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Paper No. 87-15
Presentation Time: 9:00 AM-5:30 PM
USING MULTI-CYCLIC QUARTZITES IN FLUVIAL CONGLOMERATES TO TRACK LATE CRETACEOUS-CENOZOIC PALEODRAINAGE EVOLUTION IN THE MOJAVE DESERT, SOUTHERN SAN JOAQUIN VALLEY, AND COAST RANGES, CALIFORNIA
Late Cretaceous-Cenozoic fluvial conglomerates in a 100,000 sq km area of south central California were sampled for clast composition and paleocurrents in order to test regional models of paleodrainage evolution, provenance, and sediment dispersal. To minimize sampling and counting errors, multiple samples of 100-400 clasts >2cm in long dimension were counted per sample location using the area method in outcrop, and by sieving, washing, and breaking in lab. Tests of the null hypothesis using Student’s t-test show >92% of sample means are the same at the 95% confidence interval for multiple samples from the same location regardless of operator or method. Sample means and 95% confidence intervals in the following categories were calculated: quartzite, Q; metavolcanic, V; and plutonic/metamorphic, PM. Ternary QVPM plots discriminate between three end member assemblages which are interpreted to have sources in late Precambrian-Mesozoic miogeoclinal-cratonal siliciclastic, Mesozoic arc volcanic, and Proterozoic-Mesozoic plutonic/metamorphic basement rocks respectively. The Q-rich assemblage contains metaquartzite, Qm, and orthoquartzite, Qo, subpopulations. Qo clasts are unlike known local sources and are petrographically similar to miogeoclinal and cratonal siliciclastic rocks presently exposed 150-500 km to the east. Qo clasts are locally abundant (40-70%) in fluvial conglomerates of late Cretaceous age in the Coast Ranges and in the Paleocene Goler, Eocene Tejon, and Oligocene Tecuya Formations of the northern Mojave/southern San Joaquin Valley. Late Cretaceous Qo assemblages are interpreted to record the influx of backarc, miogeoclinal-cratonal siliciclastic sediment into the forearc by S-SW flowing drainages. Paleogene Qo assemblages record recycling from inferred alluvial deposits in the Mojave Desert by W-NW flowing drainages. Late Cretaceous-Paleogene Qo-rich alluvial deposits are inferred to have been more widespread in the Mojave Desert and may yet exist in the subsurface of basins such as Antelope Valley or Fremont Valley. Qo-rich conglomerates at the base of many Tertiary sequences in the Mojave Desert may be remnants of more extensive deposits. Early Miocene and younger conglomerates in the study area are rich in local basement clasts with much lower percentages of exotic Qo.