GEOCHEMICAL STRATIGRAPHIES OF PALEOGENE LAKES UINTA, FLAGSTAFF AND CLARON: HYDROLOGY, CLIMATE AND TOPOGRAPHY OF THE LARAMIDE FORELAND OF UTAH

Isotopic and elemental compositions of primary lacustrine carbonate record the evolving hydrology of Paleogene Lakes Uinta, Flagstaff and Claron in Utah. In the Uinta basin, the δ¹⁸O values of lacustrine samples least affected by evaporative conditions correspond well with mean δ¹⁸O values of carbonates sampled from time-equivalent fluvial facies within the same basin, and the values decrease by 2-3 per mil between early Eocene and early Oligocene time. Cooling global climate and resultant enhancement of rainout since the Early Eocene Climatic Optimum (EECO) is sufficient to explain both the absolute values and the inferred decreasing trend of δ¹⁸O of surface waters in the Uinta basin. These results are consistent with sedimentological evidence of aridification and continued Laramide partitioning of basins in the region, which caused a transition from large, distally sourced drainages to smaller and increasingly localized catchments. Observed shifts in the isotopic records from the Flagstaff and Claron basins are diachronous and more sudden, further indications that tectonism played a role in such hydrologic diversions. However, data do not support significant regional or windward uplift during the Paleogene in this area of the western U.S. Instead, Laramide tectonism controlled drainage patterns by modifying mesoscale relief within an already elevated region.