DEPOSITIONAL ENVIRONMENTS AND THE EVOLUTION OF THE JURASSIC WESTERN INTERIOR SEAWAY AROUND THE CONTESTED JURASSIC J5 UNCONFORMITY OF THE SUNDANCE FORMATION, WY

The Sundance is a Middle- Late Jurassic shallow marine formation in Southeastern Wyoming that culminates in an upper transition from offshore marine mudstones of the Redwater Shale Member (RW) to an intertidal Windy Hill Member (WH). This contact has long been considered to be unconformable; however, recent publications have suggested that this transition is more of a gradual conformable shift from offshore prodeltaic muds to a tide-dominated delta. To study this contact in more detail, a section of the RS and WH was measured at Alcova Reservoir in south-central Wyoming and compared to other time- equivalent sections across Wyoming. A sharp contact was found in all sections between the interbedded muds, silts and storm beds of the RS, and the WH; containing flat topped interference ripples, cross strata and other sedimentary structures indicating a shallow intertidal environment. Evidence of a gradual transition from offshore marine to a tidal delta was not found. We suggest that an abrupt forced regression separates the end of the RS and the start of the WH. Notably, we can correlate the unconformity 230 km south in northern Colorado, where it truncates the Sundance formation down to a basal eolian member. The unconformity can be found in Utah as a sediment-starved zone of evaporites, which accumulated in incised channels and sandy beds that are similar to the Windy Hill. The Sheridan Arch in the Bighorn Basin shows evidence of tidal dune incision which indicates paleobathymetric highs affecting paleoceanographic currents and relative sea level. Confirming this unconformity will give us a clearer picture on how tectonics and climate effected the paleoceanography of the Jurassic Western Interior Seaway.