Laramide paleoseismites of the Bighorn Basin
Published:January 01, 2008
Kevin G. Stewart, Mervin J. Bartholomew, Heather A. Ballantyne, 2008. "Laramide paleoseismites of the Bighorn Basin", Roaming the Rocky Mountains and Environs: Geological Field Trips, Robert G. Raynolds
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The Paleocene Tongue River Member of the Fort Union Formation in the northern Bighorn Basin of south-central Montana and Wyoming, and the Cretaceous Eagle Formation at Elk Basin, Wyoming, contain abundant clastic dikes, sills, and other soft-sediment deformation structures. Clastic sills and tabular clastic dikes, which show evidence of forceful, upward injection of liquefied sediments, have been cited as evidence for past seismic shaking. Sills and dikes with such characteristics are common in the field trip area and we interpret them to be paleoseismites that formed during the late Cretaceous-Eocene Laramide orogeny. Other structures indicating liquefaction and flow of unconsolidated sediments are present in the same geographic areas and stratigraphic horizons as the dikes and sills. We speculate that these features may also be a result of seismic shaking. Paleoseismites in this region can be classified into two groups, clastic dikes and sills, and convolute bedding, which includes ball-and-pillow structures, slumps, and diapirs. Clastic dikes and sills are present in the proximal, conglomerate-bearing, alluvial-fan facies of the Tongue River Member and in interbedded Cretaceous sandstone and shale at Elk Basin. Clastic dikes are rare or absent in distal fluvial and lacustrine deposits, where the seismites are most commonly convolute bedding. Field mapping and stratigraphic measurements show that source-bed thickness and depositional environment are the major controls on the type of seismite that formed.
Keywords: paleoseismite, seismite, Laramide, Bighorn Basin, Elk Basin.
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Roaming the Rocky Mountains and Environs: Geological Field Trips
Prepared following the 2007 GSA Annual Meeting in Denver, Colorado, these 15 guides illustrate the latest geological and archeological thinking on a variety of current research themes. Regional-scale topics include landscape responses to dynamic processes of volcanism and uplift in Yellowstone and western Colorado, geomorphic evolution along the Front Range of Colorado and on the High Plains of South Dakota, and geoarchaeological research in central Colorado and western Nebraska. A series of papers illustrates tectonic and stratigraphic processes through time and space, with discussions of Precambrian structures in western Colorado, Jurassic deposition in south-central Colorado, and near-shore stratigraphic patterns in the Cretaceous strata of the Book Cliffs. One paper reviews potential seismic signatures in Cretaceous and Early Tertiary strata in northern Wyoming and Montana, and another discusses patterns of extension in southern Nevada and adjacent portions of California. Other topics in this well-rounded volume include the history of volcanism and gold mineralization at Cripple Creek, development of coalbed methane resources in the Powder River Basin, and a long-lived subsurface coal fire in western Colorado. Follow in the footsteps of these field trips, and see for yourself the patterns and evidence discussed.