Field Guide to Tectonic Evolution of Utah’s Central Wasatch Range

Ron Harris, Brigham Young University

Mt. Timpanogos thrust sheet, which is composed of Carboniferous limestone and sandstone that is uplifted 10 km by triple thrust stacking of this sheet over itself. The uppermost thrust fault is just above lake level on the distant side the lake. Triassic rocks in the footwall of the thrust are the in the foreground.

Introduction The Park City area is at the heart of one of the most diverse, well-exposed and accessible geological repositories on Earth. Features of nearly every geological event that shaped the face of western North America are represented. During this field trip we will visit the most accessible of these features exposed in the canyons of the central Wasatch Range (Figure 1). These canyons are like 2 km deep serial sections through a part of Earth’s crust that has experienced a complete Wilson Cycle, and provide a three- dimensional chronicle of Earth history and processes.

Stop 1 – (30 min.) Overview near Flagstaff Mountain (2797 m elevation). The purpose of this stop is to use the high elevation vantage point to see the ‘big picture’ of the field trip.

The rock we are standing on is Pennsylvanian Weber Quartzite (sedimentary quartzite), which was deposited near shore in the Oquirrh Basin (Fig. 1). This basin rapidly subsided during the end of Appalachian Orogeny much further to the south. These rocks overlie the Mississippian Manning Canyon Shale, which acts as major detachment in the section. Beneath the shale are other Paleozoic sedimentary rocks, which include a thick Cambrian transgressive section. We will see all of these rocks during the field trip and discuss their tectonic significance.

Figure 1. Top - Photograph looking south at Cascade Mt. culmination on far horizon. Cascade Mt. is near center and Mt. Timpanogos is on the right. Beyond to the west of the culmination is Great Salt Lake Rift Valley. Bottom - Balanced cross-section of the Cascade Mt. culmination (From Schelling et al., 2007). Colors correspond to column in Fig. 2.

On the far horizon to the south we can see the Cascade Mt. Culmination (duplex stack) shown in Fig. 1. Repeated stacking of the Paleozoic section upon itself causes 10 km of uplift of the Pre-Cambrian (orange) and overlying Paleozoic rocks (Fig. 2). The floor thrust for the stack uses the weak Mississippian Manning Canyon Shale, which we will see at stop 5. The roof thrust is in Triassic to Jurassic units (green, see Fig. 3). These thrust sheets restore back to footwall cut-offs around 150 km to the west.

Figure 2. Structural model for duplex stacking associated with the Cascade Mt. Culmination. Modified from Harris (2011a).

Figure 2. Stratigraphic column of the rock units we will observe during the field trip. Thinning of Late Paleozoic units to the east is due to crossing the eastern boundary of Oquirrh Basin. At stop 2 we will see the Late Pre-Cambrian Big Cottonwood Formation. The entire Paleozoic section is exposed at stop 4 and large sections of the Oquirrh Grou at stop 5. At stop 6 we will see Oligocene volcanic rocks. Colors are marker horizons that correspond to the cross section in Fig. 2. From Schelling et al. (2007).

Travel 10 miles to Stop 2. Restrooms available.

Stop 2 – Storm Mountain, Big Cottonwood Canyon (45 minutes). The purpose of this stop is to see how well time is recorded in rocks. You will see daily, bi-weekly and monthly ‘rock rings’, and some amazing mountain scenery.

The Pre-Cambrian Big Cottonwood Formation represents a 5 km thick section of clastic sediments deposited in an estuarine setting. These deposits accumulated in a basin formed by a failed rift that branched off of the dominant N-S rift that split up Rodinia (Fig. 4). The failed rift arm likely captured the drainages of much of Laurentia, which is indicated by abundant Grennville age zircon grains in the quartzite (Spencer et al., 2012).

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B. Figure 4. A) Photo of the Storm Mountain area of Stop 2 (photo by Howie Barber).

B) Reconstruction of Rodina at around 800 Ma (Harris, 2011). The Uinta Rift is a E-W failed arm of the main N-S Panthalassa Rift, which broke up Rodina. From Harris (2011a).

A modern analog of this depositional environment is found currently where the Mississippi river funnels down through an ancient north-south rift in the continent that formed during the opening of the Gulf of Mexico. The failed rift has diverted river systems of interior North America towards it for the past 200 million years. The Big Cottonwood Formation is a world famous time piece discovered by University of Utah geology professor Marjorie Chan (Chan et al., 1994). She recognized that its tidal features preserve a record of the length of the day 850-750 million years ago. Studies of modern tidal flats reveal rhythmic bundles of paper-thin layers just like what is found in the 800 million-year-old Big Cottonwood Formation. The alternating light and dark layers of sand and mud correspond to tidal variations. For example, light-colored sandy bands are deposited as the tide rises and pushes sand shoreward - the higher the tide, the thicker the white band of sand (Fig. 5). When the tide goes back out again it leaves a thin dark film of mud on top of the sand layer. These alternating thick, light and thin, dark bands of sand and mud are known as rhythmites. Like a barcode, these layers provide the most detailed record of time of any natural clock known. They record semi- daily, daily, semi-monthly, monthly, seasonal and yearly cycles.

Figure 5. Rhythmites of the Big Cottonwood Formation. The light-colored sand rich packages represent spring tidal cycles and the dark-colored argillaceous bands represent neap tidal cycles. (Photo from Marjorie Chan).

Highest tides occur when both the sun and the moon are aligned. This syzygy, as it is known, happens during a new or full moon and produces ‘spring’ tides, like a watch spring. During a half-moon, when the sun and moon are at an angle of 90 degrees, the pull of each partially cancels out the other causing lower tidal variations known as ‘neap’ tides. These two different types of tidal cycles each happen twice a month, and are clearly visible in the sedimentary record of the Big Cottonwood Formation. Spring tides express themselves as thick ‘bundles’ of white sand and silt layers, and neap tides as bundle-bounding clusters of dark bands (Fig. 5). The number of dark bands in each cycle corresponds to the number of days in a lunar month. Marjorie Chan and others found that the best-preserved cycles in the Big Cottonwood Formation show there were 38 days in a lunar month. Other patterns reveal semi-yearly and yearly (seasonal) cycles that are consistent with those preserved on a daily and monthly scale. These findings indicate that around 800 million years ago the day was only 19 hours long and a year had 461 days. This means that Earth rotated faster in the past than it does today. The lengthening of the day to the present 24 hours is also caused by the orbital dynamics of the Earth-Moon system. The Moon orbits Earth much slower than Earth spins, so the gravitational attraction between the two bodies acts as a brake on Earth’s rotation. But because the energy lost in slowing Earth’s spin must be conserved, it is transferred into speeding the orbit of the Moon, which over time moves it further away from Earth. By the Cambrian era (542 million years ago), rhythmites indicate the length of the day had increased to 22 hours (400 days a year). At that time the moon was moving away from earth about 1.3 cm/year (0.5 inch/year), which is around half of its current rate.

Travel 5 miles to stop 3. No restrooms.

Stop 3 – G.K. Gilbert Park at the Mouth of Little Cottonwood Canyon (20 minutes). The purpose of this brief stop is observe fault scarps along the active Wasatch Fault that offset ~ 15,000 year old glacial moraines (Fig. 6), and discuss the implications of these features for earthquake hazards.

Figure 6. A) Aerial photograph (circa 1970) of the mouth of Little Cottonwood and Bells Canyons. Yellow arrow is the location of G.K. Gilbert Park. The early morning photograph captures shadows outlining west- dipping scarps of the Wasatch Fault. The scarps track across lateral moraines ploughed by glaciers emptying into the Great Salt Lake Rift Valley. B) Photograph looking south at fault scarps. The scarps offset ~15,000 year old lateral moraine by up to 20 m. From Harris (2011a).

G.K. Gilbert was one of the greatest American geologists ever known. He conducted most of his research in the late 19th century in Utah. Gilbert’s geological studies along the Wasatch Front led him to many fundamental discoveries about Earth processes that took decades for the geological community to rediscover. For example, his discovery of ancient shorelines of Lake Bonneville led him to theories about climate change and isostatic rebound. His mapping scarps along the Wasatch Fault led to ideas about elastic rebound on faults as the cause of earthquakes. Based on his observations of highly eroded fault scarps adjacent to Salt Lake City he reasoned that more elastic strain energy was stored on this segment of the fault than any of the others. He actually warned Salt Lake City residents in a newspaper article that they were most at risk. Various types of strain measurements across the Wasatch Fault, such as repeated laser surveys and high-resolution GPS surveys, have proven Gilbert was right in his seismic hazards assessment of Salt Lake City. Basin and Range extension is what drives movement along the Wasatch Fault, but it moves by stick-slip. Between earthquakes when the fault is ‘stuck’ the crust is still stretching, which loads the fault with potential energy. Then, with little to no warning, the energy stored for hundreds to thousands of years is suddenly released. The sudden snapping back of the crust from the contortions caused while it was stuck is what produces an earthquake. The size of the quake depends upon how large the piece of crust that snaps back is and how far it slips. Measuring the slip potential along the Wasatch fault and determining how long the fault has been stuck provides a way to forecast the likely size of the next event. The rate of stretching of Earth’s crust across the Wasatch Fault is only 1-2 mm/yr. However, the last major earthquake was 1350 years ago. Since this time 1.3 to 2.7 meters of elastic strain energy has accumulated along the Salt Lake City segment of the Wasatch Fault. A fault slip of 1-3 meters over the 40 km length of the Salt Lake City segment will cause > magnitude 7.0 earthquake. Because the fault dips at around 45° beneath the Salt Lake Rift Valley (Fig. 7), and is locked at a depth of 10-15 km, the epicenter of the event will be in the city itself, and may go down in history as the greatest disaster in US history.

Figure 7. DEM of the Wasatch Front near Little Cottonwood Canyon showing the surface trace of the Wasatch Fault and its 3-D geometry. The fault is stuck at a depth of around 10-15 km, which means the epicenter will be in densely populated parts of the valley. The yellow arrows are the sites where trenches into the fault reveal its slip history over the past 10,000 years. From Harris (2011a).

Travel 40 miles to Stop 4. Restrooms and lunch.

Stop 4. Rock Canyon, Provo (2 hours for lunch and 0.4 mile hike). The purpose of this stop is two fold: first, we will investigate depositional environments recorded in Eocambrian to Mississippian age rocks. Second, we will see how these rocks were deformed to build the Cordillera.

The stratigraphic Story of Rock Canyon Rock Canyon opens a window into the chronicles of Panthalassa, which is when the western edge of North America was a passive continental margin much like the present tectonic setting of the eastern seaboard and Gulf of Mexico (Fig. 8). The layers exposed here provide classic examples of the effects of climate and tectonics on sedimentary environments.

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B. Figure 8. Reconstruction of Pre- Cambrian (A) and Cambrian (B) Utah, which is much like the eastern seaboard of North America is today. Thermal subsidence of the newly formed continental margin is recorded by the Cambrain transgressive sequence. From Harris (2011a)

Mineral Fork Tillite: Unconformably overlying the Big Cottonwood Formation is a strange, dark-brown deposit called a diamictite (Fig. 9), which is well exposed in Rock Canyon (Fig. 10). Diamictites are strange because they consist of pebble to boulder-sized blocks of various types encased in very fine mud particles (Fig. 9a). Herein lies the conundrum. How can both of these deposits, which require very different amounts of depositional energy, be deposited at the same time in the same place? Another enigma of this rock body is that within every cubic centimeter of its dark mud are the fossil remains of more than 100,000 plankton and bacteria (Fig. 9b). These early single-celled life forms are no larger than the width of a hair (< 1/100 mm, 1/10,000 of an inch). Why did all of these organisms die? And, how were they so well preserved. Preservation of soft-bodied organisms requires a remarkably quiet environment of deposition. Again, if the water was calm enough to give all of these tiny organisms a quiet burial, then how did the pebbles and boulders get there without disturbing the dead or the mud they are buried in?

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Figure 9 A) Detail of Mineral Fork Tillite with dropstones of granite and limestone in a fine-grained matrix. B) Single-celled organisms found in the tillite mud matrix (from Dehler et al., 2005). The width of each image is less than 1/100 mm or 1/10,000 inches.

The solution to this part of the conundrum comes from research on glacial deposits. Icebergs that calve off a glacier can transport their rocky cargo into calm water. Eventually the iceberg melts and unloads ‘dropstones’ that end up embedded in fine, fossil-rich mud (Fig9a). These deposits are known as tillite, a type of diamictite that owes its origin to glaciers. The diamictite is known as the Mineral Fork Tillite. As a glacial deposit, it documents that cool climatic conditions prevailed from 580- 740 million years ago. Glacial deposits of similar age are also found in most of the now widely dispersed fragments of Rodinia. Even the ones that were near the equator at the time have tillites. The global occurrence of these glacial deposits, even in what should have been tropical regions, presents another geologic puzzle. The plot thickens when looking more closely at the tillite and finding that it is actually inter-layered with limestone. Limestone is only deposited in warm, shallow seas rich in algae. The alternating layers of tillite and limestone found in Rock Canyon, and in other deposits of the same age throughout the world indicate that climates at that time alternated drastically between warm (greenhouse) and cold (icehouse) extremes. What could possibly account for such extreme oscillation? One of the major differences between Eocambrian Earth and now is that the only life forms on the planet were plants. Mostly consisting of algae and microbes, plants inhabited the shallow seas that flooded the edges of the Rodinia supercontinent. However, the rifting apart of Rodinia around 750 million years ago drastically increased the area of flooded continental shelf habitats, and the proliferation of new life forms ‘quickly’ followed. Plants extract CO2 from the atmosphere, and their rapid explosion could have depleted much of the atmosphere of its CO2. Since CO2 is a greenhouse gas, its depletion reduces the ability of the atmosphere to trap heat radiating from the sun, and Earth’s surface cools. Cooling then causes much of the plant life to die and become buried by sediment, which is what we find in the shale of the Mineral Fork Tillite. The burial of all of this carbon prevents it from moving through the carbon cycle, which reduces atmospheric CO2 even more and contributes to global cooling. Debates rage about how much earth cooled, but it may have resulted in a condition known as ‘Snowball Earth,’ with ice appearing even at the equator. It is estimated that these conditions could last as long as 10 million years before volcanic activity could restore CO2 concentrations to greenhouse conditions once again. However, as greenhouse conditions return, the proliferation of plant life could start the cycle all over again. The occurrence world-wide of alternating layers of fossil-rich tillite and limestone indicate there may have been as many as four extreme climate change cycles. What broke the cycle giving rise to more modulated conditions? Most scientists agree that the arrival of animals on the scene during the Cambrain explosion, especially sea- bottom dwelling creatures, changed everything.. Not only do animals exhale CO2 back into the atmosphere, but many burrow into sea-bottom sediments and release much of the CO2 trapped there back into the atmosphere. The rock record in Rock Canyon, and other parts of the world, indicates that these extreme climate oscillations ended at around the same time as the first animals appear in the fossil record.

Cambrian Transgressive Sequence: Cambrian rocks exposed in Rock Canyon were deposited on a flooded continental shelf that stretched from central Nevada to Colorado (Fig. 8). The base of the Cambrian sedimentary pile is marked by a distinctive tan-colored sandstone that unconformable overlies the Mineral Fork Tillite known as the Tintic Quartzite (Fig. 10). This extensive deposit of nearly pure quartz sand records the persistence of the huge river system that contributed to the Big Cottonwood Formation. The sand layers were part of a system of deltas that poured out over the Cambrian continental shelf of western North America. Sedimentary structures such as cross-beds, channels, and ripples indicate rapid currents from streams and waves. Cross-bed directions are mostly to the west and NW, which was off of the continental interior and toward the newly formed Panthalassic Ocean. Overlying the Tintic Quartzite are the Cambrian Ophir Shale and Maxfield Limestone (Fig. 10). This transgressive sequence reveals that the Cambrian shoreline was migrating inland (east) toward the continental interior at the time (Fig. 8). By the end of the Cambrian Period, what was initially a tropical beach where the Wasatch Range is now was much more like the shallow, clear offshore waters of the Bahamas.

Figure 10. Rock Canyon looking NE form Provo. Dark slopes at the very base of the canyon are Mineral Fork Tillite, which is overlain by orange cliffs of the Tintic Quartzite. A small slope of Ophir Shale overlies the Tintic. The rest is almost all limestone to the top of Squaw Peak (pinnacle above the cliffs). A narrow white band of dolomite separates the Cambrian limestone below from the Mississippian limestones above. The bench above Squaw Peak formed as less resistant Manning Canyon Shale eroded off. Above and beyond the bench are Pennsylvanian to Permian limestone and arenite of the Oquirrh Group. These units form the highest of a series of thrust sheets of Paleozoic units that uplifted these rocks by 10 km. Extension along the Wasatch Fault has dropped the rocks in the hangingwall (below the rift valley) back down 10 km to near their original depth

The Cambrian transgression was most likely caused by thermal subsidence and sedimentary loading of the continental margin, which let 530 meters (1740 feet) of Cambrian sediment in Rock Canyon. To the west, these units are more than 5 times this thick. The hinge-line of subsidence was near the current location of the Wasatch Range (Fig. 8). Overlying the rocks of the Cambrian Period in Rock Canyon is an erosional surface that represents a major gap in the geologic record visible on the canyon wall as a white band (Fig. 10). The color change is due to the fact that the Cambrian limestone was exposed to the surface, which bleached most of the organic material out of it and changed it to a dolomite. The long-term tectonic stability of western North America from the Neoproterozoic to the Jurassic provided ideal conditions to build a rich paleontological repository that showcases how life changed through around 400 million years of Earth’s history. Although most of the sedimentary units hosting younger fossils are to the east of the Rock Canyon divide, it is possible to move up the stratigraphic section toward Price, Utah and witness the formation by formation progressive development and diversification of life through geological time. There are only a few places on Earth where such a repository of evolution is as accessible and well exposed.

The Chronicles of Cordillera The passive continental margin of western North America was transformed into a convergent margin as the Panthalassa Ocean began to subduct and pull the continent into collision with other plates, such as the Farallon Plate (Fig. 11). The Farallon Plate is the long-lost twin to the massive Pacific Plate. The reason it is mostly lost is because it subducted beneath the edge of the America’s and in the process formed what is known as the Cordillera. Cordillera is Spanish for ‘a mountain range along a coastline.’ It is pronounced ‘cord-a-jera’. The ‘waves’ of crumpled rock caused by subduction of the Farallon Plate first arrived in Utah from the west at around 100 million years ago. There westward advance was heralded by a complete reversal in the direction of sediment supply, which for 500

Figure 11. Cartoon reconstruction of various phases of development of the Wasatch Range and western North America (Harris, 2011a). From left to right – chronicles of Panthalassa (passive margin), Cordillera phase 1 (Sevier Orogeny), Cordillera phase 2 (Laramide Orogeny) and Great Basin extension. These depositional and deformational processes represent a nearly complete Mountain Building Cycle.

million years had been from the craton to the east, but now was from rising mountain ranges to the west (Fig. 11). The layers caught up in eastward advancing mountain system in the Central Wasatch Range are different than those to the north and south due to inherited mechanical boundaries and structures of the Oquirrh Basin. This intra-cratonic basin opened at the end of the Appalachian orogeny (Pennsylvanian to Permian) and is filled with around 10 km of mostly carbonate and quartz arenite, with some shale units (Fig. 12). The brittle strength of these rocks allowed for the basin to mostly remain intact and thrust over itself to form a major structural culmination (Contractional deformation of the Sevier Orogeny is manifest in package of strong sedimentary rocks that accumulated in the Oquirrh basin created large ramp anticlines along the former basin boundaries. This reactivation of inherited structures is a characteristic of basin inversion (Fig. 12).

“Y” Mountain Anticline One of the most accessible large folds that formed during the Sevier orogen is exposed in Rock and Slate Canyons on either side of “Y” Mountain (Fig. 13). “Y” Mountain towers over the City of Provo, which is home to Brigham Young University or BYU. The original plan was to inscribe the letters ‘BYU’ on the mountain. However, constructing the “Y” expended so much time and money that the “B” and “U” were abandoned. It is likely that the engineering class assigned the task, and the mountain itself were both relieved that the project came to end. The university is now known locally and by its alumni as ‘The Y.’ The “Y” Mountain fold is a fault-propagation or tri-shear anticline (Fig. 14). The eastern forelimb is vertical to slightly overturned indicating eastward vergence. The fold is around 14 km in length and doubly plunges away from its culmination near Slate Canyon. The 10° plunge to the north forms the crest of Squaw Peak above Rock Canyon and the ‘listing ship’ landscape of Cascade Mountain (Fig. 13).

Figure 12 A. Rock Canyon Anticline panorama. Strike valley is Manning canyon Shale, which is also a major detachment horizon. These steeply dipping rocks truncate against a low angle thrust flat that dips east. Some of the east dip is due to eastward rotation of the Wasatch Range due to isostatic adjustment from 10 km of slip along the Wasatch Fault.

Figure 12B. Basin inversion model for the evolution of the southern Charleston-Nebo thrust system. The location of Sevier structures and the thick-skinned response to shortening, is mostly controlled by pre- existing weaknesses and crustal heterogeneities associated with the Oquirrh basin. From Harris (2011b).

Next Page, Figure 13. Location map of the “Y” Mountain anticline near Provo (oblique view looking east). Note plunge to the north and south, and strike valley of Manning Canyon Shale. Trace of the Wasatch Fault is in orange. From Harris (2011b).

The thrust fault that formed the “Y” Mountain anticline is interpreted as a splay of the much larger Charleston-Nebo thrust, which stretches for 175 km. The thrust sheet was initially as much as 10 km thick, but along the Wasatch Range front it is now only a few hundred of km thick at most. It is likely that the Charleston-Nebo thrust is essentially the top-to-the-east translation of the Oquirrh basin along pre-existing faults reactivated as thrust ramps.

What To See In Rock Canyon The most accessible place to view the “Y” Mountain anticline is Rock Canyon. A one-third mile paved trail leads from the parking lot at the mouth of the canyon directly to the hinge zone of the anticline. At the beginning of the trail follow the paved road to the right. For the more adventurous, there are several paths off of the paved trail that allow access to other parts of the fold. At the mouth of Rock Canyon is a fault block of bleached and highly fractured Tintic Quartzite that forms the footwall damage zone of the Wasatch fault. Farther up the canyon the contact between the dark brown Precambrian Mineral Fork Tillite and tan colored Cambrian Tintic Quartzite is well exposed (Fig. 10 and 14). This contact is unconformable with at least 200 million years of Earth history missing. The Mineral Fork Tillite displays a tightly spaced pressure-solution cleavage that is folded and faulted by top-to-the-east shear. These structures demonstrate the importance of layer-parallel shortening by pressure solution, thrusting, and folding within the clay- rich tillite. Where the canyon narrows the unconformity bends from slightly dipping northwest into a steep asymmetric anticline (Figs. 14-16). An abandoned mine shaft at the contact marks the abrupt change in dip. Many faults are found throughout the hinge zone that formed to accommodate folding. Perhaps the most obvious are the opposing thrust faults found offsetting the unconformity in the hinge zone of the fold (Fig. 15). These faults are visible in the cliff face on the north side of the canyon. They thrust the Mineral Fork Tillite upward and out of the hinge zone. The faults are an example of shortening below the neutral surface of the fold. Fold accommodation faulting also is recognized above the neutral surface in the form of an east-dipping normal fault, which decreases in slip toward the hinge zone or neutral surface. This fault juxtaposes different dip domains of the fold making it difficult to trace the fold upsection while standing in the hinge zone. The backlimb dip domain, consisting of mostly Cambrian and Mississippian limestone, rests directly on the top limb dip domain consisting of Tintic Quartzite, cutting out the Ophir Formation (see figure 16). However, this offset produces a very thick section of Ophir Formation in the hinge zone. The decrease in slip downward on the fault to accommodate thickening of shale in the hinge zone is consistent with localized extension within the fold rather than regional stress regime change (Basin and Range extension) as the cause for the normal fault. A similar structure is found in the Bridle Veil Falls fold as well (see stop 5 below).

Figure 14. Geology draped onto a DEM of the Rock Canyon Anticline. The asymmetry of the anticline is consistent with a fault-propagation folding mechanism. The Mississippian Manning Canyon Shale is shown as the basal detachment for the anticline. From Harris (2011b).

Figure 15. Looking north at core of Rock Canyon Anticline. Tintic Quartzite is folded from nearly horizontal (left) to slightly overturned (right). The most overturned beds are dragged along a thrust fault at the base of the fold. Fold accommodation structures include: opposing thrust faults moving Mineral Fork Tillite out of the hinge zone, thrust faults that die out toward neutral surface, bedding planes faults (flexural slip) with top-toward-the-hinge, dip-slip slicken lines.

West East

Figure 16. Reconstruction of the Rock Canyon Anticline (top) with normal fault cutting through hinge zone to accommodate neutral surface extension. Post folding extension (bottom) is show along the Wasatch Fault. The fault reactivates the thrust ramp in a normal sense and drops the backlimb down and to the west. From Wald et al. (2010).

Another 300 meters farther up the canyon, where a metal gate crosses the road (0.4 miles from the parking lot), are well-exposed outcrops of vertical to slightly overturned beds of the Cambrian Tintic Quartzite. Slicken sided surfaces and slicken-fibers are found along many of the bedding surfaces here with top-toward-the-hinge sense of shear. These kinematic indicators document flexural-slip fold mechanisms. The east side of ‘The Kitchen’ consists of a steeply west-dipping bedding plane with conjugate sets of en echelon fractures that strike northeast-southwest and northwest- southeast. Mode 1 fractures associated with the en echelon arrays are mostly east-west. Rotating the bedding plane back to horizontal yields an east-west maximum stress direction (σ1). The fractures are consistent with layer-parallel shortening before folding and a north-south σ3. Other indicators of an east-west σ1 are north-south fold hinge lines and cleavage planes.

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Figure 17. A) Overturned bedding plane of Tintic Quartzite on the fore-limb of the Rock Canyon anticline at ‘The Kitchen’. Tension and proto-shear fractures provide of evidence of paleostress directions during layer-parallel shortening before folding. The bedding plane strikes north-south and is parallel to the fold hinge. B) Data from deformed reduction spots in shale showing the maximum shortening is nearly E-W and extension direction is nearly N-S. the amount of strain in the shortening direction is nearly 50%. From Harris (2011b).

Drive 10 miles to Stop 5. No restrooms. Stop 5A – Oquirrh Group Sequence Stratigraphy (1 hour) The purpose of this stop to observe the sequence stratigraphy of limestone units in the lower Oquirrh Group (Fig. 18).

Figure 18. Stratigraphy of the lower Oqirrh Group. The Bridal Veil Limestone Member is well exposed on the west face of Cascade Mountain. The Bear Canyon Member forms the top of Cascade Mountain and Mt. Timpanogos. From Shore and Ritter (2007).

The Oquirrh Basin remains an enigma in the tectonic evolution of western North America during the Late Paleozoic. The Antler and Appalachian Orogenies were happening on either side of the basin, but were far enough away that their influence is questionable. Models have related this basin to foreland subsidence from the Antler Orogen and intra-cratonic extension perpendicular to the Appalachian Orogen. Scott Ritter and his student David Shore investigated the sequence stratigraphy of the Bridal Veil Limestone member of the Oquirrh Group to determine if it was driven by orbital forcing or tectonic influences (Fig. 19). The panel of rock they investigated forms the western face of Cascade Mountain near the entrance to Provo Canyon. Due to my ignorance about sequence stratigraphy I quote from their paper: “The Bridal Veil Limestone represents the basal second-order supersequence of the composite Absaroka I Supersequence in the Oquirrh basin. The 450 m thick limestone member consists of 64 coarsening-upward carbonate cycles bundled into 20 depositional sequences that range from 3 to 60 m in thickness. Sequence boundaries are marked by thin (0.5–2.0 m), regionally persistent quartz- sandstone beds and/or microkarst horizons. The typical coarsening-upward cycle comprises a 1- to 15-m-thick couplet of mud-rich, slope-forming limestone overlain by grain-rich, ledge-forming limestone…We divide the Bashkirian Bridal Veil Limestone into a hierarchy of 3 low-order, 20 intermediate-order, and 64 high-order cycles…Maximum cycle duration is around 2 million, 300,000, and 93,750 years, respectively.” This same panel of rock is folded and thrust further upstream to form the Bridal Veil Falls Fold.

Figure 19. Aerial view of the west face of Cascade Mountain near the entrance of Provo Canyon. The panel is 0.7 km across. Yellow lines are sandstone beds. Red lines are sequence boundaries. Blue arrows are individual parasequences. From Shore and Ritter (2007).

Stop 5B– Bridal Veil Falls Fold The purpose of this stop is better understand how folds develop, and how the fold geometry may relate to blind structures.

The poster child of folds in the Wasatch Range is the Bridal Veil Falls Fold (Fig. 20) in Provo Canyon. The fold likely formed as an accommodation structure of the much larger “Y” Mountain anticline. It is part of the northward plunging forelimb of the anticline, although the fold here is near the tip of the blind thrust that caused it. The plunge in this area is demonstrated most dramatically by differences in structural level. For example, the Great Blue Limestone, which forms the narrow mouth of Provo Canyon at an elevation of around 4922 feet, rises to 8859 feet southward up-plunge to form the top of “Y” Mountain (Fig. 13).

Figure 20. A) The Bridal Veil Falls Fold looking SE at the NW flank of Cascade Mountain. The cliff face is composed of Oquirrh Group carbonate (grey) and sandstone (tan). B) Detail of fault propagation folds verging SW in the opposite direction as the Rock Canyon anticline.

Traveling up Provo Canyon toward the fold it is possible to see the influence of drag along the Wasatch Fault on the footwall of the fault by tracing the contact between the Great Blue Limestone and overlying Manning Canyon Shale. At the mouth of the canyon movement along the Wasatch Fault has abruptly bent down layers of the limestone due to fault drag and smeared thick sections of the shale along the fault core zone. Several top-down-to-the-east normal faults offset the contact due to westward stretching of the footwall by the Wasatch Fault. Above the shale slopes are thick sections of the Bridal Veil Limestone and Bear Canyon Members of the Oquirrh Group (Fig. 18). These resistant units form towering cliffs decorated with waterfalls. The largest of these is Bridal Veil Falls, which drains Cascade Peak (elevation 10,908 feet) to the east (Fig. 21). The waterfalls flow out of hanging valleys caused by differences in down-cutting rates between the Provo River and its small tributaries coming from Cascade Mountain.

Figure 21. Bridal Veil Falls. The falls pours out of a hanging valley in Oquirrh Group carbonate and sandstone. Avalanches and icefalls commonly funnel through the hanging valley in the winter and have dammed the Provo River below several times.

Rivaling the scenic beauty of the falls is the large fold next to it (Fig. 20). The Bridal Veil Falls Fold is a series of nested fault-propagation style anticlines within the basal section of the Oquirrh Group limestone. The immediately underlying Manning Canyon Shale forms a major detachment zone throughout the region. The anticline is asymmetrical and verges to the southwest against the regional direction of other Sevier structures. The ‘anti-vergence’ may be explained by flexural slip in the forelimb of ‘Y’ Mountain Anticline or other accommodations associated with out-of-syncline thrusting or wedge thrusting of the Mississippian limestone into the Manning Canyon Shale. A line- length comparison of pre- and post-folding indicates at least 40-50% shortening, which does not include ductile deformation, such as pressure solution and flexural flow of shale into the hinge zone (Fig. 22). Rigid limestone and sandstone layers interbedded with relatively weak shale horizons produce mechanical anisotropies that lead to an array of fold mechanisms (Fig. 22). A.

B.

Figure 22. A) Center reference section of the Bridal Veil Falls Fold. Blue lines are parallel to pre-folding cleavage due to layer parallel shortening. White lines parallel late folding axial planar cleavage. Arrows show directions of syn- and post-axial planar cleavage flexural slip. Dashed box is area of detail. B) Detail of area in box of (A) with corresponding numbers for reference. 1 – normal fault with slip decreasing downward that accommodates extension above the neutral surface. 2 – Flexural flow of shale into hinge zone, which is overprinted by axial planar cleavage (white line), 3 – Flexural slip in shale between limestone beds that forms ‘S’ folds of axial planar cleavage (From Harris, 2011b).

Fold Geometry and Mechanisms Although the Bridal Veil Falls fold is mostly inaccessible except with climbing equipment, the BYU structure class investigates its lower sections every fall to determine how it formed. We recognize several phases of deformation, some of which are likely simultaneous. These include: (1) compactional strain that formed bed-parallel styolites; (2) layer-parallel shortening before folding, which produced layer-normal cleavage and fracture planes and layer-parallel tension fractures - structures that are folded with the layers; (3) late folding axial-planar cleavage that accounts for up to 40% volume loss in clay-rich horizons (Fig. 20); (4) syn- and post-folding flexural slip of both limbs, which opened space in the hinge zone for flexural flow of clay-rich units (Fig. 20), (5) extension above the neutral surface accommodated by normal faults with displacements decreasing toward downward (Fig. 20), and fold-hinge-parallel extension fractures on the buckled surfaces of many resistant layers, and (6) fracture development throughout various phases of deformation. The strike of the ‘Y’ Mountain Anticline is rotated counter-clockwise by 40° in the area of Cascade Mountain and Mt. Timpanogos (Fig. 23). This rotation is likely due to decreased slip along the blind thrust that caused the anticline, which is also why the fold plunges out in this direction (Fig, 10).

Figure 23. Stereographs of poles to bedding for the Bridal Veil Falls Fold (a) and Rock Canyon fold of the ‘Y’ Mountain Anticline show a rotation of 40° in strike. Decreased displacement at the northern tip of the blind thrust causing the fold is used to explain the northward plunge of the anticline and the rotation (c). A map of the Charleston-Nebo thrust sheet demonstrates the same relations at much larger scale, which is likely influenced by the inherited structure of the Oquirrh Basin. Notice how the shape of Utah Lake is influenced by topography associated with the geometry of the thrust and its influence on the geometry of subsequent normal faults (unpublished from Harris).

100 – 50 Ma

Figure 24A. Frictional resistance to subduction of the Farallon Plate causes thickening of the edge of western North America by folding and thrusting, and magmatism. The load of the mountains depresses the craton forming a foreland basin. Star is location of Park City.

50 – 30 Ma

Figure 24B. Low-angle subduction increases the friction between Farallon and North American plates causing thick-skinned deformation (basement uplifts) and a wider zone of magmatism. These processes thickened the crust of western North America to around 70 km, which caused a high plateau similar to the present day Altiplano of Peru. The Uinta Arch formed at this time, which inverted the Uinta graben that formed during the late Pre-Cambrian (see

30 Ma to Present

Figure 24C. The collapse of western North America. The demise of subduction and sinking of Farallon Plate led to weakening of the base of the North American. The foundation of the continent could no longer support the load of the high plateau causing it collapse westward towards the NW moving Pacific Plate. The Wasatch Fault forms the eastern edge of the massive crustal landslide that is the Basin and Range. All drawings in Fig. 24 from Harris (2011a).

20 Mile Drive to Stop 6. No restrooms.

Stop 6. Jordanelle Reservoir - Lahar deposits and Laramide arc. (20 min.) The purpose of this stop is to marvel at the lahar deposits full of large blocks of andesitic volcanic rocks near Park City (Fig. 24). And, to try to imagine the scale of the strato volcanoes, now completely eroded away, that sourced these deposits and others, such as the abundant Ag-Pb-Zn-Cu-Au deposits. A.

B.

Figure 24. Late Oligocene lahar deposits with large blocks of andesitic volcanic rocks. These deposits indicate that there were steep enough slopes nearby, likely strato volcanoes, to source the huge blocks encased in volcanic debris flows (lahars). A) Plutonic rocks of similar age form the mountains on the horizon. B) White porphyry overlain by lahar deposits and cut by normal faults. Most of these faults are from syn-magmatic extension and form the pathways for most ore deposits in the Park City area. The end of the Cordilleran Orogeny is marked by large eruptions, which may have been some of the largest we can document on Earth. This magmatic event may be due to rapid sinking of the Farallon Plate, which caused an influx of hot asthenosphere along the base of western North America. This event is well documented by deposits of explosive volcanic activity, intermediate composition intrusive rocks and mineralization throughout the central Wasatch Range. The intrusions and volcanoes they fed are aligned along the Uinta Arch. This arch formed during the Laramide phase of the Cordilleran Orogeny (Fig. 24B), by reactivating faults originally formed during Pre-Cambrian rifting of Rodinia (Fig. 8A). Magma generated by the sinking Farallon Plate found its way to the surface along the same fault system. A. Figure 25. A) intrusion of andesitic dikes (dark brown stripes) into quartzite along Ne-SW fissures. These fissures carry most of the ore that was mined in the Park City area. B) Replacement mineraliza- B. tion where silver and copper rich fluids intrude along fractures and react with surrounding rocks. The green mineral is a copper carbonate known as Malachite, which is a reaction between copper and limestone.

References:

Bruhn, R.L. DuRoss, C.B., Harris, R.A. and Lund, W.R., 2005, Neotectonics and paleoseismology of the Wasatch Fault. Utah. In: Pederson, J. and Dehler, C.M., eds., Interior Western U.S., Geological Soc. America F. G., 6., p. 231-250. Chan, M.A., Kvale, E.P., Archer, A., and Sonett, C., 1994, Oldest direct evidence of lunar solar tidal forcing encoded in sedimentary rhythmites, Proterozoic Big Cottonwood Formation, central Utah: Geology, v. 22, p. 791–794. Chang, W., R. B. Smith, C. M. Meertens, and R. A. Harris 2006, Contemporary deformation of the Wasatch Fault, Utah, from GPS measurements with implications for interseismic fault behavior and earthquake hazard: Observations and kinematic analysis, J. Geophys. Res, 111, B11405, doi:10.1029/2006JB004326. Hammaker, S. and Harris, R.A., 2007, Fault-related groundwater compartmentalization in the East Tintic Mining District, Utah, in Willis, Grant C., Hylland, Michael D., Clark, Donald L., and Chidsey, Thomas C., Jr., editors, Central Utah - Diverse Geology of a Dynamic Landscape: Utah Geological Association Publication 36, p. 405-423. Harris, R.A., 2011a, Exploring the Geology of Little Cottonwood Canyon: The Greatest Story Ever Told by Nine Miles of Rock, BYU Press, Provo, Utah, 80 p. Harris, R.A., 2011b, Road guide to the geology of the Sevier thrust belt in the central Wasatch Range, Utah, in Sprinkel, D.A., Yonkee, W.A., and Chidsey, T.C., Jr., editors, Sevier thrust belt: northern and central Utah and adjacent areas: Utah Geological Association Publication 40, p. 1-19. Nelson, S. and Harris, R., 2001. The role of rheology in the tectonic history of the Colorado Plateau, in Erskine, M.C., Faulds, J.E., Bartley, J.M. and Rowley, P.D., eds., The Geologic Transition, High Plateaus to Great Basin – The Mackin Volume, Amer. Assoc. Petroleum Geol., Special Publication GB78, p. 189-203. Schelling, D., Douglas K. Strickland, Keith R. Johnson, and John P. Vrona, 2007, Structural Geology of the Central Utah Thrust Belt, In: Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, UGA Publication 36, p. 1-30. Shore, D. and Ritter, S., 2007. Sequence stratigraphy of the Bridal Veil Limestone Member of the Oquirrh Formation (Lower Pennsylvanian) in the Central Wasatch Range, Utah — Towards a Bashkirian cyclostratigraphy for the Oquirrh Basin, In: Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors, UGA Publication 36, p. 58- 74. Spencer, C.J., Hoiland, C.W., Harris, R.A., Link, P.K., Balgord, E.A., 2012, A Neoproterozoic age for the Little Willow Formation, Utah: Identifying the earliest sediments of the rifting of Rodinia, J. of Pre-Cambrian Res., 204– 205, 57– 65. Wald, L.C., Kowallis, B.J. and Harris, R.A., 2010. Structural Analysis of Rock Canyon near Provo, Utah, BYU Geology Studies, V. 48, p. 53-78.