HIGH ALTITUDE PROVENANCE FOR LOW ALTITUDE LAKES, EOCENE GREEN RIVER FORMATION, WYOMING

A rapid, basinwide decrease in δ¹⁸O occurs in carbonate mudstone of the Laney Member, Green River Formation from ~26‰ to ~20‰ (VSMOW). This change coincides with a sudden change in lake type, from balance-filled (Lower LaClede Bed) to overfilled (Upper LaClede Bed). The lake-type change and isotopic shift can both be explained by sudden capture of a new drainage, derived from high elevations in central Idaho. Several lines of evidence support this conclusion. While climatic and uplift-related interpretations fail to adequately explain the observed shift, mass-balance modeling indicates that capture of a large, low δ¹⁸O river would be capable of producing the observed change. Regional sedimentologic evidence suggests that drainage from the Idaho fold and thrust belt to the Bighorn and/or Wind River basins may have been disrupted at this time, possibly by southeastward-advancing volcanic topography of the Absaroka Volcanic Province. Both the Absarokas and fold and thrust belt were areas of high topography, expected to have low δ¹⁸O water compositions. Previously published paleocurrent and provenance data from overlying deltaic sands are consistent with drainage from the north, providing support for capture of a northern drainage.

The Laney isotopic shift is present in five stratigraphic sections separated by distances as great as 75 km. Two additional sections show high δ¹⁸O in the Lower LaClede. The occurrence of the isotopic shift supports lithologic correlations based on tuffs and distinct marker beds, indicating that it is a useful stratigraphic marker where the Upper LaClede is present. Filling of Lake Gosiute to, and over, sill level at the Lower/Upper LaClede boundary is recognized in most sections by a change in mudrock composition from dolomite to calcite above a final stromatolite surface. However, in sections where shallow facies and other distinctive markers are absent, the isotopic shift is the best way to recognize the Lower/Upper LaClede boundary. This isotopic record demonstrates the utility of basinal oxygen isotope records as tracers of hydrologic change in upstream catchments. Furthermore, it shows that isotopic expression of regional events can be a useful chronostratigraphic tool in nonmarine sections that are difficult to correlate based on lithostratigraphy alone.