NEW GEOLOGIC MAP OF THE SANTA BARBARA COASTAL PLAIN AREA, SOUTHERN CALIFORNIA, REFINES UNDERSTANDING OF LATE CENOZOIC DEFORMATION

A new 1:24,000-scale digital geologic map of the Santa Barbara coastal plain provides refined stratigraphic and structural information for interpreting the timing and style of late Cenozoic deformation in the southern part of the western Transverse Ranges. The map area includes the southern flank of the Santa Ynez Mts, underlain by steeply dipping Eocene to Miocene marine to nonmarine sedimentary rocks, and the adjacent coastal plain, underlain by Miocene to Pleistocene marine sedimentary rocks and Quaternary surficial deposits. The coastal plain, home to about 200,000 people living within the Santa Barbara urban corridor, is transected by numerous potentially seismogenic structures of the Santa Barbara fold-and-fault belt.

Newly recognized unconformities in late Cenozoic strata that underlie the coastal plain suggest that contractional deformation and uplift in the area began at about 8-6 Ma. Middle Pleistocene uplift of the Santa Ynez Mts is reflected by conglomeratic alluvial-fan(?) deposits that locally interfinger with and conformably overlie marine sandstone of the Santa Barbara Fm as far west as Goleta. New paleontologic data and previous age constraints bracket the age of the Santa Barbara Fm between 790 and 400 ka; pronounced contractional deformation and uplift in the area followed deposition of this unit. Such deformation continued into the late Pleistocene, represented by several WNW-trending anticlines and synclines that fold older alluvial deposits and are expressed by aligned ridges or hills and valleys or swales.

Exposed south-dipping reverse faults in the area are flanked by asymmetric north-vergent folds, and similar blind reverse faults are inferred to underlie other north-vergent folds on the plain (e.g., Mission Ridge anticline). We have delineated several previously unmapped fault strands that cut Pleistocene deposits (e.g., splays of the More Ranch fault zone in western Santa Barbara), but we were unable to confirm the presence of some previously mapped faults (e.g., west Mission Ridge fault). Superposed slickenlines on exposed fault surfaces indicate a regional, progressive change in dominant fault slip from normal to strike-slip and reverse-oblique movement, consistent with existing models for late Cenozoic transrotational tectonics in the western Transverse Ranges.