ABSTRACT The Colorado Plateau in the southwestern United States is within the Paleozoic transcontinental arch, an area of thin, cratonic strata. The plateau was broken by latest Mississippian to early Permian Ancestral Rocky Mountain orogenesis, which produced bedrock uplifts that influenced lower Mesozoic sedimentation before Jurassic burial. Clastic sediments shed from uplifts interfinger with eolian Permian strata ultimately derived from eastern Laurentia. Triassic and Jurassic strata of the Colorado Plateau are here divided into five depositional systems, each representing a different sedimentary and tectonic setting and forming stratal associations referred to as “deposystems.” The five deposystems, which largely but not entirely correspond to formation or group names, were deposited during northward continental drift from tropical latitudes (fluvial, tidal, and nearshore marine Moenkopi and fluvial Chinle) through desert latitudes (the erg-dominated Glen Canyon and San Rafael) to temperate latitudes (fluvial Morrison). Paleomagnetically determined paleolatitudes, corrected for inclination shallowing due to postdepositional sediment compaction, place the Glen Canyon and San Rafael eolianites firmly within expected latitudes for desert environmental conditions. Lower Triassic strata of the Moenkopi deposystem form a westward-thickening wedge of fluvial and shallow marine strata and are overlain by entirely fluvial strata of the Chinle deposystem. Both contain 240–280 Ma detrital zircon populations derived from the east Mexico magmatic arc, but more northern Chinle fluvial deposits contain a higher fraction of zircons derived from Paleozoic, Neoproterozoic, and Grenville provinces in eastern Laurentia. Westward thickening of Moenkopi strata is attributed to subsidence in the proforeland basin of the east-vergent Sonoma orogeny in central Nevada, whereas accommodation space for Chinle sedimentation was provided by dynamic subsidence above the upper Triassic subduction zone behind the newly established Cordilleran magmatic arc to the southwest. Overlying, largely Jurassic Glen Canyon and San Rafael deposystems are dominantly eolian. Detrital-zircon geochronologic analysis indicates that eolian sands were derived largely from eastern Laurentia. Interbedded marginal marine, ­lacustrine-sabkha, and fluvial strata have been associated with regional unconformities, but evidence for such unconformities is here regarded as indicating facies transgressions without development of plateau-wide unconformities or disconformities. Upper Jurassic northward continental drift carried the plateau out of the desert belt and into the zone of prevailing westerly winds. This coincided with a flare up of magmatism in the Cordilleran magmatic arc, leading to transgression of Morrison fluvial sediments over erg deposits of the San Rafael deposystem. Eastward dispersal of Morrison sediments marked the initiation of the Cordilleran orogen as the dominant topographic feature of the plateau region.

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.

Abstract Area 9 of Region 2 (Pacific Coast) encompasses the Sierra Nevada, western Great Basin, and Mojave Desert geomorphic provinces. Hydrocarbons are unknown as seeps or shows in wells. As a future oil province, this area appears to be of low order of importance because of intensive tectonism, plutonic activity, and varied degrees of metamorphism imposed on the Paleozoic section; a paucity of Mesozoic strata; and the limited areas of marine Tertiary rocks. Hydrocarbon entrapment is possible in the Great Basin and in the limited areas of possibly marine Tertiary rocks adjacent to the San Andreas fault and along the Colorado River. Post-Cambrian, pre-Mississippian sequences, where present, are mainly carbonate rocks deposited in a mio- geosynclinal environment. As a result of the Antler orogeny, which occurred largely in Nevada from latest Devonian to Early Pennsylvanian time, Mississippian and Pennsylvanian rocks adjacent to this orogenic belt are made up of coarse clastic material, as they are also in the Inyo Mountains. However, carbonate rocks are predominant eastward. Post-Cambrian to Permian rocks have reservoir characteristics in the Great Basin province. The Sonoma orogeny of Late Permian and Early Triassic time resulted in the deposition of coarse clastic materials which could serve as reservoir beds in the Great Basin province. The Nevadan orogeny, associated with the Sierra Nevada and Mojave Desert plutonic events, and the Sevier orogeny on the east during Late Jurassic and Early Cretaceous time, resulted in removal of rock material from most of this area, particularly the Mojave Desert. Jurassic beds are clastic with good reservoir character but are limited to the easternmost part of the area. Cretaceous strata, if deposited, subsequently were removed. These orogenies and the Laramide orogeny in latest Cretaceous and early Tertiary time involved much of the area in extensive thrust faulting and associated faulting and folding, and, during Tertiary time, Basin- Range block-faulting was superimposed on this structural complex. Tertiary deposition was terrestrial, and volcanic activity was widespread. Sedimentary rocks of middle to late Tertiary age are confined to long narrow basins and contain much terrestrial clay, sand, and gravel; thicknesses of units are varied.