MISSISSIPPIAN TECTONIC EVOLUTION OF THE GREAT BASIN: INSIGHTS FROM COMPILATION OF A LARGE SUBSURFACE DATASET

The thick sequence of Mississippian mudrocks in Nevada colloquially and collectively known as the Chainman Shale is two distinct mudrock units separated by the regional C2 unconformity. Identification, correlation and interpretation of these two mudrock units have long been hampered by poor exposure, sparse age control and superficially monotonous appearance. New research presented here, including mudrock mineralogy, petrology, petrophysics, and organic geochemistry, confirms important shifts at the Late Mississippian (C2) regional unconformity. We limit the term "Chainman Shale" to mudrocks above the unconformity, and apply the term "Gap Wash Formation" to mudrocks below it. The geographic distribution and composition of these units record regional Mississippian paleogeographic evolution.

The results summarized here, based on more than 1400 analyses from both publicly accessible and proprietary data, demonstrate that the Gap Wash Formation and the Chainman Shale have different origins. Most important for resource assessment, the Gap Wash has significantly better potential as a hydrocarbon source than the Chainman. Electric logs in numerous wells show the unconformity clearly, allowing regional correlation and mapping. Subsurface data presented here demonstrate that the C2 unconformity extends south into southern Nevada and east into Utah.

Once the Gap Wash and Chainman mudrocks can be distinguished and considered separately, they reveal a more nuanced tectonic history of the continental margin during Mississippian time. The Gap Wash Formation was deposited into accommodation space created by the loading of the continental margin during the Antler orogeny. The Chainman Shale was deposited into a broad transgressive seaway that subsided in response to resumed continental margin contraction in late Meramecian time. The angular unconformity between them (C2) records Mississippian (Meramecian) tectonism that post-dates the Antler orogeny. The tectonic implications of this work reinforce previous evidence that the western margin of North America was not quiescent after the Devonian-Mississippian Antler orogeny. Rather, shortening continued episodically through the Mississippian and later, with far-field effects extending well to the east of the shortening-related structures.