--> Abstract: Structure in the Santa Cruz Mudstone at Davenport, CA and its Relation to San Gregorio Fault Tectonics, by R. K. Smith and L. Schipper; #90920 (1999).

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SMITH, ROBERTA K., Smith-Evernden Associates, and LOUIS SCHIPPER, RMC Lonestar, Davenport, CA

Abstract: Structure in the Santa Cruz Mudstone at Davenport, CA and its Relation to San Gregorio Fault Tectonics

Coastal Davenport, California, lies two miles northeast from the offshore San Gregorio fault. The upper Miocene Santa Cruz mudstone underlies the area. A two-mile N25-30°E transect up San Vicente Canyon from Davenport beach to the mudstone's base reveals abundant sharply intersecting fracture, fault, and bedding planes.

Approximately 500 attitudes revealed the most pervasive fracture strikes N60°W and dips 80°NE. Three other pervasive fractures average N32°E, 70°SE; N-S to N20°W, 85°NE; and N24°W, 60°SW. Faults demonstrated no compelling orientations nor relational patterns.

Fractures approximately N20°W parallel the San Gregorio fault. The fault's principal axis of compression is N25°E and the pervasive N32°E fractures may relate to this. The N60°W and N32°E strikes equal 90° and, classically, halfway between them is the San Gregorio fault strike.

The Santa Cruz mudstone's gentle oceanward bedding dips (5-15°) result from the Ben Lomond Mountain Block's uplift on its NNE side (along the N30°W-trending Ben Lomond fault) and SSW oceanward tilt. The pervasive steep NE fracture dips (and SE dips) may also relate to this uplift.

The pervasive fractures also relate to the Block's topography and drainage. Straight N25°W, steep V-shaped San Vicente Canyon's trend bifurcates sharply almost 90° 2 1/2 miles inland, to nearly parallel the N60°W fractures. Other steeply dipping fractures parallel to the canyon sides (e.g., the N32°E group) provide bedrock slide planes and may account for the canyon's shape (and instability) along with downcutting San Vicente Creek draining the uplifted Ben Lomond Mountain Block.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California