Sequence Stratigraphy of the Green River FM Fluvio-Lacustrine Clastic Tongues, Uinta Basin, USA

Steel, Ron
The University of Texas at Austin, Austin, TX

     Fluvial clastic tongues interfinger (50-100km transits) with shallow-lacustrine carbonates and oil shales, in the well-exposed Eocene Green River Formation of the Laramide Uinta Basin. Analysis of some 13 such tongues, each a few 100ky in duration, shows that the fluvial system repeatedly regressed and was transgressed during the wet intervals of wet-dry Eocene stratigraphy. The fluvial sandbodies were terminal fluvial systems (channels and frontal floodplain splays) that prograded basinwards while lake level was low, but rising. Continued and accelerated lake-level rise caused the fluvial systems to be transgressed by carbonates. Lake level fell and lake facies shallowed to dessication during dry periods. Fluvial sandbodies can be shown to be amalgamated, thick and narrow (up to 60m and a few km respectively) in the proximal reaches of their transits, becoming more isolate, thinner and wider (5-20m and >10km, respectively) distally, as might be expected in terminal systems. The progradational aspect of the fluvial system frequently causes channels to be larger at the late stages of a cycle. Lake deltas are present only in the mid-stage of the wet period, and only in the most distal segments of some of the transits, when terminal fans briefly became fan deltas. Deltas are not developed during the transgressive phases. This type of basin is contrasted with deep lake basins, where turbidites are developed abundantly.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah