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