Abstract: Pre-Miocene Locations of Gualala and Sierra Azal Blocks, Northern California

McLAUGHLIN, R. J., R. C., JACHENS, C. M. WENTWORTH, KRISTIN McDOUGALL, and W. V. SLITER, U. S. Geological Survey, Menlo Park, CA; andW. P. ELDER, California Academy of Sciences, San Francisco, CA

Restored offsets of Mesozoic basement rocks across faults of the San Andreas system permit a pre-Miocene location of the Gualala block in close proximity to the Sierra Azul block and related rocks of the San Francisco Peninsula. Rocks in these blocks were deposited, buried and deformed in lower slope basin settings along the Late Cretaceous accretionary margin, followed by northward translation and east-vergent thrusting as the San Andreas fault evolved. Stratigraphic relations in Cretaceous-Eocene sections of the Sierra Azul and Gualala areas are obscured by complex folding and attenuation and Cretaceous rocks in both areas are laumontized. In light of this, models linking these blocks must consider the following regional geologic constraints: 1) Conglomerate class suites and paleocurrent data suggest that Campanian rocks of the Gualala section had a proximal granitic source on the W-SW and an ophiolitic source to the east. 2) The Montara and Ben Lomond blocks, which are possible sources of Gualala detritus, have Paleocene-Eocene strata with NW- E-directed paleocurrents. 3) The Cretaceous section of the Sierra Azul block lacks distinctive quartz-bearing gabbro clasts present at Gualala and the basal Eocene section of the Sierra Azul area locally includes blueschistgrade Franciscan detritus, which is absent from equivalent rocks at Gualala. Meager paleocurrent data for the Sierra Azul area suggest westward paleoflow. 4) Mottled foraminiferal mudstone of early Eocene (zones P7- early P9) age, deposited at ³ 2,000 m, marks the former slope base and is found in the Sierra Azul and Gualala sections, elsewhere on the San Francisco Peninsula, the southwest side of the Diablo Range and Salinia, but not in the central Diablo Range, the Great Valley, or the Montara and Ben Lomond blocks. 5) Rudist-bearing Campanian strata of the Gualala block indicate deposition at warmer latitudes south of the Transverse Ranges, whereas somewhat older Campanian strata of the Sierra Azul block include fewer warm-water taxa.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California