INTERACTION CLIMATE DRIVEN LAKE LEVEL FLUCTUATION AND SEDIMENT SUPPLY/DISCHARGE VARIATION ON DEVELOPMENT OF THE STRATIGRAPHIC ARCHITECTURE OF THE NORTHERN BIG HORN BASIN, WYOMING

Previous studies indicate that erosion of the Bear Tooth Mountains during the Paleogene yielded a high rate of sediment supply, helping to fill the Big Horn Basin, Wyoming. High rates of basin subsidence ensured preservation of very complete terrestrial successions in the fill. Research of the stratigraphy of the basin supports the hypothesis that “short-term” (10⁴ to 10⁵ years) orbitally driven, climate cycles, during the Paleogene, produced repeated variations in moisture that triggered cyclic changes in sediment flux and river discharge; creating rhythmic alluvial and paleosol successions in the basin fill.

In our study we tested the null hypothesis that only high frequency cycles in variation of sediment supply/discharge, without the influence of lake level change, shaped the stratigraphy of the Big Horn Basin. We tested this by searching for evidence that base level change was a driver of the evolution of basin fill. We found evidence of lake level rise in outcrops around Line Creek, Wyoming. An abrupt transition from buff, sandy-conglomeratic, Fluvial Channel Facies to overlying greenish-gray, poorly-sorted lag, represents passage of a low-energy shoreline, indicating increase in lake level. Two other greenish-gray, pelecypod-bearing, Lake facies, displaying a coarsening-upward trend to a Shallow-Water Lake (SW-L) facies, overlie this Shoreline facies. This succession suggests regression in response to declining rate of rise in lake level. Concave-up discontinuities, overlain by still coarser-grained facies, lie directly above this regressive succession. These facies are buff, very coarse-grained, parallel laminated and/or dune cross-stratified, lithic arenites; interbedded with clast-supported, pebble conglomerates. These units are Alluvial deposits. The abrupt transition from the S-WL facies, beneath concave-up discontinuities, suggests incision of Alluvial facies into the Lake facies, indicating a drop in lake level. Thus climate change did more than shape basin fill by altering sediment supply and discharge. It also caused variation in lake level, producing transgressions and regressions that caused lateral and spatial migration of facies along with incision that probably produced a large-scale transfer of coarse-grained deposits to much deeper water environments in the basin.