Oxfordian Sedimentation in Western Interior United States

Abstract

Upper Jurassic (Oxfordian) sediments of the western interior United States were deposited in part of an epicontinental seaway which extended south from the present arctic. This seaway contained a complex of tectonic elements which influenced significantly the geographic distribution of depositional environments. Sedimentation was divided into an initial transgressive phase followed by a regressive phase. Four distinct facies were developed during the transgressive-regressive sequence. (1) The nearshore sand facies, restricted to the western part of the seaway, is characterized by medium- to coarse-grained chert-rich sandstone, chert conglomerate, and wood fragments. The composition, thickness, and geometry of this facies were influenced by the rising westerly source and a preexisting high (the Belt Island trend), as well as by lateral migrations of sediment depocenters. (2) The mud facies was immediately east of the nearshore marine facies. Clay shale, silty shale, siltstone, and argillaceous limestone dominate the rocks of this facies. (3) The mixed carbonate-clay facies developed only in areas removed from the westerly source of sand-size detritus. In such areas, east of the mud facies, sedimentation included both detrital (clay and silt) and chemical (calcium carbonate) components. As a result, this facies consists of highly calcareous clay shale with minor sandy limestone. (4) The marine bar-sand facies developed only during the regressive phase of sedimentation. It consists of a sheet of well-sorted, orthoquartzitic sandstone which was shaped into a eries of submarine sand bars separated by muddy interbar areas. Individual bars were several kilometers in length, and accumulated to a maximum preserved thickness of 15 m. The bars are considered to be of polygenetic origin, resulting from the interplay of storms, tidal currents, and regional circulation currents in the shallow, regressing Oxfordian sea.