CAMPANIAN PALEOSEISMITES: INDICATORS OF EARLIEST LARAMIDE DEFORMATION IN THE EAGLE FORMATION OF THE ELK BASIN ANTICLINE, WYOMING AND MONTANA

Paleoseismites, indicative of early Laramide earthquakes, occur within four Late Cretaceous (Campanian) formations on both flanks of the Elk Basin anticline, Bighorn Basin, Montana-Wyoming. Within marine units (Telegraph Creek, Claggett, and Judith River) convolute-bedding occurs in numerous sandstone beds (<1m-thick) with vertical to overturned strata on either side of numerous near-vertical vents. At the time of deposition, vents and near-vertical strata were then truncated by horizontally bedded overlying sand with no evidence of sand ejecta. The nonmarine Eagle Formation lacks surface vents, but convolute-bedding occurs in 1-5m-thick sandstones which are laterally traceable for 10's to 100's to 1000's of meters. Orientations of 145 clastic dike-segments were measured and, after bedding was rotated to horizontal, were compared with 61 previously reported joints. At a few localities, near-vertical, linear sand-dikes merge downward into convoluted sandstone beds which are interpreted as "sand-source" beds for the clastic dikes. Sand-dikes are typically planar, near-vertical dikes which are 2-50cm-wide, 1-3m-high, and 1-30m-long and were injected upward and laterally across mudstone and/or coal beds, along pre-existing near-surface joints. Dikes fill early cross-joints, consistent with their injection during earliest Laramide basement deformation before the thrust offset overlying strata. Previously reported dominant strike-joint sets reflect a similar stress field at a later Laramide deformational stage. Distribution of clastic dikes and convolute-bedding in the Eagle Formation are consistent with possible thrust-segment boundaries beneath the central (NW-trending) part of the anticline and its northern (WNW-trending) and southern (S-trending) noses.