Preliminary Geologic Map of the Santa Barbara Coastal Plain Area, Santa Barbara County, California, USGS OFR 02-126

To update the interpretation of geologic mapping and to achieve a uniform
regional geologic database. Additionally, to provide a geologic map for the
public and geoscience community to aid in assessments and mitigation of
geologic hazards in the Santa Barbara coastal plain region and to provide
sufficient geologic information for land-use and land-management decisions.

This report presents a new geologic digital map of the Santa Barbara coastal
plain area at a compilation scale of 1:24,000 (one inch on the map = 2,000 feet
on the ground) and with a horizontal positional accuracy of at least 20 m.
This preliminary map depicts the distribution of bedrock units and surficial
deposits and associated deformation underlying and adjacent to the coastal
plain within the contiguous Santa Barbara and Goleta 7.5' quadrangles. A
planned second version will extend the mapping westward into the adjoining Dos
Pueblos Canyon quadrangle and eastward into the Carpinteria quadrangle. The
mapping presented here results from the collaborative efforts of geologists
with the U.S. Geological Survey Southern California Areal Mapping Project
(SCAMP) (Minor, Kellogg, Stanley, Stone, and Powell) and the tectonic
geomorphology research group at the University of California at Santa Barbara
(Gurrola and Selting). C.L. Powell, II, performed all new fossil
identifications and interpretations reported herein. T.R. Brandt designed and
edited the GIS database, performed GIS database integration and created the
digital cartography for the map layout.

The Santa Barbara coastal plain is located in the western Transverse Ranges
physiographic province along a west-trending segment of the southern California
coastline about 100 km (62 mi) northwest of Los Angeles. The coastal plain
region, which extends from the Santa Ynez Mountains on the north to the Santa
Barbara Channel on the south, is underlain by numerous active and potentially
active folds and partly buried thrust faults of the Santa Barbara fold and
fault belt. Strong earthquakes that occurred in the region in 1925 (6.8
magnitude) and 1978 (5.1 magnitude) are evidence that such structures pose a
significant earthquake hazard to the approximately 200,000 people living within
the major coastal population centers of Santa Barbara and Goleta. Also, young
landslide deposits along the steep lower flank of the Santa Ynez Mountains
indicate the potential for continued slope failures and mass movements that may
threaten urbanized parts of the coastal plain. Deformed sedimentary rocks in
the subsurface of the coastal plain and the adjacent Santa Barbara Channel
contain deposits of oil and gas, some of which are currently being extracted.
Shallow, localized sedimentary aquifers underlying the coastal plain provide
limited amounts of water for the urban areas, but the quality of some of this
groundwater is compromised by coastal salt-water contamination. The present
map compilation provides a set of uniform geologic digital coverages that can
be used for analysis and prediction of these and other geologic hazards and
resources in the coastal plain region.

In the map area the oldest stratigraphic units consist of resistant Eocene to
Oligocene marine and terrestrial sedimentary rocks that form a mostly
southward-dipping and laterally continuous sequence along the south flank of
the Santa Ynez Mountains. Less resistant, but more variably deformed, Miocene,
Pliocene, and Pleistocene marine sedimentary rocks and deposits are exposed in
the lower Santa Ynez foothills and in the coastal hills and sea cliffs farther
south. Pleistocene and Holocene surficial alluvial, colluvial, estuarine, and
marine-terrace deposits directly underlie much of the low-lying coastal plain
area, and similar-aged alluvial and landslide deposits locally mantle the lower
flanks of the Santa Ynez Mountains.

Structurally, the Santa Barbara coastal plain area is dominated by the Santa
Barbara fold and fault belt, an east-west-trending zone of Quaternary, partly
active folds and blind and exposed reverse and thrust faults. The dominant
trend of individual structures within the belt is west-northwest -- slightly
oblique to the overall trend of the fold and fault belt. A conspicuous
exception, however, is the More Ranch fault system, which strikes
east-northeast across the fold and fault belt at a high angle to the dominant
structural grain. Based on a limited number of observations made at rare
fault-plane exposures, most of the map-scale faults in the coastal plain area
are moderately to steeply dipping and have most recently experienced reverse or
reverse-oblique slip. Multiple sets of slip lineations, including strike-slip
and, rarely, normal-slip striae, are commonly preserved on the fault planes,
however, indicating that many of the faults have a varied, complex movement
history.

Several folds within older alluvial deposits have strong geomorphic expression
that is consistent with a youthful age of deformation; commonly anticlines are
coincident with elongate ridges or hills whereas synclines coincide with
valleys or swales. The most dramatic example of such a geomorphic-structural
correlation is Mission Ridge just north of downtown Santa Barbara, which is
coincident with an anticline that is paired on its north side with a syncline
that roughly follows a linear valley. On the basis of several lines of
geomorphic evidence previous investigators have inferred that the Mission Ridge
upwarp is a fault-related fold that has propagated westward, reflecting
westward propagation of the Mission Ridge fault and resulting in progressive
westward deflection of Mission Creek. Several folds in the map area, including
the Mission Ridge anticlinal upwarp and folds just west of the Santa Barbara
Harbor, are inferred to be underlain by blind reverse and thrust faults. Other
fold axes on the coastal plain are parallel to adjacent fault traces, and in
such cases the fold on the apparent upthrown, hanging-wall side of the fault is
typically an asymmetric anticline whose steeper limb faces the fault. Nearly
all asymmetric anticlines in the map area have northward vergence, suggesting
that most associated blind reverse-thrust faults are dominantly southward
dipping similar to the exposed faults and, thus, have accommodated northward
tectonic transport of their hanging-wall blocks. Such structural geometry is
consistent with fault-propagation folding and, together with evidence of
dominant reverse fault slip, implies that faults in the map area have
accommodated significant contractional strain during the Pleistocene.

Most of the structures in the fold and fault belt deform deposits as young as
middle to late Pleistocene, but many of the west-northwest- striking faults in
the northwest part of the map area do not cut the oldest alluvial deposits or
they offset such deposits a significantly lesser amount than the underlying
bedrock units. These relations, and a moderate angular discordance that
locally exists between the middle Pleistocene Santa Barbara Formation and
overlying older alluvial deposits, indicate that pre-alluvial, possibly middle
Pleistocene, deformation occurred locally along structures that were partly
reactivated later in the Pleistocene. An erosional angular unconformity that
separates the Sisquoc Formation and older units from the Santa Barbara
Formation and partly coeval deposits suggests that significant uplift and
deformation occurred in the coastal area in the Pliocene. One or more of these
earlier deformational episodes may have been coeval with the formation of
numerous northwest- trending folds and faults in the Monterey Formation along
the sea cliffs between Santa Barbara Point and Arroyo Burro; these structures
clearly predate unconformably overlying, cliff-capping marine terrace deposits.
Possible age-equivalent folds also deform the Monterey in Sycamore Canyon area
in the east part of the map area. The late Pleistocene marine terrace
deposits, although uplifted and locally warped or gently folded, are clearly
not as strongly deformed as the older Pleistocene deposits and underlying
bedrock, and no significant deformation has been recognized in the mapped
Holocene deposits despite the historic earthquake activity in the region.
Collectively, these various structural age relations imply that deformation in
the coastal plain area was most pronounced during the Pliocene and (or)
Pleistocene prior to formation of the marine terraces in the late Pleistocene.

Compilation scale: 1:24,000

Base maps used are the U.S. Geological Survey 7.5 minute topographic
quadrangles for Goleta, Calif. and Santa Barbara, Calif.

Mapping is continuing in adjacent quadrangles and a more complete final map of
this area at an identical scale may eventually be produced.