Paul Stone

Pennsylvanian and Early Permian paleogeography of east-central California: Implications for the shape of the continental margin and the timing of continental truncation

Pennsylvanian and Early Permian paleogeographic features in east-central California include a southeast-trending carbonate shelf edge and turbidite basin that we infer paralleled a segment of the western margin of the North American continent. This segment of the continental margin was oblique to an adjoining segment on the north that trended southwestward across Nevada into easternmost California. We propose that the southeast-trending segment of the margin originated by tectonic truncation of the originally longer southwest-trending segment in Early or Middle Pennsylvanian to late Early Permian time, significantly earlier than a previously hypothesized Late Permian or Early Triassic continental truncation event. We interpret the truncating structure to have been a sinistral transform fault zone along which a continental fragment was removed and carried southeastward into the Caborca-Hermosillo region of northern Mexico, where it is now represented by exposures of Late Proterozoic and Paleozoic miogeoclinal rocks.

PALEOZOIC AND MESOZOIC EVOLUTION OF EAST-CENTRAL CALIFORNIA

East-central California, which encompasses an area located on the westernmost part of sialic North America, contains a well-preserved record of Paleozoic and Mesozoic tectonic events that reflect the evolving nature of the Cordilleran plate margin to the west. After the plate margin was formed by continental rifting in the Neoproterozoic, sediments comprising the Cordilleran miogeocline began to accumulate on the subsiding passive margin. In east-central California, sedimentation did not keep pace with subsidence, resulting in backstepping of a series of successive carbonate platforms throughout the early and middle Paleozoic. This phase of miogeoclinal development was brought to a close by the Late Devonian-Early Mississippian Antler orogeny, during the final phase of which oceanic rocks were emplaced onto the continental margin. Subsequent Late Mississippian-Pennsylvanian faulting and apparent reorientation of the carbonate platform margin are interpreted to have been associated with truncation of the c...

Correlation of metamorphosed Paleozoic strata of the southeastern Mojave Desert region, California and Arizona

Isolated outcrops of deformed, regionally metamorphosed Paleozoic strata are scattered within the southeastern Mojave Desert region of California and western Arizona. These strata unconformably overlie a basement of Proterozoic crystalline rocks and are overlain in turn by metamorphosed Mesozoic sedimentary rocks. The strata can be correlated lithostratigraphically with the classic cratonal Paleozoic section of the western Grand Canyon, Arizona, and with nonmetamorphosed Paleozoic sections transitional between cratonal and miogeoclinal in the Ship, Marble, and Providence Mountains, California. The strata evidently were once continuous with Paleozoic epicontinental strata exposed throughout the southern Great Basin and Colorado Plateau. Outcrops of Paleozoic strata and of the underlying Proterozoic basement in the southeastern Mojave Desert region define a terrane that has been disrupted by Mesozoic thrust faults and by Tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence. This coherent terrane extends at least as far west and southwest as the Big Maria, Palen, and Calumet Mountains, and possibly beyond to include Paleozoic exposures in the San Bernardino Mountains and near Victorville. Poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western Mojave Desert and the eastern Transverse Ranges.

Early Permian thrust faults in east-central California

One exposed and another inferred thrust fault in the southeastern Inyo Mountains of east-central California are interpreted as Early Permian in age. These faults, both apparently of local extent, are temporally and geographically related to a major episode of basin formation which occurred along a southeast-trending segment of the Permian North American plate margin. The close relationship between basin formation and thrust faulting supports a previous interpretation by us that this segment of the margin was undergoing active wrench tectonism during the Early Permian, resulting in the essentially simultaneous development of local extensional and compressional structures similar to those now forming in the California continental borderland.

Cretaceous age of the upper part of the McCoy Mountains Formation, southeastern California and southwestern Arizona, and its tectonic significance: Reconciliation of paleobotanical and paleomagnetic evidence

The upper part of the 7-km-thick McCoy Mountains Formation in southeastern California contains fossil angiosperm wood that closely resembles the genus Paraphyllanthoxylon , which is known only from strata of late Early Cretaceous and younger age. This wood, in conjunction with geologic field relations, supports previous interpretations that the upper part of the McCoy Mountains Formation is of late Early Cretaceous and/or Late Cretaceous age, in contrast to a more recent interpretation that the entire formation is of Jurassic age. Alternatives are therefore needed to the recent hypothesis that deposition, deformation, and metamorphism of the McCoy Mountains Formation were related to movement on the Jurassic Mojave-Sonora megashear.

Paleogeographic and structural significance of an Upper Mississippian facies boundary in southern Nevada and east-central California

A major facies boundary separating Meramecian (lower Upper Mississippian) shallow-water, carbonate-platform rocks to the south-east from coeval to slightly younger relatively deep-water siliciclastic rocks to the northwest can be traced throughout southern Nevada and east-central California. Although the precise depositional relation between the two facies is not entirely clear, the outer margin of the shallow-water carbonate rocks almost certainly marks the edge of the carbonate platform in earliest late Meramecian time. This facies boundary is distinct and unambiguous enough that it and a parallel subfacies boundary within the carbonate-platform rocks can be used to estimate amounts of displacement on Cenozoic right-slip faults in the region. The carbonate-siliciclastic facies boundary is offset by about 170 km in a right-lateral sense parallel to the Death Valley-Furnace Creek fault zone between the west side of that fault zone and the northeast side of the Las Vegas Valley shear zone. This offset is Cenozoic in age and is interpreted to be distributed as follows: 45-50 km on the Las Vegas Valley shear zone, 80 km on the Death Valley-Furnace Creek fault zone, 30 km on the Stewart Valley fault, and an additional 10-15 km that is not specifically accounted for. Restoration of these offsets successfully reconstructs the Meramecian carbonate shelf edge and aligns several thrust faults now exposed on opposite sides of the right-slip faults.

A speculative reconstruction of the Middle Paleozoic Continental Margin of southwestern North America

Geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern North America with respect to the middle Paleozoic continental margin. The southwestern margin of the middle Paleozoic continent, based on these data, is quite irregular in its present configuration, but most of the irregularities can be accounted for by strike-slip displacement of continental fragments during several post-middle Paleozoic tectonic events. We conclude that large-scale late Paleozoic sinistral displacements and Mesozoic and Cenozoic dextral displacements have been the major events affecting the configuration of this margin. Our preferred model includes the following elements: (1) 500 km of left-lateral displacement of a large part of the margin during a late Paleozoic truncation, (2) slivering of the margin by right-lateral faulting during the Mesozoic, amounting to about 275 km of total displacement, and (3) further deformation of the margin during the Cenozoic, primarily by right-lateral faulting, oroclinal bending, and crustal extension.

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

An angular unconformity in the Permian section of east-central California

The final phase of Permian continental margin tectonism in east-central California produced a widespread unconformity that separates Lower Permian strata from Upper Permian to Lower and Middle(?) Triassic strata. This unconformity has a maximum angular discordance of about 25° and locally truncates at least 1 km of Lower Permian strata. Leonardian and (or) early Guadalupian deformation that resulted in development of this unconformity may have been coeval with early phases of the Sonoma orogeny in Nevada and with Permian tectonism in the western Mojave Desert of California, but the tectonic relations among these events are uncertain. Permian deformation in east-central California took place along a southeast-trending segment of the continental margin that may have been bounded by an active sinistral transform fault zone.

New Permian Fusulinids from Conglomerate Mesa, Southeastern Inyo Mountains, East-Central California

Abstract In the Conglomerate Mesa area in the southeastern Inyo Mountains, east-central California, a series of distinctive fusulinid assemblages ranging in age from late Artinskian to Kungurian or Roadian was developed in units 7–10 of the sedimentary rocks of Santa Rosa Flat (part of the Owens Valley Group). The fauna of unit 7 shows some eastern Klamath Mountains affinity, but most of the species in unit 7 and the lower half of unit 8 are highly endemic and comprise three new genera with 12 new species, two unusual unassigned forms, and two other new species assigned to previously described genera. New taxa include: Crenulosepta new genus with five new species, C. inyoensis, C. delicata, C. fusiformis, C. rossi, and C. wahlmani; Nigribaccinus new genus with three new species, N. giganteus, N. elegans, and N.? nestelli; and the new genus Inyoschwagerina with four new species, I. magnifica, I. elayeri, I. elongata, and I.? linderae. Cuniculinella Skinner and Wilde, 1965, is represented by one new species, C. parva, and Skinnerella Coogan, 1960 by one new species, S.? mcallisteri. Faunas from the upper half of unit 8, unit 9, and unit 10 have a strong West Texas affinity. New species from these units are Skinnerella davydovi, S. hexagona, Parafusulina cerrogordoensis, P. complexa, P. halli, P. owensensis, and P. ubehebensis.