RHODES, MEREDITH K., University of WisconsinMadison, Dept. of Geology Geophysics, Madison, WI

ABSTRACT: Lacustrine Stratigraphy, Clastic Provenance, and Radiogenic Isotopic Paleohydrology of the Laney Member, Green River Formation, WY

Lacustrine basin fill is composed of continental weathering products derived from the associated drainage region of a lake. Therefore, the recognition of shifting clastic provenance and changing basin hydrology in a lacustrine basin through time provides an important proxy for tectonic and climatic events influencing the drainage basin. Strontium isotopes possess the unique ability to record tectonic changes in a drainage basin (shifting provenance) and climate dependent variations due to differential weathering of strontium bearing minerals.

The Early Eocene Green River Formation in SW Wyoming is a well documented lacustrine unit ideally suited to study chemostratigraphic packages that result from changes in basin hydrology. The Laney Member records a shift from evaporitic, saline lacustrine facies to freshwater lacustrine facies dominated by oil shale and it coincides with an overall shift from closed to open basin hydrology. The change in drainage involved two geochemically distinct source terranes, the highly radiogenic Precambrian cored uplifts surrounding the basin, and the less radiogenic Eocene Absaroka Volcanics. ⁸⁷Sr^/86Sr ratios of primary carbonate deposits should therefore reflect the lakewater provenance, or paleohydrology of the lake from which the carbonates were precipitated.

This study will aim to document changes in basin hydrology using the ⁸⁷Sr^/86Sr signature of lacustrine carbonates. The chemostratigraphy of individual lake cycles will be evaluated with respect to a strontium mass balance model created to test the sensitivity of the ⁸⁷Sr^/86Sr ratio to changes in the amount of lake plain weathering.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid