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At Pescadero State Beach, basaltic dikes and sills, pillows and peperites syndepositionally intruded shallow-marine siliciclastic conglomerate, volcanic rudite, arkosic sandstone, sandy dolomite, and rare mudrock of the Vaqueros (?) Formation. At Punta Año Nuevo, pillow basalts erupted onto the seafloor or among fine-grained sediments in a lower bathyal environment. The intrusive and extrusive rocks of these two areas have been previously described as epiclastic breccia or conglomerate.

Peperites form by mixing of magma and wet, unconsolidated sediments. Magma-wet sediment interactions within the Vaqueros (?) Formation are indicated by: (1) jigsaw-puzzle hyaloclastite peperite and intrusive pillows produced by non-explosive hydroclastic fragmentation of magma during intrusion of wet sediment; (2) more fluidal, globular peperites of less obvious origin; (3) large and small scale dikes and sills; (4) fluidized, homogenized sedimentary rocks thixotropically affected by ascending magma which host the intrusive rocks; and (5) disrupted sedimentary blocks plastically deformed by volcanic activity prior to lithification.

Extrusive volcanic rocks are also present. Sediment-lava contact relationships indicate that some of the pillows flowed along the seafloor. Matrix-supported volcanic rudites of the unit may be the product of debris or mud flows which may be sub-aerial in origin.

Megafossils indicate a late Oligocene-early Miocene age for the Vaqueros Formation of Pescadero State Beach. A potassium-argon radiometric age date of 22± 0.7 Ma. obtained from the intrusive rocks there supports contemporaneous magmatism and sedimentation.

At Punta Año Nuevo, pillow basalts of the Vaqueros (?) Formation are unconformably overlain by sedimentary rocks which contain microfossils that are Relizian to earliest Luisian in age; the substrate for the pillows is unknown.

The volcanism at Pescadero State Beach and Punta Año Nuevo may be related to displacement along the San Gregorio-Hosgri Fault and to development of the offshore Outer Santa Cruz Basin. If true, movement along the San Gregorio-Hosgri Fault and development of the Outer Santa Cruz Basin occurred earlier than previously thought.

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