Deposits of a middle Tertiary convulsive geologic event, San Emigdio Range, southern California

The upper Eocene through lower Miocene rocks of the San Emigdio Range in southern California present a remarkably well-exposed, east-west cross section of a portion of the sedimentary fill of the southern San Joaquin basin. During middle Tertiary time, a complex of west-northwestward prograding fan-deltas deposited the nonmarine Tecuya Formation and interfingering shallow marine sandstones of the Pleito, San Emigdio, and Temblor Formations. A 50-m-thick, 1.6-km-wide wedge of granitic breccia was deposited near the base of the Tecuya in the eastern (proximal) study area. Several kilometers to the west, a zone of large-scale soft-sediment deformation occurs in stratigraphically equivalent, interfingering nearshore and nonmarine deposits. Approximately 6 km to the west of this disturbed zone, in probably equivalent but relatively deep-water deposits of the lower Pleito Formation, there occurs a 15-m-thick, submarine, mass-flow unit. All three of these units apparently were produced by the depositional and deformational effects of a large earthquake or series of earthquakes. Major differences in the large-scale lithologic (facies) packaging of two thick (several hundred meters each) depositional sequences below and above the disturbed zones indicate that the architecture of the basin margin, and thus the related depositional systems, changed drastically after the postulated seismic event(s). The previously extant, stepped basin margin (shelf, slope, basin) was destroyed as the shelf edge foundered. The system of shelf fan-deltas, which occupied the basin margin, was replaced by a system of slope fan-deltas. Thus, although the disturbed zones are volumetrically minor in comparison to the sandwiching depositional sequences, they provide key insights into the long-term evolution of the basin margin.