Abstract

In the Santa Ana Mountains of southwestern California, conglomerates of late Paleocene to early Miocene age are characterized by abundant clasts of mildly metamorphosed rhyolite (and rhyodacite or dacite) pumice tuff and less abundant clasts of granite high in sodium and potassium. Piemontite is a rare but diagnostic constituent of the metatuff. The granite clasts are unlike any rock known in other conglomerates of the Los Angeles Basin or its surroundings. Metatuff clasts are somewhat less restricted. They are abundant in the Paleocene Martinez Formation of the eastern Santa Monica Mountains northwest of downtown Los Angeles. They have also been redeposited in middle Miocene and Pleistocene deposits in the Santa Ann Mountains, but they are not known as fresh importations at horizons above the lower Miocene there or at other places within 100 km of Los Angeles City Hall. In San Diego County and beyond, 100 km south of the Santa Ana Mountains, slightly different metatuff clasts are abundant in Eocene formations and are widely redeposited in younger conglomerates. The source rocks for the alkali granite and metatuff are not known. These clasts may have come from bedrock formations that have been completely eroded. Sedimentary structures and facies in the Santa Ana Mountains indicate that source masses for both rocks lay to the southeast or east in late Paleocene and Eocene times and to the northeast in early Miocene time.

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