Depositional environments of the Oligocene Alegria Formation, Santa Barbara County, California

The Oligocene Alegria Formation, which crops out only in the Santa Ynez Mountains of Santa Barbara County, California, was studied along a 16-km-long stretch of exposures east of Gaviota Canyon. Five stratigraphic sections were measured in a total of 40 days using the tape and Brunton method. The Alegria Fonnation consists primarily of trough cross-bedded sandstone. The base of the Alegria is defined by beds of gray, oyster-bearing arkose. Two sets of lenticular, high-angle, planar cross beds occur within the lower third of the section. Overlying one of the cross bed sets is a l 0- cm-thick, lenticular conglomerate. The top of the formation is characterized by bedded sandstone containing fossil fragments . Underlying the Alegria Fo1111ation are lower shoreface deposits of very fine-grained sandstone with interbedded siltstone of the upper Gaviota Formation. Overlying and interfingering with the upper part of the Alegria Formation are flu vial deposits of the lower Sespe Fonnation, which consist of grayi shorange sandstone and red mudstone. These sandstone and mudstone beds interfinger with the trough cross-bedded sandstone of the Alegria Formation. Deposition of the Alegria Formation occurred 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-bearing arkose beds represent storm beds deposited in the lower shoreface. The lenticular, planar cross-bed sets and overlying conglomerate represent a small delta front and overriding, subaqueous channel, respectively, that were deposited by a small river. The bedded sandstone that contains fossil fragments was deposited in a backshore environment. Clear evidence of foreshore deposition was not observed. Prior to Miocene rotation, the study area was south of its present location and was oriented with a general north-south trend. During the late Eocene, deposition in the study area took place in the lower shoreface environment of a wave-dominated coast that was occasionally affected by storms. By the early to early medial Oligocene the environment regressed from lower shoreface to upper shoreface. A small, east-flowing river dominated the upper shoreface for a brief period during this time and is evidence that a shoreline existed to the west of the study area. A shoreline existed within the study area at various times as regression continued during the late Oligocene. A disconforrnity that exists within the Sespe Formation and continues through the Alegria Formation to Gaviota Pass was not found within the study area.