Geologic map and digital database of the Yucaipa 7.5' quadrangle, San Bernardino and Riverside Counties, California

The Yucaipa 7.5' quadrangle is located at the southeastern margin of the San
Bernardino Basin, an extensional region situated within a right-step-over zone
between the San Jacinto and San Andreas Fault zones. The quadrangle is traversed by
several faults of the San Andreas system, including (from oldest to youngest) the
Banning Fault and the Wilson Creek, Mission Creek, Mill Creek, and San Bernardino
Strands of the San Andreas Fault.
The Mill Creek Strand of the San Andreas Fault is the easternmost strand of the San
Andreas in the Yucaipa quadrangle. It separates granitic and metamorphic rocks of
the San Bernardino Mountains block from a thin slice of similar rocks on Yucaipa
Ridge, and thus has only a small amount of strike-slip displacement.
The Wilson Creek Strand traverses Yucaipa Ridge and converges toward the Mlll Creek
Strand in the Santa Ana river Canyon. The fault has juxtaposed an igneous and
metamorphic complex (Wilson Creek block) and overlying nonmarine sedimentary rocks
(Mill Creek Formation of Gibson, 1971) against rocks of San Bernardino
Mountains-type, and thus has significant strike-slip displacement.
The Mission Creek Strand is inferred to lie beneath Quaternary surficial deposits
along the southwestern base of the San Bernardino Mountains. This fault is the
major strand of the San Andreas Fault zone, and has juxtaposed crystalline rocks of
San Gabriel Mountains-type (including Pelona Schist overlain by the Vincent Thrust
and associated upper-plate crystalline rocks) against the Wilson Creek block and the
San Bernardino Mountains.
The San Bernardino Strand defines the modern trace of the San Andreas Fault. The
strand forms primary fault features in all but the youngest Quaternary surficial
units, and is thought to have evolved in the last 125,000 years or so based on
regional fault relations.
Complications within the San Andreas Fault system over the last several hundred
thousand years have created a landscape setting in which Quaternary surficial
materials of the Yucaipa quadrangle have accumulated. Crustal extension throughout
the San Bernardino Basin region led to uplift of the Crafton Hills block and
down-dropping of the Yucaipa Valley region on faults of the Crafton Hills and
Chicken Hill complex. Subsequent middle and late Quaternary streamflows deposited
several generations of axial-valley and alluvial-fan sediment in the down-dropped
lowlands. These deposits and the older San Timoteo beds they overlie record the
history of Quaternary fault movements, and form reservoirs for ground water in the
Yucaipa quadrangle.
Digital Data:
The geologic database of the Yucaipa 1:24,000-scale 7.5' quadrangle, San Bernardino
and Riverside Counties, California, was prepared by the Southern California Areal
Mapping Project (SCAMP), a regional geologic-mapping project sponsored jointly by
the U.S. Geological Survey and the California Geological Survey. The database was
created in ARC/INFO (Environmental Systems Research Institute, ESRI), and includes
the following files: (1) a readme.txt file, (2) this metadata file, (3) coverages
containing geologic data and station-location data, (4) associated INFO attribute
data files, (5) a browse graphic (.pdf) of the geologic-map plot and map-marginal
explanatory information, (6) a PostScript graphics file of the geologic-map plot
with map-marginal explanatory information, and (7) .pdf text files describing the
map units of the Yucaipa quadrangle (Description of Map Units) and their geologic
age and correlation (Correlation of Map Units).