Stratigraphic analysis of the Branch Canyon Formation in the Sierra Madre, Santa Barbara County, California

The Branch Canyon Formation is exposed in the Caliente Range, Sierra Madre, and Sespe Creek area. In the Sierra Madre, the Branch Canyon Formation conformably overlies the Hurricane Deck Formation, overlies and interfingers with the undifferentiated Monterey Shale, and is conformably overlain by the Santa Margarita Formation. Within the formation eight lithosomes are recognized which include the following: lithosome A (conglomerate), lithesome B (fine- to coarse-grained to pebbly, abundantly fossiliferous sandstone), lithosome C (medium-grained to pebbly sandstone), lithesome D (medium-grained to pebbly, intraclast-bearing sandstone), lithesome E (fine- to medium-grained, structureless sandstone), lithesome F (fine- to medium-grained, laminated sandstone), lithosome G (fine- to coarse-grained, red sandstone), lithosome H (mudstone). The lithosomes are interpreted to represent depositional environments on a wave-dominated delta, specifically transition zone (lithosome H), shoreface (lithosomes A, B, C, and E), foreshore (lithosomes A and F), and backshore (lithosome D). Both storm deposition (lithosome B) and fair-weather deposition (lithosomes A, C, E, F, and H) are recognized. Progradation of a delta began approximately 17.5 Ma in the area of the present-day Caiiente Range and migrated westward and southward until 10 Ma, at which time a eustatic rise in sea level resulted in deposition of the offshore shelf sediment of the Santa Margarita Formation over the Branch Canyon Formation in the Sespe Creek area and Sierra Madre. An intraformational unconformity is present within the Branch Canyon Formation in the Sierra Madre. The unconformity is marked by a reddish-orange unit (lithosome G) in the eastern portion of the study area. This hiatus occurs at approximately the same time (between 11 and 15 Ma) as unconformities in the Caliente Range to the north, the Cuyama Badlands to the east, and the Sespe Creek area to the south. It is possible that initiation of movement on the San Gabriel fault, sometime between 14 and 12 Ma, is related to the regional uplift and erosion that produced the unconformities.