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The Eocene Connection Between the Channel Islands and San Diego, California

WEIGAND, PETER, California State University, Northridge, Northridge, CA; STEVEN SAUNDERS; LENI FIELD, California State University, Northridge, Northridge, CA

Ultrahard rhyodacite clasts found in Eocene alluvial and marine formations in the San Diego area and on several Channel Islands have been used to argue for correlation between these widely separated formations; separation between the San Diego area and the northern Channel Islands has been ascribed to ~110° of clockwise rotation of the western Transverse Ranges block since ~20 Ma. We have investigated in detail 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 addition, samples from both suites exhibit overlapping patterns on both rare-earth element diagrams and spider diagrams. 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 on the same river/submarine fan complex. They thus represent an excellent tie between the rotated allochthonous western Transverse anges block and autochthonous coastal southern California. Additional mineralogical and geochemical data on clasts from correlative formations on San Miguel and San Nicolas Islands will be presented.



AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California