The Mesozoic marine province of the northwestern Great Basin consists of late Early Triassic through Early Cretaceous volcanic, terrigenous clastic, and carbonate rocks. The rocks were deposited in a basin that was probably open to the north and bound on the west by the Sierran arc and on the east and south by continental uplands. Basin configuration was controlled by an enclave of noncontinental crust whose areal distribution and mechanical properties localized development of marine conditions in the region.

The deep axial region of the basin progressively shoaled during the Triassic and regionally extensive shallow marine carbonates and easterly derived clastics migrated westerly, overlapping deep marine clastics and subaerial to shallow marine volcanics interbedded with carbonates. In the Early Jurassic, depositional patterns changed. Basins, possibly related to regional extension, formed and accumulated locally derived coarse clastics and craton-derived quartz sandstones. Later, during continued synorogenic deposition, volcanogenic rocks were introduced in the west and migrated easterly.

Several hundred kilometers of stratal shortening associated with the Luning-Fencemaker thrust belt formed in response to northwest-southeast regional contraction, which began in the Middle or Late Jurassic and continued through the Early Cretaceous. Rocks deposited on the western flank of the basin did not experience northwest-southeast contraction and were separated from the thrust belt by a northwest-striking left-lateral transpressional fault, the Pine Nut fault. At about 100 Ma, thrusting ceased and transpressional motion reversed and became right-lateral.

Source-rock evaluation of thick, regionally extensive Upper Triassic carbonates has yielded TOC values ranging from 0.20 to 0.48 and conodont color indices (CAI 3.5-4.5) indicating paleotemperatures of about 180° to 250°C. Kerogen analysis indicates that the dominant organic matter is sapropelic to amorphous and in one area suggests the presence of the oil window. Hydrocarbon generation within the carbonate is a distinct possibility.

End_of_Article - Last_Page 944------------