The Trabuco and Baker Conglomerates of the Santa Ana Mountains

Abstract

A thick conglomerate series unconformably overlies Triassic metamorphics and Jura-Cretaceous intrusives and probably conformably underlies marine early Upper Cretaceous sandstones. The lower 300-400 feet of this series are deeply weathered, red, soft, massive; the upper 100 feet are fresh, greenish-gray to brown, resistant, thick bedded, and probably marine. The red lower part, named the "Trabuco formation" by E. L. Packard, was interpreted as a continental deposit older than the overlying gray conglomerate, and by Bernard Moore as a marine deposit continuous with and genetically inseparable from the gray conglomerate but weathered to its present condition by circulating iron-bearing meteoric waters after the deposition of the entire conglomerate series. Field studies of the conglomerates and pebble counts show that the upper and lower conglomerates are probably conformable and have no constant differences in composition, size, or rounding of the included pebbles. The marked and constant differences in color, consolidation, and topographic expression are believed due to differences in weathering. The Trabuco conglomerate is interpreted as a flood-plain deposit weathered deeply in place before deposition of the gray marine conglomerate overlying. The Trabuco conglomerate is thus recognized as a valid formation, confirming Packard's original interpretation.

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