Sedimentology of the Sundance Formation, northern Wyoming

The glauconitic sublitharenites and arenaceous biosparites of the upper 20 meters of the Sundance Formation are the deposits of a regressive shoreline that existed in northern Wyoming during the Late Jurassic. This regressive shoreline was characterized by discontinous barrier islands, with associated inter-barrier tidal inlets, back-barrier shoals and sandy tidal flats. In north-central Wyoming, this shoreline trended generally east-west, with the regression progressing from south to north. Within the sublitharenites of the uppermost Sundance Formation, tidal bundles, sigmoidal reactivation surfaces, herringbone cross-lamination and abundant mud drapes are a record of the influence of tidal currents during the deposition of the unit. The neap-spring cyclicity of the tidal bundles implies that they were developed in a diurnal tidal setting. The tidal range along the Late Jurassic shoreline is estimated to have been at least mesotidal (2-4 m) in north-central Wyoming. The lateral migration of inter-barrier tidal inlets along the regressive shoreline caused the sublitharenites and biosparites of the uppermost Sundance Formation to be deposited as tabular, laterally-extensive units. Earlier models, which attach an offshore environment of deposition to these units, do not explain their tabular geometries and conformable stratigraphic relationship with the overlying non-marine sediments of the Morrison Formation;The Late Jurassic seaway was a foreland basin, formed by downbowing of the North American craton in response to the load of the Cordilleran thrust belt. The areal distribution of dark chert clasts (of probably upper Paleozoic derivation) in the fragmental biosparites of the uppermost Sundance Formation suggests that areas of positive relief existed within the Late Jurassic seaway. The proximal, most obvious evidence (e.g., conglomerates overlying an unconformity) of these Late Jurassic structures is only rarely preserved. This proximal evidence exists only where the Jurassic highs have not been overwhelmed by the later development of Laramide structures