Coastal sediments, coastal processesObjectives Geol 103 Field TripThis required field trip for Geology 103 (Sedimentology/Stratigraphy) has several objectives:

Examine Franciscan sediments and metasediments that are exposed along the California coast

Study modern and ancient shallow marine sediments Compare high energy and low energy beach environments Observe sedimentary structures and large-scale bedding features in the

field Describe dunes and eolian features Practice geologic mapping and map reading

Introduction: The Great Valley Sequence, Franciscan Group, coastal sediments and coastal processes

We will look at three different sequences of rocks on this trip. The Great Valley

Sequence is a Cretaceous age deep sea fan complex that formed along the ancient margin of

California. It was deposited in a moderately deep ocean environment at a time when the

coastline of North America lay roughly along the eastern edge of California's central valley

(Figure 1, Figure 3). Water depths were probably in the range of 1000's of meters. Outcrops of

the Great Valley Sequence near Lake Berryessa are world famous for their unobstructed

exposure of the parts of a deep sea fan.

Franciscan metasediments are exposed in the Coast Range, and are similar in age to the

Great Valley Sequence. Franciscan metasediments were deposited in deep marine environments

that lay seaward of the active continental margin, and are highly deformed (Figure 2).

Lithologies in the Franciscan Group include shale and metamorphosed deep sea sediments,

metachert, metasandstones rich in volcanic clasts, pillow basalts, greenschist and blueschist

facies rocks and serpentinites. Ultramafic components of the Franciscan Group are probably

altered oceanic crust that was associated with the subduction zone. Most modern authors believe

that Franciscan sediments are the offshore equivalent of the Great Valley sequence, but have

been underplated so that Great Valley sediments lie over the Franciscan Group.

On this trip we will also look at coastal depositional environments and modern coastal

processes. We will do this by visiting modern and ancient beach deposits, eolian deposits and

shallow marine depositional systems. At each stop we will describe the bedding, sedimentary

structures and lithologic relations that lead to an environmental interpretation.

Friday Field Trip ActivitiesFriday stop #1: Schoolhouse Beach

Travel 2.5 miles north of Bodega Bay on Highway 1, and park at Schoolhouse Beach. At

this location we will have a lecture on beach deposits and observe Franciscan sediments that are

lightly metamorphosed. Franciscan sediments at this location deep are sea fan deposits, and have

many similarities to the Great Valley Sequence. At Schoolhouse Beach, massive sandstones and

interbedded siltstone and shale were originally deposited as deep sea and continental margin

sediments. These deposits are interpreted to be turbidites on the basis of their lithology and

sedimentary structures, and have been metamorphosed and sheared so that primary features are

often difficult to observe. The composition of these sandstones is very similar to the

composition of Great Valley turbidites, with a chlorite-rich matrix, minor amounts of volcanic

and sedimentary lithic grains and up to 15% potassium feldspar.

The rocks now found at Shell Beach originally formed as ocean crust and ocean

sediment, but were subducted and metamorphosed during Cretaceous? time. This outcrop, and

much of the Coast Range, is composed of Franciscan rocks. Franciscan rocks have alternately

been referred to as a formation, a group and a sequence by various authors, but for this trip we

will use the terminology of Bedrossian (1971) and refer to the Franciscan as a “group”. Dating

of Franciscan metasediments relies on a handful of poorly preserved fossils. Fossils extracted

from the Franciscan metasediments indicate ages ranging from Late Jurassic to Cretaceous, so

Franciscan rocks were deposited at approximately the same time as the Great Valley Sequence.

Tectonic activity associated with subduction along the Pacific margin deformed the Franciscan

deep sea sediments, and resulted in formation of a sheared paste (mélange) of sedimentary rocks,

ocean floor basalts, radiolarian chert, gabbro, schist, serpentine and rarely eclogite (Wahrhaftig,

1984). Late underthrusting of Franciscan metasediments beneath Great Valley sediments along

east-dipping thrust faults has resulted in the present configuration. Recent recognition of the

underthrusting and contemporaneous nature of Franciscan and Great Valley sediments has

resulted in re-interpretation of the geology of the Franciscan Group and Great Valley Sequence,

and may imply that blind fault zones extend further under the Central Valley than was previously

predicted.

Franciscan rocks can be loosely grouped as highly altered ocean floor material, and less

altered deep sea sediment. Shell Beach has good exposures of the highly altered ocean floor

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material. The jumbled outcrop that we will see today is called mélange, a French word that

means mixture. The composition of the Franciscan mélange varies through the Coast Range,

with lithologies that include shale, chert, greenstone, serpentine, eclogite and glaucophane schist.

Sediment occurs in large, discrete blocks, and neighboring blocks may have entirely different

lithology and geologic history. This is a result of represent underplating of clastic wedges that

sheared parallel to bedding.

Activities at Schoolhouse Beach:

Listen to a lecture on beach deposits.

Examine the modern beach deposits. What does the grain size on Schoolhouse

Beach indicate about the energy of this beach?

Look for features in the Franciscan complex. Describe the grain sizes,

sedimentary textures, composition, sedimentary structures and bed contacts.

Which previous field trip stop had rock outcrops that look most similar to the

exposures at this stop?

Friday stop #2: Shell Beach Continue north approximately 5 miles to Shell Beach. This is an excellent place to

observe most of the components of an ophiolite sequence. Several recent marine features are

also present at this stop. Marine terraces are visible to the east, and a Pleistocene sea stack can

be seen to the north. These terraces record uplift of the coastline at a rate of approximately 1

mm/yr (Wright, 1996).

At this outcrop, a matrix of sheared serpentine and clay surrounds blocks of rigid

metamorphic material. This diverse mixture is highly sheared and fractured, and individual rock

types occur as distinct blocks within the highly sheared matrix. Exotic metamorphic blocks that

contain high pressure, low temperature subduction zone assemblages are intermixed with the

altered sedimentary rocks. This deep crustal material may have been emplaced by thrusting and

crustal shortening, or may represent diapirism of ductile material.

High pressure, low temperature blueshist facies metamorphic rocks are visible along the

trail that leads from the parking lot to the beach. Blueshist facies refers to a suite of minerals that

form in metamorphic rocks that were subjected to high pressure but low temperature. This type

of metamorphism commonly occurs in subduction zones, where slabs of cool, dense crust are

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pushed rapidly to great depths. Later deformation and uplift along the California margin brought

the material back to the surface, where it is exposed today. Blocks of eclogite (an iron-rich, high

temperature, low pressure rock) are visible along the trail as you descend to the beach. These

ecologites contain silvery muscovite, oxidized garnet crystals, and a green pyroxene. Blocks of

blueschist are also present, and were named because of the presence of a blue amphibole, either

glaucophane or lawsonite. Serpentine is common, and formed when olivine basalts from the

ocean floor were altered by hot fluids in the subduction zone. Surfaces on blocks are often

polished or show mineral foliation (Wright, 1996). The serpentine may act as a lubricant that

helps blocks slip upward toward the surface.

An excellent example of ribbon chert can be seen at the first outcrop north of the

stairway. Ribbon chert is a deep marine deposit that forms from accumulation of siliceous

microfossils shells (tests), and is common in highly productive environments. Upwelling along

the California coast delivered nutrients to the surface, and is a likely cause of the high

productivity necessary for ribbon chert formation. This ribbon chert is reddish color, and

weathers to form resistant bands. Altered graywacke is also common along the beach, and at the

far northern end of the beach, the 7 m tall dark brown outcrop may be pillow basalt. Pillow

basalt forms in submarine environments, when lava is extruded from fissures and cracks. The

lava cools almost instantly, and the characteristic “pillow” shape is a result of quenching and

rapid cooling as the lava oozes from the subsurface. This suite of rocks would be common in an

ophiolite sequence, although the ultramafic intrusive rocks that usually form the base of an

ophiolite sequence are missing at Shell Beach.

Activities at Shell Beach:

Describe the components of an ophiolite sequence in your field notebook.

Describe a ribbon chert in your field notebook.

Friday Stop #3: San Andreas FaultDrive north on Hwy. 1 to Fort Ross. When you see the fort, turn east onto Fort Ross

road. Go about a mile until the road crosses painted marks. Stop and observe the San Andreas

fault, with sag ponds and an active fault scarp.

Activity at the San Andreas fault:

Describe the vegetation and landforms that indicate that this is a fault zone.

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Friday Stop #4: Salt Point State ParkDrive 18 miles north of Jenner on Highway 1 to Salt Point State Park. We will set up

camp, then walk or drive down to Gerstle Cove for the low tide at 4:13.

Gerstle Cove tide poolingBeaches environments can be divided into subtidal, intertidal and supratidal zones.

Today we will look at organisms that are adapted to life in a rocky intertidal zone. I will divide

the class into groups and pass out critter guides for each group. Your assignment is to

walk/crawl carefully out onto the intertidal area at Girstle Cove. Stop and retreat if you are not

comfortable on the slippery surface. Look at the plants and animals that you find, and record,

photograph and draw pictures of each. Don't disturb the animals, or make them expend extra

energy retreating or hiding from your view.

Friday evening

Group meeting around the campfire at 9:00 p.m. to discuss general geology of northern

California and plans for Saturday and Sunday field trips.

Saturday Field Trip ActivitiesOn Saturday morning we will leave our camp set up at the Salt Point campground.

Campsites should be clean and clear, because we will not return until early evening. Zip up tent

flies, secure loose items, and put away all food. Take everything with you that you will need for

a long day in the field. People will need to be in the vehicles and ready to travel by 7:30 A.M.

Directions to Point Reyes National Seashore

From Jenner take Highway 1 South to the town of Point Reyes Station. In Point Reyes

Station, take Sir Francis Drake Blvd. into the Point Reyes Nation Seashore (See Figure 4).

Saturday stop #1: San Andreas fault Travel to the second bridge on Sir Francis Drake Blvd., and pull off at a wide space on

the right side of the road. This is a narrow stop, and there is barely room for five vehicles. Front

vehicles will need to pull forward.

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Look around you. As we reached the swampy area near Point Reyes Station, we crossed

a linear trough that defines the fault trace of the San Andreas Fault. Sag ponds, displaced

streams and dating of fossil wood along this fault trace have allowed geologists to estimate the

average rate of movement for the San Andreas Fault. Right lateral displacement along this fault

has resulted in about 455 km (280 mi) of displacement since early Miocene time (Prentice and

others, 1991). Activity along this fault trace during the 1906 earthquake resulted in up to 8 m

(24 feet) of lateral movement, with an epicenter in the nearby town of Liman. The maximum

recorded displacement was at the second bridge. As we crossed the fault trace we moved from

the Franciscan complex in the east onto the Salinian Block in the west. The Salinian block is

probably an exotic terrain that probably migrated northward from the Los Angeles area. Tertiary

deposits and the geochemistry and mineralogy of the granodiorite core on Point Reyes are very

similar to deposits in the Santa Cruz mountains (to the south), leading to speculation the terrain

has migrated slowly northward through time. The Salinian terrain includes country rocks of

uncertain age that were intruded and metamorphosed during the middle Cretaceous Period, and

overlying Mesozoic to Cenozoic beach and shallow marine deposits. Follow Sir Francis Drake

Blvd. to the Point Reyes Lighthouse at the southwestern end of Point Reyes National Seashore.

Park in the parking lot, and walk toward the lighthouse.

Saturday stop #2: Point Reyes Lighthouse As you walk toward the lighthouse and visitor’s station, observe the sandstone and

conglomerate that outcrop along the path. This is the Paleocene Point Reyes Conglomerate.

Paleocene conglomerate overlies the Cretaceous granodiorite on this part of the Salinian block.

A reconstructed geologic history shows that granitic intrusions were covered by marine

sediments before being uplifted into the rocky headlands that we see today (Figure 5).

Activities at the Point Reyes Lighthouse:

Describe and sketch sedimentary structures in your field notebook.

Why is this section special to stratigraphers?

Look carefully at the clast composition in the Point Reyes granodiorite. What

clast types are present? Were all of these clasts derived from the underlying

granodiorite?

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What depositional environment was responsible for the emplacement of these

sediments?

Saturday Stop #3: Drakes Bay From South Beach: continue northward along Sir Frances Drake Blvd. until you see the

turnoff to Drake’s Bay. At Drake’s Bay we will observe the Drakes Bay Formation, a Pliocene

sedimentary unit that consists of interbedded sandstone, siltstone and mudstone. We will spend

some time looking for fossils at this stop. Be sure to look at the large blocks of sandstone behind

the visitor’s center. These blocks contain whale vertebrae, ribs and jaws, and may also contain

fragments of sea lion bones.

Activities and questions to answer at Drake's Bay:

What environment and water depth would these animals live in? These beds

were deposited in a marine environment, probably below wave base.

Walk out onto the beach, and observe the lateral continuity of a marker bed. How

far can you trace the bed before it disappears?

What grain sizes, sedimentary structures and bed contacts are found in this

depositional environment?

What other fossils can you find in the cliffs? Be careful near the cliffs, especially

if they are wet. Large blocks can fall at any time, often without warning. Small

black flecks in these sediments are fish bones, and have an unusual carbonized

mode of preservation.

What causes the large, reddish pattern in the rocks? Is there any preferential

orientation to this pattern?

At our next stop we will compare the angle of a high energy beach to the observations we

make at Drake's Bay. Describe the angle of the beach face and the width of the intertidal zone at

Drake's Bay so that we can make this comparison.

Saturday Stop #4: North Beach Return northward along Sir Frances Drake Blvd. until you see signs for North Beach.

Park in the North Beach parking lot. If time permits, we will have a lecture on eolian deposits,

and observe a modern high energy beach.

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Activities at North Beach:

Listen to a lecture on eolian deposits

Describe the shape and orientation of the Quaternary sand dunes that are common

at South Beach. These dunes have recently been stabilized by human-induced

vegetative planting, but the original dune morphology is still visible.

Draw a picture of a typical dune set in your notebook, and indicate the wind

direction on your sketch. What name (shape) is used to describe these dunes?

How common is this type of dune?

Observe the features of a high energy beach deposit. Can you identify ridge and

runnel structure? How steep is the beach face? What is the median grain size?

How wide is the intertidal zone on this beach?

Saturday stop #5 (optional)- Earthquake walk and visitor's center The visitor’s center and earthquake walk are an optional stop on the way back to camp.

This is an excellent display, and signs along the earthquake walk show the evolution of the

California coast. As you look at the displays, pay special attention to the information about the

tectonic history of California’s coast, and the timing of the shift from a subduction zone to a

transform margin.

Sunday Field Trip ActivitiesWe will break camp on Sunday morning. People should have vehicles loaded and be

ready to travel by 8:00 A.M.

Sunday stop #1: Campbell Cove (Bodega Head) Continue to drive south along Highway 1 into the town of Bodega Bay. Watch for signs

to Bodega Head and the University of California Marine Lab, and follow these signs out onto

Bodega Head. As the road starts to climb Bodega Head, turn into the parking area at Campbell

Cove.

When we left Highway 1 and crossed onto Bodega Head we crossed a sandy low-lying

area approximately 2.4 km wide that marks the surface expression of the San Andrea Fault

(Ristau and Bedrossian, 1978). Rocks exposed near Highway 1 are typical Franciscan

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metasediments, and rocks exposed on Bodega Head were deposited in a different sedimentary

and tectonic environment. Granodiorites approximately 80 to 90 million years old underlie

Bodega Head, and are part of the Salinian exotic block that is also exposed at Point Reyes (Fig.

5).

On Bodega Head, Salinian granodiorites are overlain by Quaternary marine and marine

terrace deposits. The contract between the underlying granodiorite and overlying Quaternary

marine deposits is visible along the high water line at Campbell Cove. This is an excellent

example of a rocky shoreline that has been preserved in the geologic record. Rocky shorelines

are rarely preserved in the geologic record, but on this field trip we will observe several excellent

examples of ancient rocky shorelines. The conglomeratic lag at the contact between the

granodiorite and sandstone indicates that erosion took place before deposition of the sandstone.

This excludes the possibility that Quaternary sandstones were intruded by the granodiorite.

Activity at Campbell Cove:

Describe the contact between the granodiorite and sedimentary rocks. Draw a simple

sketch in your field notebook to illustrate the relationship. What grain compositions

are present in the basal lag?

Sunday stop #4: Bodega HeadRetrace the main road downhill to the parking area on the southwest corner of Bodega

Head. We will take a brief look at the exposed granite at this locality, and discuss the

metasedimentary inclusions that are present in the granodiorite. STAY AWAY FROM THE

EDGE OF THE CLIFF. There is a drop several hundred feet to the crashing waves at the base of

Bodega Head, and if you slip you will die. When we have discussed the granodiorite, we will

follow a footpath to a narrow beach below the parking lot. Quaternary sedimentary beds above

the modern beach will be used as an “unknown” exercise to allow you to make your own

interpretations about the depositional environments of this area during Quaternary time.

Activities at Bodega Head:

Make a detailed list of the sedimentary structures that you observe, and describe the grain

sizes, textures, composition and bed contacts of each significant bed.

Interpret the depositional environment, based on your observations. Describe the energy

level, slope, type of flow(s), and type of fluid(s).

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ReferencesChipping, D.H., 1971, Franciscan Formation near Bodega Bay and Jenner, California: in

Geologic guide to the northern coast ranges to Point Reyes region, California, ed. by Lipps, J.H. and Moores, E.M, Annual field trip of the Geological Society of Sacramento, p. 47-57.

Galloway, A. J., 1977, Geology of the Point Reyes Peninsula, Main County, California: California Division of Mines and Geology Bulletin 202, Sacramento, 72 p.

Ingersoll, R. V., Rich, E. I., and Dickinson, W. R., 1977, Great Valley Sequence, Sacramento Valley: Cordilleran Section Geological Society of America Annual Meeting Field Trip8, 72p.

Prentice, C., Niemi, T.M., and Hall, N.T., 1971, Quaternary tectonics of the northern San Andrea Fault, San Francisco Peninsula, Point Reyes, and Point Arena, California in Geologic excursions in northern California: San Francisco to the Sierra Nevada, ed. by Sloan, D., and Wagner, D.L., California Department of Conservation Division of Mines and Geology special publication 109, Sacramento, p. 25-34.

Ristau, D.A., and Bedrossian, T.L., 1978, Physiographic setting of Bodega head, in, Geologic guide to the northern California coast ranges- Sacramento to Bodega Bay, ed. by J.C. Kramer, Annual field trip guidebook of the geological society of Sacramento, p. 84-90.

Wahrhaftig, C., 1984, A streetcar to subduction and other plate tectonic trips by public transport in San Francisco: Washington, American Geophysical Union, 76 p.

Wright, W.H., 1996, Shell Beach; A close look at a subduction zone. Unpublished field report, 4 p.

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Figure 1: Major geologic provinces in northern California. From Ingersoll, Rich and Dickinson, 1977.

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Figure 2: Evolution of the California coast, Great Valley sequence, and Franciscan Group. From Ingersoll, Rich and Dickinson, 1977.

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Figure 3: Submarine fan model proposed by Walker, 1978. From Boggs, Principles of Sedimentology and Stratigraphy, 1987.

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Figure 4: Geologic map of Point Reyes National Seashore.

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Figure 5: Diagrammatic history of the Salinian blocks, showing intrusion of granodiorite during Cretaceous time, deposition of marine sediments over the exposed granite, and uplift to the present position. From Galloway (1977).

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