NEW DETRITAL ZIRCON AGES AND THE PALEOGEOGRAPHY OF OLIGOCENE FLUVIAL SYSTEMS IN THE SOUTHERN GREEN RIVER BASIN, WYOMING

Investigations in the southern Green River Basin document the transition from internal drainage in the early Cenozoic fluvial systems to the present-day integrated Green River system. In Eocene time, the Green River and Uinta Basins were separated by the Unita Mountains and were occupied by Lakes Gosiute and Uinta respectively. The modern Green River begins in the Wind River Mountains and flows southward through the Green River Basin to the Unita Mountains where the river is diverted east. The Green River flows south again and cuts across the Unita Mountains at Ladore Canyon and eventually joins the Colorado River in Canyonlands.

Stratigraphic work south of Green River, WY shows evidence for two interrelated Oligocene fluvial systems. One unit is the Oligocene Bishop Conglomerate which is very poorly sorted and consists predominantly of red Proterozoic quartzite and gray Paleozoic limestone clasts. Paleocurrent data indicate the Bishop Conglomerate was deposited by northward flowing fluvial systems draining the Unita Mountains. Detrital zircon data for the Bishop Conglomerate confirm a Uinta Mountain provenance and an Oligocene age. The samples contain an abundance of 30-40 Ma grains which we interpret as syndepositional volcaniclastic material that was reworked by Bishop streams. At Antelope Butte, detrital zircon data for a tuffaceous sandstone overlying the Bishop Conglomerate provides a maximum depositional age of ~31-32 Ma for the Bishop Conglomerate.

At locations around Flaming Gorge Reservoir, Bishop Conglomerate overlies and interfingers with a second fluvial conglomerate consisting of well rounded, moderately sorted sandy gravel that is dominated by non-red quartzite clasts and rare volcanic clasts which were most likely reworked from Eocene units containing Absaroka Mountains-derived volcanics. Paleocurrent data indicate that the second fluvial unit represented an axial river system that flowed south to southwest across the Green River Basin. The Bishop streams were tributary to this axial system. Detrital zircon data from this unit show minor peaks similar to those in the Unita Mountains but also display a major component of young (30-40 Ma) grains with a major peak at 33.25 Ma. This Oligocene axial river probably flowed westward into depressions representing the remnants of Eocene Lake Gosiute.