Abstract

Sediments deposited in transitional settings between shoreface and offshore lacustrine environments in the Pliocene Glenns Ferry Formation of the southwestern Snake River Plain include extensive nearshore oolitic sequences similar to wave-built terraces along Lake Bonneville described by Gilbert (1885). These sequences record both constructional and destructional phases. Constructional units formed during basinward progradation and avalanching of oolitic sediment from a shallow bench platform down a steeply-dipping bench slope. Reversed grading seen in proximal foresets and normal grading seen in distal foresets may have resulted from sediment transport by grain flow on the upper reaches of the slope and fluidized sediment flow on the lower slope surface. Foreset beds dip basinward to the northeast at 26 degrees, and foreset units attain thicknesses of 18 meters. Destructional episodes are recorded by missing sections within the tops of constructional units. Massive beds of thickly coated ooids overlie constructional units and record ooid formation on the bench platform during transgressions which preceded progradation of subsequent foresets. The interpretation of these lacustrine bench sequences in terms of water depths during their formation demonstrates that rates of subsidence and absolute positions of lake level fluctuated greatly during their deposition. Such instability. probably related to tectonic movements, has had a significant influence on patterns of sedimentation in this area and may be characteristic of many rift-valley lacustrine systems.

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