Geology of the Northern Part of the Santa Ana Mountains, Orange County, California

Citation

Lewis, Lloyd Alan (1941) Geology of the Northern Part of the Santa Ana Mountains, Orange County, California. Master's thesis, California Institute of Technology. doi:10.7907/FS0B-A391. https://resolver.caltech.edu/CaltechETD:etd-03012007-133624

Abstract

The area covered includes parts of the northern end of the Santa Ana Mountains and the southeastern corner of the adjoining Puente Hills. The area is somewhat larger than 50 square miles. The work was done during the academic years of 1939-40 and 1940-41 while the author was the holder of the Standard Oil Company of California’s fellowship in geology. The purpose of the mapping was to work out the stratigraphic and structural relations of the rocks exposed in this area. Most of the area on the northeast side of the Santa Ana Mountains had never been mapped before and the differentiation of the Cretaceous and lower Tertiary sediments had never been attempted. The rocks of the northern Santa Ana Mountains range in age from pre-Triassic to Quaternary. The Triassic and pre-Triassic rocks are a complex of slates, sandstones, conglomerates and limestones which have been intruded by Jurassic acid intrusives and by andesitic extrusives of pre-Cretaceous age. The oldest un-metamorphosed rocks are upper Cretaceous in age. They lie with profound unconformity upon the Basement Complex of the Triassic and Jurassic rocks. The Tertiary rocks range in age from Eocene to upper Miocene. They consist of an alternating succession of conglomerates, sandstones, and shales with the sandstones and conglomerates predominating. Most of the Tertiary formations progressively overlap each other towards the southeast. The lithology of the entire sections is so similar that it has been, at times, very difficult to recognize the stratrigraphic position of sediments bounded by faults. Viewing the structure of the northern end of the Santa Ana Mountains as a whole, it seems to be a large asymmetrical anticline with gentle southwestward dips on the southwest side and nearly vertical, and frequently overturned, dips on the northeast side. This structure is complicated by considerable faulting. The faults, in general, trend northwest-southeast and they are generally parallel with the strike of the sediments. The area is also the scene of the junction of the Chino and Whittier faults which are two of the major faults of Southern California. Much of the area mapped lies between these two faults and this area is a mosaic of slice-blocks in which the dips of strata are always nearly vertical. As far as can be determined, the attitude of the faults is nearly vertical; some of them are reverse faults. Movement on many of the faults has been strike-slip as well as dip-slip.

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