The Santa Ana Mountains expose one of the most complete strati• graphic sections in coastal southern California, and much of the section contains distinctive rocks and diagnostic fossils. The section includes a basement complex of crystalline and semicrystalline rocks of Mesozoic age, unconformably overlain by as much as 5,200 m of Upper Cretaceous and Cenozoic clastic strata ranging in composition from organic shale to boulder conglomerate. The basement consists of the marine clastic Bedford Canyon Formation of Middle Jurassic age and is perhaps 5,500 m thick; the Upper Jurassic(?) and Lower(?) Cretaceous Santiago Peak Volcanics, 790 m thick; and extensive middle Cretaceous quartz plutonite bodies. The superjacent sedimentary deposits are mostly marine and of Late Cretaceous, Paleocene, Eocene, Miocene. Pliocene, and Pleistocene age. In addition, nonmarine units interfinger with marine, especially just above major unconformities. The northeastern, highest part of the Santa Ana Mountains is a sharply folded anticline, plunging northwest and having a downfaulted, narrow northeast limb and a similarly truncated nose. The downthrow to the north, on the Whittier fault zone, is variable, with an apparent stratigraphic separation of about 900 m. That to the northeast, on the Elsinore fault zone, may be slightly greater. The low western parts of the mountains are underlain by rocks that are only moderately folded, broken by numerous northwest-to north-trending normal faults. A complex, horstlike positive structure extends east-west along lower Santiago Creek. The broad Lorna Ridge syncline farther south underlies !lluch of the southern part of the mapped area. At the west edge of the mountains, the post-middle Miocene formations thicken slightly westward toward the subsiding Los Angeles basin, though both thickening and subsidence reverse directions in the subsurface anticlinal Anaheim nose, 5 or 6 km west of the mountains. The geologic map, structure sections, and supporting data show that the geologic record of the northern Santa Ana Mountains characterizes that of the northern Peninsular Ranges, including the San Joaquin Hills to the south and southwest, the Los Angles basin to the west, the Puente Hills to the north, and the Perris Block to the east. Thus, the mountains furnish evidence of regional significance: the boundary between basement and superjacent sedimentary rocks is of Early or middle Cretaceous age; northeastward transgression of Paleocene strata onto successively older units records an early Tertiary southwestward tilt of the mountain mass, and northward and eastward onlap of successively younger strata onto the basement surface is interptreted as additional evidence of an extensive, persistent early Tertiary peneplain; the tilting of the mountain mass and associated deformation continued to middle Miocene time, when relative depression of the Los Angeles basin began; and continuing deformation produced pronounced erosional unconformities in upper Miocene, Pliocene, and upper Pleistocene strata. TABLE ...
