THE ELUSIVE LATE PALEOZOIC AND EARLY MESOZOIC HYDROCARBON-SEEP RECORD OF NEVADA

Hydrocarbon seeps are unique geobiological targets as they fuel a complex chemosynthetic ecosystem in otherwise depauperate conditions. The global record of Paleozoic and early Mesozoic seeps is very spotty due largely to their formation in tectonically active zones such as forearcs and transpressional basins. In the Cordillera, seeps have been described from the Upper Devonian of Nevada and Mexico, Triassic of Oregon, and Jurassic of British Columbia, Oregon, and California. By the early Cretaceous, seeps become more common in the forearc (Great Valley Group). This temporal shift is coincident with a global change in macrofaunal dominance from dimerelloid brachiopods to mollusks. In order to better understand the older dimerelloid seep record, more localities are needed from the Paleozoic and early Mesozoic.

Nevada and neighboring regions provide ample targets for exploration due to the accretionary arcs and bounding basins. There are several components to the desktop search strategy. First is identifying ideal geological conditions, typically deep marine slopes adjacent to tectonically active areas. Often, these facies are associated with limestone olistostromes. Second is combing museum and literature records for dimerelloid brachiopods. Historically, most were either identified as the generic Rhynchonella, the Triassic genus Halorella, or Leiorhynchia. Third is recognition of isolated anomalous ‘lentils’ of limestone in otherwise muddy deposits.

Specific targets that have escaped intensive metamorphism include the Black Rock and Jackson terranes, in particular the Permian Bilk Creek Limestone and the Permian-Triassic Quinn River Formation as there are currently no known Permian dimerelloid brachiopods. Equivalent strata to the south in the Southern Sand Springs Range record Triassic back-arc sedimentation with records of Halorella as well as rare megalodontid bivalves. The Triassic Pit and Hosselkus formations in northern California as well as Triassic(?) to Jurassic forearc deposits of the Sierra Nevada foothills and northern Mexico are also known to host olistrostromes as well as rare ‘lentils’ of limestone that need to be resurveyed.