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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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