The Origin of Folded Chert Horizons in the Monterey Formation, Lions Head, California

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Education



Major Advisor

Walter S. Snyder


Small scale folds that deform the chert rich horizons in siliceous facies of the Monterey Formation, in Santa Maria Basin, at Lions Head, California are of tectonic origin. Folding is well developed in the chert-dominated horizons and dies out a short distance into the adjacent siliceous mudstones. This style has been attributed to gravity-driven, penecontemporaneous folding, but a tectonic origin is evidenced by the dominantly brittle deformation of the competent chert layers. Mechanically, the folds formed through complex interaction between fracture and flexural- slip and flexural-flow. Opal-CT and quartz beds display brittle fractures and rotated fracture blocks that responded to early shortening. Variable fold styles, typical of disharmonic folding, are displayed in association with well developed slip surfaces. Dilation breccia and void space occur in the hinges and reflect volumetric problems developed during fold propagation. Layer parallel shortening, manifested as thickening and pressure solution, dominated the deformation in the mudstone horizons. After a period of initial shortening, the entire Monterey section began to fold. The highly layered, fractured, chert horizons acted partially as slip zones during the development of the large scale flexural-slip folds. continued folding in the chert horizons generated the variable fold styles that tend to have a consistent vergence. Subsequent diagenesis has partially healed the fractures and slip surfaces, creating the appearance that the chert beds deformed as a ductile medium during folding.

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