Subtidal to intertidal deposits in a mixed clastic-carbonate epicontinental seaway, the Windy Hill Sandstone and Upper Sundance Formation (Oxfordian), Wyoming, U.S.A.

Abstract

Oxfordian deposits in northern Colorado and Wyoming, USA preserve proximal, intertidal, clastic, coastal deposits and distal, offshore to nearshore, subtidal bioclastic facies that accumulated during a forced regression of the Jurassic epeiric Sea. This contrasts with the common association between tidal deposits, carbonate facies, and rising sea level common to many depositional models of mixed clastic/carbonate systems. Ichnology, sedimentology, and architecture of these deposits were documented along a 460 km outcrop transect to test previous depositional interpretations and decipher the mechanisms driving the change from proximal, siliciclastic, intertidal flats to distal, bioclastic, subtidal macroforms. In southeastern Wyoming, the Windy Hill Sandstone (WH) is composed of very fine, SiO2-dominated, intertidal facies that truncate offshore to lower shoreface, storm-dominated deposits of the Redwater Shale Member (RS) of the Sundance Fm. Eolian and small fluvial systems delivered sand to the coastline after subaerial erosion reworked it from older, uplifted Jurassic strata onshore. The regional and time-transgressive J-5 unconformity separating the WH from the RS is readily identifiable using ichnological and sedimentological criteria. In the Wind River and Bighorn basins to the north, the Upper Sundance Fm (USF) is time-equivalent to the WH but is composed of glauconitic, silt-prone sandstone and meter to decimeter-scale, bioclastic, cross-bedded sandstone bodies. The abundance of molluscan shell material and limited volume of siliciclastic sediment in the geographically widespread outcrops suggests that nearby, marine shoals were the source of the coarse-grained material. Bioclastic, cross-stratified sandstone bodies represent two architectural elements: 1) coarsening-upward bodies with seaward-dipping foresets arranged into complexes bound by seaward-directed bounding surfaces interpreted to represent subtidal compound dunes and 2) landward- and laterally accreting tidal inlet fills composed of meter-scale, landward-accreting bodies with some landward-directed current ripples. The presence of transgressive tidal inlets supports previous interpretations that the WH and USF record high frequency transgressions superimposed on a tectonically-driven forced regression.