NEW GEOLOGIC MAPS OF THE PUENTE HILLS, SOUTHERN CALIFORNIA

The geology of the Puente Hills east of Los Angeles, as mapped by T. W. Dibblee, Jr., is now published as two maps based on the Whittier/La Habra and Yorba Linda/Prado Dam quadrangles. Dibblee mapped this area in 1943-44, prior to its widespread urbanization. Dibblee's mapping pertaining to structure and stratigraphy differs, in places significantly, from the subsequently published 1960s USGS maps of the same area and there exist intrinsic values in showing these differences.

The Puente Hills expose the Miocene marine sedimentary sequence composed of the Monterey (Puente) Formation, late Miocene, roughly divided into two siliceous shale members separated by a sandstone member, overlain by the Sycamore Canyon Formation, latest Miocene, of clastic sediments. Along the NW margin of these hills and SSW of the Whittier fault, this formation is overlain by the mostly marine clastic Fernando Formation, Pliocene, unconformably overlain by the San Pedro shallow-marine sand, and La Habra alluvial sediments, both Pleistocene.

Structurally the Puente Hills are bounded roughly by the active Whittier fault on the SSW and partly by the Chino fault on the NE. These faults diverge northwestward from the SE end of the Puente Hills. Movement on both faults, which dip steeply into the Puente Hills, is upward on the hills' side and right lateral. Both faults die out northwestward. Structurally the Miocene sequence in these hills is anticlinal with maximum uplift adjacent to the Whittier fault along a series of axes that plunge diagonally eastward from the fault. At the SE end of the Puente Hills and southward the Monterey Formation is underlain by successively older clastic formations, from middle Miocene to Upper Cretaceous age, that dip steeply NNE from the Whittier fault. Southwest of the Whittier fault the Miocene and Pliocene sediments dip SSW from the fault but are complexly faulted. Structural closures entrap oil in upper Miocene sands in the oil fields of this strip.