FLUVIAL SYSTEMS IN THE SIWALIK MIOCENE AND WYOMING PALEOGENE

The 3 km thick Miocene Siwalik Group (Himalayan foredeep in northern P akistan) and the 2 km thick Paleogene Fort Union/Willwood formations ( Bighorn Basin in Wyoming) both preserve long records of fluvial deposi tion adjacent to rising mountain belts. Depositional environments and associated habitats change across large basins along with changing phy siography and with the location of different river systems that may ha ve varied greatly in patterns of channel deposition and the drainage o f adjacent floodplain areas. Deposits exposed in these two basins prov ide very different records of shifting paleoenvironments and patterns of basin filling. These differences reflect distinct patterns and scal es of depositional environments, the nature of the exposures, and the types of sedimentologic studies that have been carried out in each bas in. The Siwalik Group fills a basin that extended at least 1000 km alo ng its axis and 150-250 km away from the mountain front. Comparison of Siwalik deposits and modern drainages in the Himalayan foredeep sugge sts the ancient Siwalik basin was filled by large rivers that deposite d low gradient sediment fans covering areas on the order of 1000 km(2) , and by smaller intrafan rivers with more poorly drained floodplains. Despite the scale of these river systems relative to Siwalik exposure s in Pakistan, transitions between different systems have been recogni zed. Deposits of coeval river systems in the Siwalik Basin show pronou nced differences in alluvial architecture, the character of overbank d eposits, and the abundance and taphonomy of organic remains. In contra st, the Bighorn Basin in Wyoming is a relatively small intermontane fo reland basin extending 200 km along its axis and about 80 km across. B ighorn Basin strata were deposited by a river that flowed south to nor th along the basin axis and by smaller rivers that flowed transverse t o the basin axis. Much of this basin is exposed and thus it is possibl e to reconstruct changing patterns of deposition and environments thro ugh time in more detail than in the Siwalik Basin. These patterns indi cate changes in basin-wide drainage conditions and environments throug h time, but there are also important differences among coeval strata. Upsection shifts in environments and vertebrate faunas within both the Siwalik and Bighorn Basins may reflect tectonic or climatic forcing, but this comparison emphasizes the importance of recognizing deposits from different contemporaneous river systems before inferring such lar ge-scale controls on paleoecological change through time.