Siliceous Tuff Clasts in the Upper Paleogene of Southern California

Abstract

Pebbles, cobbles, and boulders of rhyolitic and other siliceous tuffs are abundant in upper Paleogene conglomerates that crop out along and near the California coast between Los Angeles and San Diego. Typical clasts are made up of feldspar and quartz crystals in a fine-grained groundmass that is nonporous and in large part crystalline, although relics of the clastic texture are usually present, and some examples contain ghosts of pumice fragments. Possible source rocks of similar lithology and more or less appropriate age include the Sidewinder Volcanics of the Mojave Desert, northeast of Los Angeles and 70 to 140 miles north of the conglomerate outcrops, and the Imuris Volcanics of northern Sonora, 400 miles southeast of San Diego. Petrographic and chemical differences between the conglomerate clasts and the Sidewinder Volcanics show that few, if any, of the clasts can have been derived from the Mojave Desert formation. The Imuris Volcanics are more similar but perhaps too young; moreover, they seem to require 140 to 165 miles of fluviatile transport, as well as 300 miles of strike-slip movement on the San Andreas and related faults, with separation of a hypothetical Peninsular-Sonoran batholith into a very large California-Baja California portion and a very small Sonora portion. Possibly the conglomerate clasts were derived from much nearer but now unavailable sources, either higher levels of the Peninsular Ranges of southern California (above the present erosion surface) or rocks down-faulted beneath the Salton Trough east of these ranges. Exceptional tuff-clast conglomerates at and near the base of the Rose Canyon Formation (middle Eocene), in the La Jolla section of San Diego, were probably fed from very close, perhaps southern, sources.