HR: 0800h
AN: H31A-1281    [Abstracts]
TI: Geomorphic Drainage Capture Recorded by Oxygen Isotopes of Green River Formation Lacustrine Mudstone, Eocene, Wyoming
AU: * Doebbert, A C
EM: doebbera@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin, Madison, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Booth, A L
EM: mbooth@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, 367 Panama St., Stanford, CA 94305 United States
AU: Carroll, A
EM: carroll@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin, Madison, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Chamberlain, C
EM: chamb@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, 367 Panama St., Stanford, CA 94305 United States
AU: Rhodes, M
EM: Meredith.Rhodes@bp.com
AF: Department of Geology and Geophysics, University of Wisconsin, Madison, 1215 W. Dayton St., Madison, WI 53706 United States
AB: The isotopic composition of cement and other meteoric precipitates are increasingly being used to interpret orogenic uplift histories, based on the relationship between altitude and rainwater δ¹⁸O. However, other variables such as changing regional drainage patterns may also affect the downstream composition of surface waters, especially when multiple drainages commingle in a lake. The Green River Formation contains some of the best documented lacustrine deposits in the world, making it ideal for examining such issues. Carbonate mudstone in balanced-fill facies of the lower LaClede Bed averages 3.41‰ (PDB), and records a deep, saline to brackish lake that fluctuated near its sill. In contrast, overfilled facies of the upper LaClede Bed record a freshwater lake, and δ¹⁸O reaches values as low as -9.72‰. This transition occurred shortly after deposition of the Analcite Tuff at 48.94 ± 0.12 Ma (Smith et al., 2003), and was geologically abrupt. Based on ⁴⁰Ar/³⁹Ar-calibrated sediment accumulation rates it required no more than 200-300 ky. An almost identical transition occurs in two cores separated by about 30 km, making local diagenesis an unlikely cause. The magnitude of δ¹⁸O change is similar to that in some uplift studies, but its rapidity virtually excludes uplift as a controlling mechanism. Instead, we propose that both the change in sedimentation and the sharp decrease in δ¹⁸O are the result of a drainage capture event. The addition of a new drainage to the basin may have adjusted isotopic values in two ways: by introducing runoff with relatively low δ¹⁸O, and by decreasing residence time (and therefore evaporation) of lake water. Decreasing ⁸⁷Sr/⁸⁶Sr across the same transition suggests that the newly added waters may have been sourced from rising volcanic topography to the north in the Absaroka province. Although this rising topography allows for the possibility of some uplift component, the rate of change in lacustrine δ¹⁸O is consistent with geomorphic processes rather than uplift. These results indicate the need for considerable caution when examining uplift records from other ancient lake deposits.
DE: 0746 Lakes (9345)
DE: 1824 Geomorphology: general (1625)
DE: 4870 Stable isotopes (0454, 1041)
SC: Hydrology [H]
MN: Fall Meeting 2005