ABSTRACT

Exotic, purple-red rhyolitic clasts are abundant constituents in upper Paleocene and Eocene conglomerates in the California Continental Borderland. Four families of rhyolitic rocks have been recognized. 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 and invariably include quartz; they dominate conglomerates in the San Diego and northern Channel Islands areas. Las Palmas clasts have a distinctive micropore-microgranular groundmass; they abound in Baja California strata. Black Rhyodacites are much darker and more brittle and were derived from a closer source terrane of bedrock and Upper Cretaceous conglomerates; they are most abund nt in the earlier deposited conglomerates of the San Diego and northern Channel Islands areas. The striking similarities between San Diego and northern Channel Islands conglomerates demonstrate that they were once part of an integrated, east-west-oriented depositional system that acts as a piercing point to document 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 ang e of Farallon Plate subduction.