RADIOISOTOPIC DATING OF EOCENE FOSSIL LAKE, GREEN RIVER FORMATION, WYOMING

The Green River Formation of Wyoming, Colorado, and Utah represents a series of topographically partitioned paleolakes that existed during the Eocene Epoch; Fossil Lake, Lake Gosiute, and Lake Uinta. Fossil Lake, near Kemmerer Wyoming, is the smallest of these paleolakes, yet contains the greatest abundance of exceptionally preserved vertebrate fauna. Despite being world-renowned for its fossils -and even though the Green River Formation of neighboring paleolake Gosiute has been the subject of several geochronology studies- the age of Fossil Lake has remained poorly constrained by a single date. As such, paleontological and paleoclimate studies of Fossil Lake are limited without better age constraints of the lake strata. Through engagement with the Kemmerer community including several commercial quarries and the Fossil Butte National Monument, we sampled ashes across all three members of the Green River Formation representing the entire paleolake sequence: the lower Road Hollow Member, middle Fossil Butte Member, and upper Angelo Member.

Here we present preliminary results from the study; mostly from zircon U-Pb measurements via LAICPMS intended to pre-screen zircon grains for any detrital components prior to higher-resolution CAIDTIMS measurements. Radioisotopic dates from five new ash samples are consistent with stratigraphic order and suggest deposition of Fossil Lake only lasted ~2 Ma.

Lower Road Hollow Member yields a zircon ²⁰⁶Pb/²³⁸U weighted mean date by LAICPMS of 53.19 ± 0.42 Ma, from 12 analyses. From the lower Fossil Butte Member, a sample of the tri-ash (bottom lower sandwich-bed ash) by CAIDTIMS yields a preliminary date from two concordant analyses with a Concordia Age of 52.9 ± 0.4 Ma. A second sample collected from the upper-upper sandwich-bed ash of the Fossil Butte Member yields a zircon U-Pb date of 52.51 ± 0.41 Ma (LAICPMS, 17 analyses). Two ashes from the Angelo Member yield zircon U-Pb dates via LAICPMS of 51.74 ± 0.83 Ma (6 analyses) and 51.50 ± 0.34 Ma (15 analyses), respectively. Future results from CAIDTIMS dating of the LAICPMS pre-screened zircon grains, with anticipated precisions of 0.1% (+/- 50 ka), will facilitate precise calculations for the lake lifespan and depositional rates between important fossiliferous beds within Fossil Lake.