Geology of the Humphreys Station area, Los Angeles County, California

Abstract

Six formations of Tertiary and Quaternary Age are present south of the Santa Clara River in the Easternmost Ventura Basin. The upper Miocene non-marine Mint Canyon formation is successively overlain by the uppermost Miocene marine "Modelo" formation, the lower Pliocene marine Pico formation, the Pliocene-Pleistocene non-marine Saugus formation, and by Pleistocene terrace deposits and Recent alluvium. Angular unconformities characterize most of the boundaries between these formations, but disconformable relationships appear to exist locally. As a result of two or more major periods of deformation, the rocks are highly folded, with broken anticlines and dips that vary from horizontal to vertical. Overturning occurs at least at one place. Compressive stresses from the north appear to have been the most active agents of deformation; but the block-type San Gabriel fault, which runs through the Southern part of the Humphreys Station area, may have had some tilting effect upon the beds throughout this whole region. In general, the structural trend is northwest-southeast. Broad folds in the Mint Canyon formation plunge northwestward at low to moderate angles. Post-Modelo thrusting from the north has developed minor folds on the limbs of the major folds. Superimposed on the highly folded formations are the later formations, themselves slightly folded. As a result of Quaternary erosion, the topography is one of late youth early maturity; with maximum relief in the mapped area of about 1000 feet.