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Charles E. Weaver
Charles E. Weaver
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October 01, 1949

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 north-westward-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 fresh-water 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.

The Paleocene is represented by the Martinez formation, and the Eocene in ascending order by the Capay shale and the Domengine and Markley sandstones. The formations of the Paleocene and Eocene series consist of marine sediments ranging in thickness from 2000 to 5000 feet that were deposited in embayments far more restricted in area than the seas of the Upper Jurassic and Cretaceous time. The marine sedimentary formations of the Oligocene and lower part of the Miocene series occupy still more restricted areas than those of the Paleocene and Eocene, and near Carquinez Strait are more than 5000 feet thick. The upper Miocene sand-stones of the San Pablo group are far more widely distributed and are nearly 2500 feet thick. They are characteristically coarse-grained and were deposited in moderately shallow water which locally was brackish or fresh. The Pliocene rocks crop out extensively in the north-central part of the area and consist largely of alternating flows of andesite, basalt, dacite, and rhyolite together with associated tuffs and agglomerates, whose total thickness is 100 to 1200 feet. In Santa Rosa and Petaluma quadrangles marine sandstones contain invertebrate fossils closely allied to those of the Merced formation in San Francisco. The beds in Marin and Sonoma counties are about 250 feet thick and rest unconformably upon the Franciscan group. Near Petaluma Valley they interfinger with tuffs.

Before the Pliocene volcanics accumulated the entire area east of the San Andreas fault was folded and faulted, after which it was deeply eroded. The Pliocene volcanics were laid down on the beveled surface of the older rocks and later were moderately folded and broken by normal faults and locally overturned and broken by thrust faults.

During the Quaternary period the Coast Ranges in Middle California, in common with areas to the north and south, were undergoing crustal deformation by differential elevation and depression of the numerous fault blocks, some of which at times were temporarily beneath sea level. Stream erosion adjusted to the differentially elevated blocks and left a somewhat obscure record of the complex Quaternary history.

Igneous activity was confined mainly to three widely separated epochs. The pre-Knoxville was characterized by intrusions of quartz diorite. The peridotites, gabbros, and basalts associated with the Franciscan group formed during the second epoch of activity in the early part of the late Jurassic. The third was characterized by extrusion of andesitic and rhyolitic materials during the middle and late Pliocene. Volcanoes may have been active during the intervening periods in areas not far from that studied, as tuflaceous products are present in the Oligocene and Miocene sediments.

Important associated mineral resources include mercury, magnesite, limestone, road metal, building stone, clays, and surface and ground water.

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GSA Memoirs

Geology of the Coast Ranges Immediately North of the San Francisco Bay Region, California

Charles E. Weaver
Charles E. Weaver
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Geological Society of America
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Publication date:
October 01, 1949




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