27 The Enigmatic Late Cretaceous-Paleogene Tectonic Evolution of the Southwestern USA

Wednesday, 13 May 2020
1:30 PM - 5:30 PM
San Rafael

PROVENANCE, PALEOGEOGRAPHY, AND CROSS-FAULT CORRELATIONS OF LATE CRETACEOUS-PALEOGENE POLYCYCLIC FLUVIAL AND SHALLOW MARINE CONGLOMERATES, WESTERN MOJAVE, COAST RANGES, AND TEHACHAPI MTNS, CA

Paper number: 27-8
Program: Theme Sessions
Time: 3:45 PM – 4:05 PM

Authors

Abstract

New data and statistical analysis of the composition, paleocurrents, and sedimentology of Late Cretaceous-Paleogene conglomerates in southern and central California are integrated with previous studies of Seiders and Cox (1992), Grove (1993), and Howard (2000) to test estimates of fault offset and supplement provenance studies based on detrital minerals. Compositions can be differentiated based on percentages of resistant metavolcanic and orthoquartzite clasts and labile plutonic and metamorphic clasts. Campanian-Maastrichtian fluvial and shallow marine conglomerates in the Salinian block were deposited south of the Mojave. To the east, in the La Panza range, conglomerates with high percentages of orthoquartzite sourced in miogeoclinal siliciclastics of the eastern Mojave define the Q assemblage. To the west, in the Santa Lucia range, high percentages of siliceous-felsic porphyries sourced, in part, from Jurassic metavolcanics of the central Mojave define the Mv assemblage. We infer two compositionally distinct sources that were dispersed by SSW-flowing fluvial systems and that late Cretaceous fluvial conglomerates were once more extensively exposed in the Mojave. In the Paleocene, Q and Mv assemblages were recycled in W-flowing fluvial/submarine canyon systems. By the late Paleocene-early Eocene, we infer W-flowing rivers in the Tehachapi Mountains deposited recycled Q and Mv assemblages sourced from the Mojave in the Tejon Embayment and were a possible source of Q and Mv assemblages in the La Honda basin. Correlation of Q assemblages in the Oligocene Zayante Fm of the Santa Cruz Mtns and in the late Eocene-early Oligocene upper San Emigdio-lower Tecuya Fms is evidence for ≈70 km of late Paleogene dextral offset along a proto-San Andreas fault at the latitude of the Tehachapi Mtns. In the mid-Oligocene, two adjacent alluvial fans with different conglomerate compositions were deposited in the western Tehachapi-eastern San Emigdio Mtns. The change in composition between the two fans is a 1 km wide linear feature overlain by early Miocene rhyodacitic tuff breccia that trends at a high angle to the San Andreas fault. This linear feature has been displaced 326±5 km to the northern Gabilan Range since the late Oligocene.

Geological Society of America Abstracts with Programs. Vol. 52, No. 4

doi: 10.1130/abs/2020CD-347144

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