Tectonic Implications of Newly Documented Early(?) Through Late Paleozoic Age Strata, Black Rock Desert Region, Northwest Nevada
WYLD, S. J., University of Houston, Houston, TX
Early Paleozoic to Pennsylvanian age strata are exposed in several of the Paleozoic arc sequences of the western United States cordillera
(northern Sierra Nevada, eastern Klamath Mountains, Chilliwack terrane) where they form depositional basement for thick volcanic and volcaniclastic sequences of Permian age. Volcanic rocks of Permian age are also present in the Black Rock Desert (BRD) of northwest Nevada, but previous studies have been unable to clearly document the existence of any older Paleozoic rocks. New work in the Pine Forest range, BRD, however, indicates that this range contains a previously unrecognized, but extensive and structurally intact section of metamorphosed middle (and early?) Paleozoic through Late Permian age strata. This section has been dated paleontologically (M. Orchard, C. Stevens, B. Wardlaw, personal communication, 1989) and radiometrically (U/Pb zircon; J. Wright, personal communication, 1 89), and consists of the following (protolith descriptions in ascending stratigraphic order): (1) quartzo-feldspathic sedimentary and mafic volcanic rocks of probable early Paleozoic and/or Devonian age, and mafic volcanic rocks of probable Devonian age; (2) generally deep marine, chert/argillite/quartz-rich clastic rocks of post-Late Devonian(?) to Late Mississippian age; (3) shallow marine carbonate of middle Carboniferous age; (4) volcanic and volcaniclastic rocks of Early Pennsylvanian age; (5) shallow marine limestone and volcaniclastic rocks of Early Permian(?) to Guadalupian age; and (6) chert and argillite of Late Permian(?) age. Slightly angular unconformities separate units 4 and 5, and units 5 and 6. This stratigraphic succession documents a history that is very similar to tha recorded in other Paleozoic arc sequences of the western United States cordillera, including a record of early Paleozoic to Devonian(?) quartzose sedimentation and volcanism, Mississippian deep marine siliciclastic sedimentation, some renewed volcanism in Pennsylvanian time, and generally shallow marine to emergent conditions in Pennsylvanian to Early Permian time. A significant difference between the Paleozoic record in the BRD and that in other arc sequences further west, however, is the much greater thicknesses of volcanic rocks in the latter areas. This relation suggests that the BRD lay in a back-arc position relative to the main axis of Paleozoic arc volcanism.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)