THE NIOBRARA FORMATION FROM PROXIMAL TO DISTAL: GAMMA RAY REVERSAL LINKED TO ORGANIC MATTER DEPOSITIONAL PROCESSES

The Smoky Hill Member of the Late Cretaceous Niobrara Formation is a proven hydrocarbon reservoir in Rocky Mountain region basins. While a regionally extensive formation, the lithology and organic matter (OM) type and concentrations change laterally and stratigraphically depending upon the depositional location and processes within the Western Interior Seaway. The Smoky Hill lithologies consist of alternating chalk/marl cycles. The chalks and marls are composed of varying amounts of coccolith-rich peloids, foraminifera, and other bioclasts within a mixed micrite, clay, and silt matrix. Previous studies have defined the peloids as fecal pellets, but petrographic observations in this study have identified three types of calcareous peloids – fecal pellets, intraclasts, and lithoclasts.

A gamma ray (GR) reversal is observed between two Smoky Hill depositional environments. The Denver Basin preserves eastern distal ramp environments. Here OM is concentrated within the marls, which show a high GR response, while more organic lean chalks have a low GR response. The more proximal Sand Wash and Washakie Basins preserve eastern foredeep environments, where the equivalent chalk/marl cycles contain higher percentages of detrital silt and clay due to the more proximal setting. In contrast, OM concentration is associated with the calcareous members, which show a high GR response, while more organic lean marl equivalents show a low GR response. Elemental data show the GR response is related to fluctuations in uranium associated with changes in OM content, resulting in the observed reversal of GR response with respect to lithology and location.

OM concentration is correlative with depositional processes. Eastern distal ramp marls were influenced by detrital input from the west, suggesting eastward transport of OM in addition to pelagic deposition. Within the foredeep, the marine OM is primarily located within the peloids, which originated in the distal ramp setting and were transported westward. This likely resulted from winter storms ripping up peloids and transporting them westward in hyperpycnal and other storm-derived flows. The presence of the intraclasts and lithoclasts combined with sedimentary structures indicate periodic high energy depositional processes which reworked sediment and transported OM.