Geological Society of America Memoir 195 2002 Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block Elizabeth R. Schermer Department of Geology, Western Washington University, Bellingham, Washington 98225, USA Cathy J. Busby James M. Mattinson Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA ABSTRACT Sedimentologic, stratigraphic, and geochronologic data from strata of early Me- sozoic age in the central Mojave block elucidate the paleogeographic and tectonic evolution of the magmatic arc in the southern U.S. Cordillera. A sequence of calcar- eous siltstone, volcaniclastic conglomerate, tuff, and quartzose sandstone records the transition from shallow-marine rocks of the Fairview Valley Formation to the sub- aerial Sidewinder volcanic series. Quartzose sandstones occur below, within, and above the transitional sequence and indicate that texturally mature, craton-derived quartz sand gained access to the arc during the initial stages of volcanism. U-Pb data indicate that explosive volcanism began at 179.5 ؓ 3.0 Ma and continued until 151 Ma (Lower Sidewinder volcanic series). A rhyolite dike of the Independence 1.3 ؓ dike swarm (Upper Sidewinder volcanic series) that postdates normal faulting and tilting of the ignimbrites yielded a U-Pb date of 151.9 ؓ 5.6 Ma. The data define the age of extension and development of the angular unconformity between the Upper and Lower Sidewinder volcanic series at ca. 151 Ma. The data suggest that at least part, and possibly all, of the Fairview Valley For- mation is late Early Jurassic in age. We correlate the Fairview Valley Formation with Mesozoic metasedimentary rocks in the Rodman Mountains and Fry Mountains, and at Cave Mountain to the east. Eolian quartz arenites in these sequences suggest a coastal environment coeval with the Navajo Sandstone on the Colorado Plateau. The reinterpretation of the shallow-marine rocks as Jurassic instead of Triassic suggests a period of uplift and erosion or nondeposition extending from the Early Triassic into the Early Jurassic, followed by a return to marine conditions. Shallow-marine con- ditions persisted until the beginning of arc volcanism in the late Early Jurassic time. Similarities to the early Mesozoic arc of the Sierra Nevada, together with the struc- tural evolution of the region, suggest that the change from high-standing to low- standing paleogeography reflects a large-scale tectonic control on relative sea level related to a period of intra-arc extension or transtension. Schermer, E.R., Busby, C.J., and Mattinson, J.M., 2002, Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences in the central Mojave block, in Glazner, A.F., Walker, J.D., and Bartley, J.M., eds., Geologic Evolution of the Mojave Desert and Southwestern Basin and Range: Boulder, Colorado, Geological Society of America Memoir 195, p. 93–115. 93 94 E.R. Schermer, C.J. Busby, and J.M. Mattinson INTRODUCTION Jurassic and Cretaceous batholithic rocks intrude the supra- crustal rocks and are widely exposed throughout the Mojave Understanding of the paleogeography and tectonic evolu- Desert (Fig. 1). tion of the early stages of the continental-margin magmatic arc The shallow-marine Fairview Valley Formation (Bowen, in the southern U.S. Cordillera has been hampered by incom- 1954; Dibblee, 1960a, 1960b; Miller, 1978b, 1981) forms part plete knowledge of the ages and depositional environments of of Walker’s (1988) overlap assemblage in the Victorville region sedimentary and volcanic rocks in the greater Mojave Desert (Fig. 2) and was interpreted to be Early Triassic in age. Coarse region (e.g., Glazner et al., 1994). In the central Mojave block, conglomeratic units within the Fairview Valley Formation were well-preserved volcanic and sedimentary sequences record the interpreted by Miller (1978b, 1981) as alluvial-fan facies re- initiation of arc volcanism and provide insight into the paleo- flecting intra-orogenic deposition following a Permian–Triassic geography of the early arc. In this study we present sedimen- orogenic event. Quartzose sandstone overlies the shallow- tologic, stratigraphic, and U-Pb geochronologic data on the marine rocks and the conglomerate. The contact of the Fairview Fairview Valley Formation and the overlying Sidewinder vol- Valley Formation and the quartzose sandstone with overlying canic series, a sedimentary and volcanic sequence of early Me- volcanic rocks is parallel to bedding in the underlying strata sozoic age exposed in the Victorville area of the Mojave block across the region and appears to be conformable. (Fig. 1). Our results suggest new correlations of lower Meso- An important aspect of early Mesozoic arc paleogeography zoic sequences across the Mojave block and a revised interpre- is reflected in the observed association of quartzose sandstones tation of the paleogeographic evolution of the region. Our new and volcanic rocks in the early Mesozoic arc. Busby-Spera U-Pb ages also define the duration of explosive volcanism and (1988) noted that supermature eolian quartz arenites are com- set limits on the ages of intra-arc deformational events. Inas- monly associated with proximal volcanic rocks throughout the much as direct dating of specific structures has been difficult, southwestern Cordillera, and she interpreted the association to we also attempt to relate the style of intra-arc sedimentation reflect trapping of eolianites in a low-standing arc graben- and magmatism to the tectonic setting. depression. The eolianites were correlated by earlier workers with the Lower Jurassic Navajo Sandstone of the Colorado Pla- GEOLOGIC SETTING teau and the Aztec Sandstone of the Las Vegas region (Cameron et al., 1979; Hewett, 1931, 1954; Marzolf, 1980, 1983; Miller The Mesozoic magmatic arc in the Mojave Desert was built and Carr, 1978). More recent work (Busby-Spera, 1988; Busby- across Precambrian–Paleozoic cratonal-miogeoclinal strata that Spera et al., 1990; Fackler-Adams et al., 1997; Riggs et al., were deformed and metamorphosed in Pennsylvanian–Triassic 1993) has indicated that some eolianites intercalated with arc- time, possibly during strike-slip truncation of the continental type volcanic rocks are age-equivalent to several younger margin (Burchfiel and Davis, 1972, 1981; Miller and Cameron, quartz arenites of the Colorado Plateau, including the Middle 1982; Stone and Stevens, 1988; Walker, 1988; Martin and Jurassic Temple Cap and Page Sandstones and the upper Middle Walker, 1995). Permian or Early Triassic alkalic plutonic rocks Jurassic Carmel Formation. Quartzites and quartz-rich sand- intrude deformed Paleozoic strata and record the initiation of stones also occur in sequences interpreted by Walker (1987, subduction-related magmatism (Barth et al., 1990; C. Miller, 1988) to be Early Triassic in age, but most of these occurrences 1978; Miller, 1978b; Miller et al., 1995). Shallow-marine rocks are too metamorphosed to determine whether they were depos- that unconformably overlie the Paleozoic rocks have been in- ited in an eolian environment. To understand the paleogeogra- terpreted as a Lower Triassic overlap assemblage deposited phy of coeval backarc and arc environments and to determine across the deformed margin from the Victorville region to Cave whether deformation events in the arc are related to those in Mountain (Fig. 1) and across undeformed rocks farther east the backarc, better understanding of the ages and depositional (Walker, 1987, 1988). Facies boundaries within the overlap se- environments of these quartzose strata is required (e.g., Bjerrum quence strike northwest, indicating that the change in trend of and Dorsey, 1995; Burchfiel and Davis, 1981; Lawton, 1994). the continental margin from northeast prior to the truncation Most of the pre-Tertiary volcanic rocks in the Mojave Des- event to northwest afterward was accomplished by Early Tri- ert are silicic and intermediate-composition rocks of Jurassic assic time (Walker, 1988). The shallow-marine rocks are typi- age. The largest exposure, the Sidewinder volcanic series cally overlain by thick sequences of volcanic rocks that reflect (Bowen, 1954) (Figs. 1, 2, 3), consists of a Ͼ4-km-thick se- the transition to the tectonics of the fully active magmatic arc. quence of Jurassic rhyolitic to dacitic intracaldera ignimbrites In several areas, including the Victorville region, Cave Moun- (Lower Sidewinder volcanic series) overlain with angular un- tain, and the Soda Mountains (Fig. 1), quartzite and quartz-rich conformity by a thin sequence of rhyolite to basalt lavas (Upper sandstone occur between and are locally interfingered with the Sidewinder volcanic series); (Karish et al., 1987; Schermer and shallow-marine rocks and the volcanic rocks. The age of the Busby, 1994). Recent dating of ignimbrites and lavas in the Ma defines the 4 ע6to167 ע quartzose sandstones has been somewhat controversial, and Cowhole Mountains at 172 some exposures have been considered Triassic whereas others age of magmatism in that region as Middle Jurassic (Busby- have been interpreted as Jurassic in age (e.g., Walker, 1987). Spera et al., 1989; Busby et al., this volume). Dating of the Paleogeographic and tectonic implications of Jurassic sedimentary and volcanic sequences 95 A SR GM EP N Garlock fault SO TM I-15 Baker GLM CR CH C PR Barstow IM I-15 RM I-40 SM PC basement, PC-Pz Metasedimentary rocks QM Pz eugeoclinal rocks 34°30'N Mz plutonic rocks Victorville San Andreas Fault Mz volcanic, sed rocks ° 117 15W 116°30W area of J extension 50 km area of J shortening B ° 0510 km 34 45'N 34°45'N RZ NRM SR OM SWM SRM Fig. 2 BM QM FV Fig. 3 FM N ° Victorville 34 30'N 117°15'W 116°30'W Figure 1. (A) Generalized geologic map of the western Mojave Desert, showing Mesozoic supracrustal and plutonic rocks, pre-Mesozoic strata, and localities mentioned in text. Abbreviations: C—Cave Mountain, CH—Cowhole Mountains, CR—Cronese Hills, EP—El Paso Mountains, GLM—Goldstone-Lane Moun- tain, GM—Granite Mountains, IM—Iron Mountain, PR—Providence Mountains, QM—Quartzite Moun- tain, RM—Rodman Mountains, SM—Shadow Mountains, SO—Soda Mountains, SR—Slate Range, TM— Tiefort Mountains.