Diagenesis of Miocene siliceous shales, Temblor Range, California

Journal of Research of the U.S. Geological Survey
By:  and 



Siliceous Monterey Shale and related shales of the Temblor Range, Calif., are subdivided into three depth-controlled zones characterized by different forms of silica. These are, in descending stratigraphic order: (1) Biogenic opal zone, with remains of diatoms and other siliceous organisms, (2) diagenetic cristobalite zone, and (3) diagenetic quartz zone. Using the top of the youngest marine unit, the overlying Etchegoin Formation, as datum, the transition from biogenic opal to disordered cristobalite occurs within the Monterey Shale of Chico Martinez Creek at -730 m, and the ordered cristobalite-to-microquartz transition at about -2,030 m. Temperatures that prevailed at these transition depths while the sedimentary pile lay at the bottom of the sea are estimated at about 50° and 110°C, respectively. Diagenetic cristobalite manifests, downward through a 1,300-m interval of section, a progressive decrease in its d(101) spacing because of a gradual ordering of its internal structure through adjustments in the solid state. Diagenetic microquartz forms only from well-ordered cristobalite that provides the most appropriate concentration of dissolved silica for precipitation of microquartz. Scanning electron micrographs of the silica mineral in pores of rocks made up of disordered cristobalite show aggregates of well-formed bladed crystals, like those described from deep-sea cherts. The pore silica minerals in rocks made up of ordered cristobalite occur as dendritic growths of poorly formed stubby crystals, and the change in crystal habit could be an external expression of the internal ordering process.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Diagenesis of Miocene siliceous shales, Temblor Range, California
Series title Journal of Research of the U.S. Geological Survey
Volume 3
Issue 5
Year Published 1975
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Description 13 p.
First page 553
Last page 566
Country United States
State California
Other Geospatial Temblor Range
Online Only (Y/N) N
Additional Online Files (Y/N) N
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