Recognition of a Middle Eocene Unconformity in Transverse Ranges, Utilizing Satellite Imagery and High-Altitude Photography

K. M. Campion, S. R. Morgan, Z. Berger

Integration of satellite imagery and high-altitude photography with ground observations has helped document the existence of an unconformity in uppermost Eocene rocks in the Transverse Ranges of California. The surface is expressed locally as an angular unconformity that can be mapped from the western Transverse Ranges near Gaviota Gorge to Matilija Hot Springs north of Ojai, California. This unconformity separates the Gaviota Formation from the Sacate Formation in the western Transverse Ranges and exists within the Coldwater Formation from San Marcos Pass to Matilija Hot Springs. Rocks below the unconformity are variable in age, but are generally older near Matilija Hot Springs than on the west near Gaviota Gorge. This indicates greater truncation below the unconformity o the east, in a direction up the paleoslope. This unconformity formed about 39.5 Ma and coincides with a sequence boundary and lowstand of sea level interpreted from middle Eocene sections elsewhere.

Low-amplitude folds in middle Eocene strata are accentuated by the regional unconformity. Middle Eocene sections above anticlinal features are relatively thin, owing to truncation by the unconformity. In contrast, relatively thick sections of middle Eocene strata are present in synclines, such as at San Marcos Pass. Upper Eocene rocks are concordant with the unconformity surface, which locally brackets the timing of deformation. Previously, this surface was unrecognized by field mapping as an angular unconformity; facies changes had been used to explain discontinuities across the unconformity and the abrupt lateral changes in lithology. The geometry of this angular unconformity on satellite images is similar to sections that are offset by faulting and indicates the need for field chec s before final interpretations are made. However, in areas with high-angle dips, bedding geometry can be identified using satellite and high-altitude photos. Unconformities and sequence boundaries can be recognized on high-altitude photos by identifying truncation patterns and stratal geometries analogous to those observed on seismic data.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.