Sedimentological Model for Lacustrine Shoreline Deposition, Lower Green River Formation (Eocene), Northeastern Uinta Basin, Utah

James W. Castle

Sedimentological analysis and subsurface paleoenvironmental mapping in a 250-mi² area of the northeastern Uinta basin indicate that laterally continuous sandstone bodies represent deposition in lacustrine barrier-beach complexes. A lacustrine shoreline depositional model was developed using interpretations from approximately 1,500 ft of continuous core from 24 wells in the general area of Wonsits Valley and Red Wash oil fields. The model can be applied to interpreting depositional environments of ancient lacustrine deposits and to predicting subsurface distribution of lacustrine shoreline sandstones.

Coarsening-upward sequences observed in cores represent deposition in high-energy, wave-dominated shoreline systems that prograded into low-energy, open lacustrine environments. The vertical sequence consists of: lacustrine shale overlain by transitional siltstone and silty sandstone beds; lower shoreface very fine-grained sandstone; upper shoreface fine-grained sandstone; swash-backwash fine to medium-grained sandstone; and backshore fine to medium-grained sandstone and interbedded algal-laminated shale.

Paleoenvironmental mapping using interpretations from cores and well logs indicates that maximum dimensions of the barrier-beach complexes studied are approximately 10 mi long and 3 mi wide. Thickness ranges from approximately 10 to 35 ft. Mapping shows that barrier complexes were separated from onshore fluvial systems by a mudflat-lagoon depositional environment.

The barrier-beach deposits in the lower Green River Formation are similar to some modern and ancient, marine, siliciclastic shoreline deposits formed along microtidal coasts.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.