BOLES, JAMES R., University of California, Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93106; MARK GRIVETTI, Dames and Moore, Santa Barbara, CA 93111; BILL TRACY, County of Santa Barbara, Department of Public Works, Santa Barbara, CA 93101

Abstract: Calcite Cementation Along the Refugio, Fault: A Link between Tectonism, Fluid Movement and Fluid-Rock Interaction at a Basin Margin

Fluid movement is believed to accompany faulting, but physical evidence is usually lacking. Cementation along the steepdipping, east-west trending Refugio Fault, 30 km west of Santa Barbara, and along similar faults as far east as Goleta, offer a strong case for interrelated faulting and fluid movement. The Refugio fault juxtaposes Oligocene sandstone, with about 300 m of apparent vertical offset, against overlying Miocene shale. Locally, the fault zone is filled with a 1.5 meter thick calcite cement which also extends via small fractures into the adjacent sandstone.

Both in outcrop and thin section the calcite exhibits evidence of multiple episodes of deformation including brecciation, shear bands, and finely spaced twinning in calcite crystals. There is also evidence for multiple fluid flow episodes. Multiple bands of calcite, each about 0.5 to 2 mm wide contain an initial layer of detrital fine sand to silt-size particles of siliciclastic and other detritus. These clasts are clustered along the terminus of the calcite crystals of the preceding band. Calcite nuclei form within the detrital-rich band and grow perpendicular outward as fan-shaped crystals. In some case where the crystals extend through the detrital bands they are off set laterally a few microns. These bands are interpreted to be the result of episodic fluid movement, possibly during faulting.

Preliminary oxygen, carbon, strontium isotopic values, and fluid inclusion data support a basin margin model of low temperature interaction between oxidized groundwater, silicates, and hydrocarbon escaping from the basin flanks.

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