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.
Description:
Includes bibliographical references (pages 132-137)
California State University, Northridge. Department of Geological Sciences.