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The AAPG/Datapages Combined Publications Database
Pacific Section of AAPG
Abstract
Clast Populations in Sespe and Poway Conglomerates and Their Possible Bearing on the Tectonics of the Southern California Borderland
Abstract
Numerous investigators have noted a similarity in the conglomerates of the Sespe Formation (Santa Ana Mountains) and the Poway Group (San Diego area). Although the formations differ slightly in age, both rock units are fan-delta accumulations of river-transported detritus, and both feeding channels have been traced east of the Elsinore fault zone.
Clast populations of the conglomerates contain some similarities, but principally they reveal pronounced and diagnostic differences in the content of the two formations. For example, red to purple quartz-bearing rhyolitic clasts (commonly referred to as “Poway-type” clasts) are exceedingly abundant (80 to 86 percent) in the Poway conglomerates but constitute only 10 to 14 percent of the Sespe assemblage. Quartzite clasts constitute 5 percent in the Poway and 7 to 17 percent in the Sespe. A distinctive gray schistose metavol-canic rock type is abundant in Sespe conglomerates, but is unknown in the Poway assemblage. The Sespe contains abundant porphyritic metavolcanic clasts lacking quartz phenocrysts; this rock type is rare in the Poway assemblage.
Altogether, clast populations of the Sespe and Poway are distinctively different and easily recognized. Although both assemblages contain “Poway-type” rhyolitic clasts, the total population of the assemblages implies markedly different source areas. Paleogeographic and palinspastic reconstructions of the adjacent Continental Borderland Province should not be based on the mere presence of “Poway-type” clasts in offshore Paleogene conglomerates, but rather the percentage of “Poway-type” clasts together with the presence or absence of the other distinct clast types of the two assemblages. In this regard, the offshore Eocene conglomerates most strongly resemble the conglomerates of the San Diego area.
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