LARAMIDE PALEOSEISMITES IN THE CLARK'S FORK BASIN, MONTANA AND WYOMING

Paleoseismology relies on interpretation of paleoliquefaction structures and other kinds of paleoseismites, yet few sites exist where well-exposed paleoseismites are present both in a significant stratigraphic interval and over a wide geographic area in sediments of varied depositional environments. We have discovered one such area along the eastern edge of the Beartooth Mountains in south-central Montana and Wyoming. Paleoseismites here are in the Tertiary Fort Union Formation of the Clark's Fork Basin, which is composed of sediment derived from the uplift and erosion of the Beartooth Mountains during the Laramide orogeny. Although the stratigraphic and geographic occurrence of these features is widespread throughout the basin, detailed mapping of the Tongue River Member of the Fort Union Formation in study areas northwest of Clark, WY and southeast of Red Lodge, MT revealed abundant cross-sectional exposures of a wide variety of paleoseismites, including clastic dikes and sills, diapirs, convoluted bedding, and ball-and-pillow structures. The kind of paleoseismite produced was dependent on both proximity to the Beartooth fault zone and characteristics of the enveloping sediment. Upwardly injected tabular clastic dikes are the most common seismite in the conglomerate-bearing, alluvial-fan facies proximal to the mountain front. Clastic dikes are rare or absent in sediments deposited in more distal fluvial and lacustrine environments. In these finer grained rocks, seismites are most commonly convoluted bedding, diapirs, and ball-and-pillow structures. The distinct change in liquefaction structures is likely a result of differences in shaking intensity, grain size, and water content of sediments. Establishing the relationship between these characteristics and the kind of liquefaction structure provides another tool for paleoseismological analysis.