Depositional Environments of the Oligocene Alegrian Formation, Santa Barbara County, California

M. P. Sgriccia and A. E. Fritsche
Dept. of Geological Sciences, California State Univ, Northridge, Northridge, CA

The Oligocene Alegria Formation, occurring only in the Santa Ynez Mountains of Santa Barbara County, California, was studied along a 16-km stretch of exposures east of Gaviota Canyon. The formation consists primarily of trough cross-bedded sandstone. Interbedded with the cross-bedded sandstone are a few lenticular, graded, thin to medium beds of oyster limestone at and near the base of the formation; a single, tabular set of large-scale, tangential cross beds overlain by a lenticular conglomerate about one-third of the way up the section; and sandstone with thin beds of fossil fragments near the top. Underlying the Alegria is fine sandstone and siltstone of the Gaviota Formation, a shelf to lower shoreface deposit, and overlying the Alegria is reddish sandstone, mudstone, and conglomerate of the Sespe Formation, a nonmarine, fluvial deposit.

The Alegria Formation was deposited mostly in a shallow-marine, wave-dominated environment. The dominant trough cross-bedded sandstone represents deposition in the upper shoreface either by longshore currents or wave currents in the breaker zone. The lenticular oyster limestone beds represent storm beds deposited by gradient currents in channels on the shoreface. The tangential cross beds and overlying conglomerate lens represent a small delta front that was overridden by a subaqueous channel deposit of a small, eastwardflowing river in very shallow marine water. Current direction is presently south, but after 90
of counterclockwise rotation is restored, original flow direction was eastward. An original eastward-flowing current direction indicates the presence of an exposed forearc ridge to the west of the site of deposition during the Oligocene. The sandstone with the thinly bedded fossil fragments was deposited in a backshore environment. Clear evidence of foreshore deposition was not observed. A persistent red mudstone that occurs near the top of the formation may represent the location of an intraformational disconformity.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California