Graduate and undergraduate students enrolled in Sedimentary Basin Analysis (GEOS 417/517) flew to Salt Lake City in late September for a five-day fieldtrip across the Sevier fold-thrust belt and foreland basin system in Utah and Wyoming. Among the 17 participants were representatives from six different countries, including China, Italy, Turkey, Argentina, United Kingdom, and the US. The trip was led by Professor Peter DeCelles with assistance from Dr. Gian Paolo Cavinato from the University of Rome, and ExxonMobil provided generous support for part of the costs involved.
The purpose of the trip was to familiarize students with regional-scale structure of fold-thrust belts and the relationships between structure and sediment accumulation in foreland basins. The fieldtrip area is an outstanding natural laboratory for such studies, as it contains classic examples of both fold-thrust belt and foreland basin features, including six major thin-skinned thrust systems, a huge basement-involved duplex, excellent examples of growth structures in wedge-top foreland basin deposits, and a wide variety of alluvial, fluvial, and marine facies. Moreover, extensive hydrocarbon exploration and production for many decades provides a nearly unrivalled understanding of subsurface structure.
The trip began at the airport in Salt Lake City, where the students were given an overview of the Wasatch front and eastern Basin and Range. Proceeding north to Ogden, we observed the internal geometry of the Ogden basement duplex and the Wasatch anticlinorium. The Willard thrust, one of the world’s great thrust faults with more than 50 km of slip, was our target at Pineview Reservoir in the northern Wasatch Range. All of our activity on day two was focused on the Crawford thrust system, which is exposed at several different structural levels along an approximately 100 km strike length in northern Utah. Spectacular growth structures, passive roof backthrusts, and triangle zones distinguish the Crawford thrust. The third day provided a chance to observe coarse-grained proximal foreland basin deposits along the Crawford and Medicine Butte thrusts, and an introduction to the Absaroka thrust system. Day four began with a detailed inspection of synorogenic Cretaceous fan-delta deposits that document Absaroka thrust timing, followed by a traverse of the Green River basin to the southwestern flank of the Wind River Range. Students pondered the possible relationships between Laramide contractional structures exemplified by the ‘Winds’ and the more conventional thrust belt to the west. The last day of the trip was spent in a large lacustrine wedge-top basin at Fossil Butte National Monument, and, en route to Salt Lake City, in the earliest back-bulge and foredeep deposits of the foreland basin system.
The group camped out every night but the weather was lovely and the fall colors were stunning. Even in field area that is as scientifically ‘mature’ as the Wyoming-Utah thrust belt and foreland basin, which has been the focus of so much study by academic and industry geologists and geophysicists, the students found much to debate and question.