Permian and Triassic Arc Sequences and Accretionary Complexes
Published:January 01, 1990
1990. "Permian and Triassic Arc Sequences and Accretionary Complexes", Paleozoic and Early Mesozoic Paleogeographic Relations; Sierra Nevada, Klamath Mountains, and Related Terranes, David S. Harwood, M. Meghan Miller
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New geologic mapping and fossil data from the Pine Forest Range, Black Rock Desert, northwest Nevada, indicates that the range contains a structurally intact sequence of variably metamorphosed middle (and early?) Paleozoic through latest Triassic strata. The oldest rocks in the range include metamorphosed quartzo-feldspathic sedimentary rocks and mafic volcanic and volcaniclastic rocks of Mississippian and/or older age. Overlying fan facies chert/argillite/quartz-rich clastic rocks are of post–Late Devonian(?) and pre–Late Mississippian age, and are succeeded by shallower marine Upper Mississippian to Lower Pennsylvanian(?) volcanic rocks, volcanic-lithic–rich clastic rocks, and limestone. The remainder of Paleozoic time is characterized mostly by shallow marine conditions and the development of several unconformities. A thin sequence of shallow marine carbonates and clastic sediments, yielding early Late Permian fossils at the top, overlies Pennsylvanian(?) strata across an unconformity that may span early Pennsylvanian through Early Permian time. Upper Permian(?) chert and shale unconformably overlie older rocks and reflect some subsidence in Late Permian(?) time. A third unconformity separates Paleozoic and Triassic rocks and spans latest Permian(?) through Middle or Late Triassic time. Triassic strata in the Pine Forest Range record two distinct periods of deposition: (1) fan facies sedimentary-lithic–rich sediments and basinal carbonates were deposited from Ladinian or Carnian (late Middle or early Late Triassic) through early Norian (late late Triassic) time, and (2) mafic to intermediate composition lavas and associated volcanic-lithic– and crystal-rich fan facies sediments were deposited during most of the remainder of Norian time. Lavas exhibit the trace-element characteristics of volcanic arc magmas. Relatively deep marine conditions of deposition occurred throughout Middle(?) to Late Triassic time.
The Paleozoic stratigraphic record in the Pine Forest Range shows important similarities to that of other Paleozoic arc sequences in the western U.S. Cordillera, including those in the northern Sierra Nevada and eastern Klamath Mountains (California), Blue Mountain province (Oregon), and Chilliwack terrane (Washington). These similarities support an interpretation of paleogeographic and tectonic ties between the Black Rock Desert and these other arc sequences in Mississippian (and early Paleozoic?) through Permian time. In addition, the presence of a Permo-Triassic unconformity in the Pine Forest Range represents new evidence that these Paleozoic arc sequences were characterized by uplift and erosion during the time of the Sonoma orogeny. Early Mesozoic strata in the Pine Forest Range provide a record of volcanism and sedimentation that is similar to that in other early Mesozoic volcanic arc sequences from the southwestern United States through northern California. These similarities support an interpretation that early Mesozoic arc sequences in northwest Nevada, as well as northern California, form the northern continuation of the west-facing early Mesozoic arc documented in the southwestern United States. In addition, the Triassic record in the Pine Forest Range suggests that extension-related intra-arc subsidence, inferred to have characterized the southwestern United States during early Mesozoic time, may also have affected early Mesozoic rocks of the Black Rock Desert.