Depositional subenvironments in an arid closed basin; the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming, U.S.A.



The Wilkins Peak Member of the Green River Formation is a non-marine dolomitic deposit that formed in an arid, hydrologically closed basin. A transect from the margin to the centre of the Wilkins Peak basin was examined in outcrop exposures in the south-eastern corner of the basin and was supplemented with published core descriptions. The rocks were divided into seven facies based upon lithology, layering style and sedimentary fabrics: (1) AF facies—quartzose conglomerates and sandstones representing sandy alluvial fans coming from the Uinta Mountains. (2) SF facies—quartzose, peloidal, dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans. (3) DM facies—irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat. (4) EL facies—dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds. (5) PL facies—finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes. (6) SM facies—trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology. These represent a central salt pan and the surrounding saline mudflat. (7) AS facies—arkosic sandstones, siltstones and mudstones which represent braided river and more basinal deltaic plain deposits.

The five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations. There are at least forty 3–4 m thick cycles in the Wilkins Peak and one large-scale cycle comprising the whole member, which mimics the facies changes of the smaller cycles. This is compatible with the transitions of the Wilkins Peak Member to the Tipton and Laney members as climatic responses and not shifts in drainage. The sheet flood sandstones, actively aggrading subaerial mudflats, and bedded evaporites of the Wilkins Peak are apparently unique within the Green River Formation. They reflect the unusual provenance and hydrology of the Wilkins Peak and suggest chemical and sedimentary models established for the other Green River members will not be applicable.