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):

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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

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Stop #1 — Bodega Head

Start at SFSU

Stop #2 — Shell Beach

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Geologic Map of California

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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.

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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.

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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.

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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.

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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.

These contain the secrets of the sequence of events present on the west side of the San Andreas fault. As you descend the trail, you can see a set of layered rocks above the diorite. These are part of the younger sedimentary rocks; they are layered and contain fragments of the underlying diorite.

The rock sequence exposed here includes the major players in the historical drama that constructed the land we call Salinia. The stretch of coast including the Santa Cruz Mountains and Big Sur are part of the same terrane. The players include a sequence of older metamorphic rocks, intruded by 80-100 m.y. plutonic igneous rocks and a mantle of much younger sedimentary rocks.

Take a close look at the main granitoid rock. What is its mineralogy? List approximate percentages for each phase below:

Give the granitoids the appropriate name based on the IUGS classification system:

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Sur Series inclusion in Bodega Head granitoid

Look for dark enclaves within the granitoids like in the above photo. What kind of rocks are these — sedimentary, igneous, metamorphic?

What is the relationship between the dark enclaves and the granitoid? Explain their origin. Which is older — the granitoids or the enclaves?

These dark rocks are known as the Sur Series after similar rocks exposed in the Big Sur Mountains. We do not know the age of these rocks, but recent age dates on grains of zircon indicate they are very ancient, part of the original continent more than 1 billion years old. These grains could have been eroded from older rocks, but we do not know their complete history because it has been masked by metamorphism. Limestone changed to marble, shale to schist or gneiss, with no trace of the original rock. Here they are visible only as ghostly dark patches in the granitoid. On Point Reyes, we find the same rock sequence with large blocks of gneiss and marble preserved.

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Standing in the small sandy slot near the water on the north side of the beach, you can see a smooth wall of granitoids. On the low ledge to your right, you can see a dark patch, remains of the Sur Series. The fragments were so small that they were completely changed to a dark granular rock by metamorphism from the heat of the diorite. Looking up at chest level, we can see a pink stripe cutting across the ledge. These pink stripes also paint other cliffs.

Pink intrusions cross-cut the granitoid and a recently uplifted marine terrace erosional surface at Bodega Head

What kind of intrusive feature is this?

What minerals comprise this instrusion? Name this intrusion.

What is the relationship between the intrusion and the granitoid? Which is younger? Older? Explain your answer using Bowen’s reaction series.

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Closeup of granitoid below, pink intrusion and fault gouge (dark layer) and fault breccia (rubbly material above)

On top of the pink stripe we can see more granitoid, but it is in fragments, as if it were broken and shattered. Also, just above the pink intrusion there is a thin layer of clay, with fragments of quartz and pink feldspar in it. Why is there this sudden difference in the granitoid and what clues to this mystery can we see in the clay layer? The broken up rock tells us that high pressures have been active and the clay layer looks like ground up flour from the granitoid. The grinding and pressure tells us that this is a fault, more personally called the Windmill cove fault. Movement along this plane has fractured the rocks above and milled the rock to a clay-rich vein we call fault gouge. The fault runs along the shore to the north and can be followed easily across the bare rock terraces.

Scrambling uphill from here, we come to a point where sandy, layered rocks overlie the broken granitoid. This is a profound contrast in rock type and indicates an important turn of events in the geological history. The granitoid is very old and formed deep under the earth's surface. The sand layers look like the ones forming today on the beach, so they must be very young and formed at the surface. There is a lot of geologic time missing here — around 100 million years are not represented by rocks. Also there is a radical difference in environment of rock formation. This is a geologic unconformity, where part of the rock sequence is missing.

What could cause such a contrast? The only way we can expose granite at the surface is by erosion, and in this case deep erosion can only be caused by uplift of the surface of the land. These granites formed deep in the bowels of a mountain range, which eventually was worn down to sea level. The waves ground up granite and made it into sand, which formed the layers we see. Wave erosion penetrates to a level about 15 feet below the ocean surface, forming a relatively flat surface called a marine terrace. We can see flat terraces carved on diorite to the north that have been uplifted just above sea level. The arkosic (feldspar-rich) sandstones above have horizontal layers, formed on a flat surface that has been pushed up to present elevation. Uplift here has been steady for the last 3 million years.

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Layers of marine terrace sandstones, topped by soil with shell midden

The Coast Ranges are caught between the Pacific and North American plates with horizontal movement along the SAF and convergent motion as well. This forces the marine terraces upward at rates of between 0.5-1.0 mm per year. We find 40,000 year age dates for these rocks using radioactive carbon (C-14) from dark fragments of carbonized wood. We can see large chunks of quartz, granitoid and pink feldspar in the sandstone just above the granitoid.

Look down at the granitoids and notice that its texture changes as you look inland from the ocean. The granitoid looks foliated closer to the sandstones. What’s going on here? Explain this foliation.

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Close-up of the marine terrace deposit basal breccia. Look for fragments of granodiorite and Sur Series rocks.

These show that pieces of rock may have fallen from nearby cliffs and settled into the sand. If you look carefully, you might be able to find fragments of shiny schist or striped gneiss, recycled remnants of the Sur Series. Farther up the cliff, the sand becomes finer-grained, but still contains fragments of quartz and pink feldspar, evidence that they come from erosion of the underlying granitoid.

The same process that formed these rocks goes on today, forming a terrace under the waves offshore and accumulating sand in layers on the beach. The waves bottom out below the ocean surface to erode the rocks to a flat surface, with occasional resistant blocks of diorite forming islands that we see above water level as sea stacks. Light-colored sand made of quartz and feldspar and larger chunks eroded from cliffs accumulate in layers on this surface. Slow uplift will expose these layers as yet another marine terrace in the geologic future.

This is a perfect example of “uniformitarianism”, one of the foundation theories of geology. The concept can be summarized by "the present is the key to the past". We can observe processes going on today that are the same as processes active in the past, and the result is similar rocks. Here we can see the older marine terraces exposed by uplift and erosion and we can see the same process forming future marine terraces today. . We can also see the future unconformity between the sand forming today on the beach and the diorite bedrock everywhere they come into contact.

An epic environmental drama unfolded here in the early 1960s where PG&E started construction on a nuclear power plant on Bodega Head. Nuclear Power plants need cooling water, and the most abundant supply of cold water is along the coast. Unfortunately, there are many active faults along the coast, and this site was right next to the San Andreas fault. They excavated a huge pit for the foundation, and encountered the sandstones.

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A geologist happened to be studying these layers and found that they were offset by a fault. This indicated that there had been an earthquake with offset along this fault less than 40,000 years ago.

The fault also may be connected to the San Andreas fault. We can see minor faults and folds caused by earthquake shaking in the sandstone bluffs along Windmill Beach. Public pressure of the danger of nuclear power plants and the presence of this fault prompted PG&E to decide that it would not be a good idea to build a nuclear power plant on a fault and so they abandoned the project here.

They looked at a site to the north at Point Arena and next to the San Andreas fault, but found that the marine terraces had been tilted recently by large earthquakes. They finally decided on a site to the south at Diablo Canyon. Their studies of that site indicated that there were no active faults in the vicinity. However, during construction, geologists found a new fault 5 miles off shore, and the design had to be altered for more earthquake protection. Unfortunately, workers installed new bracing for pipes from blueprints printed backwards, so more delays occurred before plant completion, all because of earthquakes and geology.

At the very top of the sandstone, we can see a dark layer of organic soil with white flecks in it. The white turns out to be shells of local clams, mussels, etc. These are part of a midden, a shell dump from generations of Native Americans who called Bodega Head their home. Until the 1930s there was an active Miwok community located to the south in a sheltered ravine. Geologic History

This area demonstrates the geologic history of Salinia, the rocks to the west of the San Andreas fault in Sonoma County. The first event is formation of the original rocks of the Sur series, perhaps more than 1 billion years ago on a continental shelf. These rocks were intruded and metamorphosed by the granodiorite of Bodega Head 100 million years ago. Granite pegmatites probably were the last dregs of magma to intrude and crystallize. The Windmill Cove fault cut the older rocks and fractured and ground up the granodiorite. A long period of uplift and erosion followed, with northward movement along the SAF of at least 200 miles in the last 29 Ma. Finally, the marine terraces formed on the wave-eroded granodiorite causing an unconformity. Presently, uplift and erosion raise old terraces and set the stage for new terraces to form.

At Point Reyes, visible to the south on a clear day, we see a similar set of rocks and history. There is a thick sequence of layered rocks between the granodiorite and the marine terraces preserved there also. Much of the history missing in the unconformity at Bodega Head is filled in to the south.

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STOP #2 — SHELL BEACH Shell Beach: A close look at a subduction zone http://www.sonoma.edu/geology/wright/shellb.html

Modified from Terry Wright ©1996

Shell Beach — Sonoma Coast State Beach Highway 1, 8 miles N of Bodega Bay, 3 miles S of Jenner.

Shell beach is a scenic and geological jewel on the northern Sonoma County coast. A short trail leads to the beach where the complex structure and rocks of the Franciscan Complex lie out like a smorgasbord for hungry geologists. It is a classic field area, world- famous for its perfect exposures of an incredible variety of rocks and structures. All natural things are protected by state law, so please leave your hammer in the car.

The beach is in a sheltered inlet, so it is a pleasant place to visit even on the most windswept days. Lower tides or low high tides afford the most area for study, but you can study rocks along the trail and the high beach at all tide levels.

When you arrive, take a look around the area to the east of the parking lot. The landscape reflects the underlying geologic structure. Crags of gray rock poke through a smooth blanket of grassy slopes. The underlying rocks have a structure called "block in matrix". We call this structure “mélange” from the French word for mixture. It is a true mixture of many different kinds of rocks sheared together in a subduction zone. This creates topography like slightly melted rocky road ice cream. Hard blocks of rock are like nuts and marshmallows in ice cream. They form the craggy outcrops. These blocks sit in soft matrix of sheared shale and serpentinite that is easily eroded into grassy slopes. This creates a topography that looks like slightly melted ice cream. Each of the hard blocks is a very different rock type born on the ocean floor many miles from here. The spreading

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ocean floor carried them to the continent where subduction faulted, deformed, and mixed them up. Recent uplift and erosion exposed the depths to our view. The matrix consists of softer rocks of shale and serpentinite pulverized in fault zones.

This faulted mixture of different rocks is called "mélange"; the French word for mixture. Most geologists agree that it forms when one plate slides underneath another in a subduction zone. Mélange may also form along great fracture zones in ocean crust, where movement is dominantly horizontal. Soft-sediment landslides on the ocean floor can also mix rocks but they usually consist of rocks of the same type. At Shell Beach we see fragments of originally continuous sedimentary layers, metamorphic rocks, volcanic and plutonic igneous rocks. Original rocks range in age from 150-100 million years. The subduction and mixing occurred between 100 and 10 million years ago.

The parking lot is on a gradual slope that descends from Highway 1 to the tops of the sea cliffs. Layers of sand and gravel underlie the surface. These layers are similar to sediment accumulating today on the beaches below. This is an uplifted marine terrace, so named because of its flat surface and its similarity to rocks forming today under the waves offshore. The marine terrace now is uplifted 100 feet above sea level.

To the north a rock monument rises from the terrace plain. This is a Pleistocene sea stack, formed like the towers rising above the surf today along the coast (photo 1). When the terrace was at sea level, pounding waves washed away soft mélange matrix and left the hard block standing like a sentinel. Abalone divers report a sandy shelf underwater with resistant stacks rising above, similar to this marine terrace.

The elevation of this ancient shore shows that rapid uplift of this area occurs today from pressures along the San Andreas fault. The area around Cape Mendocino to the north is presently rising at 1.4 mm per year.. Local terrace deposits contain wood fragments dated at 40,000 years so the uplift here is about 1 mm/year. Marine terraces rise like a flight of stairs up the high ridges to the east, telling us that uplift has persisted here for much of Pleistocene time.

There are several scenic trails in the area. The Kortum Trail leads across the marine terrace north to Goat Rock, and south to Wright's Beach campground. This is part of a trail system that leads along the coast of Sonoma County named for Bill Kortum, a Petaluma veterinarian, who was the leader in the fight to guarantee access to the coastline for the public over developed private lands.

The Pomo canyon trail starts across Highway 1 from the parking lot and leads east up over the ridge into Willow Creek valley. The Pomo canyon walk-in campground is at the east end of the trail. Campsites in deep redwoods or high on a ridge are accessible from Willow Creek road off Highway 1 at the Russian River bridge.

The Shell Beach trail leads from the plateau down a winding trail to the beach. A rebuilt trail winds down to the beach. Several sections of the trail have unique cable stairs made from hexagonal wood rungs strung along two heavy cables and set in rock. From the top

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of the trail you can see the level marine terrace and underlying bluffs made of orange layers of sand and gravel deposited on ancient beaches. Steep gray rocky slopes below consist of mélange, with resistant blocks surrounded by matrix. Along the beach the resistant blocks look like dice tossed into the sea.

The main trail to the beach winds down the side of a gully covered with vegetation. The area was formed by a landslide during a major storm on January 2, 1982, which caused millions of dollars of damage in the Point Reyes area and along this part of the coast.

The first outcrops buried in the bushes to the left of the trail are conglomerates of the Marine terrace. These are made of small pebbles of hard chert and quartz from the underlying Franciscan Complex.

Large boulders along the trail are hard blocks of the Franciscan Complex mélange. The first has a tilted flat surface with crystals of green and silver with red dots. The grass green crystals are omphacite pyroxene, with silver flaky muscovite mica pockmarked by red garnets. This is an eclogite, a very iron-rich metamorphic rock, formed under the high pressures, but relatively low temperatures of a subduction zone environment

The next block in the path below is blueschist a metamorphic rock containing a blue amphibole, glaucophane or lawsonite, as the principle mineral. The green and blue streaks are alternating layers of blueschist and eclogite. Streaks come from flow in solid rock that occurred under intense squeezing in the subduction zone. This is metamorphic foliation, so called because of the layers formed during flow by the flat or platy minerals that line up parallel to flow.

Blueschist along the trail to Shell Beach

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The ridge to the left of the trail is light green clay with dark green fragments of shiny serpentine embedded in it. Serpentine forms when very iron-rich rocks of the mantle combine with hot water. These fluids change olivine to serpentine. Slick surfaces are polish formed by shear in fault zones. Serpentinite is relatively light and weak so it moves by directed pressures like a watermelon seed, or lemon pit squeezed between the fingers.

Serpentinite is intimately associated with high-grade metamorphic rocks, and may act to carry blocks of blueschist and eclogite from deep in the subduction zone up into the mélange. Blueschist blocks have an age 10 my older than the rest of the Franciscan complex, and may have risen from a “cryptic plate” hidden deep beneath the Franciscan. Recent seismic studies of the Coast Ranges show another plate at depth that could be the hidden source of metamorphic blocks.

The trail skirts a steep slope down to a gully with boulders scattered about. The north slope is light green serpentinite, a continuation of the ridge next to the first outcrops. The boulders in the creek bed and on the beach are resistant blocks that have weathered out of the mélange matrix.

Mélange matrix and blocks from Shell Beach trail

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The far slope of the gully is gray sheared shale of the mélange matrix with several large resistant sandstone blocks. The matrix erodes into badlands topography with turrets and tiny gulleys. This is the result of torrential rains of winter storms falling on soft shale.

The beach is a smorgasbord of different resistant blocks weathered out of the mélange. The sand on the beach is black, colored from the erosion of predominantly dark rocks of the mélange that are its source.

In your own words, define “mélange”:

How does a mélange form?

Identify the many Franciscan rocks present at this beach and check them off as you find them:

Eclogite Basalt

Blueschist Greenstone

Amphibolite Serpentinite

Graywacke Peridotite

Chert

Shale

Conglomerate

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Gray boulders with sandpaper feel are graywacke sandstone with black layers of shale and white streaks of quartz veins. A prominent boulder south of the stream mouth has shiny slick surfaces with grooves, evidence that a fault moved, polishing and scraping the soft sandstone surface. Sandstone formed as turbidity currents of sand and mud that flowed rapidly down the continental slope to form layers in an oceanic trench. Alternating layers of black mud and gray sand result from the heavy sand sinking first followed by the lighter clay minerals in the mud. The layers are said to be “graded” with coarse sand at the bottom and finer mud at the top.

Smooth brown rocks are also sandstone, with more quartz in them so giving a lighter color. Blocks made of pebbles are conglomerate, formed from stream gravels.

Light green rocks with black surfaces and green to white veins are peridotite. These are a message from the mantle. They originate in the mantle deep below the crust and faulted into the mélange. Shiny, bronze pyroxene crystals are cut by green veins of serpentine. One block of peridotite has white fibrous veins of asbestos in it. In some outcrops, the peridotite has weathered orange, from oxidation of iron. Rectangular vein patterns on some rocks come from serpentine formed in fractures. Shiny boulders of green and black are serpentine, many have grooves on the surface from fault scraping.

Block of peridotite at Shell Beach

Chert appears as light colored shiny boulders with hard layers. Color ranges from white, to red, orange or dark green. The red colors come from iron in various combinations with oxygen. Chert is made up of silicon and oxygen in the form of amorphous (structureless) silica. Much of the silica comes from microscopic floating organisms called radiolaria. Silica is not abundant in sea water so it is a mystery how layers of chert form. Perhaps during volcanic eruptions, silica-rich ash falls on the surface of the sea and the fine silica

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in the ash dissolves, providing material for the radiolaria to multiply rapidly in a "bloom". When the volcanic eruption stops, the silica supply is cut off, the radiolaria die and fall to the bottom of the ocean to form chert. The presence of chert is further evidence that ocean floor has been plastered against the side of the continent and uplifted.

Chert, sedimentary rock from the deep ocean floor

Greenstone appears as light green massive rocks. These are metamorphosed basalt ash and lavas. One outcrop on the beach at the mouth of the gully has bulbous pillow structures with dark mud surrounding them. Pillow lavas form when basalt lava erupts underwater at the mid-ocean ridge or on other volcanoes such as the Hawaiian islands. Divers have taken movies of pillow lava forming from undersea flows from Kilauea volcano.

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Pillow structure in basalt at Shell Beach, formed as lava erupted underwater

A jet black rock at the foot of the path is amphibolite, a foliated metamorphic rock with garnets and tiny folds visible in the foliation. Amphibolite is also metamorphosed basalt which is heated to much higher temperatures and pressures than greenstone.

The best place to see the true texture of the mélange is at the base of the slope south down the beach (Photo 8). Winter storm waves washing up on the base of the slope expose streamlined blocks of sandstone surrounded by sheared shale. The sandstone is more resistant to fault shearing than the shale, so it gets milled by the moving shale matrix into a rounded or streamlined shape.

This outcrop is a microcosm of the structure of the entire Franciscan complex from southern California to Oregon. Blocks miles in dimension, called terranes, originate as fragments of ocean floor or continental margin. They are transported on the moving ocean floor to be stuck into the subduction zone between ocean floor and the continent. Blocks of limestone in Laytonville, 100 miles to the north, have fossils and an ancient magnetic field that tells us they came from 17 degrees south of the equator. Like the mélange at Shell Beach, the entire Franciscan Complex consists of a collage of different pieces of geologic real estate, each with a unique history and surrounded by faults.

To the north of the path a gray ridge with vertical brown and black stripes on it is a massive block of graywacke sandstone. The dark splotches in the sand are pieces of shale, picked up by a turbidity current as it swept across a deposit of mud formed by a previous current. North of this ridge is a small sheltered cove with a slope between the graywacke mass and the main ridge. We can see the relationship between blocks and matrix clearly in the mélange here. Outcrops on the left are a cliff of folded layered chert.

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The gully to the right has dark shale matrix with a block of chert and a light green block of serpentinite. The gray matrix turns green near the serpentinite, demonstrating that the matrix is serpentinite which has been chipped off by faults. The matrix comes from the blocks by fault shearing. On the right the large block of graywacke remains as a resistant block.

North up the beach, more boulders of the same rocks are exposed, and tide pools are rich with sea life at low tide. At low tide, an optional rough hike around the north point of the beach is possible. The rocks are slippery, so good footwear is necessary here. The beach leads past sloping layers of graywacke sandstone with beautiful graded bedding onto a long crescent beach. Near the end of the beach, look up to the right at the foot of the cliff where a vertical face has gently sloping lines on it. These are slickensides or scratches on a fault plane showing the movement direction.

At the end of the beach a tricky scramble up a notch involving 20’ of rock climbing leads to a gully, across a beach and eventually to a flat-topped ridge and a spectacular view of a dark cliff with bulbous forms of pillow lava. They formed as iron-rich lava erupted under the ocean on the ocean floor. They were carried on the ocean floor to the edge of the continent where they were incorporated in the mélange. A rough, steep path leads to the left of the cliff and up to the marine terrace above. You can hike back south to the parking lot along the flat with occasional views down to the beach below.

The Shell Beach mélange is a perfect illustration of the structure and mixture that takes place in a subduction zone at a convergent plate boundary. Different rocks from all over the ocean floor can be mixed together by faulting in the accretionary wedge. The granites of Yosemite formed when the same ocean floor slid deep down to the east, melted and floated upward as light blobs of molten magma. The walls of Yosemite are close cousins to the mélange at Shell Beach.

26 San Francisco State University Petrology — 2006

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