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Mechanical Development of Folded Chert Beds in Monterey Formation, California

Derrick Crowther, Walter S. Snyder

Small-scale folds in the upper siliceous facies of the Miocene Monterey Formation, at Lions Head, California (Santa Maria basin) are of tectonic origin. Folding is well developed in the chert-dominated zones and dies out rapidly in the adjacent siliceous mudstones. A tectonic origin is evidenced by the dominantly brittle deformation of the competent chert layers. Mechanically, the folds formed through a complex interrelationship between fracture and flexural slip. Opal-CT and quartz-chert layers display brittle fractures and rotated fracture blocks that responded to shortening. Thrusting of the chert layers is common in folds where fold propagation was impeded. Dilation breccia and void space occur in the hinges and reflect room problems during development of these dishar onic folds. Subsequent diagenesis has partially healed the fractures and slip surfaces, creating the erroneous appearance that ductile deformation was an important factor in the formation of the folds.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.