Abstract

The Lower Mississippian (Kinderhook-Osage) Tripon Pass Limestone in northwestern Utah and northeastern Nevada comprises dominantly calciclastic turbidite and debris-flow facies deposited within a submarine fan complex in the Antler foredeep depozone. Six depositional facies are recognized, including turbidite assemblages of distal fan origin, channelized conglomerate and coarse-grained sand bodies of the middle fan, and hemipelagic calcareous siltstone and fossiliferous limestone deposited in a slope/outer shelf setting.

Provenance studies of the Tripon Pass Limestone unequivocally define an eastern margin or forebulge source of detritus in western Utah. The Tripon Pass contains dominantly carbonate clasts and lithic sand grains, flute casts that indicate paleoflow directions of west to southwest, and depositional facies that are distributed with proximal facies located in the east and distal facies in the west. The western margin of the foreland in central Nevada comprises a series of east-verging, imbricated thrust sheets containing middle Paleozoic, deep basinal siliceous lithologies collectively termed the Roberts Mountains allochthon. In contrast, the eastern margin of the foredeep comprises middle to late Paleozoic preorogenic and synorogenic shelfal carbonates, which are the only possible sources of Tripon Pass calciclastic facies.

The stratigraphic architecture of the Tripon Pass Limestone is divided into three informal members (lower, middle, and upper) that reflect varying rates of subsidence, eustatic sea-level change, and sediment supply to the basin. The lower member comprises dominantly thin bedded calcareous siltstone deposited in a sediment starved basinal setting during rapid transgression and tectonic subsidence. The middle member comprises a progradational submarine fan complex indicating increased sediment supply from local tectonic uplift of the forebulge margin during regression. The upper member is characterized by mostly fine-grained facies and eastward retrogradation of the fan complex in response to waning sediment supply and flexural subsidence.

Estimates of the cumulative amount of lowermost Mississippian and Devonian forebulge strata erosionally removed and incorporated into the Tripon Pass submarine fan complex range between 300 and 750 m. The Tripon Pass accounts for nearly 75% of the preserved foreland basin fill near the eastern forebulge margin and progressively decreases in importance toward the west.