FLUVIAL SYSTEMS IN THE SIWALIK MIOCENE AND WYOMING PALEOGENE

The 3 km thick Miocene Siwalik Group (Himalayan foredeep in northern Pakistan) and the 2 km thick Paleogene Fort Union/Willwood formations (Bighorn Basin in Wyoming) both preserve long records of fluvial deposition adjacent to rising mountain belts. Depositional environments and associated habitats change across large basins along with changing physiography and with the location of different river systems that may have varied greatly in patterns of channel deposition and the drainage of adjacent floodplain areas. Deposits exposed in these two basins provide very different records of shifting paleoenvironments and patterns of basin filling. These differences reflect distinct patterns and scales of depositional environments, the nature of the exposures, and the types of sedimentologic studies that have been carried out in each basin. The Siwalik Group fills a basin that extended at least 1000 km along its axis and 150-250 km away from the mountain front. Comparison of Siwalik deposits and modern drainages in the Himalayan foredeep suggests the ancient Siwalik basin was filled by large rivers that deposited 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 exposures in Pakistan, transitions between different systems have been recognized. Deposits of coeval river systems in the Siwalik Basin show pronounced differences in alluvial architecture, the character of overbank deposits, and the abundance and taphonomy of organic remains. In contrast, the Bighorn Basin in Wyoming is a relatively small intermontane foreland basin extending 200 km along its axis and about 80 km across. Bighorn Basin strata were deposited by a river that flowed south to north along the basin axis and by smaller rivers that flowed transverse to the basin axis. Much of this basin is exposed and thus it is possible to reconstruct changing patterns of deposition and environments through time in more detail than in the Siwalik Basin. These patterns indicate changes in basin-wide drainage conditions and environments through 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 large-scale controls on paleoecological change through time.