Stratigraphy of the Lower Mancos Shale in the San Juan Basin

Abstract

The lower half of the Mancos Shale in the northern San Juan Basin can be divided into the following stratigraphic units, in ascending order: Graneros Shale, Greenhorn Limestone, Carlile Shale, and Niobrara Shale. The presence of the Juana Lopez Member of the Mancos allows the Carlile to be further divided into an upper and lower Carlile Shale. Tongues of shallow marine and continental sediments extend into the Mancos from the south, representing several transgressions and regressions of varying extent. These sediments are not seen along the northern side of the basin, but the changing conditions of sedimentation are reflected in vertical changes of the Mancos microfauna. Six distinct foraminiferal zones are present in the lower Mancos. In ascending order these are the: Rotalipora zone, Bigenerina zone, Gavelinella tumida. zone, Trochammina wickendeni zone, Gavelinella ammonoides zone, and the Globotruncana zone. Completely contained within the Trochammina wickendeni zone is the Ammomarginulina subzone. The faunal assemblages within these zones are used to give an indication of the physical conditions which prevailed within the basin during the time represented by the Mancos sediments. The basal Graneros on the northeast side of the basin contains the Rotalipora fauna, and represents the beginning of the first definitely marine transgression in the Upper Cretaceous. The sandstones of middle Graneros age, called “upper Dakota” sandstones, and the contained Bigenerina fauna give evidence of a slowing of the transgression in the San Juan Basin area. The Gavelinella tumida fauna, and the calcareous beds of the Greenhorn and lowermost Carlile which enclose it, record the most extensive transgression of the early Mancos sea. The change to the arenaceous fauna of the Trochammina wickendeni zone marks the end of the transgression and the beginning of a regression that continued all through the Carlile, although the overlying Gavelinella ammonoides fauna within the upper Carlile shows that the character of the regression changed with time. The unconformable boundary between the Carlile and Niobrara sediments is marked by a change to the Globotruncana fauna. This fauna and the nature of the basal Niobrara sediments indicate that the amount of sediment being brought into the basin during early Niobrara time was small. This means that the sand bodies, lying on the eroded upper surface of the Carlile rocks, are made up of sediments which were derived from these pre-existing Carlile sediments. These sand bodies were deposited as “strike valley” sands, and owe their position to irregularities on the upper surface of the Carlile beds. The basal Niobrara sands give way to calcareous shales both laterally, in a basinward direction, and vertically.