Thomas B. Anderson
Salinian block Paleocene sedimentary rocks crop out on the Point Reyes Peninsula and along the Sonoma-Mendocino County coastline to the north. At the extreme tip of Point Reyes, the Point Reyes Conglomerate crops out locally in contact with the Cretaceous granitic basement and is interpreted to be a submarine canyon deposit. To the north along the Sonoma and Mendocino County coastline, turbidites of the German Rancho Formation on the Gualala block are well-exposed in sea-cliff outcrops.
The Point Reyes Conglomerate is composed of 3 m thick sequences of massive, normally graded conglomerates overlain by thinner bedded sandstones with minor mudstones and is interpreted to be part of the arenaceous conglomerate turbidite facies. The clast population is primarily locally derived granite (20%) with maximum sizes of 2.5 m and a distinctive purple potassium feldspar porphyry (25%) which occurs as smaller, rounded cobbles. Other common clasts are chert and siliceous volcanics. Load structures occur in the finer grained beds near the top of each sequence. Potassium/argon ages from feldspar separates from the purple porphyry range from 82-84 Ma. The Paleocene age has been determined from its faunal content. The relationship to the grainitic basement, the large size of the gran tic clasts, and the structures of the conglomerate/sandstone beds suggest deposition within a submarine canyon.
The German Rancho Formation is composed primarily of turbidite sequences deposited on a submarine fan system in the southeast-northwest trending Gualala Basin. The sandstones are primarily feldsarenites and contain a variety of structures and sequences characteristic of deposition from both high- and low-density turbidity currents. Reworked macrofossils and deep water truce fossils including Planolites and Thalassinoides are locally abundant. The Paleocene age of the unit is based on macrofossil assemblages. A preliminary conclusion is that the submarine canyon system in which the Point Reyes Conglomerate was deposited was the source for the basinal turbidites of the German Rancho Formation.
Correlation of the Point Reyes Conglomerate with the Carmelo Formation at Point Lobos and the German Rancho Formation with the Butano Sandstone in the Santa Cruz Mountains indicates up to 180 km of offset along the San Gregorio Fault. Recent workers have used geochronology, isotope studies, and elemental chemistry of clasts in the German Rancho conglomerates to suggest that the unit may be derived from as far south as the southern Sierras or the Mojave Desert area indicating total offset across the San Andreas Fault Zone of 600 km or more.
AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California