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The Cowhole Mountains, near Baker, California, are an east-tilted remnant of the Jurassic magmatic arc that extends from western Nevada through southeastern California into Arizona and Sonora, Mexico. In Middle Jurassic time, two northeast-erly trending grahens bounded by syndepositional faults accumulated 550 m and 700 m of predominantly eolian sandstone (Aztec Sandstone). Because east dip in one of the grabens increases from 30° at the base of the sandstone section to 65° at its top, we infer that structure to have developed during tilting to the west on east-dipping, listric normal faults. Westward tilting continued to accumulate during extrusion of an overlying Middle Jurassic volcanic series (Cowhole volcanics). Felsic volcanic rocks that buried the grabens comprise ignimbrites, volcaniclastic rocks, and flow breccias, emplaced in a proximal but extra-caldera setting. Plutons and sills were intruded at shallow levels, possibly during both graben filling and extrusion of volcanics. Local magmatism of significant volume ended with intrusion of a dike swarm that may be correlative with the Independence dike swarm (148 Ma). After the range tilted 90° toward the east, small glide blocks and landslides detached from high portions of the eroding range and descended into the southern graben. Our model suggests that two sets of high-angle normal faults, oriented nearly at right angles to each other, were active during the same Middle Jurassic interval. A modern analogue may be provided by the Central American arc graben, in which similar transtensional structures have developed perpendicular to the regional graben where it is intersected obliquely by the Chiapas shear zone.

Abstract Geologic relationships observable at this site are displayed Paleozoic sedimentary and Mesozoic sedimentary and rocks preserved beneath a cap of Jurassic rhyolite on a herein referred to as Rhyolite Ridge. Rhyolite Ridge liesat northeastern corner of the Cowhole Mountains approximately 13 mi (21 km)south of Baker, California, comprising portions sections 12 and 13 of T12N, R9E and 7 and 18 of T12N, R1OE, San Bernardino Principle Point. Rhyolite Ridge (not labeled) be found in the southwest corner of the Seventeenmile Quadrangle, 7 1/2-minute Topographic Series of the U.S. Geologi cal Survey. The ridge may be reached by taking the paved baker Road south from the four-way stop in central Baker or turning south from either of the central Baker off-ramps 1–15. Figure 1 indicates mileage between points mentioned in following text.

The Bird Spring Shelf in southeastern California, along with coeval turbidite basins to the west, records a complex history of late Paleozoic sedimentation, sea-level changes, and deformation along the western North American continental margin. We herein establish detailed correlations between deposits of the shelf and the flanking basins, which we then use to reconstruct the depositional history, paleogeography, and deformational history, including Early Permian emplacement of the regionally significant Last Chance allochthon. These correlations are based on fusulinid faunas, which are numerous both on the shelf and in the adjoining basins. Study of 69 fusulinid species representing all major fusulinid-bearing Pennsylvanian and Lower Permian limestone outcrops of the Bird Spring Shelf in southeastern California, including ten new species of the genera Triticites , Leptotriticites , Stewartina , Pseudochusenella , and Cuniculinella , forms the basis for our correlations. We group these species into six fusulinid zones that we correlate with fusulinid-bearing strata in east-central and southern Nevada, Kansas, and West Texas, and we propose some regional correlations not previously suggested. In addition, we utilize recent conodont data from these areas to correlate our Early Permian fusulinid zones with the standard Global Permian Stages, strengthening their chronostratigraphic value. Our detailed correlations between the fusulinid-bearing rocks of the Bird Spring Shelf and deep-water deposits to the northwest reveal relationships between the history of shelf sedimentation and evolution of basins closer to the continental margin. In Virgilian to early Asselian (early Wolfcampian) time (Fusulinid Zones 1 and 2), the Bird Spring Shelf was flanked on the west by the deep-water Keeler Basin in which calcareous turbidites derived from the shelf were deposited. In early Sakmarian (early middle Wolfcampian) time (Fusulinid Zone 3), the Keeler Basin deposits were uplifted and transported eastward on the Last Chance thrust. By middle Sakmarian (middle middle Wolfcampian) time (within Fusulinid Zone 4), emplacement of the Last Chance allochthon was complete, and subsidence caused by thrust loading had resulted in development of a new turbidite basin (Darwin Basin) along the former western part of the Bird Spring Shelf. At the same time, farther east into the craton, paralic facies began prograding westward, so that the youngest fusulinid-bearing limestones on the shelf in this area become progressively younger to the west. Eventually, in Artinskian to Kungurian (late Wolfcampian to Leonardian) time (Fusulinid Zones 5 and 6), deposition of fusulinid-bearing limestone on the shelf was restricted to a marginal belt between the prograding paralic facies to the east and the Darwin Basin to the west. Development of the Keeler Basin in Pennsylvanian to earliest Permian time was approximately coeval with collision between South America-Africa (Gondwana) and North America (Laurentia) on the Ouachita-Marathon orogenic belt. This basin developed inboard of a northwest-trending, sinistral fault zone that truncated the continental margin. Later, in the Early Permian, the Last Chance allochthon, which was part of a northeast-trending belt of deformation that extended into northeastern Nevada, was emplaced. This orogenic belt probably was driven by convergence at the continental margin to the northwest. This work adds significant detail to existing interpretations of the late Paleozoic as a time of major tectonic instability on the continental margin of southeastern California as it changed from a relatively passive margin that had characterized most of the Paleozoic to an active convergent margin that would characterize the Mesozoic.