A DEBRIS FLOW DEPOSIT IN ALLUVIAL, COAL-BEARING FACIES, BIGHORN BASIN, WYOMING, USA - EVIDENCE FOR CATASTROPHIC TERMINATION OF A MIRE

Coal and clastic facies investigations Of a Paleocene coal-bearing suc cession in the Grass Creek coal mine, southwestern Bighorn Basin, Wyom ing, USA, suggest that disruption of peat accumulation in recurrent mi res was caused by the repetitive progradation of crevasse splays and, ultimately, by a catastrophic mass movement. The mass movement, repres ented by deposits of a debris flow, marked the termination of signific ant peat accumulation in the Grass Creek coal mine area. Megascopic an d microscopic analyses of coal beds exposed along the mine highwalls s uggest that these deposits developed in low-lying mires, as evidenced primarily by their ash yields and maceral composition. Disruption of p eat accumulation in successive mires was caused by incursions of sedim ent into the mire environments. Termination by crevasse splay prograda tion is represented by coarsening-upward successions of mudrock and ta bular, rooted sandstone, which overlie coal beds in the lower part of the coal-bearing interval. A more rapid process of mire termination by mass movement is exemplified by a debris flow deposit of diamictite, which overlies the uppermost coal bed at the top of the coal-bearing i nterval. The diamictite consists of a poorly sorted, unstratified mixt ure of quartzite cobbles and pebbles embedded in a claystone-rich or s andy mudstone matrix. Deposition of the diamictite may have taken plac e over a matter of weeks, days, or perhaps even hours, by catastrophic flood, thus reflecting an instantaneous process of mire termination. Coarse clastics and mud were transported from the southwest some 20-40 km as a viscous debris flow along stream courses from the ancestral W ashakie Range to the Grass Creek area, where the flow overrode a low-l ying mire and effectively terminated peat accumulation.