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The Salinian Terrane and Franciscan Complex of the Bodega Bay Area

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The Salinian Terrane and Franciscan Complex of the Bodega Bay Area San Francisco State University Petrology — 2006 The Salinian terrane and Franciscan Complex of the Bodega Bay area Saturday, May 18, 2006 Petrology Geology 426 Field trip guide compiled by Mary Leech San Francisco State University Name: Score (out of 50): 1 San Francisco State University Petrology — 2006 Driving directions Time Mileage Directions 8:30 0.0 Meet in the COSE van parking lot behind Thornton Hall, SFSU 0.5 R on Lake Merced Drive 1.3 Veer Right onto Sunset 3.8 R on Martin Luther King, Jr. Drive in Golden Gate Park 4.9 L on Park Presidio Blvd. toward Hwy 1 north Note: 19th Ave. becomes Park Presidio at GG Park Stay in the right lanes for Hwy 1 north 8.4 Cross the Golden Gate Bridge and continue to drive north on Hwy 101 11.5 Look left — after the tunnel, look left to see good examples of Franciscan pillow basalts 9:20 44.3 Exit 101 at Hwy 116 to Sonoma/Napa 44.5 Follow around to R to Lakeville Hwy west/116/Petaluma Blvd. 46.6 L on E. Washington at traffic light 54.5 E. Washington becomes Bodega Ave. 65.0 Bodega Ave. becomes Valley Ford Rd. 73.5 Valley Ford Rd. joins Hwy 1 & becomes Valley Ford cut-off 10:00 73.6 L onto East Shore Rd. 74.0 R onto Bay Flat Rd. (becomes West Shore Rd.) 77.4 Stay R at a fork in the road 10:10 77.6 Park in the dirt lot (toilets available here) STOP 1: BODEGA HEAD AND THE SALINIAN TERRANE 13:00 77.6 Depart Bodega Head 81.2 L onto East Shore Rd. 81.6 L onto Hwy 1 north (slow through towns) 88.7 L toward Shell Beach (sign on L of Hwy) 13:15 88.8 Park in dirt lot near the trail head (toilets available here) STOP 2: SHELL BEACH AND THE FRANCISCAN COMPLEX 17:00 Field trip ends — Return to San Francisco State University 2 San Francisco State University Petrology — 2006 Stop #1 — Bodega Head Start at SFSU Stop #2 — Shell Beach 3 San Francisco State University Petrology — 2006 Geologic Map of California 4 San Francisco State University Petrology — 2006 5 San Francisco State University Petrology — 2006 6 San Francisco State University Petrology — 2006 OVERVIEW OF TODAY’S GEOLOGY Franciscan Complex Some descriptions rock types are modified from the California Department of Conservation Special Publication 119, Geologic field trips in northern California) Franciscan high-grade metamorphic rocks and ophiolites The Franciscan Complex contains world famous high-grade metamorphic rocks (high-pressure and temperature) that formed at great depths in a subduction zone. Blocks include amphibolites, eclogites, and blueschists that exhibit the highest grade of metamorphism of any rocks in the Franciscan. These high-grade rocks are found in a shale and serpentinite matrix mélange that give the local topography is distinctive look — large blocks of resistant metamorphic rocks in a matrix of soft, easily erodable shale and serpentinite. Minerals to look for in these rocks include garnet, amphibole, epidote, omphacite (clinopyroxene), and a blue amphibole called glaucophane. Geochronologic data indicate that the high-grade blocks are the oldest rocks in the Franciscan Complex having been metamorphosed about 160 Ma. Franciscan rocks form the east wall of the San Andreas fault for virtually its entire course through the Coast Ranges of central and northern California, although the Franciscan is concealed along some reaches of the fault by overlying rocks. The Franciscan is a heterogeneous assemblage that consists largely of dismembered sequences of graywacke, shale, and lesser amounts of mafic volcanic rocks, thin- bedded chert, and rare limestone. These rocks also occur with serpentinite and tectonic pods of blueschist in mélange zones that are the locus of much shearing within the Franciscan and that generally separate blocks of the more coherent sequences. The sedimentary and volcanic Franciscan rocks were formed in a deep marine environment, as attested by the abundance of foraminifers in the limestone and by radiolarians in the chert. Most of these rocks are probably Late Jurassic and Cretaceous in age, c. 160 to 100 Ma (Bailey and others, 1964), but some of the chert and associated volcanic rocks are as old as Early Jurassic, c. 200 Ma (Irwin and others, 1977; Blome and Irwin, 1983). In the northern Coast Ranges, some of the rocks assigned to the coastal belt of the Franciscan assemblage are as young as late Tertiary and are thought to have accreted to North America during post-middle Miocene time (McLaughlin and others, 1982). The geochemistry of the basalt is consistent with formation at an oceanic spreading center – most Franciscan volcanic rocks appear to have formed at spreading ridges or were erupted off-axis at seamounts or oceanic rises. Pillow structures are occasionally visible in the basalts. As the oceanic plate moved toward the Franciscan subduction zone, greywacke was deposited on top of the chert at ~95 Ma as ocean floor rocks neared the Franciscan trench. The sequence of basalt- chert-graywacke is repeated many times at the Marin Headlands by thrust faults that formed during the underplating of the Marin headlands units. The age and origin of Franciscan mélange is problematic. Mid-Cretaceous limestone in mélange near Laytonville in the northern Coast Ranges, 225 km northwest of San Francisco, has a paleomagnetic inclination that indicates an origin several thousand kilometers to the south (Alvarez and others, 1980). Similarly, Franciscan pillow basalt about 45 km northwest of San Francisco is thought to have moved northward 19° of latitude (approx 2,000 km) from its site of origin (Gromme, 1984). These and other features indicate that some, possibly much, of the Franciscan has been transported great distances northward along the Pacific margin relative to a stable North America. 7 San Francisco State University Petrology — 2006 Typical ophiolite sequence — oceanic crust, upper mantle rocks and deep-ocean sediments Salinian Terrane Sierran granites or an "exotic" origin? For a long time geologists pointed to the origins of the Salinian granites by tracing back along the San Andreas fault to the Tehachapi Mountains where the first granites can be found on the east side of the San Andreas. Recent studies however indicate that 60 million years ago Pt. Reyes was attached to the west of Monterey, California where similar Salinian granitic rocks are common. Fault movement along a large, largely offshore, fault of the San Andreas System is believed responsible for moving Pt. Reyes from this location. Support for the non-Sierran, "exotic" nature of the Salinian block comes from geochemical studies of the southern Sierra and Salinian granitics that indicate some large discrepancies in the two areas. 8 San Francisco State University Petrology — 2006 The Geology of Bodega Head: The Salinian Terrane west of the San Andreas fault http://www.sonoma.edu/geology/wright/Bhead.html Modified from Terry Wright ©1996 Bodega Bay is a natural harbor resulting from movement along the San Andreas fault. The eastern shore is straight and parallel to the edge of a wide zone of faulting that extends across the bay to the hills on Bodega Head. During the 1906 earthquake, 15' of movement displaced the harbor to the north relative to the mainland. Downward movement of the fault zone and erosion of rocks shattered by faulting gave us the depression of the bay. A sand spit closes the bay to the south at Doran Beach and a wide reach of sand dunes forms a northern barrier along Salmon Creek Beach. The rock contrast across the fault is profound. We see oceanic rocks of the Franciscan Complex Complex to the east and continental granites exposed on Bodega Head, a fragment of southern California or perhaps Baja California dragged north along the fault. If we try to match rocks from Bodega Head to rocks east of the fault, we have to go at least to the Tehachapi Mountains, 500 Km to the south to find similar granites. Some people feel the match is best in Baja California, several thousand kilometers to the south. Besides the many attractions of good food and tourism, natural attractions abound. The Bodega Marine Lab, run by the University of California has tours Friday afternoons and many ongoing research programs on marine biology. The beaches and landscape surrounding the bay are a natural lab for geology and biology. 9 San Francisco State University Petrology — 2006 Our field trip here takes us to the tip of Bodega Head to see perfect exposures of the areas underpinnings and reveals the events that shaped the history of these rocks. Access to Bodega Head is via the road that turns west off of route 1 north of the fire station at the “Bodega Head” sign to Bay Shore Road that leads around the bay. A sharp right turn up the hill by the jetty leads to another junction where a right leads to the parking lot at Windmill Cove. The windmill is long gone, but this is still a great spot to watch the whales on their migration and to see outrageous exposures of the rocks of this side of the SAF. Looking down to the north, you can see a small beach with rocky outcrops on the north side. Check the tide tables before you go — this stop is best at low tide. Hike down the path that leads down the first main gulley north of the parking lot. This is a bit muddy in winter and spring, and slippery, so good foot ware is advisable. Study the gray rocks just above water level on the north side of the beach. 10 San Francisco State University Petrology — 2006 Outcrop above beach at Bodega Head showing the granitoids, with a dark spot of Sur Series, stripe of light-colored dike, fault, and upper breccia.
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