Subsurface Geology of the San Fernando Valley, California
T. L. Wright1, D. Okaya2, R. S. Yeats3
1Consulting Geologist, San Anselmo, CA
2Southern California Eartquake Center, Univ of
Southern California, Los Angeles, CA
3Oregon
State University, and Hiroyuki, Kyoto University, Japan
Seismic and well data from the San FernandoValley (SFV) document evolution of that region from mid-Miocene rifting to north-south contraction. Formations in the western SFV subsurface (Cretaceous to Paleogene strata, and Miocene Topanga and Modelo Formations) trace southward to outcrops in the Santa Monica Mountains that constrain faulting along the valley’s south basin edge. Cretaceous strata in the Simi Uplift to the west are >2 kmhigher than equivalent strata beneath the western SFV across a boundary marked by the Chatsworth Reservoir fault, and Neogene thinning and offlap.
The Santa Susana Mountains and south-vergent Santa Susanna
fault (SSF) form the north edge of the SFV and represent Pleistocene
inversion of the East Ventura basin, a Miocene rift feature. Contraction
related to the SSF formed sub-parallel structures to the south
represented by the Mission Hills fault, and active Northridge Hills
anticline and blind
thrust. Beneath these structures, an older
zone
of
ESE-trending, down-to-the-north faults includes the Devonshire and
subsurface Frew faults and marks the northeast edge of the Mio-
Pliocene Oak Ridge-Simi Hills shelf. Southeast and en echelon to
this, WNW-trending Miocene normal faults form the north edge of
the Leadwell granitic high beneath the southeastern SFV.
The Verdugo fault (east edge southern SFV) is a NE-dipping reverse
oblique fault in Pacoima oil field (east end Northridge Hills
anticline). Projected northwest, this fault coincides with the abrupt
transition eastward from the Santa Susana Mountains to the Sylmar
basin, where Quaternary sediments exceed 3-km thickness, and from
the SSF to basement-involved reverse faults north of Sylmar basin.
This may reflect a postulated Miocene connection with the northern
San Gabriel fault as a locus of San Andreas displacement 12 to 6 Ma,
and of mid-Miocene transrotation. Although this alignment may still
represent an important crustal boundary, the near-surface Verdugo
fault north of Pacoima field now curves west to join the Mission Hills
fault, indicating absorption of dextral slip by thrusting and folding in
the Mission Hills zone
.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California