Provenance and Diagenesis of Oligocene Sandstones, Southern San Joaquin Basin, California

Michael J. Hayes

Oligocene Vedder sandstones and correlatives from the southern San Joaquin basin provide an opportunity to compare diagenesis resulting from variable provenance, depositional environment, and burial or tectonic history. Sandstones were examined from 15 cores in this basinwide petrographic survey. Oligocene sandstones typically are fine to medium grained, moderately sorted, subangular, and quartzofeldspathic, although sandstones from the western and east-central basin are enriched in potassium feldspar and intermediate volcanic rock fragments, respectively. Detrital compositions are transitional between uplifted continental block and undissected to dissected magmatic arc on provenance diagrams. Sediment provenance included the granodioritic Sierra Nevada-Tehachapi Mountain magmatic arc complex and perhaps granitic terranes to the west. Volcanic detritus was shed from southern and eastern sources.

Diagenesis varies within the basin, partly reflecting local pore-fluid evolution and detrital composition. The effects of variable geothermal gradients, sedimentation rate, and tectonism on diagenesis await evaluation. In the composite basin-wide paragenetic sequence, calcite, dolomite, siderite, and chlorite-smectite are earliest authigenic phases. Variable cement and compaction relationships indicate nonuniform depth and timing of cementation. Early cements inhibited subsequent diagenesis and compaction. With deeper burial, plagioclase, potassium feldspar, and carbonates dissolved, pore-filling kaolinite precipitated, plagioclase was albitized and zeolitized, and late-stage carbonates crystallized. Further geochemical analyses will better define composition and origin of authigenic hases and document reaction progress. Cement zones and sources may be identified and spatially and temporally linked in an attempt to constrain scales and rates of mass transfer of calcium basin wide.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.