Chaotic structure and possible décollement relations, previously little known in Cenozoic rocks of the Coast Ranges, have recently been exposed 25 miles southeast of San Francisco, California. Orderly Eocene sandstone-mudstone sequences are interrupted by chaotic zones consisting of disordered mudstone with scattered and rotated bodies of sandstone. It is believed that the chaotic structure resulted in part from Eocene submarine sliding. After the Eocene rocks were moderately folded, Miocene strata were deposited unconformably upon them. Pliocene-Pleistocene thrusting and folding moved the Miocene strata northeastward relative to the underlying Eocene rocks, apparently producing a surface of décollement. Continued folding involved the décollement surface itself, increased the structural complexity of the chaotic zones, and locally overturned intact strata.

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