REGIONAL VARIABILITY OF A MIXED CLASTIC-CARBONATE, FORCED REGRESSIVE, TIDAL SYSTEM: THE WINDY HILL SANDSTONE AND UPPER SUNDANCE FORMATION, WYOMING, USA

Ichnology, sedimentology, and architecture of the regressive, Oxfordian Windy Hill Sandstone (WH) and Upper Sundance Formation (USF) were documented along a 460 km proximal-distal transect from northern Colorado (NC) to northern Wyoming. In NC, floodplain deposits of the Morrison Fm accumulated unconformably upon eolian dunes of the Pine Butte Member of the Sundance Fm (SF). In SE Wyoming, the WH is composed of rounded, very fine, SiO2-dominated sandstone containing a Pteraichnus Ichnofacies unconformably overlies offshore to lower shoreface deposits of the Redwater Shale Member (RS) of the SF. This association represents intertidal flats that accumulated on the J-5 unconformity and records > 15 m of sea level fall driven by a forced regression. In the Wind River and Bighorn (BHB) basins, the USF is characterized by meter-scale, bioclastic macroforms overlying burrowed, glauconitic, very fine-grained sandstone or scoured into the underlying RS. Down-current-directed foresets and master surfaces, coarsening-upward motifs, and lack of lateral accretion surfaces in many of these bodies are characteristic of compound tidal dunes rather than tidal bars. Scours within the macroforms contain planar-tabular bedded bioclastic dunes directed to the north (basinward) or south (landward). This part of the USF accumulated in a subtidal region of the basin with low rates of clastic input, a nearby source of shell material, and high tidal energy. In the northernmost outcrops at Red Gulch, rip-up clasts of sandstone identical in lithology to parasequences in the underlying RS are preserved in landward- and laterally-accreting tidal inlet deposits composed almost entirely of shell and shell fragments. This suggests that the underlying sand was lithified prior to inlet incision and supports interpretation of the J-5 Unconformity as an unconformity in the BHB. Compound dunes and tidal inlet fills host low-diversity, low-abundance ichnofossil assemblages including diminutive Ophiomorpha. A new depositional model for the WH and USF recognizes that while a forced regression was the dominant control on accumulation, local variables like paleobathymetry and availability of clastic and carbonate material were important influences on facies distribution, stacking, and clastic vs. carbonate lithology.