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Silica Diagenesis in Santa Cruz Mudstone, Late Miocene, California

Dallilah El-Sabbagh

The silica-rich upper Miocene Santa Cruz Mudstone is similar to the Miocene Monterey Formation. Previous studies have suggested the Santa Cruz Mudstone was not buried deeply nor had it undergone extensive diagenesis. Because opaline diagenesis is temperature dependent, I examined the silica diagenesis of the Santa Cruz Mudstone using scanning electron microscopy and x-ray diffraction methods to study its burial history.

In a series of samples from Santa Cruz to Davenport, California (over 16 km), opal-CT is the dominant silica phase present and clay minerals are notably absent. The d(101)-spacing values of opal-CT range from 4.11 A (Santa Cruz area) to 4.06 A (north of Santa Cruz), exhibiting the complete range of d(101)-spacing values found in opal-CT zones.

Scanning electron micrographs of crystalline microtextures show rosettes of opal-CT (lepispheres) in cavities of samples with medium to high d(101)-spacing values. The morphology of lepisphere crystallites grades from bladed to spiny with decreasing d(101)-spacing values, reflecting an internal crystal ordering with increased diagenesis. Further diagenetic changes occurred in a sample with 4.06 A d(101)-spacing where incipient quartz crystals signal the initial conversion of opal-CT to microcrystalline quartz.

Silica diagenesis demonstrates that burial temperatures surpassed the range of opal-A to opal-CT conversion and approached conversion temperatures (55°C to 110°C) of opal-CT to microcrystalline quartz. The conversion occurred when the Santa Cruz Mudstone was buried over 1,900 m (depth calculated from a geohistory diagram). This burial temperature brings the Santa Cruz Mudstone within the oil generation window, and could account for the presence of hydrocarbons in the unit.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.