DEPOSITIONAL ENVIRONMENTS OF CLASTIC-DOMINATED INTERVALS OF THE WILKINS PEAK MEMBER (EOCENE), GREEN RIVER FORMATION, SOUTHWEST WYOMING

The Wilkins Peak Member of the Green River Formation (Eocene, Southwestern Wyoming) includes nine clastic-dominated intervals that truncate hypersaline lacustrine deposits of Eocene Lake Gosiute. This study uses detailed characterization of hydrodynamic lithofacies, paleocurrent and accretion direction measurements, and field-checked photomosaic interpretations to elucidate the depositional environments of these clastic intervals. Based on this work, the Wilkins Peak arkosic intervals are interpreted as a well-channelized fluvial system dominated by lateral-, downstream-, and obliquely- accreting macroforms.

Nineteen hydrodynamic lithofacies recur predictably within six discrete lithofacies assemblages. These include: 1) organic-rich and carbonate-rich strata that are interpreted as lacustrine mudflat deposits; 2) massive mudrock with carbonate nodules interbedded with unchannelized thin-bedded sandstones, interpreted as pedogenically altered overbank deposits and crevasse splays; 3) intraclastic conglomerate in upper fine- to medium-grained sandy matrix, interpreted as channel lag; 4) cross-stratified and plane-bedded sandstones organized in accretion macroforms, interpreted as lower bar deposits; 5) current-ripple, supercritical ripple, upper-flow regime dominated deposits organized in accretion macroforms interpreted as sand-rich upper bar deposits; 6) mudrock and thin-bedded sandstones organized in accretion complexes, interpreted as mud-rich bar top deposits.

Changes in fluvial architecture reflect changes in the preservation of fluvial macroforms, rather than a fundamental shift in fluvial style. These differing degrees of preservation are a result of changes in sediment partitioning in the landscape, reflecting the evolving accommodation-to-sediment-supply (A/S) ratio within the system. Conglomeratic and lower bar dominated assemblages reflect periods of relatively more erosion and bypass. These are overlain by accretionary intervals with more completely preserved macroform architecture, including ripple cross-laminated upper bar and fine-grained bar top deposits. These deposits demonstrate increasing in-channel sediment storage and concomitantly less clastic delivery to more distal locations in the lacustrine depositional system.