rodinian continental margins

2 The Paleozoic 2.1 The Setting • The beginning of the Phanerozoic represents the largest punctuation mark of all time because of the appearance of complex life forms. • This section treats the interval from ~700-500 million years ago (Vendian, Cambrian, and Early Ordovician). • Profound unconformities mark both the top and the base of the sequence. The Sauk sequence was deposited during the Sauk transgression, a world-wide sea level rise which culminated in the flooding of nearly the entire North American craton in the early Ordovician 500 million years ago. 2.2 The Cambrian Craton • There is a profound thickening of Cambrian rocks along the continental margins. • Cratons are not perfectly flat, there are domes and arches. These can be caused by compression, extension, or thermal subsidence. • Very few faults of any significance, most probably represent the surficial expression of basement fault zones. • Transcontinental Arch, a broad upwarp. Along the arch, no Cambrian sediments are represented, the sea did not raise fast enough. 2.3 The Sauk Transgression • General Setting 1.By the end of the Cambrian, Nearly three-fourths of the continent was flooded during the Sauk transgression. This sea was much like modern continental shelves except it was much larger. 2. Transgression rate: About ten miles per million years on average, the actual rate was more spasmodic. The maximum flood before the Sauk seas retreated was during the early Ordovician.

3. Paleomagnetic data indicate that North America Lay along the Equator and was oriented 90° clockwise to its present position. • Quartz rich sand was dominant in most areas, some of which was glauconite bearing, shale was minor. When you think Cambrian, think extensive quartz sandstones. 1. The sand was derived from the craton. The craton was weathered and eroded for more than 1/2 billion years, so a huge volume of clastic material must have been available. 2. It was probably eroded and redeposited a number of times before it was eventually deposited in the eperic seas. 3. Cambrian Cratonic sandstones rank among the maturest in the world. They are noted for how perfectly the grains are rounded and sorted, and are almost pure quartz. • By the close of the Cambrian, the vast supply of weathered quartz was used up, and much of the sedimentary source land was gradually being submerged. A great carbonate factory was turned on and worked its way toward the center of the continent. 1. Many of the limestones consist of shell debris, and display ripple marks, cross beds and other features indicating physical transport. 2. The critters present required agitated, well-oxygenated water with plenty of sunlight. 3. Oolites 4. Most of the limestones have subsequently been converted to dolomite through diagenesis. • Since there is abundant evidence of wave and current action and a lack of deep water facies, it is interpreted that the Sauk sea was very shallow, probably no deeper than 100 m anywhere on the craton. 1. The presence of stromatolites suggests the approximate presence of sea level 2. Dessication (caused by drying out) cracks 3. Flat Pebble conglomerates 4. Glauconite forms in shallow seas where deposition rates are extremely slow. 2.4 The Later Ordovician • Widespread unconformity was produced across the mid continent during the early Ordovician. At the height of the Sauk transgression, dolomite deposition was dominant. In the midcontinent, the unconformity is overlain by the St. Peter Sandstone. 1. The sea retreated to the continental margins. 2. Because the continental interior is very flat, and only a slight drop in sea level would have been needed to expose a considerable area. 3. The underlying Cambrian sands were the likely source. Only a little erosional truncation of the overlying dolomites would have been needed to reach these sands. 4. Classic transgressive deposit. Virtually all of the sand is nearshore facies. • After deposition of the SPS, marine carbonate sediments formed across the entire craton 1. Many are >80-90% skeletal material, and are among the most fossiliferous in the world.

2. Like the during the Cambrian, the craton was nearly completely flooded again. 3. Probably somewhat deeper. • In the Appalachian region, the later Ordovician sequence is quite different than what has just been described. 1. More deeper water facies, with black shales, graywackes, and cherts. 2. Pelagic (deep water) sediment is an oddity for continental interiors, and must record a change in source areas. 3. Northern Appalachians are almost all dark shales, and thicker. 4. Farther east, volcanic rocks occur within the Ordovician Succession, indicating that volcanic islands were forming here. • Taconic Orogeny 2.5 The Silurian and Devonian • Aftermath of the Taconic Orogeny • Silurian and Devonian strata are scattered widely over the craton. This could be the result of nondeposition or erosion during a significant Hiatus in the early Devonian about 400 million years ago. This is the boundary between the Tippecanoe and Kaskaskia sequences. Middle Silurian facies are dominated by marine carbonates and lesser shale. The fossils present indicate a shallow sea with uniform conditions over an immense region. Similar conditions persisted in the Devonian as well. • Lots of Carbonate Reef environments • Evaporites 2.5.1 Middle-Upper Devonian Strata of North America (Early Kaskaskia) • Kaskaskia strata rest unconformably above older strata 1. Similar to the base of the St. Peter Sandstone 2. Emerged lowland for a few million years • Basins and arches became more sharply delineated 1. Upper Devonian Strata is more widespread 2. Michigan basin continued to subside, the Williston and Forrest City basins were also actively subsiding. 3. Acadian Orogeny 4. Arctic-Cordilleran Orogenies • Franklin Orogenic Belt • Ellesmerian • Antler Orogeny 2.6 Mississippian Rocks (362-322 my ago) • Last widespread carbonate producing epeiric sea in north America • Limestones rich in crinoid fragments predominated and display conspicuous clastic textures including ripple marks, cross bedding and oolites. • Organic reefs were present but not as widespread. • The end of the Mississippian is marked by a major p between the Kaskaskia and Absaroka Sequences. 2.7 Pennsylvanian Rocks A. Beginning in the late Mississippian and persisting through the early Permian time, strata deposited within North American craton display a striking repetitive pattern.

B. In the eastern States, upper Mississippian strata display ss-sh-ls sets repeated several times vertically. These probably represent deltaic deposits from rivers draining from Canada and the Appalachians. C. Practically all Pennsylvanian strata display a repetitive pattern, the most striking of which occurs in the coal-bearing sequences. 2.8 Permian-Early Triassic Rocks • The close of the Paleozoic is marked by the fall in sea level and represents a period of time when North America was characterized by extensive emergence. • Permian Basin, Phosphoria sea. • This period of time is marked by extensive redbeds and evaporites throughout the western U.S. • Great desert that rivaled the Sahara in the Rocky Mountain-Grand Canyon area 2.9 Tectonics • Marathon-Ouachita Orogeny (Oklahoma /Arkansas, Texas) • Ancestral Rocky Mountain Orogeny (Western OK, Colorado, NM) • Appalachian (Alleghenian) Orogeny . The final closing of the Iapetus ocean and the collision of Gondwanaland and North America.

3 The Mesozoic Era 3.1 The Opening of the North Atlantic 3.2 Triassic Period A. Triassic strata is very similar to Permian discussed earlier B. Red sandy Alluvial plane, mostly terrestrial sediments C. Marine Conditions existed only in the west D. Still a volcanic margin arc along the western margin 3.3 Jurassic Period A. Navajo Desert B. Sundance Formation C. Morrison Formation. 3.4 Cretaceous Period A. Major transgression and seas were as high then as ever before. Some epeiric seas experienced unprecedented depths exceeding several hundred meters. It was the last major flood of the continent. B. Seas transgressed from the north and south C. Most K rocks are in the west, others are part of the coastal plains D. clastic wedges shed from the mountains to the west 3.5 Cordilleran Tectonics A. Cordillera is the name for the western margin of North America. B. Remained a passive margin for much of the early Paleozoic. Became an active margin an has remained such since the Devonian. Subduction became more rapid in the Mesozoic. C. Sonomia and the Sierran Arc D. Sevier Orogeny. Late Cretaceous E. Laramide Orogeny. Late Cretaceous and Early Tertiary

4 The Cenozoic Era 1.Introduction and Chronology

2.Coastal plain strata 3.Tertiary strata of the Cordilleran Region. 4.Cordilleran Tectonic History A. Laramide Orogeny Continued B. Intermontane Basins -- Thick accumulations of materials shed from uplifted ranges C. Resumption of Arc Volcanism D. San Andreas Fault E. Basin and Range F. Columbia Plateau G. Colorado Plateau H. Cascade Volcanism I. Yellowstone Hot Spot and Snake River Plain

TECTONIC HISTORY OF THE U.S. CORDILLERA

1. Continental Scale Structure of North America North America consists of an older interior craton rimmed by five Phanerozoic (570 – 0 Ma) orogenic belts A. North American Craton • the craton is an amalgamation of older (> 1.0 Ga) crustal fragments that were accreted during a variety of collisional events and failed rifting events - these rocks form the “crystalline basement” of the interior of the continent • subdivision of basement rocks of the craton is based on tectonic affinity and age • basement rocks are commonly overlain by a veneer of lightly deformed, subhorizontal, Paleozoic sedimentary rocks • portions of the craton and shield are tectonically active (i.e. seismically active) due to the reactivation of older faults and crustal discontinuities due to tectonic events along the continental margin or due to epeirogenic processes B. North American Phanerozoic Orogens • these orogenic belts formed largely by continent-continent or subduction-related collisions - continent-continent collision results in accretion of mature (> 200 Ma old) lithosphere - subduction collision results in accretion of immature (< 200 Ma old) oceanic material including volcanic arcs, accretionary wedges and volcanic plateaus 1. Cordillera 2. Franklin 3. Caledonide 4. Appalachian-Ouachita 5. Sierra Madre

2. Tectonic Subdivisions of the Craton A. Grenville Province (1.3-1.0 Ga) • collisional orogen extending nearly 5000 km from Mexico to Labrador • formed during assembly of a supercontinent named Rodinia • dissected during later continental rifting and pieces are now preserved in many continents, including South America and Africa B. Keweenawan Rift Province (~1.1 Ga) • failed rifting event causing large outpourings of basaltic lavas • evident in subsurface from gravity and magnetic anomalies C. Yavapai and Mazatzal Provinces (1.8-1.6 Ga) D. Trans-Hudson and Penokean Provinces (1.9-1.8 Ga) E. Superior, Slave and Wyoming Provinces (Archean, > 2.5 Ga)

3. The Cordilleran Orogenic Belt • perhaps the longest-lived orogenic belt on the face of the Earth - subduction convergence has continued here since the rifting and breakup of Rodinia between 730 –550 Ma - in total, this orogenic belt is a continuous chain of mountains extending from Alaska to the tip of South America • geology here is a result of every type of plate interaction except continent-continent collision 38

• this mountain belt is active today and along it we have a variety of tectonic styles - oblique subduction convergence south from Alaska to the Mendocino triple junction - strike-slip deformation south from Cape Mendocino to the Gulf of California - active spreading in the Gulf of California - subduction convergence and local strike-slip deformation from Central America to Argentina • classic setting for the identification of “suspect terranes,” or fragments of crust or sedimentary rock that were created elsewhere and subsequently accreted to the continental margin A. Paleozoic and Mesozoic Tectonics in the U.S. Cordillera 1. Antler Orogeny (Devonian - earliest Mississippian) • accretion of an island arc formed above a westward-dipping subduction zone • sediment deposited in foreland basins in eastern Nevada 2. Sonoma Orogeny (Permian-Triassic) • accretion of an island arc formed above a westward-dipping subduction zone • polarity of subduction flips during this event - continental arc magmatism begins along western North America 3. Nevadan Orogeny (Late Jurassic) • accretion of several island arcs or microcontinents - arc-arc collision • highly oblique convergence - creates right-lateral strike-slip fault zones and sutures • closes a small, doubly subducting oceanic basin - creates an Andean-style margin along western North America - to the west there is now a significant expanse of the Farallon oceanic plate 4. Sevier Orogeny (Latest Jurassic to Late Cretaceous) • fold and thrust belt formation along the majority of the continental margin, from Canada into Mexico - major remnants of this thrust belt are found in southwestern Wyoming • roots of Sierra Nevada batholith intruded during continental arc magmatism - extensive thickening of crust • Great Valley sequence deposited in fore-arc basin • Franciscan Complex rocks deposited and deformed in accretionary prism • extensive deposition in foreland basins on the eastern side of tectonic highlands 5. Laramide Orogeny (Latest Cretaceous to Eocene) • basement-involved reverse faults cause “thick-skinned” deformation in the Colorado Plateau, Rocky Mountain foreland and Black Hills of South Dakota - creates, among others, the Bighorn Mountains, Wind River Mountains, Uinta Mountains, Uncompahgre Uplift and Rocky Mountain Front Range - some uplifted as much as 30,000 feet • monoclinal folding of sedimentary cover over uplifted basement rocks • intermontane basins develop between basement uplifts - coeval stratigraphy in the basins thickens and coarsens toward the uplifts • uplifts are enigmatic because they are so far from the inferred continental margin and because they have widely varying orientations - hypothesized to be inverted normal faults of Proterozoic age

- thought to be associated with accelerated convergence rates, low-angle subduction, and the subduction of young, hot, buoyant oceanic crust

B. Cenozoic Tectonics in the U.S. Cordillera • at the end of the Laramide, when motions between the North American, Kula and Farallon plates changed dramatically, convergence rates decreased rapidly • oblique subduction of a spreading ridge beginning around 30 Ma - creates a triple junction and an ever-lengthening San Andreas transform boundary • 10-15 million years after ridge subduction, extensional deformation begins in the Rio Grande Rift and the Basin and Range Province of Nevada - general structural style is one of alternating horsts and grabens, or half-grabens - extensional deformation has migrated outward and northward over the last 10 million years - crosscuts Sevier age fold-and-thrust belt structures in Utah and Wyoming - creates the Teton uplift in northwestern Wyoming, beginning around 9 Ma • eruption of Columbia River Flood Basalts between 17-6 Ma, most between 17-15.5 Ma • initiation of Yellowstone hot spot.

Recent & Gravels & Approximate Thickness General outcrop description Main outcrop locality Pleistocene Terraces Gravel terraces or benches near major drainages. Near major basin drainages.

Absaroka Pliocene igneous flows and Igneous flow rocks, volcanic-derived sediments, dikes, sills, plugs. sediment Miocene Absaroka Plateau, west of State highway 120. Gray, white, volcanic-derived shale, sandstone, conglomerate, Oligocene Wiggins 0-4000' breccia. Plugs, dikes, and sills prevalent.

Tatman Drab sand and shale. may be part volcanic-derived. Remnants East of State 120 near Gooseberry 0-600' form highest topography in central part of basin. Cutoff. South of U.S. 20-14 (Tatman Mt.).

Eocene Brightly colored (mostly red) shale, claystone, sandstone (conglomeratic in part) on west flank, grading to pastel to drab Willwood 0-8000'+- on east flank. Forms Badlands.

Central basin area and on flank of basin near most major highways.

Drab sandstone, mudstone, and coal beds. Some fossil leaves. General tan appearance on outcrop. Coal mines at Bear Creek Paleoocene Fort Union 0-8000'+- and Red Lodge, Montana.

Sand, shales, and few coals. Forms resistant ridges. Outcrop parallels many basin roads. Lance 700-1200'

Meeteetse-Lewis Massive white sandstones, and many thin shales and coals. State 120, near Meeteetse, Wyoming Bearpaw 700-1500' Generally less resistant than formations above or below. (type section).

Sandstone, shale, coal. Forms resistant ridges. Outlines many Mesaverde Group 1500' +- anticlines in the basin. Most major highways are near Cretaceous outcrops in less resistant shale valleys.

Gray, black shales. Few thin sandstones. Type section in Shoshone River out-west of Cody, WY. 2120' measured. Microfossils and megafossils abundant. Forms valley Near many highways. Good exposure at type section locality. Cody 1400-2100' and flat areas.

Carlile Forms an alternating series of sandstone ridges and shale valleys. Frontier 500-1000' Most major highways rimming the basin. Cretaceous Forms rounded steep slopes with some resistant siliceous bands. Mowry 370' Silver-gray color on outcrop. Thermopolis Dark gray to black shale. Muddy sandstone member, near center Muddy 700' of section, generally poorly resistant. Type section near Thermopolis. WY. Cloverly Group 300' Thin sands and shales, tan to brown, siderite. Some conglomerate lenses in lower part. Morrison 200' Shales, sub-waxy, and some sandstones and conglomerate. Sundance 200' White and light red appearance on outcrop. Near most major highways around Jurassic Glauconite sandstone, thin lines and shale. Greenish appearance the basin. Gypsum Springs 275' on outcrop. Gypsum, shale, and thin limestone. Light red, pink, and white Crow Mt. on out crop. Alcove Triassic Chugwater 500-1200' Red Peak Red, maroon sand and shale. Paler Dinwoody at base. U.S. 20-State 789. Good section in Dinwoody 50' road cut south of Thermopolis. Permian Phosphoria 50-300' Gray. Forms resistant dip slope. Tensleep 50-300' Cliff-former, cross-bedded sand. Pennsylvanian U.S. 20-State 789 in the Wind River Amsden Darwin 300' Dolomite, red shale, sandstone. Darwin Sandstone at base. Canyon, south of Thermopolis. Mississippian Madison Group 330-900' Cliff-forming. Light gray limestone group.

Devonian Three Forks? Jefferson? 0-400' Shale and dolomite. U.S. 20-14 West of Cody, Wyoming.

Ordovician Big Horn 0-500' Massive, tan, cliff-forming dolomite.

Gallatin 1000- U.S. 20-739 Wind River Canyon. Poorly resistant limestones, sandstones, and shales. Generally Cambrian Gros Ventre 1200' hold up by the overlying resistant Big Horn Dolomite.

Flathead

U.S. 20-78 Wind River Canyon. Other Granite, with a few metamorphic rocks noted in some areas. exposures along highways through the mountain passes.

ROCK SEQUENCE IN THE BIG HORN BASIN

BLACK HILLS PRECAMBRIAN GEOLOGY 44

1. Archean Rocks A. Present in the Black Hills but are limited to two small areas 1. Along Little Elk Creek (NE) 2. Near Bear Mountain (SW) B. Part of the Wyoming Archean province C. Both have granites (tonalites) that are about 2.5 billion years old • These granites intrude older high grade metasedimentary successions. D. Likely the foundation for the rest of the Precambrian rocks in the Black Hills. These rock are unconformably overlain by younger metasedimentary rocks.

2. Proterozoic Rocks A. 99% of all exposed Precambrian rocks in the Black Hills area B. Consists of metasedimentary, metavolcanic and intrusive granitic rocks C. At least two depositional successions 1. ~2.2 bya 2. ~1.9-2.0 bya D. As many as five deformational and metamorphic events have been documented • typically tightly folded and have well-developed, steeply inclined foliation • dominant structural grain is NNW G. Intruded by the ~1.7 bya post tectonic, S-type Harney Peak Batholith H. Part of the Trans-Hudson belt

3. Summary of Major Mapped Early Proterozoic Rocks A. Xqi • Occur in the Nemo area • Metaconglomerate, Quartzites, BIF. Some conglomerates are uraniferous • Deposited in a rift environment • Unconformably overlie the Little Elk Granite • Intruded by the 2.17 bya gabbroic, gravity differentiated Nemo Sill (Xmg) B. Xgf • Exposed in the Nemo area • Conglomerate, quartzite, marble (shallow water facies) • Lateral equivalent to southern units? C. Xpb • Greenstones (pillowed), BIFs, slates • 1.97 bya • Homestake Formation and associated rocks in the Lead and Rochford areas • Occurrence of Black Hills gold. D. Xgw-Xif-Xcg-Xbs-Xms • Schists, BIFS, metagraywackes, metaconglomerate, quartzite, phyllite, metatuff, minor greenstones. Mainly deep marine sedimentary and volcanic rocks. • Graywackes are the most abundant • Conformably overlie Xpb • Present in the southern and eastern Black Hills

E. Xp • Youngest metasedimentary succession • Mainly poorly exposed black slate and phyllite • Repeated several times by folding • Present in the Deerfield Lake and Pactola Lake areas in the western Black Hills F. Harney Peak Granite • Present in the southern Black Hills including Mount Rushmore • Topographic high of the uplift, very resistant rock • Associated pegmatites • Contact metamorphic zones • Mechanism of intrusion has a significant effect on the country rock

4. Dominant Structures in Black Hills Precambrian Rocks 1. Primary structures 2. Cleavage/Schistosity 3. Lineations 4. Folds 5. Ductile shear zones

REGIONAL STRATIGRAPHY PROJECT

Purpose and Introduction 46

The purpose of this project is to introduce you to the stratigraphy of the western Powder River Basin and instruct you in the methods of measuring and presenting stratigraphic columns. You will spend three days measuring and describing the stratigraphic units along the eastern flank of the Big Horn Mountains. You will first measure and describe the Archean and Cambrian rocks along US 14 west of Dayton, WY near Steamboat Mountain. Next you will measure and describe Ordovician through Pennsylvanian strata exposed along Crazy Woman Canyon. Lastly, you will measure and describe Permian through Cretaceous units in the Bud Love Big Game Winter Refuge northwest of Buffalo. Base maps of these areas will be provided daily. Each day you will record this stratigraphic information in your field notebook. Once completed, you will be required to submit three individual stratigraphic columns, one for each day's work, for grading. It would be wise to work on each day’s column that evening. Here are the guidelines for the formal drafts of the columns:

Helpful Hints • use 1” : 50’ as the vertical scale in your notebook. • try to capture the variability of units on the scale of 20-40’ • use the evening of each day to work on that day's column • calculate the maximum thickness of your three sections before you cut your paper; you don't want to end up with a piece that is too small to fit your largest section! • use the standard lithologic symbols found at the back of the guidebook when filling in your sections • the right side of each of your sections should be a weathering profile that reflects each units’ relative resistance to erosion. • use the reference section for the Big Horn Basin as a template for the construction of your sections.

Project Components, Guidelines and Layout 1. Your final stratigraphic columns should be drafted at a scale of 1 inch to 100 feet. 2. They should be drafted as neatly as possible on the provided graph paper. 3. They should be in appropriate detail that reflects the variability of each formation. 4. The following information should be included in each of the columns: • a descriptive title of each section including the location where it was measured, • the lithostratigraphic and chronostratigraphic nomenclature, • the thickness that you measured, • appropriate patterns and symbols, • a weathering profile, • brief lithologic descriptions.

At the end of this project you will turn in: 1. Three graphical stratigraphic columns, all on one sheet of paper 2. Brief written descriptions of each formation, including thicknesses, all written adjacent to the part of the stratigraphic column to which they pertain (i.e. on the same large sheet of paper as your graphical stratigraphic columns) 3. Your field notebook

Project Grading • Stratigraphic Columns (3 @ 30 pts ea) 90 pts • Field notebook 30 pts 120 pts total

AMSDEN CREEK MAPPING PROJECT

Purpose and Introduction This project is a two-day introduction to geologic mapping. During this project you will begin to learn how to locate yourself on a topographic map, recognize, define and map rock units, collect structural data, locate formation contacts, take quality field notes and make field cross sections. The field area is located within state-owned land of the Amsden Creek Big Game Winter Range, northwest of Dayton, Wyoming. Because this project is an introduction to geologic mapping, we will begin mapping as a single large group. The faculty will remain with the group for a least one half of the first day, and you will be free to map the area with your partner thereafter. The geology of the area consists of homoclinally dipping Mesozoic and Paleozoic strata: the same rocks that the class described and measured in the Regional Stratigraphy Project. No mappable faults or stratigraphic pinch-outs are present in this area.

Helpful Hints • review your field notes from the Regional Stratigraphy Project to aid in defining rock units. • structure, unit thicknesses, and geomorphic expression must agree with one another. • draw cross sections in your field notes to determine structural and stratigraphic relationships. • be sure to have complete coverage of structural data throughout the map area; you should have at least fifteen strike and dip measurements on your finished map.

Project Components and Layout Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and cross section should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.

When mapping in the field or drawing your cross section, observe the following guidelines: • rock units below the Cretaceous should be mapped separately. • in the Cretaceous succession, map the Cloverly Fm. as a separate unit, and lump all younger Cretaceous units together as Ku (Cretaceous Strata, undivided). • draw your geologic cross section down to an elevation of 1000' above sea level. At the end of this project you will turn in 1. a final colored and inked map, 2. the map explanation with brief descriptions and formal names for each map unit, 3. one geologic cross section assigned by the instructor(s), 4. your field notebook. 5. your field map

Project Grading • Final Map 40 pts • Explanation 10 pts • Cross Section 30 pts • Field Maps 10 pts • Field Notebook 10 pts 100 pts total

STEERHEAD MAPPING PROJECT

Purpose and Introduction The purpose of this project is to expose you to the mapping of faults, Paleozoic bedrock and syntectonic growth strata associated with a classic Laramide uplift. You are given two field days and one drafting day to complete a geologic map and two cross sections of a portion of the Steerhead Ranch, roughly ten miles west of Buffalo, Wyoming. The area consists of Archean igneous and metamorphic basement rocks nonconformably overlain by Paleozoic (Cambrian- Pennsylvanian) cratonic sedimentary rocks. Both Archean and Paleozoic rocks were deformed during the Laramide Orogeny and numerous faults of various orientations and slip sense occur within the area. The principal structure of the area is the Clear Creek Thrust, one of the major range-bounding faults that carries the Bighorn Mountains above it. A thick succession of syntectonic Tertiary gravel occurs within both the hanging wall and footwall of the fault. When constructing your map and cross sections, be mindful of what we know about the regional structural geology. The map area is largely a hanging-wall anticline developed above a basement fault and any subsidiary faults and folds should have geometries similar to those documented in similar structures. Tear faults and extensional faults both exist in this mapping area.

Helpful Hints • review the criteria for recognizing faults in the field (e.g. stratigraphic omission or duplication, abrupt changes in strike, juxtaposition of units not normally adjacent). • review and understand the 3-D geometries of tear faults and extensional faults associated with Laramide uplifts - don't interpret structures that differ vastly from these well- documented types without very compelling evidence. • draw schematic maps and cross sections in your field notes to help you figure out complex geometries or structural relationships. • the resolution of your map constrains what you can put on it; in other words, if a feature is too small to appear on your map (e.g. a fault with 5' of offset), then you can't map it!

Project Components, Guidelines and Layout Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and both cross sections should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.

When mapping in the field or drawing your cross sections, observe the following guidelines: • map all gravel in the area as Tertiary Wasatch (Tw) Formation. • map the Gallatin and Gros Ventre formations as one unit, the Gallatin – Gros Ventre (Cgg) • fill in the footwall of your cross section with a "normal" stratigraphic sequence. • draw cross sections down to a depth of 4000' above sea level.

At the end of this project you will turn in: 1. a final colored and inked map, 2. the map explanation with brief descriptions and formal names for each map unit, 3. two geologic cross sections assigned by the instructor(s),

4. your field notebook, and 5. Your field map

Project Grading • Final Map 60 pts • Explanation 10 pts • Cross Sections (2 @ 30 pts ea) 50 pts • Field Map 10 pts • Field Notebook 20 pts 160 pts total

ALKALI ANTICLINE MAPPING PROJECT

Purpose and Introduction The purpose of this project is to give you experience mapping folded sedimentary rocks as well as understanding the stratigraphy of Cretaceous source and reservoir rocks in a petroleum system. Folds in this area are classic, Laramide-style, basement-involved folds that trend northwest-southeast. The general subsurface geometry of these kinds of structures is well known because oil is produced from several Paleozoic and Mesozoic formations in the Bighorn Basin. The petroleum traps occur in the crestal areas of folds, or in association with facies changes or in stratigraphic pinchouts in fold limbs. This area is also a major source of bentonite. Bentonite was surface mined (and reclaimed) from the Thermopolis Formation in the southern portion of the project area. The field area is located on Bureau of Land Management property along the eastern edge of the Bighorn Basin, and comprises 2-3 large-scale anticlines and synclines that at the surface, mostly involve Mesozoic strata; Paleozoic strata are primarily in the subsurface. The principal anticline in the area is named Alkali anticline. The stratigraphic column and associated unit thicknesses you should use for your cross section are as follows:

Formation/Unit Thickness (ft) Cody calculate from your map Frontier calculate from your map Mowry calculate from your map Thermopolis 500’ Cloverly 180’ Morrison 350’ Sundance 350’ Gypsum Springs 200’ Chugwater 550’ Dinwoody - Phosphoria 300’ Tensleep – Amsden (combine) 370’ Madison 450’ Bighorn 250’ Gallatin – Gros Ventre 400’ Flathead 100’ Precambrian undivided

Helpful Hints • in the Bighorn Basin, two main kinds of faults often form in association with map-scale folds: basement-involved, contractional faults that underlie and uplift the entire structure, and steeply dipping, cross-strike, extensional faults that often have displacements of only a few meters or tens of meters. The latter type will be exposed at the surface in this area, whereas the former will only exist in the subsurface. Review the relations between apparent fault separation and true fault displacement. • consider the various models (e.g., folded basement, faulted basement, trishear, etc.) for how to interpret the subsurface structure beneath the major folds; use the one that best matches your field data.

• be consistent with your subsurface interpretations: faults should not rapidly change dip or displacement along strike. • this area is almost 100% exposed, so you should have no trouble finding places to collect structural data. Plan to have at least 40 data that are evenly distributed over the map. • use your air photos to plan your mapping strategy, and to logically project contacts through areas you did not visit. • a more detailed stratigraphic column of the surface-exposed rocks is given in the adjacent figure. Use this column to help you locate yourself in the stratigraphic section.

Project Components and Layout This is a two-day project that will be completed in the field. At the end of this project you will turn in 1. a final field map, 2. the map explanation with brief descriptions and formal names for each map unit, 3. one geologic cross section 4. your field notebook, 5. your field map, and

Project Grading • Final Map 50 pts • Explanation 10 pts • Cross Section 20 pts • Field Notebook 20 pts 100 pts total

SOUTH FORK MAPPING PROJECT

Overview The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 49.19 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (TuffZirc calculation) of 51.80 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n=379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fact that nearly a third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanically twinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shortening strain axes (31/31 samples; n p 1231 twins) are now in a random orientation. Synemplacement calcite veins record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record of a strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwestsoutheast motion. Poorly constrained cross sections and palinspastic restorations indicate 125 km of shortening (and uncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightly earlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintained its internal stratigraphy and did not disaggregate while in motion.

Purpose and Introduction This is a new mapping are for 2017. Like with the Mickelson project that we will do in a few days, this area is in the vicinity of the Heart Mountain slide. While not as famous or well studied as its younger cousin, the geology of this area is rich in complexity and will be a nice challenge for you. This project is entirely on private land, and the only road access is from the Carter Mountain road. It will be best to start high each day, and then walk down hill in a series of strike parallel traverses. The stratigraphy will include Mesozoic rocks and include the section from the Sundance through the Frontier. The most notable difference here as compared to where you have seen these rocks before is that the Morrison Formation is much thicker, and contains chert pebble conglomerate in the middle of the section. Eocene rocks of the Willwood Formation also are present here.

The structure here is rich in complexity. You will find both folds and faults. The exposure is excellent as well. So be careful that you spend the time needed to solve the structure.

When mapping in the field or drawing cross sections, observe the following guidelines: Draw you cross section deep enough to show all of the structure. This will be the most challenging cross section that you will draw at camp.

At the end of this project you will turn in 1. a final colored and inked map, 2. the map explanation with brief descriptions and formal names for each map unit, 3. a geologic cross section assigned by the instructor(s), 4. your field notebook

Project Grading • Field Map 10 pts • Final Map 60 pts • Explanation 10 pts • Cross Sections (2 @ 25 pts ea) 50 pts • Field Notebook 20 pts 150 pts total

MICKELSON (HEART MOUNTAIN) MAPPING PROJECT

Purpose and Introduction 56

This mapping area is in part of the Heart Mountain Slide area. The Heart Mountain Slide is a classic structure whose important features are outlined in the accompanying field guide (Malone and others, 1999). This area consists of gently dipping upper Paleozoic and Mesozoic strata that are cut by steeply-inclined dip-slip faults, allochthonous Paleozoic rocks that are bounded below by the gently inclined Heart Mountain Slide base, and Eocene volcanic and plutonic rocks of the Absaroka Volcanic Supergroup. You will have three field days and one drafting day to complete this project.

Be Careful. Grizzly bears and moose use this area.

In this area, you will be mapping in an area with limited outcrop, and little alluvium. . Float mapping is important. You may find yourself using more dashed contacts on this map.

Project Components and Layout Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and cross sections should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper.

When mapping in the field or drawing cross sections, observe the following guidelines: • Draw cross sections down to a depth of 5000’ above sea level

At the end of this project you will turn in 1. a final colored and inked map, 2. the map explanation with brief descriptions and formal names for each map unit, 3. two geologic cross sections assigned by the instructor(s), 4. your field notebook

Project Grading • Field Map 10 pts • Final Map 60 pts • Explanation 10 pts • Cross Sections (2 @ 25 pts ea) 50 pts • Field Notebook 20 pts 150 pts total

Stratigraphic Units in The Mickelson (Heart Mountain) Area Tertiary Wapiti Formation (Tw) – Interbedded andesitic/mafic breccia, lava flows, and volcanic siltstone, sandstone, and conglomerate; weathers brown and gray, thickness >400 ft in the map area. All volcanic rocks are allochthonous. Crandall Conglomerate (Eocene) – Limestone cobble and boulder conglomerate. Locally > 50 ft. thick. Small patches in pre-volcanic paleotopography. Triassic Chugwater Formation (TRc) – Red shale, siltstone, and sandstone, gypsiferous in places. Thickness, 650-750 ft. Triassic-Permian Park City and Dinwoody Formations (TR-Ppcd) Dinwoody is Triassic in age and consists of yellow-brown, gray, and red siltstone, gypsum, and dolomite. Thickness 20-50 ft. Park City is Permian in age and consists of siliceous limestone and dolomite, nodular chert, and brown shale. Thickness 70-110 ft. Pennsylvanian Tensleep Sandstone (lPt) – Light reddish gray well-sorted, cross-bedded, massive sandstone; thin beds of limestone and dolomite in the lower part. Thickness: 170-220 ft. Amsden Formation (IPa) – Interbedded red shale, purple shale, dolomite, limestone and minor sandstone. Thickness 250-300 ft. Mississippian Madison Limestone (Mm) – Massive light gray limestone. Thickness 700-900 ft in the lower plate of the Heart Mountain Detachment; the thickness is variable in the upper plate. All Madison exposed here is allochthonous. Devonian Jefferson – Three Forks Formations (Djtf) -- (present only in subsurface here). Thickness 250 ft. Ordovician Big Horn Dolomite (Ob) -- (present only in subsurface here). Thickness 400 ft. Cambrian Gallatin Group – Gros Ventre Formation (Cgg) – (present only in subsurface here). Thickness 1200 ft. Flathead Sandstone (Cf) -- (present only in subsurface here). Thickness 150 ft. Precambrian Granite and Gneiss, undivided (PCg) -- (present only in subsurface here). • References Malone, D.H., Craddock, J.P., Schmitz, M.D., *Kenderes, S., #Kraushaar, B., #Murphy, C.J., Nielson, S., and Mitchell, T.M., 2017, Volcanic initiation of the Eocene Heart Mountain slide, Wyoming, USA: In press at the Journal of Geology. Craddock. J.P., Malone, D.H., Porter, R., MacNamee, A.F, Mathisen, M., Leonard, A.M., and #Kravits, K., 2015, Structural Evolution of the Eocene South Fork Landslide, Northwest Wyoming: Journal of Geology, v. 123, n. 5, p. 311-335. Malone, D.H., Craddock, J.P., and #Mathesin, M.K., 2014, Age and Provenance of Allochthonous Volcanic Rocks at Squaw Peaks, WY: Implications for the Heart Mountain Slide: The Mountain Geologist, v. 51, n. 4, p. 321-336.

Malone, D.H., Craddock, J.P., Tranel, L.R., and #Mustain, M.R., 2014, Age and Provenance of Eocene Volcanic Rocks and Hominy Peak, Northern Teton Range, WY: Implications for the Emplacement of the Heart Mountain Slide: The Mountain Geologist, v. 51, n. 4, p. 295-320. Malone, D.H., Craddock, J.P., and #Schroeder, K.A., 2014, Detrital Zircon Age and Provenance of Wapiti Formation (Eocene) Tuffaceous Sandstones, South Fork Shoshone River Valley, Wyoming: The Mountain Geologist, v. 51, n. 4, p. 279-294. Malone, D.H.,*Breeden, J.R., Craddock, J.P., Anders, M.H., and #Macnamee, A.F., 2014, Age and Provenance of the Eocene Crandall Conglomerate: Implications for Heart Mountain Faulting: The Mountain Geologist, v. 51, n. 4, p. 249-278. Malone, D.H., Craddock, J.P., Anders, M.H., and Wulff, A.P., 2014, Constraints on the Emplacement Age of the Massive Heart Mountain Slide, Northwestern Wyoming: Journal of Geology, v. 122, n. 6, p. 671-685. Craddock, J.P., *Geary, J., and Malone, D.H., 2012, Vertical Injectites of Detachment Carbonate Ultracataclasite at White Mountain, Heart Mountain Detachment, Wyoming. Geology, v. 40, p. 463-466; doi: 10.1130/G32734.1 King, E.M., Malone, D.H., and *DeFrates, J., 2009, Oxygen isotope evidence of heated pore fluid interaction with mafic dikes at Cathedral Cliffs, Wyoming: Mountain Geologist, vol. 46, no. 3, p. 1-18. Craddock, J.P., Malone, D.H., Magloughlin, J., Cook, A.L., #Rieser, M.E., and Doyle, J.R., 2009, Dynamics of the Emplacement of the Heart Mountain Allochthon at White Mountain: Constraints from Calcite Twin Strain, Anhysteretic Magnetic Susceptibility, and Thermodynamic Calculations: Geological Society of America Bulletin, vol. 121, no. 5/6, p. 919-938. DeFrates, J., Malone D.H., and Craddock, J.P., 2006, Anisotropic Magnetic Susceptibility (AMS) Analysis of Basalt Dikes at Cathedral Cliffs, WY: Implications for Heart Mountain Faulting: Journal of Structural Geology, v. 28, n. 1, p. 9-18. Craddock, J.P., Neilsen, K.J., and Malone, D.H., 2000, Calcite Twinning Strain Constraints on the Hearth Mountain Detachment Emplacement Rates and Kinematics: Journal of Structural Geology, v. 22 p. 983-994. Malone, D.H., 1997, Recognition of a Distal Facies Greatly Extends the Domain of the Deer Creek Debris-Avalanche Deposit (Eocene), Absaroka Range, Wyoming: Wyoming Geological Association Annual Field Conference Guidebook, v. 48, p. 1-9. Nelson R.S. and Malone, D.H., 1997, Stratigraphy and Economic Geology of the LaSalle/Ottawa, Illinois Area. Guidebook for the 60th Annual Tri-State Geologic Field Conference, 84 p. Malone, D. H., 1996, Revised Stratigraphy of Eocene Volcanic Rocks in the Lower North and South Fork Shoshone River Valleys, Wyoming: Wyoming Geological Association Annual Field Conference Guidebook, vol. 47, p. 109-138. Malone, D.H., 1995, A very large debris-avalanche deposit within the Eocene volcanic succession of the Northeastern Absaroka Range, Wyoming: Geology, v. 23, n .7, p. 661- 664. MOONSHINE MAPPING PROJECT

Purpose and Introduction

The purpose of this project is to introduce you to field mapping of metamorphic rocks. The project will help you develop the strategies and unique field techniques used when working in layered crystalline rocks that are often complexly deformed and difficult to distinguish from one another. In these types of rocks, a firm grasp and intuitive application of basic structural and stratigraphic principles is essential for success! You are given four field days and one drafting day to complete a geologic map and cross- section of part of the Precambrian core of the Black Hills. The area is comprised of ~ 2 billion year old (Early Proterozoic), low to medium grade meta-sedimentary and meta-igneous rocks that are deformed into upright, moderately- to steeply-plunging, tight to isoclinal folds. You will subdivide and name the exposed rocks into mappable units (i.e. Formations), providing detailed descriptions of these units in your map explanation. Depending on how finely you subdivide the section and where you happen to walk, you should have from 4-6 Formations on your map. Throughout the area, you will collect orientation data on tectonic foliations that are subparallel to the axial planes of the map-scale folds in the region, as well as lineations defined by the axes of mesoscopic folds throughout the area. These data are extremely important because they define the 3D orientation of the map-scale folds in the area and will be used to construct your geologic cross section.

Helpful Hints • the deformation occurred here at medium to high temperatures and pressures. Consequently, the rocks were fairly ductile and the folds likely formed by flexural flow or passive shear mechanisms. Folds that form by these mechanisms often exhibit a “similar” geometry in that layers tend to thicken in the fold hinges and thin in the fold limbs. In other words, layer thickness is not maintained throughout these structures! Constant layer thickness is a property of folds formed during lower temperature (sub-greenschist-grade, < 250˚ C) deformation where substantial mechanical differences exist between different rock types. • remember that a Formation is a mappable rock UNIT, not necessarily a single rock TYPE. You can lump several rock types into a single mappable Formation in the area. Draw contacts between your Formations at the most significant, easily recognizable, laterally continuous rock types. • be wary of collecting your data on steep hillsides; weak rocks often slump out of place. • deciphering the structure of the area is impossible without knowing the sequence of Formations in the area. Because these are mostly layered metasedimentary rocks, they abide by all basic stratigraphic principles. Formations that were not in original sedimentary contact with one another cannot now be in contact with one another without a fault being present. There are no mappable faults in this study area.

Project Components, Guidelines and Layout Follow the Guidelines for Map and Cross Section construction when preparing your project. The map, explanation and cross sections should be constructed on the paper you are provided. You are free to design the layout of these items any way you like, but they all must fit on the same sheet of paper. Construct your cross section using the grid method as presented in lecture. Project the folds above the topographic surface, but color and label only those parts of the folds that are beneath the ground. Contacts showing the folds above ground should be dashed. Be

60 sure to calculate the height of your cross section when designing the layout of your project; depending on your plunge data, the section may be twice as tall as the map is long!

At the end of this project you will turn in: 1. a final colored and inked map, 2. the map explanation with complete descriptions and formal names for each map unit, 3. one geologic cross section assigned by the instructor(s), 4. your field notebook, 5. your field map, 6. a stereoplot and data table showing all the foliations (at least 20) and lineations (at least 8) you measured throughout the map area. Show the average foliation and lineation orientation on your plot.

Project Grading • Final Map 50 pts • Field Map 10 pts • Explanation/Strat Column 30 pts • Cross Section 30 pts • Stereonet 10 pts • Field Notebook 20 pts 150 pts total

APPENDIX 1: ESSENTIAL ANALYSIS TOOLS AND TECHNIQUES FOR FIELD GEOLOGY

FIELD DESCRIPTION OF ROCKS

Field description of rocks is an essential component of geological fieldwork. Geologists must be able to use hand-specimens and outcrop-scale observations to derive a thorough and accurate description of any rock they encounter in the field. Following are some brief outlines to help you remember the steps involved in basic rock description. These are not meant to be comprehensive, but are intended to remind beginning geologists what to look for and what kinds of questions to ask themselves when they are examining rocks in the field.

Sedimentary Rocks 1 General Rock Type (conglomerate, sandstone, shale, mudstone, etc.) 2 Color (fresh and weathered) 3 Texture 3.1 clastic 3.1.1 % grains • grain size: coarse (> 2 mm), medium (2 - 1/16 mm), fine (< 1/16 mm) • sorting (well, moderate, poorly) • surface texture (e.g., polished, frosted, etc.) • roundness 3.1.2 carbonate allochems • % intraclasts • % ooids • fossil and fossil fragments (identify when possible) 3.1.3 % detrital matrix • sand or mud? • micrite (i.e. microcrystalline calcite mud) 3.1.4 cement • composition • degree of cementation 3.2 crystalline 3.2.1 sizes: coarse (> 2 mm), medium (2 - 1/16 mm), fine (< 1/16 mm) 3.2.2 crystal shapes: euhedral, subhedral, anhedral, etc. 4 Mineral Composition (%) quartz, feldspar, muscovite, biotite, chert, calcite, dolomite, rock fragments, etc. 5 Sedimentary Structures 5.1 bedding 5.2 nodules, concretions, geodes 5.3 sole marks 7 Rock Name

Igneous Rocks 1. Color (fresh and weathered) 2. Texture 2.1 aphanitic, phaneritic, glassy, pyroclastic 2.2 porphyritic or non-porphyritic 2.3 grain sizes (if large enough to see) 2.4 fabric 2.4.1 panidiomorphic-, hypidiomorphic-, allotriomorphic-granular 2.4.2 ophitic, diabasic, trachytic, etc. 3 Structures 3.1 flow features 3.2 gas features 3.3 devitrification features 62

3.4 schlieren, augen, orbicules 3.5 inclusions (kind, size, shape, quantity) 4 Mineralogy (identify types, colors, shapes, sizes and quantities of each) 4.1 essential 4.2 varietal 4.3 accessory 4.4 alteration/secondary 4.5 ratio of alkali feldspar to total feldspar 5 Rock Name

Metamorphic Rocks 1 Color (fresh and weathered) 2 Type of Metamorphism 3 Texture 3.1 foliated 3.1.1 slaty cleavage 3.1.2 phyllitic 3.1.3 schistosity 3.1.4 gneissic banding 3.2 non-foliated 3.2.1 granoblastic 3.2.2 hornfelsic 3.3 lineated 3.3.1 discrete or constructed? 3.3.2 3.4 porphyroblastic (give sizes of matrix grains and porphyroblasts) 4 Mineralogy 4.1 mineral types 4.2 sizes 4.3 shapes 4.4 color 4.5 quantity 5 Rock Name 6 Metamorphic Facies

Dott’s (1964) Classification of Sandstones.

Rock names for various sedimentary components. Note that sand, silt, and clay do not include detrital grains of calcite or dolomite. Modified after Williams et al. (1954).

Folk’s (1962) classification of limestones.

Dunham’s (1962) classification of limestones.

Generic classification scheme for metamorphic rocks.

MEASURING STRATIGRAPHIC SECTION

We measure stratigraphic sections to acquire basic, fundamental geologic information. Observations and numerical data collected while measuring a section can be useful in a wide variety of geological subdisciplines, including at least: sedimentology, structural geology, and paleontology. Measuring a stratigraphic section may help a geologist establish the stratigraphic position and characteristics of marker horizons that will assist in mapping a region. Measuring a section may facilitate stratigraphic correlations by establishing the age or lithologic properties of units in an area. Measuring a stratigraphic section may have structural significance because it may allow us to recognize deformation-induced thickening or thinning of stratigraphy. Measured sections can provide detailed information about lithologic facies, thereby facilitating an analysis of regional or local depositional history or environments. The best place to measure a stratigraphic section is where the rocks are very well exposed. Complete exposure and easy access are two of the most important considerations when choosing where to measure a stratigraphic section. The next choice in deciding where to measure a stratigraphic section depends in part on one's purpose for measuring it. If your goal is simply to find the true depositional thickness of a unit or units, then the section must be measured in an area free of significant deformation that could alter original stratigraphic thicknesses. If, however, you seek to recognize tectonic thickness changes, then you must measure both an undeformed stratigraphic section and one or more deformed sections. It is only by having an undeformed thickness of the rocks that you can recognize stratigraphic sections that have been thickened or thinned. Measuring stratigraphic sections is an inherently imprecise process and there are many sources of errors. Mistakes measuring strikes and dips, measuring lengths, shooting bearings, or measuring slope angles are all compounded when you measure a section. Because of this, and because the original, depositional thickness of stratigraphic units can change over a region, it is wise to always include an estimate or calculation of the error in your final measurements. It is very unlikely that a unit will be exactly 111 m thick. It is more realistic that the unit is 110 ± 5 m thick.

Measurement Techniques Figure A1-1 shows the general variables involved in measuring the thickness of layered rocks. Note that T is the true thickness of the rocks, and can only be measured in a plane normal to bedding strike. It is possible to measure layer thickness along some line or in a plane that is not normal to strike, but corrections must be made to your "apparent thickness" measurements when this is done. There are a wide variety of methods available for measuring stratigraphic sections. Each of them can be adapted to the particular environment in which you are operating. In general, the more precise the method, the more time consuming it is.

Compass and Tape Methods Figure A1-2 shows an example of how to measure a stratigraphic section using a compass and measuring tape. This method can be subdivided into several general steps: 1. Measure the strike and dip () of rocks in the section of interest.

2. Extend a measuring tape parallel to the dip direction of the rocks and across the outcrop width of the section of interest. If the section is longer than your tape, you will have to proceed in several increments. 3. Measure the slope distance s for the interval of interest. 4. Determine the slope angle () by sighting up and down the tape. 5. Calculate the true thickness from:

T  s  sin    

use + when the slope and bedding dip in opposite directions and - when they dip in the same direction

Note that there are several important sources of error that can creep into sections measured in this manner. • error in measuring s,  or  • changes in  or  that might occur over the length of your measured interval • misorientation of your traverse so it is not perpendicular to strike

Hewett (1920) Method This method eliminates the need to measure the surface slope length (s), and slope dip (), and thereby increases the speed which with a section can be measured. In place of these two parameters, the Hewett method requires the geologist to determine their eyeheight (E), and use this distance as a unit measuring length to calculate section thicknesses. After measuring her eyeheight, the vertical distance from the ground to their eyes, a geologist can make fairly rapid measurements of stratigraphic thicknesses in the following manner: 1. Measure the strike and dip () of rocks. 2. Set the compass inclinometer to this dip angle. 3. While looking in the direction of a traverse line perpendicular to bedding strike, site through the compass to some point on the ground that lies along the traverse line (Figure A1-3). 4. Move to the newly sited position and record one eyeheight of measured stratigraphic thickness 5. Repeat steps 1-4 until reaching the end of the section of interest. Since a section is unlikely to be an integer number of eyeheights or staff heights, the geologist will commonly have to estimate the fractional component of the section he or she last encounters. 6. After traversing the section of interest, calculate the section thickness (T) using:

T  nE  cos  ,

where n is the number of eyeheights it took to traverse the section, E is the eyeheight of the measuring geologist, and  is the dip of the rocks.

Note that when measuring sections by the Hewett (1920) method, geologists must be careful to exactly occupy each station they successively site to, and they must also carefully adjust for changes in dip (), that might occur between the top and bottom of the measuring interval. In 70 practice, a geologist will first determine the dip of rocks in the area to be measured. Using this dip, one would then insert n=1 into the equation above and determine the number of feet of section for each eyeheight of the measuring geologist. By doing this, one will immediately know for each eyeheight, how many feet of section one has measured; no later conversion is necessary and one can log the section much more efficiently.

Mertie (1922) Method It is often impossible to conduct a traverse perpendicular to the strike of the section of interest. In these circumstances, when the geologist can follow a straight-line, strike-oblique traverse, another geometric parameter enters into the equations for calculating section thickness. This new parameter is the acute angle () between the line of strike of the section of interest, and the trend of the traverse line (Figure A1-4). When calculating section thickness in this way, we must know four geometric parameters, the slope length (s), the slope dip (), the bedding dip () and the traverse trend (). When these numbers are known, the section thickness (T) can be calculated from:

T  s sin  cos  sin    s sin  cos  

As in the compass and tape methods, the (+) sign is used when the slope and bedding dip in opposite directions, and the (-) sign is used when they dip in the same direction. Note that when the traverse is perpendicular to strike (i.e.  = 90˚), this equation reduces to the simple equation given under Compass and Tape Methods. The Mertie (1922) method can also be modified and combined with the Hewett (1920) method to create a rapid technique for measuring section thickness along a straight-line strike- oblique traverse. To do this, one would set the compass clinometer to the apparent dip of bedding observed along the traverse line and then sight each new position along the strike- oblique traverse using this apparent dip angle. The number of eyeheights (E) along the traverse are counted accordingly and used in the equation given under Hewett's method. Note, however, that you must still use the true dip () in the equation for calculating section thickness.

Practical Field Procedures When Measuring Stratigraphic Sections The items in the list below should be incorporated in your field notes when you are measuring stratigraphic sections. Remember that you should draw a balance between the detail of your description and your purpose for measuring the section. For example, you need not describe variations that occur on the scale of centimeters if your main purpose for measuring a section is to establish for mapping purposes, the aggregate thickness of a formation whose thickness is 100 m. In this case, you need to accurately establish the total formation thickness, the variation in total thickness, and the internal variations in the formation that may help you identify it in the field and which may be large enough to appear on the scale of your map.

1. Title: Include the formation(s) measured, the geographic location, and the method of measurement. If you are using eyeheight, record your eyeheight as the standard used.

2. Starting and Ending Points: use either a stratigraphic top or bottom as the starting and/or ending points of your measured section. Give a geographic description of both the starting and ending points, making sure your description is sufficient to permit others to find the

place where you measured your section and therefore follow your work. Stratigraphic limits, such as adjacent formations, alluvium, hilltops, covered intervals, etc., should be indicated at the proper places in the section.

3. Attitude of Beds: If the attitude is uniform, it should be stated in the heading, otherwise, the attitude of a bed or group of beds should be indicated at the appropriate places in the section. Even if the attitude seems uniform, make sure to measure and be certain; dip changes of only a few degrees can cause significant errors in your thickness measurements.

4. Unit Numbers: Each described unit should be given a number, in serial order. It does not matter whether Unit 1 is the oldest or youngest bed in the section.

5. Unit Descriptions: Provide a complete description of each unit in your section. See the section titled “Field Description of Rocks,” in Appendix 1 for some guidelines on what to include in your rock descriptions. Your description should include at least the following: • type of rock (e.g. ss, sh, ls, dol, mudst) • color of fresh and weathered surfaces • texture (grain size and character, sorting, porosity or density) • mineralogy • bedding (thickness of bed or laminae, geometry of beds, continuity) • outcrop description (cliff former, slope former, ledges, rounded knobs, etc.) Often it is best to indicate this characteristic with a small diagrammatic profile in the margin of your notebook. • other features such as fissility, cementation, concretions, vugs, veins, organic content, and fossils • relation to enclosing beds (e.g. sharp contact, gradational contact, unconformity)

6. Side Stepping and Traverse Maps: Remember that you can pass obstacles along your section line by "side-stepping," or moving laterally along strike at the same stratigraphic position, and then continuing along with your strike-normal traverse. If you do this, or if your traverse is anything but a straight line, you should include a map of your traverse, indicating where you side-stepped or altered the course of your traverse. Show the length of each straight section of the traverse, as well as the bearing of the traverse line.

7. Data Table: Keep your data organized and spatially related in your notebook. It will be easier to reconstruct what you did if you keep your data in some sort of table that associates your observations with your measurements of thickness, bedding orientation, traverse trend, etc. Begin at the bottom of each pair of pages and work your way to the top of a page. Because we usually measure sections from bottom to top, this will keep your notes in proper stratigraphic order, with older rocks at the bottom and the beginning and bottom of your notes, and younger rocks at the top and end of your notes. The diagram below is an example of how to lay out your field notebook when measuring stratigraphic sections. Think of the diagram as spreading across two facing pages of your notebook. Unit numbers and their corresponding descriptions go on the left-hand page of your notebook, whereas the graphic representation of the column, along with strike and dip and traverse bearing data all go on the facing, right-hand page. Leave some space along the far right edge for notes about

72 sidestepping, the difficulty of the terrain, and other things like who was doing the measuring, etc.

(eg., strike &dip, traverse bearing, Obstacles or complications in traverse

Binding of field notebook

Example layout of your field notebook when measuring stratigraphic sections.

True dip

Alignment diagram (nomogram) for use in solving apparent-dip problems. After Palmer (1918).

Place a straight edge on the two known values to determine the unknown value. As shown on the diagram, if the true dip of a plane is 43o and the angle between the strike and the apparent dip direction is 35 o, then the apparent dip is 28 o.

Calculating Strike and Dip of a Surface from Contacts on a Map

Follow the above four steps to use a three-point problem to determine the attitude of a homoclinally dipping surface. (a) Locate three points (A,B,C) that lie on th esurface, but which crop out at different elevations. (b) Draw a line connecting the points at the hightest (C) and lowest (A) elevations and find a new point (B’) along that line that has the same elevation as your third map point (B). (c) Draw a line connecting points B and B’, this is the line of strike for the surface. The value of the strike can be measured with a protractor. (d) Draw a lineperpendicular to the strike line and through the lowest identified point on the surface. (A). Mearure the length of this line using the map scale and then calculate the dip of the surface by dividing the elevation between A and B by the length of the line you just drew (i.e. rise over run). The dip of th esurface is the inverse tangent of the qoutient you just obtained. The dip direction is from the line of the strike toward the lowest identified point (A).

Calculating Outcrop Patterns from Field Observations – Constructing a Map

You can construct a geologic map or simple outcrop pattern of homoclinally-dipping rocks by finding the intersection lines between the surface topography and the formation contacts in the region. (a) Conduct a simple three-point problem to find the strike and dip of the surface of interest. Draw in the line of strike and after picking a useful contour interval, determine the horizontal distance between structure contours that would occur on the surface. The spacing is given by the contour interval divided by the tangent of the surface dip. (b) Draw in equally- spaced ctructure contours that cover the entire map area. (c) Place the structure contour map over or under the contours that have equal elevations. (d) Connect the dots to make a continuous line marking the intersection of your surface and the topography.

Calculating Outcrop Patterns from Field Observations – Constructing a Map

To create the outcrop pattern of the bottom of the formation you simple move the struction contour map and repeat steps (a) through (d) for the new surface. (e) Find a point (Z) at the bottom of the formation and re-register the structure contour map at that point, with the structure contour lines parallel to the local strike of the surface. (f) Connect the dots marking the intersection between the surface topography and the formation bottom. Color in the formation and you’re done.

Remember that you cannot do this simple contruction method if the layer changes thickness significantly or if it significantly changes strike or dip over the region of the construction. However, if you can construct structure contours on complex structural surfaces such as folded rock layers, where strike and dip change over the map area, you can still contruct a map with this method. In that case your structure contour map will look something like a topographic map, rather than a series of straight, parallel lines.

APPENDIX 2: GENERAL GUIDELINES FOR MAP PREPARATION

The following is a list of guidelines useful in preparing geologic maps. Before submitting a project, make sure your map abides by all these guidelines. If it does, you will get a better grade.

Contacts and Symbols • use a different symbol to indicate the confidence of your contact interpretation - solid lines mean you know exactly where the contact is located and should be accurate to within 1-2 ft. - dashed lines mean you an only approximate where the contact is located, and should be accurate to within 20-30 ft. (often you can state the limits of accuracy here as geologists do not necessarily agree) - dotted lines mean a contact is covered, for example, by alluvium, and its location is therefore only inferred • the use of dashed or dotted lines is appropriate for any feature that appears as a line on the map, for example, if you can only approximate the location of a fault, draw that fault as a dashed line • strike and dip symbols should be drawn so that the center of the symbol lies exactly at the point where you took the measurement

Explanation • every map should have an explanation section that includes: 1. your name and the date 2. the names of your partners for the project 3. index boxes for each stratigraphic unit observed in the area or shown on the cross section - write the age and formal name of the unit next to the corresponding box - put the formal unit abbreviation in the box - color each box a unique color that matches the color used for that unit on the map - arrange the boxes in proper stratigraphic sequence, with the youngest at the top and oldest at the bottom 4. the map scale in text and graphically, as a scale bar 5. the map contour interval, for example C.I. = 20 feet - because some of our projects use two or more maps “pasted” together, the contour interval may be different in different sections of your base map. If this is so, you will want to indicate the different contour intervals, as well as where on the map they apply 6. All symbols used on either the map or section (e.g., strike and dip, fold axes, contacts, faults) 7. a brief title such as “Map and Cross Section Explanation”

Geologic Reality • make sure your map is geologically reasonable

- don’t violate the laws of physics or geology (e.g., stratigraphic sequences must be maintained, faults don’t have reverse displacement at one point and normal displacement at another, etc.) Other Examples: • faults should exhibit geometries similar to those observed regionally (e.g. don’t interpret a low angle thrust fault if the tectonic style of the region is dominated by high angle normal faults). If you do make a radically different or new interpretation, you’d better be able to support it with your field observations! • intrusions should exhibit geometries similar to those observed regionally • local strike of sedimentary bedding is almost always parallel to contacts between conformable units • obey the rule of V’s

Outcrop Widths and Unit Thicknesses • although unit thicknesses are relatively uniform throughout a map area of the size we typically work with, the horizontal width of an outcrop belt measured perpendicular to the contacts (i.e. the outcrop width) is not constant. - outcrop width depends on unit thickness, unit dip, and the dip of surface topography, the last two of which can vary significantly throughout even a small map area - there are explicit relations between these parameters that must be obeyed if your map is to be geologically reasonable - for simple horizontal topography, outcrop width equals unit thickness divided by the sine of the unit’s dip angle) • be sure your unit thicknesses fall within the known limits for the region

Presentation Style and Grace • a part of every grade is attributed to the overall aesthetic appeal of your map • consider the following questions before you hand in your section (perhaps even before you start it!) - did you color evenly? - is your lettering neat and legible? - is the layout graphically economic? - is information organized so that users can quickly find whatever they need (e.g., the scale or a symbol definition, etc.)

APPENDIX 3: ESSENTIAL ANALYSIS TOOLS AND CROSS SECTION CONSTRUCTION

The following is a list of guidelines to be used in preparing geologic cross sections. Before submitting a project, make sure your cross section abides by all these guidelines. If it does, you will most likely get a better grade.

Apparent Dip Data • show the location and orientation of all the strike and dip data you projected into the plane of your section. - illustrate apparent dip values using “tadpoles” whose heads lie on the topographic profile, and whose tails dip in the appropriate direction at the appropriate apparent dip angle.

Geologic Reality • this is the most difficult part of section construction, and where your understanding of geology and how to extrapolate into the subsurface is most important - developing and refining these extrapolation skills is an essential part of this course! • there are few, if any, “absolutely right” answers in field geology, but the idea in section construction is to use the surface data (i.e. your field observations compiled on your map and in your notes) as the basis for a subsurface interpretation - if your interpretation does not violate the laws of physics and geology, matches the surface outcrop data exactly, and is compatible with what we know about the regional geology (e.g., igneous, tectonic and metamorphic history and style), then your section is just as “right” as any other section that abides by the same constraints. - geologists are constantly arguing over which interpretation is “best”, but, there should be no arguing over where a contact lies on the map, whether that contact is located in the right place on your section, or the orientation of bedding at a certain location. • how to make sure your section is geologically reasonable? - don’t violate the laws of physics or geology (e.g., stratigraphic sequences must be maintained, faults don’t have reverse displacement at one point and normal displacement at another, etc.) - other things to check include 1. contacts on your section must be accurately located so that they exactly match your map 2. unit thicknesses must be kept uniform (see below) 3. interpret only natural fault and fold geometries 4. cross sections must exactly match at intersection points

Presentation Style and Grace • a part of every grade is attributed to the overall aesthetic appeal of your section • consider the following questions before you hand in your section (perhaps even before you start it!) - did you color evenly and lightly? - is your lettering neat and legible? - is the layout graphically economical? - is information organized so that users can quickly find whatever they need (e.g., the scale or a symbol definition, etc.) - are all words spelled correctly? - are geologic units in the Explanation in the proper age order? - are all map and cross-section symbols shown in the Explanation?

Scale • every cross section must have a vertical and horizontal scale, written in text, and illustrated graphically along the sides of the section. • if the vertical and horizontal scales are equivalent, note it on the section. If they are not, write out the vertical exaggeration that was used. - an example scale would be 1 : 50,000. Note that this scale indicates 1 unit on the section represents 50,000 equivalent units in the real world. - the correct separator between the two parts of the scale is a colon (:), not an equal (=) sign

Section Endpoints • label each end of the cross section according to the corresponding endpoints of the section line on the map - for example, label ends A and A’, B and B’, etc. - often it is also a good idea to label the compass direction of each end of the section, for example, N and S, or NE and SW.

Section Intersection Points and Bends • if your section crosses another section line, label the intersection between the two sections on each of the two sections. - this is usually done with an arrow drawn slightly above the topographic profile and some adjacent text noting for example, “intersection with B-B’”. • if your section bends, label the point at which it bends. - this is usually done with an arrow drawn slightly above the topographic profile and some adjacent text noting for example, “bend in section”.

Topographic Profile • your cross section must accurately show the topography along the section line

Units Labeled and Colored • each formal stratigraphic unit should be colored to match your map • each formal stratigraphic unit should be labeled with the formal abbreviation for the map unit - if the label won’t fit, draw it nearby and draw an arrow from the label to the corresponding unit

Unit Thicknesses • is the thickness of a given unit uniform throughout the area? - unless you have direct evidence to the contrary, the thickness of each unit should be uniform throughout your cross section • are the unit thicknesses within the range of known thicknesses in the area? - if your section thickness is significantly thicker (e.g., 10-20%) than the known greatest thickness in the region, chances are, you’ve made a mistake. The same is true if your thickness is significantly less than the known minimum thickness in the region. • is the thickness consistent with the mapped outcrop width and measured dip?

Anaphylaxis: First aid

A life-threatening allergic reaction (anaphylaxis) can cause shock, a sudden drop in blood pressure and trouble breathing. In people who have an allergy, anaphylaxis can occur minutes after exposure to a specific allergy-causing substance (allergen). In some cases, there may be a delayed reaction or anaphylaxis may occur without an apparent trigger.

If you're with someone having an allergic reaction with signs of anaphylaxis:

 Immediately call 911 or your local medical emergency number.

 Ask the person if he or she is carrying an epinephrine autoinjector to treat an allergic attack (for example, EpiPen, Twinject).

 If the person says he or she needs to use an autoinjector, ask whether you should help inject the medication. This is usually done by pressing the autoinjector against the person's thigh.

 Have the person lie still on his or her back.

 Loosen tight clothing and cover the person with a blanket. Don't give the person anything to drink.

 If there's vomiting or bleeding from the mouth, turn the person on his or her side to prevent choking.

 If there are no signs of breathing, coughing or movement, begin CPR. Do uninterrupted chest presses — about 100 every minute — until paramedics arrive.

 Get emergency treatment even if symptoms start to improve. After anaphylaxis, it's possible for symptoms to recur. Monitoring in a hospital setting for several hours is usually necessary.

If you're with someone having signs of anaphylaxis, don't wait to see whether symptoms get better. Seek emergency treatment right away. In severe cases, untreated anaphylaxis can lead to death within half an hour. An antihistamine pill, such as diphenhydramine (Benadryl), isn't sufficient to treat anaphylaxis. These medications can help relieve allergy symptoms, but work too slowly in a severe reaction.

Signs and symptoms of anaphylaxis include:

 Skin reactions including hives, itching, and flushed or pale skin

 Swelling of the face, eyes, lips or throat

 Constriction of the airways, leading to wheezing and trouble breathing

 A weak and rapid pulse

 Nausea, vomiting or diarrhea

 Dizziness, fainting or unconsciousness

Some common anaphylaxis triggers include:

 Medications

 Foods such as peanuts, tree nuts, fish and shellfish

 Insect stings from bees, yellow jackets, wasps, hornets and fire ants

If you've had any kind of severe allergic reaction in the past, ask your doctor if you should be prescribed an epinephrine autoinjector to carry with you.

Blisters: First aid

If a blister isn't too painful, try to keep it intact. Unbroken skin over a blister provides a natural barrier to bacteria and decreases the risk of infection. Cover a small blister with an adhesive bandage, and cover a large one with a porous, plastic-coated gauze pad that absorbs moisture and allows the wound to breathe. If you're allergic to the adhesive used in some tape, use paper tape.

Don't puncture a blister unless it's painful or prevents you from walking or using one of your hands. If you have diabetes or poor circulation, call your doctor before considering the self-care measures below.

How to drain a blister To relieve blister-related pain, drain the fluid while leaving the overlying skin intact. Here's how:

 Wash your hands and the blister with soap and warm water.

 Swab the blister with iodine or rubbing alcohol.

 Sterilize a clean, sharp needle by wiping it with rubbing alcohol.

 Use the needle to puncture the blister. Aim for several spots near the blister's edge. Let the fluid drain, but leave the overlying skin in place.

 Apply an antibiotic ointment to the blister and cover with a bandage or gauze pad.

 Cut away all the dead skin after several days, using tweezers and scissors sterilized with rubbing alcohol. Apply more ointment and a bandage.

Call your doctor if you see signs of infection around a blister — pus, redness, increasing pain or warm skin.

Blister prevention To prevent a blister, use gloves, socks, a bandage or similar protective covering over the area being rubbed. Special athletic socks are available that have extra padding in critical areas. You might also try attaching moleskin to the inside of your shoe where it might rub, such as at the heel.

Cuts and scrapes: First aid

Minor cuts and scrapes usually don't require a trip to the emergency room. Yet proper care is essential to avoid infection or other complications. These guidelines can help you care for simple wounds:

1. Stop the bleeding. Minor cuts and scrapes usually stop bleeding on their own. If they don't, apply gentle pressure with a clean cloth or bandage. Hold the pressure continuously for 20 to 30 minutes and if possible elevate the wound. Don't keep checking to see if the bleeding has stopped because this may damage or dislodge the clot that's forming and cause bleeding to resume. If blood spurts or continues flowing after continuous pressure, seek medical assistance.

2. Clean the wound. Rinse out the wound with clear water. Soap can irritate the wound, so try to keep it out of the actual wound. If dirt or debris remains in the wound after washing, use tweezers cleaned with alcohol to remove the particles. If debris still remains, see your doctor. Thorough cleaning reduces the risk of infection and tetanus. To clean the area around the wound, use soap and a washcloth. There's no need to use hydrogen peroxide, iodine or an iodine-containing cleanser.

3. Apply an antibiotic. After you clean the wound, apply a thin layer of an antibiotic cream or ointment such as Neosporin or Polysporin to help keep the surface moist. The products don't make the wound heal faster, but they can discourage infection and help your body's natural healing process. Certain ingredients in some ointments can cause a mild rash in some people. If a rash appears, stop using the ointment.

4. Cover the wound. Bandages can help keep the wound clean and keep harmful bacteria out. After the wound has healed enough to make infection unlikely, exposure to the air will speed wound healing.

5. Change the dressing. Change the dressing at least daily or whenever it becomes wet or dirty. If you're allergic to the adhesive used in most bandages, switch to adhesive-free dressings or sterile gauze held in place with paper tape, gauze roll or a loosely applied elastic bandage. These supplies generally are available at pharmacies.

6. Get stitches for deep wounds. A wound that is more than 1/4-inch (6 millimeters) deep or is gaping or jagged edged and has fat or muscle protruding usually requires stitches. Adhesive strips or butterfly tape may hold a minor cut together, but if you can't easily close the wound, see your doctor as soon as possible. Proper closure within a few hours reduces the risk of infection.

7. Watch for signs of infection. See your doctor if the wound isn't healing or you notice any redness, increasing pain, drainage, warmth or swelling.

8. Get a tetanus shot. Doctors recommend you get a tetanus shot every 10 years. If your wound is deep or dirty and your last shot was more than five years ago, your doctor may recommend a tetanus shot booster. Get the booster as soon as possible after the injury.

Fractures (broken bones): First aid

A fracture is a broken bone. It requires medical attention. If the broken bone is the result of major trauma or injury, call 911 or your local emergency number. Also call for emergency help if:

 The person is unresponsive, isn't breathing or isn't moving. Begin cardiopulmonary resuscitation (CPR) if there's no respiration or heartbeat.

 There is heavy bleeding.

 Even gentle pressure or movement causes pain.

 The limb or joint appears deformed.

 The bone has pierced the skin.

 The extremity of the injured arm or leg, such as a toe or finger, is numb or bluish at the tip.

 You suspect a bone is broken in the neck, head or back.

 You suspect a bone is broken in the hip, pelvis or upper leg (for example, the leg and foot turn outward abnormally).

Don't move the person except if necessary to avoid further injury. Take these actions immediately while waiting for medical help:

 Stop any bleeding. Apply pressure to the wound with a sterile bandage, a clean cloth or a clean piece of clothing.

 Immobilize the injured area. Don't try to realign the bone or push a bone that's sticking out back in. If you've been trained in how to splint and professional help isn't readily available, apply a splint to the area above and below the fracture sites. Padding the splints can help reduce discomfort.

 Apply ice packs to limit swelling and help relieve pain until emergency personnel arrive. Don't apply ice directly to the skin — wrap the ice in a towel, piece of cloth or some other material.

 Treat for shock. If the person feels faint or is breathing in short, rapid breaths, lay the person down with the head slightly lower than the trunk and, if possible, elevate the legs.

Frostbite: First aid

When exposed to very cold temperatures, skin and underlying tissues may freeze, resulting in frostbite. The areas most likely to be affected by frostbite are your hands, feet, nose and ears.

If your skin looks white or grayish-yellow, is very cold and has a hard or waxy feel, you may have frostbite. Your skin may also itch, burn or feel numb. Severe or deep frostbite can cause blistering and hardening. As the area thaws, the flesh becomes red and painful.

Gradually warming the affected skin is key to treating frostbite. To do so:

 Protect your skin from further exposure. If you're outside, warm frostbitten hands by tucking them into your armpits. Protect your face, nose or ears by covering the area with dry, gloved hands. Don't rub the affected area and never rub snow on frostbitten skin.

 Get out of the cold. Once you're indoors, remove wet clothes.

 Gradually warm frostbitten areas. Put frostbitten hands or feet in warm water — 104 to 107.6 F (40 to 42 C). Wrap or cover other areas in a warm blanket. Don't use direct heat, such as a stove, heat lamp, fireplace or heating pad, because these can cause burns before you feel them on your numb skin.

 Don't walk on frostbitten feet or toes if possible. This further damages the tissue.

 If there's any chance the affected areas will freeze again, don't thaw them. If they're already thawed, wrap them up so that they don't become frozen again.

 Get emergency medical help. If numbness or sustained pain remains during warming or if blisters develop, seek medical attention.

Head trauma: First aid

Most head trauma involves injuries that are minor and don't require hospitalization. However, call 911 or your local emergency number if any of the following signs or symptoms are apparent.

Adults

 Severe head or facial bleeding

 Bleeding or fluid leakage from the nose or ears

 Severe headache

 Change in level of consciousness for more than a few seconds

 Black-and-blue discoloration below the eyes or behind the ears

 Cessation of breathing

 Confusion

 Loss of balance

 Weakness or an inability to use an arm or leg

 Unequal pupil size

 Slurred speech

 Seizures

If severe head trauma occurs

 Keep the person still. Until medical help arrives, keep the injured person lying down and quiet, with the head and shoulders slightly elevated. Don't move the person unless necessary, and avoid moving the person's neck. If the person is wearing a helmet, don't remove it.

 Stop any bleeding. Apply firm pressure to the wound with sterile gauze or a clean cloth. But don't apply direct pressure to the wound if you suspect a skull fracture.

 Watch for changes in breathing and alertness. If the person shows no signs of circulation (breathing, coughing or movement), begin CPR.

Heat cramps: First aid

Heat cramps are painful, involuntary muscle spasms that usually occur during heavy exercise in hot environments. The spasms may be more intense and more prolonged than are typical nighttime leg cramps. Inadequate fluid intake often contributes to heat cramps.

Muscles most often affected include those of your calves, arms, abdominal wall and back, although heat cramps may involve any muscle group involved in exercise.

If you suspect heat cramps

 Rest briefly and cool down

 Drink clear juice or an electrolyte-containing sports drink

 Practice gentle, range-of-motion stretching and gentle massage of the affected muscle group

 Don't resume strenuous activity for several hours or longer after heat cramps go away

 Call your doctor if your cramps don't go away within one hour or so

Heat exhaustion: First aid

Heat exhaustion is one of the heat-related syndromes, which range in severity from mild heat cramps to heat exhaustion to potentially life-threatening heatstroke.

Signs and symptoms of heat exhaustion often begin suddenly, sometimes after excessive exercise, heavy perspiration, and inadequate fluid or salt intake. Signs and symptoms resemble those of shock and may include:

 Feeling faint or dizzy

 Nausea

 Heavy sweating

 Rapid, weak heartbeat

 Low blood pressure

 Cool, moist, pale skin

 Low-grade fever

 Heat cramps

 Headache

 Fatigue

 Dark-colored urine

If you suspect heat exhaustion:

 Get the person out of the sun and into a shady or air-conditioned location.

 Lay the person down and elevate the legs and feet slightly.

 Loosen or remove the person's clothing.

 Have the person drink cool water or other nonalcoholic beverage without caffeine.

 Cool the person by spraying or sponging with cool water and fanning.

 Monitor the person carefully. Heat exhaustion can quickly become heatstroke.

Call 911 or emergency medical help if the person's condition deteriorates, especially if fainting, confusion or seizures occur, or if fever of 104 F (40 C) or greater occurs with other symptoms

Heatstroke: First aid

Heatstroke is the most severe of heat-related problems, after heat cramps and heat exhaustion. Heatstroke often results from exercise or heavy work in hot environments combined with inadequate fluid intake.

Young children, older adults, people who are obese and people born with an impaired ability to sweat are at high risk of heatstroke. Other risk factors include dehydration, alcohol use, cardiovascular disease and certain medications.

What makes heatstroke severe and potentially life-threatening is that the body's normal mechanisms for dealing with heat stress, such as sweating and temperature control, become inadequate. The main sign of heatstroke is a markedly elevated body temperature — generally greater than 104 F (40 C) — with changes in mental status ranging from personality changes to confusion and coma. Skin may be hot and dry — although if heatstroke is caused by exertion, the skin may be moist.

Other signs and symptoms may include:

 Rapid heartbeat

 Rapid and shallow breathing

 Elevated or lowered blood pressure

 Cessation of sweating

 Irritability, confusion or unconsciousness

 Feeling dizzy or lightheaded

 Headache

 Nausea

 Fainting, which may be the first sign in older adults

If you suspect heatstroke:

 Move the person out of the sun and into a shady or air-conditioned space.

 Call 911 or emergency medical help.

 Cool the person by covering with damp sheets or by spraying with cool water. Direct air onto the person with a fan or newspaper.

 Have the person drink cool water or other nonalcoholic beverage without caffeine, if he or she is able.

Hypothermia: First aid

When exposed to cold temperatures, especially with a high wind chill factor and high humidity, or to a cool, damp environment for prolonged periods, your body's control mechanisms may fail to keep your body temperature normal. When more heat is lost than your body can generate, hypothermia, defined as an internal body temperature less than 95 F (35 C), can result.

Wet or inadequate clothing, falling into cold water and even not covering your head during cold weather can increase your chances of hypothermia.

Signs and symptoms of hypothermia include:

 Shivering

 Slurred speech

 Abnormally slow breathing

 Cold, pale skin

 Loss of coordination

 Fatigue, lethargy or apathy

 Confusion or memory loss

 Bright red, cold skin (infants)

Signs and symptoms usually develop slowly. People with hypothermia typically experience gradual loss of mental acuity and physical ability, so they may be unaware that they need emergency medical treatment.

Older adults, infants, young children and people who are very lean are at particular risk. Other people at higher risk of hypothermia include those whose judgment may be impaired by mental illness or Alzheimer's disease and people who are intoxicated, homeless or caught in cold weather because their vehicles have broken down. Other conditions that may predispose people to hypothermia are malnutrition, cardiovascular disease and an underactive thyroid (hypothyroidism).

To care for someone with hypothermia:

 Call 911 or emergency medical assistance. While waiting for help to arrive, monitor the person's breathing. If breathing stops or seems dangerously slow or shallow, begin cardiopulmonary resuscitation (CPR) immediately.

 Move the person out of the cold. If going indoors isn't possible, protect the person from the wind, cover the head, and insulate the individual from the cold ground.

 Remove wet clothing. Replace wet things with a warm, dry covering.

 Don't apply direct heat. Don't use hot water, a heating pad or a heating lamp to warm the person. Instead, apply warm compresses to the center of the body — head, neck, chest and groin. Don't attempt to warm the arms and legs. Heat applied to the arms and legs forces cold blood back toward the heart, lungs and brain, causing the core body temperature to drop. This can be fatal.

 Don't give the person alcohol. Offer warm nonalcoholic drinks, unless the person is vomiting.

 Don't massage or rub the person. Handle people with hypothermia gently because their skin may be frostbitten, and rubbing frostbitten tissue can cause severe damage.

Insect bites and stings: First aid

Signs and symptoms of an insect bite result from the injection of venom or other substances into your skin. The venom causes pain and sometimes triggers an allergic reaction. The severity of the reaction depends on your sensitivity to the insect venom or substance and whether you've been stung or bitten more than once.

Most reactions to insect bites are mild, causing little more than an annoying itching or stinging sensation and mild swelling that disappear within a day or so. A delayed reaction may cause fever, hives, painful joints and swollen glands. You might experience both the immediate and the delayed reactions from the same insect bite or sting. Only a small percentage of people develop severe reactions (anaphylaxis) to insect venom. Signs and symptoms of a severe reaction include:

 Nausea

 Facial swelling

 Difficulty breathing

 Abdominal pain

 Deterioration of blood pressure and circulation (shock)

Bites from bees, wasps, hornets, yellow jackets and fire ants are typically the most troublesome. Bites from mosquitoes, ticks, biting flies, ants, scorpions and some spiders also can cause reactions. Scorpion and ant bites can be very severe. Although rare, some insects also carry disease such as West Nile virus or Lyme disease.

For mild reactions

 Move to a safe area to avoid more stings.

 Remove the stinger, especially if it's stuck in your skin. This will prevent the release of more venom. Wash the area with soap and water.

 Apply a cold pack or cloth filled with ice to reduce pain and swelling.

 Try a pain reliever, such as ibuprofen (Advil, Motrin, others) or acetaminophen (Tylenol, others), to ease pain from bites or stings.

 Apply a topical cream to ease pain and provide itch relief. Creams containing ingredients such as hydrocortisone, lidocaine or pramoxine may help control pain. Other creams, such as calamine lotion or those containing colloidal oatmeal or baking soda, can help soothe itchy skin.

 Take an antihistamine containing diphenhydramine (Benadryl, others) or chlorpheniramine maleate (Chlor-Trimeton, others).

Allergic reactions may include mild nausea and intestinal cramps, diarrhea, or swelling larger than 4 inches (about 10 centimeters) in diameter at the site, bigger than the size of a baseball. See your doctor promptly if you experience any of these signs and symptoms.

For severe reactions Severe reactions affect more than just the site of the insect bite and may progress rapidly. Call 911 or emergency medical assistance if the following signs or symptoms occur:

 Difficulty breathing

 Swelling of the lips or throat

 Faintness

 Dizziness

 Confusion

 Rapid heartbeat

 Hives

 Nausea, cramps and vomiting

Take these actions immediately while waiting with an affected person for medical help:

1. Check for medications that the person might be carrying to treat an allergic attack, such as an autoinjector of epinephrine (EpiPen, Twinject). Administer the drug as directed — usually by pressing the autoinjector against the person's thigh and holding it in place for several seconds. Massage the injection site for 10 seconds to enhance absorption.

2. Loosen tight clothing and cover the person with a blanket. Don't give anything to drink.

3. Turn the person on his or her side to prevent choking if there's vomiting or bleeding from the mouth.

4. Begin CPR if there are no signs of circulation, such as breathing, coughing or movement.

If your doctor has prescribed an autoinjector of epinephrine, read the instructions before a problem develops and also have your household members read them.

Puncture wounds: First aid

A puncture wound doesn't usually cause excessive bleeding. Often the wound seems to close almost instantly. But this doesn't mean treatment isn't necessary.

A puncture wound — such as from stepping on a nail — can be dangerous because of the risk of infection. Wounds resulting from human or animal bites may be especially prone to infection. If the bite was deep enough to draw blood and bleeding persists, seek medical attention.

Otherwise, follow these steps:

1. Stop the bleeding. Apply gentle pressure with a clean cloth or bandage. If bleeding persists after several minutes of pressure, seek emergency assistance.

2. Clean the wound. Rinse the wound with clear water. Use tweezers cleaned with alcohol to remove small, superficial particles. If debris remains embedded, see your doctor. Clean the area around the wound with soap and a clean cloth.

3. Apply an antibiotic. After you clean the wound, apply a thin layer of an antibiotic cream or ointment.

4. Cover the wound. Bandages can help keep the wound clean and keep harmful bacteria out.

5. Change the bandage regularly. Do so at least daily or whenever it becomes wet or dirty.

6. Watch for signs of infection. See your doctor if the wound doesn't heal or if you notice any redness, drainage, warmth or swelling.

See your doctor if the puncture wound

 Is deep

 Is in your foot

 Has been contaminated with soil or saliva

 Is the result of an animal or human bite

If you haven't had a tetanus shot within five years, your doctor may recommend a booster within 48 hours of the injury.

If an animal — especially a stray dog or a wild animal — inflicted the wound, you may have been exposed to rabies. Your doctor may give you antibiotics and suggest starting a rabies vaccination series.

100

Shock: First aid

Shock may result from trauma, heatstroke, blood loss, an allergic reaction, severe infection, poisoning, severe burns or other causes. When a person is in shock, his or her organs aren't getting enough blood or oxygen. If untreated, this can lead to permanent organ damage or death.

Various signs and symptoms appear in a person experiencing shock:

 The skin is cool and clammy. It may appear pale or gray.

 The pulse is weak and rapid. Breathing may be slow and shallow, or hyperventilation (rapid or deep breathing) may occur. Blood pressure is below normal.

 The person may be nauseated. He or she may vomit.

 The eyes lack luster and may seem to stare. Sometimes the pupils are dilated.

 The person may be conscious or unconscious. If conscious, the person may feel faint or be very weak or confused. Shock sometimes causes a person to become overly excited and anxious.

If you suspect shock, even if the person seems normal after an injury:

 Have the person lie down on his or her back with feet about a foot higher than the head. If raising the legs will cause pain or further injury, keep him or her flat. Keep the person still.

 Check for signs of circulation (breathing, coughing or movement) and if absent, begin CPR.

 Keep the person warm and comfortable by loosening any belts or tight clothing and covering the person with a blanket. Even if the person complains of thirst, give nothing by mouth.

 Turn the person on his or her side to prevent choking if the person vomits or bleeds from the mouth.

 Seek treatment for injuries, such as bleeding or broken bones.

101

Snakebites: First aid

Most North American snakes aren't poisonous. Some exceptions include the rattlesnake, coral snake, water moccasin and copperhead. Their bites can be life-threatening.

Of the poisonous snakes found in North America, all but the coral snake have slit-like eyes and are known as pit vipers. Their heads are triangular, with a depression (pit) midway between the eye and nostril on either side of the head.

Other characteristics are unique to certain poisonous snakes:

 Rattlesnakes rattle by shaking the rings at the end of their tails.

 Water moccasins' mouths have a white, cottony lining.

 Coral snakes have red, yellow and black rings along the length of their bodies.

To reduce your risk of snakebite, avoid touching any snake. Instead, back away slowly. Most snakes avoid people if possible and bite only when threatened or surprised.

If a snake bites you

 Remain calm.

 Immobilize the bitten arm or leg, and stay as quiet as possible to keep the poison from spreading through your body.

 Remove jewelry before you start to swell.

 Position yourself, if possible, so that the bite is at or below the level of your heart.

 Cleanse the wound, but don't flush it with water, and cover it with a clean, dry dressing.

 Apply a splint to reduce movement of the affected area, but keep it loose enough so as not to restrict blood flow.

 Don't use a tourniquet or apply ice.

 Don't cut the wound or attempt to remove the venom.

 Don't drink caffeine or alcohol.

 Don't try to capture the snake, but try to remember its color and shape so you can describe it, which will help in your treatment.

102

Spider bites: First aid

Only a few spiders are dangerous to humans. Two that are present in the contiguous United States, and more common in the Southern states, are the black widow spider and the brown recluse spider. Both prefer warm climates and dark, dry places where flies are plentiful. They often live in dry, littered, undisturbed areas, such as closets, woodpiles and under sinks.

Most presumed spider bites are actually bites from other bugs. If you suspect you have been bitten by one of these spiders, check to see if the spider lives in your area. Black widow spider Black widow spider

Although serious, a black widow bite is rarely lethal. You can identify this spider by the red hourglass marking on its belly. The bite feels like a pinprick. You may not even know you've been bitten. At first you may notice slight swelling and faint red marks. Within a few hours, though, intense pain and stiffness begin. Other signs and symptoms include:

 Chills

 Fever

 Nausea and vomiting

 Severe abdominal pain Brown recluse spider Brown recluse spider

You can identify this spider by the violin-shaped marking on its back. The bite produces a mild stinging, followed by local redness and intense pain within eight hours. A fluid-filled blister forms at the site and then sloughs off to leave a deep, enlarging ulcer. Reactions from a brown recluse spider bite vary from a mild fever and rash to nausea and listlessness. On rare occasions death results, more often in children. If bitten by a spider Try and identify the type of spider that bit you. Clean the site of the spider bite well with soap and water. Apply a cool compress over the spider bite location. If the bite is on an extremity, elevate it. Aspirin or acetaminophen (Tylenol, others) and antihistamines may be used to relieve minor signs and symptoms in adults. Use caution when giving aspirin to children or teenagers. Though aspirin is approved for use in children older than age 2, children and teenagers recovering from chickenpox or flu-like symptoms should never take aspirin. Talk to your doctor if you have concerns. If bitten by a brown recluse or black widow spider 1. Cleanse the wound. Use soap and water to clean the wound and skin around the spider bite.

2. Slow the venom's spread. If the spider bite is on an arm or a leg, tie a snug bandage above the bite and elevate the limb to help slow or halt the venom's spread. Ensure that the bandage is not so tight that it cuts off circulation in your arm or leg.

3. Use a cold cloth at the spider bite location. Apply a cloth dampened with cold water or filled with ice.

4. Seek immediate medical attention. Treatment for the bite of a black widow may require an anti-venom medication. Doctors may treat a brown recluse spider bite with various medications.

103

Sprain: First aid

Your ligaments are tough, elastic-like bands that connect bone to bone and hold your joints in place. A sprain is an injury to a ligament caused by tearing of the fibers of the ligament. The ligament can have a partial tear, or it can be completely torn apart.

Of all sprains, ankle and knee sprains occur most often. Sprained ligaments swell rapidly and are painful. Generally, the greater the pain and swelling, the more severe the injury is. For most minor sprains, you probably can treat the injury yourself.

Follow the instructions for R.I.C.E.

1. Rest the injured limb. Your doctor may recommend not putting any weight on the injured area for 48 hours. But don't avoid all activity. Even with an ankle sprain, you can usually still exercise other muscles to minimize deconditioning. For example, you can use an exercise bicycle with arm exercise handles, working both your arms and the uninjured leg while resting the injured ankle on another part of the bike. That way you still get three-limb exercise to keep up your cardiovascular conditioning.

2. Ice the area. Use a cold pack, a slush bath or a compression sleeve filled with cold water to help limit swelling after an injury. Try to ice the area as soon as possible after the injury and continue to ice it for 15 to 20 minutes, four to eight times a day, for the first 48 hours or until swelling improves. If you use ice, be careful not to use it too long, as this could cause tissue damage.

3. Compress the area with an elastic wrap or bandage. Compressive wraps or sleeves made from elastic or neoprene are best.

4. Elevate the injured limb above your heart whenever possible to help prevent or limit swelling.

After two days, gently begin using the injured area. You should feel a gradual, progressive improvement. Over-the-counter pain relievers, such as ibuprofen (Advil, Motrin, others) and acetaminophen (Tylenol, others), may be helpful to manage pain during the healing process.

See your doctor if your sprain isn't improving after two or three days.

Get emergency medical assistance if:

 You're unable to bear weight on the injured leg, the joint feels unstable or numb, or you can't use the joint. This may mean the ligament was completely torn. On the way to the doctor, apply a cold pack.

 You develop redness or red streaks that spread out from the injured area. This means you may have an infection.

 You have re-injured an area that has been injured a number of times in the past.

 You have a severe sprain. Inadequate or delayed treatment may contribute to long-term joint instability or chronic pain.

104

Sunburn: First aid

Signs and symptoms of sunburn usually appear within a few hours of exposure, bringing pain, redness, swelling and occasional blistering. Because exposure often affects a large area of your skin, sunburn can cause headache, fever and fatigue.

If you have a sunburn

 Take a cool bath or shower. You can also apply a clean towel dampened with cool water.

 Apply an aloe vera or moisturizing lotion several times a day.

 Leave blisters intact to speed healing and avoid infection. If they burst on their own, apply an antibacterial ointment on the open areas.

 If needed, take an over-the-counter pain reliever such as aspirin, ibuprofen (Advil, Motrin, others), naproxen (Aleve) or acetaminophen (Tylenol, others). Use caution when giving aspirin to children or teenagers. Though aspirin is approved for use in children older than age 2, children and teenagers recovering from chickenpox or flu-like symptoms should never take aspirin. Talk to your doctor if you have concerns.

Don't use petroleum jelly, butter, egg whites or other home remedies on your sunburn. They can prevent or delay healing.

If your sunburn begins to blister or if you experience immediate complications, such as rash, itching or fever, see your doctor.

105

Tick bites: First aid

Some ticks transmit bacteria that cause illnesses such as Lyme disease or Rocky Mountain spotted fever. Your risk of contracting one of these diseases depends on where you live or travel to, how much time you spend in wooded areas, and how well you protect yourself.

What to do if a tick bites you

 Remove the tick promptly and carefully. Use tweezers to grasp the tick near its head or mouth and pull gently to remove the whole tick without crushing it.

 If possible, seal the tick in a container. Put the container in your freezer. Your doctor may want to see the tick if you develop signs or symptoms of illness after a tick bite.

 Use soap and water to wash your hands and the area around the tick bite after handling the tick.

 Call your doctor if you aren't able to completely remove the tick.

See your doctor if you develop:

 A rash

 A fever

 A stiff neck

 Muscle aches

 Joint pain and inflammation

 Swollen lymph nodes

 Flu-like symptoms

 Light sensitivity to the eyes or skin (photosensitivity)

If possible, bring the tick with you to your doctor's appointment.

Call 911 or your local emergency number if you develop:

 A severe headache

 Difficulty breathing

 Paralysis

 Chest pain or heart palpitations

106

Field Camp Emergency Contacts

Chamberlain, SD Sanford Chamberlain Clinic 300 S Byron Blvd (605) 234-5511 https://www.sanfordhealth.org/medical-services/emergency-medicine

Chamberlain Police Dept. 715 N Main Street (605) 234-4406 http://www.chamberlainsd.net/police_5.html

Sheridan, WY Sheridan Memorial Hospital 1401 W 5th St (307) 672-1000 https://www.sheridanhospital.org/

Sheridan Police Dept. 45 W 12th Street (307) 672-2413 http://www.sheridanpolice.com/

Cody, WY West Park Hospital 707 Sheridan Ave (307) 527-7501 http://www.westparkhospital.org/

Cody Police Dept. 1402 River View Drive (307) 527-8700 http://cityofcody-wy.gov/91/Police

Spearfish, SD Spearfish Regional Hospital 1440 N Main Street (605) 644-4000 https://www.regionalhealth.com/location/spearfish-regional-hospital

Spearfish Police Dept. 625 N. 5th Street (605)642-1305 http://cityofspearfish.com/departments/police/index.php

107

Structural and sedimentological development of footwall growth synclines along an intraforeland uplift, east-central Bighorn Mountains, Wyoming

Richard G. Hoy Department of Earth and Atmospheric Sciences, Purdue University, Kenneth D. Ridgway*} West Lafayette, Indiana 47907-1397

ABSTRACT 24.1% shortening and 9.7 km of uplift had occurred along the eastern margin of the Bighorn Mountains. Structural, sedimentological, and provenance data from Paleogene The caliber of synorogenic deposition in the Powder River basin was synorogenic deposits of the east-central flank of the Bighorn Moun- linked directly to the lithologic composition of the Bighorn Mountains. tains provide new information about the development of footwall Approximately half of the 3.6-km-thick source-stratigraphic section of growth synclines, the evolution of fault-related folds, and the erosional the eastern Bighorn Mountains was eroded prior to accumulation of unroofing history of intraforeland uplifts. Three conglomerate units, conglomerate. The majority of this eroded material was derived from the upper conglomerate member of the Fort Union Formation and the Mesozoic mudstone and poorly indurated sandstone that were incap- Kingsbury and Moncrief Members of the Wasatch Formation, are in- able of generating coarse detritus. The first Paleogene conglomerates corporated within an asymmetric, east-verging growth syncline in the deposited along the east-central Bighorn Mountains, therefore, do not footwall of the main range-bounding thrust system. Three stages of represent the initiation of Laramide uplift, but instead represent the footwall deformation are recorded within these conglomerates. Analy- exposure of coarse-clastÐforming rock types from the lower half of the sis of mapped progressive unconformities, retrodeformed balanced hanging-wall stratigraphic section (i.e., the Mississippian Madison cross sections, and conglomerate clast composition data define these Limestone and Ordovician Bighorn Dolomite). stages as part of a continuum of deformation associated with the development of footwall growth synclines. INTRODUCTION Development of an anticline-syncline pair marked the earliest stage of growth syncline formation (stage I). Rotation of the shared fold For over a decade, there has been increasing interest in the relationship limb resulted in amplification of the growth syncline. Fine-grained, between thrust fault deformation and synorogenic sedimentation within synorogenic sediment derived from easily eroded Mesozoic mudstone foreland and intraforeland basins (e.g., Lawton, 1985; Steidtmann and bypassed the growth syncline during this stage. By the end of Lebo Schmitt, 1988; Heller et al., 1988; Jordan et al., 1988; Lawton et al., 1994; Shale deposition, an average of 12.1% of shortening and 6.46 km of up- DeCelles and Mitra, 1995). Several studies have shown the importance of lift had occurred along the range margin. Continued growth syncline using synorogenic deposits in the structural analysis of foreland basins (De- development was marked by the deposition of the Kingsbury Conglom- Celles et al., 1991; Burbank et al., 1992a; Burbank and Vergés, 1994). A few erate. The Kingsbury Conglomerate was derived from resistant, mid- studies have integrated both structural data and the provenance of synoro- dle and lower Paleozoic carbonate strata in the uplifted source terrane. genic conglomerates to better determine the link between thrusting and dep- Intraformational unconformities, recording as much as 55¡ of bed ro- osition (Graham et al., 1986; Lawton, 1986; Pivnik, 1990; DeCelles et al., tation, were developed within the Kingsbury Conglomerate as fold 1991). Several recent studies have also recognized the development of foot- limb rotation occurred coeval with deposition. Cross sections indicate wall growth synclines adjacent to major thrust faults as a characteristic that during this early stage of fault-related folding, an average of 16.9% structural element (DeCelles et al., 1987, 1993; Vergés et al., 1996; Ridg- shortening and 8.12 km of uplift occurred along the eastern flank of the way et al., 1997). Footwall growth synclines, as defined here, are footwall Bighorn Mountains (end of stage I). The intermediate stage (stage II) folds in which sedimentation took place during structural growth of the syn- of footwall growth syncline development involved partial truncation of cline. Footwall growth synclines have been interpreted as forming in re- the growth syncline by the advancing thrust faults and deposition of the sponse to fault-propagation folding (Suppe and Medwedeff, 1990; DeCelles Moncrief Conglomerate. The lower portion of the Moncrief Conglom- et al., 1991), fault-bend folding (Medwedeff, 1989), and detachment fold- erate was rotated basinward in the developing growth syncline. The fi- ing (Hardy and Poblet, 1994; Vergés et al., 1996; Espina et al., 1996). The nal stage of deformation (stage III) was dominated by the thrust fault- presence of rotated strata, progressive unconformities, and abrupt changes ing of middle and lower Paleozoic strata eastward over steeply dipping in conglomerate clast composition, documented within growth synclines, Mesozoic strata and rotated Eocene synorogenic conglomerate. During attests to the dynamic relationship between thrust fault deformation, fold this stage of deformation, the Moncrief Conglomerate was deformed, development, and synorogenic sedimentation. as the initially blind thrusts propagated into the near-surface conglom- One goal of this study is to develop a better understanding of the struc- erate deposits, truncated the entire footwall syncline, and overrode the tural evolution of footwall growth synclines associated with fault-related synorogenic conglomerate package. Cross sections in areas where this folds. The east-central Bighorn Mountains, deformed by west-dipping final stage of deformation is well developed indicate that an average of thrust faults (Fig. 1), are a natural laboratory for studying the evolution of growth synclines. Here, synorogenic conglomerates in the upper conglom- *E-mail: [email protected] erate member of the Paleocene Fort Union Formation and in the Kingsbury

GSA Bulletin; August 1997; v. 109; no. 8; p. 915Ð935; 16 figures; 1 table.

915 HOY AND RIDGWAY

Twm MONCRIEFFE RIDGE 107° 00'W 106° 45'W Twk N D' Tfuc 2 Tfu

Mz Pz BUFFALO DEEP FAULT 1 pC STORY

05 Thrust Fault km PINEY CREEK THRUST Figure 1. General geologic map of

U High Angle Fault the east-central flank of the Bighorn D U D Mountains, Wyoming, showing ma- 1 Petroleum Well 44° 30'N jor structural features and location

C' of the study area (Hose, 1955; D Mapel, 1959). This part of the range

MOWRY BASIN THRUST can be subdivided into four struc- MOWRY BASIN tural units (from north to south):

U D Piney Creek thrust block, Mowry basin, Clear Creek thrust, and

Kingsbury Ridge. Paleozoic strata JOHNSON CREEK (block pattern) crop out as flatirons CLEAR CREEK THRUST along the range margin. Mesozoic C JOHNSON CREEK THRUST B' BUFFALO strata (striped pattern) are exposed 4 3 in the Mowry basin and at Kings-

NORTH RIDGE ′ ′ ′ Shell bury Ridge. Lines AÐA , BÐB , CÐC , Lineament and DÐD′ indicate locations of cross Bighorn B KINGSBURY RIDGE Mountains sections shown in Figures 6, 9, 11, and 12. Location of petroleum wells 5 A' with geophysical logs used for this 44° 15'N

study: 1—Granite Ridge #1-2-9D; A SISTERS HILL 2—Granite Ridge velocity test hole; THRUST 3—Arco Kenny Ranch well #1-4; Tensleep Fault 4—Buffalo Federal 1-1 well; 5— Mapco I-30 Federal well. Absaroka Big Black Mountains Powder Hills Horn River Basin Basin

Owl Creek Mts. Wind River Basin Laramie Mts. Wind River Mts. Sweetwater Uplift

Precambrian Rocks WYOMING

and Moncrief Members of the Eocene Wasatch Formation have been incor- cases, the rate of basin subsidence has been proposed as the controlling fac- porated within an asymmetric east-verging growth syncline along the main tor on synorogenic conglomerate deposition (Beck et al., 1988; Heller et al., range-bounding thrust fault system (Figs. 1 and 2) (Ridgway et al., 1992; 1988; Burbank et al., 1992b; Heller and Paola, 1992). Results of our study Hoy and Ridgway, 1995). Mapping of progressive unconformities within in the Bighorn Mountains indicate that an additional variable, the lithologic this footwall growth syncline and conglomerate compositional data provide composition of the intraforeland uplift, had an important influence on the constraints needed for retrodeforming balanced cross sections that delineate caliber of synorogenic sediment deposited in the adjacent basin. the various stages of fault-related fold development. Another goal of this study is to closely document the uplift and unroofing STRUCTURE AND PRE-LARAMIDE STRATIGRAPHY OF THE history of an intraforeland uplift and associated sediment deposition within BIGHORN RANGE the adjacent basin. Many recent studies have shown that coarse-grained deposition in foreland and intraforeland basins may be controlled by a num- The Bighorn Mountains of north-central Wyoming and south-central ber of interacting variables (e.g., Flemings and Jordan, 1990). In some Montana (Fig. 1) are part of the Laramide Rocky Mountain foreland (see cases, conglomerate deposition has been attributed mainly to uplift on Brown, 1988, for a review). The Bighorn Mountains are an asymmetric, nearby thrust sheets (Burbank et al., 1988; Steidtmann and Middleton, basement-cored uplift and form the western margin of the Powder River 1991; Jordan et al., 1993; DeCelles, 1994; Pivnik and Khan, 1996). In other basin. Demorest (1941) subdivided the range into three segments on the

916 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Figure 2. Stratigraphic column of the western Powder River basin and east-central Bighorn Mountains. Pre-Laramide strata (pC-K)ÐPre- cambrian (pC): granite and gneiss; Cambrian (C): Flathead Sandstone (locally quartzitic, 105 m) and Gros Ventre Formation (195 m); Ordovi- cian (O): Bighorn Dolomite (110 m); Mississippian (M): Madison Lime- stone (190 m); PennsylvanianÐPer- mian (IP-P): Pennsylvanian Amsden and Tensleep Formations (75 and 83 m, respectively), Permian Goose Egg Formation (55 m); TriassicÐ Jurassic (Tr-J): Triassic Chugwater Formation (240 m), Jurassic Gyp- sum Spring (50 m), Sundance (85 m), and Morrison Formations (55 m); Cretaceous (K): Cloverly Formation (45 m), Skull Creek Shale (50 m), Newcastle Sandstone (15 m), Mowry Shale (160 m), Frontier Formation (150 m), Cody Shale (1080 m), Park- man Sandstone (220 m), Bearpaw Shale (60 m), and Lance Formation (580 m). Tertiary synorogenic strata (T): Paleocene Fort Union Formation (Tullock, Lebo Shale, and Tongue River Members; 360Ð1180 m) and Eocene Wasatch Formation (Kingsbury and Moncrief Conglomerate Members; 300Ð910 m). Thickness of individual synorogenic conglomerates discussed in text: Fort Union conglomerate (430 m), Kingsbury Conglomerate (240 m), and Moncrief Conglomerate (430 m). Thickness data from Hose (1955) and Mapel (1959).

basis of the configuration of dominant structural features. Uplift in the preted horizontal compression as the primary deformational agent (Black- northern and southern segments of the range was controlled by west-verg- stone, 1981; Jenkins, 1986; Brown, 1988; Hudson, 1992; Stone, 1993). ing thrust faults, whereas the central segment, delineated by the Tensleep The pre-Laramide stratigraphy of the Bighorn Mountains can be divided fault on the south and the Shell lineament on the north, was controlled by into three general rock types: Precambrian granite and gneiss, lower and east-verging faults (Fig. 1). Initial geologic mapping (Darton, 1906; De- middle Paleozoic carbonate strata (610 m), and upper Paleozoic and Meso- morest, 1941; Hose, 1955; Mapel, 1959) identified the major faults and sub- zoic mudstones (3020 m) (Hose, 1955; Mapel, 1959) (Fig. 2). The thickest divided the east-central flank of the range into four structural units. These formations in the lower and middle Paleozoic carbonate package are the Or- are (from north to south) the Piney Creek thrust block, the Mowry basin, the dovician Bighorn Dolomite (110 m) and the Mississippian Madison Lime- Clear Creek thrust, and Kingsbury Ridge (Fig. 1). Two roughly north- stone (190 m). The thickest formation in the upper Paleozoic and Mesozoic southÐstriking fault systems are present along the east-central flank of the package is the Cretaceous Cody Shale (1080 m) (Fig. 2). Bighorn Mountains. The western fault is the Bighorn fault, but different names have been applied to this fault in each of the subdivisions: the Piney STRATIGRAPHY AND SEDIMENTOLOGY OF Creek thrust, the Mowry basin thrust, the Johnson Creek and Clear Creek SYNOROGENIC DEPOSITS OF THE POWDER RIVER BASIN thrusts, and the Sisters Hill thrust at Kingsbury Ridge (Fig. 1). The eastern fault, identified in seismic sections by Foster et al. (1969) and later named The Powder River basin is one of several nonmarine intraforeland basins the Buffalo Deep fault by Blackstone (1981), roughly parallels the range that formed in response to basement deformation during the Laramide orog- margin (Fig. 1). This fault has been interpreted as a reactivated basement eny (Dickinson et al., 1988). The lower Tertiary fill of the Powder River fault with as much as 1.2 km of displacement (Foster et al., 1969). basin is more than 2000 m thick and consists of the Paleocene Fort Union Initial interpretations of the structures along the east-central part of the Formation and the Eocene Wasatch Formation. The Fort Union Formation Bighorn Mountains proposed thrust faulting as the dominant uplift mecha- has three members: the early Paleocene Tullock Member, the middle Paleo- nism (Demorest, 1941). Later studies suggested a block-uplift mechanism as cene Lebo Shale, and the late Paleocene Tongue River Member (Fig. 2) the primary deformational style, particularly along the Piney Creek thrust (Hose, 1955; Mapel, 1959). The Wasatch Formation has two members: the (Palmquist, 1978; Stearns, 1978). Seismic surveys (Foster et al., 1969; Rob- Kingsbury Conglomerate and the Moncrief Conglomerate (Fig. 2). The two bins and Grow, 1990) and wells drilled through the Precambrian rocks in the formations contain three synorogenic conglomerates, the upper conglomer- hanging wall of the Bighorn thrust have demonstrated that the central part of ate member of the Paleocene Fort Union Formation (Tongue River equiva- the range is structurally controlled by east-verging, relatively shallow dip- lent), and the Kingsbury and the Moncrief conglomerates (Fig. 2). These ping thrust faults (approximately 30¡W). More recent studies have inter- conglomerates are located along the western flank of the Powder River

Geological Society of America Bulletin, August 1997 917 HOY AND RIDGWAY basin adjacent to the central segment of the Bighorn Mountains (Fig. 1) Balancing of the cross sections was based upon three assumptions. First, it (Sharp, 1948; Obernyer, 1979). They have been interpreted as alluvial-fan is assumed that all uplift along the current trend of the Bighorn Mountains oc- deposits (Sharp, 1948; Obernyer, 1979), and contain proximal fan, medial curred during the Laramide orogeny. Central Wyoming has certainly under- fan, and distal fan facies (Flores and Warwick, 1984; Ridgway et al., 1991). gone earlier and later episodes of deformation, but on the basis of previous Isopach data indicate that initiation of the Powder River basin as a struc- tectonic studies of the Bighorn Mountains (Demorest, 1941; Hoppin, 1961; tural and depositional basin occurred during deposition of the middle Paleo- Curry, 1971; Jenkins, 1986), these other deformational events had little influ- cene Lebo Shale (Curry, 1971; Ayers, 1986). Several studies have shown ence on the present structural configuration. The second assumption is that the that basin subsidence was greatest adjacent to the thrusted east-central seg- climatic controls on weathering of the source section did not significantly al- ment of the Bighorn Mountains (Curry, 1971; Obernyer, 1979; Ayers, 1986; ter the composition of detritus transported and deposited in the basin. Despite Flores, 1986), where the Lebo Shale and Tongue River Members of the Fort the interpretation that the Paleogene synorogenic conglomerates were de- Union Formation and the Wasatch Formation are thickest. Depositional en- posited by wet alluvial fans (Flores and Warwick, 1984), and that the annual vironments during Fort Union and Wasatch deposition were predominantly temperature increased from 10 ¡C at 58.5 Ma to 18 ¡C at 50.5 Ma (Hickey, alluvial fan, fluvial, and lacustrine (Seeland, 1976; Ethridge et al., 1981; 1980; Wing et al., 1991), the consistent abundance of pebble- to cobble-sized Flores and Hanley, 1984; Ayers, 1986; Weaver and Flores, 1987). Major limestone clasts within the synorogenic deposits indicates that chemical Paleocene and Eocene fluvial systems flowed northward through the Pow- weathering was not a controlling factor. The third assumption is that very lit- der River basin toward the Williston basin (Flores and Ethridge, 1985). tle longitudinal transport of sediment occurred in the alluvial-fan systems. Pa- leocurrent data from this and previous studies (Flores and Ethridge, 1985; Flo- METHODS res, 1986) support this assumption. Therefore, we would not expect conglomerate clasts derived from adjacent erosional drainages along strike to Sedimentological data collected from the three Tertiary conglomerate significantly influence clast composition data at a given locality. units along the western edge of the Powder River basin include conglomer- One cross section was constructed through each of the four structural seg- ate clast composition, paleocurrent directions, and maximum particle size. ments of the east-central flank of the range (Fig. 1). Analysis of the stratig- These data were collected within the context of measured stratigraphic sec- raphy, clast composition, and deformation of the synorogenic conglomer- tions. Structural data used for construction of cross sections were collected ates within the framework of cross sections is used to interpret the structural on traverses perpendicular to the range margin. These data include orienta- evolution of each segment. tions of bedding units and faults in the Paleozoic and Mesozoic strata in the hanging wall of range-bounding thrusts and also in the footwall synorogenic ANALYSIS OF STRUCTURAL EVOLUTION Tertiary strata. Angular unconformities separating the three conglomerates were mapped at a scale of 1:24 000 and the amount of discordance was Kingsbury Ridge measured. This information, along with data obtained from petroleum well logs, was used to construct four balanced cross sections. The cross sections Stratigraphy. Kingsbury Ridge is the type locality of the Eocene Kings- were both line and area balanced using the standard methods established by bury Conglomerate (Twk) (Fig. 1). At this location, the Kingsbury Con- Dahlstrom (1969) and Woodward et al. (1985). glomerate is 240 m thick. The conglomerates of this formation are massive, Each cross section was sequentially retrodeformed to show the relative clast-supported units with lenticular geometries (Fig. 3A). Maximum parti- positions of the hanging wall and footwall during the various stages of syn- cle size data range from an average of 5Ð10 cm at the eastern edge of Kings- orogenic deposition. Assuming a 10¡ depositional slope for the proximal bury Ridge to 25Ð30 cm in the west-central part (Fig. 4A). Pebble imbrica- sections of the currently rotated alluvial-fan deposits, the strata can be re- tions indicate that paleoflow was to the east-northeast (Fig. 4A). stored and the approximate structural level of the feeder canyon can be pro- Matrix-supported conglomerate units interpreted as debris-flow deposits are jected into the hanging-wall stratigraphy. This projection delineates which present, but uncommon, and have an average maximum particle size of stratigraphic units were exposed on the hanging wall of the thrust fault, and, 80 cm. Sandstone beds are predominantly massive, but in some cases have therefore, which rock types were available as source rocks for the synoro- interbedded siltstones (Fig. 3A). The Kingsbury Conglomerate in this area genic deposits in the Powder River basin during each stage of deformation. has been interpreted as middle to distal alluvial-fan deposits (Flores and Each interformational and intraformational angular unconformity mapped Warwick, 1984). in the synorogenic conglomerates was restored in a similar fashion during Clast Composition. The composition of the Kingsbury Conglomerate the retrodeformation process. The match between measured conglomerate indicates derivation from the Cambrian through Mississippian formations, clast composition in the footwall and the possible source stratigraphic sec- and a minor contribution from Precambrian sources (Fig. 5A). At the base tion (based on projection into the hanging wall) helped determine geologi- of the Kingsbury Conglomerate section at Kingsbury Ridge, 80% of the cally plausible solutions during each stage of retrodeformation. This conglomerate composition is Mississippian Madison Limestone (Mm) and method is similar to that developed by DeCelles et al. (1991) in their study Ordovician Bighorn Dolomite (Ob) (Fig. 5A). Approximately 11% of the of the Beartooth Conglomerate of Wyoming and Montana. clasts were derived from Cambrian formations: the flat-pebble conglomer- Pinning point locations for the cross sections were chosen in the follow- ate and purple limestone of the upper Gros Ventre Formation (Cgv), and the ing manner. The eastern pinning point in the footwall was located in the un- Flathead Sandstone (Cf). Precambrian granite and gneiss constitute only deformed Powder River basin, close to the basin axis. Because the majority 8% of the clast types. At the top of the section, only 30% of the clasts were of rocks exposed in the hanging wall are Precambrian crystalline basement, derived from the Mississippian Madison Limestone and Ordovician Big- there is no easily chosen location for the western pinning points based on horn Dolomite, whereas the Cambrian and Precambrian contribution in- stratigraphy. To find a western pinning point, the Paleozoic sedimentary sec- creased to approximately 68% (43% Cgv and Cf; 25% pC). At the top of the tions were projected from bedding attitudes on both flanks of the range un- Kingsbury Conglomerate section, 2% of the conglomerate clasts are re- til the hinge of the fold was determined. Assuming concentric flexural-slip worked basal Kingsbury Conglomerate (Twk in Fig. 5A). folding where no slip occurs between units at the hinge, this location was Deformation of Synorogenic Conglomerate. Growth strata are well used as the western pinning point for each of the cross sections. developed in the Kingsbury Conglomerate at Kingsbury Ridge. The con-

918 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Figure 3. Log of measured stratigraphic sections from the Kings- bury Conglomerate including maximum particle sizes (MPS) and paleocurrent directions. (A) Mid- fan deposits at Kingsbury Ridge. Location 3A in Figure 4A. (B) Mid- fan deposits in the Mowry basin. Location 3B in Figure 4C. Grain- size abbreviations: m—mud, s— silt, vf—very fine sand, f—fine sand, m—medium sand, c—coarse sand, vc—very coarse sand, p— pebble, b—boulder. Arrows show paleocurrent directions where north is taken as the top of the figure. Paleocurrent directions from imbricated conglomerate clasts and trough cross-stratification. Note scale changes between individual sections.

tact with the underlying Fort Union deposits is an angular unconformity Structural Evolution. Cross section AÐA′ shows that two faults, the Sis- with as much as 54¡ of discordance. A traverse perpendicular to the re- ters Hill thrust (SHT in Fig. 6) on the west and the Buffalo Deep fault (BDF gional strike of the Kingsbury Conglomerate reveals gradual rotation of in Fig. 6) on the east, controlled the deformation observed at Kingsbury the strata (Fig. 6C). The steepest dip of the basal Kingsbury Conglomer- Ridge. The Mapco I-30 Federal well (well #5 in Figs. 1 and 6) provided ate is approximately 65¡E, whereas deposits at the top of the ridge to the control for the footwall stratigraphy. Uplift calculated from cross section east are dipping about 10¡E (Fig. 6C). Assuming an initial 10¡ deposi- AÐA′ at the Precambrian-Phanerozoic contact is 8.10 km, and shortening is tional slope, the conglomerates have undergone as much as 55¡ of pro- calculated as 5.73 km (15.5%) (Fig. 6C). Cross section AÐA′ was retrode- gressive rotation. formed in three stages. (1) Initial uplift of the Bighorn Range occurred along

Geological Society of America Bulletin, August 1997 919 HOY AND RIDGWAY

A) Twm KINGSBURY RIDGE B) NORTH RIDGE CLEAR CREEK THRUST 3A

Twk

7A 7B Tfu

Mz Pz

Pz Twk Figure 4. Sedimentologic data of synorogenic conglomerates from Twm each of the four regions in the study area (Fig. 1). Data include the aver- pC age maximum particle sizes (MPS, Pz Mz Twk dots) and average paleocurrent directions (arrows) from imbricated 0 5 10 conglomerate clasts (average mea- km 0 5 10 km surement of 10 imbricated clasts per station) and trough cross-stratification. Circles containing numbers C) D) MOWRY BASIN B show location of measured strati- 10 7C Tfu graphic sections. The letters in D Mz represent locations of conglomerate Twk A Pz clast compositions shown in Fig- Twk ure 5D. pC—Precambrian crys- Tfu Twm

talline basement; Pz—Paleozoic strata; Mz—Mesozoic strata; Tfu— Mz Pz C Fort Union Formation; Tfuc—Fort PINEY CREEK THRUST BLOCK Union conglomerate; Twk—Kings- Pz Tfuc 3B pC bury Conglomerate; Twm—Mon- D crief Conglomerate. Twk

Twk

Mz 0 5 10 0 5 10 km km Pz N Clast Sizes Paleocurrent Direction <10 cm 25-50 cm Twm Twk Tfuc 10-25 cm 50-100 cm >100 cm 3A Measured Section Locations

Figure 4

the Sisters Hill thrust (Fig. 6A). This uplift exposed the easily eroded Meso- of the basin. (2) Thrust displacement was transferred from the Sisters Hill zoic and upper Paleozoic mudstones (Fig. 2), which are interpreted as the thrust to the Buffalo Deep fault (Fig. 6B). The Buffalo Deep fault is inter- source terrane for the Lebo Shale Member of the Fort Union Formation. Be- preted to be part of a large fault-related fold. Its high angle may be indica- cause displacement on the Sisters Hill thrust is approximately 300 m, uplift tive of reactivation of a basement fault (Foster et al., 1969), but its expres- associated with this fault cannot account for the unroofing of the entire sec- sion in the Phanerozoic units between the two range bounding faults is that tion of Mesozoic rocks (approximately 2 km) or the thickness of the Lebo of a large hanging-wall anticline. This anticline is in part derived from a Shale (915 m). Uplift at the end of movement along the Sisters Hill thrust basement fault (FS1 in Fig. 6), which is a splay of the Buffalo Deep fault. was 1.85 km with 0.73 km of shortening (2.0%) (Fig. 6A). At this time, During displacement on FS1, the hinge of this anticline migrated to the west most sediment eroded from the hanging wall bypassed the proximal region (e.g., rolling-hinge model of Schmidt et al., 1993). The initiation of uplift on

920 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Figure 5. Histograms of conglomerate clast compositions in the four regions of the study area. (A) Upsection change in composition of the Kings- bury Conglomerate at Kingsbury Ridge. Note the decrease in Mississippian Madison Limestone (Mm) and the increase in Cambrian Gros Ven- tre (Cgv) and Precambrian basement clasts (pC) upsection. Cf—Cambrian Flathead, Ob—Bighorn Dolomite, Twk—reworked Kingsbury Con- glomerate clasts. (B) Average clast composition of the Kingsbury Conglomerate (Twk) and Moncrief Conglomerate (Twm) exposed at North Ridge along the Clear Creek thrust and at Johnson Creek. n—number of clasts identified. (C) Average composition of the Fort Union conglomerate (Tfuc) and Kingsbury Conglomerate (Twk) in the Mowry basin. Note the greater contribution of clasts from the Cambrian Gros Ventre (Cgv) and Cambrian Flathead (Cf) Formations in the Kingsbury Conglomerate relative to the Fort Union conglomerate. (D) Composition of the conglomerate outcrops AÐD along the Piney Creek thrust block. See Figure 4D for outcrop locations. Note that the compositions are intermediate between those of the Kingsbury Conglomerate and those of the Moncrief Conglomerate exposed elsewhere along the range margin. Compo- sitions of conglomerates at top of Moncrieffe Ridge (Twm) are shown for comparison.

the Buffalo Deep fault allowed for continued exposure and unroofing of the flected in the Kingsbury Conglomerate clast composition by an upsection nonresistant Mesozoic and upper Paleozoic units (deposited as the Lebo increase in Precambrian and Cambrian clast types and a decrease in Missis- Shale) until the more durable middle and lower Paleozoic carbonate rocks sippian clast types (Fig. 5A). With continued deformation, proximal Kings- were exposed in the hanging wall. Initial exposure of the Mississippian bury Conglomerate deposits were incorporated into the steeply dipping Madison Limestone and Bighorn Dolomite marked the onset of Kingsbury western limb of the growth syncline (Fig. 6C). The proximal Kingsbury Conglomerate deposition; calculated uplift is 6.55 km and shortening is Conglomerate on the steep limb of the growth syncline became a source of 2.13 km (5.8%) (Fig. 6B). During this stage of deformation, further devel- sediment for younger Kingsbury Conglomerate strata being deposited in the opment of the growth syncline allowed for sediment deposition in the foot- hinge of the growing footwall syncline. This cannibalization is documented wall of the Buffalo Deep fault (Fig. 6B). Incorporation of fine-grained Fort by the introduction of conglomerate clasts of the Kingsbury Conglomerate Union deposits into the steep limb of the footwall growth syncline, prior to into younger Kingsbury Conglomerate deposits (Fig. 5A). With continued deposition of the Kingsbury Conglomerate, resulted in 54¡ of angular dis- rotation of the western limb of the growth syncline, progressive unconfor- cordance between the two units. (3) As displacement on the Buffalo Deep mities developed in the Kingsbury Conglomerate; 55¡ of syndepositional fault proceeded, uplift and shortening in the region increased, causing bed rotation occurred within the Kingsbury Conglomerate during this deeper incision into the hanging wall. Deeper hanging-wall dissection is re- stage of deformation.

Geological Society of America Bulletin, August 1997 921 HOY AND RIDGWAY

A) Start of Lebo Shale Deposition Uplift 1.85 km Shortening 0.73 km (2.0%) A'

SHT BDF

B) Start of Kingsbury Conglomerate Deposition Uplift 6.55 km Shortening 2.13 km (5.8%) Tfu Fort Union Formation Figure 6. Sequential restoration of Kc Cody Shale cross section AÐA′ at Kingsbury Ridge A' Kingsbury Conglomerate (see Fig. 1 for location of cross section). Amount of uplift and shortening is shown Mississippian Madison Limestone- Ordovician Bighorn Dolomite for each stage of retrodeformation. (A) Initial displacement along the Sisters Hill Precambrian Basement SHT Tfu thrust (SHT) and beginning of Lebo Unconformity Shale deposition in the Powder River Kc Bedding Attitude basin. (B) Transfer of displacement to the Buffalo Deep fault (BDF) and onset of FS1

BDF Kingsbury Conglomerate deposition. 0 1 2 3 6 km Note the development of the footwall growth syncline by this stage. (C) Present C) A structural configuration of Kingsbury Ridge. Note the progressive decrease in

dip within the Kingsbury Conglomerate. Petroleum well logs used for construction: 5 5—Mapco I-30 Federal well. Surface data A' from Hoy (1996). Present Configuration Total Uplift 8.10 km SHT Tfu Total Shortening 5.73 km (15.5%)

Kc FS1

BDF A A'

SHT FS1 BDF

Figure 6

Clear Creek ThrustÐNorth Ridge show evidence of channeling (Fig. 7B). Maximum particle size data in the proximal deposits range from about 20 cm near the base of the section to as Stratigraphy. Boulder conglomerates exposed west of Buffalo, much as 400 cm near the top of the section (Fig. 4B). Due to the large, Wyoming, are part of a large Eocene alluvial fan system (Sharp, 1948; Nel- rounded clasts, imbrication is not well developed in the proximal deposits; son, 1968). The deposits are exposed along Clear Creek and along U.S. however, the distal deposits show paleocurrent directions ranging from Highway 16 at North Ridge (Fig. 1). This ancient fan system is composed of northeast to southeast (Fig. 4B). the Moncrief Member of the Wasatch Formation (Twm) and is about 430 m Clast Composition. The clast composition of the Moncrief Conglomer- thick. The proximal deposits of the Moncrief Conglomerate are clast-sup- ate shows that the majority of the clasts were derived from Precambrian ported, boulder conglomerates that are predominantly massive, although up- gneiss and granite (Fig. 5B). The conglomerate composition indicates that ward fining is evident in some units. Mid-fan deposits are composed of thick, even at the onset of Moncrief deposition, the source terrane was composed upward-fining cobble conglomerate beds with thin interbeds of coarse sand- predominantly of Precambrian rocks. Along the line of cross section BÐB′ stone (Fig. 7A). The more distal sandstone deposits are finer grained and (Fig. 1), the Kingsbury Conglomerate is observed only in the ARCO Kenny

922 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Figure 7. Log of measured stratigraphic sections of the Mon- crief Conglomerate with maximum particle size (MPS) and paleocurrent data. (A) Mid-fan deposits at North Ridge. Location 7A in Figure 4B. (B) Distal-fan deposits at North Ridge. Location 7B in Figure 4B. (C) Mid-fan deposits at Moncrieffe Ridge. Loca- tion 7C in Figure 4D. See Figure 3 for lithology key and grain-size explanation.

Ranch well logs (well #3 in Figs. 1 and 9). Well cuttings indicate that its CCTF2 in Fig. 9) and the Buffalo Deep fault (BDF in Fig. 9). The Clear clast composition is similar to correlative deposits located elsewhere along Creek thrust system is interpreted to have directly influenced both the dep- the range margin (i.e., predominantly Paleozoic carbonates with a minor osition and deformation of the synorogenic deposits. Uplift along cross sec- Precambrian component). The Kingsbury Conglomerate is exposed along tion BÐB′ is 9.57 km; shortening is 9.30 km (26.3%) (Fig. 9D). Two wells, the Clear CreekÐJohnson Creek thrust fault to the north along Johnson the Arco Kenny Ranch well #1-4 (well #3 in Figs. 1 and 9) and the Buffalo Creek (Fig. 1). Here, the majority of the conglomerate clasts are Madison Federal 1-1 well (well #4 in Figs. 1 and 9), were used to determine the rela- Limestone and Bighorn Dolomite (Fig. 5B). tive position of footwall stratigraphy. The Arco Kenny Ranch well (for Deformation of Synorogenic Conglomerates. The Moncrief Conglom- which a dipmeter survey is available) drilled through 731 m of basement erate exposed along U.S. Highway 16 is in fault contact with the Clear Creek granite in the hanging wall before penetrating the CCTF1 and into 305 m of thrust fault (Fig. 8). Locally, directly beneath the Clear Creek thrust (within Kingsbury Conglomerate (Fig. 9D). The well then penetrated an overturned 15 m below the thrust fault), a thick sequence of upward-fining beds has been section of Fort Union shales (dipping 70¡ to the west) before crossing a sec- rotated basinward to near vertical orientations. Fault gouge and fractured ond fault (CCTF2) at a depth of 1707 m into a normal stratigraphic section clasts along bedding planes indicate flexural-slip folding. Directly below this dipping 20¡ to the west (Fig. 9D). The hanging wall of the Clear Creek panel of vertically dipping beds is a panel of horizontal strata. Back thrusts thrust system places middle and lower Paleozoic carbonate strata on top of and bedding-plane faults have offset conglomerate clasts and locally caused the Moncrief Conglomerate (Fig. 8), which is composed of nearly 100% elongation of clasts due to shearing within this lower panel. The lowest ex- Precambrian clasts. posed and unfaulted proximal fan deposits dip 45¡E. Overall, the Moncrief Cross section BÐB′ was retrodeformed in four stages. (1) The initial uplift Conglomerate in this area is interpreted to have undergone a minimum of 35¡ of the Bighorn Mountains in the North Ridge area began with displacement of bed rotation (assuming an initial 10¡ depositional slope). along the Clear Creek thrust system (CCTF1 and CCTF2 on Fig. 9). This led Structural Evolution. The two primary fault systems present in the to the exposure and unroofing of the Mesozoic mudstones and poorly in- vicinity of North Ridge are the Clear Creek thrust system (CCTF1 and durated sandstones in the hanging wall. The bulk of eroded hanging-wall

Geological Society of America Bulletin, August 1997 923 HOY AND RIDGWAY

Figure 8. Photograph of the Clear Creek thrust fault along U.S. Highway 16 showing relationship between thrust- faulted basin margin and synorogenic Moncrief Conglomerate. View is toward the north. Solid black line and large arrow indicate thrust fault. Middle and lower Paleozoic strata have been emplaced over Eocene Moncrief Conglomerate. Note the steeply dipping Moncrief Conglomerate (Twm) in footwall (small arrow). Verti- cally dipping hanging-wall units include: Cgv—Cambrian Gros Ventre Formation, Ob—Ordovician Bighorn Dolomite, and Mm—Mississippian Madison Limestone.

sediment was transported out of the most proximal part of the basin, result- (Fig. 1). The Fort Union conglomerate is predominantly a clast-supported, ing in the deposition of the Lebo Shale Member of the Paleocene Fort Union channelized conglomerate interbedded with fine sandstone-siltstone pale- Formation. At this stage, uplift was 3.96 km with 2.05 km of shortening osols (Fig. 10). In surface exposures, the Fort Union conglomerate is 430 m (6.0%) (Fig. 9A). During this initial stage of deformation, the footwall thick, and maximum particle size data from the top 160 m show a progres- growth syncline was an open fold hinged at the tip of CCTF2 (Fig. 9A). sive upward-fining sequence, from around 20 cm at the base of the mea- (2) Continued displacement along the CCTF1 exposed the more durable sured section to about 9 cm at the top (Fig. 10). Occasional matrix-sup- middle and lower Paleozoic units in the hanging wall. Unroofing of these ported conglomerate units, interpreted as debris-flow deposits, have a units led to the deposition of the Kingsbury Conglomerate observed in the maximum particle size close to 50 cm. Many of the conglomeratic intervals Kenny Ranch well logs. Uplift at the onset of the Kingsbury Conglomerate exhibit pebble imbrication showing two directions of paleoflow alternating deposition was 6.01 km, and shortening was 5.86 km (16.6%) (Fig. 9B). between nearly due south and northeast (Fig. 4C). The Kingsbury Con- During this stage of deformation, rotation of the steeper, western limb of the glomerate overlies the Fort Union conglomerate in the Mowry basin and growth syncline was caused by coeval displacement on the CCTF1 and the contains mainly clast-supported, channelized conglomerates. The Kings- CCTF2 as CCTF2 began to propagate through the fold hinge (Fig. 9B). bury Conglomerate has more trough cross-stratification than the Fort Union (3) After deposition of approximately 300 m of the Kingsbury Conglomer- conglomerate, as well as a greater abundance of interbedded sandstone and ate, displacement was transferred to a splay of the CCTF1 (FS1 in Fig. 9C). mudstone (Figs. 3B and 10). Maximum particle size data show an upward- Displacement along this splay resulted in the exposure of the Precambrian coarsening package from 10 cm at the base of our measured section to near basement rocks that were the source terrane for the Moncrief Conglomerate. 20 cm at the top (Fig. 3B). Paleocurrent measurements from cross-stratifi- Uplift at the onset of the Moncrief Conglomerate deposition was 7.44 km, cation and pebble imbrication show sediment transport directions to the and shortening was 6.65 km (18.8%) (Fig. 9C). During this stage of defor- east-southeast (Fig. 4C). mation, the Moncrief Conglomerate was deposited along the hinge of the Clast Composition. The Fort Union conglomerate consists of nearly footwall growth syncline above the Kingsbury Conglomerate (Fig. 9, C 90% Mississippian Madison Limestone and Ordovician Bighorn Dolomite and D). The Moncrief Conglomerate appears to have undergone 35¡ of east- clasts and 10% Cambrian Gros Ventre Formation and Cambrian Flathead ward syndepositional rotation as it was incorporated into the growing foot- Sandstone clasts (Fig. 5C). There is little change in clast composition up- wall syncline. (4) After deposition of the Moncrief Conglomerate, reactiva- section (Hoy, unpub. data). The Kingsbury Conglomerate in the Mowry tion of the CCTF1 truncated the western limb of the footwall syncline coeval basin contains 75% Mississippian Madison Limestone and Ordovician Big- with propagation of CCTF2 through the hinge of the syncline. This last horn Dolomite clasts (50% and 25%, respectively), 25% Cambrian Flathead stage of deformation juxtaposed the lower Paleozoic strata over the Eocene and Gros Ventre clasts, and <1% Precambrian basement clasts (Fig. 5C). synorogenic deposits, as observed along the U.S. Highway 16 road cut Deformation of Synorogenic Conglomerates. The synorogenic con- (Figs. 8 and 9D). Note that, unlike at Kingsbury Ridge, the Buffalo Deep glomerates in the Mowry basin have undergone significant progressive bed fault does not appear to directly influence synorogenic deposition, despite its rotation. In the Fort Union conglomerate, the basal strata are dipping as much most likely displacement during the Kingsbury Conglomerate deposition. as 40¡E, whereas strata at the top of the section dip about 20¡E. An angular unconformity of 10¡ exists between the Fort Union conglomerate and the Mowry Basin Kingsbury Conglomerate. Intraformational rotation in the Kingsbury Con- glomerate is less evident than in the underlying Fort Union conglomerate. Stratigraphy. Synorogenic conglomerates in the Mowry basin include The basal Kingsbury strata locally dip as much as 20¡E, but the predominant the Paleocene Fort Union conglomerate and the Kingsbury Conglomerate bedding attitudes at the base of the Kingsbury Conglomerate are 10¡E.

924 Geological Society of America Bulletin, August 1997 Moncrief Conglomerate Tfu Fort Union A) Start of Lebo Shale Deposition Formation Uplift 3.96 km Kingsbury Conglomerate Shortening 2.05 km (6.0%) Kc Cody Shale B' Mississippian Madison Limestone- Ordovician Bighorn Dolomite Unconformity

Bedding Precambrian Basement Attitude Kc CCTF1

0 1 2 3 6 km BDF CCTF2

B) C) Start of Moncrief Conglomerate Deposition

Uplift 7.44 km

Start of Kingsbury Conglomerate Deposition Shortening 6.65 km (18.8 %) Uplift 6.01 km Shortening 5.86 km (16.6%) FS1 B' B'

Tfu Tfu

Kc Kc CCTF1 CCTF1

CCTF2 BDF CCTF2 BDF D) B

FS1

3 4 B'

Present Configuration Total Uplift 9.57 km Tfu Total Shortening 9.30 km (26.3%) CCTF1 Kc

B CCTF2 BDF B' CCTF1

FS1 CCTF2 BDF

Figure 9. Sequential restoration of cross section BÐB′ at North Ridge (see Fig. 1 for location of cross section). Amount of uplift and shortening is shown for each stage of retrodeformation. (A) Initial displacement along the Clear Creek thrust system (CCTF1 and CCTF2) and beginning of Lebo Shale deposition in the Powder River basin. (B) Continued displacement along CCTF1 and start of Kingsbury Conglomerate deposition.

(C) Displacement transferred to splay FS1 of the CCTF1 marks onset of Moncrief Conglomerate deposition. (D) Reactivation of CCTF1 to present-day structural configuration. Petroleum well logs used for construction: 3—Arco Kenny Ranch #1-4 well; 4—Buffalo Federal 1-1 well. BDF—Buffalo Deep fault.

Geological Society of America Bulletin, August 1997 925 HOY AND RIDGWAY

Figure 10. Log of measured stratigraphic section of mid-fan deposits of the Fort Union conglomerate in the Mowry basin with maximum particle size (MPS) and paleocurrent data. Location 10 in Figure 4C. See Figure 3 for explanation of lithology key and grain-size explanation.

Structural Evolution. Deposition of the conglomerates in the Mowry along cross section CÐC′ is 8.14 km, and shortening is 5.1 km (14.2%) basin resulted from basinward rotation between the Mowry basin thrust (Fig. 11D). fault and the Johnson Creek thrust fault (MBTF and JCTF in Fig. 11). As at Cross section CÐC′ was retrodeformed in four stages. (1) Initial move- North Ridge (Fig. 9), the Buffalo Deep fault is located farther east in the ment along the Mowry basin thrust resulted in deposition of the Lebo Shale, Powder River basin, well away from the synorogenic deposits, and as a re- which was derived from the erosion of Mesozoic and upper Paleozoic units. sult did not influence conglomerate deposition in the Mowry basin. Uplift During initial growth syncline development, the syncline was hinged at the

926 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Kingsbury Conglomerate A) Start of Lebo Shale Deposition Uplift 4.25 km Mississippian Madison Limestone- Shortening 1.38 km (3.4%) C' Ordovician Bighorn Dolomite

Precambrian Basement Kc

Unconformity 0 1 2 3 6 km Tfu Fort Union Formation MBTF Kc Cody Shale Bedding Attitude JCTF BDF

C) B) Start of Fort Union Conglomerate DepositionStart of Kingsbury Conglomerate Deposition Uplift 6.17 km Uplift 6.80 km Shortening 3.17 km (7.9%) Shortening 4.26 km (10.6%)

C' C'

Tfu Tfu

Kc Kc MBTF MBTF JCTF BDF JCTF BDF D) Present Configuration C Total Uplift 8.14 km Total Shortening 5.10 km (14.2%)

Tfu MBTF

JCTF BDF C C'

MBTF JCTF BDF

Figure 11. Sequential restoration of cross section CÐC′ at the Mowry basin (see Fig. 1 for location of cross section). Amount of uplift and shortening is shown for each stage of retrodeformation. (A) Initial displacement along the Mowry Basin thrust fault (MBTF) and start of Lebo Shale deposition in the Powder River basin. (B) Farther displacement along MBTF and the Johnson Creek thrust fault (JCTF) and onset of Fort Union

conglomerate deposition. (C) Continued displacement along the MBTF and the JCTF and onset of Kingsbury Conglomerate deposition. (D) Dis- placement along the MBTF and the JCTF to present structural configuration. BDF—Buffalo Deep Fault.

tip of the JCTF (Fig. 11A). Uplift at this stage was 4.25 km with 1.38 km of exposed the Mississippian Madison Limestone and Ordovician Bighorn shortening (3.4%) (Fig. 11A). Much of the sediment derived from the up- Dolomite and resulted in the deposition of the Fort Union conglomerate lifted source terrane bypassed the proximal region and was deposited farther (Fig. 11B). Uplift at this phase was 6.17 km, and shortening was 3.17 km out in the basin. (2) Continued displacement on the Mowry basin thrust fault (7.9%) (Fig. 11B). During this stage the Johnson Creek thrust fault began to

Geological Society of America Bulletin, August 1997 927 HOY AND RIDGWAY propagate through the hinge of the fold. Continued basinward rotation of the controlled by the two imbricate faults of the Piney Creek thrust fault system strata between the Mowry basin thrust fault and the Johnson Creek thrust (PCTF1 and PCTF2 in Fig. 12) that dip approximately 25¡W. The Buffalo fault produced the progressive unconformities found within the Fort Union Deep fault is located under the distal deposits at Moncrieffe Ridge (Figs. 1 conglomerate. (3)Additional displacement and uplift along the Mowry basin and 12D). However, the fault does not appear to have influenced deposition thrust fault allowed for the increased dissection of the hanging wall or deformation within these synorogenic deposits. Data from the Granite (Fig. 11C). This accounts for the increased contribution of Cambrian clasts Ridge well help determine the location of the Piney Creek thrust fault and within the Kingsbury Conglomerate. Uplift at the onset of the Kingsbury footwall strata (well #1 in Fig. 12D). This well was spudded in Precambrian Conglomerate deposition was 6.80 km, and shortening was 4.26 km (10.6%) granite and drilled through 753 m of granite before it penetrated a thrust (Fig. 11C). During this stage the Johnson Creek thrust fault propagated fault (PCTF1). After crossing the thrust fault, the well entered an overturned through the entire hinge of the growth syncline. (4) During the latest stages section of Upper Cretaceous strata, then crossed a second fault (PCTF2) of displacement, block rotation between the Mowry basin thrust fault and the into nearly horizontal strata of the same age (Fig. 12D). Uplift along cross Johnson Creek thrust fault caused the western limb of the growth syncline to section DÐD′ is 9.90 km, and shortening is 8.89 km (21.9%) (Fig. 12D). become slightly overturned (Fig. 11D). As much as 10¡ of syndepositional Cross section DÐD′ was retrodeformed in four stages. (1) Initial dis- rotation occurred within the Kingsbury Conglomerate during this stage. placement and uplift along the Piney Creek thrust fault (PCTF1) initiated unroofing of the Mesozoic and upper Paleozoic source section and deposi- Piney Creek Thrust Block tion of the Lebo Shale in the Powder River basin (Fig. 12A). Uplift at this stage was 5.1 km, and shortening was 2.61 km (6.6%) (Fig. 12A). During Stratigraphy. The northernmost synorogenic conglomerates are exposed this early stage of fold development, the growth syncline was hinged at the adjacent to the Piney Creek thrust block, a large basement-involved thrust top of PCTF2. (2) Continued thrust displacement and erosion exposed the block flanked on the north and south by northeast-southwestÐstriking tear middle and lower Paleozoic carbonate rocks in the hanging wall and gener- faults (Fig. 1). Two conglomerates, the Kingsbury Conglomerate and the ated the Kingsbury Conglomerate (Fig. 12B). The fine-grained members of Moncrief Conglomerate, are exposed basinward of the Piney Creek thrust the Fort Union Formation were rotated into the steeper limb of the growth (Fig. 1). The Kingsbury Conglomerate is exposed at the base of Moncrieffe syncline during this stage. This rotation produced the unconformity be- Ridge and in one location farther south along the Piney Creek thrust block tween the Fort Union Formation and the Kingsbury Conglomerate. During (Fig. 1). Maximum particle size data from the Kingsbury Conglomerate av- this stage the PCTF2 propagated partially through the hinge of the growth erage between 15 and 30 cm (Fig. 4D). The best exposures of the Moncrief syncline. Uplift at the onset of Kingsbury Conglomerate deposition was Conglomerate occur on Moncrieffe Ridge (Fig. 1). This alluvial fan deposit 6.49 km, and shortening was 6.15 km (15.5%) (Fig. 12B). (3) Additional (Sharp, 1948; Obernyer, 1979) is lithologically very similar to that of North displacement along the Piney Creek thrust fault exposed Precambrian base- Ridge (Fig. 1); clast-supported, boulder conglomerate facies in proximal fan ment rocks, resulting in the deposition of the Moncrief Conglomerate deposits grade laterally into finer mid-fan (Fig. 7C) and distal fan deposits. (Fig. 12C). Uplift at this stage was 8.99 km, and shortening was 7.95 km Less-extensive deposits of Moncrief Conglomerate occur along the length of (19.2%) (Fig. 12C). (4) In the final stage of deformation, the Piney Creek the Piney Creek thrust block (Fig. 1). Maximum particle size data from the thrust block was thrust over the conglomerates at Moncrieffe Ridge, result- Moncrief Conglomerate range from about 9 cm in the distal sections to as ing in the configuration observed today (Fig. 12D). The change in con- much as 200 cm in the proximal deposits (Fig. 4D). The Kingsbury Con- glomerate clast composition (Fig. 5D) suggests that the structural evolution glomerate in this area has paleocurrent indicators that document north-north- from stage 2 through stage 4 in Figure 12 was a gradual transition rather eastward and southeastward paleoflow, whereas paleocurrent indicators in than a series of discrete events, as indicated by angular unconformities be- the Moncrief Conglomerate display southeastward paleoflow (Fig. 4D). tween the Kingsbury Conglomerate and the Moncrief Conglomerate in Clast Composition. Conglomerate clast composition is well defined on other areas along the range margin. Moncrieffe Ridge. A velocity test hole was drilled on Moncrieffe Ridge (well #2 in Figs. 1 and 12) in association with the Granite Ridge well (well GENERAL IMPLICATIONS #1 in Figs. 1 and 12) drilled through the hanging wall of the thrust block. The test hole sample logs (i.e., mud logs) did not show an abrupt composi- Structural Development of Growth Synclines tional change from Kingsbury Conglomerate (predominantly Paleozoic clasts) to Moncrief Conglomerate (nearly 100% Precambrian basement Our analysis of the configuration of the east-central Bighorn Mountains clasts), but instead showed a gradual change from 100% sedimentary clasts identified three structural stages of footwall growth syncline development of Mississippian Madison Limestone, Ordovician Bighorn Dolomite, and associated with fault-related folding. Cambrian Gros Ventre Formation at a depth of 533 m, to 100% basement Stage I. The structural configuration of two areas, Kingsbury Ridge clasts of granite and gneiss at a depth of 232 m. Sample logs indicate that (AÐA′ in Figs. 1 and 6) and the Mowry Basin (CÐC′ in Figs. 1 and 11), is in- the contact between the Kingsbury and Moncrief Members was arbitrarily terpreted to be the product of deformation during the earliest part of fault- placed at the first appearance of sedimentary clasts in the drill cuttings. Out- related fold development. Our cross sections (Figs. 6 and 11) show that the crops of the Kingsbury Conglomerate, adjacent to the Piney Creek thrust structural configuration of the range margin in both areas was controlled by (labeled AÐD in Figs. 4D and 5D), also have clast compositions that docu- imbricate basement-involved blind thrusts. The surface expression of the ment a gradual transition from primarily sedimentary clasts to crystalline blind thrusts is an anticline-syncline pair (Fig. 13A). Paleozoic strata in the Precambrian clasts. shared fold limb dip between 20¡ and 65¡ toward the Powder River basin Deformation of Synorogenic Conglomerates. The footwall synoro- (Fig. 14). Eastward, the Mesozoic strata show a steepening of dips upsec- genic conglomerates of the Piney Creek thrust block show little deformation in the shared fold limb and locally become overturned (Fig. 14). Coarse tion. The Kingsbury Conglomerate shows a maximum of 20¡ of rotation synorogenic sediments were deposited in the developing growth syncline (assuming an original 10¡ depositional slope), but deformed conglomerate and in places onlapped the eroded, steeply dipping Mesozoic strata clasts were not found. (Fig. 14). The onlap relationship indicates that significant limb rotation and Structural Evolution. Uplift along the Piney Creek thrust block was erosion occurred prior to deposition of the conglomerates.

928 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Moncrief Conglomerate Tfu Fort Union Formation A) Start of Lebo Shale Deposition Uplift 5.1 km Kingsbury Conglomerate Kc Cody Shale Shortening 2.61 km (6.6%)

Mississippian Madison Limestone- D' Ordovician Bighorn Dolomite

Precambrian Basement Kc Unconformity 0 1 2 3 6 km PCTF1 Bedding Attitude PCTF2 BDF B) C) Start of Moncrief Conglomerate Deposition

Start of Kingsbury Conglomerate Deposition Uplift 6.49 km Uplift 8.99 km Shortening 6.15 km (15.5%) Shortening 7.95 km (19.2%)

D' D'

Tfu Tfu Kc Kc PCTF1

PCTF1 PCTF2 BDF PCTF2 BDF

D) D

2 D'

Present Configuration Total Uplift 9.90 km Tfu

Total Shortening 8.89 km (21.9%) Kc PCTF1 PCTF2 D BDF D'

BDF PCTF1 PCTF2

Figure 12. Sequential restoration of cross section DÐD′ at Moncrieffe Ridge (see Fig. 1 for location of cross section). Amount of uplift and shortening is shown for each stage of retrodeformation. (A) Initial displacement along the Piney Creek thrust system (PCTF1 and PCTF2) and beginning of Lebo Shale deposition in the Powder River basin. (B) Further displacement along PCTF1 and start of Kingsbury Conglomerate deposi-

tion. (C) Continued displacement along PCTF1 and onset of Moncrief Conglomerate deposition. (D) Displacement along PCTF1 and PCTF2 to present structural configuration. Petroleum well logs used for construction: 1—Granite Ridge 1-2-9D well, 2—Granite Ridge velocity test hole. BDF—Buffalo Deep fault.

Two depositional episodes occurred along the western margin of the there was an average of 6.46 km of uplift and 4.60 km of shortening along Powder River basin during stage 1 deformation. First, deposition of the the eastern Bighorn Mountains (Table 1). During the second depositional Lebo Shale occurred during the initial displacement on the range-bounding episode, the Fort Union conglomerate and the Kingsbury Conglomerate faults. Our cross sections show that at the end of Lebo Shale deposition, were deposited (Fig. 13A). Fold limb rotation due to fault propagation in-

Geological Society of America Bulletin, August 1997 929 HOY AND RIDGWAY

fluenced coarse-grained synorogenic sedimentation in two ways. First, alluvial-fan deposits underwent a gradual eastward rotation that resulted, over time, in a progressive fanning and decrease of dips upsection (Fig. 14). The most proximal fan deposits were incorporated into the shared fold limb and were uplifted, eroded, and redeposited basinward. Second, the continued uplift of the proximal part of the growth syncline caused a basinward progradation of subsequent alluvial-fan deposits (Fig. 13A). The Kingsbury Conglomerate is the most widespread of all the synorogenic conglomerates in the study area and was deposited along the entire east-central margin of the Bighorn Mountains. Our cross sections and the distribution of the Kingsbury Conglomerate indicate that the deformation represented by stage I occurred throughout the study area. Stage I is interpreted as the earliest stage of fault-related fold development. After Kingsbury Conglomer- ate deposition, there was an average of 8.17 km of uplift and 6.36 km of shortening (Table 1). Stage II. The exposures on the north side of Johnson Creek (Fig. 1) are representative of stage II (Fig. 13B). The Moncrief Conglomerate was deposited during stage II, and overlies the Kingsbury Conglomerate with as much as 95¡ of angular discordance (Fig. 15). The lower portion of the Moncrief Conglomerate has been rotated basinward in the developing growth syncline in a manner similar to that of the Kingsbury Conglomerate during earlier stages of deformation (Fig. 9, C and D). At Johnson Creek, the Johnson Creek thrust has overridden the Kingsbury Conglomerate, which is overturned in the footwall (Fig. 15). The overlying Moncrief Con- glomerate, however, is undeformed and dips approximately 10¡E (Fig. 15). In this case, the advancing thrust fault partly truncated the footwall growth syncline, but did not override the entire synorogenic package, as seen in stage III at North Ridge (Fig. 8). Stage III. The final stages of fault-related fold development are represented by the present configuration of the Clear Creek thrust fault (BÐB′ in Figs. 1 and 9D) and the Piney Creek thrust block (DÐD′ in Figs. 1 and 12D). Surface expressions of stage III are thrust faults that place middle and lower Paleozoic strata in fault contact with the synorogenic Moncrief Conglomer- ate of the Wasatch Formation (Fig. 13C). The Paleozoic strata are the leading edge of a large hanging-wall anticline and range in dip from 30¡E in the Cambrian Flathead Sandstone at the core of the fold, to as much as 50¡W overturned in the Mississippian Madison Limestone at the edge of the advancing limb. Hanging-wall Mesozoic strata had been removed by prior unroofing associated with earlier stages of deformation. Figure 13. Schematic cross sections illustrating various stages of During the final stages of fault-related folding, the initially blind thrusts footwall growth syncline development along the east-central flank of propagated into the near-surface conglomerate deposits, truncated the foot- the Bighorn Mountains. (A) The end of stage I deformation as seen wall syncline, and overrode the entire synorogenic package (Fig. 13C). Dur- at the Mowry basin represents the early development of the growth ing fault truncation of the growth syncline, adjacent footwall synorogenic syncline. Note the anticline-syncline pair above the blind thrust conglomerate deposits were deformed. Along U.S. Highway 16, for exam- faults and the deposition of synorogenic sediments along the hinge of ple, beds of the Moncrief Conglomerate have been rotated to near vertical the syncline. Key for figure: jackstraw pattern represents Precam- (Fig. 8), and boulder-sized clasts have been deformed along numerous back brian crystalline basement rocks; beds without patterns represent thrusts and flexural slip surfaces (Hoy, 1996). Cross sections through the Paleozoic and Mesozoic rocks; Tfuc—Fort Union conglomerate; Clear Creek thrust fault and Piney Creek thrust block indicate that these ar- Twk—Kingsbury Conglomerate; Twm—Moncrief Conglomerate. eas, characteristic of stage III, have undergone an average of 9.74 km of up- (B) Stage II represents the intermediate stage of growth syncline evo- lift and 9.10 km of shortening (Table 1). lution as documented at Johnson Creek. Note that the thrust system partially truncated the growth syncline. Note rotation of Paleozoic Implications for Fault-Related Fold Models and Mesozoic strata in the shared fold limb to a steeper dip (relative to stage I). Fold limb rotation resulted in development of progressive The deposition and deformation of the synorogenic deposits along the unconformities (not shown) in synorogenic conglomerate within the east-central Bighorn Mountains are a direct result of the formation of folds growth syncline. (C) Stage III deformation as represented by the associated with basement-involved faults. The fold styles shown by our cross structural configuration seen along the Clear Creek thrust system at sections are similar to “thick-skinned” folding styles associated with base- North Ridge. Note that the thrust system has completely truncated ment faults that have propagated upsection into sedimentary cover rocks the footwall growth syncline and overrode the entire synorogenic (Brown, 1983, 1988; DeCelles et al., 1991; McConnell and Wilson, 1993; package. Schmidt et al., 1993; Stone, 1993). The folds along the east-central Bighorn

930 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

Figure 14. Shared limb of an anticline- syncline pair characteristic of stage I deformation in the Mowry basin. View is to the north. Tadpole symbols indicate dips, and sinusoidal lines indicate unconformities. Paleozoic strata (Pz) in the shared fold limb dip approximately 50¡E (to right) toward the Powder River basin. Eastward, Mesozoic strata (Mz) show a steepening of dips upsection and eventually become overturned in the shared fold limb. Arrow points to a well-exposed overturned Cre- taceous outcrop dipping to the west (left). Shallow-dipping synorogenic Tertiary conglomerates, the Fort Union conglomerate (Tfuc) and the Kingsbury Conglom- erate (Twk) were deposited in the hinge of the developing growth syncline. Note that the synorogenic deposits onlap the eroded, steeply dipping Mesozoic strata with an angular unconformity. This onlap relationship indicates that significant limb rotation and erosion occurred prior to deposition of the conglomerates. Continued limb rotation during conglomerate deposition produced an additional angular unconformity that separates Tfuc and Twk (note the change in dip between the two units). Compare this figure with schematic diagrams shown in Figure 13 (A and B).

Mountains are different than those described by “thin-skinned” classic fault- The Lebo Shale has been interpreted as an extensive fluvial-lacustrine facies propagation fold models (Suppe, 1983; Mitra, 1990). In our study area, for that formed along the western margin of the Powder River basin (Lake example, angular and progressive unconformities in the proximal limb of the Lebo) (Flores and Ethridge, 1985; Flores, 1986; Ayers, 1986). Previous footwall growth syncline are evidence that the forelimb was progressively ro- studies concluded that Lake Lebo was filled entirely by deltas prograding tated to a steeper dip. In the classic geometric models of fault-propagation from the ancestral Black Hills, Casper Arch, and Laramie Mountains. The folding, limb dips are attained instantaneously and do not change after fold lack of coarse detritus in the lower Fort Union deposits was interpreted as growth begins (see Fischer et al., 1992, for discussion). These results support evidence that the adjacent Bighorn Mountains provided little sediment to a growing number of studies suggesting that limb rotation is an important the forming basin (Curry, 1971; Tewalt et al., 1983; Flores and Ethridge, component in the growth of some fault-related folds (Anadon et al., 1986; 1985; Ayers, 1986; Flores, 1986). These studies suggested that the first in- DeCelles et al., 1991; Holl and Anastasio, 1993; Poblet and Hardy, 1995; dication of uplift and erosion of the Bighorn Mountains was contained in Vergés et al., 1996; Zapata and Allmendinger, 1996). the sandstones of the Tongue River Member and the Fort Union conglom- McConnell (1994) proposed a basement-involved model in which the erate (Fig. 2), and that major uplift began during the Eocene with deposition thrust fault propagates through the forelimb of the fold. In his model, the of the Kingsbury and Moncrief conglomerates. footwall syncline is hinged where the fault intersects the basementÐsedi- In contrast to the traditional interpretation outlined above, results of this mentary cover contact. Continued fault propagation rotates the forelimb study indicate that, by the onset of middle Paleocene Lebo Shale deposition, strata in both the hanging wall and footwall. McConnell’s model describes an average of 3.8 km of uplift and 4.50% of shortening had already occurred many of the structural features documented in the eastern Bighorn Moun- (Figs. 6, 9, 11, and 12). The subsidence indicated by isopach data of the tains, but does not include the imbricate fault pair observed in the study area. Lebo Shale (Curry, 1971; Ayers, 1986) had to be accompanied by erosion Block rotation between imbricate fault pairs interpreted for the Bighorn of approximately 2 km of mainly Mesozoic mudstone from the hanging Mountains is similar to that discussed by Kellogg et al. (1995) for Laramide uplifts in Montana.

TABLE 1. RESULTS OF UPLIFT AND SHORTENING CALCULATIONS Implications for Laramide Uplift and Unroofing of the Bighorn Depositional Map location Cross Uplift Shortening Shortening Mountains stage* section (km) (km) (%) Onset Twk Kingsbury Ridge AÐA′ 6.55 2.13 5.8 Estimates of the initiation of Laramide uplift of the Bighorn Mountains North Ridge BÐB′ 6.01 5.86 16.6 ′ have ranged from Late Cretaceous (Gries et al., 1992; Hansley and Brown, Mowry basin CÐC 6.80 4.26 10.6 Piney Creek thrust block DÐD′ 6.49 6.15 15.5 1993), to late Paleocene (Sharp, 1948; Hose, 1955; Mapel, 1959; Merin and End Twk Kingsbury Ridge AÐA′ 8.10 5.73 15.5 Lindholm, 1986). Isopach maps of the members of the Fort Union Forma- North Ridge BÐB′ 7.44 6.65 18.8 Mowry basin CÐC′ 8.14 5.10 14.2 tion indicate that subsidence in the Powder River basin began in the early to Piney Creek thrust block DÐD′ 8.99 7.95 19.2 middle Paleocene, as indicated by thickening of the Lebo Shale member End Twm North Ridge BÐB′ 9.57 9.30 26.3 from about 8 m in the eastern part of the basin to 915 m adjacent to the east- Piney Creek thrust block DÐD′ 9.90 8.89 21.9 central Bighorn Mountains (Curry, 1971; Flores and Ethridge, 1985, Fig. 4). *Twk—Kingsbury Conglomerate, Twm—Moncrief Conglomerate.

Geological Society of America Bulletin, August 1997 931 HOY AND RIDGWAY

Figure 15. Photograph of structural relationship exposed on Johnson Creek that is characteristic of stage II deformation. View is looking north at the Johnson Creek thrust (thick solid white line). Tadpole symbols indicate dip directions, thin sinusoidal line indicates angular unconformity, and arrow indicates displacement direction on thrust fault. In the hanging wall, Mississippian Madison Limestone (Pz) is dipping 70¡E. In the footwall, overturned Eocene Kingsbury Conglomerate (Twk) is dipping 60¡W. The Moncrief Con- glomerate (Twm), which unconformably overlies the Kingsbury Conglomerate, dips approximately 10¡E. The lack of deformation and bed rotation in the Moncrief Conglomer- ate indicates that it has not been overridden by the Johnson Creek thrust (cf. Fig. 13B). In this example, the thrust fault has partially truncated the growth syncline but did not override all of the synorogenic conglomerates in the footwall.

wall of the Bighorn thrust system. The Permian through Cretaceous hang- Cretaceous, our cross sections indicate about 9.5 km of total rock uplift, ing-wall stratigraphic-source section is composed predominantly of mud- about 5.5 km of exhumation, and about 4 km of net surface uplift. Assum- stone and subordinate poorly indurated sandstone (Fig. 2) that is incapable ing a 20 m.y. active tectonic period (middle Paleocene to middle Eocene), of generating coarse detritus. Prior to the accumulation of the Kingsbury bulk (rock) uplift of the Bighorn Mountains was on the order of approxi- Conglomerate, roughly 2.8 km of stratigraphic-source section had been mately 50 cm/1000 yr. eroded from the eastern Bighorn Mountains. Stripping of these nonresistant rocks from the Bighorn Mountains must be accounted for by deposition CONCLUSIONS within the Powder River basin, because it is unlikely that the sediments generated by erosion of the entire Mesozoic section bypassed the local depo- (1) The earliest stage of growth syncline formation (stage I) along the east- center within the growing basin (i.e., Lake Lebo). Our results suggest that ern Bighorn Mountains was characterized by development of an anticline- the 914 m of Lebo Shale adjacent to the thrusted east-central segment of the syncline pair. Initial uplift exposed easily eroded Mesozoic mudstones that Bighorn Range is at least partly the product of unroofing of the Mesozoic were transported through the embryonic growth syncline and deposited far- strata in the nearby hanging-wall section (Fig. 16A). The lack of coarse de- ther out in the basin. An average of 6.46 km of uplift and 12.1% of shortening tritus in the thick sections of the Lebo Shale along the western basin margin occurred along the range margin during this earliest stage of fault-related fold is in fact what should be expected from the uplift of the Bighorn source ter- development. Continued growth syncline development was characterized by rane. In addition, the presence of Lake Lebo adjacent to the east-central uplift and exposure of resistant Paleozoic carbonate rocks. This resulted in Bighorn Mountains can be used to infer that there was enough local relief deposition of the Fort Union and Kingsbury conglomerates along the hinge of to result in ponding of water. Our interpretation does not preclude the pos- the growth syncline. Fold limb rotation, development of progressive uncon- sibility of additional sediment contribution from other nearby uplifts into formities, reworking of proximal facies along the steeper limb of the syncline, Lake Lebo (Flores and Ethridge, 1985; Ayers, 1986). By the end of the and basinward progradation of alluvial-fan deposits are indicative of this Paleocene, Lake Lebo had been filled and a regional fluvial system had de- stage. An average of 8.17 km of uplift and 16.9% of shortening had occurred veloped (Tongue River deposits). During this time, the alluvial-fan system along the range margin by the end of this stage (end of stage I). that deposited the Fort Union conglomerate found in the Mowry basin was (2) The intermediate stage (stage II) of footwall growth syncline devel- active. The Fort Union conglomerate is found only in the Mowry basin, in- opment involved partial truncation of the growth syncline by the advancing dicating that the fan was of local significance (Fig. 16A). The initiation of thrust faults and deposition of the Moncrief Conglomerate. The lower por- Kingsbury Conglomerate deposition marks the exposure of durable middle tion of the Moncrief Conglomerate was rotated basinward in the develop- and lower Paleozoic strata in the hanging wall along the entire east-central ing growth syncline in a manner similar to that of the Kingsbury Conglom- range margin (Fig. 16B). The onsets of the later stages of fault-related fold- erate during the early stage of deformation. ing were marked by the exposure of Precambrian rocks in the hanging walls (3) A final stage of deformation (stage III) along the east-central Bighorn of Clear Creek and Piney Creek thrust faults, and deposition of the Moncrief Mountains was characterized by thrust faulting of middle and lower Paleo- Conglomerate (Fig. 16C). zoic strata over Eocene synorogenic conglomerate. The Moncrief Conglom- The calculated 8 to 10 km of maximum uplift for the east-central Bighorn erate was deformed during this stage of fault-related fold development, when Mountains is in general agreement with estimates of 5 to 10 km of uplift as- the initially blind thrusts propagated into the near-surface conglomerate de- sociated with other Laramide uplifts (see Dickinson et al., 1988, for review). posits, truncated the entire footwall syncline, and overrode the synorogenic Assuming that the Bighorn Mountains were at sea level at the end of the conglomerate package. An average of 9.7 km of uplift and 24.1% of short-

932 Geological Society of America Bulletin, August 1997 FOOTWALL GROWTH SYNCLINES, BIGHORN MOUNTAINS, WYOMING

A) Middle - Late Paleocene Fort Union Lebo Shale and Conglomerate Member Deposition

Pz Mz

Tfuc

Pz N pC Tullock (Tfu) LAKE LEBO Figure 16. Sequence of schematic block diagrams depicting the depositional and structural history of the east-central Bighorn Mountains. (A) Middle to late Paleocene: Ini- tial uplift along the Bighorn thrust system Early Eocene caused unroofing of Mesozoic mudstones B) Kingsbury Conglomerate Deposition (Mz) and deposition of the Lebo Shale in the Powder River basin. The Fort Union con- Coalescing alluvial fans Tfuc glomerate (Tfuc) of the Mowry basin was de- Braid-plain deposits posited in the late Paleocene when resistant mid-lower Paleozoic strata (Pz) were locally exposed in the hanging wall of the Bighorn Pz Twk pC thrust. pC—Precambrian rocks. (B) Early Mz Eocene: Regional exposure of lower Paleozoic Pz pC strata in the hanging wall leads to deposition N of the Kingsbury Conglomerate (Twk) along Tw the east-central range margin. Tw—Wasatch Tfu Formation. (C) Middle Eocene: Localized shortening resulted in additional displacement along the Clear Creek and Piney Creek Middle Eocene thrust faults, exposure of Precambrian rocks C) Moncrief Conglomerate Deposition (pC) in the hanging wall, and deposition of Mowry Basin Piney Creek Thrust Block the Moncrief Conglomerate (open circle pat- Clear Creek Thrust tern) in footwall growth synclines. Johnson Creek

pC Moncrieffe Ridge

pC Pz Pz

Pz pC Pz

Tw pC Tfu Mz Mz N Kingsbury Ridge

Tfu

North/Bald Ridge Mz Buffalo Deep Fault

ening occurred along the range margin, where this final stage of deformation tial uplift of the Bighorn Mountains occurred during the early to middle is well developed. Paleocene and is represented by deposition of the Lebo Shale in the Powder (4) The type of synorogenic detritus deposited along the eastern margin River basin. The Lebo Shale is at least partly the product of unroofing of the of the Bighorn Mountains was influenced by source terrane lithology. Ini- Mesozoic mudstones carried in the hanging wall of the Bighorn thrust fault

Geological Society of America Bulletin, August 1997 933 HOY AND RIDGWAY

system. The late PaleoceneÐEocene conglomerates do not represent the ini- DeCelles, P. G., Pile, H. T., and Coogan, J. C., 1993, Kinematic history of the Meade thrust based tiation of Laramide uplift, but instead represent the exposure of coarse- on provenance of the Belcher Conglomerate at Red Mountain, Idaho, Sevier thrust belt: Tec- tonics, v. 12, p. 1436Ð1450. clastÐforming rocks in the hanging-wall stratigraphic section. Demorest, M. H., 1941, Critical structural features of the Bighorn Mountains, Wyoming: Geo- logical Society of America Bulletin, v. 52, p. 161Ð176. Dickinson, W. R., Klute, M. A., Hayes, M. J., Janecke, S. U., Lundin, E. R., McKittrick, M. A., ACKNOWLEDGMENTS and Olivares, M. D., 1988, Paleogeographic and paleotectonic setting of Laramide sedimentary basins in the Rocky Mountain region: Geological Society of America Bulletin, v. 100, p. 1023Ð1039. Espina, R. G., Alonso, J. L., and Pulgar, J. A., 1996, Growth and propagation of buckle folds de- This study was part of a Master’s thesis completed by Hoy at Purdue Uni- termined from syntectonic sediments (the Ubierna Fold Belt, Cantabrian Mountains, N. versity. Hoy’s graduate studies were supported by Mobil Oil, Geological So- Spain): Journal of Structural Geology, v. 18, p. 431Ð441. ciety of America, American Association of Petroleum Geologists, Sigma Xi, Ethridge, F. G., Jackson, T. L., and Youngberg, A. D., 1981, Floodbasin sequence of a fine-grained meander belt subsystem; the coal bearing lower Wasatch and upper Fort Union Formations, Colorado Scientific Society, and the Department of Earth and Atmospheric southern Powder River Basin, Wyoming, in Ethridge, F. G., and Flores, R. M., eds., Recent Sciences at Purdue University. Ryan Nichols provided field assistance for and ancient nonmarine depositional environments, models for exploration: Society of Eco- Hoy. Ridgway thanks Ron Cole and Peter DeCelles for field work and dis- nomic Paleontologists and Mineralogists Special Publication 31, p. 191Ð209. Fischer, M. P., Woodward, N. B., and Mitchell, M. M., 1992, The kinematics of break-thrust folds: cussions during the initial stages of this study; Jean Weaver for a field trip that Journal of Structural Geology, v. 14, p. 451Ð460. introduced him to the geology of the western Powder River basin; and Jay Flemings, P. B., and Jordan, T. E., 1990, Stratigraphic modeling of foreland basins: Interpreting thrust deformation and lithosphere rheology: Geology, v. 18, p. 430Ð434. Scheevel for useful discussions and an aerial tour of the study area. We thank Flores, R. M., 1986, Styles of coal deposition in Tertiary alluvial deposits, Powder River Basin, Arvid Johnson for helpful discussions on folds and the many ranch owners for Montana and Wyoming, in Lyons, P. C., and Rice, C. L., eds., Paleoenvironmental and tec- access to outcrops. 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Geological Society of America Bulletin, August 1997 935 Inversion of Proterozoic extensional faults: An explanation for the pattern of Laramide and Ancestral Rockies intracratonic deformation, United States

Stephen Marshak Department of Geology, University of Illinois, Urbana, Illinois 61801, USA Karl Karlstrom Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA J. Michael Timmons

ABSTRACT platform in the United States includes both the The Rocky Mountains, Colorado Plateau, and Midcontinent, regions of the North Ameri- Midcontinent and the Rocky Mountain–Colorado can cratonic platform, display similar styles and patterns of Phanerozoic deformation. In Plateau province (Fig.7361). The latter consists these regions, movement on basement-penetrating faults during the late Paleozoic Ancestral of the Rocky Mountains, south of the Lewis and Rockies event and/or during the Mesozoic-Cenozoic Laramide event generated flat-topped Clark fault zone, and the Colorado Plateau uplifts bordered by outward-verging, monoclinal forced folds. We suggest that these struc- tablelands. tures, divided into two sets on the basis of orientation (west to northwest and north to north- Two Phanerozoic deformation events affected east), formed by inversion of Proterozoic extensional-fault systems. In this model, Proterozoic the Rocky Mountain–Colorado Plateau province. rifting events formed weak faults in the cratonic platform crust, and these faults were reacti- The late Paleozoic Ancestral Rockies event created vated by stress transmitted during Phanerozoic compressional orogenies. If this model is cor- basins and basement-cored uplifts both in the rect, the pattern of Ancestral Rockies and Laramide contractional structures reflects the region occupied by the present-day Rocky Moun- trends of Proterozoic extensional faults, and regional variation in forced-fold vergence reflects tains, and in the Midcontinent (Fig. 2; McBride the control of antecedent fault dips on fault-propagation fold geometry during inversion. Late and Nelson, 1999). These basins and uplifts rep- Proterozoic rifts formed throughout Rodinia, so inversion tectonics likely occurred in cratonic resent the craton’s response to stress generated by platforms worldwide. collisional orogeny along North America’s eastern and southern margins (Kluth and Coney, 1981; Keywords: Laramide, Ancestral Rockies, inversion tectonics, Colorado Plateau, Proterozoic rifting, Ye et al., 1996). The Laramide orogeny occurred Rocky Mountains, intracratonic deformation, Midcontinent, fault-propagation folding. in Cretaceous-Tertiary time. This event, a response to plate convergence along the west coast, INTRODUCTION sion of extensional faults formed during the as- overlapped in age with the end of the Jurassic- Ranges of the Rocky Mountains, monoclines sembly and breakup of supercontinents during Cretaceous Sevier orogeny. South of the Lewis of the Colorado Plateau, and fault-and-fold zones the Proterozoic. This paper builds on ideas pre- and Clark fault zone in the United States, the of the U.S. Midcontinent differ dramatically in sented in Marshak and Paulsen (1996), Karl- inception of the Laramide represents a change in structural relief and degree of exposure, but strom and Humphreys (1998), and Timmons the locus and style of deformation, relative to the resemble each other in structural style and re- et al. (2000). Sevier deformation belt. Specifically, Sevier gional orientation (Marshak and Paulsen, 1996; North America’s craton, continental crust that deformation shortened the former passive margin McBride and Nelson, 1999). Here we character- is relatively cool and mechanically strong, con- of western North America to form an east-verging ize this intracratonic deformation, focusing on sists of a shield in which Precambrian crys- fold-and-thrust belt (Fig. 2A). Laramide deforma- that of the Rocky Mountains and Colorado talline basement crops out, and a platform in tion, in contrast, formed basins and basement- Plateau (Fig. 1). We argue that both Ancestral which a section of Phanerozoic strata as much cored uplifts in the cratonic platform, well to the Rockies and Laramide deformation reflect inver- as 7 km thick covers the basement. The cratonic east of the fold-and-thrust belt front (Fig. 1B).

Figure 1. A: Map of United States A 100° RM-CP province D n = 96; circle = 10% showing location and trend of N MCR faults and folds in cratonic plat- LCFZ MDS form. Rocky Mountain–Col- Fig. 2b orado Plateau province (RM-CP) 40° is shaded. OA—Oklahoma aulacogen; LCFZ—Lewis and Clark fault zone, TWL—Texas- Walker line, MSM—Mojave- USA Midcontinent n = 143; circle = 23% OA Sonora megashear, MCR—Mid- N continent rift. B: Cross-sectional 0 500 km TWL 1.3-1.0 Ga sketches indicating contrasts in MSM deformational style among dif- E ferent continental provinces. C: probable Precambrian rift edge of the coastal plain Rose diagrams illustrating ap- approximate trace of a fold or edge of foreland fold-and-thrust belts proximate dominant fault trends fault in the cratonic platform east edge of the RM-CP province C in (upper) Rocky Mountain–Col- B + basement cover marker bed fault + orado Plateau province and + + + + + + + + + + + + (lower) Midcontinent (derived + + + + + + + + + from map in A). D: Map indicat- foreland fold-and-thrust belt Rocky Mtn. region Colorado Plateau Midcontinent 900-700 Ma ing faults and dikes (MDS— McKenzie dike swarm) formed between 1.3 and 1.1 Ga. E: Map indicating structures formed between 900 and 700 Ma (D and E modified from Marshak and Paulsen, 1996;Timmons et al., 2000).

Geology; August 2000; v. 28; no. 8; p. 735–738; 3 figures. 735 Montana 0MT km 400 A 0 500 km B foreland edge of the fold-and-thrust belt South LCFZ Dakota Figure 2. A: Ancestral Rockies area of B 45°N structures of Rocky Mountain– Big Horn Idaho Y Z Colorado Plateau province, and Owl Creek Black Midcontinent equivalents (modi- Hills fied from Kluth and Coney, 1981; Snake River Plain McBride and Nelson, 1999). X B: Laramide structures of Rocky Wind River Wyoming Mountain–Colorado Plateau Prov- Nevada ince, showing variations in ver- 40°N Uinta Front gence of Laramide structures Range (location shown in Fig. 1A; modi- Utah central Colo- fied from Hamilton, 1988; Davis, Basin & Range rado 1978; Gries, 1988; Yin, 1994). Diagonal pattern delineates Colo- rado Plateau, darker shaded Ancestral Rockies uplift areas are uplifts. X-Y-Z delineates section line in Figure 3A. future Rocky Mountain front Arizona Explanation for B Colorado New LCFZ—Lewis and Clark fault σ stress trajectory (from 35° N Plateau 1 Laramide uplift Mexico zone, TWL—Texas-Walker line. van der Pluijm et al., 1997) TWL trace of fault active during fault or fold trace of a structure active the Laramide (tooth is on southern Rio Grande rift during the Ancestral Rockies event the hanging-wall side) 115°W Basin & Range 105°W

CHARACTERISTICS OF by west to northwest and north to northeast the origin of the faults—when and why they INTRACRATONIC PLATFORM trends (Figs. 1 and 2; e.g., Davis, 1978; Erslev were first formed by rupturing of previously intact DEFORMATION and Wiechelman, 1997). Thus, in marked con- crust—remains an enigma. Two end-member Erosion and post-Paleozoic deformation ob- trast with structures in foreland fold-and-thrust explanations have been suggested for their scure Ancestral Rockies fault geometries, so to belts, intracratonic folds of the Rocky Mountain– origin. (1) They were formed during the Ances- characterize the style of intracratonic platform Colorado Plateau province do not display re- tral Rockies and Laramide events in response to deformation, we focus on Laramide examples in gional parallelism. Furthermore, the folds verge compressive stress. (2) They were formed prior the United States. Laramide uplifts are basement in various directions (Fig. 2B); the vergence of to the Ancestral Rockies event and thus their cored (thick skinned) in that their formation re- these folds depends on the dip of the underlying movement during the Ancestral Rockies and sults from reverse-sense to transpressional dis- fault. Specifically, folds over west-side-up faults Laramide events represents fault reactivation placement on underlying basement-penetrating verge east, folds over north-side-up faults verge (e.g., Davis, 1978). Here we provide arguments faults. (In this regard, Laramide faulting contrasts north, and so on. It has been argued that the two that favor the second explanation, and we sug- with that of a foreland fold-thrust belt, in which trends formed during distinctly different stages of gest further that weak faults of the cratonic thrusts affect only sedimentary cover above a the Laramide orogeny, but a consensus is build- platform in the Rocky Mountain–Colorado subhorizontal detachment.) In the present U.S. ing that both sets were active coevally, but with Plateau province, as in the Midcontinent Rocky Mountains, displacements on Laramide different local kinematics (e.g., Erslev and (Marshak and Paulsen, 1996), formed by crustal faults reach 15–20 km, whereas in the Colorado Wiechelman, 1997). rupturing during Proterozoic rifting. Thus, we Plateau, they reach 1–2 km. Cenozoic erosion ex- In addition to faults and folds, strata in the propose that Phanerozoic movement on faults posed basement in uplifts of the Rocky Moun- Rocky Mountain–Colorado Plateau province lo- in the Rocky Mountain–Colorado Plateau tains, a region of 3–4-km-high mountains. In the cally developed strain by growth of mesoscopic province represents intracratonic rift inversion. Colorado Plateau region, uplifts are gently tilted fault arrays (Varga, 1993; Molzer and Erslev, Inversion of normal faults refers to reactiva- or flat-topped tablelands; erosion breached the 1995) and by development of cataclastic defor- tion of the faults in a manner that generates cover to expose underlying basement in relatively mation bands (Davis, 1999). These strata do not reverse or oblique-slip movement. During in- few examples. Close to the surface, faults border- contain widespread tectonic cleavage, however, version, the normal faults act as crustal weak- ing Laramide uplifts steepen and divide updip indicating that, in contrast to foreland fold-and- nesses that allow slip under low differential into splays that die out in a monoclinal forced thrust belts, significant (>10%) layer-parallel stress when the crust undergoes later shortening fold involving cover. Laramide monoclines are shortening did not broadly develop in the prov- (Cooper and Williams, 1989). The stress neces- fault-propagation folds, the steep limb of which ince. Notably, studies of calcite twinning indicate sary to initiate sliding on preexisting faults is faces the downdropped block (Erslev, 1991; that strata have been subjected to differential less than that needed to form new faults in intact Mitra and Mount, 1998; Allmendinger, 1999). stresses of <100 MPa (van der Pluijm et al., rock, partly because frictional resistance is gen- The shape of Laramide faults at depth remains 1997). The orientation of stress fields during erally less than shear rupture strength under the debatable. In some examples, faults dip steeply deformation of the province remains controver- same confining pressure (Etheridge, 1986), (>60°) near the surface, but the fault dip de- sial, due to the wide range structural trends. partly because alteration in fault zones produces creases at depth, whereas elsewhere, thrusts dip weak rocks (Holdsworth et al., 1997), and partly at about 35° to a depth of 20 km (Stone, 1993). ORIGIN OF ROCKY MOUNTAIN– because the presence of preexisting fractures The faults may merge with a mid-crustal detach- COLORADO PLATEAU PROVINCE provides a reservoir for groundwater, which ment (Gries, 1988; Erslev, 1993). FAULTS: A RIFT-INVERSION MODEL may become overpressured during compres- In map view, Laramide faults and associated Although the geometry and kinematics of sion, thereby decreasing the effective normal folds of the Rocky Mountain–Colorado Plateau Phanerozoic faulting in the Rocky Mountain– stress holding opposing walls of the fault to- province define a rhombohedral grid dominated Colorado Plateau province are well understood, gether (Sibson, 1993).

736 GEOLOGY,August 2000 CASE FOR RIFT INVERSION IN THE X Wind River Mts. Owl Creek Mts. Big Horn Mts.Y YZ Black Hills GRB WRB BHB PRB ROCKY MOUNTAIN–COLORADO PLATEAU PROVINCE To justify applying the rift-inversion concept mid-crustal weak zone A Moho 30 km to the Rocky Mountain–Colorado Plateau prov- Proterozoic sea level speculative model of Proterozoic rifts from which the Rocky Mountain evolved ince, we show (1) that the province contains Proterozoic rifts, (2) that inversion has affected documented rifts, (3) that uplifts in the province Moho parallel documented rifts, and (4) that the geom- B future level of erosion etry of uplifts resembles that of inverted rifts. We Figure 3. A: Schematic cross section of Rocky Mountains (location shown in Fig. 2). Modified also argue that the rift-inversion concept explains from Brown (1988) and Stone (1993). GRB—Green River basin,WRB—Wind River basin, BHB— how significant uplifts can develop in the interior Bighorn basin, PRB—Powder River basin. B: Hypothetical east-west cross section illustrating of a craton, a region that has not been subjected how cross section in A may have looked prior to Phanerozoic inversion. to large differential stress. zones of the province that do not carry Protero- folds formed over border faults on opposite sides Existence of Rifts zoic rift strata in their hanging wall closely re- of a rift have opposite vergence. Within rifts, Several areas in the cratonic platform preserve semble those that do—both penetrate basement at inversion of pairs of synthetic and antithetic evidence of pre–Ancestral Rockies rifting. depth and die out in a monoclinal flexure updip. faults, on a local scale, form local flat-topped up- Specifically, the exposure of locally thick suc- In this regard, the geometry of intracratonic fault- lifts bordered by opposite-dipping faults. These cessions of rift strata (with or without volcanics) ing in the Rocky Mountain–Colorado Plateau relations have been observed in the field and have in the Grand Canyon, southern Arizona, the province closely resembles that of documented been produced with sandbox models (McClay, Uinta trough, and the Beltian embayment, along inverted rifts elsewhere in the world (e.g., the 1995). As noted earlier, structures of the Rocky with the exposure of widespread mafic magma- Pampean ranges in Argentina; Schmidt, 1996) Mountain–Colorado Plateau province include tism (including dike swarms), demonstrates that and that of inverted rifts in sandbox models local flat-topped uplifts (e.g., the Uinta uplift) the craton was pervasively rifted in the Protero- (McClay, 1995). bordered by monoclinal folds, and folds on oppo- zoic. Marshak and Paulsen (1996) suggested that site sides of the uplifts have opposite vergence. It the rifts developed during three events: northeast- Regional Map Pattern is significant that fault dips (and associated fold southwest extension at 1.5–1.3 Ga, east-west to To explain the presence of the two nonparallel vergence) on opposite sides of the entire Laramide northwest-southeast extension at 1.1 Ga (coeval fault sets of the Rocky Mountain–Colorado deformation belt are opposite (Fig. 3). The rift- with the Grenville orogeny and the opening of Plateau province requires that (1) faults initiated inversion model explains this geometry. the Midcontinent Rift), and east-west to northwest- as coevally conjugate shear ruptures, (2) faults southeast extension at 0.7–0.6 Ga (breakaway of initiated at two different times in response to dif- Stress State at the Time of Faulting East Gondwana). Timmons et al. (2000) recog- ferent stress states, (3) stress fields progressively The stress necessary to initiate frictional slid- nized two stages of rifting: northeast-southwest rotated during fault formation, (4) fault slip oc- ing on preexisting faults is less than that neces- extension between 1.3 and 1.0 Ga, and east-west curred on two nonparallel preexisting foliation sary to generate new faults in intact rock to northwest-southeast extension between 0.9 sets, or (5) fault slip occurred on two nonparallel (Etheridge, 1996). Furthermore, preexisting and 0.7 Ga. Regardless of the exact timing, by preexisting fault sets. Observations favor the last normal faults may be particularly weak, because the end of these events, the North American solution. In the Laramide structures, for example, dilatancy during normal faulting leads to rock- craton contained two sets of basement-penetrating evidence suggests that the region did not endure water interaction that forms weak fault rocks faults (one trending north to northeast and the two mutually orthogonal compressional phases, (Holdsworth et al., 1997), and may leave resid- other trending west to northwest; Fig. 1D and and that slip directions on faults vary with fault ual fractured rock that can become overpres- 1E) and related grabens, composing a rhombo- orientation (dip slip occurs on some faults while sured during inversion (Sibson, 1993). Calcite hedral network. strike-slip displacement occurs on others; Molzer twinning studies and the lack of penetrative fab- and Erslev, 1995; Erslev and Wiechelman, 1997), rics indicate that differential stress affecting the Geometric Similarity to Documented as would be expected if the faults were preexist- cover of the Rocky Mountain–Colorado Plateau Examples of Rift Inversion ing and were reactivated during subsequent province during the Phanerozoic was smaller Several intracratonic fault-and-fold zones asso- crustal shortening. Furthermore, fault-slip direc- than the differential stress affecting orogenic ciated with uplifts in the Rocky Mountain– tions are not in the same plane, as would be ex- belts along the continental margin (van der Colorado Plateau province demonstrably result pected for conjugate faults, and faults only lo- Pluijm et al., 1997). Specifically, their measure- from inversion of preexisting rifts. For example, cally parallel regional foliations (Prucha et al., ments suggest that stress state in the craton dur- reverse faulting emplaced the Precambrian rift 1965). However, fault sets of the Rocky Mountain– ing marginal orogenies is in the range of only strata of the Uinta trough over younger strata on Colorado Plateau province are parallel to the 20–40 MPa, less than the experimental failure the trough’s margins, reverse faulting occurred on two major sets of Proterozoic extensional faults strength of rock under confined compression at the margins of the Beltian embayment in Montana and dikes of the U.S. cratonic platform (see shallow crustal levels (cf. Handin, 1966). The (Huntoon, 1993; Schmidt, 1996), and reverse Brown, 1988). Thus, the rift-inversion model rift-inversion model explains how faulting can faulting occurred along fault blocks of Protero- provides an explanation for observed fault occur in a region where stresses may have been zoic strata in the Grand Canyon (Timmons et al., trends and slip directions. too low to generate new faults. 2000). If all hanging-wall blocks of the Rocky Mountain–Colorado Plateau province uplifts con- Vergence Variations AN INTEGRATED IMAGE OF tained rift strata, as in the examples herein, then Border faults on opposite sides of a rift com- INTRAPLATFORM DEFORMATION the rift-inversion model would be self evident. Be- monly have opposite dip directions (even though We argue that intracratonic faults, such as cause they do not, the ancestry of their bounding within the rift, faults dominently dip in the same those of the Rocky Mountain–Colorado Plateau faults remains a question. We note here that fault direction), and as a consequence, fault-propagation province, originated as normal faults during

GEOLOGY,August 2000 737 Proterozoic extensional tectonics, and were reac- belt: Geological Society of America Memoir 171, intracratonic extensional tectonism?: Geology, tivated as reverse or transpressional faults during p. 1–25. v. 24, p. 151–154. Cooper, M.A., and Williams, G.D., eds., 1989, Inver- McBride, J.H., and Nelson, W.J., 1999, Style and origin the Phanerozoic. This hypothesis is compatible sion tectonics: Geological Society [London] Spe- of mid-Carboniferous deformation in the Illinois with the observed map pattern of faulting, with cial Publication 44, 375 p. Basin, USA—Ancestral Rockies deformation?: regional variations in vergence of related folds, Davis, G.H., 1978, Monocline fold pattern of the Colo- Tectonophysics, v. 305, p. 275–286. and with fault geometry. To explain the lack of rado Plateau, in Matthews, V., III, ed., Laramide McClay, K.R., 1995, The geometries and kinematics of rift strata in the hanging walls of uplifts, we note folding associated with basement block faulting inverted fault systems: A review of analogue in the western United States: Geological Society model studies: Geological Society [London] Spe- that regional exhumation stripped about 10 km of of America Memoir 151, p. 215–233. cial Publication 88, p. 97–118. crust off the surface of the province prior to the Davis, G.H., 1999, Structural geology of the Colorado Mitra, S., and Mount, V.S., 1998, Foreland basement- deposition of Phanerozoic cover strata (Timmons Plateau region of southern Utah: Geological So- involved structures: American Association of et al., 2000). The “Great Unconformity” at the ciety of America Special Paper 342, 168 p. Petroleum Geologists Bulletin, v. 82, p. 70–109. Erslev, E.A., 1991, Trishear fault propagation folding: Molzer, P.C., and Erslev, E.A., 1995, Oblique con- base of the Paleozoic is called great for a Geology, v. 24, p. 617–620. vergence during northeast-southwest Laramide reason—this erosion stripped away Proterozoic Erslev, E., 1993, Thrusts, back-thrusts, and detach- compression along the east-west Owl Creek and rift strata in all but the deepest rifts, but did not ments of Rocky Mountain foreland arches, in Casper Mountain Arches, central Wyoming: remove basement-penetrating faults. The faults Schmidt, C.J., et al., eds., Laramide basement American Association of Petroleum Geologists remained as weaknesses in the crust, available for deformation in the Rocky Mountain foreland of Bulletin, v. 79, p. 1377–1394. the western United States: Geological Society of Prucha, J.J., Graham, J.A., and Nickelsen, R.P., 1965, reactivation (Fig. 3). America Special Paper 280, p. 339–358. Basement-controlled deformation in Wyoming In light of our proposal, we suggest the follow- Erslev, E., and Wiechelman, D., 1997, Fault and fold province of Rocky Mountains foreland: Ameri- ing tectonic evolution of the North American cra- orientations in the central Rocky Mountains of can Association of Petroleum Geologists Bul- tonic platform (the Rocky Mountain–Colorado Colorado and Utah, in Hoak, T.E., et al., eds., letin, v. 49, p. 966–992. Fractured reservoirs: Characterization and mod- Schmidt, C.J., 1996, Inversion of Cretaceous rift basins Plateau province and the Midcontinent). After eling: Denver, Colorado, Rocky Mountain Asso- and reversion of Carboniferous uplifts in the cratonization, two or more extensional events, be- ciation of Geologists, p. 131–136. Pampean Ranges, Argentina, with implications tween 1.3 and 1.1 Ga and between 0.9 and 0.7 Ga, Etheridge, M.A., 1986, On the reactivation of extensional for reactivation in the Laramide Rocky Moun- produced basement-penetrating faults. These fault systems: Royal Society of London Philosoph- tains: Geological Society of America Abstracts faults remained as permanent weaknesses in the ical Transactions, ser. A, v. 317, p. 179–194. with Programs, v. 28, no. 7, p. A-112. Gries, R., 1988, Oil and gas prospecting beneath Pre- Sibson, R.H., 1993, Load-strengthening vs. load-weak- upper crust. Late Proterozoic exhumation stripped cambrian of foreland thrust plates in Rocky ening faults: Journal of Structural Geology, v. 15, away the rift fill in all but the deepest rifts. Mountains: American Association of Petroleum p. 123–128. Phanerozoic continental-margin orogenies in- Geologists Bulletin, v. 67, p. 1–28. Stone, D.S., 1993, Basement-involved thrust generated verted the weak faults, causing reverse-transpres- Hamilton, W., 1988, Laramide crustal shortening, in folds as seismically imaged in the subsurface of Schmidt, C.J., and Perry, W.J., Jr., eds., Interac- the central Rocky Mountain foreland, in Schmidt, sional displacements that generated basement- tion of the Rocky Mountain foreland and the C.J., et al., eds., Laramide basement deformation cored uplifts and associated monoclines. Because Cordilleran thrust belt: Geological Society of in the Rocky Mountain foreland of the western not all faults were oriented appropriately to de- America Memoir 171, p. 27–39. United States: Geological Society of America form by dip-slip deformation, regional deforma- Handin, J., 1966, Strength and ductility, in Clark, S.P., Special Paper 280, p. 271–318. tion was partitioned between reverse-, oblique-, Jr., ed., Handbook of physical constants: Geolog- Timmons, J.M., Karlstrom, K.E., Dehler, C.M., ical Society of America Memoir 97, p. 223–290. Geissman, J.W., and Heizler, M.T., 2000, Protero- and strike-slip movements (Varga, 1993; Karl- Holdsworth, R.E., Butler, C.A., and Roberts, A.M., zoic multistage (ca. 1.1 and ca. 0.8 Ga) extension strom and Daniel, 1993; Tindall and Davis, 1999). 1997, The recognition of reactivation during con- in the Grand Canyon Supergroup and establish- Notably, west-trending faults could have served as tinental deformation: Geological Society [Lon- ment of northwest and north-south tectonic grains accommodation zones bounding crustal blocks don] Journal, v. 154, p. 73–78. in the southwestern United States: Geological So- Huntoon, P.W., 1993, Influence of inherited Precambrian ciety of America Bulletin (in press). that underwent different amounts of stretching basement structure on the localization and form of Tindall, S.E., and Davis, G.H., 1999, Monocline develop- during the Proterozoic, and different amounts of Laramide monoclines, Grand Canyon, Arizona, in ment by oblique-slip fault-propagation folding: The shortening during the Phanerozoic. Schmidt, C.J., et al., eds., Laramide basement East Kaibab monocline, Colorado Plateau, Utah: The rift-inversion model, which may apply in deformation in the Rocky Mountain foreland of Journal of Structural Geology, v. 21, p. 1303–1320. cratonic platform regions worldwide, can be the western United States: Geological Society of van der Pluijm, B.A., Craddock, J.P., Graham, B.R., America Special Paper 280, p. 243–256. and Harris, J.H., 1997, Paleostress in cratonic tested by collection of subsurface data that may Karlstrom, K.E., and Daniel, C.B., 1993, Restoration of North America: Implications for deformation of document preserved normal-sense displacement Laramide right-lateral strike slip in northern New continental interiors: Science, v. 277, p. 794–796. on faults at depth. Further evidence could come Mexico by using Proterozoic piercing points; tec- Varga, R.J., 1993, Rocky Mountain foreland uplifts: from reexamination of slip kinematics on ex- tonic implications from the Proterozoic to the Products of a rotating stress field or strain parti- Cenozoic: Geology, v. 21, p. 1139–1142. tioning?: Geology, v. 21, p. 1115–1118. posed faults and from systematic study of cooling Karlstrom, K.E., and Humphreys, E.D., 1998, Per- Ye, H., Royden, L., Burchfiel, C., and Schuepbach, M., histories across block-bounding faults in the re- sistent influence of Proterozoic accretionary 1996, Late Paleozoic deformation of interior gion (Karlstrom et al., 1999). boundaries in the tectonic evolution of south- North America: The greater Ancestral Rocky western North America: Interaction of cratonic Mountains: American Association of Petroleum ACKNOWLEDGMENTS grain and mantle modification events: Rocky Geologists Bulletin, v. 80, p. 1397–1432. We are grateful for reviews and/or discussions with Mountain Geology, v. 33, p. 161–179. Yin, A., 1994, Mechanics of monoclinal systems in the Bruce Douglas, Eric Erslev, Chuck Kluth, Tim Paulsen, Karlstrom, K.E., Cather, S.M., Kelley, S.A., Heizler, Colorado Plateau during the Laramide orogeny: and Alan Whittington. This work was supported in part M.T., Pazzaglia, F.J., and Roy, M., 1999, Sandia Journal of Geophysical Research, v. 99, through funds from the National Science Foundation Mountains and Rio Grande Rift: Ancestry of p. 22,043–22,058. Mid-America Earthquake Center Project SG-5. structures and history of deformation: New Mexico Geological Society 50th field conference Manuscript received January 25, 2000 REFERENCES CITED Guidebook, Albuquerque Geology, p. 155–165. Revised manuscript received May15, 2000 Allmendinger, R.W., 1999, Inverse and forward numer- Kluth, D.F., and Coney, P.J., 1981, Plate tectonics of the Manuscript accepted May 19, 2000 ical modeling of trishear fault-propagation folds: Ancestral Rocky Mountains: Geology, v. 9, Tectonics, v. 17, p. 640–656. p. 10–15. Brown, W.G., 1988, Deformational style of Laramide Marshak, S., and Paulsen, T., 1996, Midcontinent U.S. uplifts in the Wyoming foreland, in Schmidt, C.J., fault and fold zones: A legacy of Proterozoic and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and the Cordilleran thrust

738 Printed in USA GEOLOGY,August 2000

GEOLOGICAL NOTE Detrital Zircon Geochronology of the Bighorn Dolomite, Wyoming, USA: Evidence for Trans-Hudson Dust Deposition on the Western Laurentian Carbonate Platform

David H. Malone,1,* John P. Craddock,2 Alex Konstantinou,3 Patrick I. McLaughlin,4 and Krista M. McGillivray1

1. Department of Geography-Geology, Illinois State University, Normal, Illinois, USA; 2. Department of Geology, Macalester College, St. Paul, Minnesota, USA; 3. Cyprus Hydrocarbons Company, Nicosia, Cyprus; 4. Indiana Geological Survey, Indiana University, Bloomington, Indiana, USA

ABSTRACT This study uses detrital zircon U-Pb geochronology from shallow-water carbonates of the Bighorn Dolomite in Wyoming, USA, to provide robust evidence for long-distance eolian sediment transport during the Ordovician. The Bighorn Dolomite was deposited in a shallow-water carbonate platform that developed approximately 107 south of the Ordovician paleoequator on the western edge of Laurentia. The ages and textures of detrital zircons from the Bighorn indicate that the grains were transported by winds through saltation and suspension from the paleo east where rocks of the Paleoproterozoic Trans- Hudson orogenic belt were exposed in present-day Manitoba and Saskatchewan. Our interpretation of long-distance eolian transport is consistent with the paleogeography of Laurentia and expected prevailing wind directions and draws on modern analogs where Saharan sediment is transported by trade winds for distances of more than 3000 km.

Online enhancements: supplemental table.

Introduction Late Ordovician rocks in the Rocky Mountain area dovician. The spheroidal, frosted zircons were en- record important information about the tectonic, trained and transported by wind 1500 km to the Late stratigraphic, sedimentologic, and paleogeographic Ordovician (∼450 Ma) Laurentian carbonate plat- evolution of North America. Detrital zircon stud- form in Wyoming. ies are used extensively to determine sediment dis- During the Ordovician, Wyoming was about 107 persal, provenance, and maximum depositional ages south of the equator and rotated about 607 clock- for eolian siliciclastic rocks (e.g., Pell et al. 1997; wise with respect to today (ﬁg. 1). The study area was Soreghan et al. 2002; Dickinson and Gehrels 2003, 500–1000 km east of the western edge of the exposed 2009; Stephens et al. 2010; Konstantinou et. al. 2014). Laurentian land mass (Scotese and McKerrow 1991; Here we present and interpret data for zircons ex- Blakey 2016). The Upper Ordovician Bighorn Dolo- tracted from the Bighorn Dolomite, which we believe mite, which is as much as 150 m in thickness, is a is the ﬁrst study that focuses on zircons deposited prominent stratigraphic unit that occurs through- by wind in carbonate rocks. We argue that the zircon out southern Montana, northern Wyoming, eastern grains were derived from Trans-Hudson (1.8–2.1 Ga) Idaho, and northern Utah (Zenger 1992; Sansom and and Archean (12.5 Ga) crystalline rocks that were Smith 2005; Holland and Patzkowsky 2009, 2012). exposed in western Laurentia during the Late Or- The Bighorn Dolomite is also present in the subsurface of the Williston Basin. In Wyoming, the Big- Manuscript received June 24, 2016; accepted November 13, horn Dolomite disconformably overlies the Cam- 2016; electronically published January 27, 2017. brian Snowy Range Formation (Saltzman 1999), with * Author for correspondence; e-mail: [email protected]. a hiatus of about 40 Ma. The Bighorn Dolomite de-

261 Figure 1. A, Paleogeogaphic reconstruction of the Laurentian Ordovician western shelf (modified from Blakey 2016). The locations of the Precambrian basement provinces are overlain (modified from Whitmeyer and Karlstrom 2007). The dashed green arrow is the proposed trajectory of sediment transport from the Trans-Hudson and adjacent Archean source areas. The location of the depositional area is indicated by the red star. The paleoequator is indicated by the red line. B, Geologic map of the Precambrian terranes in Saskatchewan and Manitoba (modified from Bickford et al. 1990 and Ansdel 2005). Journal of Geology GEOCHRONOLOGY OF THE BIGHORN DOLOMITE 263 position began at about 450 Ma as part of a major Methods Late Ordovician (Tippecanoe, C2) transgression that We collected ∼50 kg of sample from the basal Big- flooded the southwestern Laurentian continental horn Dolomite at White Mountain. The zircon grains shelf (Sloss 1963; Haq and Schutter 2008; Holland were extracted from bulk carbonate samples by tra- and Patzkowsky 2012). The Bighorn Dolomite is dis- ditional methods of crushing and grinding, followed conformably overlain by Devonian strata of the Darby, by panning, heavy liquids, and a Frantz magnetic sep- Jefferson, and Three Forks Formations. arator. The entire zircon population was handpicked The study area is the western side of White Moun- from the nonmagnetic residue and analyzed at the tain in the northern Absaroka Range about 50 km Arizona LaserChron Center (Gehrels et al. 2006, 2008). east of Yellowstone National Park (fig. 2). White Table S1 (available online) summarizes the details of Mountain consists of allochthonous Paleozoic car- the analytical methods and provides a table of zircon bonate and Eocene volcanic and plutonic rocks that ages. The resulting interpreted ages are shown on an were emplaced as part of the massive Heart Moun- age-probability diagram using the routines in Isoplot tain Slide at 48.87 Ma (Malone 1995; Craddock et al. (Ludwig 2008). 2000, 2009, 2012; Anders et al. 2010; Malone et al. 2014; Mitchell et al. 2015). The base of the slide mass is along a detachment surface in the lower Bighorn Results Dolomite, about 1–2 m above the contact with the underlying Cambrian Snowy Range Formation. A The zircons analyzed range in size from 100 to 125 mm 1–2 m veneer of carbonate ultracataclasite (see Crad- (very fine sand) in maximum dimension (fig. 3). dock et al. 2009 for a complete description), which Most are pink, and some are colorless. Most grains was formed by crushing of the Bighorn Dolomite dur- are very well rounded and have frosted grain surface ing slide emplacement, occurs above the detachment textures that are compatible with wind abrasion surface. The sampling locality is the autochthonous, (Kuenen and Perdok 1962; Krinsley and Doornkamp undeformed Bighorn Dolomite below the slide base. 1973; Dott 2003). All zircons have uranium concen- May et al. (2013) provide an excellent summary trations lower than 250 ppm, and all ages are con- of zircon ages for Archean and Phanerozoic rocks cordant. in northwest Wyoming. The basement rocks of the Figure 4 is a probability density plot of the ages for Wyoming craton area are 2.7–3.2 Ga granites and the 57 zircons analyzed. Thirty-nine zircons yield gneisses (Frost and Fanning 2006). May et al. (2013) Trans-Hudson ages, which range from 1804 to 1973 Ma. report two zircon age spectra from the Flathead Two peak ages are evident in the data: 1836 and Sandstone, which is about 250 m beneath the Big- 1921 Ma. Nine zircons yield Paleoproterozoic ages horn Dolomite and was not exposed in this part of between 2030 and 2090 Ma. The oldest fraction is dom- Wyoming during the Ordovician as it was buried inated by very early Paleoproterozoic and Archean by hundreds of meters of Gros Ventre and Gallatin grains and range in age from 2345 to 3017 Ma. Two formationstrata.TheoldestFlatheadsamplethatMay grains yield Grenville Province ages (1051–1121 Ma). et al. (2013) collected is from just above the Archean The Trans-Hudson orogenic belt is a principal gneiss unconformity, and most detrital zircons in Paleoproterozoic suture zone in North America. The these samples are Archean in age. A second Flat- Trans-Hudson orogeny occurred 1.8–2.0 Ga when the head sample taken 10 m above the unconformity was Superior (east), Wyoming (south), and Rae-Hearne dominated by Yavapai-Mazatzal ages (1.6–1.8 Ga; (northwest) Archean blocks converged and were su- Bickford et al. 2008), with a peak age of 1.785 Ga. tured together (Hoffman 1988; Bickford et al. 1990; Based on detrital zircon geochronology and paleo- Ansdel 2005; Whitmeyer and Karlstrom 2007) to current patterns, Yonkee et al. (2014) interpreted form the core of the Laurentian continent (fig. 1). The northwestern transport of siliciclastic sediment from deformed rocks associated with the Trans-Hudson the Laurentian craton to the Cordilleran continen- orogen extend from the subsurface in the US High tal passive margin during Neoproterozoic and Cam- Plains, northward into Manitoba and Saskatchewan, brian times. where they are well exposed. In northern Manitoba, No significant sandstone units occur locally in the belt turns east and extends through northern the Ordovician-Mississippian stratigraphic section Canada to Greenland. of Wyoming. The 1–2 m thick Lander Sandstone is The Archean zircons may have originated from beneath the Bighorn Dolomite in the Big Horn and either the Rae-Hearne or Superior cratons on either Wind River ranges 1200 km to the south and east, side of the Trans-Hudson orogenic belt. Granites but this sandstone is not present in northwest Wyo- with ages between 2.0 and 2.4 Ga occur in the Rae ming (Holland and Patzkowsky 2012). craton north of Lake Athabasca (Bickford et al. 1990). Figure 2. A, White Mountain sampling locality in the northeastern Absaroka Range (4474507.1600N, 109730ʹ43.8200W). The lower slope is shale and limestone of the Cambrian Snowy Range Formation. The Heart Mountain detachment occurs 1–2 m above the contact with the overlying Bighorn. A ∼1–2 m layer of carbonate ultracataclasite was generated during Heart Mountain slide emplacement at 48.87 Ma. About 100 m of allochthonous, thermally metamorphosed Bighorn Dolomite overlies the detachment. B, Closer view of the basal Bighorn and overlying carbonate ultracataclasite. The inset in A is a generalized stratigraphic column of the northern Absaroka Range (modified from Malone et al. 2014). A color version of this figure is available online. Journal of Geology GEOCHRONOLOGY OF THE BIGHORN DOLOMITE 265

zircon age peak may be associated with the late stages of the emplacement of the Wathaman Bath- olith, which was formed as part of the continental margin arc. Younger plutons such as the Junction Granite, which range in age from 1800–1830 Ma, were emplaced into the Rae-Hearne craton boundary during the ﬁnal continent-continent collision (Bickford et al. 1990). The two Grenville-age grains were transported to western Laurentia before eventually becoming part of the Ordovician eolian sediment load transported to Wyoming (Rainbird et al. 1992).

Discussion Provenance of Bighorn Detrital Zircons. The zircons from the Bighorn Dolomite most likely originated in the Saskatchewan and Manitoba Trans- Hudson Province, as the underlying Flathead Sandstone and basement rocks in Wyoming have distinctly different detrital zircon age spectra (May et al. 2013; Malone et al. 2014). An Ordovician fluvial transport Figure 3. Scanning electron microscope backscatter im- of zircons to the south is not admissible, as the Wil- age of zircon grains from the Bighorn Dolomite. The main liston Basin lies between the sediment source and image shows grain surface textures compatible with ex- depositional areas. tensive wind abrasion. The upper left inset displays zircon The dominance of Trans-Hudson–age zircons in grain shape and size. The lower left inset is a cathodolu- Upper Ordovician strata of western Laurentia is not minescence image of an analyzed zircon. Spot size p 30 mm. unique to the Bighorn. Although thick sections of laterally persistent Upper Ordovician quartz arenite Alternatively, the latest Archean and earliest Paleo- are rare in western Laurentia, where present along proterozoic granites, including a substantial volume the Cordilleran continental margin, Trans-Hudson of those emplaced at ∼2450 Ma, occur in the Sask grains dominate. Gehrels and Pecha (2014) compiled craton (Ansdell 2005). detrital and Hf isotopic data for deep-marine Or- The origin of the 2.0–2.1 Ga zircons is more elu- dovician sandstones from Sonora to British Colum- sive, as igneous rocks of this age in the region are rare. Continental margin successions of this age occur on the margins of both the Superior and Rae- Hearne (Wollaston metasedimentary domain) cratons, each of which have interbedded volcanic rock of this age (Ansdell 2005). Bickford et al. (1990) report concordant zircons of this age in granulites along the margin of the Rae-Hearne craton. Patterson and Heaman (1991) recognized gabbros of this age in the Hurwitz Group in the Trans-Hudson hinterland, about 250 km north of the Reindeer Zone. These Bighorn zircons may also be derived from the Marathon large igneous province (LIP), which is a profuse mafic dike swarm that occurs throughout the Superior Province (Halls et al. 2008). Figure 4. Probability density plot of the age spectrum The zircons associated with the 1921 Ma age peak of Bighorn Dolomite zircons. The principal peaks are at are likely derived from island arc plutonic and vol- 1836, 1921, and 2099 Ma. Grenville p 980–1300 Ma, canic rocks in the Flin Flon domain (Ansdell 2005). midcontinent granite-rhyolite (MCGR) p 1300–1500 This igneous activity occurred between 1925 and Ma, Yavapai-Mazatzal (Y-M) p 1600–1800 Ma, Trans- 1880 Ma, which was prior to the main collision of Hudson (T-H) p 1800–2100 Ma, and Archean Wyoming, the Superior and Rae-Hearne cratons. The 1836 Ma Superior, Rae, or Hearne cratons 12500 Ma. 266 D. H. MALONE ET AL. bia. They interpreted the likely source area to be the decades (Goudie and Middleton 2001). Prospero et al. Peace River Arch because of the presence of un- (1970) reported that particles as large as 20 mmmax- common 2.1–2.0 Ga grains. We noted above that imum dimension were transported from Africa for such ages are also present in and adjacent to the as much as 4000 km to the Caribbean. Similarly, Trans-Hudson orogeny in Saskatchewan and Mani- Arimoto et al. (1997) found Saharan dust as large as toba; we prefer this area as the source for Bighorn 57 mm in maximum dimension in Bermuda. Giant Formation zircons as it is much closer to the depo- Saharan dust particles more than 62.5 mm in max- sitional area than is the Peace River Arch. imum dimension have been found in the British Isles, More recently, Pope et al. (2016) recognized the which is ∼3000 km from the sediment source areas distinct difference in provenance between Ordo- (Middleton et al. 2007). vician quartz arenites east and west of the Trans- Glaccum and Prospero (1980), in their atmo- continental Arch, a structure which had long been spheric sampling of Saharan dust of the North At- recognized as an early Paleozoic sediment dispersal lantic found quartz grain more than 100 mm and barrier and drainage divide. Link (2016) analyzed mica grains more than 350 mm in diameter more detrital zircons in the Kinnickinnic and Swan Peak than 500 km west of the African coast. Molinaroli formations in central Idaho and found these sand- et al. (1993) reported dust particles as large as 102 mm stones to contain 11850 Ma grains, including some deposited in Sardinia from Saharan dust storms more rare 2100–2000 Ma. This work provides further than 500 km away. In their study of suspended dust evidence for the distinct eastern and western Lau- transport by measuring the dust accumulation in rentian sediment sources divided by the Transcon- mountaintop filters at Canyonlands and Mesa Verde tinental Arch. National Monuments on the Colorado Plateau, Neff St. Peter Sandstone: Eolian Characteristics and De- et al. (2013) found maximum particle sizes ranging trital Zircon Geochronology. The middle Ordovician from 128 to 296 mm in diameter, but these sand- St. Peter Sandstone, which is famous for its com- sized particles were rare. They further noted that the positional and textural maturity, is the basal Tip- paucity of coarse grains may be the result of ineffi- pecanoe Sequence (Sloss 1963) unit present in the cient sampling of very large size particles because of north-central United States. The St. Peter Sand- sampler design. stone was initially interpreted to be only marine in origin, but textures and structures indicative of eolian transport are widely recognized (e.g., Dott et al. Conclusions 1986; Nadon et al. 2000). Sediment dispersal patterns as indicated by zircon geochronology of Cambro- The only admissible interpretation of well-rounded Ordovician sandstones in the midcontinent indi- zircons of Trans-Hudson age in the Ordovician Big- cate that local basement rocks supplied most of horn Dolomite in northern Wyoming is long-distance the sediment for Cambrian sandstones, while distal (1500 km) eolian transport from the western mar- Grenville and Archean sources provided most of gin of Laurentia west to the shallow carbonate plat- the sediment for the St. Peter Sandstone (Konstan- form. This interpretation is consistent with the pa- tinou et al. 2014). leogeography and the expected trade-wind patterns Modern Analogs of Long-Distance Sand Emission and in tropical latitudes. Moreover, the western Lauren- Transport. The Upper Ordovician Laurentian sed- tian land mass was arid, as indicated by the presence imentary record includes ample evidence of eolian of extensive evaporite deposits in the Williston Ba- processes. Huff et al. (1992) and Kolata et al. (1996) sin (Husenic and Bergstrom 2015). The influence of report the occurrence of more than 50 Ordovician eolian processes during the Ordovician was perhaps K-bentonite ash beds interbedded with carbonate much more significant than after higher land plants rocks and shales in eastern North America. These became widespread during the Silurian (Dott 2003). ash beds range in age from 450 to 460 Ma. These This new evidence of Ordovician eolian transport deposits are interpreted to be formed through a se- and deposition in a carbonate platform has as series ries of Plinian-style volcanic eruptions along the Ta- of regional and global implications. The transport conic arc as it converged on North America. Some of of sediment across the Williston Basin would likely the beds are as much as 1 m in thickness and cover have impacted biogeochemical processes and weather millions of square kilometers (Kolata et al. 1996) in patterns similar to dust deposition in the Atlantic eastern Laurentia. Ocean today (Prospero 2013). In recent years, there Modern analogs for long-distance transport of has been a paradigm shift from a view of the Late dust are numerous. The transport of Saharan dusts Ordovician as a greenhouse world punctuated by a for thousands of kilometers has been recognized for short-lived ice age restricted to the Hirnantian to a Journal of Geology GEOCHRONOLOGY OF THE BIGHORN DOLOMITE 267 time of long-lived glaciation punctuated by a short ther consideration. This study indicates that the anal- interval of warming (i.e., Boda event; e.g., Vanden- ysis of detrital zircons from carbonate rocks is useful broucke et al. 2010; Finnegan et al. 2011; Ghienne for reconstructing ancient eolian sediment dispersal et al. 2014). Support for this new view is robust; and further constraining maximum and absolute documentation comes from a variety of sources (e.g., ages in sedimentary successions. paleobiogeography, sequence stratigraphy, stable isotope geochemistry). The implications include con- tracted low-latitude climate belts and a more vigor- ACKNOWLEDGMENTS ous atmospheric circulation. While the former has been documented using paleobiogeography, the lat- This project was funded by Keck Consortium grants ter is more elusive using traditional approaches. to J. P. Craddock and D. H. Malone in 2010 and 2011. Historically, zircons for provenance analysis have J. Trela, J. Calhoun, A. Hinds, J. Geary, K. Kravitz, and been sought mainly from siliciclastic rocks, where K. Schroeder provided able field assistance and small numbers of zircons are transported along with helped with sample preparation and analysis. The ubiquitous quartz. For nearly a century, conodont efforts of the University of Arizona LaserChron cen- paleontologists and biostratigraphers have reported ter staff are appreciated. We also appreciate E. Finzel, zircons and other detrital siliciclastic material in B. Hampton, and W. Thomas for their kind and sup- sample residues, but these grains received little fur- portive reviews of the manuscript.

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David H. Malone,1,* John P. Craddock,2 Mark D. Schmitz,3 Stuart Kenderes,4 Ben Kraushaar,5 Caelan J. Murphey,1 Stefan Nielsen,6 and Thomas M. Mitchell7

1. Department of Geography-Geology, Illinois State University, Normal, Illinois 61790, USA; 2. Geology Department, Macalester College, St. Paul, Minnesota 55105, USA; 3. Department of Geosciences, Boise State University, Boise, Idaho 83725, USA; 4. Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA; 5. Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82701, USA; 6. Department of Earth Sciences, Durham University, Durham DH13LE, United Kingdom; 7. Department of Earth Sciences, University College London, London, United Kingdom

ABSTRACT The Eocene Heart Mountain slide of northwest Wyoming covers an area of as much as 5000 km2 and includes allochthonous Paleozoic carbonate and Eocene volcanic rocks with a run-out distance of as much as 85 km. Recent geochronologic data indicated that the emplacement of the slide event occurred at ∼48.9 Ma, using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) extracted from U-Pb zircon ages from basal layer and injectite carbonate ultracataclasite (CUC). We now reﬁne that age with U-Pb results from a lamprophyre diatreme that is temporally and spatially related to the CUC injectites. The ages for the lamprophyre zircons are 48.97 5 0.36 Ma (LA-ICPMS) and 49.19 50.02 Ma (chemical abrasion isotope dilution thermal ionization mass spectrometry). Thus, the lamprophyre and CUC zircons are identical in age, and we interpret that the zircons in the CUC were derived from the lamprophyre during slide emplacement. Moreover, the intrusion of the lamprophyre diatreme provided the trigger mechanism for the Heart Mountain slide. Additional structural data are presented for a variety of calcite twinning strains, results from anisotropy of magnetic susceptibility for the lamprophyre and CUC injectites and alternating-ﬁeld demagnetization on the lamprophyre, to help constrain slide dynamics. These data indicate that White Mountain experienced a rotation about a vertical axis and minimum of 357 of counterclockwise motion during emplacement.

Online enhancements: supplemental tables.

Introduction The Heart Mountain slide has been one of the most Mountain slide and is an allochthon of Paleozoic controversial and enigmatic features in North Amer- carbonate and Eocene volcanic rocks. The western ica for nearly 120 y. The slide covers an area of at least half of White Mountain, which is in the distal por- 3500 km2 (Hauge 1993 and references therein) and tion of the bedding-parallel section of the Heart perhapsasmuchas5000km2, with a run-out distance Mountain slide, is the only occurrence of marble of as much as 85 km (Malone et al. 2014b). White within the entire Heart Mountain system. This ther- Mountain is one of the best exposures of the Heart mal metamorphism took place before slide emplacement and likely accompanied intrusion of the diorite stock at the northeast end of White Mountain a Manuscript received January 11, 2016; accepted March 13, (Malone et al. 2014 ). 2017; electronically published May 16, 2017. The base of allochthonous rocks at White Moun- * Author for correspondence; e-mail: [email protected]. tain is deﬁned by a horizontal, 2-m-thick carbonate

439 440 D. H. MALONE ET AL. ultracataclasite (CUC; Craddock et al. 2009) that on this lamprophyre to unravel the details of the re- fed eight injectites of the same material, which are lationship of emplacement kinematics and mechan- as much as 120 m above the slide base within mar- ics of the Heart Mountain slide. ble and Madison Group limestones (Craddock et al. 2012). Existing structural, mineralogical, and geo- Previous Work chemical constraints indicate that the Heart Moun- tain slide moved at ∼100 m/s (Craddock et al. 2009). The Heart Mountain slide is one of the largest sub- New field and lab work has identified a coeval aerial landslides on Earth, with the only analog be- tabular lamprophyre dike (striking N557E) at White ing the 21 Ma Markagunt slide in Utah (Hacker Mountain that is the remnant of a lamprophyre dia- et al. 2014). The Heart Mountain slide initiated on a treme that may have triggered the Heart Mountain 50-km-long, gently dipping bedding plane within slide. We present calcite twinning-strain, AMS (an- the Ordovician Bighorn Dolomite, cutting upsection isotropy of magnetic susceptibility), paleomagnetic, for ∼5 km and out onto the Eocene landscape (Ma- compositional, petrological, and geochronologic data lone et al. 2014a; figs. 1–3). Most of those working

Figure 1. Generalized geologic map of northwest Wyoming, illustrating the areal extent of the Heart Mountain slide and locations of features discussed in the text. 1 p White Mountain, 2 p Heart Mountain, 3 p McCullough Peaks, 4 p Squaw Peaks, 5 p Upper South Fork Shoshone River Valley. Modiﬁed from Malone et al. (2014b, 2014c). Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 441

Figure 2. Generalized geologic map of the Heart Mountain slide area. Modiﬁed from Hauge (1993) and Malone et al. (2014a). on the slide regard it as a catastrophic event asso- Aharanov and Anders 2006; Craddock et al. 2009, ciated with widespread igneous activity in the Eo- 2012; Anders et al. 2010; Malone et al. 2014a). Tim- cene Absaroka volcanic province (e.g., Bucher 1947; ing relations are key to understanding various em- Nelson and Pierce 1968; Pierce 1973; Malone 1995; placement models, which include a catastrophic Beutner and Craven 1996; Beutner and Gerbi 2005; landslide followed by volcanic burial (Pierce 1973), 442 D. H. MALONE ET AL.

Figure 3. Oblique view of White Mountain and the basal detachment of the Heart Mountain slide, including the positions of the carbonate ultracataclasite (CUC) injectites and lamprophyre. Injectites are numbered 1–8 from west to east. Open circles indicate calcite twin samples, black circles CUC injectite AMS samples, and the gray circle the lamprophyre AMS and paleopole sample. Sample sites are indicated in subsequent ﬁgures. A color version of this ﬁgure is available online.

incremental movement potentially lasting millions in Craddock et al. 2009, 2012; previous workers have of years (Hauge 1985, 1990; Templeton et al. 1995; referred to this as breccia, microbreccia, or the “basal Swanson et al. 2016), and catastrophic emplacement layer”) occurs as a centimeters-to-meter-thick ve- of Paleozoic carbonate and Eocene volcanic rocks neer along the contact of allochthonous and autoch- over a period of minutes to hours (Craddock et al. thonous rocks. The basal layer is best exposed at 2009). The mechanism of upper-plate movement that White Mountain (Nelson et al. 1972), along with allows for catastrophic emplacement along a low- discordant injectite dikes of the same material found angle surface with only minor associated deforma- up to 120 m above the slide base (Craddock et al. tion preserved above, along, or below the detachment 2012). U-Pb ages from zircons extracted from the is poorly understood. Contemporaneous dike intru- basal-layer CUC and injectites (White Mountain sion (Aharanov and Anders 2006; DeFrates et al. and Silver Gate) yield an age of ∼48.9 Ma (Malone 2006; King et al. 2009), various detachment ﬂuids et al. 2014a). (Templeton et al. 1995; Aharanov and Anders 2006; The purpose of this article is to provide additional

King et al. 2009), and CO2 generation by limestone- constraints on the timing and duration of the Heart on-limestone sintering (Beutner and Gerbi 2005; Mountain slide, using U-Pb geochronology, magnetic, Craddock et al. 2009; Anders et al. 2010; Mitchell and structural techniques on rocks formed during et al. 2015) have been proposed to facilitate move- emplacement. We describe and interpret a newly dis- ment of the slide mass. Hauge (1993) provides an covered lamprophyre igneous suite at White Moun- excellent detailed summary of all aspects of the first tain that bears on the Heart Mountain problem, and century of Heart Mountain research. Malone and we present an analysis of the lamprophyre diatreme Craddock (2008) summarized the research conducted and slide dynamics. Thus, this work is a follow-up to from 1990 to 2008. Malone et al. (2014a), who directly tested the rapid- Throughout the proximal areas of the Heart Moun- and gradual-emplacement hypotheses. Field evidence tain slide, carbonate-dominated cataclastic mate- indicates that the lamprophyre igneous suite is geo- rial (first reported by Pierce 1979; referred to as CUC metrically and temporally related to the CUC inject- Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 443 ites at White Mountain. The concept presented and ature was calculated from the Ti-in-zircon ther- argued here is that the emplacement/eruption of the mometer (Watson et al. 2006), with a nominal ac- lamprophyre suite may have provided the elusive tivity of TiO2 of 0.6 used for both lamprophyre breccia triggering mechanism for slide emplacement. zircons and the Orapa kimberlite zircon standard. For U-Th-Pb age analysis, instrumental fractionation of the 206Pb/238U, 207Pb/206Pb, and 208Pb/232Th ratios was corrected and ages calibrated with re- Methods spect to interspersed measurements of the Plešo- The igneous-suite energy-dispersive X-ray fluores- vice zircon standard (Sláma et al. 2008). Signals at cence (XRF) data were collected at Western Kentucky mass 204 are indistinguishable from 0 for most zir- University and the Illinois State Geological Survey cons after subtraction of mercury backgrounds mea- (table S1; tables S1–S6 available online). Mineral sep- sured during the gas blank (!150 counts/s 202Hg); arations and heavy-mineral analysis via wavelength- thus, ages are normally reported without common- dispersive X-ray spectroscopy on a scanning elec- Pb correction, although our data acquisition and re- tron microscope were done at Macalester College. duction scheme allows for a variety of 204Pb-, 207Pb-, Fabric analysis via electron backscatter diffraction or 208Pb-based corrections. Radiogenic-isotope ratio (EBSD) was done at the University of Minnesota- and age error propagation includes uncertainty con- Duluth. Zircon crystals were analyzed for trace- tributions from counting statistics, background sub- element concentrations and U-Pb age via laser ablation– traction, and regression errors. Standard calibration inductively coupled plasma mass spectrometry uncertainty is propagated in quadrature following (LA-ICPMS) and subsequently dated at high precision group statistics (e.g., weighted mean calculations); by chemical abrasion–isotope dilution thermal ioni- standard calibration uncertainties (2j) over an experi- zation mass spectrometry (CA-IDTIMS) at Boise State ment range from 1% to 2% for 206Pb/238Uandfrom University. Diamonds were analyzed with Raman 0.5% to 1% for 207Pb/206Pb. The LA-ICPMS U-Pb iso- spectroscopy at Western Kentucky University and tope and trace-element results are reported in tables S2 X-ray diffraction and electron microprobe analysis at and S3. the University of Minnesota. Paleomagnetic measure- For more precise dates, the same zircon crystals ments were conducted at the University of Minnesota were analyzed via CA-IDTIMS at Boise State Uni- Institute of Rock Magnetism. Calcite twinning-strain versity (table S4). Selected zircon crystals were re- analysis was conducted at Macalester College. moved from the epoxy mount and annealed at 9007C Zircon Geochronology. In situ zircon U-Th-Pb iso- for 60 h. Individual crystal fragments were then tope and trace-element concentration measurements “chemically abraded” in 120 mL of 29 M HF for 12 h were conducted at Boise State University with a at 1907C in 300-mLTeflon perfluoroalkoxy (PFA) ThermoElectron X-Series II quadrupole ICPMS and microcapsules. The residual crystals after this partial a New Wave Research UP-213 Nd:YAG ultraviolet dissolution were fluxed in 3.5 M HNO3 in an ultra- (213-nm) laser ablation system. Lamprophyre brec- sonic bath and on a warm hotplate for 60 min and cia zircons were analyzed with a suite of standards then rinsed twice in ultrapure H2O before being including a large zircon megacryst from the Orapa reloaded into microcapsules and spiked with the kimberlite as a reference material. Zircons were ab- mixed U-Pb isotope tracer ET535 (Condon et al. lated with a laser diameter of 25 mm using fluence 2015). Additional details of chemical separations, and pulse rates of 5–6 J/cm2 and 10 Hz, respectively, mass spectrometry, and data analysis are described during a 45-s analysis (15-s gas blank, 30-s ablation), in Rivera et al. (2013). Errors on CA-IDTIMS U-Pb which excavated a pit approximately 25 mm deep. isotope ratios and dates are reported at the 95% Ablated material was carried by a 1-L/min He gas confidence interval (2j) in table S4. Uncertainty in stream to the plasma. Dwell times were 5 ms for Si the weighted mean date is given as 5x(y)[z], where x and Zr; 40 ms for 49Ti, 238U, 232Th, 202Hg, and all Pb is the internal error based on analytical uncertainties isotopes; and 10 ms for all other high-field-strength only, including counting statistics, subtraction of elements and rare earth elements (REEs). Background tracer solution, and blank and initial common-Pb count rates for each analyte were obtained before subtraction; y includes the tracer calibration uncer- each spot analysis and subtracted from the raw count tainty propagated in quadrature; and z includes the rate for each analyte. For concentration calculations, 238U decay constant uncertainty propagated in background-subtracted count rates for each analyte quadrature. The latter uncertainty should be con- were internally normalized to 29Si and calibrated sidered when comparing our dates with those de- with respect to National Institute of Standards 610 rived from other decay schemes, such as 40Ar/39Ar. Ã and 612 glasses as the primary standards. Temper- The resulting interpreted ages are shown on Pb =U 444 D. H. MALONE ET AL. concordia and ranked-age diagrams using the rou- highest deviations (Groshong et al. 1984). This pro- tines in Isoplot (Ludwig 2008). cedure has been used if the number of measured AMS and Demagnetization Techniques. The “Roly- grains was large (n 1 20). In cases where the data ap- Poly” is an AC susceptibility bridge with an auto- pear to be inhomogeneous, the separation of incom- mated sample handler for determining anisotropy of patible twins (negative expected values [NEVs]) from low-field magnetic susceptibility at room tempera- compatible twins (positive expected values) of the ture. An alternating current in the external “drive” initial data set allows separate calculation of two or coils produces an alternating magnetic field in the more least squares deviatoric-strain tensors. Thus, sample space with a frequency of 680 Hz and an am- the CSGT can be used to obtain information on su- plitude of up to 1 mT. The induced magnetization of perimposed deformations (Groshong 1972, 1974) and a sample is detected by a pair of “pickup” coils, with differential stress magnitudes (Rowe and Rutter 1990). a sensitivity of 1.2 # 1026 SI volume units. For an- The validity of this stripping procedure was dem- isotropy determination, a sample is rotated about onstrated in experimental tests where the reliability three orthogonal axes, and susceptibility is measured depends on the overall complexity of deformation at 1.87 intervals in each of the three measurement and the number of grains with twins (Groshong 1974; planes. The susceptibility tensor is computed by least Teufel 1980). The stripping procedure was used in squares from the resulting 600 directional measure- cases of high proportions of NEVs and a large number ments. Very high precision results from the large of measured grains. An experimental reevaluation of number of measurements; in most cases principal- the CSGT has shown that measurements of about axis orientations are reproducible to within 27 and 50 grains on one thin section or 25 grains on two axial ratios to within about 1%. For each measured mutually perpendicular thin sections yield the best core, one unique magnetic ellipsoid is produced and results (Groshong et al. 1984; Evans and Groshong plotted. Sample anisotropy percentages ranged from 1994; Ferrill et al. 2004). The chance to extract the 5% to 14% for the eight samples and 193 cores taken records of more than two deformations from one from the CUC injectites, CUC basal layer, and lam- data set is limited when dealing with natural rocks prophyre. The AMS data are provided in table S5. (Burkhard 1993). Individual analyses of veins, matrix, Calcite Twin Analysis. The calcite strain-gage tech- nodules, and so on allows the acquisition of several nique (CSGT) of Groshong (1972) allows investiga- strain tensors without applying statistical data strip- tion of intraplate stresses as constrained by intra- ping. The complexity of rotational strains in fault crystalline twinning of rock-forming calcite grains. zones has limited the application of this method to Although the result is a strain tensor, a similar orien- the efforts of Gray et al. (2005). Application of the tation of the stress tensor appears likely in case of CSGT requires the following assumptions to be valid: coaxial deformation (Turner 1953, 1962). The CSGT (1) low temperatures (dominance of Type I and Type has been used to constrain strain tensor directions II twins), (2) random c-axis orientations of calcite, in veins (Kilsdonk and Wiltschko 1988; Paulsen et al. (3) homogenous strain, (4) coaxial deformation, (5) vol- 2014), limestones (Engelder 1979; Spang and Gro- ume constancy, (6) low-porosity materials, and (7) low shong 1981; Wiltschko et al. 1985; Craddock and strain (!15%). If these conditions are not fully met, van der Pluijm 1988; Mosar 1989; Ferrill 1991; Crad- the underlying data set of the calculated strain tensor dock et al. 2000), marble (Craddock et al. 1991), amyg- could be biased, modified, or random. Strain tensors daloidal basalts (Craddock and Pearson 1994; Crad- were calculated from calcite e-twin data sets with dock et al. 1997, 2004), and lamprophyres (Craddock the software package of Evans and Groshong (1994). et al. 2007). The calcite twinning-strain data are presented as Below temperatures of ca. 2007C, intracrystalline table S6. deformation of calcite results in the formation of e-twins. The formation of calcite e-twins requires a Results shear stress exceeding ca. 10 MPa (Wenk et al. 1987; Burkhard 1993; Lacombe and Laurent 1996; Ferrill Field Relations. White Mountain is composed of 1998). Calcite offers three glide systems for e-twinning. Ordovician Bighorn Dolomite and Mississippian Mad- From U-stage measurements of width, frequency, ison Limestone that were thermally metamorphosed and orientation of twins and the crystallographic into marbles before emplacement of the Heart Moun- orientation of the host crystals, a strain tensor can tain slide. This thermal metamorphism was associ- be calculated with a least squares technique (Gro- ated with the intrusion of a diorite stock at ∼49.8 Ma shong 1972). In order to remove “noise” from the (Malone et al. 2014a).Thelamprophyrediatremedike data set, a refinement of the calculated strain tensor is at the west end of White Mountain and is hosted in can be achieved by stripping the 20% of twins with the Bighorn and Madison marble. The marbles are flat Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 445 lying near the base of the allochthon, where the in table S1. The diorite and granodiorite are locally lamprophyre has intruded, but are more complexly foliated, and the diorite has a U-Pb zircon age of deformed and steeply dipping near the contact of the ∼49.9 Ma and is therefore older than the zircons in diorite stock. The marbles are crosscut by eight sub- the CUC injectites and basal layer (Malone et al. vertical CUC injectites that occur as much as 120 m 2014a). The lamprophyre (fig. 4) has a brown, apha- above the SE-dipping, 3-m-thick CUC basal layer that nitic groundmass with numerous armored lapilli and, defines the base of the Heart Mountain slide (Crad- aside from being interlayered with the clinopyro- dock et al. 2012). Andesitic, basaltic, and clastic dikes xenite, looks much like the CUC injectites but does also crosscut the marble and diorite stock. Lower- not react with HCl. The lamprophyre margin also plate rocks at White Mountain are flat lying and un- contains diorite, granodiorite, and minor andesite, metamorphosed basal Bighorn Dolomite and Cam- all of which are crosscut by the lamprophyre brec- brian Snowy Range Limestone (Craddock et al. 2000, cia. Both units are crosscut by aragonite and zeolite 2009). veins. The lamprophyre is found only above the The west end of White Mountain contains steeply base of the slide and is allochthonous (fig. 3). included volcanic and plutonic rocks that terminate Lamprophyre Description. Lamprophyres are vol- along the basal detachment. The contact between the canic peralkaline and ultrapotassic igneous rocks that igneous rocks and the marble strikes N557E, can be initiate in the subcontinental lithospheric mantle at traced for ∼3 km, and is ∼300 m high. Whole-rock 6–8-GPa pressures and temperatures of 13007C. As a compositional analyses of the igneous series, includ- result of volatile saturation, they have an enormous ing diorite (fresh, weathered, foliated), lamprophyre, pressure differential during ascent, similar to that of lamprophyre breccia, and clinopyroxenite, are shown kimberlites (70 GPa; e.g., MacGregor 1970; Mitchell

Figure 4. Field photographs of the White Mountain lamprophyre and related rocks. A, Lamprophyre (top) intruding diorite; B, foliated diorite; C, brecciated clinopyroxenite; and D, lamprophyre. A color version of this figure is available online. Figure 5. Photomicrographs of the White Mountain lamprophyre breccia. A, Foliated nature of the breccia (sample 11WY-55) from which the euhedral zircons were extracted (cathodoluminescence image, inset). B, C, Thin section of analcime (white) with inclusions of sphene (lighter inclusion in C), Cr-diopside (darker inclusions in C), and diamond (darkest areas in E) in plane light. D, Scanning electron microscopy backscatter image of C. E,Magnified view of diamond with Raman spectra spots. A color version of this figure is available online. Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 447

loss-on-ignition values (∼30%; see CUC injectite XRF data in Craddock et al. 2012), whereas the lampro-

phyre breccia is SiO2 rich (46%) and CaO rich (24%) and ultramafic in composition. The lamprophyre breccia contains olivine (forsterite), Cr-diopside, chro- mite, pyrope garnet, sphene, spinel, zircon, apatite, and secondary aragonite and analcime. Heavy-mineral separations yield significant amounts of pyrite, mag- netite, ilmenite, and hematite, plus euhedral, weakly zoned zircons. The lamprophyre breccia contains euhedral, single-phase zircons, whereas many of the zircons in the CUC are rounded and polished and found with euhedral spinel. The breccia textures are chaotic but weakly foliated, with mantled lapilli, pyroxene with apatite needles, and rolled analcime clasts with included Cr-diopside, sphene, and diamonds (fig. 5). The diamonds (30–50 mmindiam- eter) are found in green clinopyroxenite and are pink in color. Raman spectroscopy (fig. 6) identifies a 1301-nm diamond peak, confirmed by ion microprobe and in situ X-ray diffractometry (2v p 47.77 and 907). Whole-rock compositional analyses yielded results comparable to those for other North American lamprophyres (fig. 6; Cullers and Medaris 1977; Laughlin et al. 1986; LeCheminant et al. 1987; Sage 1987; Wyman and Kerrich 1989, 1993; Barrie and Shirey

Figure 6. A, Major-element oxides from the White Mountain lamprophyre (diamonds) and published North American lamprophyres (plus signs). North American lamprophyre data compiled from the GEOROC (GEOchemistry of Rocks of the Oceans and Continents) database (Cullers and Medaris 1977; Laughlin et al. 1986; LeCheminant et al. 1987; Sage 1987; Wyman and Kerrich 1989, 1993; Barrie and Shirey 1991; Fitton et al. 1991; Wyman et al. 1995; Hattori et al. 1996; Sevigny and Thériault 2003; Tappe et al. 2004, 2008). B, Laser Raman spectroscopy of diamonds present in figure 5. and Bergman 1991; Heaman et al. 2004; Skinner and Marsh 2004; Sparks et al. 2006; Wilson and Head Figure 7. Log Zr-versus-Nb (ppm) plot comparing the 2007; Cas et al. 2008). White Mountain lamprophyre to alkaline igneous rocks The lamprophyre breccia at White Mountain was in Montana. The plot shows that the White Mountain initially thought to be a CUC injectite (Malone et al. lamprophyre resembles a subtype of lamprophyre called 2014a) until thin sections, heavy-mineral analysis, a minette. Diagram after Hearn (2004), with lamprophyre and XRF data became available. The CUC injectites and minette fields from Mitchell and Bergman (1991) and are SiO2 poor (7%) and CaO rich (38%) and have large kimberlite and orangeite fields from Mitchell (1995). 448 D. H. MALONE ET AL.

1991; Fitton et al. 1991; Wyman et al. 1995; Hattori termined. Sample 11WY-55 also contained zircons et al. 1996; Sevigny and Thériault 2003; Tappe et al. separated from the lamprophyre diatreme. These same 2004, 2008). With the exception of CaO, all other ma- zircon crystals were reoccupied with laser spot anal- jor elements fit within the compositional ranges of yses for a suite of trace elements including REEs, Y, published North American lamprophyre data. The Ti, Nb, Ta, Hf, Th, and U. Figure 8 illustrates the high CaO values may be a result of a sampling bias, trace-element concentrations of lamprophyre breccia with disproportionately high clinopyroxene within the zircons and Orapa kimberlite megacryst, superim- sample. Trace elements Zr and Nb show that the posed on the compositions of a diverse suite of ig- rocks from White Mountain are similar to minettes neous zircons from Belousova et al. (2002). The trace- (fig. 7), which are associated with arc magmatism element compositions of the Orapa kimberlite zircon (Hearn 2004). Compositional and petrologic evidence megacryst clearly overlap the distinctive fields of suggests that the rocks on White Mountain are likely kimberlite zircon geochemistry defined by Belou- a diamond-bearing lamprophyre and that the rocks sova et al. (2002), typified by relatively high Hf con- likely originated from the subcontinental lithospheric tents, low REE and U contents, low Nb/Ta, and the mantle. lack of a Eu anomaly in the REE pattern. Compared Geochronology. Lamprophyre breccia–derived zir- to kimberlitic zircons, the lamprophyre breccia zircons are euhedral, without older cores, and were cons span a larger range of correlated temperatures originally dated at 48.9 5 0.5 Ma, as part of sample and Eu anomalies, indicative of magmatic differen- 11WY-55 (White Mountain injectite, as reported by tiation in the presence of feldspar. The lamprophyre Malone et al. 2014a), with LA-ICPMS techniques at breccia zircons are clearly distinct from kimberlite the University of Arizona Laserchron Center. Sam- zircon compositions, with higher REE contents, a mi- ple 11WY-55 included zircons separated from sev- nor negative Eu anomaly, and higher U contents and eral CUC injectites. As zircons for each individual Nb/Ta. The geochemical characteristics of the lam- injectite dike were sparse, the data from the set of prophyre breccia zircons do, however, overlap the dikes were compiled, and a resultant age was de- compositions of lamproite zircon from Belousova

Figure 8. Trace-element geochemistry of lamprophyre breccia zircon crystals with respect to measurements of the Orapa kimberlitic zircon megacryst, and the ranges of compositions for a diverse suite of igneous zircons from Belousova et al. (2002). The compositions of the Orapa kimberlite and White Mountain lamprophyre zircons are themselves distinct but overlap the ﬁelds of kimberlite and lamproite zircon, respectively, from Belousova et al. (2002). Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 449

lamprophyre preserve a Kmax ﬂow fabric parallel to the lamprophyre dike (N557E, 907). Additional oriented marble and vein samples were collected from White Mountain to complement the calcite strain results in Craddock et al. (2009; ﬁg. 11; table S6). Samples 2a (marble) and 5 (limestone) preserve the pre–Heart Mountain layer–parallel shortening strain of the Sevier orogeny (Craddock and van der Pluijm 1989; Craddock et al. 2000). Sample 5 remained autochthonous with the E-W shortening orientation, whereas sample 2a has been rotated about a vertical axis by the landslide. Sample 6 is a calcite vein discovered in the footwall of White Mountain (N-S, 907) that is truncated by the detachment and preserves a preslide horizontal shortening

Figure 9. U-Pb concordia and ranked-age plots of chemical abrasion–isotope dilution TIMS results for zircons from lamprophyre breccia sample 11WY-55. The 49.19 5 0.02 Ma date significantly refines the age of lamprophyre intrusion and is within the error of the 48.9 5 0.5 age for carbonate ultracataclasite zircons at Silver Gate (Malone et al. 2014a). et al. (2002), being less trace element enriched than most granitoid zircons. On the basis of cathodoluminescence imagery and LA-ICPMS results, a selection of five grains were plucked from the epoxy mounts and analyzed via CA-IDTIMS. All grains yielded concordant and equivalent isotope ratios with a weighted mean 206Pb/238U date of 49.19 5 0.02(0.04)[0.06] Ma (fig. 9). Given the consistency of this result and the simplicity of the zoning and compositions of this population of zircons, we interpret this result as estimating, within its analytical uncertainty, the emplacement age of the lamprophyre diatreme. Rock Fabrics. Anisotropy of magnetic susceptibility (AMS) is a sensitive magnetic technique used as a proxy for measuring magmatic flow. Craddock et al. (2009) report a subhorizontal flow fabric (NW- SE) in the CUC basal layer, and we report here the results of AMS measurements in four CUC injectites and the lamprophyre (fig. 10; table S5). White Mountain injectites 1, 2, and 4 (fig. 2) represent one sample each from the vertical base of the structure, and injectite 3 has results from the bottom, middle, and top spanning ∼100 m of relief. We also have one sample from the injectite at Silver Gate, 50 km to fl Figure 10. Lower-hemisphere projections of AMS (an- the northwest. There is no ow fabric in any of the isotropy of magnetic susceptibility) data (filled circles are CUC injectite AMS results except the top of White for Kmax, open circles for Kmin; large circles illustrate ori- Mountain injectite 3, where the Kmax values are all entations of the lamprophyre, injectites, or detachment). subvertical and the Kmin values are subhorizontal See figure 2 for sample locations and table S5, available and normal to the injectite. The AMS results for the online, for a summary. CUC p carbonate ultracataclasite. 450 D. H. MALONE ET AL.

Figure 11. Schematic cross section of White Mountain and calcite twinning-strain data (table S6) plotted on lower- hemisphere projections (ε1 p shortening axis; ε2 p intermediate axis; ε3 p extension axis). Sample details: 1. Madison Formation marble in contact with lamprophyre; 2a. Madison marble with pre–Heart Mountain layer–parallel shortening (N507E); 2b. vertical-shortening overprint (negative expected values); 3. calcite vein parallel to the lamprophyre margin; 4. vertical shortening in detachment-parallel vein; 5. Pre–Heart Mountain layer–parallel shortening in footwall limestones; and 6. Laramide shortening in a vein truncated by the slide. AMS p anisotropy of magnetic susceptibility); CUC p carbonate ultracataclasite. strain parallel to the vein with no strain overprint. Bighorn Dolomite (n p 169); neither material pre- Sample 1 is Madison Group marble (N557E, 557NW) serves an optic axis fabric (ﬁg. 12). in contact with the lamprophyre margin, sample 2b is the strain overprint (NEV) from the marble of sam- Discussion ple 2a, and sample 4 is a detachment-parallel calcite vein; all three record a vertical shortening strain re- Enigmatic White Mountain, with its allochthonous lated to postslide burial. Differential stress magni- marbles and its comparatively thick basal layer (2 m tudes are remarkably consistent (235 MPa) for twin- of CUC), now reveals additional information re- ning events that were tectonic or related to postslide garding the initiation and dynamics of the Eocene burial. Electron backscatter diffractometry was mea- Heart Mountain slide. Detailed ﬁeld mapping has sured on the basal CUC (n p 183) in contact with documented the eight CUC injectites, which be- Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 451

Figure 12. Electron backscatter diffraction pole results for the CUC (carbonate ultracataclasite; top) and Bighorn Dolomite (bottom) contact.

come listric (and/or join) with the basal CUC, and thereby also genetically and dynamically related. the presence of older intrusions (diorite, granodio- We suggest that the emplacement of the lampro- rite; 49.9 5 0.5 Ma) and the younger lamprophyre phyre diatreme was the triggering mechanism for the suite (breccia, clinopyroxenite, lamprophyre). Zir- catastrophic Heart Mountain slide and that White cons in the lamprophyre breccia and CUC (injec- Mountain represents a remnant of the eruption center. tites and basal layer; Malone et al. 2014a)arethe Eruption and Landslide Dynamics. Figures 13 and same age (49.19 Ma 5 0.02 Ma TIMS age) and are 14 portray the interpreted sequence of events at 452 D. H. MALONE ET AL.

White Mountain. The diorite-granodiorite intruded at 49.9 5 0.5 Ma and thermally metamorphosed the host carbonate rocks. The lamprophyre diatreme intrusion at 49.19 5 0.02 Ma is preserved as a dike oriented at N557E, 907. The lamprophyre dike pinches out to the northeast, suggesting that the west end of the dike was connected to the main diatreme body. The lamprophyre diatreme intrusion, which triggered the emplacement of the Heart Mountain slide, included generation of the CUC fault gouge and injection (!120 m) of CUC into the upper plate. The AMS proxy for magmatic flow preserves a southeastward, subhorizontal flow in the CUC basal layer (Craddock et al. 2009) and chaotic flow in the injectites, except at the top of injectite 3. The lamprophyre AMS results suggest intrusion to the north-

east (N557E), as the Kmax directions are subhorizontal and parallel to the intrusion. This AMS ﬂow proxy is also parallel to the pre–Heart Mountain slide layer– parallel shortening strain preserved in the host Mis- sissippian Madison limestones, which were oriented N907E (Craddock et al. 2000) before the landslide. This means that the lamprophyre erupted in an E-W direction and then slid down the bedding-plane portion of the slide to its resting place at White Moun- tain (N557E), which requires a horizontal rotation about a vertical axis and a minimum of 357 of counterclockwise motion. Alternatively, the White Moun- tain allochthon could have rotated to N557E early in its motion and slid into place heading to the southeast, as the detachment striations and AMS fabric

(Kmax) at White Mountain trend S407E. Gentle N-S folds are present in the marble of western White Mountain. The marbles are termi- nated against the lamprophyre suite and preserve a vertical shortening strain. The lamprophyre-margin sheared calcite vein preserves a horizontal shortening strain parallel to the lamprophyre dike and no strain overprint; this is presumably deformation related to shearing along the lamprophyre margin during emplacement (ﬁg. 11). Preslide calcite veins preserve a Laramide shortening strain (Craddock and van der Pluijm 1989) and no strain overprint. The marbles away from the lamprophyre preserve a vertical Figure 13. Schematic sequence of events at White Moun- shortening-strain overprint (NEVs), and the coeval tain (modiﬁed from Malone et al. 2014a; not to scale). Before vein preserves vertical shortening; these are both in- the initiation of the Heart Mountain slide, a diorite stock terpreted to be related to postslide burial by younger intruded a succession of Paleozoic carbonate and Eocene vol- Eocene volcanic rocks. ∼ canic rocks at 49.9 Ma. The emplacement of a lampro- The lower-plate (subsurface) location of the lam- phyre intrusion at ∼49.19 Ma caused failure and collapse of prophyre diatreme is unknown and presumed to be the upper plate. This slab was catastrophically emplaced to the south and east into the adjacent Bighorn and Absaroka west or northwest of White Mountain. The lampro- Basins. The basal layer carbonate ultracataclasite (CUC) phyre and CUC along the base of the slide and as- contains cataclastic carbonate material derived from the sociated injectites were part of the same intrusion- upper plate, euhedral and abraded zircons from the lam- landslide collapse event, as their zircons are the prophyre and other intrusions, and delicate volcanic glass. same age. Journal of Geology VOLCANIC INITIATION OF THE HEART MOUNTAIN SLIDE 453

Figure 14. Map view schematic of the sequence of events at White Mountain. These diagrams are not to scale; the area shown is 5–10 km2.At∼50 Ma, trachyandesite lavas and other volcanic rocks were deposited on Paleozoic carbonate rocks. The carbonate rocks preserve the Sevier Laramide E-W layer parallel strain. A diorite stock was emplaced at ∼49.9 Ma. At ∼49.19 Ma, during eruption, a lamprophyre diatreme intruded the diorite stock and host carbonate rock, with a Kmax ﬂow direction parallel to the to the Sevier-Laramide E-W layer parallel strain. The eruption caused the upper plate to fail, and the allochthonous rocks at White Mountain were transported to the southeast while rotating counterclockwise about a vertical axis of rotation, before coming to rest at their current location. LPS p layer-parallel shortening.

Regional Field Relations. Heart Mountain and (Humphreys 1995; Smith et al. 2014) and is an anom- McCullough Peaks are the most obvious and distal alous thick, horizontal cap in northwest Wyoming, upper-plate slide blocks of Madison Group lime- especially when compared to the variety of other al- stones in the Bighorn Basin. It is now apparent that kaline Eocene intrusions in the area (Sundell 1993; the vertical volcaniclastic rocks atop Squaw Peaks Feeley and Cosca 2003) in the southern Bighorn Basin are also part of the CO2 Generated by the Heart Mountain Slide. The Heart Mountain slide run-out, along with Eocene emplacement of the Heart Mountain slide has been volcanic rocks in the upper South Fork Shoshone proposed to be the result of a reduction in friction River Valley and Carter Mountain (Malone 1995, along the detachment, perhaps accommodated by

1996, 1997; Malone et al. 2014b, 2014c). The Ab- CO and CO2 generated by limestone-on-limestone saroka volcanic ﬁeld covers an area of ∼8,000 km2 sintering (Han et al. 2007; Mitchell et al. 2015) and with a composite thickness of ∼5 km extruded be- the introduction of mantle CO and CO2 when the tween 55 and 38 Ma (Sundell 1993). This extensive lamprophyre erupted. We have calculated the amount maﬁc-intermediate outpouring may in some way of CO2 generated by the Heart Mountain slide to be be related to Eocene rollback of the Farallon Plate 7 billion kilograms (0.07 gigatons), which is a mi- 454 D. H. MALONE ET AL.

2 nuscule amount of CO2 when compared to the an- of the slide (∼5000 km ) to include the Eocene vol- nual human (29 gigatons) and oceanic (332 gigatons) caniclastic rocks at Squaw Peaks and Hominy Peak

CO2 emissions. The observation and interpretation (100 km to the west). The location of the lampro- that a lamprophyre eruption occurred and triggered phyre root remains unknown but is presumed to be the event provides an answer to the question, what upgradient to the west of White Mountain and is caused this slab of rock to move to begin with? Once part of the regional grouping of Cretaceous-Eocene the emplacement was triggered, the various mech- alkaline and ultramafic intrusions that intruded as anisms suggested by Beutner and Gerbi (2005), Aha- the Farallon slab rolled back to the southwest be- ronov and Anders (2006), Craddock et al. (2009), and tween 50 and 45 Ma. Anders et al. (2010) that aided continued movement It is interesting that we may have come full circle may have operated. with respect to an explanation for the initiation of the Heart Mountain slide. Bucher (1933) was the first to suggest that Heart Mountain was propelled to the Conclusions east by a gigantic volcanic explosion. His explanation was Our understanding of the Heart Mountain slide has been refined during the past decades. There are now The writer thinks it possible that the limestone-plates data for (1) the structure and stratigraphy of the rocks which constitute the thrust-masses of this region were involved (Pierce 1973; Malone 1995, 1996, 1997), thrust eastward and scattered much as they are today by (2) emplacement duration (Craddock et al. 2000, 2009), the horizontal component of the force of a large volcanic (3) emplacement timing (Malone et al. 2014a;this explosion . . . volcanic gases, perhaps largely pent-up steam, report), (4) areal extent (Malone et al. 2014b,2014c), exploded in such a way as to shear off large sheets of limestone from the highly micaceous Cambrian shales (5) the relationship between the South Fork and Heart and to drive them eastward (or, perhaps better, south- Mountain slide masses (Craddock et al. 2015), (6) the eastward) down the pediment slope into the plain. characteristics of the CUC (Beutner and Craven 1996; Beutner and Gerbi 2005, Craddock et al. 2009, 2012; Anders et al. 2010), and (7) the regional paleogeography at the time of collapse (Rhodes et al. 2007; ACKNOWLEDGMENTS Malone et al. 2014b, 2014c; Craddock et al. 2015). White Mountain, with its allochthonous marble, This project was funded by the Keck Geology Con- has always been an anomaly within the chaos of the sortium (2010–2012). J. Geary was also supported by Heart Mountain slide. We argue that the marble is the Minnesota Space Grant consortium. Field as- the result of contact metamorphism adjacent to an sistance from J. Trela, J. Calhoun, A. Hinds, J. Geary, older diorite intrusive suite and that the younger K. Schroeder, J. Bardwell, K. Kravitz, R. McGoughlin, lamprophyre was the triggering mechanism for the M. Mathisen, A. MacNamee, G. and J. Miller, and volcanically induced landslide at ∼49.19 Ma. Zir- L. Seiple is greatly appreciated. Diamond X-ray and cons in the lamprophyre breccia and resultant CUC microprobe analyses were done at the University of (basal layer and injectites) are the same age and Minnesota. M. Jackson and his colleagues at the In- thereby genetically related. A supersonic lampro- stitute for Rock Magnetism, University of Minne- phyre eruption helps explain earlier computations sota, were very helpful. A. Wulff provided the pho- about the mechanism and rapid initiation speed tomicrographs of the diamond-bearing thin section. (100 m/s) of the landslide, which also makes plau- The reviews of B. Wernicke and an anonymous re- sible the increase in the chaotic, rotational run-out viewer significantly improved this article.

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Peter S. Dahl* Daniel K. Holm Department of Geology, Kent State University, Kent, Ohio 44242 Edward T. Gardner} Fritz A. Hubacher Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210 Kenneth A. Foland}

ABSTRACT Collectively, these data suggest that the sociated with the Hearne-Superior and Hearne and Wyoming provinces were once sep- Wyoming-Superior collision(s) is required to A question regarding the 1900–1600 Ma arate continents that were ultimately welded to answer this question. Synchroneity of these col- assembly of Laurentia is whether the Wyo- the Superior province (and to each other) dur- lisions would imply that a discrete Hearne- ming province (west-central United States) ing distinct Early Proterozoic orogenies. Re- Wyoming continent existed prior to Laurentia. was part of the Hearne province (west-cen- gional relationships further suggest that final However, diachroneity of collision would sug- tral Canada) prior to the Early Proterozoic docking of the eastern Wyoming province with gest (1) that docking of this presumed continent Trans-Hudson orogeny, or a separate entity Laurentia began during the ca. 1780–1740 Ma was time transgressive from north to south, or welded later to a Hearne-Superior continent interval of island-arc accretion along the south- (2) that the Hearne and Wyoming provinces (central Canada). New 40Ar/39Ar mica dates ern margin of the growing craton. were both discrete entities, which docked sepa- help to address this question by extending a rately with the Superior province during distinct Middle Proterozoic geochronologic front, INTRODUCTION orogenies. long established along the southern Wyoming The Early Proterozoic Trans-Hudson orogen province, into the Black Hills of South Research in Precambrian tectonics during extends westward beneath Hudson Bay into the Dakota. This suggests that previously unex- the past decade has led to the important realiza- northern Saskatchewan-Manitoba border region, plained, north-directed fold nappes (F1) in tion that episodic supercontinent assembly and then curves southward beneath Phanerozoic cover the Black Hills resulted from island-arc ac- breakup might reflect large-scale mantle dy- (Green et al., 1979, 1985; Lewry and Collerson, cretion to the south ca. 1780 Ma. North- namics. For example, it has been postulated 1990; see Fig. 1). Previous geochronologic stud-

northwest–trending upright F2 folds, which that the rapid aggregation of Archean and juve- ies within the well-exposed west-to-south bend of formed during east-west collision of the nile crust between 1900 and 1600 Ma, which the orogen (the Reindeer zone) have shown that Wyoming and Superior provinces, must resulted in Laurentia (Hoffman, 1988; Fig. 1), the Hearne and Superior provinces collided with therefore be younger. New 40Ar/39Ar horn- may have occurred above, and been caused by, an intervening Archean block (and overlying ju- blende dates, recently published age data, large-scale mantle downwelling (Gurnis, 1988; venile crust) during separate thermotectonic and crustal heat-flow considerations further Hoffman, 1989). Although appealing, notions events. These events have been dated as ca. suggest that this collision began at or before of large-scale downwellings and resultant su- 1860–1845 Ma in the northwestern, outer bend, ca. 1770 Ma and culminated with posttectonic percontinent cycles are critically dependent on and as ca. 1825-1790 Ma in the southeastern, in- magmatism beginning ca. 1715 Ma (the Har- the recognition and precise dating of ancient ner bend (U/Pb zircon, monazite, titanite; e.g., ney Peak granite). This tectonic-magmatic in- orogenic belts in the interiors of now-stable Bickford et al., 1990; Machado, 1990; Ansdell terval is ~50–60 m.y. younger than that re- cratons. et al., 1995). Postcollisional granite and/or peg- ported for the Hearne-Superior collision An important question regarding the Early matite magmas were emplaced in the Reindeer (Trans-Hudson orogeny in Canada). Compa- Proterozoic assembly of Laurentia is whether zone as early as ca. 1780–1770 Ma (U/Pb zircon: rably young metamorphic dates (1810–1710 the Archean Wyoming province (west-central Van Schmus et al., 1987; Bickford et al., 1990; see Ma) also typify the eastern and northern United States) was welded to the Hearne Fig. 1, northernmost black dot) during a pro- Wyoming province periphery (western Dako- province (west-central Canada) prior to the su- tracted interval of intracontinental shortening that tas and southwestern Montana). percontinent Laurentia, or was a separate mi- ended ca. 1690 Ma (e.g., Hajnal et al., 1996). crocontinent rafted in during its assembly. A The buried collision zone between the *E-mail: [email protected]. precise age comparison of thermotectonism as- Wyoming and Superior provinces extends from

Data Repository item 9975 contains additional material related to this article.

GSA Bulletin; September 1999; v. 111; no. 9; p. 1335–1349; 8 figures; 1 table.

1335 DAHL ET AL. southeasternmost Saskatchewan to northwestern Nebraska (Figs. 1 and 2 [right inset]). This buried zone is widely interpreted (from geophysical data) as the southern lithotectonic extension of the Trans-Hudson orogen (Green et al., 1979, 1985; Sims and Peterman, 1986; Van Schmus et al., 1987; Klasner and King, 1990; Houston, 1993; Baird et al., 1996). Metamorphic rocks obtained from boreholes in the Wyoming-Superior collision zone have yielded mostly ca. 1810– 1710 Ma dates for isotopically retentive minerals (U/Pb zircon and Rb/Sr whole rock; compilation of Sims et al., 1991), but the geological significance of these dates regarding the timing of Wyoming-Superior convergence is unclear. The ca. 1810–1710 Ma dates suggest a younger Wyoming-Superior collision compared to the Trans-Hudson (ca. 1860–1845 Ma and ca. 1825– 1790 Ma) collisions well documented in Canada and cited here. However, regional maps of gravity and magnetic anomalies suggest that the southernmost Wyoming-Superior collision zone was truncated by, and is older than, the ca. 1790–1630 Ma collage of southward-younging island-arcs terranes composing the Central Plains orogen (U/Pb zircon; Sims and Peterman, 1986; Bickford et al., 1986; Sims et al., 1991). This apparent truncation is depicted in Figures 1 and 2 (right inset). Other regional geophysical data may bear on these apparently contradictory indi- cations for the timing of Wyoming-Superior collision. It has been proposed that a prominent conductivity trend, the North American Central Figure 1. Precambrian tectonic elements of central portion of North American craton Plains anomaly (Fig. 1), may delineate loci of (modified after Hoffman, 1989). Inset shows location within North America. Abbreviations: Proterozoic collisional suturing (Camfield and BH—Black Hills outlier of Archean Wyoming province (and location of Fig. 2); GFtz—Great Gough, 1977; Dutch and Nielsen, 1990). Arc- Falls tectonic zone; KR—Keweenawan rift; N—approximate trace of North American Cen- continent sutures coincident with the anomaly tral Plains conductivity anomaly. Other dated localities exposed within the Wyoming province have been dated as ca. 1860–1845 Ma in northern periphery include CFB—Cheyenne fold belt (and environs), southeastern Wyoming; Saskatchewan (Trans-Hudson orogen; Bickford SWM—block uplifts of southwestern Montana; and LR—Little Rocky Mountains, north- et al., 1990) and ca. 1780–1740 Ma in southern central Montana. Foliation symbols indicate orientations of dated compressional fabrics, ex- Wyoming (Medicine Bow orogen, Cheyenne belt; cept for the LR, where the symbol is interpretative only. See text for discussion. The designa- Premo and Van Schmus, 1989; Chamberlain, tion Central Plains juvenile crust includes not only the ca. 1790–1630 Ma Central Plains 1998). However, the diachroneity of collisional orogen (Sims and Peterman, 1986), but also the newly recognized ca. 1780–1740 Ma Medicine sutures coincident with the northern and southern Bow orogen (Chamberlain, 1998); the latter is exposed within a restricted area immediately Central Plains Anomaly termini obscures the south of the CFB and may extend northeastward, south of the Black Hills. time significance (if any) of the anomaly elsewhere along its length; e.g., in the Wyoming- Superior collision zone of the western Dakotas beled BH in Fig. 1). The crystalline core of the ming province and the presumed Dakota block (Fig. 1). In summary, the timing of Wyoming- Black Hills exposes part of a northerly trending (COCORP transect; Baird et al., 1996). Previous Superior collision relative to the Trans-Hudson deformed belt of Early Proterozoic continental geochronologic work in the Black Hills deformed- collisions in Canada, and the nature of a crucial margin rocks (Hoffman, 1989). The Central Plains belt exposures (see geologic map, Fig. 2) has con- orogenic junction in the north-central United anomaly approximates the boundary of this de- clusively bracketed the main collisional thermo- States, remain obscured beneath Phanerozoic formed belt with an apparent crustal fragment tectonism within a ca. 1880–1715 Ma interval cover, hence the designation “Trans-Hudson?” in caught up within the Wyoming-Superior conver- (U/Pb zircon and monazite; Redden et al., 1990). Figure 1 (following Hoffman, 1989). gent zone (i.e., the Dakota block inferred by Baird However, the breadth of this interval still leaves The only surface rocks conceivably affected by et al., 1996, Fig. 3). Deep-crustal seismic reflec- unanswered this question: was it Trans-Hudson the Trans-Hudson orogeny in the United States are tions imply that the relatively narrow deformed orogeny, or younger Wyoming-Superior collision, in a Laramide-style uplift in the Black Hills, South belt (including the Black Hills, Fig. 1) comprises that affected the Black Hills and environs? Dakota, which is located in the westernmost an Early Proterozoic tectonic wedge that formed Thus, current geophysical and geochrono- Wyoming-Superior convergent zone (black dot la- above a west-dipping suture between the Wyo- logic data for the Hearne-Superior and Wyo-

1336 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA

ming-Superior collision zones permit multiple hypotheses regarding the details of the assembly history of western Laurentia. The hypotheses include: (1) the Trans-Hudson orogeny was synchronous, time transgressive, or progressively overprinted from northern Saskatchewan- Manitoba to western South Dakota; or (2) these two localities were affected by distinct Early Proterozoic orogenies (widely) separated in time. Discerning among these possibilities requires a more precise constraint for the timing of Wyoming-Superior collisional orogeny, such as previously established for the Trans-Hudson and Medicine Bow orogenies in northern Manitoba- Saskatchewan and southeastern Wyoming, respectively. The Black Hills uplift offers a prime opportunity to accomplish this goal, given its excellent exposures of multiply deformed rocks and strategic location near the enigmatic junction of major Early Proterozoic orogens (Fig. 1). This paper presents new 40Ar/39Ar incremental-release data for 17 mostly fabric-defining amphiboles and micas in amphibolites and metapelites from the northern and west-central Black Hills (see Fig. 2 for sample locations). Integrating these data with published geologic and geochronologic data for the Black Hills and environs indicates that thermotectonism in this part of the Wyoming-Superior collision zone began before or ca. 1770–1780 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonothermal interval is at least ~50–60 m.y. younger than that associated with terminal Trans-Hudson orogeny in northern Saskatchewan-Manitoba. Considering this age difference, and published reports of ca. 1800 Ma (collisional?) thermotectonism along the northwestern Wyoming province margin (e.g., O’Neill et al., 1988; Erslev and Sutter, 1990), the new 40Ar/39Ar results presented herein are consistent with a discrete Wyoming microcontinent having docked with Laurentia during Early Proterozoic time.

BLACK HILLS—EARLY PROTEROZOIC GEOLOGIC SETTING AND PREVIOUS GEOCHRONOLOGY

Figure 2. Generalized geologic map of Early Proterozoic crystalline core of the Black Hills de- The Precambrian crystalline core of the Black lineating north-northwest–trending folds (F2 and refolded F1) and showing sample locations for Hills uplift (Fig. 2) exposes small epicratonic rift hornblende (H), muscovite (M), and biotite (B) dated as part of this study (see Table 1 for age basins floored by Archean basement of the eastern data). LEG and BMG indicate Little Elk and Bear Mountain granite gneiss, respectively, which Wyoming province and infilled with Early Prot- represent exposed outliers of Archean Wyoming province basement. Geology is based on DeWitt erozoic clastic sedimentary rocks and mafic ig- et al. (1989). Right inset locates Laramide Black Hills uplift and depicts structural grain of its neous rocks (Gosselin et al., 1988). Windows of Early Proterozoic crystalline core; left inset locates metamorphic isograds within crystalline this basement in the Black Hills are evident as in- core. Inset abbreviations: STA—staurolite; GAR—garnet; BIO—biotite; KY—kyanite; liers of gneissic granite near Nemo and west of SILL—sillimanite; KSP—K feldspar; WC and SC—Wyoming and Superior cratons; Hill City (the Little Elk granite and the granite at THO?—Trans-Hudson(?) orogen; CPO—Central Plains (and older Medicine Bow) orogens, Bear Mountain, respectively). The oldest Early undifferentiated; MT—Montana; WY—Wyoming; NE—Nebraska; ND—North Dakota; and Proterozoic supracrustal rocks in the Black Hills SD—South Dakota. (ca. 2500–2170 Ma) are exposed only in Nemo

Geological Society of America Bulletin, September 1999 1337 DAHL ET AL.

and environs (Fig. 2), and preserve folds related to designated here as S2, because an earlier fabric be directed outside the ~5–10-km-wide hydrother- cryptic tectonism between ca. 2170 Ma and ca. (denoted here as S1) is preserved as oblique in- mal envelope surrounding the main granite pluton, 1970 Ma (Redden et al., 1990). Nonconformably clusion trails of fine-grained quartz in regional- where exposed regional-metamorphic rocks are overlying these deformed rocks are younger Early metamorphic garnets. However, the apparent likely to be somewhat less overprinted.

Proterozoic supracrustal rocks, which were de- lack of metamorphism associated with F1 folding 40 /39 posited between ca. 1970 Ma and before 1880 Ma implies that S1 was imposed during the early RESULTS— Ar Ar and which dominate the Black Hills basin(s). The stages of F2. The absolute timing of F1 and F2 THERMOCHRONOLOGY OF THE Archean basement and Early Proterozoic supra- folding is bracketed by an 1883 ± 5 Ma extrusion NORTHERN AND WEST-CENTRAL crustal rocks were tectonically buried, intensely age (U/Pb zircon) obtained from the youngest BLACK HILLS deformed, and regionally metamorphosed during basinal rift basalt exhibiting both F1 and F2 folds major crustal shortening between ca. 1880 Ma and by ca. 1715 ± 3 Ma intrusion ages (concor- Samples and ca. 1715 Ma. In addition, these rocks were dant U/Pb and Pb/Pb monazite) preserved in the thermally metamorphosed during intrusion of the oldest dikes of the Harney Peak granite (late syn- Minerals were selected for 40Ar/39Ar dating

Harney Peak granite (Fig. 2) beginning ca. 1715 ± F2; Redden et al., 1990). In the northern and cen- from a total of 17 metamorphic rocks collected 3 Ma (U/Pb ages of igneous zircon and monazite; tral Black Hills, F1 and F2 are locally refolded by over the northern two-thirds of the Black Hills Redden et al., 1990). The entire Precambrian cross-folds (F3) with steep, nonpenetrative folia- (Fig. 2, Table 1): they include 6 hornblendes, 2 complex was unroofed by Cambrian time and was tion (S3) trending more westerly than S2; both the muscovites, and 9 biotites. These phases were nonconformably overlain by Phanerozoic clastic age and cause of this deformation (designated separated from: (1) drill core supplied by the sedimentary rocks; however, the complex was ul- here as D3) are uncertain. However, D3 deforma- Homestake Mining Company (amphibolites CM- timately reexposed during Laramide doming tion is probably older than the doming associated 2242 and HM-1652; micaceous selvages associ- (Lisenbee and DeWitt, 1993). with granite emplacement (Redden et al., 1990). ated with gold-bearing quartz veins, HM-1007 East-west closure of the Black Hills basin(s) is Terminal closure of the Black Hills basin(s) and HM-32); (2) exposures sampled near Tinton, associated with terminal collision of the Wyoming culminated in structural unloading followed by Central City, Lead, and Hill City (amphibolites and Superior provinces, and was commonly re- intrusion of Harney Peak granite into the mid- T93-6, CC93-1, WP-3, and BH-3); and (3) expo- ported in older literature as having occurred ca. crust. Extensive low-pressure (P) (regional con- sures sampled along an ~10 km traverse near 1840 Ma (i.e., during the Trans-Hudson orogeny). tact) metamorphism and localized doming of Nemo (granitic gneisses and metapelites of the This estimate originated from a reset or disturbed country rocks accompanied granite emplacement Little Elk granite [LEG] and Nemo [NM] series; “isochron” date obtained from the Little Elk gran- (Redden et al., 1990; Holm et al., 1997). Doming n = 9). The two dated amphiboles analyzed thus ite (whole-rock Rb/Sr; Zartman and Stern, 1967), (designated here as D4) produced local folds and far by electron microprobe (T93-6 and CM-2242) coupled with the subparallelism of its gneissosity schistosity (F4 and S4) conforming to the granite range in composition from magnesio-hornblende, with regional north-northwest–trending structural contact, and warped older S2 fabric into paral- [(K0.02Na0.27)(Ca1.86Na0.02Fe1.43Mg2.89Al0.71Ti0.07 trends (see Fig. 2). However, reset whole-rock lelism with S4 (Redden et al., 1990). Following Mn0.02) (Al1.14Si6.86)O22.0 (OH)2.0] to ferro- Rb/Sr “isochrons” are commonly suspect because intrusion and cooling to ambient mid-crustal (tschermakitic-) hornblende [(K0.05Na0.39)(Ca1.82- of limits to Sr rehomogenization beyond whole- temperatures, the country rocks remained ther- Fe2.65Mg1.47Al0.99Ti0.05Mn0.04)(Al1.56Si6.44)O22.0 rock scale (Lanphere et al., 1964; Heaman et al., motectonically quiescent for ~200 m.y. before fi- (OH)2.0], respectively. Most dated amphiboles 1986). Moreover, our recalculation of the 1840 Ma nally cooling through Ar closure temperatures of and micas exhibit preferred orientation and ap- 87 date (using the Rb decay constant of Steiger and micas ca. 1500–1300 Ma, possibly during slow pear to comprise the S2 (i.e., syncollisional) fab- Jäger, 1977) yields an 1800 ± 70 Ma estimate, uplift (Holm et al., 1997). A post-granite, north- ric. Vein micas, however, are randomly oriented which, although now consistent with the indepen- east-trending fabric (denoted here as S5) is par- and thus late-S2 to post-S2 in origin. The 60–80 dent 1880–1715 Ma thermotectonic interval indi- ticularly evident across the central Black Hills; mesh fractions of all amphiboles and micas were cated here, offers virtually no refinement of colli- this fabric was imposed by a cryptic deformation separated and purified using standard magnetic- sional age within that interval. (denoted here as D5) provisionally considered to separation and hand- picking techniques. The main 1880–1715 Ma structural elements be ca. 1535 Ma in age (Redden et al., 1990). in the Black Hills crystalline core are east-north- Regional metamorphism accompanying F2 Analytical Methodology east–trending fold nappes (F1) that were subse- folding attained garnet-grade conditions in mid- quently refolded into north-northwest–trending crustal rocks (Norton and Redden, 1990; tempera- The 40Ar/39Ar measurements were performed upright folds (F2) accompanied by a steep, lo- ture, T = 400–500 °C; Friberg et al., 1996), al- in the Radiogenic Isotopes Laboratory at Ohio cally penetrative, axial-planar foliation (DeWitt though deeper crustal rocks now exposed in the State University using normal procedures that et al., 1989; Redden et al., 1990). The F1 folding domal areas record locally higher metamorphic have been described previously (Foland et al., occurred during a north-directed, thin-skinned P-T conditions (Terry and Friberg, 1990). The pre- 1984, 1993). Each sample was analyzed by in- thrusting event (D1) of uncertain origin—but cise timing of F2 folding—and thus of Wyoming- cremental heating to successively higher temper- with no metamorphism or foliation evident, even Superior continental collision—has proven very atures in ≥12 steps. Some were analyzed in du- at F1 fold hinges—whereas F2 folding and re- difficult to establish. This difficulty stems from plicate or by total-fusion analysis. Aliquots of gional metamorphism accompanied tectonic bur- the widespread thermal and mineralogical over- about 6–10 mg for micas and 80–200 (or 3–5) ial associated with the east-west D2 collision printing of older assemblages caused by intrusion mg for hornblendes were irradiated in the Ford (Redden et al., 1990; Redden and DeWitt, 1996). of the Harney Peak granite and/or pegmatite com- Nuclear Reactor of the Phoenix Memorial Labo-

Large-scale F2 synforms and antiforms are delin- plex and related aqueous fluids (T = 480–650 °C, ratory at the University of Michigan. The irradi- eated by metamorphosed iron formations and P = 3–4 kbar; Helms and Labotka, 1991; Friberg ated aliquots were heated by resistance heating in muscovite-rich schists in the northern two-thirds et al., 1996). Consequently, precise dating of the a high-vacuum, low-blank, double-vacuum fur- of the area (Fig. 2). The axial planar foliation is collision requires that thermochronologic efforts nace to successively higher temperatures for a pe-

1338 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA

TABLE 1. SUMMARY OF NEW 40Ar/39Ar RESULTS FOR SAMPLES FROM THE NORTHERN BLACK HILLS Total gas Plateau Sample Mineral K* K/Ca K/Cl Age† Temp§39Ar# K/Ca K/Cl Age† Number (%) (Ma) (°C) (%) (Ma)** CM-2242 Hornblende 0.11 0.0158 34.3 1959 1075–1150 55 0.0161 43.3 1787 ± 8 [±19] WP-3 Hornblende 0.11 0.0250 27.7 1037 WP-3 Hornblende 0.22 0.0247 36.9 1134 WP-3 Hornblende 0.0235 35.4 1154 WP-3 Hornblende 0.0239 34.1 1069 T93-6 Hornblende 0.18 0.0284 11.3 1736 1100–1300 86 0.0275 12.2 1736 ± 13 [±21] T93-6 Hornblende 0.19 0.0340 14.8 1665 1000–1350 74 0.0304 15.6 1714 ± 10 [±20] T93-6 Hornblende 0.0352 16.3 1567 1221–1550 22 0.0299 16.2 1742 ± 5 [±18] T93-6 Hornblende 0.0340 15.2 1679 1051–1551 79 0.0297 15.8 1742 ± 6 [±18] CC93-1 Hornblende 0.15 0.0238 67.8 1563 HM-1652 Hornblende 0.12 0.0271 221 2187 HM-1007 Muscovite 5.5 24.3 1570 1226 1025–1350 91 27.7 289 1221 ± 9 [±19] HM-32 Biotite 6.5 11.4 >9900 1301 725–1350 98 11.5 >9900 1304 ± 4 [±14] LEG-1 Biotite 6.3 22.4 186 1323 800–1250 90 35.8 223 1341 ± 4 [±14] LEG-8 Biotite 6.9 14.8 196 1307 800–1350 97 16.1 199 1307 ± 7 [±14] LEG-8b Biotite 6.6 34.8 132 1123 LEG-10 Biotite 5.7 25.7 508 1396 875–1350 88 2.30 157 1396 ± 5 [±15] LEG-15 Biotite 4.8 8.91 165 1169 775–1275 76 14.4 171 1286 ± 7 [±15] LEG-23 Biotite 4.8 29.7 233 1339 LEG-25 Biotite 4.7 7.82 640 1422 800–1350 86 8.28 763 1457 ± 7 [±16] NM-3 Biotite 4.8 31.1 231 1159 800–1350 67 28.4 254 1308 ± 8 [±15] NM-1 Muscovite 2.3 287 540 1351 800–1000 85 45.6 >9900 1361 ± 13 [±18] BH-3 Hornblende 0.22 0.0273 17.2 1692 1125–1400 80 0.0269 17.8 1691 ± 5 [±17] Note: Sample localities are shown on Figure 1. Samples are listed above by location, north to south. *Weight percent K in sample calculated from 39Ar yield and sample weight. †Age calculated using the constants in Steiger and Jäger (1977). §Temperature range of increments included in the plateau. #Percent of total 39Ar included in plateau. **Uncertainties at the one sigma level. The uncertainties in brackets provide for a 1% systematic uncertainty in the age of the neutron-fluence monitor. riod of ~30 min at each temperature. The results tirely attributable to sample heterogeneities. With not give reasonable plateaus but rather highly dis- of the incremental-heating measurements are the exception of two samples, the micas yield cordant, “staircase” spectra (no plateau) and total- summarized in Table 1, which gives information plateaus or near plateaus; most of the plateau seg- gas dates of 1123 and 1339 Ma. Three biotites that on the K, Ca, and Cl contents and age for the to- ments are broad (>~85% of the 39Ar), with discor- are among the micas yielding the best developed tal (or integrated) gas and for the plateau (if ob- dance for most samples seen for small, low-tem- plateaus (HM-32, NM-3, and LEG-8; see Fig. 3) served) fractions. An overall systematic uncer- perature fractions. Only half of the hornblendes yield analytically indistinguishable plateau dates tainty of ±1% is assigned to J values to reflect yield plateaus and these are less well defined; of 1304 ± 4 Ma to 1308 ± 8 Ma. Biotite LEG-15 uncertainty in the absolute age of the monitor. where plateaus exist, the main discordance occurs gives a slightly younger plateau (1286 ± 7 Ma) Typically, this uncertainty is not included when for lower temperature fractions. Most, but not all, that is not well developed, and its more discordant age uncertainties are quoted here, in order to em- of the age discordance is accompanied by con- spectrum is probably not meaningfully different phasize the level of apparent age dispersion comitant variations in the K/Ca and K/Cl ratios from this group. Only one plateau date is dis- among plateau fractions in terms of internal con- and thus reflects mineralogical heterogeneities. tinctly younger (HM-1007, 1221 Ma), whereas cordance and to compare plateaus among sam- The lower temperature discordance observed for four are clearly older. All eight biotites from a re- ples using a common monitor. However, it is ap- micas is characterized by lower K/Ca and K/Cl ra- stricted area north of Nemo (the LEG series and propriate to use an uncertainty that includes the tios compared to the higher temperature fractions. NM-3) show spectra with distinct, albeit very mi- total J uncertainty (see Table 1) when comparing For hornblendes, the lower temperature discor- nor in some cases, discordance with lower age, the 40Ar/39Ar ages to other isotopic ages. The dance is generally characterized by significantly low-temperature increments. These discordant complete listing of 40Ar/39Ar analytical data and higher K/Ca values than the bulk of the sample. fractions are typically small (<1% or a few per- procedural information is available from the This suggests the presence of an alteration or im- cent of the total 39Ar) but are appreciable GSA Data Repository.1 purity phase of higher K content in the hornblende. (7%–9%) for LEG-15 and NM-3. The apparent Incremental-heating 40Ar/39Ar age results are il- ages of these fractions, sometimes having large lustrated in normal age spectra diagrams in Fig- Muscovites and Biotites uncertainties, range from about 250 to 700 Ma ures 3 and 4. The age spectra range from highly and are generally characterized by low K/Ca and concordant to highly discordant. Replicate mea- The S2 micas in the Little Elk granite and K/Cl ratios. The correlation of age and chemical surements on the WP-3 and T93-6 hornblendes nearby Nemo schists (LEG and NM series, n = 9; parameters indicates the presence of mineralogi- show very good agreement (Fig. 4), and the rela- Nemo area, Fig. 2), and random micas in quartz cal heterogeneities occurring as intergrown tively minor differences of replicate runs are en- veins deep in the Homestake Mine (HM series, n phases or alteration or impurities. The cause of the = 2; Lead, Fig. 2), all yield total-gas (or integrated spectral discordance is not obvious; it could re- gas) dates ranging from 1123 to 1422 Ma. Nine flect real age differences or 39Ar recoil redistribu- 1 GSA Data Repository item 9975, supplemental ta- mica samples yield good plateaus or near plateaus tion during irradiation. However, all but two bles, is available on the Web at http://www .geosociety.org/pubs/ftpyrs.htm. Requests may also be ranging from 1221 to 1457 Ma, with most (n = 6) (LEG-8 and LEG-10) of these eight samples give sent to Documents Secretary, GSA, P.O. Box 9140, plateaus between 1286 and 1396 Ma (Fig. 3). initial low-temperature fractions of 250–300 Ma Boulder, CO 80301; e-mail: [email protected]. However, two biotites (LEG-8b and LEG-23) do (e.g., see LEG-15 and NM-3). This behavior is

Geological Society of America Bulletin, September 1999 1339 Figure 3. 40Ar/39Ar age spectra for muscovite and biotite mineral separates obtained from Early Proterozoic metamorphic rocks, northern Black Hills. See Figure 2 for sample locations, Table 1 for summary of ages, and Table DR-1 (see text footnote 1) for complete listing of analytical data.

1340 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA

Figure 4. 40Ar/39Ar age spectra for hornblende mineral separates obtained from Early Proterozoic metamorphic rocks, northern Black Hills. See Figure 2 for sample locations, Table 1 for summary of age data, and Table DR-1 (see text footnote 1) for complete listing of analytical data.

not expected for recoil, suggesting that the low- not record the timing of the pre-granite continental their Ar closure temperatures) long after their last temperature age discordance reflects, instead, an collision that caused the micas to (re)crystallize, crystallization, as did the southern Black Hills mi- alteration or “younger” phase that releases Ar at despite low metamorphic grade and great distance cas (Holm et al., 1997). lower laboratory temperatures. from the main Harney Peak granite pluton (Fig. 2). The reason the northern Black Hills micas give The plateau dates of the mica samples show a Nor is it likely that their ca. 1400–1300 Ma dates (apparently) younger 40Ar/39Ar ages than their relatively large range (>150 m.y.) among the sam- reflect Middle Proterozoic thermal resetting, be- southern counterparts and why both mica suites ples from a small area north of Nemo. In addition, cause the nearest ca. 1500–1300 Ma anorogenic preserve such a wide range of dates is uncertain these northern Black Hills dates are collectively plutons (Fig. 1) are ~200 km from the Black Hills. (Fig. 5A). These features may be attributed to one younger than post-Harney Peak granite uplift and Although these micas probably originated during or more of the following: (1) The dates with cooling ages in the southern Black Hills (see Holm Wyoming-Superior continental collision, they ap- plateaus record variably mixed ages reflecting a et al., 1997; cf. Fig. 5A). The new plateau dates do parently resided at mid-crustal levels (i.e., above Proterozoic mica cooling age and overprinting in

Geological Society of America Bulletin, September 1999 1341 DAHL ET AL.

Figure 5. Summary histograms of Proterozoic age data for the Black Hills (South Dakota) and environs. (A) Mean mica and K-feldspar age data from within and south of the Black Hills (excluding Early Proterozoic [1660–1600 Ma] dates of Holm et al., 1997, obtained from a somewhat shallower structural level). (B) Mean dates obtained from the Wyoming-Superior collisional province (including the Black Hills); one date from southeastern Wyoming is included. Geochronologic data are compiled from Sims et al. (1991; exclusive of one ca. 1970 Ma date); Krogstad et al. (1993; a U/Pb K-feldspar–whole-rock deformation date); Resor et al. (1996; a U/Pb age for titanite growth, southeastern Wyoming); Dahl et al. (1998; a Pb/Pb age for garnet growth); Dahl and Frei (1998; two Pb/Pb ages for garnet and staurolite growth); and this paper (two Ar/Ar dates for hornblende). See text for discussion.

the form of 40Ar loss or mineralogical alteration, Hornblendes tinguishable. Although the hornblende appears during or subsequent to mid-Paleozoic time. optically to be very fresh and inclusion free, the Widespread Tertiary intrusions in the northern The hornblendes yield quite variable ages and separate contains minor, unavoidable plagio- Black Hills are the most logical source of the heat age spectra. The total-gas ages range from 1037 clase impurities. White-mica alterations within and/or fluids needed to overprint these micas, as to 1959 Ma (Table 1); only four samples produce the plagioclase, observed optically, may consti- also suggested by Zartman et al. (1964). (2) Some results even approaching plateaus, with the others tute the K-rich material responsible for high K/Ca micas contain excess 40Ar that is not apparent displaying highly discordant spectra (Fig. 4). signatures (and low apparent ages) in the spectra. from the step-heating analysis (e.g., Foland, Hornblendes in three amphibolites yield apparent The T93-6 amphibolite is adjacent to granite 1983). (3) The mica samples reflect different Ar ages older than the oldest dated Harney Peak and/or pegmatite of uncertain Precambrian age blocking temperatures perhaps due to composi- granite. Some hornblende separates obviously and is overlain by a Tertiary laccolith that may tional and microtextural factors (e.g., Dahl, contain excess 40Ar. There is also indication of have played a role in producing the low apparent 1996a). (4) The dates record closure of the K-Ar overprinting of some hornblende ages. ages. The plateaus are compromised for this system at the same temperature, but for tectonic The spectra of four hornblende aliquots from “mixed-phase” but they are very consistent and in- reasons various samples cooled below it at differ- amphibolite T93-6 (Tinton, Fig. 2) yield inte- terpretable as minimum ages for the hornblende. ent times (e.g., Holm et al., 1997). These possibil- grated dates ranging from 1786 to 1569 Ma. To Moreover, 1742 ± 5 Ma probably represents a ities cannot be confirmed or eliminated conclu- varying degrees these exhibit broad, imperfect minimum age for D2 hornblende growth, rather sively with present data, and the exact causes are “plateau” regions; the best defined of these are than post-D2 resetting and/or cooling or pre-D2 in- not important to the tectonic issues that are the the same at 1742 ± 5 Ma. It is clear from the heritance, because (1) peak metamorphic temper- main concern in this paper. Moreover, because K/Ca relationships that the lower ages seen in in- atures of ~500–550 °C at Tinton (J. Berry, 1995, age-composition-microstructure relationships cremental heating are due to more K-rich mater- written commun.) are comparable to Ar closure among the Black Hills micas are currently being ial of lower age in the hornblende separate. The temperatures for actinolitic hornblendes (Dahl, examined as part of a separate study, full consid- aliquot with the lowest integrated date (and 1996b); and (2) metamorphism prior to D2 is not eration of these four possibilities is more appro- plateau region) contains more of this material recognized at Tinton. Additional support for the priately deferred to presentation of the results of and, therefore, less reliably defines a plateau 1742 Ma hornblende growth age (overall uncer- this new investigation. date than the others that are analytically indis- tainty is ±18 m.y.; see Table 1) is provided by a

1342 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA

207Pb/206Pb age of 1746 ± 10 (±2 σ) Ma for syntectonic garnet growth in the Homestake Mine area in nearby Lead (Fig. 2; Dahl et al., 1998; Terry et al., 1998). The 1742 Ma growth age for T93-6 hornblende is included in the age histogram (Fig. 5B). Hornblende CM-2242 (Crook Mountain drill core; Fig. 2) records a total gas age of 1959 Ma (Fig. 4), and its highly discordant, U-shaped spectrum indicates incorporation of excess 40Ar. A narrow plateau (1787 ± 8 Ma) is permissible, but the 1770 ± 6 Ma minimum increment observed is interpretable as a maximum age for this hornblende (cf. Bowtell et al., 1994). With this interpretation, the 1770 ± 6 Ma date reflects the time of argon closure following hornblende growth during the D2 event or (more likely) following resetting by the subjacent Harney Peak granite documented by Bachman et al. (1990). Hornblende HM-1652 (Homestake Mine drill core; Lead) yields a highly disturbed age spectrum with a total gas date of 2187 Ma (Fig. 3). This spectrum, which shows clear evidence of excess Ar, is not readily interpretable, and is not considered further in this paper. Hornblendes in three other amphibolites give dates that are significantly younger than their probable pre-Harney Peak granite growth ages. Am- phibolite BH-3 is from the Bear Mountain dome (see Fig. 2), where subjacent granite intrusion has been inferred from gravity data (Duke et al., 1990). Hornblende in this rock defines a gentle northeast- dipping foliation that strikes parallel to the adjacent kyanite isograd, so it probably formed during

D2 compression and was mechanically reoriented or recrystallized during D4 doming before ca. 1715 Ma. The BH-3 hornblende yields a 1691 ± 5 Ma plateau date, which agrees with S2 hornblende dates obtained in staurolite-zone amphibolites northeast of the main Harney Peak granite pluton, near Keystone (Berry et al., 1994; Figs. 2 and 5B). Near concordance of these dates with the known ca. 1720–1690 Ma interval of granite and/or pegmatite intrusion strongly suggests that the dated S2 Figure 6. Map showing major tectonic divisions and locations of isotopically dated samples hornblendes were all thermally reset during the in- (after Sims et al., 1991). BH—Black Hills crystalline core (exposed); HP and THO?—Hearne trusive episode. This interpretation is consistent province and (widely presumed) southern extension of Trans-Hudson orogen (both buried un- with the ~500–550 °C temperatures of retrograde der Phanerozoic cover). Great Falls tectonic zone is possible Hearne-Wyoming continental su- metamorphism preserved in garnet rims of adja- ture. Geochronologic front previously established in southeastern Wyoming province (thick cent metapelites (Friberg et al., 1996). dashes) is provisionally extended eastward in this study (thin dashes) and represents northern

The results for the other two hornblendes, limit of <1520 Ma ages for mineral systems with relatively low closure temperatures (TC). See CC93-1 and WP-3, which yield total gas dates of text for discussion and Figure 5 for summary of supporting isotopic dates from the BH and re- 1563 Ma and ca. 1100 Ma, indicate overprinting. gions immediately north and south. Apparently synchronous, northeast- and north-north- The spectrum for hornblende CC93-1 (sampled west–trending, compressional fabrics are depicted by foliation symbol (Laramie Peak shear from a road cut in Central City, near Lead) is zone, Wyoming) and dashed pattern (Black Hills uplift, South Dakota). See text for discussion. highly discordant; the lower temperature fractions show severe discordance correlated with bodies located within 100–200 m of the host am- mum estimate of true hornblende age. K/Ca and K/Cl variations. The classic “staircase” phibolite on the hornblende and/or plagioclase Hornblende WP-3 was separated from the cen- pattern of steadily increasing apparent age exhib- impurities. If this is true, the maximum age inter of a 600 × 300 m raft of Proterozoic amphibo- ited between ca. 1420 Ma and ca. 1640 Ma most crements of 1641 ± 5 Ma and 1642 ± 5 Ma, rep- lite enclosed by a Tertiary rhyolite stock that in- likely indicates the effects of Tertiary intrusive resenting ~15% of the 39Ar, provide only a mini- truded Phanerozoic strata ~1.5 km west of

Geological Society of America Bulletin, September 1999 1343 DAHL ET AL.

Figure 7. Schematic composite cross section across Cheyenne belt suture between Proterozoic island-arc terrane (Medicine Bow orogen, south) and Archean Wyoming province (cross-hatch pattern, north), ca. 1780–1760 Ma. Both Laramie Mountains (Wyoming) and Black Hills (South Dakota) localities are depicted; see Figure 6 for map locations. Bold dashes highlight different crustal levels presently exposed in the two localities. LPsz (Laramie Peak shear

zone) is dated as 1763 ± 7 Ma (Resor et al., 1996), and D1 thrust wedges (Black Hills) formed between ca. 1780 Ma and ca. 1760 Ma (this study). See text for discussion. Detailed geological cross sections of the Cheyenne belt and environs were presented elsewhere (e.g., Chamberlain et al., 1993; Houston, 1993; Resor et al., 1996; Chamberlain, 1998).

Whitewood Peak. All four aliquots of this horn- Ma and ca. 1300 Ma, following the discussion Dakota, as shown in Figure 6 (thin dashed line). blende exhibit highly discordant, two-tiered, here. This event is perhaps better constrained By analogy, the Black Hills extension of this front 40Ar/39Ar spectra in which age and K/Ca are well within a ca. 1500–1400 Ma interval by the south- may represent the northern extent of a thin- correlated (Fig. 4). The spectra are clearly the re- ern Black Hills micas, for which overprinting by skinned thrust sheet originating from island-arc sult of a mixture of higher and lower K phases, the Tertiary intrusive activity may be ruled out (Holm accretion to the south, which would account for former having a much lower age contributing et al., 1997). Regionally, mica cooling ages of ca. the shallow, north-directed F1 fold nappes (see about 40% of the K in the bulk sample. The four 1500–1400 Ma are not unique to the Black Hills. Fig. 7). According to this scenario, micas in the incremental-heating runs of this sample show very Comparable Middle Proterozoic cooling ages Black Hills could not cool, and could not retain good agreement. One defines well a lower age of were obtained by Peterman and Hildreth (1978) in Ar, during Early Proterozoic time (unlike their 75–80 Ma for the first 28% of the 39Ar released. the southeastern Wyoming province, between the northern counterparts; Figs. 5 and 6), because the This can be interpreted as the maximum for the Cheyenne fold belt (Figs. 1 and 6) and a subparal- postulated thrust sheet caused greater crustal time of overprinting or alteration of the Protero- lel geochronologic front they delineated ~150 km thickening in the south and therefore contributed zoic hornblende, which was likely during em- to the north (bold dashed line, Fig. 6), beyond to prolonged thermal incubation there. Moreover, placement of the Tertiary stock dated as ca. 58 Ma which micas yield only Archean dates. because the oldest known island-arc terranes (J. Kirchner, 1995, written commun.). The horn- The Cheyenne fold belt is a southeast-dipping south of the Wyoming province (and its eastern- blende separate contains abundant intergrown pla- shear zone delineating the boundary of ca. most exposures in the Black Hills) are ca. 1790 gioclase impurities, which could not be avoided in 1780–1740 Ma island-arc accretion along the Ma in age (Premo and Van Schmus, 1989), the separation, but otherwise appears optically to be southeastern margin of the Wyoming Province thrust sheet (and thus the F1 fold nappes) must very fresh and inclusion free, revealing no obvious (Medicine Bow orogeny; Chamberlain, 1998, therefore be younger. Presuming that the south- phases of higher K/Ca ratio. The spectra indicate a and references therein). In three dimensions, the east-dipping Cheyenne belt suture extends to the younger phase with K/Ca of ~1, suggesting that Cheyenne belt has been interpreted as the root of northeast under Phanerozoic cover (Fig. 6), and (sub)microscopic micaceous alteration could have a now-eroded, north-directed, thin-skinned, that island-arc accretion was synchronous along formed in the host hornblende and/or the plagio- foreland-thrust complex that overrode the south- its entire length, it would follow that F1 folding in clase impurities during emplacement of the stock. eastern Wyoming craton as far north as the geo- the Black Hills began early within the ca. 1780– In the incremental heating, after this phase is chronologic front (Condie and Shadel, 1984; 1740 Ma Medicine Bow orogeny. Previous stud- 39 largely degassed, the next ~50%–55% of the Ar Duebendorfer and Houston, 1987; Premo and ies that considered F1 fold nappes in the Black appears to be derived mostly from the hornblende. Van Schmus, 1989; Houston et al., 1989). This Hills to be older than ca. 1840 Ma could not ade- The apparent ages on these portions of the WP-3 interpretation is incorporated into Figures 6 and quately account for these prominent structures. spectra rise to about 1600 Ma, which is interpreted 7. It is believed that Archean micas were ther- This is because the north-directed compression as a minimum age for the hornblende. mally reset beneath this thrust sheet (Karlstrom along the southern flanks of the Wyoming and Su- and Houston, 1984) and remained open to Ar perior cratons did not occur until ca. 1780 Ma TIMING OF EARLY PROTEROZOIC loss until Middle Proterozoic uplift and /or cool- (i.e., the maximum age of metamorphism in the TECTONISM IN THE BLACK HILLS ing between 1600 and 1400 Ma (Peterman and Medicine Bow and Central Plains orogens). AND ENVIRONS Hildreth, 1978).

Paralleling the age trends in southeastern F2 Folding and Regional Metamorphism F1 Folding Wyoming, micas (and feldspars) within the Wyoming-Superior collision zone have yielded It follows that the north-northwest–trending F2 Even though the Black Hills micas (Figs. 3, 5, mostly younger (Middle Proterozoic) dates within upright isoclinal folds and S2 axial planar folia- and 6) do not directly date the east-west Wyoming- and south of the Black Hills; and only older (Early tion that define regional structural grain (Figs. 1 Superior collision that produced them, their dates Proterozoic) dates north of the Black Hills (cf. and 2) were imposed after ca. 1780 Ma (i.e., af- nevertheless support a ca. 1790 Ma or earlier time Fig. 5, A vs. B; Fig. 6, solid circles vs. triangles). ter F1 folding) but before intrusion of the earliest frame for the earlier, north-vergent F1 fold nappes. Provisionally, therefore, the geochronologic front granite at ca. 1715 ± 3 Ma. Consistent with this 40 39 The biotite data point to cooling through the char- of southeastern Wyoming is now extended east- scenario, the new Ar/ Ar dates for S2 horn- acteristic closure temperature between ca. 1500 ward over the Black Hills and into central South blendes narrow the beginning of syncollisional

1344 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA regional metamorphism in the northern Black the southern Black Hills. Provisionally, this meta- gion and the relative abundance of younger (ca.

Hills to within about 1770 to 1740 Ma. As out- morphism is associated with D2 compression 1810–1710 Ma) dates there (Figs. 5B and 6), we lined in the following, this time frame is inde- (pending confirmation that no metamorphism ac- currently regard the younger dates as recording pendently supported by (1) precise Pb/Pb min- companied D1). In the northern Black Hills, the the main phase of collisional thermotectonism in eral dates recently produced in the Black Hills; younger 40Ar/39Ar and 207Pb/206Pb ages for garnet- the western Dakotas, including the Black Hills. and (2) U/Pb and Rb/Sr dates (generally less pre- zone hornblende (1742 Ma) and garnet (1746 Ma), Within the Black Hills, the onset of this event is cise) from the buried part of the Wyoming-Supe- respectively, suggest an ~15–20 m.y. delay in the better constrained ca. 1770–1740 Ma (depending rior collision zone. onset of prograde metamorphism at the somewhat on location), according to the new thermochrono- Within the Black Hills, tectonic burial and pro- shallower crustal levels currently exposed there logic data reported in this paper. grade metamorphism in the kyanite zone (Bear (Lead and Tinton domes, Fig. 2). All five mineral-

Mountain dome, Fig. 2) have been dated as 1760 ± growth ages are included in Figure 5B. F3 Folding and Subsequent Deformation 7 Ma (±2σ). This result represents a combined gar- Other considerations point to a ca. 1770–1740 net-staurolite age recently obtained in a metapelite Ma time frame for the onset of D2 thermotecton- The age and therefore the origin of the F3 (sampled near amphibolite BH-3), in which the ism in the Black Hills and environs. Previous cross-folds and their steep axial planar foliations garnet and staurolite also yielded separate growth studies here have linked crustal thickening, F2 (S3) can now be considered, following the ther- ages of 1762 ± 15 Ma and 1759 ± 8 Ma, respec- folding, regional metamorphism, and S-type motectonic synthesis developed here. Whereas tively (Dahl and Frei, 1998; 207Pb/206Pb step-leach granite magmatism as manifestations of terminal the original age of these structures was only method of Frei and Kamber, 1995). Garnet in this Wyoming-Superior convergence (Redden et al., broadly known within a ca. 1880–1715 Ma inter- metapelite is rotated within the same gently dip- 1990; Terry and Friberg, 1990; Helms and val, this interval is now compressed to ca.

ping, matrix schistosity (S2) exhibited by horn- Labotka, 1991; Nabelek et al., 1992; Friberg et al., 1760–1715 Ma, based upon the new age con- blende in sample BH-3, which was flattened dur- 1996). Analogous interrelationships are also en- straint for F2 folding reported here. It is believed ing D4 doming. Therefore, its 1762 Ma growth age visioned in younger collision belts (e.g., Turner that F3 folding may have occurred in response to represents a maximum age for the S2 matrix schis- et al., 1992). Moreover, in contrast to the ~130 a major shear couple affecting the Black Hills tosity and a minimum age for S1 (the oriented m.y. time lag between tectonism and posttectonic and environs (Redden et al., 1990). Following quartz inclusions). In contrast, southern Black magmatism previously inferred in the Black Hills, this interpretation, the new 1760–1715 Ma age

Hills metapelites dated outside the kyanite zone the narrower ~25–55 m.y. interval proposed here constraint for F3 folding suggests that this event contain only younger garnets (ca. 1755–1710 Ma, is more consistent with independent estimates represents the waning stages of continental as- n = 14), which appear to establish a continuum of proposed for other orogenic terrains. These esti- sembly in the Black Hills region. regional to thermal metamorphism (at shallower mates include (1) ~20–55 m.y. and ~50–60 m.y. The younger than 1715 Ma (ca. 1535 Ma?) crustal levels) that culminated in Harney Peak (from thermochronologic studies in the Trans- age of the S5 foliation and its steeply dipping granite emplacement (207Pb/206Pb step-leach Hudson and Penokean orogens; Bickford et al., northeasterly trend (Redden et al., 1990) permit method; Dahl et al., 1998). Thus, because the old- 1990; Holm and Lux, 1996; respectively; solid speculation that this fabric records the inboard ef- est garnet (1762 Ma) was sampled from the deep- dots in Figure 1 locate dated posttectonic plu- fects of late-phase arc accretion far to the south of est exposed crust (kyanite zone), it most likely tons); and (2) ~10–50 m.y., ~30–100 m.y., and ap- the Black Hills, along the southern margin of a pinpoints the onset of post-1880 Ma compres- prox 50 m.y. (from conductive heat-flow model- previously welded Wyoming-Superior continent. sional thermotectonism in the Black Hills region ing of collisionally thickened crust; England and (Dahl and Frei, 1998). Thompson, 1986; Glazner and Bartley, 1985; IMPLICATIONS FOR EARLY Krogstad et al. (1993) reported a 1768 ± 16 Ma Stüwe and Sandiford, 1995; respectively). Thus, PROTEROZOIC DOCKING OF THE reset date for the Bear Mountain granite the petrogenetic links inferred among Early Prot- WYOMING PROVINCE WITH (207Pb/206Pb, K feldspar–whole rock), also within erozoic structural, metamorphic, and plutonic ele- LAURENTIA the kyanite zone (Fig. 2), and argued for at least ments in the Black Hills demand a corresponding partial Pb reequilibration of the K feldspar during temporal link that only a ca. 1770 Ma or younger Prior to this study, the timing of Wyoming- metamorphism accompanied by shearing. Com- collision of the Wyoming and Superior provinces Superior collision during the 1900–1600 Ma as- plete Pb reequilibration during this shearing provides. sembly of Laurentia was constrained within a would seem likely, however, given the relative Outside the Black Hills, where the Wyoming- broad 1880–1715 Ma interval. The new scenario ease with which K feldspar dynamically recrys- Superior zone is buried under Phanerozoic cover developed here narrows this interval from ca. tallizes in nature (Tullis and Yund, 1985). More- (Fig. 6), our observation is that the dates for iso- 1770 Ma or later to ca. 1715 Ma, which supports over, Pb loss from the K feldspar of Krogstad et al. topically retentive minerals (as compiled by Sims the notion that earlier, north-directed thrust (1993) was probably governed by recrystalliza- et al., 1991) tend to cluster within a ca. 1810– wedges (related to arc-continent collision) over- tion, not diffusion, because the nominal Pb clo- 1710 Ma range (open circles; Fig. 6). Included rode an open epicratonic basin (or basins) in the sure temperature (~600 °C; Cherniak, 1995) ex- within this cluster is a relatively precise 1781 ± 16 future Black Hills beginning ca. 1780 Ma. More-

ceeds peak metamorphic conditions for the Ma U/Pb age for zircon (mafic granulite, north- over, these thrust wedges and the resultant F1 fold sample locality (~550 °C; Friberg et al., 1996). eastern Montana drill core, concordia intercept nappes were presumably evolved at shallow These observations support interpretation of the date). These ca. 1810–1710 Ma dates have been crustal levels, as depicted schematically in Figure 1768 ± 16 Ma date as approximating a true defor- interpreted as related to collision in a very general 7, given the lack of evidence for accompanying mational age. sense (Peterman et al., 1983; Sims et al., 1991), metamorphism (Redden et al., 1990; Redden and To summarize, the 207Pb/206Pb ages for kyanite- but no specific connections between mineral age DeWitt, 1996). In contrast, the virtually synchro- zone garnet (1762 Ma), staurolite (1759 Ma), and and deformational fabric have been reported. nous (ca. 1780–1760 Ma) thrusting along the K feldspar (1768 Ma) are concordant within error, However, given the rarity of Trans-Hudson (ca. Cheyenne belt and environs was accompanied by and date the earliest prograde metamorphism in 1860–1790 Ma) dates in the western Dakotas re- (high-grade) metamorphism, and thus occurred at

Geological Society of America Bulletin, September 1999 1345 DAHL ET AL. relatively deeper crustal levels (see Fig. 7). Beginning ca. 1770 Ma or later, the episode of north-directed arc-continent collision south of the Black Hills was superseded by east-west collision of the Wyoming and Superior provinces. This collision: (1) closed the Black Hills rift basin(s); (2) tectonically buried and metamorphosed the basin sediments; (3) refolded the early fold nappes to produce the F2 north-northwest–trending upright folds and penetrative S2 foliation; and (4) initiated or mobilized major gold mineralization in the F2-folded Homestake iron formation (Lead, Fig. 2). Collisional thermotectonism apparently continued in the Black Hills over a protracted, ~40–50 m.y. interval (continuous range of 207Pb/206Pb garnet-growth ages; Dahl et al., 1998), and was essentially completed by ca. 1715 Ma, when the posttectonic Harney Peak granite began to intrude. South-southwest of the Black Hills, however, the western fault-bounded edge of the Wyoming-Superior collision zone (Hartville uplift; Sims, 1995; see Fig. 6) apparently underwent this thermotectonism (or local reactivation thereof) somewhat later, ca. 1715–1666 Ma (U/Pb zircon and monazite; Krugh et al., 1996). The proposed scenario of sequential arc-continent (ca. 1780 Ma or later) and continent-continent (ca. 1770 Ma or later) collisions further accounts for the occurrences of apparently synchronous, northeast- and north-northwest–trending, compressional fabrics in southeastern Wyoming (Laramie Peak shear zone) and the Black Hills, respectively (Fig. 6). Specifically, in southeastern Figure 8. Schematic diagrams showing proposed docking history of the Wyoming province Wyoming, compression along the northeast- with Laurentia between ca. 1880 Ma and 1760 Ma; see text for discussion. HP—Hearne trending Laramie Peak shear zone has been dated province; GFtz—Great Falls tectonic zone, and may represent the Wyoming-Hearne continen- as 1763 ± 7 Ma (U/Pb titanite growth; Resor et al., tal suture (but see text). CPO—Central Plains (ca. 1780–1630 Ma) and Medicine Bow (ca. 1996; see Fig. 6, open circle). This age is identi- 1780–1740 Ma) orogens, undifferentiated. Shading depicts regions principally affected by colli- cal (within error) to the 1762 ± 15 Ma maximum sional or accretionary deformation and metamorphism ca. 1770–1760 Ma. Clockwise rotation age inferred for the penetrative, north-north- depicted for Wyoming province prior to docking follows Roscoe and Card (1993), but is not a re- west–trending compressional fabric (S2) in the quirement of the proposed docking model. Bold ellipse on Wyoming province margin (labeled Black Hills (207Pb/206Pb growth age for rotated “BH”) outlines future exposure of Black Hills crystalline core, and opposing arrows represent garnet in locally flattened S2 fabric; Dahl and pre-1880 Ma rifting therein. North-south–trending, west-dipping, subduction zone depicts Frei, 1998). Wyoming-Superior convergent zone, highly simplified; recent COCORP data (Baird et al., In contrast to the ca. 1770–1715 Ma thermotec- 1996) suggest that this suture involves not the Superior province, but an intervening crustal frag- tonic interval for Wyoming-Superior collision ment (Dakota block) bounded to the east by an east-dipping Dakota-Superior suture. Presumed (north-central United States), as developed here, Dakota block and its eastern suture are omitted for clarity. Diagrams are not drawn to scale. the latest phase of thermotectonism associated with Hearne-Superior collision (central Canada) began ca. 1830–1825 Ma and ended by ca. provinces were discrete continents welded to the print (K-Ar, hornblende, and biotite; Peterman, 1790–1780 Ma (e.g., Bickford et al., 1990). There- Superior continent (and to each other) by diachro- 1981). Near the western terminus of the Great fore, the Trans-Hudson orogeny was: (1) over- nous collisions in Early Proterozoic time, as Falls tectonic zone, in southwestern Montana printed by a younger orogeny unique to the west- schematically depicted in Figure 8. Evidence for (Fig. 1), reworked Archean basement and overly- ern Dakotas; (2) time transgressive from northern a possible Hearne-Wyoming collision is consid- ing metasupracrustal rocks are well exposed in Saskatchewan-Manitoba to the western Dakotas; ered below. Laramide block uplifts, and a pervasive north- or (3) absent in the western Dakotas altogether. Along the northeast-trending Great Falls tec- east-trending fabric in these rocks may be Early The lack of relict Trans-Hudson (i.e., ca. 1860– tonic zone of Montana (Figs. 1 and 6), reworked Proterozoic in age, not Archean, according to the 1790 Ma) dates in the Black Hills rules out sce- Archean rocks from the Little Rocky Mountains following recent studies. (1) In the Highlands nario 1, barring a complete overprinting there (Fig. 1) have yielded ca. 1750–1715 Ma mineral Range, O’Neill et al. (1988) documented syntec- (both mineralogic and isotopic). Following sce- dates, which have been considered as provisional tonic gneiss doming which they dated as ca. 1800 narios 2 or 3, however, the Hearne and Wyoming evidence of a ca. 1800 Ma thermotectonic over- Ma (U/Pb zircon) and attributed to thermotecton-

1346 Geological Society of America Bulletin, September 1999 EARLY PROTEROZOIC TECTONISM IN THE BLACK HILLS, SOUTH DAKOTA ism at the southwestern end of the Great Falls Wyoming province as a discrete Early Proterozoic lation, Fig. 5B) and ended by ca. 1715 Ma (posttectonic zone. Harlan et al. (1996) reported ca. entity prior to Laurentia, based upon independent tectonic Harney Peak granite). This new synthesis 1820–1795 Ma 40Ar/39Ar cooling ages for foli- evidence. In particular, they observed the striking invites reconsideration of an old term, “Black ated hornblendes and biotites in adjacent meta- lithostratigraphic resemblances and comparable Hills [collisional] orogeny” (Goldich et al., 1966), morphic rocks. (2) In the Madison Range, Erslev depositional ages of the Early Proterozoic Huron- to distinguish the ca. 1770–1715 Ma interval of and Sutter (1990) identified an Early Proterozoic ian Supergroup (southern margin of Superior Wyoming-Superior convergence from the older, northeast-trending mylonite zone, in which they province; Fig. 1) and the Snowy Pass Supergroup strictly Trans-Hudson (i.e., ca. 1860–1790 Ma), dated major southeast-directed thrusting as be- (southern margin of Wyoming province; Fig. 1). interval of Hearne-Superior convergence to the fore ca. 1800 Ma (40Ar/39Ar hornblendes and They concluded that these now-isolated (passive north. Thus, a discrete Wyoming continent ap- muscovite). (3) In the intervening Ruby and To- margin) sedimentary suites were deposited in a pears to have docked with a newly formed bacco Root Ranges, Dahl et al. (1998) reported once-contiguous basin. Roscoe and Card further Hearne-Superior continent beginning ca. 1820 an 1822 ± 12 Ma (±1σ) 207Pb/206Pb step-leach postulated that the Wyoming province originated Ma in southwestern Montana (southern Alberta?)

age for garnet and monazite growth, whereas in the area now occupied by the Keweenawan rift and ca. 1770 Ma in the western Dakotas (D2 in the Brady et al. (1994) and Kovaric et al. (1996) doc- and Penokean orogen (Fig. 1) and became a rifted, Black Hills; Fig. 8). Island-arc accretion along the umented younger 40Ar/39Ar cooling ages of ca. then rotated, segment off the southern Superior southern Wyoming province margin (beginning 1800–1700 Ma for fabric-defining hornblendes province beginning ca. 2150 Ma (i.e., the age of loca. 1780 Ma; Medicine Bow orogeny) explains

and biotites. The inboard limit of Early Protero- cal basalt dikes). earlier thrusting in the Black Hills (D1; Fig. 8). zoic thermotectonic overprinting of Archean The main focus of this discussion has been the In conclusion, the 1900–1600 Ma assembly rocks in southwestern Montana is approximated Early Proterozoic docking history of an appar- of Laurentia appears to have involved a pro- by the northeast-trending “Giletti Line” (Giletti, ently discrete entity, the Archean Wyoming gression of collisional and/or accretionary ther- 1966; see also Erslev and Sutter, 1990). province, with Laurentia. However, the scenario motectonism. This thermotectonic progression These cited authors attributed their age data to developed here invites preliminary consideration (1) began in central Canada (ca. 1860–1790 an enigmatic collisional reworking of Archean of its location prior to Laurentia. Roscoe and Card Ma; e.g., Bickford et al., 1990; Machado, crust in southwestern Montana during Early Prot- (1993) advanced one possibility, as outlined here. 1990); (2) continued later in the north-central erozoic time. We propose that this collision oc- Their hypothesis, or some variant, is appealing United States (ca. 1820–1740 Ma; this study, curred between the Wyoming and Hearne because it removes the need for a presumed south- and references therein); and (3) ended in the provinces, ca. 1820–1800 Ma, and that the Great ern Archean landmass (e.g., Houston, 1993) to ex- central United States (ca. 1700–1600 Ma; Falls tectonic zone was the continental suture (or plain the Early Proterozoic (ca. 2000 Ma or older) Van Schmus et al., 1987). This apparently con- arc-continent suture if an intervening island-arc passive-margin sequence exposed in southern tinuous, thermotectonic younging from north to complex was involved; see Mueller et al., 1995). Wyoming (also exposed in Nemo and environs, south supports the notion that Early Proterozoic This scenario, as depicted schematically in Fig- Black Hills; e.g., localities NM-1 and NM-3, mantle downwelling, perhaps centered in ure 8, could account for the Early Proterozoic Fig. 2). However, basalt dikes in southeastern north-central Canada, was ultimately responsi- thermotectonic overprint of Archean basement Wyoming have yielded no evidence for ca. 2150 ble for the assembly of supercontinent Lauren- and overlying metasupracrustal rocks previously Ma rifting (e.g., Houston, 1993; R. Bauer, 1998, tia (Gurnis, 1988; Hoffman, 1989). documented along a prominent, yet cryptic, written commun.), which would argue against the crustal lineament in the northern Wyoming Roscoe-Card hypothesis. In addition, the chemi- ACKNOWLEDGMENTS province. Hoffman (1989, 1990) also invoked the cal and isotopic signatures of the Wyoming and Great Falls tectonic zone as the likely Wyoming- Superior provinces are very different (e.g., We gratefully acknowledge many colleagues Hearne suture. The subparallel Vulcan structure Mueller and Wooden, 1988; Gosselin et al., 1988; for helpful geologic discussions, including Bob of southern Alberta (just south of the “Hearne” Card, 1990; Frost et al., 1998), which indicates Bauer, John Brady, Kevin Chamberlain, Jack label in Fig. 1) has been suggested as an alterna- that these two provinces were always discrete en- Cheney, Ed Duke, Robert Frei, Verne Friberg, tive candidate for this suture (DEEP PROBE tities even if once connected prior to ca. 2150 Ma Steve Lucas, Dan Lux, Peter Nabelek, Jim seismic transect; Henstock et al., 1998). Hoffman (Houston, 1993). Although the thermochrono- Ratté, and Jack Redden. Special thanks go to (1989, 1990), however, interpreted this feature as logic data presented in this paper support the exis- Rick Bachman, James Berry, and Mike Terry an Archean crustal lineament within the southern tence of a discrete Wyoming microcontinent (Fig. (Homestake Mining Company) for providing Hearne province, citing equivalent cratonization 8; see also Roscoe and Card, 1993), the pre-2150 key samples from Homestake properties and de- ages (ca. 2800–2900 Ma) immediately to its Ma location of the Wyoming microcontinent and tailed perspectives on local geology. We also north and south (cf. older than 3100 Ma for the its possible ca. 2150–1900 Ma movements (i.e., thank Jeff Linder and Frank Huffman for help in Wyoming province). prior to Laurentia) remain unknown. maintaining laboratory equipment; members of A hypothesis that the Hearne and Wyoming the Ford Reactor staff for sample irradiations; provinces were discrete entities welded to the Su- CONCLUSIONS and Mike Dorais for assistance with the electron perior province (and to each other) during distinct microprobe analyses. In addition, we greatly ap- orogenies as Laurentia was assembled is sup- There is no conclusive evidence that the west- preciate the insightful journal reviews provided ported by the diachroneity of continental collision ern Dakotas region (north-central United States) by Bob Bauer, Dave Mogk, and John Geissman. from central Canada to the north-central United underwent collisional thermotectonism ca. 1860– This work was financially supported by grants States. Further support for this hypothesis is pro- 1845 Ma or ca. 1825–1790 Ma, as occurred dur- from the National Science Foundation (grant vided by recent evidence for (collisional?) ther- ing the Trans-Hudson orogeny of central Canada. EAR-9614822 to Dahl, grants EAR-9701496 motectonism along the Great Falls tectonic zone. Instead, it now appears that the western Dakotas and EAR-9220344 to Foland); Homestake Min- The tectonic scenario is depicted in Figure 8. underwent a distinctly younger collisional event, ing Company (Dahl); and the Kent State Uni- Roscoe and Card (1993) also interpreted the which began ca. 1770–1740 Ma (see age compi- versity Foundation (Holm, Dahl).

Geological Society of America Bulletin, September 1999 1347 DAHL ET AL.

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K., 1995, Archean and Early Proterozoic tectonic logic relationships of the Little Elk Granite, northern Early Proterozoic gneiss dome in the Highland Moun- framework of north-central United States and adjacent Black Hills, South Dakota: U.S. Geological Survey Pro- tains, southwestern Montana, in Lewis, S. E., and Berg, Canada: U.S. Geological Survey Bulletin 1904-T, 12 p. fessional Paper 575-D, p. D157–D163. R. B., eds., Precambrian and Mesozoic plate margins: Sims, P. K., and Peterman, Z. E., 1986, Early Proterozoic Cen- Zartman, R. E., Norton, J., and Stern, T., 1964, Ancient granite Montana Bureau of Mines and Geology Special Publica- tral Plains orogen: A major buried structure in the north- gneiss in the Black Hills, South Dakota: Science, v. 145, tion 96, p. 81–88. central United States: Geology, v. 14, p. 488–491. p. 479–481. Peterman, Z. E., 1981, Archean gneisses in the Little Rocky Sims, P. K., Peterman, Z. E., Hildebrand, T. 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Geological Society of America Bulletin, September 1999 1349 Structure, Timing, and Kinematics of the Early Eocene South Fork Slide, Northwest Wyoming, USA

† John P. Craddock,1,* David H. Malone,2 Ryan Porter,3 Alison MacNamee,4, † ‡ Maren Mathisen,5, Katherine Kravitz,6, and Andrea Leonard2,§

1. Geology Department, Macalester College, St. Paul, Minnesota 55105, USA; 2. Department of Geography-Geology, Illinois State University, Normal, Illinois 61761, USA; 3. School of Earth Sciences and Environment Sustainability, Northern Arizona University, Flagstaff, Arizona 86001, USA; 4. Geology Department, Colgate University, Hamilton, New York 13346, USA; 5. Geology Department, Augustana College, Rock Island, Illinois 61201, USA; 6. Geology Department, Smith College, Northampton, Massachusetts 01063, USA

ABSTRACT The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 48.87 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (TuffZirc calculation) of 51.80 11.04/21.08 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n p 379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fact that nearly a third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanically twinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shortening strain axes (31/31 samples; n p 1231 twins) are now in a random orientation. Synemplacement calcite veins (3207, 907; n p 2) record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record of a strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwest- southeast motion. Poorly constrained cross sections and palinspastic restorations indicate 125 km of shortening (and uncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightly earlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintained its internal stratigraphy and did not disaggregate while in motion.

Online enhancements: appendixes, supplementary tables.

Introduction The tectonic evolution of northwest Wyoming is cratonic strata. During Cretaceous time, the region rich in complexity. Archean basement rocks are was part of the Western Interior Basin, and several overlain by about 1000 m of Paleozoic and Mesozoic thousand meters of marine, marginal marine, and terrestrial strata accumulated in the area. By latest Cretaceous time, the Sevier orogeny caused the de- Manuscript received November 18, 2014; accepted May 11, 2015; electronically published June 19, 2015. velopment of the north-south-trending east-verging * Author for correspondence; e-mail: craddock@macalester overthrust belt along the Idaho-Wyoming border. As .edu. these structures were active, 11000 m of Paleocene † Present address: Department of Geology and Geophysics, synorogenic strata of the Fort Union Formation was University of Texas, Austin, Texas, USA. deposited in the foreland east of the Sevier belt. ‡ Present address: Geology Department, Colorado School of Mines, Golden, Colorado 80401, USA. During this time, thick-skinned Laramide foreland § Present address: Geology Department, Boise State Uni- deformation began, which resulted in the uplift of versity, Boise, Idaho 83725, USA. the Beartooth Mountains (DeCelles et al. 1991),

311 312 J. P. CRADDOCK ET AL.

Cody Arch, Teton Range, Wind River Range (Steidt- Previous Work mann and Middleton 1991), Rattlesnake Mountain The HMS is one of the most controversial and (Omar et al. 1994; Neeley and Erslev 2009), and enigmatic geologic features, representing the larg- Washakie Range as well as the coeval subsidence of est and best-preserved volcanic landslide structure the Absaroka and Bighorn Basins (Armstrong and on Earth (Bucher 1933, 1947; Malone 1995; Crad- Oriel 1965; Dorr et al. 1977; Wiltschko and Dorr dock et al. 2000; Beutner and Gerbi 2005; Craddock 1983; Bird 1998; Peyton et al. 2012; Painter and et al. 2009; Anders et al. 2010). Allochthonous, up- Carrapa 2013; Fan and Carrapa 2014). As much as right, mountain-sized blocks of Paleozoic carbon- 1500 m of lower Eocene Willwood Formation con- ate and Eocene volcanic rocks are dispersed over an glomerate, sandstone, and mudstone filled these area of 3400 km2, with a displacement of as much as adjacent basins, and a regional unconformity exists 110 km on a surface that dipped !27 (Beutner and at the base of the Willwood Formation. During the Gerbi 2005; Malone and Craddock 2008; Craddock late stages of the Laramide orogeny, beginning in et al. 2009). Over the past 20 yr, most researchers Middle Eocene time, widespread volcanism began have accepted that emplacement of allochthonous in the Absaroka Range far from any plate bound- rocks was initiated by the catastrophic collapse of ary (Smedes and Protska 1972; Sundell 1993). The a stratovolcano ∼50 m.yr. ago and that the entire emplacement of the Heart Mountain slide (HMS) event took place over a period of a few minutes to and the South Fork slide (SFS) were broadly con- hours (Malone 1995; Beutner and Gerbi 2005; Ahara- temporaneous with the inception of Absaroka vol- nov and Anders 2006; Craddock et al. 2009; Anders canism (fig. 1; Feeley et al. 2002). et al. 2010; Malone et al. 2014b). The SFS is exposed over an area of ∼800 km2,and The SFS is exposed in the valley of the South Fork the internal structural grain has an orientation of the Shoshone River, which has relief of ∼1500 m, of ∼N507 E, which is oblique to the Sevier (north- and the SFS is overlain by hundreds of meters of flat- south structures; east-west shortening; Craddock lying Absaroka Volcanics (fig. 2). Dake (1918) first 1992; Weil et al. 2009) and Laramide (northwest- reported the complexities of these folded Jurassic- southeast structures; southwest-northeast short- Eocene sediments exposed southwest of Cody, ening as slab dip shallowed; Craddock and van Wyoming, which include a 10-km-wide belt of folded der Pluijm 1999; Weil and Yonkee 2012) struc- strata with northwest- and southeast-bounding tures, thereby suggesting that if the SFS is a tec- faults and internal folds all oriented N 507 E(figs. 1, tonic feature, it formed as part of a local, unknown 2). The SFS system consists of as much as 1200 m Eocene orogeny (Blackstone 1985; Clarey 1990). An (stratigraphic thickness) of allochthonous Jurassic alternative explanation, based on the proximity of through Eocene strata, and transport distances of at the SFS to the allochthonous HMS mass, is that least 10 km are evident; bounding faults are clear, but the SFS is an Early Eocene landslide event (Pierce there is sparse kinematic data. Drill data suggest a 1957; Clarey 2012) that just preceded the HMS detachment at a depth of ∼250 m for oil wells drilled within a very short temporal window (Malone et al. to ∼1500 m in the producing Permian Phosphoria 2014b). (Pierce and Nelson 1969; Clarey 2012). HMS and SFS Spatial and temporal relationships have been es- details are lost on the scale of the published geologic tablished for the HMS (see below), whereas the gen- maps (1∶62,500; Pierce and Nelson 1968, 1969; esis of the SFS is constrained only as being younger Pierce 1970, 1997). than the Eocene Willwood Formation, which it Allochthonous rocks of the SFS have been inter- crosscuts and overlies. The goals of our work were preted as the front of a massive southeast-directed fourfold: (1) to remap and describe in detail the crit- gravity slide that both predates and is unrelated to ical, well-exposed regions of the SFS in the vicinity the HMS (Pierce 1957, 1973; Blackstone 1985), as the of Belknap Creek, a north-draining tributary to the easternmost expression of the Cordilleran over- South Fork of the Shoshone River; (2) to constrain thrust belt (Clarey 1990), and as the coevally em- the regional paleogeography at the time of South placed toe of the HMS (Beutner and Hauge 2009; Fork deformation using U-Pb detrital zircon geochro- Clarey 2012). nology of the Eocene Willwood Formation; (3) to determine the strain relationships of upper- and lower- plate rocks during deformation using mechanical Methods twins in calcite; and (4) to develop a kinematic model for the emplacement of the SFS allochthonous rocks Detrital Zircon Geochronology. U-Pb geochronol- that is consistent with these new data. ogy of zircons was conducted using a laser ablation Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 313

Figure 1. Geodynamic base map of Wyoming (A) with the location of the South Fork detachment structures (open star) in the South Fork of the Shoshone River valley (B). Inset B is overlain on a LIDAR base (inset C)toshowthe locations of the Tertiary detrital zircon (DZ) sample sites (fig. 2C). Cross sections of the northwest-bounding fault are shown in figure 3. Seismic station SNFF is denoted by a filled star. multicollector inductively coupled plasma (ICP) tube of sufficient width that U, Th, and Pb isotopes mass spectrometer (MS) at the Arizona LaserChron are measured simultaneously. All measurements are Center (Gehrels et al. 2006, 2008). The analyses made in static mode, using Faraday detectors with involve ablation of zircon with a Photon Machines 3 # 1011-Ω resistors for 238U, 232Th, and 208Pb-206Pb Analyte G2 excimer laser (or, prior to May 2011, a and discrete dynode ion counters for 204Pb and 202Hg. New Wave UP193HE excimer laser) using a spot Ion yields are ∼0.8 mv/ppm. Each analysis consists diameter of 30 m. The ablated material is carried of one 15-s integration on peaks with the laser off in helium into the plasma source of a Nu high- (for backgrounds), fifteen 1-s integrations with the resolution ICP-MS, which is equipped with a flight laser firing, and a 30-s delay to purge the previous Figure 2. Localized geologic features of the South Fork slide (SFS) in the valley of the South Fork of the Shoshone River, including a schematic cross section (A), the local structural complexities seen along Rock Creek (see fig. 3, section F–F0; B), and the ages of detrital zircons in the Willwood, the youngest rocks deformed in the SFS (C). See figure A1 (available online) for descriptions of labels. Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 315 sample and prepare for the next analysis. The abla- contains the same area) and then stacks the prob- tion pit is ∼15 m in depth. ability curves. For each analysis, the errors in determining Calcite Twin Analysis. The calcite strain-gage 206Pb/238Uand206Pb/204Pb result in a measurement technique (CSGT) of Groshong (1972) allows inves- error of ∼1%–2% (at 2j level) in the 206Pb/238U age. tigation of intraplate stresses as preserved by me- The errors in measurement of 206Pb/207Pb and 206Pb/ chanical twinning of rock-forming calcite grains re- 204Pb also result in ∼1%–2% (at 2j level) uncer- solving three-dimensional stress (Turner 1953) and tainty in age for grains that are 11.0 Ga, but they strain tensors. The Groshong technique has been are substantially larger for younger grains because used to measure strain tensor directions in calcite of the low intensity of the 207Pb signal. For most veins (Kilsdonk and Wiltschko 1988; Craddock and analyses, the crossover in precision of 206Pb/238U Pearson 1994), limestones (Engelder 1979; Spang and and 206Pb/207Pb ages occurs at ∼1.0 Ga. Groshong 1981; Wiltschko et al. 1985; Mosar 1989; 204Hg interference with 204Pb is accounted for by Evans and Dunne 1991; Ferrill 1991; Craddock and measurement of 202Hg during laser ablation and van der Pluijm 1999; Craddock et al. 2000), marble subtraction of 204Hg according to the natural 202Hg/ (Craddock et al. 1991; Craddock and Craddock 2012), 204Hg ratio of 4.35. This Hg correction is not sig- and calcite fault gouge (Gray et al. 2005). nificant for most analyses because our Hg back- Under temperatures of ∼2007C, intracrystalline grounds are low (generally ∼150 cps at mass 204). deformation of calcite mainly leads to e-twins. The Common Pb correction is accomplished by using formation of calcite e-twins requires a shear stress the Hg-corrected 204Pb and assuming an initial Pb exceeding ∼10 MPa (Wenk et al. 1987; Burkhard composition from Stacey and Kramers (1975). Un- 1993; Lacombe and Laurent 1996; Ferrill 1998). Cal- certainties of 1.5 for 206Pb/204Pb and 0.3 for 207Pb/204Pb cite offers three glide systems for e-twinning. From are applied to these compositional values on the U-stage measurements of width, frequency, and ori- basis of the variation in Pb isotopic composition in entation of twins and the crystallographic orien- modern crystal rocks. tation of the host crystals, a strain tensor can be cal- Interelement fractionation of Pb/U is generally culated using a least squares technique (Groshong ∼5%, whereas apparent fractionation of Pb iso- 1972). To remove noise from the data set, a refine- topes is generally !0.2%. In-run analysis of fragment of the calculated strain tensor can be achieved ments of a large zircon crystal (generally every fifth by stripping 20% of the twins with highest devia- measurement) with known age of 563.5 5 3.2 Ma tions (Groshong et al. 1984). This procedure was used (2j error) is used to correct for this fractionation. if the number of measured grains were large (n 1 20). The uncertainty resulting from the calibration cor- In cases where the data appear to be inhomoge- rection is generally 1%–2% (2j)forboth206Pb/207Pb neous, the separation of incompatible twins (nega- and 206Pb/238U ages. Concentrations of U and Th are tive expected values [NEVs]) from compatible twins calibrated relative to our Sri Lanka zircon, which (positive expected values) in the initial data set contains ∼518 ppm of U and 68 ppm of Th. allows separate calculation of two or more least The analytical data are reported in table S1 squares deviatoric strain tensors. Thus, the CSGT (tables S1–S3 are available online). Uncertainties can be used to obtain information on superim- shown in these tables are at the 1j level and in- posed deformations (Groshong 1972, 1974) and dif- clude only measurement errors. Analyses that are ferential stress magnitudes (Rowe and Rutter 1990). 120% discordant (by comparison of 206Pb/238Uand Fabric interpretations are based on the proximity 206 207 Pb/ Pb ages) or 15% reverse discordant (ta- of the shortening axis (ϵ1) and contoured compres- ble S1) are not considered further. The resulting sion axes maxima (Turner 1953) to a sample’s bed- interpreted ages are shown on Pb /U concordia di- ding plane. Layer-parallel shortening (LPS) results agrams and relative age-probability diagrams using when the bedding plane great circle intersects the the routines in Isoplot (Ludwig 2008). The age- compression axis contours and the shortening axis probability diagrams show each age and its uncer- is 5207 to the bedding plane anywhere along the tainty (for measurement error only) as a normal great circle. distribution and sum all ages from a sample into a The data-stripping procedure was validated in single curve. Composite age probability plots are experimental tests that showed the reliability of made from an in-house Excel program (see the Anal- the technique in multiply deformed limestones ysis Tools section on the LaserChron website for (Groshong 1974; Teufel 1980). The stripping proce- a link) that normalizes each curve according to the dure was used in cases of high proportions of NEVs number of constituent analyses (such that each curve and a high number of measured grains. An experi- 316 J. P. CRADDOCK ET AL. mental reevaluation of the CSGT has shown that ment Center using SOD (Owens et al. 2004) for measurements of about 50 grains in one thin sec- all events 257–957 distant between April 1997 and tion or 25 grains from two mutually perpendicular April 2014 with magnitudes 15.8. A total of 568 ra- thin sections yield the best results (Groshong et al. dial receiver functions were calculated from 356 1984; Evans and Groshong 1994; Ferrill et al. 2004). events using a Gaussian pulse width of ∼1 s. To re- The chance to extract the records of more than two move erroneous receiver functions, those with low deformations from one data set is limited when deal- variance reduction (80% on the radial component ing with natural rocks (Burkhard 1993); more mea- and 60% on the tangential) were removed. Addi- sured twins increases this possibility of detecting tionally, all receiver functions were visually ana- multiple deformations. Individual analyses of veins, lyzed to assess quality, and those with extreme matrix, nodules, and so on allows the acquisition of amplitudes, few or numerous zero crossings, and several strain tensors without applying statistical negative first arrivals were removed. Receiver func- data stripping. Application of the CSGT requires the tions were migrated to depth using a P-wave veloc- following assumptions to be valid: (1) low temper- ity of 6.4 and a Vp/Vs ratio of 1.75 and are shown as atures (dominance of type I and II twins; see below), elevation below sea level. H-K stacking (Zhu and (2) random c-axis orientation of calcite, (3) homog- Kanamori 2000) was attempted but proved largely enous strain, (4) coaxial deformation, (5) volume unreliable due to the complicated crustal structure constancy, (6) low-porosity materials, and (7) low and the lack of a clear Moho conversion. strain (!15%). If these conditions are not fully met, For station SNFF, located within the South Fork the underlying data set of the calculated strain ten- valley, we analyzed receiver functions for indi- sor could be biased. Strain tensors were calculated cations of dipping crustal layers and anisotropy. from calcite e-twin data sets using the software pack- Anisotropy and dipping layers produce distinctive age of Evans and Groshong (1994). polarity reversals on the tangential component of Seismic Investigations. P-wave seismic receiver receiver functions when plotted by back azimuth. functions utilize scattered energy from teleseismic To evaluate the possibility of dipping layers and earthquakes to image seismic velocity contrasts crustal anisotropy, we used a neighborhood algo- within Earth (e.g., the Moho discontinuity), and our rithm search (Fredericksen et al. 2003) to analyze goal was to use this method to generate imagery these signals, as outlined in Porter et al. (2011). The of seismic discontinuities, if any, within the SFS search was conducted to fit the signal between 0 area. The depth of interfaces within the subsurface and 6 s in the receiver functions, assuming a three- are determined by measuring the delay time be- layer model consisting of an upper crustal layer with tween the initial P-wave arrival and the P- to S-wave a P-wave velocity of 5 km/s, a lower crustal layer conversions that form at these velocity contrasts. with a P-wave velocity of 6.4 km/s, and an upper In radial receiver functions, positive polarity P to mantle layer with a P-wave velocity of 7.8 km/s. A

S conversions are indicative of an increase in veloc- fixed Vp/Vs ratio of 1.75 was assigned to the crust, ity with respect to depth, while negative conver- while 1.74 was used for the upper mantle. sions represent a decrease in velocity with respect to depth. Higher amplitudes are indicative of sharper and/or greater velocity contrasts within Earth, while Results lower amplitudes suggest the opposite. In a region with isotropic seismic velocities and flat-lying lay- Field Relationships. We have mapped the Belknap ers, all converted energy is contained within the ra- Creek quadrangle in the north-central portion of the dial component of the receiver function. By analyz- SFS (1∶24,000; Malone et al. 2012; app. A; apps. A–D ing the tangential component of receiver functions, are available online). For mapping purposes, we se- it is possible to quantify seismic anisotropy and dip- lected the best-exposed and most centrally located ping layers within the subsurface. area in Pierce’s broader South Fork Fault interpre- We calculated receiver functions from seismic tations. The best-exposed structure in the SFS is an stations in the vicinity of the South Fork of the upright, open, periclinal syncline that trends N 507 E Shoshone River valley and surrounding area to ex- and plunges 307 to the northeast and southwest. amine subsurface structure. Receiver functions The northwest-bordering fault (N 507 E) can be in- were calculated using a time-domain pulse-stripping terpreted as both a normal and a thrust fault along technique (Ligorría and Ammon 1999) on data from strike (see below) and places the Jurassic Sundance six stations in the vicinity of the SFS. Earthquake Formation in fault contact with the Eocene Willwood data were downloaded from the Incorporated Re- Formation. The southeast border (N 507 E, 457 SE) search Institutions for Seismology Data Manage- includes upright, southeast-dipping Cretaceous Fron- Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 317 tier Formation in contact with overturned Creta- nous Wapiti volcanic rocks range in age from 49.44 ceous sediments. The gross structure is an upper to 48.99 Ma. The HMS occurred at 48.87 Ma (Malone plate of upright Jurassic Sundance to Cretaceous et al. 2014b). These geochronologic ages constrain Mowry Formations that is a periclinal syncline. the SFS structures to have formed in a ∼2,400,000-yr This fold overlies an upright sequence of Sundance- window between 51.80 and 49.44 Ma (fig. 2; table S1). Morrison Formations that is subhorizontal along Moreover, the SFS was likely emplaced during the the northwest side of the structure but is subvertical latter stages of this time interval to afford time to and overturned on the southeast side. The Sundance- incise and erode several hundred meters of the Will- Morrison Formations overlie a complex section of wood Formation and underlying strata. Jurassic Morrison-Cretaceous Frontier strata that Northwest Border. Theboundingfaultonthe are primarily subvertical, overturned, and in contact northwest side of the SFS is oriented N 507 E, with with southeast-dipping Cretaceous Frontier strata variable dip and dip direction along a length of on the southeast side (fig. 2). ∼40 km (fig. 3). Six northwest-southeast cross sec- Detrital Zircon Geochronology. Uppermost Will- tions (A–F) demonstrate the variability of structural wood Formation sandstone units were sampled at style and offset along strike (fig. 4). In the five north- five localities in the South Fork Shoshone River val- eastern profiles (A–E), the Sundance Formation over- ley: 11-WY-32 (Mariposa Ranch), 11-WY-33 (Car- lies the younger Eocene Willwood Formation. In ter Mountain Road), 11-WY-34 (Siggins Ranch), 11- the southwestern profile (TE Ranch), subvertical WY-35 (Rock Creek), and 11-WY-36 (Castle Rock Cody Formation is in fault contact with horizontal Ranch; fig. 1; table S1). Figure 2C is a histogram of Willwood Formation. From northeast to southwest, the detrital zircon age spectrum (n p 379) from all samples. The majority of Willwood detrital zircons (71%) are Archean in age, which most likely were derived from the Archean Beartooth uplift. Proterozoic zircons amount to 15% of the detrital zircon spectrum; Paleozoic rocks comprise ∼2% of the spectrum. Mesozoic zircons amount to 8% of the age spectrum and are most likely first-cycle sediment derived from the Idaho Batholith to the west. Eocene zircons amount to 3% of the Willwood detrital zircon spectrum and were most likely derived from volcanic and plutonic rocks in the northern Absa- roka Range. Ludwig (2008) provides a macro that uses a variant of the Monte Carlo statistical technique for determining the youngest TuffZirc age of the sandstone. This technique is used when the youngest grain split was formed at the same time as sandstonedeposition(Maloneetal.2014b,2014c).By this methodology, the youngest Willwood zircon is 51.80 (11.04/21.28) Ma, based on the 10 Eocene zircons analyzed. This means that the uppermost Willwood Formation that rests directly beneath the SFS allocthonous rocks must be younger than its youngest included fragment and that the upper Will- wood Formation cannot be any older than this age (51.80 Ma). The South Fork of the Shoshone River valley has ∼1500 m of relief from the river bottom to the tops of the Absaroka Volcanics at Carter Mountain and Wapiti Ridge. The SFS is overlain by autochtho- Figure 3. Schematic drawings, from the northeast to nous volcanic rocks of the Wapiti Formation, which the southwest, along the northwest border fault of the themselves are overlain by allochthonous Paleo- SouthForkslide.Seefigure 1 for cross section locations zoic and volcanic rocks of the HMS. Malone et al. and local stratigraphy. See figure A1 (available online) for (2014c) determined that the age of the autochtho- descriptions of labels. 318 J. P. CRADDOCK ET AL.

Cretaceous Thermopolis-Frontier section is in thrust contact (top to the northwest?) with a thrust-repeated section of Sundance-Morrison. Fault kinematic indicators are common in float but were not found in situ (see below), and, in the context of the entire SFS, the absolute fault motions are not constrained, nor do the local, relative fault offsets show any structural consistency. Internal Deformation. Detailed mapping has revealed a variety of small-scale structures that bear on the evolution of the SFS (see fig. 4A for field locations of figs. 4–8; apps. A–C). The northwest side of the structure places an upright syncline in fault contact with Tertiary Willwood Formation (fig. 4B). The southeast half of the structure is composed of Jurassic Morrison–Cretaceous Frontier Formation sequences that are overturned, dip to the northwest, and are in fault contact with Cretaceous strata that dip to the southeast and are upright (fig. 5A,5B). This overturned sequence has local, internal folds and is overlain by a chaotic sequence of Sundance and Morrison Formations (fig. 5A). In addition, within the overturned Morrison-Frontier Formation section we discovered faults that strike N 507 E with

Figure 4. A, Air photo basemap of the South Fork slide area with inset figure numbers of photos of internal deformation. B, View to the southwest of the syncline that overlies the horizontal Willwood Formation. See figure 2 for stratigraphic column. See figure A1 (available online) for descriptions of labels. the offsets are as follows: (1) a southeast-dipping fault with hanging wall anticline (Castle Rock Angus Ranch); (2) a northwest-dipping normal fault (fault drag) with hanging wall anticlines (north Carter Moun- tain Road); (3) a southeast-dipping fault with synthetic small-offset thrusts in the footwall Willwood Formation (Carter Mountain Road); (4) a southeast- dipping fault with a truncated footwall syncline in the Willwood Formation and disharmonic folds in the Cody Formation (southwest Carter Mountain Road); (5) a southeast-dipping fault with Sundance klippe (vertical bedding) resting on flat-lying Will- wood and Cody Formations (north TE Ranch); and Figure 5. View northeast of Jurassic Sundance klippe (6) a vertical fault that juxtaposes horizontal Will- over Morrison Formation and the Sundance (Js) in fault wood against subvertical Cody and that includes contact with the overturned Jm-Kt section (A). B is sim- northwest-dipping thrusts (top to the southeast; JE ilar but is viewed toward the southwest. See figure A1 Ranch). Just to the southeast, the northwest-dipping (available online) for descriptions of labels. Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 319

dicators were not observed, so this may be a paleovalley wall. Barrel Fold. Sheath folds were first described in high-grade rocks as tubelike structures that have a closed hinge in three dimensions and are often useful as a kinematic indicator (Cobbold and Quinquis 1980; see the review in Alsop and Holdsworth 1999). Within the middle portion of the SFS structure, seen in two cross sections (fig. 9) and restored (fig. 10), we discovered a large feature (∼1 km by 0.5 km in a vertical face) that appeared to be a faulted, refolded fold (fig. 11). Closer inspection revealed that the fold has a core of Morrison Formation mudstones (no top and bottom indicators were observed) surrounded completely by fossiliferous Sundance For- mation shales juxtaposed against a subvertical section of overturned Cloverly Formation sandstone. The subvertical Cloverly-Sundance contact is a fault with top-to-the-southeast striations. The structure is overlain (surrounded?) by Morrison Formation (no

Figure 6. Two examples of internal faulting in the Morrison Formation. Lip balm is shown for scale in A, and students are shown for scale in B.

∼457 dips that, in one case, places upright sections of Morrison against each other (fig. 6A). Along strike, we found a fault with the same strike, an opposing northwest dip, and two overturned sections of Mor- rison Formation in fault contact (fig. 6B; see bottom- on-bottom structure of Van Hise and Leith 1911). The northwest continuation of the uppermost Sun- dance Formation (Js in fig. 5A) is preserved as a variety of klippe with vertical bedding resting on horizontal Willwood Formation (fig. 7A). We also discovered a sliver of Cretaceous Thermopolis For- mation between the overturned Cretaceous Clo- verly and Jurassic Morrison Formations (fig. 7B; see the stratigraphic section in fig. 2). Southeast Border. The southeast border is also 7 1 Figure 7. Vertical klippe of Jurassic Sundance Forma- oriented N 50 E and can be traced for 20 km. This tion on horizontal Tertiary Willwood Formation (A)and boundary juxtaposes southeast-dipping, upright Cre- a fault-bounded inlier of Cretaceous Thermopolis For- taceous Thermopolis-Frontier Formation strata in mation between overturned Morrison and Cloverly For- contact with an overturned section of Morrison- mations (B). See figure A1 (available online) for descrip- Frontier Formation (figs. 5, 8). Fault kinematic in- tions of labels. 320 J. P. CRADDOCK ET AL.

surface with interlayered calcite, sulfur, and organ- ics (“oil”;see“Acknowledgments”). Above the fault gouge locale, the Sundance Formation is a fossiliferous limestone (N 407 E, 907), the twinned calcite records a shortening strain (2167,207)thatislayer parallel, and there is a modest strain overprint that indicates a secondary LPS perpendicular to the original shortening strain (28% NEVs; samples 13 and 14; table S2; see below). Both shortening axes trend parallel to the barrel fold, but the primary shortening strain axis plunges to the southwest, and the overprint shortening strain plunges to the northeast. Calcite Twinning Strains. Thirty-three strain results from the Sundance Formation limestones are reported in figures 12, 13, and 14 and in table S2. Sample locations are shown in appendix B. We have Figure 8. View approximately N 807 E of overturned chosen three sample divisions: (1) samples clearly Frontier Formation. Shown is Kf in contact with southeast- outside the SFS, namely, the foreland group in the dipping, upright Frontier Formation, which we interpret vicinity of Rattlesnake Mountain (n p 3); (2) sam- as a steeply inclined paleovalley wall. Section 21, Belknap ples in proximity to the SFS but clearly in the autoch- Quadrangle. See figure A1 (available online) for descrip- thonus stratigraphic section below the main SFS tions of labels. structure (n p 5); and (3) samples from the SFS structure (figs. 2B,3,4A–10; n p 23). The lower plate top and bottom indicators were observed), and fault contains the upright Sundance-Willwood section ex- drag indicates that the Morrison was thrust over posed along the South Fork of the Shoshone River the Sundance and internal Morrison Formations ∼300 m below and to the northwest of where the from the southeast to the northwest. Since the Sun- Willwood is offset by the overlying northwest- dance is a continuous layer around the structure, bounding fault of the SFS (figs. 1, 2A,3,4A). The we have named this a “barrel fold,” indicative of ro- shortening strain magnitudes (24%), differential tation about a horizontal northeast-southwest axis. stress magnitudes (220 MPa), and NEVs (low) are uni- The interior of the fold is Morrison, which is every- form throughout the sample suite (table S2; Rowe where in fault contact with the older Sundance. and Rutter 1990). The rotation direction is clockwise relative to the Foreland Twinning Strains. The three foreland kinematic sense along the Cloverly-Sundance con- Sundance samples all come from the limbs of the tact (overturned section, with a younger-over-older Laramide, basement-cored Rattlesnake Mountain relationship; normal faulting) but counterclockwise structure, are not part of the SFS, and are thereby in relative to the continuous layer of the Sundance For- the SFS foreland (fig. 12; table S2). All three samples mation (older-over-younger relationship; thrusting). preserve an LPS strain with none of the shortening A top-to-the-northwest interpretation also gives a axes parallel to the distal Sevier orogeny east-west sense of kinematic rotation. The Cloverly Forma- shortening (Craddock and van der Pluijm 1999) or in tion complicates the interpretation because the ki- any consistent relationship to the Laramide Rattle- nematics indicate that it was thrust up into the snake Mountain structure (see Neeley and Erslev structure, yet it is younger than the Sundance and 2009). See “Discussion” for references to the distal Morrison Formations. The absolute sense of rota- Sevier strain field. tion is mind-boggling, but the rotation direction is Lower-Plate Sundance Formation. Five lower- northwest-southeast about a horizontal axis. A pal- plate samples to the southwest and northeast of the inspastic reconstruction of the outcrop indicates SFS upper plate were analyzed (fig. 12; table S2). All that the Sundance Formation thickness is reduced lower-plate samples preserve LPS. The lower-plate 1500%, from 220 to 0.3 m (at its thinnest); that the samples to the southwest all contain a minimal Morrison Formation is reduced 50%, from 133 to number of NEVs (average: 6%), which indicates only 66 m; and that the Cloverly Formation is reduced one twinning event. Shortening axes in the lower- 500%, from 100 to 5 m. We did not observe a con- plate samples proximal to the SFS have no clear tinuation of the structure along trend, as the barrel pattern. fold is in the lower plate of the SFS. The southeast Upper-Plate Sundance Formation. All of the 23 contact of the Morrison-Sundance contains a faulted upper-plate Sundance Formation twinning samples Figure 9. Schematic cross section of the South Fork slide (see fig. 2A) inferred at depth. See figure A1 (available online) for descriptions of labels. Figure 10. Palinspastic restoration across the South Fork slide based on figure 9 and the interpretation that there are two structural levels, the lower of which is an overturned fold (see “Discussion”). See figure A1 (available online) for descriptions of labels. Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 323

shortening axes trending northwest-southeast with very shallow plunges and no strain overprint. Seismic Reflection Structure. The limited industry- generated seismic reflection data across the Sho- shone valley is old, grainy, and does not document any basement rock involved in the SFS (Britten- ham and Tadewald 1985). The TE Ranch oilfield was developed in 1959, and five wells currently produce ∼160 barrels of oil per month from the Phosphoria Formation at a depth of ∼1500 m, which is well below the SFS deformed interval (Wyoming Oil and Gas Commission website). We used the seismic stations in the area (fig. 15) and historic seismic data to better resolve local seismic discontinuities beneath the SFS. To calculate receiver functions for northwest Wyoming, we utilized a time-domain pulse-stripping technique (Ligorría and Ammon 1999) to calculate receiver functions for seven stations in the vicinity of the SFS. Earthquake data were downloaded using SOD (Owens et al. 2004) for all events 257 to 957 distant between April 1997 and April 2014 with magnitudes greater than 5.8. A total of 577 radial receiver functions were calculated from 356 events using a Gaussian pulse width of ∼0.75 s. To remove erroneous receiver functions, those with low variance reduction (80% on the radial component and 60% on the tangential) were removed. Additionally, all receiver functions were analyzed visually to assess quality, and those with extreme amplitudes, few or numerous zero crossings, and negative first arrivals were removed. Receiver functions were fi Figure 11. Barrel fold with eld photo (top) and inter- migrated to depth using a P-wave velocity of 6.4 and pretation (bottom). Photo width is ∼1 km. See figure A1 a V /V ratio of 1.75 and are shown as elevation be- (available online) for descriptions of labels. p s low sea level. H-K stacking (Zhu and Kanamori 2000) was attempted but proved unreliable due preserve an LPS fabric with low NEVs (12.2%; ta- to the complicated crustal structure and the lack of ble S2; fig. 13). We presume that the LPS twinning a clear Moho conversion. Notably, receiver func- strains are related to the Sevier orogeny (Craddock tions at all of the stations analyzed have a strong 1992; Craddock and van der Pluijm 1999; Craddock positive conversion in the first 1–2 s, consistent with et al. 2000) and are older than the SFS structure. a sharp increase in velocity at depth less than or Samples from the western edge, central area, or east- equal to ∼16 km (fig. 15; app. D). ern area of the SFS allochthon do not appear to have To better understand shallow crustal structure any distinct spatial strain pattern, even when bed- within the region of the SFS, we focused our analysis ding and the corresponding shortening axes are ro- on station SNFF, which lies within the South Fork tated to their presumed original horizontal orien- Valley and sits on allochthonous rocks of the SFS. tation (fig. 14). There is no evidence of a twinning The tangential receiver functions, when plotted by strain overprint. back azimuth, show strong polarity reversals at 0 s, Upper-Plate Calcite Veins. Although the two indicative of a dipping layer (e.g., Porter et al. 2011) calcite veins analyzed are from different locations at a depth of !1 km. We utilized a neighborhood within the upper plate, hosted in Sundance lime- algorithm search to determine the orientation of stones, their twinning strain results are nearly iden- the strike and dip of the layer and estimate that it tical (fig. 13; table S2). Sundance Formation bed- has a strike of ∼0677 and dip of ∼97 SE. Although our ding is horizontal, vein orientations are the same grid search does not match the amplitudes well, the (N 407 W, 907), and they each exhibit an LPS with the strike is determined by the location of the polarity 324 J. P. CRADDOCK ET AL.

Figure 12. Lower-hemisphere projections of calcite twinning strain data for the foreland adjacent to the South Fork slide (SFS; upper) and for the lower plate of the SFS. All samples are from the Sundance Formation. See table S2 and appendix A. Turner (1953) compression axes are contoured; ϵ1 is the maximum shortening strain; ϵ2, the intermediate axis; and ϵ3, the extension axis. Great circles are bedding. reversal and is a much more robust measurement Craddock 1992; Peyton et al. 2012; Fan and Carrapa than dip. This dipping layer is geometrically com- 2014). The Sevier structures resulted from east- patible with the projected geometry of the basal SFS west shortening with folds and thrusts that are ori- detachment. ented north-south, whereas Laramide basement uplifts have many orientations due to the shallowing slab-dip along the western margin of North Amer- Discussion ica (Bird 1998; Craddock and van der Pluijm 1999; The Wyoming province is dominated by late Jurassic- DeCelles 2004; Painter and Carrapa 2013) and are Paleocene Sevier thin-skinned structures that over- not found north of ∼467N (Gries 1983, 1990). The SFS lap in space and time with late Cretaceous-Eocene structural grain is not oriented parallel to either Se- thick-skinned Laramide uplifts (Armstrong and Oriel vier or Laramide structures and thus is unique in the 1965; Wiltschko and Dorr 1983; Steidtmann 1991; Wyoming province (ﬁgs. 1–3). Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 325

Figure 13. Lower-hemisphere projections of Sundance Formation calcite twinning strains from the upper plate of the South Fork slide. See ﬁgure 12 for details.

The valleywide field relationships constrain the which is as old as 49.44 Ma, and eventually the SFS to having formed after deposition of the youn- HMS, at 48.87 Ma (fig. 2). At first glance, the SFS gest detrital zircon in the Eocene Willwood Forma- appears to be thin skinned in origin but lacks the tion (51.80 Ma) and before being overridden and typical LPS strains, axial planar cleavage, and joint buried by volcanic rocks of the Wapiti Formation, sets of a thrust sheet. We interpret the southeast 326 J. P. CRADDOCK ET AL.

Figure 14. Lower-hemisphere projections of layer-parallel shortening strain axes from the Sundance limestones for the foreland (left), lower (center), and upper (right) plates of the South Fork slide, with strain axes plotted in situ (top) and with bedding rotated to horizontal (bottom). See table S2. boundary to be a paleovalley wall (fig. 8), where the All calcite twinning data reported here, with mod- southeast half of the structure is overturned (figs. 4A, est strain magnitudes (24.3% average shortening), 9) and the northwest-bounding fault changes dip low NEVs, and modest differential stress magni- and offset direction along strike and lacks any in tudes (220 MPa), support the twinning fabric being situ fault kinematic indicators (figs. 3, 4B,7A). The a pre-SFS LPS strain fabric that formed as the older Sundance klippe on the northwest side of the struc- Sevier orogen evolved. Mechanically twinned cal- ture rests on flat-lying Willwood Formation, but the cite in Paleozoic through Mesozoic carbonates pre- Sundance bedding is vertical (fig. 7A). The other in- serves an LPS fabric that is predominately east-west ternal structures (figs. 4–9) are atypical of a thrust throughout the Idaho-Wyoming thrust belt (Crad- belt, especially the barrel fold (fig. 11), a structure dock 1992; Craddock and van der Pluijm 1999; Weil never reported. and Yonkee 2012) and is present 2000 km into the The receiver function seismic anisotropy result plate interior, where calcite twins from the Creta- also supports a shallow (!1 km) P-wave discontinuity ceous Greenhorn Limestone in western Minne- that strikes parallel to the SFS, which could be a de- sota also preserve a horizontal, east-west-shortening tachment surface at the top of the Gypsum Springs strain. This pattern then serves as a passive strain Formation (i.e., the base of the SFS) and that the dips marker for the region because it makes it possible of this surface may represent paleotopographic relief. to track the younger allochthonous movement of Attempts at palinspastic reconstructions were rocks that have a preexisting twinning strain fabric based on the cross sections in figures 2B and 9, and (fig. 16; Craddock and van der Pluijm 1999). Lara- none of them are satisfactory (fig. 10; see MacNamee mide structures also contain the Sevier LPS and a 2012). Each of the structural layers is eroded off, so regional rotation with a consistent north-south sub- there is no constraint on the length of any SFS layer horizontal shortening recorded by Laramide calcite (no pinpoints; see below). A minimum of 25 km is veins (van der Pluijm et al. 1997; Craddock and van required to unfold and undeform the sequence, but der Pluijm 1999; Craddock and Relle 2000). Craddock there are unknown erosional gaps between segments, et al. (2000) used this Sevier LPS fabric to document so the total shortening (25 km) is a minimum and the that the autochthonous Cambrian limestones in total displacement of wither plate is unknown. the HMS footwall preserve this east-west LPS strain Figure 15. Location map of seismic stations surrounding the South Fork of the Shoshone River valley (A) and the seismic velocity profile beneath station SNFF (B), showing a strong velocity increase at a depth of ∼15 km. See appendix D for seismic profiles from other stations. C shows the radial and tangential components of receiver functions calculated at station SNFF plotted by back azimuth. The color image shows the calculated receiver functions, while the thick lines show the forward-modeled results from the neighborhood algorithm search. The 0-lag reverse in tangential polarities between 1807 and 2407 back azimuth is attributed to a dipping midcrustal layer. 328 J. P. CRADDOCK ET AL.

Most folds record an LPS strain normal to a fold’s axis. To evaluate this traditional observation, we plotted all the shortening axes from the Sundance limestones from the contact between the syncline and the northwest border fault. There are eight samples, from the northeast to the southwest, numbered 33, 32, 28, 27, 26, 25, 30, and 22 that are from the same topographic level in the syncline (ﬁg. 13; app. B; table S2) but not the same structural level (ﬁgs. 9, 10). All the samples have an LPS fabric and the same location on the fold

but very different ϵ1 orientations with respect to bedding. If this LPS fabric is Sevier orogen in age, then a very complex folding style is required to rotate a layer-parallel, east-west-shortening orientation to the current pattern (fig. 17). This syncline is the highest part of the SFS chaos (figs. 2C, 9). There is no SFS-related strain overprint except within the barrel fold (samples 13 and 14; fig. 13; table S2), which preserves an orthogonal shortening axis that trends parallel to the SFS but plunges in the opposite direction normal to the rotation direction of the barrel fold (northwest-southeast). Neither of these strain observations has been reported in Figure 16. Lower-hemisphere projections of calcite twin thin-skinned structures, again suggesting that the shortening axes from the autochthonous Heart Mountain SFS formed as a landslide deposit. footwall (A), the upper-plate allochthonous Heart Moun- The HMS liberated the Ordovician Bighorn- tain slide blocks (B; Craddock et al. 2000), the extended Mississippian Madison Group limestone section into foreland of the Sevier orogen (C; Craddock and van der a volcanic-sedimentary slurry that flowed to the Pluijm 1999), and the frontal Prospect (filled circles) and southeast. The upper-plate Paleozoic section broke distal, allochthonous Paris (open circles) thrusts of the Idaho-Wyoming thrust belt (D; Craddock 1992). Each plotted point represents the shortening strain axis for a sample where the shortening axis intersects local bedding and is a layer-parallel shortening strain. and that the allochthonous upper-plate Madison limestones have translated and rotated into a chaotic pattern as part of the Heart Mountain landslide (fig. 16). There is no calcite twinning strain overprint as a result of the younger HMS landslide, and the 23 upper-plate LPS twinning results in the SFS have a chaotic pattern (fig. 13; in situ or with bedding rotated to horizontal) much like the Heart Moun- tain allochthon plot (fig. 16). The absence of any twinning strain overprint in the HMS footwall (Crad- dock et al. 2000) and the SFS structures, which were overridden by the younger HMS, supports a landslide origin for both. Foreland LPS twinning results do not show an east-west-shortening pattern, which we attribute to their location near the Laramide Rattlesnake Mountain structure (Neeley and Erslev 2009 and references therein). Lower-plate Sundance Figure 17. Lower-hemisphere projection of layer-parallel 7– 7 limestones dip to the west and northwest (12 40 ) shortening axes (ϵ1) for samples located along the north- and preserve an LPS fabric but not in an east-west west side of the upper-plate syncline limb (N 507 E, 457 SW). orientation. See figures 4A and 11B, table S2, and appendix A. Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 329

ever, as the Willwood Formation strata studied here are Early Eocene in age, the ancestral Idaho River, as envisioned by Chetel et al. (2011), may have developed later, after Eocene volcanism in the Absa- roka Range would have formed an east-west drainage divide, forcing the regional drainage to shift from east-flowing to south-flowing rivers. The Early Eo- cene westerly sediment source is consistent with the paleogeographic reconstruction of the Crandall Con- glomerate (49.57 Ma; Malone et al. 2014a)farther to the north. All our local Bighorn Basin detrital zircon results indicate the basin had zircon sources from the west and north and an outlet to the south, which is not in agreement with the regional results of May et al. (2013). In any case, it is clear that a vigorous synorogenic sedimentary system was in operation during the early and middle Eocene in the Absaroka Basin and correlated with an uplift rate of ∼.38–.60 mm/yr (3 km) during the Early Eocene (Fan and Carrapa 2014). The Willwood Formation overlies the Lara- mide angular unconformity, which has 1500 m of paleotopographic relief evident in the vicinity of the Figure 18. Paleodrainage map of northwest Wyoming SFS. Furthermore, the Willwood Formation ranges in the Eocene, with the star showing the position of the from 0 to 300 m in thickness in the study area. This South Fork slide. variability in thickness is attributed to the paleotopography of the lower contact, as discussed, as well into hundreds of independent blocks (e.g., Heart as active erosion during and after the deposition Mountain) during its motion. If the SFS structure of the Willwood. Another major unconformity was formed as a landslide, it moved as a cohesive entity developed on top of the Willwood Formation prior (i.e., slump?), as there is continuity to the structure to deposition of the Eocene Wapiti Formation. This along strike. There is also plenty of evidence of surface also has several hundreds of meters of re- complex internal deformation, including the cha- lief, indicating that the potential for landslides into otic twinning strain patterns that suggest that the the Absaroka Basin was possible, supported by a structure formed as a landslide without additional structure contour map of the top of the Willwood twinning strain overprint as the slide moved. It Formation (fig. 19), which shows that the paleo- is easier to envision an additional landslide that is valley was steepest on the south side and may have nearly contemporaneous with the world’s largest been a breakaway of the SFS. catastrophic volcanically induced Heart Mountain Our field, structural, seismic, and geochronology landslide (Malone 1995; see also Hacker et al. 2014) results for the SFS suggest a genesis as a landslide, than propose a new orogeny that effected one small which, in proximity to the Heart Mountain land- intracontinental area in a time period of ∼2 m.yr. slide chaos and the steep stream gradients drain- The Willwood detrital zircon spectrum is doming the Laramide highlands, allows us to propose inated by Archean ages, indicating that the Bear- a kinematic model for the observations along the tooth uplift was supplying significant sediment to South Fork of the Shoshone valley. The crux of the this part of the Absaroka Basin. Smaller but signifi- problem is how to explain the repeated (two levels) cant zircon populations were derived from westerly Jurassic Sundance-Cretaceous Frontier sections, sources, most likely portions of the Idaho Batho- much of which is overturned, having arrived on top lith, which suggests through-going drainage from of upright Eocene Willwood Formation mudstones. west to east within the Absaroka Basin during Early The base of the Sundance Formation in the SFS is Eocene time (fig. 18). This is in contrast to the re- 300 m topographically higher than its autochtho- gionalpaleogeographic reconstruction of Chetel et al. nous equivalent lower in the South Fork of the Sho- (2011), where the principal river system in opera- shone valley. The nearest upright Sundance-Frontier tion during middle Eocene (47–49 Ma) time would section is ∼15 km to the northwest along the south have drained the Sevier Highlands to the south. How- flank of east-trending Pat O’Hara Mountain, and this 330 J. P. CRADDOCK ET AL.

Figure 19. Structure contour map of the top of the Willwood Formation. The Early Eocene (i.e., pre–South Fork slide) paleodrainage was roughly parallel to the current South Fork Shoshone River drainage. is the traditional up-gradient source of the SFS, al- then displaced (collapsed?) as a normal fault (top to though the area is now buried in Absaroka Volcanics the northwest), placing Sundance on a northwest- (Feeley et al. 2002). We propose two scenarios to ex- dipping upright section of Cretaceous sediments. A plain our field and structural observations. In sce- period of erosion ensued, after which a second slide nario 1, a slide block of Jurassic Sundance–Cretaceous was placed on the nappe (fig. 20). Scenario 2 is the Frontier Formations slid from the northwest along same sequence without the erosional period, where a base of Gypsum Springs evaporites on the Will- the nappe immediately collapses on itself, form- wood mudstones. This slide block was ∼20 km wide, ing the upper syncline, which moved to the north- ∼40 km long, and ∼1.2 km thick (stratigraphic thick- west (fig. 21). In either case, ∼40 km of the Jurassic- ness). As the block approached the paleo-Shoshone Cretaceous section is found stacked in a paleovalley valley with its steep valley walls (∼307 in fig. 5B; ∼10 km wide that corresponds to the seismic discon- figs. 3, 8), the leading edge of the slide overturned tinuity at ∼1 km depth. So, palinspastically, shorten- and a nappe evolved that slid to the southeast up ing is ∼30 km with an unknown translation (slide) the southeast valley wall. The hinge of the nappe, distance. The internal barrel fold is then a smaller cored in the Gypsum Springs–Sundance Formations, version of the overturned slide nappe in which it Journal of Geology STRUCTURE, TIMING, KINEMATICS OF SOUTH FORK SLIDE 331

to the South Fork structures. Submarine slides generally fall into three geologic categories: (1) deltaic slides, (2) volcanic slope slides, and (3) continental slope slides related to postglacial rebound (table S3). None of the submarine examples, despite some having long runout dimensions, maintain the primary stratigraphy (1.2 km thick) or layered structure we see with the SFS, making the SFS and the HMS geologic anomalies. So, if the scenarios presented in ﬁg- ures 20 and 21 are plausible, the SFS is the single largest slide block (40 # 10 # 1.2 km) that main- tains its original stratigraphy even as it folded into the Shoshone paleovalley.

Conclusions On ﬁrst inspection, the SFS looks like a traditional thin-skinned fold structure, but the lack of an axial planar cleavage and joint sets as well as the preva- lence of unique internal deformation structures and

Figure 20. Schematic chronology of the evolution of the South Fork slide. A, Upright slide block arrives from the northwest. B, Slide block begins to fold into paleovalley. C, Slide block folds over itself, forming a nappe that blackslides along the basal Js (D). Erosion follows (E) before arrival of the upper-plate syncline (F). See figure A1 (available online) for descriptions of labels. formed and is the only example of drastic variations in sediment thinning (attenuation); the larger nappe gives a sense of fold vergence to the southeast, which then suggests a source area to the northwest. Either scenario in figures 20 and 21 could also have a source from the southeast, as supported by the steeper paleovalley walls on the southeast side of the valley (fig. 19). The SFS landslide then dammed the ancestral Shoshone River (see below) and was buried by the HMS (48.87 Ma) that is found above the SFS on Figure 21. Schematic chronology of the evolution of both sides of the valley (fig. 2). the South Fork structures. A, Upright slide block arrives Terrestrial landslides are well documented as cha- from the northwest. B, Slide block begins to fold into fl otic, poorly layered ows, and we have reviewed paleovalley. C, Slide block folds over itself, forming a these features relative to the HMS (Malone 1995; nappe that blackslides along the basal Js (D), which then Craddock et al. 2009, 2012; Malone et al. 2014b). forms the upper-plate backslide syncline (E). See figure A1 Submarine landslides may present another analog (available online) for descriptions of labels. 332 J. P. CRADDOCK ET AL. a rootless, overturned section nullify this interpre- tain event and other large gravity-driven structures, tation. The LPS calcite strains are of Sevier origin, such as the Castle Rocks chaos and Enos-Owl Creek and the SFS evolution rotated what was once a con- slide (Sundell 1993), it seems even more unlikely to sistent east-west-shortening fabric into a chaotic invoke a south-directed, gravity-driven thrust sheet shortening axis pattern with no significant strain of which the SFS structure is the “toe” of a slide block overprint. We favor an interpretation where the Juras- that is rooted in the Jurassic Gypsum Springs and sic Sundance–Eocene Willwood section collapsed as moved a thickness of 1500 m of rock over hundreds a landslide (or series of landslides) into the dynamic of square kilometers (a distance of 120 km) for un- fluvial environment that existed in the Absaroka known geologic reasons (Clarey 2012). and Bighorn Basins at that time. The SFS section slid along the Gypsum Springs Formation and Willwood ACKNOWLEDGMENTS mudstones, presumably to the southeast into a paleovalley along what is now the Shoshone valley. The Generous funding for this research was provided by source area, often attributed to be either the south grants by the Keck Geology Consortium to D. H. side of the Laramide Pat O’Hara Mountain structure Malone and J. P. Craddock in 2010–2012 and by the or north of the North Fork of the Shoshone River, USGS EDMAP program in 2011–2012. These mon- is now buried in Absaroka Volcanic rocks, each of ies supported fieldwork and laboratory analysis. Ad- which is ∼20 km northwest of the SFS. Potential ditional support was provided by the Illinois State source areas for landslides to the southeast are also University Foundation. Field assistance was provided buried by Absaroka Volcanics (e.g., Carter Moun- by Keck students A. Kelly, S. Kenderes, B. Kraushaar, tain and the Washakie Wilderness). If true, the SFS K. Schroeder, K. Kravitz, and R. McGaughey. J. Trela, is an additional landslide deposit (∼800–2000 km2) J. Calhoun, A. Hinds, and J. Bardwell also assisted that is contemporaneous and beneath the HMS run- with the fieldwork, rock preparation, and zircon geo- out (3400 km2). It is also unique that the slide blocks chronology. Colleagues at ConocoPhillips analyzed are as much as several kilometers in maximum di- the barrel fold hydrocarbons. We also thank the own- mension and maintained their stratigraphy with- ers of the TE and Hoodoo ranches for permission to out disaggregating during motion. While yet another access their properties. K. Sundell provided a help- Eocene landslide is consistent with the Heart Moun- ful review of the manuscript.

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