Miocene geology of the east-central portion of the San Rafael wilderness, Santa Barbara County, California

Abstract

In the remote east-central San Rafael Wilderness, Santa Barbara County, California a sequence of Oligocene to Miocene strata was mapped in detail in order to determine stratigraphic and structural relationships between individual rock units. The oldest rocks in the area are of Late Cretaceous age and are interpreted to have been deposited in a submarine- fan environment. These rocks were uplifted, slightly folded, and eroded prior to deposition of unconformably overlying middle Tertiary rocks. During late Oligocene to early Miocene time, continued erosion was accompanied by the formation of alluvial fans, as represented by the light-olive-gray cobble conglomerate and lithic arkosic sandstone of the Simmler Formation. Near the beginning of the Miocene, the area subsided. This allowed the sea to transgress and deposit the shallow-water marine arkosic sandstone of the Vaqueros Formation (Zemorrian to early Saucesian). Continued rapid subsidence accompanied by transgression allowed for the rapid deposition of the Rincon Shale (early Saucesian to Saucesian) and the undifferentiated member of the Branch Canyon Sandstone (early Saucesian to Saucesian) in a submarine-fan environment. The Rincon contains foraminifera which are suggestive of a lower bathyal environment. The interchannel and slope deposits of the Rincon consist of light- to dark-gray mudstone with some interbedded yellowish-gray, medium-grained, arkosic sandstone, whereas the submarine-fan channel deposits of the Branch Canyon consist of very light-gray, medium-grained, arkosic sandstone with very minor thin interbeds of mudstone. The interfingering of these two formations and the westward pinching out of the Rincon suggest channel migration to the west. Conformably overlying the Branch Canyon in gradational contact is an unnamed Miocene unit (Relizian) which contains interbedded yellowish-gray, medium-grained, arkosic sandstone; pale yellowish-brown, fossiliferous limestone; and poorly resistant yellow-brown mudstone. These rocks represent channel (only near the base), interchannel and basin-fill deposits. A facies change occurs laterally in a westward direction between this unit and the undifferentiated member of the Branch Canyon Sandstone suggesting westward channel migration. The yellow-brown mudstone and minor interbedded sandstone of the undifferentiated Monterey Shale (Relizian to Luisian) conformably overlies the unnamed Miocene unit. The rocks in this unit represent basin-fill deposits. Following the deposition of the undifferentiated Monterey Shale the area was uplifted, folded, and faulted. The major east- to west-trending Hurricane Deck syncline, along with a few other folds, deform both middle Tertiary and Cretaceous rocks. Four minor folds deform only Miocene rocks. Faults in the area include the major Nacimiento fault and three minor faults all of which trend northwest. Stream terraces which are not faulted or folded were deposited during the Pleistocene.