A composite section of more than 30,000 feet of Mesozoic and Cenozoic rocks is present in the Nipomo quadrangle, San Luis Obispo County, California, ranging from the characteristic but heterogeneous mixture of Jurassic Franciscan rock types to a few remnant patches of upper Pliocene Careaga Formation.

Mudstone and conglomerate of the Lower Cretaceous Jollo Formation with Buchia pacifica Jeletzky and Jurassic Franciscan group locally containing B. piochii (?)(Gabb) are unconformably overlain by Upper Cretaceous Carrie Creek Formation (new name). Acila demessa from this Upper Cretaceous formation dates the sequence of sandstone, siltstone, and conglomerate west of the Nacimiento fault as middle Coniacian to uppermost Campanian.

The early Tertiary (i.e., Zemorrian to Relizian) marine Vaqueros, Rincon, and Point Sal Formations are thin and discontinuous in most places and display changes in facies from clastic sedimentary rocks to basaltic and andesitic lavas or to tuffs of the Obispo Formation. Middle and upper Miocene rocks of the Monterey (Luisian to upper Mohnian) and Santa Margarita (upper Mohnian to upper Delmontian or “Repettian”?) Formations are characterized by sandy or in places oytser- and echinoid-rich eastern facies and siltstone or silty claystone western facies largely devoid of macro-invertebrate fossils.

The Huasna syncline which lies between the East and West Huasna faults and 35 miles west of the San Andreas fault is the dominant structural feature of the area. Many smaller folds are near the bordering faults.

Although the major Huasna faults and the Nacimiento fault which occurs in the eastern part of the quadrangle are conspicuous features, evidence is not conclusive as to the history of their movement. Axial traces of folds along the West Huasna fault, different rock types or facies relationships and thicknesses of the Monterey Formation exposed on opposite sides of the fault, and offset contacts within the fault zone suggest strike-slip and probably left slip. The East Huasna fault also has a probable strike-slip component because of the marked differences in stratigraphy and facies on its opposite sides, and drag features suggest right slip. Movement along the Nacimiento fault zone may have begun near the end of or after Upper Cretaceous sedimentation. Owing to its apparently long and complex history, several periods or types of movement may have occurred. The last record of movement in this area is post-late Miocene reverse slip.

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