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SCHULTZ, J.L., J.R. BOLES, and G.R. TILTON, University of California, Santa Barbara, CA

Title: ABSTRACT: Isotopic Evolution of Strontium in Carbonate Cements from the Stevens Sandstone: Implications for Calcium Mass Transfer in the San Joaquin Basin, California

Strontium isotopes have been used as geochemical tracers of calcium mass transfer and pore water evolution in the south-central San Joaquin basin, California. Carbonate cements in the Miocene Stevens sandstone record a systematic decrease in the 87Sr/86Sr ratios of the pore water with time (temperature) since deposition in response to plagioclase alteration, and indicate three phases in the evolution of central basin pore waters: (1) Early dolomite cements (< 35 degrees C) have Sr ratios consistent with their formation from entrapped seawater. (2) Early calcites (35 to 75 degrees C) record decreasing Sr ratios with increasing temperature. 87Sr/86Sr values of the earliest calcites vary significantly by geographic location, progressing from lower, more evolved water ratios in the dee er central parts of the basin (0.7080 at 40 degrees C, Coles Levee field) to higher, less evolved values toward the flanks (0.7087 at 40 degrees C near Fruitvale). This eastward progression toward higher ratio pore waters may be recording a flux of more evolved water from the center of the basin upward and outward toward the flanks. At Coles Levee, the input of low ratio Sr implies large-scale mass transfer, as at least 1 km of additional depth is required to reach temperatures at which plagioclase alteration occurs. (3) At > 80 degrees C, all calcite cements have ratios in the 0.7072-0.7075 range, and appear strongly influenced by plagioclase alteration. At Coles Levee, this low-ratio Sr can be accounted for by local plagioclase dissolution, and implies a significant decrease in regi nal mass transfer in the later stages of the basin's history.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)