Sur Series schist, quartzite, marble, gneiss and granulite in the Salinian block likely deposited in a marine shelf late in the Paleozoic. Some Great Valley rocks are exposed in the Coastal Ranges as well, like the Jurassic Knoxville Formation shale. These rocks were likely moved to their current location by the two major faults. In the southern part of the range, between the Nacimiento and San Andreas fault zones, the metamorphic rocks and granite plutons of the Salinian block make up the basement rock. In some places, ophiolite sequences preserve remnant oceanic crust rock on land, while other parts of the range are intruded with ultrabasic igneous rocks that have serpentinized to peridotite. Examples of rock formations in the Franciscan subduction complex include the Calera limestone east of the San Andreas fault or the Laytonville limestone which extends north of San Francisco to Eureka.įranciscan rocks have a complex geological structure. In some places they are interbedded with shale, limestone and radiolarian chert. These rocks are rich in plagioclase, quartz and chlorite mica (which gives them a greenish color) and range up to 25,000 feet thick. The Franciscan subduction complex makes up the basement rock for part of the range, with a mélange dominated by graywacke first deposited in offshore deep ocean basins. Per square mile of drainage, the Eel River has the highest suspended sediment load of any river in the US, exceeding the Colorado River and Mississippi River.
The mountains are drained by the Russian, Eel, Mad and Klamath rivers which tend to follow faults and folds. The California Coast Ranges extend from the Oregon line southward 600 miles to the Santa Ynez River.
Regional geology California Coast Ranges The San Andreas Fault became perhaps most active after the Miocene, potentially resulting in up to 350 miles of offset in some locations. Examples of filled basins included the Los Angeles Basin, the Eel River Basin around Eureka or the 50,000 foot thick sedimentary sequences of the Ventura Basin. ĭuring the Cenozoic, the volcanic and deep water sedimentary Franciscan rocks were accreted to the edge of California and vast areas of marine sedimentary rocks deposited in the Central Valley and what would become the Transverse and Coast Ranges. Simultaneously island arcs and small sections of continental crust rafted onto the edge of North America, building out the continent. These basins steadily filled with sediment, with one famous example preserved as the Great Valley beds in the Coast Ranges. Throughout the Jurassic the Nevadan Orogeny accelerated with large-scale granitic intrusions and erosion into deep marine basins. The rocks of eastern California formed a shallow continental shelf, with massive deposition of limestone during the Paleozoic, and sediments from this time are common in the Sierra Nevada, Klamath Mountains and eastern Transverse Range.Īctive subduction began in the Triassic during the Mesozoic, producing large granite intrusions and the beginning of the Nevadan Orogeny as well as more dryland conditions and the retreat of the ocean to the west.
The oldest rocks in California date back 1.8 billion years to the Proterozoic and are found in the San Gabriel Mountains, San Bernardino Mountains, and Mojave Desert. This produced major crustal stretching, volcanism and displacement of up to 125 miles. One of the most important events was the advent of the San Andreas Fault around 29 million years ago in the Oligocene, when the region subducted a spreading center in the East Pacific Rise. The area formed as a series of small island arcs, deep-ocean sediments and mafic oceanic crust accreted to the western edge of North America, producing a series of deep basins and high mountain ranges. The geology of California is highly complex, with numerous mountain ranges, substantial faulting and tectonic activity, rich natural resources and a history of both ancient and comparatively recent intense geological activity.
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