ALTERNATING PROGRADATIONAL AND DEGRADATIONAL REGIMES IN AN OPEN-COAST TIDAL FLAT: THE JURASSIC REDWATER–WINDY HILL–MORRISON TRANSITION, WYOMING, USA

The Upper Jurassic Windy Hill Sandstone of Wyoming exhibits many of the criteria consistent with deposition on an open-coast tidal flat, a mixed-energy environment sharing features of a wave-dominated shoreface and a tidal flat. As such, the Windy Hill Sandstone records a net regression from the offshore mudstones of the Redwater Shale (Sundance Formation) to the terrestrial Morrison Formation, although the sequence stratigraphic architecture of this transition varies along depositional dip from south to north. In the southern portion of the study area, the Windy Hill Sandstone is thin and difficult to distinguish from surrounding strata. Surfaces of forced regression exist in the underlying Redwater Shale, and possible evidence for subaerial exposure is recognizable at some outcrops. The Windy Hill Sandstone thickens to the north in the Bighorn Basin, and multiple surfaces of forced regression are present in both the Windy Hill and Redwater Shale, placing these units in the falling-stage systems tract. This degradationally stacked package is overlain by aggradationally stacked tidal-flat parasequences, indicating deposition in the lowstand systems tract. The same degradational to aggredational pattern is also present farther to the north. No major or regionally-extensive flooding surface caps the Windy Hill Sandstone, and the Morrison Formation records continued progradation of the coastal plain; in other words, there is no evidence that the lowstand systems tract is overlain by a transgressive systems tract, deviating from a more typical stratigraphic architecture. The identification of surfaces of forced regression at stratigraphic positions previously interpreted as the J5 unconformity calls for a re-evaluation of the placement of this surface, suggesting that any unconformity would be higher in the section than previously thought, perhaps at the Windy Hill/Morrrison contact in central and southern Wyoming, then passing into a correlative conformity to the north. This interpretation implies that the Redwater Shale, Windy Hill Sandstone, and overlying Morrison Formation are partly contemporaneous within Wyoming. The Windy Hill Sandstone re-enforces that tidal deposition can occur in a wide variety of sequence stratigraphic settings, and is not limited to the lowstand systems tract.