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). A. 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.