ABSTRACT

The middle Tertiary (Oligocene-Early Miocene) strata of the San Emigdio Range in southern California were deposited along a shoreline oriented north-northeast to south-southwest, adjacent to a tectonically active highland that formed the southern margin of the San Joaquin Basin. Abundant storm deposits (sandstone and conglomerate) are preserved in the San Emigdio, Pleito, and lower Temblor Formations. Extensive lateral exposures allow recognition of fair-weather and storm-dominated facies in contemporaneous nearshore, inner-shelf, and outer-shelf deposits.

The nearshore deposits consist of coarse-grained progradational sequences that are similar to those described by Clifton (1981) in Miocene rocks of the nearby Caliente Range. Each complete sequence consists of, in ascending order, a basal transgressive lag, followed by inner-shelf, surf-zone, longshore-trough, and beach-foreshore facies. The inner-shelf deposits consist of fair- and foul-weather deposits. The former deposits comprise massive, locally pebbly, very fine-grained sandstone, whereas the latter deposits consist of alternating intervals of cross-stratified (landward-oriented) conglomerate and hummocky cross-stratified fine-grained sandstone. The outer-shelf deposits are composed of massive, very fine-grained to silty sandstone, and sequences of mollusk beds and hummocky cros -stratified sandstone. The massive outer-shelf sandstones were deposited during periods of dominantly fair weather. The sequences of hummocky cross-stratified sandstone were deposited during and immediately after storms.

The outer-shelf storm deposits apparently were preserved on an event-by-event basis, whereas the inner-shelf storm deposits were preserved in amalgamated packages. Unmodified storm deposits are rare in the nearshore deposits, owing to the reworking capabilities of high-energy nearshore processes.

Reconstructions of wave conditions based on Airy wave theory, paleogeographic constraints, and storm facies suggest that peak storm waves were more than 5 m high, with periods of 7-9 seconds. The study area was situated at the end of a long fetch, in a favorable position to receive the full brunt of northwesterly wind-driven waves.