ABSTRACT: Diagenetic Pathways of Sandstone-Shale Sequences: An Example from a Shelf-Turbidite System, Southern San Joaquin Basin, California

Ronald C. Surdam, Thomas L. Dunn, Donald B. MacGowan

Sandstone and shale sequences undergo progressive organic and inorganic diagenetic transformations from the time of erosion through deposition, early burial, and down to depths corresponding to the onset of metamorphism. Many of the important reactions are mediated by organic-inorganic interactions, which in part affect the diagenetic pathway of a sand-shale sequence from early burial until incipient metamorphism. Factors governing this evolutionary path include provenance and depositionally controlled compositional and textural elements; near-surface

redox reactions in the zone of early burial (sediment-water interface to burial temperatures of about 80°C), which control the composition of early carbonate cements and the fate of iron in the system; organic-inorganic interactions within the zone of intermediate burial diagenesis (about 80 to 160°C), which control carbonate and aluminosilicate mineral stability; deep diagenetic reactions, including abiotic, thermal sulfate reduction, carbonate mineral alteration, and quartz cementation; and telogenetic alteration including meteoric influx and renewal of near-surface processes due to uplift, exposure, and/or base level fluctuation. These factors are represented as divides on a diagenetic pathway diagram. The divides are arranged depending upon the presence, absence, or rela ive abundance of critical components. Critical components include sulfate, iron, and organic material (early burial diagenesis); bicarbonate, carboxylic acid anions, and iron (intermediate burial diagenesis); and sulfate, iron, and hydrocarbons (deep burial diagenesis). An example of burial diagenesis in the Stevens and Santa Margarita sandstones in the San Joaquin basin of California illustrates the pathway diagram.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990