Upper Cretaceous Conglomerates from Salinian Terrane, Central California

Karen Grove

Upper Cretaceous conglomeratic sediments in the Salinian terrane were extensively sampled and analyzed to obtain information about the sediment source area that would improve tectonic reconstruction models. This allochthonous crustal block has moved about 300 km north as a result of Neogene San Andreas-related activity; however, its prior history is poorly understood. Salinian basement rocks were uplifted during the Late Cretaceous. Upper Cretaceous depositional sequences have recorded the subsequent period of basin formation and sediment infilling. Conglomerate clasts, because they are whole rock samples of the source area, are excellent provenance indicators. The Salinian clasts are interesting because they are primarily lithotypes unknown from within the underlying bas ment rocks.

Over 55 clast composition counts, made up of 100 randomly selected samples per count, were completed throughout the Salinian terrane. Sites were located in the unnamed Upper Cretaceous rocks along the Big Sur Coast, in the Ventana Wilderness Area, the Hunter-Liggett Military Reservation, around Nacimiento and Santa Margarita Lakes, in the La Panza Range, the Pigeon Point Formation south of San Francisco, and the Gualala Formation south of Point Arena. Over 100 samples representing a broad suite of lithologies were collected, thin sectioned, and analyzed petrographically. The compositional range is greater than has previously been described and includes basaltic to rhyolitic volcanic rocks, gabbroic to granitic plutonic rocks, various metamorphic rocks (notably quartzites), and rare se imentary rocks. The compositional clast counts and the petrographic analyses are used to compare the Upper Cretaceous sedimentary sections to each other and to potential source areas, suggesting a model for tectonic reconstruction of the basin(s).

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.