Paleo-Hydrology of the Monterey Formation, California, Based on Structural and Geochemical Characteristics of Veins.

EICHHUBL, Peter, and BOLES, James R.,
Geological Sciences, University of California, Santa Barbara, CA 93106, U.S.A.;

Veins provide insight into the hydrologic conditions of the Miocene Monterey Formation throughout its burial and uplift history. The timing of fracture formation and cementation is inferred based on cross-cutting relations with folds, on the orientation of geopetal, detrital fracture fill, and on isotopic and fluid inclusion characteristics of vein cement. Veins which predate folding contain geopetal, detrital infill which has been tilted with the bedding during regional folding. Vein dolomite with carbon and oxygen isotopic values of up to +15 and +1 per mill PDB, respectively, indicates precipitation in the zone of methanogenesis at shallow burial and low temperatures. Continued cementation shows increasingly lighter isotopic values indicating vein cementation and fluid flow during progressive burial up to maximum burial conditions. This trend suggests that early fluid flow is driven by compaction.

Veins which postdate folding form during uplift of the basin margins and are spatially and genetically related to mesoscale faults. Hydrocarbon inclusions relate fluid flow to hydrocarbon maturation and migration. Rates of fluid flow on the order of meter per second are obtained by calculating the minimum flow velocity necessary for entrainment of centimeter-sized rock fragments found in geopetal, graded fracture fill. Fluid flow along one particular fault causes a temperature anomaly of about 15 degrees Celsius above ambient temperatures based on the oxygen isotopic composition of dolomite cement in fault-related fractures. The strontium isotopic composition of carbonate vein fill suggests upward flow distances in late fracture systems of a few hundred meters.