LATE TERTIARY DRAINAGE EVOLUTION IN THE SOUTHERN GREEN RIVER BASIN, WYOMING

Drainage changes in the southern Green River Basin are recorded by compositional differences between deposits of the modern Green River and ancient (Oligocene?) rivers draining Laramide highlands. These changes reflect the transition from internal drainage during the early Tertiary to development of an integrated Green River system.

Gravels of the modern Green River near Green River, Wyoming consist primarily of various quartzite clasts and a small but significant percentage of granitic clasts representing Archean rocks of the Wind River Mountains. Green River terraces that are up to 180 m higher than the modern river contain broadly similar gravel compositions. In contrast, ancient river gravels that are up to 400 m higher than the modern river south of Green River, Wyoming, have a markedly different composition. The ancient river gravels consist of 2 units the uppermost of which correlates to the Oligocene Bishop Conglomerate. Bishop gravels are poorly sorted and consist predominantly of red Proterozoic quartzite and gray Paleozoic limestone clasts. The Bishop Conglomerate was deposited by north-flowing streams whereas the modern Green River flows south towards the Uinta Mountains.

At locations east of Flaming Gorge Reservoir, outcrops of Bishop Conglomerate overlie a second fluvial unit consisting of rounded pebble- to cobble-sized gravel that is dominated by non-red quartzite clasts, and which also lacks Archean granitic clasts. Geomorphic relations and limited paleocurrent data indicate that the river represented by these gravels flowed east to southeast from Green River, Wyoming, but that it did not drain Archean rocks of the Wind River Mountains.

Detrital zircon data for the Bishop Conglomerate and the underlying fluvial gravels confirm that these two river systems drained different regions. The Bishop Conglomerate contains few early Tertiary-aged (30-40 Ma) grains and numerous Precambrian-aged grains. In contrast, the underlying fluvial gravels contain many Tertiary-aged grains and only 3 Archean-aged grains. The detrital zircon data suggests that the maximum age for the fluvial gravels is ~32 Ma. Regional uplift and stream capture are most likely responsible for the observed drainage changes.