--> Abstract: The Nature of the Black Point Spilite, Basement Rock of the Gualala Block, Northern California, by J. R. Phillips and T. B. Anderson; #90935 (1998).
[First Hit]

Datapages, Inc.Print this page

Abstract: The Nature of the Black Point Spilite, Basement Rock of the Gualala Previous HitBlockNext Hit, Northern California

PHILLIPS, J. RICHARD, and THOMAS B. ANDERSON, Department of Geology, Sonoma State University, Rohnert Park, CA 94928

The Black Point Spilite (BPS) crops out near Sea Ranch in northwestern Sonoma County and consists of a series of finegrained, dark bluish to greenish gray, hydrothermally altered, internally-faulted basalt flows. The unit is generally massive, but locally flow bands, dikes, and pillow structures are present. Epidote replaces amygdules and the cores and rinds of the pillows, silica and calcite veins commonly fill minor fractures, and pyrite and jasper are common.

The BPS is in fault contact with the Stewartis Point Member of the Gualala Formation at the south end of Black Point Beach. Most internal faults in the BPS are generally parallel to individual flows in the unit, which dip to the south. The BPS crops out along the sea cliffs for nearly 2 km to the south end of Pebble Beach where it is also in fault contact with highly folded Stewartis Point strata. The northernmost exposure of the BPS is a fault Previous HitblockNext Hit present just south of Smuggleris Cove.

Published paleomagnetic studies indicate that the Previous HitblockNext Hit originated at low latitudes, but the initial tectonic setting is unknown. Whole-rock chemical analysis, when plotted on a TiO2-K2O-P2O5 discrimination Previous HitdiagramTop, suggests that the BPS is an ocean-floor basalt, but its chemistry is different from basalts of the Franciscan Complex. A study including detailed mapping and trace element and REE analysis is in progress to determine more precisely the nature and origin of this enigmatic unit of basalt west of the San Andreas Fault Zone.

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