San Francisco State University Petrology — Spring 2006 1 Geology of the California central coast April 21-22, 2006 Field trip guide compiled by Mary Leech Name: Score (out of 100):COAST RANGES PROVINCE The Coast Ranges extend from the southern and western edge of the Klamath Mountains near the Oregon border, to the Transverse Ranges in southern California, a distance of more than 1000 km; in addition, the geology found in the Coast Ranges continues as basement through the western Transverse Ranges, then offshore from the Peninsular Ranges and Baja California, finally to reappear on land at Cedros Island, offshore Mexico. The Coast Ranges got their name because of their proximity to the Pacific Ocean, and the interface between shoreline processes and the geology of the province dominates the scenery found in the province, as seen in the view of a wave-carved sea tunnel near Mendocino in northern California, eroded into Franciscan Complex sandstones. In the Coast Ranges Province proper, three linear belts are recognized: (1) an eastern belt composed of Franciscan Assemblage (Franciscan Complex)rocks (graywacke and shale in coherent units, often metamorphosed to blueschist facies, together with melanges composed not only of sheared shale and graywacke blocks but also exotic blocks of some or all of the following rock types: serpentinite, chert, greenstone (as in the photo to the right), eclogite, and blueschist, overlain by the Jurassic Coast Range ophiolite (in thrust contact), which is in turn depositionally overlain by the Jurassic to Cretaceous Great Valley Sequence of turbidites and fan deposits; (2) a central belt composed of Mesozoic plutons that had intruded metamorphic basement rocks, overlain by younger sediments, and called the Salinian Block; and (3) a western belt which consists once again of the sequence of Franciscan Assemblage, Jurassic Coast Range ophiolite, and remnants of the Great Valley Sequence, but with some differences -- less blueschist metamorphism and more age restriction of Franciscan sediments (Jurassic to Paleocene in the northern part of the eastern belt, but lack of Paleocene-age sediments in the western belt), and more restricted Jurassic age for Great Valley sequence equivalent rocks. The three belts are separated by major faults, the eastern belt from the Salinian Block by the active San Andreas strike-slip system, and the Salinian Block from the western belt by the more enigmatic Sur-Naciemento fault system, a complex system of apparent strike-slip faults (e.g., Rinconada fault), but with some faults that have reverse components and are thrusts (Page, 1981). These boundary fault systems have been responsible for several hundreds of kilometers of right lateral motion in the Neogene, but paleomagnetic and micropaleontologic evidence in studies of Franciscan cherts suggests that these rocks have travelled from low latitudes, more than 1000 km (Champion, 198?; Wahrhaftig, 1987). While the San Andreas system of faults is clearly active, and has caused numerous historical earthquakes in the Coast Ranges (the most famous being the large 1906 San Francisco event), the Sur-Nacimiento is considered to be inactive (Alt and Hyndman, 2000). The origin of the Franciscan Complex has been assumed to be related to subduction processes. A simplified version of the geological history was synthesized by Blake and Jones (1981): "The close temporal and spatial relations of Franciscan rocks with rocks of the structurally overlying Great Valley Sequence and the volcanoplutonic rocks that lie to the east in the Klamath Mountains and the Sierra Nevada have led to the widely accepted hypothesis that rocks of the Franciscan assemblage were deposited in an active trench contemporaneously with deposition of the Great Valley sequence in a forearc basin adjacent to the active Klamath-Sierraarc complex. These three elements comprise the arc, arc-trench gap, and trench setting . . . typical of convergent plate boundaries (Dickinson, 1971). A large component of subduction generally has been thought to be eastward normal to the trend of the continental margin (e.g., Hamilton, 1969)." Blake and Jones (1981) go on to explain that, although meritorious in a general sense, the hypothesis is too simplistic to explain some anomalous relationships, including the nearby presence of additional arc terranes, and sedimentological problems (compositional characteristics and correlations). GREAT VALLEY PROVINCE The Great Valley Province consists of the San Joaquin and Sacramento Valleys that together extend about 700 km from the Klamath Mountains in the north to the Transverse Ranges in the south; the Great Valley spans a width of about 150 km from the Sierra Nevada in the east to the Coast Ranges in the west. The valley reaches depths of about 10,000 m at its southern end, and is filled with a large volume of sediments of Mesozoic through Recent age, reflecting a variety of types and provenances; for example, inland shallow Miocene seas deposited Monterey Formation shales with more siliceous clasts than coastal Monterey exposures due to their origin from the Coast Range batholith, and Tertiary through Recent alluvium has been largely derived from the adjacent, well-watered Sierra Nevada. The older alluvium sometimes is exposed as inverted topography on the eastern edge of the valley, where older river channels, filled with fluvial debris, were covered with more resistant sediments or volcanics, and adjacent materials were eroded away following uplift; an example of this type of topography northnortheast of Fresno is shown in the photo to the right. Recent alluvium covers nearly the entire valley floor, so that the underlying geology has largely been deduced from exposures in the adjoining mountain ranges, from geophysical studies, and from deep drill holes, particularly along the western margin of the valley where oil is found in a series of domes and other types of traps. So voluminous has been the deposition from Recent alluvium that the surface topography has little expression, consisting mostly of the inverted topography (discussed above) locally exposed along the eastern margin, gently sloping fan deposits along both margins, and the Sutter Buttes, a Tertiary volcanic feature of older Cascades origin exposed west of Yuba City. The alluvial materials have such small gradients and are sufficiently unconsolidated that river systems easily