Chertification in the Monterey Formation of California

Conference · Tue May 01 00:00:00 EDT 1990 · AAPG Bulletin (American Association of Petroleum Geologists); (USA)

OSTI ID:7033835

Behl, R J ^[1]

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1. Univ. of California, Santa Cruz (USA)

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.

• OSTI ID: 7033835
• Report Number(s): CONF-900605--
• Journal Information: AAPG Bulletin (American Association of Petroleum Geologists); (USA), Journal Name: AAPG Bulletin (American Association of Petroleum Geologists); (USA) Vol. 74:5; ISSN AABUD; ISSN 0149-1423
• Country of Publication: United States
• Language: English

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CALCIUM COMPOUNDS
CALIFORNIA
CARBON COMPOUNDS
CARBONATE MINERALS
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CHALCOGENIDES
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MINERALS
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OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PETROLEUM DEPOSITS
POROSITY
PYRITE
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RESOURCES
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SEDIMENTARY ROCKS
SILICON COMPOUNDS
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TECTONICS
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USA