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.

Details of the stratigraphy, depositional setting, and clastic petrography of the upper Paleozoic Schoonover sequence in the Independence Mountains, northern Nevada, provide the basis for a better understanding of the paleogeography of the continental margin of western North America during the late Paleozoic. The Schoonover sequence represents the northernmost exposures of the Golconda allochthon, which was thrust over the outer edge of the continental margin of western North America during the Permo-Triassic Sonoma orogeny. The Schoonover sequence, like the Havallah sequence and other units of the Golconda allochthon, is an imbricated assemblage of thrust-bound packages of radiolarian “ribbon” chert, basaltic greenstone, silty limestone turbidites, and siliciclastic sandstones that range in age from Late Devonian to Early Permian. Detailed mapping, and facies and petrographic analysis of lithostratigraphic units, combined with paleontologic control, indicate that the Schoonover sequence consists of a coherent stratigraphic succession of basinal deposits with paleogeographic ties to the autochthonous shelf margin and to a volcanic arc. Latest Devonian- to earliest Mississippian-age basaltic and andesitic greenstones and tuffaceous sedimentary rocks form the stratigraphic base of the sequence and are succeeded by Lower Mississippian siliciclastic sandstones. The petrography of these sandstones indicates that they consist of mixed detritus derived from volcanic and continental shelf source terranes, indicating the proximity of an arc to the continental margin. In addition, stratigraphic relations between autochthon and allochthon indicate that the Schoonover basin evolved adjacent to the continental shelf throughout the late Paleozoic, its history punctuated by periods of basaltic volcanism and pulses of continent-derived clastic input. The onset of the Sonoma orogeny resulted in closure of the basin and emplacement of its deposits on the continental margin.

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.

Rocks in the eastern Klamath Mountains were affected by polyphase tectonism. The older Callahan tectonic event, of Late Ordovician–Early Silurian age, produced high-pressure and epizonal metamorphism and is expressed in the Yreka-Callahan nappes by intrafolial folds. The main tectonic event occurred during Late Devonian and Early Mississippian time and is interpreted here to be coeval with the Antler orogeny, defined in the Roberts Mountain allochthon to the east in Nevada. It resulted in the amalgamation of lower Paleozoic Trinity ophiolite and the pre-Devonian to Early Devonian intra-oceanic island arc with flysch-type sediments deposited in a continent margin (Yreka-Callahan assemblage), and a metamorphic sequence of amphibolite, marble, and quartz-mica-schist of the Central Metamorphic Belt. The magmatic evolution of Paleozoic and lower Mesozoic rocks in the eastern Klamath Mountains is characterized by the predominance of convergent plate boundary magmatism. Nevertheless, lower Paleozoic pillow basalts associated with coralline limestone occur as olistoliths in the Yreka-Callahan assemblage. They display either T mid-ocean ridge basalt (MORB) affinities with flat rare earth element (REE) patterns or mild alkalic affinities marked by higher contents in TiO 2 and HFS elements and LREE-enriched patterns. The Ashgillian calc-alkaline differentiated suite, characterized by LREE-enriched patterns and low ɛ Nd (T) + 3.5, was erupted through continental crust in an Andean margin setting. The pre-Devonian to Early Devonian volcanic rocks that show tholeiitic affinities with very depleted to somewhat enriched LREE patterns and high ∊ Nd (T) values represent the remnants of an intra-oceanic immature island arc. After the Late Devonian(?)–Early Mississippian tectonic event, island-arc magmatism resumed with the eruption of depleted low-K tholeiitic basalt and andesite, characterized by depleted LREE patterns during Late Devonian(?) or Carboniferous-Permian(?) time (Gregg Ranch and Copper Peak sections). A major pulse in volcanism occurred during Middle to Late Permian and is now represented by the voluminous sequence of volcanogenic debris and flows of the Nosoni, Dekkas, and Bully Hill Formations. This volcanism, formed of predominantly low-K tholeiitic basalt and andesite, characterized by flat REE patterns and high ɛ Nd (T) + 4 to 8, with minor calc-alkaline, alkaline (enriched LREE patterns), and boninitic (Mg-Cr-rich) basalts, was erupted through a thickened crust. At the end of the Permian island-arc volcanism, uplift and faulting occurred in the arc. The magmatic affinities of the middle to late Paleozoic volcanic rocks in the eastern Klamath Mountains suggest an evolution through time from an immature to a mature island-arc setting. The Permian-Triassic history of the Blue Mountains (Seven Devils Group; Huntington Formation) presents many similarities with that of the eastern Klamath terrane. In both areas, evidence for Permian volcanism is similar with flat REE patterns and high ɛ Nd (T) + 6.7 to 8.4. Triassic volcanic rocks display low-K tholeiitic affinities (flat REE patterns) with minor calc-alkaline flows in the uppermost levels of the volcanic pile. Apparently, compressive structures related to the Sonoma orogeny are absent in both areas. Thus the eastern Klamath and Blue Mountains could have formed part of the same arc in Permian-Triassic time.