EVIDENCE FOR METEORIC RECHARGE IN THE SAN-JOAQUIN BASIN, CALIFORNIA PROVIDED BY ISOTOPE AND TRACE-ELEMENT CHEMISTRY OF CALCITE

Calcite cements were analysed petrographically and geochemically to document patterns of meteoric recharge into marine sandstones in the San Joaquin basin, California. Calcite cements in shallow marine sandstones in the eastern basin are pure (>96 mol.% Ca, <2 mol.% Mg, <1 mol.%Fe) relative to low temperature marine calcites (91-94 mol.% Ca) in central basin turbidite sandstones. Calcite cements with volumes >32% and deltaO-18PDB < -10 parts per thousand record early meteoric flushing of marine sandstones. Timing of meteoric infiltration varied on a bed scale; calcite cements with meteoric and marine deltaO-18 and point-counted volumes of 26-27% appear 5 m apart. The purity of low temperature calcite (<70-degrees-C crystallization) may be the most effective regional indicator of meteoric exposure. The magnesium, iron and manganese content of calcite is distinct from reservoir to reservoir. Pore fluids were apparently compartmentalized on reservoir scales (10(8)-10(9) m3) and rates of locally variable dissolution reactions and calcite crystallization exceeded the flushing rates of reservoirs. If meteoric recharge began shortly after the deposition of marine shelf sandstones (approximately 23 Ma), then calcite cements record downdip infiltration at an average rate of 1 mm/year, less than some recorded rates of gravity-driven flow.