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Home , Casper Mountain, Ferris Mountains, Gannett Peak, Laramie Peak, Medicine Bow Peak, Structural basin

Chapter 4 Geologic Setting

Alan Ver Ploeg, Karl Taboga and Paul Taucher

4-27 he Platte River Basin drainage basin configuration of these elements and relationships comprises approximately 24,106 square among them influence the availability of miles (15.43 million acres) in , . The geologic history relevant to Teffectively covering the southeast corner of groundwater resources of the Platte River Basin the state. The geologic setting for the basin is, begins with the nonconformable deposition of therefore, both extensive and complex in that it transgressive marine sediments onto Precambrian encompasses 4 major Laramide structural basins, basement rocks during Middle Cambrian time overlies small parts of five other Laramide basins (Libra and others, 1981; Richter, 1981; Snoke, and includes five mountain ranges of the southern 1993). From that time forward, a general geologic as well as several smaller history that describes the development of the uplifts. So, a complete description of the geologic stratigraphic and structural elements the Platte framework of the Platte River Basin must include River Basin is as follows: summary accounts of the assemblages of geologic and hydrogeologic units and structural elements 1. Paleozoic strata in the Platte River Basin that define each of these major structures. To were deposited in marine and nonmarine accomplish this, an extensive set of figures and transgressive /regressive environments. maps, presented as plates, have been prepared for Marine limestones and dolomites are the inclusion in this report: dominant lithologies of the Paleozoic sequence, with less extensive sandstones • Plate 1 illustrates the bedrock and shales that represent beach and near- of the Platte River Basin in Wyoming, shore environments. Deposition in the and overlain on a base Paleozoic Era was broken by long periods map that shows highway, township, state of erosion, indicated in the and county data. Inset maps present the by several regional unconformities. elevations of the Precambrian basement 2. The early Era was a time when and lineaments (linear geologic features). shallow seas deposited interbedded layers Appendix A contains detailed descriptions (in decreasing abundance) of sandstone, of the geologic units shown in Plate 1. siltstone, shale, carbonates, and evaporites. • Plate 2 displays an outcrop map of An emergent transition to a terrestrial hydrogeologic units in the Platte River environment during the Late Triassic Basin developed by correlating the and Early Jurassic Epochs resulted in the geospatial data of hydrogeologic units with deposition of marginal marine, eolian, hydrostratigraphic nomenclature charts fluvial, and paludal sandstones and shales. (Plates J, K, M, S, T, and U; Figure 7-2). 3. During the Early Epoch The general hydrogeology of the Platte a thick section of interbedded shale, River Basin is discussed in Chapter 7. sandstone, siltstone, and claystone was Individual Platte River Basin aquifers are deposited under terrestrial, shallow marine discussed in detail in Chapter 7, also. and deltaic conditions. Late Cretaceous • Nineteen cross sections, contained in this transgressions and regressions resulted in a chapter (Figures 4-2 through 4-20), show thick sequence of interbedded sandstone, typical subsurface structure in the Platte siltstone, claystone, and shale deposited River Basin. Isopach maps with substantial in marine, marginal marine, coastal coverage of the major aquifers in the Platte plane, and deltaic environments. Crustal River Basin are not available. deformation associated with the Laramide began in the Late Cretaceous; 4.1 General/historical geology the Lance Formation recorded the final, eastward retreat of the Cretaceous seas The Platte River Basin contains simple to followed by the deposition in terrestrial complex stratigraphic and structural elements. The environments that would prevail

4-28 throughout the Tertiary Period. feet (Richter, 1981) of Cenozoic, Mesozoic, and 4. Laramide compressional deformation Paleozoic sediments deposited on Precambrian continued through the early crystalline basement rocks (Libra et al., 1981; with large-scale reverse and thrust faults Richter, 1981). The structural basins are bordered forming the basement-cored mountain by compressional uplifts cored by Precambrian ranges and uplifts that surrounded and granite and mantled by moderately to steeply separated the concurrently subsiding dipping sedimentary formations (Libra et al., 1981). structural basins that make up the Platte Paleozoic and Mesozoic formations exposed along River Basin. The uplifted areas were the flanks of the mountain ranges surrounding the source of several thousand feet of the Platte River Basin were folded, faulted, and Tertiary sediments composed of Mesozoic, eroded from the highest areas of the uplifts during Paleozoic and Precambrian rocks that the ; they now dip basinward were eroded from the uplifts and filled at angles ranging from approximately 5 degrees the basins to the extent that all but the to vertical, and some are overturned. Strata of highest areas of the surrounding uplifts Paleocene through early Eocene age are also were buried. These strata are composed of deformed around the perimeters of these structural conglomerates, sandstones, and claystones basins but are mostly flat lying in the interior basin deposited primarily in fluvial, alluvial fan, areas. Numerous anticlinal structures with associated and lacustrine environments. faults that formed during the Laramide Orogeny 5. Late Tertiary normal faulting concurrent crop out along the margins of the structural basins. with modest extension occurred Section 5.4 discusses the substantial influence that throughout Wyoming. Uplift during the structures, primarily those located around the basin past 5 million years over a broad area perimeters, exert on groundwater recharge, flow and that encompasses the Platte River Basin storage. resulted in the erosion and removal of The topography of the Platte River Basin an enormous volume of Tertiary strata, generally reflects the structure and topography exhuming the Laramide framework and of the Precambrian basement surface formed by sculpting the present physiography of the uplift, folding, faulting, and erosion of the earth’s Platte River Basin crust under compressional during the Sevier 6. The youngest geologic units in the basin are and Laramide . Downwarping of the unconsolidated Pliocene and Quaternary structural basins and upwarping and faulting of the terrace deposits and Quaternary alluvial uplifts were concurrent; and the upper strata within deposits of various thicknesses. These the basin interiors are composed of Tertiary-age deposits, some as much as several hundred sediment that was eroded from the adjacent uplifts. feet thick, are composed of conglomerate, The structure contour map of the Precambrian gravel, sand, and finer-grained clastic basement surface in the Platte River Basin shown material. The age and occurrence of these on the lower inset map on Plate 1 shows a general deposits have been correlated with recent northwest-southeast structural trend. The geologic glacial and interglacial periods by Mackin cross sections on Figures 4-2 through 4-20 (1937). show Precambrian basement rocks overlain by varying thicknesses of Paleozoic through Cenozoic 4.2 formations, all deformed by large-scale folding and faulting. The Laramide age basins, contained either The major Laramide structural elements of the wholly or partly within the Platte River drainage Platte River Basin (Figure 4-1) comprise: basins, are small (Carbon basin) to large (Denver • The folded and faulted Precambrian basin) asymmetric intermontane structural basins basement formed during the Laramide Orogeny (Late • The deeply buried downwarped areas of Cretaceous-Eocene) that contain up to 35,000 the Denver, Laramie, Hanna-Carbon,

4-29 Shirley, and Powder River in the basins is an ongoing process. Accordingly, basins the stratigraphic sections preserved in interior • The mountain ranges and uplifts that basin areas are most complete, and stratigraphic surround and separate the basins: sections are less complete to non-existent at o The Laramie, Medicine Bow and higher elevations in the surrounding mountain Sierra Madre/Park mountain ranges. In some places Tertiary and Quaternary ranges deposits directly overly Precambrian basement o Green Mountain, Ferris rocks. Mountains, Seminole, and Shirley Mountains 4.4 Granite Mountains, Green o The Granite Mountains Mountain, , Seminoe o The Casper Arch and Hartville Mountains, Shirley Mountains and Uplift Freezeout Mountains (Lillegraven and o The Rawlins Uplift Snoke, 1996; WSGS, 2013) o The o The structural basins Together, these Rocky Mountain foreland . Denver Basin features form the southern flank of the Sweetwater . Laramie Basin Arch which extends over 100 miles from the . Shirley Basin western edge of the Granite Mountains to the . Hanna-Carbon Basin eastern Freezeout Mountains. This area separates . Saratoga Valley the from the Great Divide and . Hanna basins, and in a general sense connects the . southern end of the Wind River Range with the . Wind River Basin northern .

There are many subsidiary structures within 4.4.1 Granite Mountains (Love, 1970; WSGS, the Platte River Basin, some of which are or may 2013 ) be important elements of existing or potential sites for local groundwater development, but The Granite Mountains, (Plate 1; Figures 4-1, discussion of these features is beyond the scope 4-2) which consist of massive pink granite shot of this study. through with black diabase dikes, form a number of landmarks such as Independence Rock, Devils 4.3 Basin stratigraphy Gate, Split Rock, and Sweetwater Crossing along the Sweetwater River. The rocks that form these Geologic units within the structural basins features crop out in bald, stark knobs that stand of the Platte River drainage basin vary widely above light-colored, flat-lying Tertiary aged in lithology and distribution, and range in age sedimentary rocks. from Precambrian crystalline rocks to recent The area of the Granite Mountains is unique alluvial and terrace deposits. The structural basins because it was once a much higher mountain range, that compose the Platte River Basin contain up similar to other mountains in the state that formed to 35,000 feet, of Cenozoic through Paleozoic during the Laramide Orogeny. For example, the sedimentary strata. The explanation on Plate Crooks-Green-Ferris-Seminoe-Shirley mountain 1identifies the geologic units present in the basin; complex that bounds the Granite Mountains on the individual geologic units are described in the south has structural features like those found Appendix A. The distribution of geologic units on the flanks of other Laramide mountain ranges reflects several periods of deposition, uplift, in Wyoming; however, the Granite Mountains erosion, volcanism, and reworking/re-deposition lack a high central core of Precambrian . of older units as younger strata. The erosion of Also like other Wyoming mountain ranges, rocks exposed in upland areas and re-deposition sedimentary basins on both sides of the Granite

4-30 Major Laramide structural elements, Platte River Basin. Laramide structural River elements, Platte Major Figure 4-1. Figure

4-31 Figure 4-2. Geologic cross section A-A’.

4-32 Mountains contain a thick sequence of lower the northern flank of this mountain complex and Tertiary rocks, including thick conglomerate separate it from the Granite Mountains. Crooks sequences that become coarser grained as they Mountain and Green Mountain are both low approach the Granite Mountains. The rock types elevation features composed of Paleocene through found in the conglomerates, such as the giant Oligocene aged conglomerates containing giant boulders of Precambrian granitic rocks found in Precambrian granite boulders that have the same and on top of Green and Crooks Mountains south composition as the granite knobs of the Granite of the Sweetwater River, are exactly the same rock Mountains. types found in the core of the Granite Mountains. The Ferris Mountains, farther to the east During early Tertiary (Paleocene and especially are more rugged and over 2,000 feet higher in early Eocene) time, there must have been high elevation. The mountains’ flanks consist of an mountains in the area to produce these structural assemblage of upturned Cambrian – Upper features and the conglomerates. Cretaceous sedimentary strata and the Precambrian The south and north margins of the Granite granite core is exposed along the summit ridge. Mountains are defined by an obvious system of The most striking geologic features of the Ferris east-west trending normal faults which preserve late Mountains are the sub vertical exposures of Tertiary sedimentary rocks in the downdropped resistant Mississippian Madison Limestone that blocks. These rocks were almost completely form the flatirons along its southern flank. removed by erosion elsewhere in Wyoming. At The Seminoe Mountains (elevation 7,421 ft one time, all Wyoming basins were completely above MSL) have a similar geologic setting to the filled with these late Tertiary rocks, and only the Ferris Mountains in that Paleozoic and Mesozoic highest mountain peaks stood above this fill. The sedimentary units cover its southern flank which granite knobs along the Sweetwater represent the also exhibits Madison Limestone flatirons along buried crest of the central core of a mountain range the mountains eastern half. The Precambrian core that collapsed and subsided in the late Tertiary is more complex than the granite core of the Ferris (mostly in the Pliocene Epoch). It is estimated Mountains, however. The core of the eastern two that at least several thousand feet of the Granite thirds of this small mountain range is Late Archean Mountains collapsed along these normal faults into granite while the western third consists of Late a downthrown trough (). This collapse was Archean metavolcanic and metasedimentary rocks probably caused by crustal extension that affected which contain iron and gold ores. Several small other Laramide uplifts as well, but evidently not to gold mines were in operation for a brief period in the same extent it did the Granite Mountains. The the late 1800s in this area but no large scale mining proximity of early Tertiary volcanic rocks of the operation ever developed. on the north edge of the Granite The Shirley Mountains constitute the Mountains may also be a factor, and the debate easternmost part of the Green Mountains- continues as to why this entire mountain range Ferris Mountains-Seminoe Mountains-Shirley collapsed. Mountains structural complex, which defines the southern part of the Granite Mountains uplift. 4.4.2 Crooks Mountain, Green Mountain, The Shirley Mountains contain Precambrian Ferris Mountains, Seminoe Mountains, Shirley rocks in its western core and a large anticlinal Mountains (Lillegraven and Snoke, 1996) structure involving Paleozoic and Mesozoic rocks in the eastern part. The mountains separate the The geology of these mountains (Plate 1; Shirley Basin to the north from the Hanna Basin Figures 4-1 through 4-3), which parallel the to the south. The Shirley Mountains were thrust Granite Mountains to the north, suggests that southward, overriding the deep synclinal axis of the this structural complex constitutes the remnant Hanna Basin. The southeastern part of the Shirley of the foothills of the Granite Mountains prior Mountains that contains the large draped to their collapse. A long system of thrust faults in with Paleozoic and Mesozoic rocks is sometimes the west and normal faults in the east, run along called the Freezeout Mountains (or Hills).

4-33 Figure 4-3. Geologic cross section B-B’-B”. 4-34

4-34 4.5 Hanna-Carbon Basin and The Tertiary Hanna and Ferris formations surrounding mountain ranges (Richter, contain thick beds in both the Hanna 1981; WSGS, 2013) and Carbon basins. Coal was originally mined underground at old Carbon (now a ghost town) This consists of the larger and later at Hanna, and was used to fuel steam Hanna Basin (Plate 1; Figures 4-1, 4-4) and the locomotives on the Union Pacific Railroad. After smaller, subsidiary Carbon Basin to the southeast, the railroads switched from coal-fired to diesel- separated by the Simpson Ridge anticline. This electric locomotives, coal mining practically ceased basin is sandwiched between the Seminoe and in the basin. Coal mining by both underground Shirley Mountains to the north, the Medicine Bow and surface methods resumed in the 1970s due to Mountains to the south, and the Rawlins uplift to increased demand for coal to fuel electric power the west. It is separated from the Laramie Basin to plants (brought about by passage of the Federal the southeast by several folds in Cretaceous rocks. Clean Air Act). Today, most coal mining activity in The Hanna Basin is quite small as intermontane the basin has once again nearly ceased because this basins go—only about 35 miles long by 20 miles coal must compete with the easily mined and much wide—but it is unique because of the great depth cheaper coal from the Powder River Basin. to which the sedimentary rocks are depressed. The Precambrian floor beneath the sedimentary rocks 4.6 Casper Arch (WSGS, 2013) in the deepest part of the basin north of Hanna lies approximately 30,000 feet below sea level. A structural arch represents a transitional Structural relief ranges from 38,000 feet, measured area between Wyoming’s basins and mountains. from the highest point on the Shirley Mountains, It is primarily an uplifted area, but the extent of to more than 41,000 feet, measured from the top uplift has not been enough to form a mountain of Elk Mountain (a horizontal distance of only 15 range with an exposed core of Precambrian or 20 miles to the deepest part). The basins contain basement rocks. Relief is usually about the same a thick sequence (up to 23,000 feet) of Upper as in the adjacent basins, but the uplift has Cretaceous and Tertiary clastic sedimentary rocks exposed rocks older than those in the basins. derived in part from adjacent uplands. Tertiary The Casper arch (Plate 1; Figures 4-1, 4-5) is a rocks in the northern part of the basin adjacent large, northwest-trending asymmetric anticlinal to the Shirley Mountains include a 10,000- to structure that connects the 15,000-foot-thick succession of vertically dipping with the Laramie Mountains and separates the conglomerates containing clasts eroded from nearly Powder River Basin (to the northeast) from the every sedimentary and Precambrian rock exposed Wind River Basin (to the southwest). Rocks on in the surrounding uplifts. Overall, the thickness the northeast flank dip gently into the Powder of the sedimentary section ranges up to 35,000 feet River Basin while steeply dipping rocks are thrust (Richter, 1981). southwestward, overriding the synclinal axis of The structure of the Hanna-Carbon Basin is the Wind River Basin. Upper Cretaceous marine complex. Even the Tertiary rocks that are relatively shale crops out in the center of the arch, with flat lying in most other Wyoming basins are rocks as young as Paleocene on the flanks of the complexly folded and faulted, especially on the arch. Several large oil fields are found on the arch, edges of the basin. Only the western part of the including the Salt Creek/Teapot , Pine basin is relatively undeformed, with the rocks Mountain/Poison Spider, and Tisdale Mountain dipping eastward off the Rawlins uplift. Near , which expose older Cretaceous rocks in Hanna, where the basin is deepest, even the their cores. youngest (Eocene) part of the Hanna Formation is folded into a small . In the northern part 4.7 Shirley Basin (WSGS, 2013) of the basin, Upper Cretaceous rocks are highly overturned and overlain unconformably by lower The Shirley Basin (Plate 1; Figures 4-1, Tertiary rocks that may also have steep dips. 4-6) is a small relatively undeformed

4-35 Geologic cross section C-C’. cross Geologic Figure 4-4. Figure

4-36 Geologic cross section D-D’. cross 4-5. Geologic Figure

4-37 Figure 4-6. Geologic cross section E-E’.

4-38 between the Laramie Range and Freezeout or the highest area of the uplifts is about 16,000 feet Shirley Mountains. Its floor is full of flat lying defined by the difference between elevations in the sandstones of the Eocene Wind River formation, deepest part of the , approximately which contain large deposits of uranium. Like 4,000 feet below sea level and on Medicine Bow other uranium deposits in Wyoming, the uranium Peak at 12,006 feet above sea level in the Snowy leached from Precambrian granitic rocks and Range Mountains (Blackstone, 1993). moved in percolating ground waters into porous sandstones of the Wind River formation. The 4.9 Laramie, Medicine Bow and Sierra Shirley Basin contains large reserves of uranium, Madre Mountain Ranges (WSGS, which are minded from open pits. The north end 2013) of the Shirley Basin is covered by white, tuffaceous claystones and sandstones of the Oligocene White Just south of the Colorado-Wyoming River formation, dated at 31 to 35 million years boundary, the mountain masses of the Colorado old. The basin contains 8,000 feet of Paleozoic Front Range of the and Mesozoic rock unconformably overlain by a divide into three prongs that extend northward Cenozoic section (Richter, 1981). into Wyoming. These three prongs are known, from east to west, as the Laramie Mountains, the 4.8 Laramie Basin (Richter, 1981; , and the Sierra Madre. WSGS, 2013) ) The Laramie Mountains (Plate 1, Figures 4-1, and 4-12 through 4-16) are most closely related to, and Between the Laramie Mountains and are an extension of, the Colorado Front Range. the Medicine Bow Mountains is a complexly In Wyoming, they separate the Denver Basin and downfolded area, the trend of which roughly Hartville uplift to the east from the Laramie and parallels the sweeping arc of U.S. Highway 30 Shirley basins to the west. , a and the Union Pacific Railroad from Laramie to northern salient of the Laramie Mountains, borders Rawlins. The area is comprised of the Laramie and the Casper arch on the south. The northeastern Hanna-Carbon Basins. The Laramie basin (Plate 1, Laramie Mountains borders the Powder River Figures 4-1 and 4-7 through 4-11) is a small (60 Basin on the south. miles north to south by 30 mile) trapezoidal shaped The Laramie Mountains are a moderately high asymmetrical Laramide structural basin located in (8,000 – 9500 feet above sea level) mountain the southern part of this downwarped area. The range. The high relief landscape in the northern Laramie Basin is bounded on the northeast and part of the range is capped by east by the Laramie Mountains, on the south and (elevation 10,274 feet), a famous landmark well west by the Medicine Bow Mountains and is open known to emigrants traveling westward along to the northwest. The Laramie Basin contains the . Farther south, near the border a maximum thickness 12,000 feet of Cenozoic between Wyoming and Colorado, the Laramie through Paleozoic sediments near the basin axis. Mountains have been reduced by erosion to low The surface geology consists of nearly relief. The basic geologic structure of the range is horizontal unconsolidated Quaternary deposits of a large asymmetric arch, steepest on the east. which overlay Tertiary units that lie unconformably Precambrian rocks are extensively exposed in its on Mesozoic and Paleozoic rocks downfolded core, which is flanked by sedimentary strata. into a large trough. The structure contour map Crossing the Laramie Mountains between (Plate 1, inset) of the Precambrian basement and Laramie, Interstate 80 and the surface shows that the synclinal axis of the Laramie Union Pacific Railroad first traverse a relatively Basin is located along the west side of the basin flat surface of late Tertiary sedimentary rocks and trends generally north-south along the arc of called the “Gangplank,” finally reaching the the mountain front of the thrust/reverse faulted Precambrian core of the mountains and a rolling Medicine Bow Mountains. Maximum structural upland of low relief at about 8,000 feet above sea relief between the deepest area of the basin and level called the Sherman surface. The Gangplank

4-39 Geologic cross section F-F’. cross Geologic Figure 4-7. Figure

4-40 Geologic cross section G-G’. cross Geologic Figure 4-8. Figure

4-41 Geologic cross section H-H’. cross Geologic Figure 4-9. Figure

4-42 Geologic cross section I-I’. cross Geologic Figure 4-10. Figure

4-43 Geologic cross section J-J’. cross Geologic Figure 4-11. Figure

4-44 Geologic cross section K-K’. cross 4-12. Geologic Figure

4-45 Geologic cross section L-L’. section L-L’. cross Geologic Figure 4-13. Figure

4-46 Geologic cross section M-M’. cross Geologic Figure 4-14. Figure

4-47 Geologic cross section N-N’. cross Geologic Figure 4-15. Figure

4-48 Geologic cross section O-O’. cross Geologic Figure 4-16. Figure

4-49 is the only place along the entire eastern mountain called the Park Range. The Sierra Madre Range is front where the Tertiary rocks that once buried the western prong of the Colorado Front Range the Laramie Mountains are still preserved and that projects into Wyoming. This uplift separates are in contact with the Precambrian core of the the Saratoga Valley from the Washakie Basin, and mountains. Further erosion during later Tertiary may be structurally connected to the Rawlins uplift time left a few higher granite boulder knobs such to the north. The Continental Divide runs along as the spectacular area at Vedauwoo. Sherman the crest of the Sierra Madre, and the highest point Pass, at the summit of the Laramie Mountains on is Bridger Peak (11,007 feet). Drainages on its west Interstate 80, is the highest point (8,640 feet above flank flow to the Colorado River by way of the sea level) on this transcontinental highway. The Little Snake River; drainages on its east flank flow Pennsylvanian Casper formation outcrops to the . along the western flank of the Laramie Mountains Geologically, the Sierra Madre are quite from Sherman Pass to the city limits of Laramie similar to the Medicine Bow Mountains, but lack and its heavily fractured alternating limestone- the broad upland surface of the latter. The same sandstone strata can be seen in the road cuts along major zone (or ) that cuts across the I-80. Medicine Bow Mountains is also present in the The Medicine Bow Mountains are the middle Sierra Madre. This suture, called the Cheyenne prong of the Front Range that projects into belt, separates the oldest Archean rocks of what Wyoming, and the northward extension of is known as the Wyoming Province to the north the Never Summer Range in Colorado. This from younger igneous and metamorphic rocks mountain range is separated from the Laramie to the south. Attached to the rocks north of the Mountains by the Laramie Basin, a structural basin Cheyenne belt are some younger Early Proterozoic underlying the Laramie plains. The mountains metasedimentary rocks similar to those in the are characterized by a rather broad, rolling upland Medicine Bow Mountains to the east. The Archean surface approximately 9,000 feet above sea level, rocks of the Wyoming Province extend northward above which rise two areas of higher relief. The from the Sierra Madre to form the Precambrian highest part of the Medicine Bow Mountains, cores of nearly all Wyoming mountain ranges. the Snowy Range, consists of a thick sequence South of the Cheyenne belt in the Sierra Madre of metasedimentary rocks, including a thick, are well-preserved metamorphosed volcanic and very resistant white quartzite that culminates in sedimentary rocks (including basalt, andesite, at 12,006 feet. and rhyolite flows and tuffaceous rocks, shales, The Medicine Bow uplift contains an excellent and greywackes), as well as some younger granitic geologic record of two major ages of Precambrian intrusions approximately 1.7 and 1.4 billion years rocks in the western : the older old. Archean (more than 2.5 billion years old) and the younger Proterozoic (2.5 billion to 540 4.10 Rawlins Uplift (WSGS, 2013) million years old). A thick series of Proterozoic metasedimentary rocks is in contact along a The Rawlins Uplift defines the eastern margin major (or suture) with an older sequence of the Great Divide Basin, the western end of of Archean-age metamorphic rocks. Stromatolites southeastern Wyoming, and the beginning of the occur within 1.7 billon-year-old metasedimentary desert basin of the west. It is a small Laramide rocks. Stromatolites are fossil remains of blue-green Uplift that has many of the characteristics of larger algae, among the oldest and earliest forms of life Wyoming ranges. Precambrian basement rocks known. They resemble giant cabbage heads and can are exposed in the core of the uplift, and its flanks be seen along the road to Lewis Lake en route to are composed of outwardly-dipping Paleozoic and the Sugarloaf Recreation Area. Mesozoic strata. A dips beneath the The Sierra Madre is the name applied in uplift along the west and south sides. Rawlins lies Wyoming to a section of mountainous terrain that at its south end. is continuous with a mountain mass in Colorado

4-50 4.11 Saratoga Valley (WSGS, 2013) River Formation. A thick, early Eocene arkosic conglomerate called the Battle Spring Formation This area is a northwest-trending structural crops out in the northeast third of the basin and low between uplifts of the Medicine Bow interfingers with finer-grained rocks in the rest of Mountains to the east and the Sierra Madre to the basin. This conglomerate was derived from the the west. The North Platte River flows north Granite Mountains that once stood high above through this valley. Rocks of Miocene age overlie the northern part of the basin. The maximum (or are faulted against) Precambrian rocks in the thickness of the sedimentary units in that part of southern Saratoga Valley near the Colorado border the Great Divide Basin located within the Platte and progressively truncate Paleozoic and lower River watershed exceeds 29,000 feet (Blackstone, Mesozoic rocks in the subsurface to the north. 1993). Upper Cretaceous rocks crop out in the northern Because of its location at the top of the Saratoga Valley and form a small sub-area known as continent, and the fact that no high mountain the Kindt Basin. The east-trending Grenville Dome masses exist to the west or the east, the Great and Fort Steele anticlines on the northern flank of Divide Basin presents no restriction to the westerly the Kindt Basin approximately define the northern airflow pattern of western North America. This boundary of the Saratoga Valley, separating it from westerly flow is “funneled” through southwestern the Hanna Basin to the north, while the Rawlins Wyoming between the mountain masses of the uplift and Miller Hill anticline (the northern Overthrust Belt/Wind River Range in Wyoming projection of the Sierra Madre) bound the valley to and the Wasatch/ in . In the northwest. other words, the wind blows, and it blows a lot, in this part of Wyoming. Because southwestern 4.12 Great Divide Basin, Wind River Wyoming is a desert environment and many of Basin, Powder River Basin (WSGS, the outcropping rocks are poorly consolidated and 2013) easily eroded, it is no coincidence that the Great Divide Basin is home to extensive sand dune fields. The Platte River Drainage Basin contains For example, the Killpecker dune field, which small portions of the northern Great Divide, extends across the Great Divide Basin from near southeastern Wind River, southwestern Powder Farson to Seminoe and Pathfinder Reservoirs, is the River and northeastern Green River structural largest continuous area of active sand dunes in the basins. The Green River and Great Divide U.S. In the Platte River Basin, the Killpecker dune structural basins constitute two subbasins within field extends eastward from the south side of Green the Greater Green River Basin. Mountain across the northern Great Divide basin to Seminoe Reservoir, passing along the south side 4.12.1 Great Divide Basin of the Ferris Mountains and Seminoe Mountains. Another arm of the dune field runs northeast Geologically, the Great Divide Basin is one past the west end of the Seminoe Mountains to of the two eastern sub-basins within the Greater Pathfinder Reservoir. Surrounding the areas of Green River Basin. It is bounded on the east by active sand dunes are even larger areas of stabilized the Rawlins uplift; on the north by the Granite or inactive dunes. Mountains (and related Crooks Mountain-Green Mountain uplifts) and the southern Wind River 4.12.2 Wind River Basin Range; and on the west by the north plunge of the Rock Springs uplift. The deepest part of the basin Almost exactly in the center of Wyoming lies along the steep east flank of the Rawlins uplift is a rhomboidal topographic depression known and the northern mountains. The Great Divide as the Wind River Basin (Plate 1, Figure 4-5). basin contains a thick sequence of marine Upper Mountains and uplifts surround the basin, and Cretaceous and continental early Tertiary rocks as nothing about the area is simple. The east and west well as lacustrine rocks related to the Eocene Green sides of the rhomb are bounded by the northwest-

4-51 trending Casper arch and Wind River Mountains, and northwestern edges of the basin ever since. respectively; the north side is bounded by the Extensive oil and gas development has occurred Bridger-Owl Creek-Washakie ranges; and the in nearly all parts of this basin, and reservoirs of south side is bounded by the Granite Mountains. nearly every age have produced. The basin contains In general, the basin is a highly asymmetrical a thick sequence of Upper Cretaceous and lower syncline, with the basin axis nearest and parallel Tertiary sedimentary rocks that contain important to the northern mountains and the Casper reserves of coal, oil, and especially natural gas. arch. The deepest part of the trough lies in the north, immediately adjacent to the Owl Creek 4.12.3 Powder River Basin Mountains, where the Precambrian basement may be displaced vertically more than 30,000 A very wide and moderately deep basin, feet. The maximum thickness of the sedimentary the Powder River Basin (Plate 1; Figures 4-1 units in that part of the Wind River Basin located and 4-17) lies between the and the within the Platte River watershed is nearly 24,000 Bighorn Mountains. It is separated from the feet (Blackstone, 1993) and occurs farther to the Wind River Basin by the Casper arch and from southeast along the basin axis. the Denver Basin by the Hartville uplift. The The western boundary of the basin is the basin is asymmetrical, with the steepest dips and east-dipping flank of the Wind River Range, the basin axis on the western side of the basin, characterized by northwest-trending hogbacks near and parallel to the eastern Bighorn Mountain of Paleozoic sedimentary rocks. Structurally, front. In several places along the western and the southern boundary is the northern flank southern edges, the basin has been overridden by of the Granite Mountains, defined by a series thrust faults that place Precambrian and Paleozoic of northwest-trending features including the rocks against Tertiary rocks of the basin. The Rattlesnake Hills, Conant Creek, and Alkali Butte maximum thickness of the sedimentary units in anticlines. These anticlines lie north of the North that part of the Powder River Structural Basin Granite Mountains fault system. Topographically, situated within the Platte River watershed is most of the southern boundary is a north-facing almost 19,000 feet (Blackstone, 1993). escarpment of Middle Eocene, Oligocene, and The basin edges are defined by a relatively thin Miocene rocks known as Beaver Rim and Shirley band of Cretaceous marine sedimentary rocks; the Rim, an erosional feature analogous to the Pine basin interior is characterized by a wide expanse of Ridge escarpment in eastern Wyoming and lower Tertiary rocks, i.e., the Paleocene Fort Union Nebraska. Formation and the Eocene . The Casper arch, a broad upfold of In many places, the natural burning of thick coal sedimentary rock, forms the eastern boundary. beds initiated by lightning strikes, grass fires, and Topographic relief is low, but the rocks are greatly spontaneous combustion has baked the rocks elevated relative to their depth in the Powder River overlying the coal beds in a manner similar to the and Wind River Basins on opposite sides of the action of a brick kiln. The resulting baked rock, arch. Exploratory drilling revealed that the west known variously as clinker, red dog, or scoria, is side of the arch is in the hanging wall of a thrust usually red from oxidation of iron in the sediments; fault with low dip to the east. There is more than it is more resistant to erosion than the unbaked 5 miles of westward movement on this hanging rocks below, and caps buttes and mesas over a large wall, and the fault has actually overridden the area of the basin where coal beds are at or near the synclinal axis of the Wind River Basin. Oil and gas surface. production in deep reservoirs has been established This basin is one of the most important energy- in the footwall of this thrust fault. producing areas in Wyoming (and the nation). Its The first oil well in Wyoming was drilled vast coal resources are augmented by oil and natural southeast of Lander at Dallas Dome in 1884, and gas resources, including natural gas produced from oil has been produced from this and a series of coal beds, and uranium deposits. Over broad areas, northwest-trending anticlines along the western the Tertiary rocks contain extensive low-sulfur coal

4-52 Figure 4-17. Geologic cross section P-P’.

4-53 deposits that are surface-mined near Gillette and 4.13 Wind River Range (WSGS, 2013) Wright. Most of Wyoming’s coal production comes from the Powder River Basin, and the state has The Wind River Range is the largest and been the leading coal producer in the U.S. since highest discrete mountain mass in Wyoming, the late 1970s. The important Wyoming-type or containing most of the state’s highest summit “roll-front” uranium deposits were discovered at peaks, such as (at 13,804 feet, the Pumpkin Buttes in the center of the basin in the highest point in Wyoming), (13,745 1950s, and led to the state’s uranium boom. feet), and (13,192 feet). Trending N 40° W from South Pass City on the south 4.12.4 Green River Basin (WSGS, 2013) to Fish Lake Mountain on the north, the range is a major barrier between western and central This is a large, complex intermontane area Wyoming; it separates the Wind River Basin to that covers much of southwestern Wyoming; it the northeast from the Greater Green River Basin is used as a general term to include a number of to the south and southwest. The northeast flank of separate structural arches and sedimentary basins the Wind River Range consists of a 96-mile-long, as described below. Mountain ranges (uplifts) continuous band of Paleozoic sedimentary rocks border the Greater Green River Basin on all but the exposed in a series of hogbacks and dip slopes that western edge, where the Thrust Belt borders it. The dip eastward into the Wind River Basin. The core Greater Green River Basin is bounded on the south of the range exposes an extensive area of some by the Uinta Mountains and Cherokee Ridge; on of the oldest Precambrian rocks in the state. The the southeast, east, and northeast by the Sierra southwest flank of the range is characterized by an Madre, Rawlins uplift, Granite Mountains, and extensive thrust fault system, with Precambrian Wind River Range; and on the north by the Gros rocks thrust south and southwest onto relatively Ventre Range. Three major sedimentary basins are flat-lying Tertiary rocks in the Green River found within the Greater Green River Basin —the Basin. The fault system extends eastward where Green River, Washakie, and Great Divide basins. Precambrian rocks in the South Pass area were The Green River Basin occupies the western half thrust over Tertiary rocks of the northern Great of the Greater Green River Basin, and is separated Divide Basin. A survey by the Consortium for from the Great Divide and Washakie basins in the Continental Reflection Profiling (COCORP) used east half of the Greater Green River Basin by the reflection seismology across the southern Wind north-trending Rock Springs uplift. More specific River Range along Wyoming State Highway 28 details for each of these basins are described in their to gather new data on crustal structure. The data own separate sections. The Wamsutter arch, an revealed that the thrust fault along the western eastward extension of the Rock Springs uplift, is a boundary of the range dips to the east at an angle structural divide that separates the Washakie Basin of 30° and extends to a depth of approximately from the Great Divide Basin. 18.7 miles. The Greater Green River Basin is part of Glaciated peaks mark the high central region the Wyoming Basins geomorphic province; as of the range; glaciers and permanent snowfields one might expect, topographic relief within the lie in the higher valley heads. U-shaped glaciated Greater Green River Basin is much less than the valleys issue from both the east and west sides of surrounding mountains, with the lowest areas the Continental Divide, which forms the backbone occupied by stream drainages and the higher of the range. On the west side of the range, a broad areas by cuestas, mesas, and plateaus. Only a very bench-like at about 9,400 feet above small part (about 30 square miles) of the Green sea level is dotted by a myriad of glacial lakes. River Structural Basin falls within the Platte River This high, relatively flat subsummit surface above drainage on the far western end of the watershed timberline is dissected, as if by a biscuit cutter, by at the headwaters of the Sweetwater River. This steep-sided glacial valleys. The range’s highest peaks upland area is overlain by undifferentiated Tertiary stand high above the subsummit surface. At the rocks. base of the mountain flanks, long, deep, moraine-

4-54 dammed lakes—such as Fremont and New Fork Uplift and on the west by the Laramie Mountains on the west side and Dinwoody and Bull lakes on (Figure 3-2); the Cenozoic through Paleozoic the east side—occupy valleys where glacial ice once sedimentary strata reach a maximum thickness flowed down from the higher regions onto the of 12,000 feet near Cheyenne. The surface rocks basin floor. here are nearly horizontal strata of Late Tertiary The South Pass area at the south end of the age, which lie unconformably on Paleozoic and Wind River Range hosts a sequence of Archean Mesozoic rocks downfolded into a large trough. (more than 2.5 billion years old) supracrustal The trough extends southward from Cheyenne to metamorphic rocks called a greenstone belt that Colorado Springs, Colorado. The axis of the trough contains the state’s principal gold deposits as well lies a few miles east of the mountain front (Plate as iron ore deposits. Besides being an important 1, inset). The maximum structural relief between point along the Oregon Trail, South Pass was also the deepest area of the basin and the highest area the site of extensive lode and placer gold mining of associated uplift in the Laramie Mountains in the 19th century and of a surface iron ore mine exceeds 17,000 feet. This is the difference between and mill in the 20th century. The area remains elevations in the deepest part of the structural basin attractive for gold prospectors and has a rich and at approximately 7,000 feet below sea level near well-preserved history. Cheyenne and at Laramie Peak, which is 10,274 feet above sea level. 4.14 Hartville Uplift (WSGS, 2013) 4.16 Geothermal resources (Heasler The Hartville Uplift (Plate 1; Figures 4-1 and and Hinkley, 1984; Hinkley and 4-17 through 4-20) and is a structural arch that Heasler, 1986) - Figure 4-21 separates the Denver Basin from the Powder River Basin. The arch extends from the northeast end of The geothermal resources of the Platte River the Laramie Range to the south end of the Black Basin are of the low-temperature hydrothermal type, Hills. Precambrian, Paleozoic, and Mesozoic rocks occurring where groundwater exists at anomalously were raised along the arch during the Laramide elevated temperatures (relative to the average orogeny, but were covered by the blanket of geothermal gradient). These elevated occurrences Cenozoic sediments. Today, there is very little are not typically found at a depth where they can be topography to suggest that this is a structurally put to beneficial use. Hydrothermal resources of the uplifted region. However, south of Glendo, to the Platte River Basin are primarily suited to local, small- east, Permian and Pennsylvanian-aged limestones, scale projects that utilize low-temperature waters for sandstones, and red shales poke through the space-heating, de-icing, and recreational/ therapeutic blanket of Cenozoic strata to form tree-covered applications (e.g., Saratoga Hot Springs). hills along the crest of the Hartville arch. These Generally, groundwater is heated as it flows Paleozoic strata are also exposed in a large interstate downdip into a structural basin in accord with the highway roadcut at the north end of the Glendo local geothermal gradient resulting from heat flow Reservoir. from deep in the earth toward the land surface. Platte River Basin hydrothermal resources occur 4.15 Denver Basin (Belitz and primarily where the heated groundwater rises to Bredehoeft, 1988, WSGS, 2013) shallower depth under artesian hydraulic pressures at velocities that preclude dissipation of the heat The Denver Basin (Figures 4-1 and 4-20), acquired at depth. This requires vigorous upward also known as the Denver Julesburg basin is a large flow through permeable up-folded strata or up (350 miles by 150 miles) oval shaped asymmetrical faults, systems, or wells. In general, the Laramide structural basin located in southeastern conditions that control hydrothermal resources Wyoming, eastern Colorado, and western occur only within the more productive Mesozoic Nebraska. In Wyoming, the Denver structural and Paleozoic aquifers in the Platte River Basin. basin is bounded on the north by the Hartville The locations of known and potential areas of

4-55 Geologic cross section Q-Q’. cross Geologic Figure 4-18. Figure

4-56 Geologic cross section R-R’. cross Geologic Figure 4-19. Figure

4-57 Geologic cross section S-S’. cross 4-20. Geologic Figure

4-58 Geothermal resources, Platte River Basin. River Platte resources, 4-21. Geothermal Figure

4-59 hydrothermal resource development are shown on The Wyoming State Geologic Survey (WSGS) Figure 4-21. has evaluated many Wyoming sites for potential mineral development. These include precious 4.17 Mineral resources metals (Hausel, 1989, 2002), gemstones (Hausel and Sutherland, 2000), base metals (Hausel, 1993, The development of mineral resources generally 1997), industrial minerals (Harris, 1992, 1996), requires the use and proper disposal or surface decorative stones (Harris, 2003), coal (Jones and discharge of groundwater and has a high potential others, 2011), coal bed natural gas (WSGS, 2005) to create avenues for groundwater contamination. and oil and gas (Lynds, 2013). Although these Figures 5-4, 5-7, 5-8, and 5-9 show the reports and maps generally discuss groundwater, distribution of oil-and-gas operations and other they do provide information on areas of future active and historic mineral development locations development that could create potential impacts on within the Platte River Basin (Section 5.7.2). state groundwater resources. Mineral development Even in areas without active mineral development, in the Platte River Basin as a source of potential the presence of some naturally occurring minerals contamination to groundwater resources is further such as those that contain uranium, arsenic, and discussed in Chapter 5. hydrocarbons, can, at significant concentrations, negatively impact groundwater quality. Some small communities in the northern part of the Platte River Basin have had to develop mitigation plans to address exceedances for naturally occurring radium, uranium and/or arsenic in their public water systems (WWC, 2011; Olsson Associates, 2008). Significant quantities of oil and gas have been developed in the Platte River Basin primarily in the Casper Arch, High Plains and northern Great Divide Basin (Figure 5-4). In the last few years, increased oil and gas exploration and production operations are focused on the Cretaceous Niobrara and Frontier Formations from Cheyenne north to Douglas. This trend has continued over the last year (June 1, 2012 to June 1, 2013) with over 140 Applications for Permit to Drill (APD) filed with the Wyoming Oil and Gas Conservation Commission (WOGCC) for wells in Converse County and 67 APDs approved for Laramie County. Again, most of these permits target the Niobrara Formation. Substantial uranium and coal have been commercially developed in the Platte River Basin. Currently, there is no active coal mining in the Platte River Basin but uranium is mined by in-situ recovery extraction at the Smith Ranch-Highland operation 23 miles northwest of Douglas. Industrial minerals including sand, gravel, clay, limestone, dolomite, feldspar, shale, bentonite, and gypsum have been extensively mined within the Platte River Basin, and still are produced in many locales (Figures 5-7, 5-8, and 5-9).

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