Full Title: Geology of the Coast Ranges Immediately North of the San Francisco Bay Region California
Author: Charles E. Weaver
This report discusses the geography, systematic geology, structure, and economic deposits in the Coast Ranges of Central California involving nine quadrangles with an area of approximately 2215 square miles of which about a fifth is water. The hills of the Coast Ranges immediately north of San Francisco occupy three northwestward- trending areas which are separated by the San Andreas fault and northern extension of the Haywards fault system. Each area is composed of distinct groups of rock formations. The oldest formations investigated, perhaps of Paleozoic age, consist of metamorphosed sediments, volcanic rocks, and quartz diorite and are exposed in Point Reyes Peninsula. They form a small residual mass of what was probably an extensive coastal land that lay west of the present coast of California but which late in the Tertiary or early in the Pleistocene foundered beneath the Pacific Ocean. This former land area is believed to have furnished many of the sediments laid down during Mesozoic and Tertiary time that are now exposed in the mountains between the San Andreas fault and the Sacramento Valley. The next younger rocks are a thick series of sandstones and associated basic igneous rocks which are widely distributed throughout the Coast Ranges of California and southern Oregon and are known as the Franciscan group. They constitute the surface rock exposures in most of the area between the San Andreas fault and the northern extension of the Haywards fault zone. The Franciscan has a thickness of 7000 feet or more. It contains no fossils except poorly preserved radiolarians in the cherts and Foraminifera in the limestones, which do not permit of an exact age determination. Recently discovered ichthyosaur head bones and teeth from cherts thought to belong to the Franciscan group are closely allied to a species from the lower Portlandian of Europe. From evidence obtained within the mapped area the Franciscan is younger than the time of intrusion of the quartz diorite into the Sur series which may have been during the late Paleozoic. Its upper age limit is uncertain because of fault contacts with the Knoxville formation. It has been considered by other writers from evidence obtained outside of the mapped area as in part contemporaneous with the Knoxville. The area between the Petaluma-Cotati Valley trough and the Sacramento Valley is composed of more than 30,000 feet of Jurassic to Quaternary marine and freshwater sediments, together with about 1200 feet of Pliocene andesites, rhyolites, and tuffs. These sediments probably accumulated in structural troughs whose areas and physical environments changed greatly during the Cretaceous and Tertiary. The lower portion consists of clay shales and subordinate amounts of sandstone and conglomerate as much as 17,000 feet thick, containing a marine fauna of ammonites, pelecypods, and gastropods. These rocks include the Jurassic and Lower Cretaceous portions of the Knoxville formation and the Upper Cretaceous Chico. Several faunal zones may be distinguished in the Knoxville, but the formation in the mapped area cannot be subdivided on a lithologic basis. The Chico formation consists of interbedded shales and sandstones about 7000 feet thick…
Published: 10/20/1949
Pages: 242
Product Category: EBooks