WEIGAND, PETER W., JENNIFER A. WHITNEY, STEVEN C. SAUNDERS, MARTHA P. SGRICCIA*, MARY S. STECHESON, LENI G. FIELD, JOHN C. DEMARTINO, and JESSE F. HOLT
Department of Geological Sciences, California State University, Northridge, CA 91330-8266

Abstract: Poway Clasts Revisited - The Connection between San Diego and Santa Cruz Island, California

Ultrahard rhyolite clasts found in upper Eocene alluvial and marine formations in the San Diego area and on Santa Cruz and San Miguel Islands have been used to argue for correlation between these widely separated formations; separation has been ascribed to strike-slip translation and more recently to ~110<deg> of clockwise rotation of the western Transverse Ranges block since ~20 Ma. We have reinvestigated the petrography and geochemical composition of one suite of clasts from the Stadium Conglomerate of the Poway Group collected near San Diego and another from the Jolla Vieja Formation on Santa Cruz Island. Both suites are characterized by phenocrysts of vacuolized plagioclase, embayed quartz, and altered biotite, devitrified groundmass, and metamorphic epidote and piemontite. Rocks in both suites range in composition from trachydacite to rhyolite (SiO₂ = 66 to 76 wt %). When all nine major oxides and 33 trace elements are plotted against SiO₂, both suites form completely overlapping fields. In both suites, patterns of the REE decrease moderately from La to Sm, show a small Eu anomaly, and are flat from Gd to Lu. Spider diagrams for both suites show identical depletions for Ba, Nb, Ta, Sr, P, and Ti. These identical petrographic and geochemical characteristics confirm previously made conclusions that these two suites of clasts were derived from the same source and were deposited by the same river/submarine fan complex. They thus represent an excellent tie between the rotated allochthonous western Transverse Ranges block and autocthonous coastal southern California.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California