Abstract: The Point Reyes Conglomerate: a segment of the Carmelo Formation, displaced 150 to 185 km by the San Gregorio Fault, in west-central California

BURNHAM, KATHLEEN, Stanford University, Stanford, CA94305-2115 ([email protected])

Virtually identical suites of distinctive conglomerate clasts, together with other shared characteristics, indicate that the Point Reyes Conglomerate and the Carmelo Formation (south of Monterey, at and near Point Lobos), were originally deposited in the same submarine canyon system. Essentially all the abundant, and several of the rare, class varieties of each location appear in the other.

The distinctive purple porphyry clasts of both conglomerates are devitrified semi-welded tuff, having a zircon U/Pb age of 151.6± 2.6 Ma, as well as a peculiar arrangement of euhedralzircon-studded titanomagnetite. Petrography and microprobe analysis suggest a mutual parent rock, depositional history, and diagenesis.

Certain boulders within the Point Reyes Conglomerate bear a greater likeness to basement of Point Lobos than to local basement. Although the basement granodiorites of Point Reyes and of Monterey are so similar that nothing precludes their correlation, they exhibit three notable differences, enough to show the unusual Porphyritic Granodiorite of Monterey is the best candidate for origin of these Point Reyes clasts.

Matrix of the two conglomerates is substantially identical, indicating a common origin and sedimentary history.

Similarity of the two conglomerates contrasts with twenty other Upper Cretaceous and Paleogene conglomerates in the California Coast Ranges, none of which share the Point Reyes/Point Lobos suite of particular class varieties. These findings imply that the Point Reyes Conglomerate is a segment of the Carmelo Formation, displaced 150 to 185 km by right-lateral motion of the San Gregorio Fault since the Lower Eocene.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California