Interpretation of Carbonate-Detrital Silicate Transitions in the Carboniferous of Western Wyoming

Abstract

A conceptual model of sedimentation in a complex of subtidal, tidal-flat, sabkha, and dune environments facilitates the interpretation of stratigraphic contacts between carbonate and detrital silicate rocks. The boundary between carbonate and detrital silicate sedimentation approximates the reach of the highest tides, and is a zone of transition between the predominantly carbonate coastal sabkha and the predominantly detrital silicate continental sabkha. With regressive sedimentation, the horizontal sequence of facies from marine carbonate sediment landward to the detrital silicates of the continental sabkha results in a stratigraphic sequence of carbonate rock conformably overlain by detrital silicate rock. The Lower Carboniferous strata in western Wyoming appear consistent with that model; thus, the contact between the carbonate strata of the Madison Group and the detrital silicate rock at the base of the Amsden Formation is conformable. The Lodgepole Formation is predominantly of shallow subtidal origin. The Mission Canyon Formation is the product of tidal-flat sedimentation. Regressive sedimentation of the Madison Group culminated with the creation of a land surface on which sabkhas and dune fields developed. The lower beds of the Amsden Formation are the continental sabkha and dune deposits. In the Upper Carboniferous, the middle part of the Amsden Formation records a period of time when an increased supply of terrestrial detritus shifted the carbonate-detrital silicate boundary from the high-tide line into the shallow subtidal environments. Carbonate tidal-flat deposits at the base of the upper part of the Amsden Formation indicate a return to the sabkha environment. The repeated interbedding of carbonate tidal-flat and sabkha deposits with detrital silicate sabkha and dune deposits throughout the upper Amsden and Tensleep Formations resulted from numerous local transgressions and regressions, during which the area never was far removed from the coastal-continental sabkha boundary.