SHELL BEACH: A CLOSE LOOK AT A SUBDUCTION
Terry Wright, PhD.
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 'melange" 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 serpentine which 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 millions of years ago and hundreds of miles from here. The spreading 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 serpentine pulverized in fault zones
This faulted mixture of different rocks is called "melange"; the
French word for mixture. Most geologists agree that it forms
In fault zones, and typically when one plate slides underneath another in a subduction zone. Melange 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 mixed together. 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 forming a yellow band of material along the tops of seacliffs. 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 uplifted100 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.
When the terrace was at sea level, pounding waves washed away soft melange
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
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 parking lot 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. These “cable stairs” are used by the state parks to form a safe and almost indestructible access to the beach. After a storm has dislodged the stairs, they can be easily repaired.
From the top 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 melange, with resistant blocks surrounded by matrix. Along the beach the resistant blocks erode out of the matrix and look like dice tossed into the sea.
Trail and Beach Geologic guide
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 melange. 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. Above this block and below the orange marine terrace deposits look for a streaked dark block of meta gabbro poking out from the poison oak.
The next block on the path below on the right (Photo 4) is a blueschist; ametamorphic 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 which 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.
The ridge and slope 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 olivene to serpentine. Slick surfaces are polish formed by shear in fault zones. Serpentine is relatively light and weak so it moves by directed pressures like a watermelon seed, or lemon pit squeezed between the fingers
Serpentine 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 melange. Blueschist blocks are 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 which 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 serpentine, a continuation of the ridge next to the first outcrops. The boulders in the
gully and on the beach are resistant blocks that have weathered out of the melange matrix.
The far slope of the gully is gray sheared shale of the melange matrix with several large resistant blocks.
The matrix erodes into badlands topography with turrets and tiny gulliess. 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 melange. The sand on the beach is black, colored from the erosion of predominantly dark rocks of the melange which are its source. Contrast this with the light colored quartz and feldspar sands at Bodega Head (bodega head field trip).
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 or turbidity currents.
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 melange.. Shiney 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.
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 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 deep ocean floor has been plastered against the side of the continent and uplifted. Folded layers here show the effects of stress in the subduction zone.
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 volcanos such as the Hawaiian islands. Divers have taken movies of pillow lava forming from undersea flows from Kiluea volcano.
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 melange is at the base of the
slope south down the beach behind a gray sandstone cliff. 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 implying a trip of 6000 miles from original deposition in the southeast Pacific. Like the melange 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 melange here. Outcrops on the left are a cliff of folded layered chert .
The gully to the right has dark shale matrix with a block of chert and a light green block of serpentine. The gray matrix turns green near the serpentine, demonstrating that the matrix is serpentine 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.
Hike at low tide to the north—Pillow Lavas
North up the beach, more boulders of the same rocks are exposed, and tidepools
are rich with sea life at low tide. At low tide, an optional rough hike (not
recommended for neophyte hikers) 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 bench a tricky scramble up a notch involving 20 feet 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 basalt 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 melange. 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 a total distance of about 2 miles.
The Shell Beach melange 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 melange at Shell Beach.