--> ABSTRACT: Chertification in the Monterey Formation of California, by Richard J. Behl; #91003 (1990).

Datapages, Inc.Print this page

ABSTRACT: Chertification in the Monterey Formation of California

Richard J. Behl

Quartz cherts of the Miocene Monterey Formation of California are complex rocks that reflect varied environments, mechanisms, and timing of chertification. Because cherts do not reflect just the simple prograde diagenesis of siliceous sediments, it is important for sedimentologists and petroleum explorationists to understand their origin. Both early and late forming cherts exist; however, only cherts formed prior to tectonic deformation can become fractured reservoirs so important to oil production from the Monterey.

Monterey cherts occur as beds, nodules, lenticular bodies, spheroids, dikes, and breccias. All are characterized by their hardness, density, vitreous luster, and smooth, conchoidal fracture. These properties result from localized addition of silica (silicification), or by closed-system changes in already siliceous rocks. Silicification includes (1) replacement of a compositionally distinct host, and (2) pore-filling cementation. Additional silica may be supplied to cherts from adjacent, interlayered organic-rich and clayey lamina. Closed-system chert formation results from stratal compaction and porosity loss caused strictly by in-situ silica dissolution and reprecipitation during the opal-CT to quartz phase change; silicification can occur before or after the change.

Rates and textures of chertification are controlled by local primary composition. Microscopically, quartz crystallite size varies inversely with the concentration of disseminated organic matter, pyrite, and secondary dolomite euhedra, reflecting different rates of quartz nucleation and growth. Megascopically, quartzification of opal-CT chert occurs preferentially along specific lamina, regardless of the degree of deformation, indicating that primary composition is the principal control of quartz chert formation.

An integrated field, petrographic and isotopic study of cherts in the Monterey Formation will help our understanding of these enigmatic rocks.

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