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The Underappreciated Middle Mississippian (C2) Unconformity in the Great Basin: Its Tectonic and Stratigraphic Significance

Don French, James H. Trexler, Jerome P. Walker, and Patricia H. Cashman
3704 Hayden Drive, Billings, MT

In Nevada, the two Mississippian mudrock units that can be distinguished from geochemical and petrophysical log data correspond to lithotectonic units previously recognized from surface geology. Locally exposed paleosols, truncated sub-unconformity structures, synorogenic sediments, and contrasting depositional environments (or provenance) across the unconformity are all unequivocal evidence for mid-Mississippian uplift and erosion. However, the poor exposure and superficial similarity of the mudrocks above and below the C2 unconformity make it difficult to map at a regional scale. The new geochemical and petrophysical-log results enable us to locate the unconformity with outcrop or well-log data, and therefore to better evaluate the extent and tectonic significance of the units below and above it. Rocks below the unconformity include sedimentary rocks of the Antler foredeep that are as old as Kinderhookian and as young as middle Osagean, as well as strata associated with the Roberts Mountains allochthon. At the longitude of Eureka and Elko, these rocks are deformed into east-vergent folds and east-directed thrust faults, accommodating east-west shortening. In this Antler foredeep section, coarse-grained rocks have been mapped as Dale Canyon, Diamond Peak, and Melandco formations, and fine-grained rocks as Chainman Shale. However, this stratigraphic nomenclature must be revised because some formation names, particularly 'Chainman Shale' and 'Diamond Peak Formation', have been applied to rocks both above and below the C2 unconformity. We use the unique, previously defined term 'Gap Wash Formation' for mudrock below the C2 unconformity. The C2 unconformity is buried by synorogenic and post-orogenic sediments, as old as late Meramecian in age, that were shed eastward from the mid-Mississippian tectonic highland. Coarse-grained rocks of this section have been mapped as Tonka and Diamond Peak formations, and fine-grained rocks have been mapped as Chainman Shale. We retain the term 'Chainman Shale' for these mudrocks, because they include the type section of this unit. In middle Osagean-Meramecian time, ~30-40 million years after the Antler orogeny, Antler foreland was deformed, uplifted and eroded, forming the C2 unconformity. This unconformity was covered by late Meramecian to Chesterian overlap strata. It is essential to use correct stratigraphic terminology that distinguishes units below the C2 unconformity from those above it.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013