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ABSTRACT

The Cemex Inc. limestone quarries are located in the Sidewinder Mountain–Black Mountain area northeast of Victorville in the Mojave Desert, California. Bedrock in this area includes Neoproterozoic and Paleozoic miogeoclinal carbonate-dominated metasedimentary roof pendants, and is the type location of the Jurassic Fairview Valley Formation, an overlying unnamed quartzite unit, and the Jurassic Sidewinder volcanic rocks. A variety of Triassic, Jurassic, and Cretaceous intrusive rocks and dikes also are abundantly exposed.

Complex geologic structure in the area resulted from a prolonged geologic history that includes multiple Permian–Triassic and Mesozoic contractional, metamorphic, intrusive, and extensional deformation events, and younger Cenozoic deformation that has continued into recent time. Permian–Triassic deformation includes complex polyphase deformation and metamorphism of Paleozoic rocks, and intrusion of Triassic monzonite, followed by uplift and erosion. Lower Jurassic Fairview Valley Formation unconformably overlies Paleozoic rocks and Triassic monzonite and was largely derived from them. The Fairview Valley Formation was deformed and eroded prior to juxtaposition with overlying quartzite. Overlying the quartzite are Middle Jurassic Lower Sidewinder Volcanic rocks (181–165 Ma). During and following Lower Sidewinder volcanism, faulting related to regional caldera collapse events occurred. Extensional deformation occurred prior to and after intrusion of late Middle Jurassic granitic rocks. Northwest-trending dikes correlated with the Late Jurassic Independence dikes cut previously juxtaposed Fairview Valley Formation, quartzite, and Lower Sidewinder Volcanics. During Cretaceous time, intrusion of quartz monzonite plutonic rocks occurred.

The Helendale fault, one of several active major northwest-striking faults of the Eastern California shear zone, has been traced for ~90 km and passes within 2 km of the Cemex operations. The location of the Helendale fault zone is thought to be a long-lived zone of structural weakness. Recurrent surface rupture along the modern Helendale fault in Holocene and recent time (200–10,000 yr) has been documented. Data suggest that right-stepping en echelon strands formed a pull-apart basin as much as 3 km wide in the Fairview Valley area, with up to 3 km of right lateral offset.

The Cemex Inc. cement production facility is the largest in California. Deposits mined include the Reserve (White Mountain) quarry and the Black Mountain quarry. The Reserve quarry produced an estimated 25 million tons of calcite marble between the early 1940s and late 1970s. The deposit located on the south slope of Black Mountain was formed from metamorphosed Upper Paleozoic limestones. The Black Mountain quarry was opened in the early 1950s and originally formed a mountain. Currently, the quarry is an open pit ~1200 m long, 800 m wide, and >60 m deep. Production is estimated at 4 million tons per yr. Cement-grade ore mined at the Black Mountain quarry is limestone-cobble conglomerate of the upper member of the Jurassic Fairview Valley Formation, which was derived from the Pennsylvanian–Permian Bird Spring Formation, and only occurs in the quarry area. The deposit is folded into a tight northwest-trending syncline and bounded to the south, east, and north by faults. This unique deposit is suitable as a long-term (>100 yr) source of limestone for cement manufacture.

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