VARIATIONS IN SHORELINE MORPHOLOGY AND STRATIGRAPHIC EXPRESSION OF DEPOSITIONAL SEQUENCES AROUND DOMINANTLY OPEN HYDROLOGY (OVERFILLED) LAKE BASINS

Diagnostic stratal stacking patterns and geometries characterize depositional sequences deposited under dominantly open hydrologic conditions (i.e., overfilled lake basin settings). Stratal architecture, however, can vary significantly around the lake margin. Near point sources of advected clastics, well-expressed parasequences of planar-parallel, graded, and current- and wave-ripple sandstone beds atop laminated to bioturbated calcareous mudstones form in stream-dominated shorelines. Away from these input points, parasequences are more subtly expressed in coarsening-upward packages from profundal laminated, organic-rich calcareous claystones through bioturbated marls and mudstones, wackestone, and packstones, to skeletal grainstones at wave-dominated shorelines.

Two intervals of the Eocene Green River Formation illustrate these variations. The Sand Butte Bed (near Fort LaClede, WY) has wedge-shaped foresets that dip at up to 20° and change downslope into flat-lying beds of ripple- or parallel-bedded sandstones and mudstones, with a distinct down-flow change in grain size. Parallel laminations, current, wave, and wave-current ripples, abundant plant fragments, syndepositional faults, and sandstone pillows all point to a delta-front origin. Foreset beds grade upward into flat-lying topset beds or are erosionally truncated and overlain by lag beds. Sandstone units are elongate to fan-shaped, up to 24 m thick, covering 3.3 km in dip, 2.5 km in strike. The Luman tongue at Hiawatha, WY, records progradation of an interdeltaic, muddy shoreline dominated by gastropod packstones and grainstones, most notably in decreasing lamina continuity and increasing skeletal content. Littoral coquinas are common and widespread in tabular bodies, less than1-m thick, and rather muddy. Coquinas contain relatively well-sorted accumulations of Goniobasis and Viviparvus shells. Parasequences and sequences are well expressed in profiles of organic carbon content and hydrogen indices.

Depositional sequences in overfilled lake basins can have architectures similar to shallow marine siliciclastic sequences, but with subtle, yet important differences. These differences affect the fundamentals of stratal correlation and estimation of distributions of fine- and coarse-grained lithosomes.