PROVENANCE OF ANCIENT AND MODERN SEDIMENT DERIVED FROM THE UINTA UPLIFT, WESTERN U.S

Ancient drainage basins provided sediment from Laramide uplifts to expansive lacustrine basins in the North American Cordilleran broken foreland. Eocene clastic sediment accumulation at the center of the Green River Formation lakes was derived from the east-west trending anticlinal Uinta Mountains. The Uinta Uplift’s well documented bedrock geology provide a unique opportunity to quantify the catchment geometry of the streams that draining across alluvial portions of the basin during the Eocene. This synthetic provenance study examines modern-day sediments collected from streams draining the Uinta Uplift to provide a basis for interpreting ancient drainage networks from sediment composition. Interpretation of sediment petrography derived composition of Eocene sediments will be accomplished by analyzing modern sources and compositions, permitting corrections for differences in durability and sediment grade that are lithology-controlled.

We collected sediments from streams draining varied Proterozoic through Mesozoic lithologies along the flanks of the Uinta Mountains at the margins of the Greater Green River Basin, the Piceance Creek Basin, and the Uinta Basin. Petrographic analysis of modern sediments was completed by washing and separating sediments into size fractions. Loose grains and thin sections of each sample were then point counted to compare dominant lithologies across sand- and gravel-sized grains. Point counting reveals sediment compositions that reflect significant variations in the weathering and durability of basement lithologies within modern catchments. Although all sediments are lithic to quartzose, several important differences exist between the different source areas. Sediments derived from mud and sand rich Mesozoic rocks are dominated by monolithic quartz, quartz sandstone clasts, and chert clasts. Sediments derived from the Paleozoic bedrock are rich in carbonate clasts, reflecting the carbonate units present. Sediment draining from Mesozoic strata contains volcanic lithic clasts. Catchments draining the Proterozoic Uinta Mountain group produce quartzose sand containing predominantly sedimentary lithic clasts.