Abstract

Four families of exotic, purple-red rhyolitic clasts have been recognized in upper Paleocene and Eocene conglomerates in the California continental borderland. Owl Creek clasts have low percentages of phenocrysts that never include quartz; they are primarily found in northern localities such as the Santa Ana Mountains. Poway rhyolites are packed with phenocrysts, including quartz; they dominate conglomerates in San Diego and the northern Channel Islands. Las Palmas clasts have a distinctive micropore-microgranular goundmass; they abound in Baja California. Black rhyodacites are darker and more brittle and were derived from a closer source; they are most abundant in the earlier deposited conglomerates of the San Diego and northern Channel Islands areas. The similarities between San Diego and northern Channel Islands conglomerates demonstrate that they were once part of an integrated, east-west-oriented depositional system that documents major post-Eocene strike slip in the offshore California continental borderland. The sudden appearance of exotic rhyolitic clasts in mixed suites in upper Paleocene conglomerates, and the subsequent increases in their abundance and decreases in their variety testify to the growth of fewer, longer, and less overlapping rivers through the Eocene. The requisite integration of fluvial drainages followed the eastward-moving front of the Laramide Orogeny, caused by a shallowing angle of Farallon Plate subduction.--Modified journal abstract.

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