Sedimentation on the Front of Eocene Gilbert-type Deltas, Washakie Basin, Wyoming

Abstract

ABSTRACT The transition from alluvial to lacustrine rocks in the Laney Member of the Green River Formation includes Gilbert-type deltas whose depositional features provide information about the history and nature of Lake Gosiute, and document proximal to distal sedimentation on the delta fronts. Gilbert-type deltaic rocks in the Laney are composed of cycles each consisting of constructional and destructional phases. The constructional phases are represented by an upward-coarsening sequence produced by lakeward progradation of delta topset and foreset beds over prodeltaic sediments. During the destructional phase erosion and reworking of delta-plain and -slope beds resulted in formation of an erosional surface, and locally, a sandstone shoreline veneer. These features of Laney deltaic deposits re similar to constructional and destructional phases of late Quaternary and modern Gilbert-type delta deposits of the Truckee River at Pyramid Lake, Nevada. They are attributed to fluctuation in lake level or shifting of the stream mouth entering the lake. The most characteristic feature of the delta deposits is foreset bedding with depositional dips of up to 20 degrees. The maximum thicknesses of foreset deposits range from 10 to 25 meters. The geometry of the foreset deposits suggests that the lake was shallow with maximum depths of 25 meters where deltas were constructed. Foreset beds grade upward into flat-lying sandstone and mudrock topset beds, and grade downward into flat-lying mudrock bottomset beds. The delta fronts consist of repetitive sequences of sandstone and mudrock with sporadic, thick, massive sandstone beds. In the Bridger and Washakie Basins local foundering of deltaic sandstone and mudrock into lacustrine oil shale resulted in large load casts, slump folds, and sandstone pillows that partly or wholly obliterated orig nal bedding features of topset, foreset, and bottomset beds. Proximal to distal delta-front sedimentation patterns include: thinning of stratigraphic intervals of 50 to 70 percent; increase in the ratio of mudrock to sandstone; and decrease in the total number of beds in a stratigraphic interval. Bedding sequences in delta-front rocks are 1) ripple-bedded fine-grained sandstone overlain by parallel-laminated mudrock; 2) ripple-bedded fine-grained sandstone, parallel-laminated fine-grained sandstone, and parallel-laminated mudrock; and 3) parallel-laminated fine-grained sandstone overlain by parallel-laminated mudrock. These sequences were produced by interflow and mixing of sediment-laden stream water in lake water of equal density, and by wave action on the delta front.