This guide is intended for those students who missed the field trip and must make it up on
their own.
It is highly recommended that you go to the class web site at
http://www.mpcfaculty.net/alfred_hochstaedter/geology.htm
and download a color version of this guide and print it out for yourself.
Click on “Field Trip #1 – key with pictures”
Introduction
Exploring sediments and rocks of the seafloor is one of the themes of our first
weekend field trip. As we travel from stop to stop imagine yourself traveling around on
the seafloor. Through the combination of this field trip and the Pt Lobos field trips, we’ll
be able to visit many of the seafloor areas we’ve talked about in class, including the
submarine canyon like the Monterey Canyon, a submarine fan, a mid-ocean ridge, and
the deep sea floor of the abyssal plain.
Some of the other themes are the deposition of river gravels, and the contrasts
between the rocky emergent coastline of Big Sur and the smooth, sandy coastline of a
more stable environment like Marina and Ft Ord.
Field Trip stops
Monterey Formation on Olmstead
Road.
The main thing to notice here
is the Monterey Formation
Diatomaceous Shale. To the naked
eye, this rock looks like mudstone.
But if we had powerful microscopes,
we could see all of the diatoms that
comprise this rock. Since there is very
little terrigeneous material in the rock,
we know that it was formed far from
continents. It was probably formed in
the deep oceans of the abyssal plains.
What is the
evidence that
tectonics moved
these rocks up to
this spot and
higher on the hill
rather than the
sea level
dropping to
expose these
rocks?
Folding and Faulting in the rocks. Rocks higher on hill would mean that sea level was
once several hundred meters higher, which is unlikely because the current inter-glacial
period suggests we are now at a sea-level high.
Monterey Formation on Olmsted Road. Note continuous,
coherent beds. The Monterey Formation is composed of claysized particles and diatoms, silica-rich organisms that die and fall
to the bottom of the sea and accumulate there.
How did the rocks
move from the seafloor
to the North American
continent? Through
tectonic processes. When
plates or blocks of rocks
converge, the rocks in the
middle deform in such a
way that they get thicker.
Getting thicker means
creating mountains and
mountain ranges. Imagine
continents colliding and
mountain ranges forming.
Mountaineers at the top of
Mt Everest in the Himalayas
have found fossils of
creatures that lived in the
oceans.
Ike looking closely at the Monterey
Formation. Or maybe he’s looking at
a spider.
Jack pointing out cherty layers in the
Monterey Formation. The cherty layers
probably contain more diatoms than
the layers above and below (buff
colored layers at left), which contain
more clay. Chert, almost entirely SiO2,
weathers in a more brittle manner than
the more clay-rich layers.
Garland Park. Park in the
main parking area for
Garland Park. Cross the onelane bridge to the other side
of the river. Look at the
hydrograph on the bridge.
On the other side, examine
the flood-plain and check
out the pictures of the last
big flood at the visitor
center.
On the way back, go down
below the bridge to see the
river gravels in the Carmel
River.
What kind of sediments
occur in the river bed? What kind of rocks occur? What is the size of the clasts? How is
the sorting?
Well sorted and well rounded river gravels. Rock types include granite, metamorphic
rock, and a little bit of Monterey formation, exactly those types of rock that occur
upstream in the Carmel Valley from Garland Park.
In the visitor center, when did the last major flood occur in the Carmel River drainage
system?
1995.
What information can you gleam from the hydrograph on the bridge over the Carmel
River?
What the total amount of flow down the Carmel River was for every year.
Well sorted, large grain
size river gravels. The
class is examining the
large assortment of rock
types in the river here.
We found nice granites,
schists, and gneisses. All
of these come from the
crystalline core of the
Santa Lucia Mountains.
See examples of rock
types on the next page.
A nice gneiss and a
potassium feldspar grain
found in the Carmel River
gravels
Timmy holds up a
metamorphic rock with nice
garnets in it. Those reddish
blobs are garnet.
More rock types found in the Carmel
River gravels include milky quartz
grains, a mafic basalt or metamorphic
rock (the big dark one), and some
smaller crystalline samples.
Carmel Valley Road
Outcrop There are two
types of sediments or
rocks exposed here. One
of them is the Monterey
Formation.
What process formed the
other deposit?
The other deposit overlies
the Monterey Formation
and consists of river
gravels. River processes
deposited these gravels.
They look just like the
gravels viewed along the
Carmel River in Garland
Park.
What is the geologic history of this place?
First the Monterey Formation was deposited on an abyssal plain, uplift of the Monterey
Formation to the elevation of the ancestral Carmel River, the ancestral Carmel River
flowed through and deposited the river gravels, the Carmel River continued to erode
downwards to form the present-day Carmel Valley that we can see before us.
A photograph of the Carmel
Valley gravels from a previous
field trip. Note the Monterey
Formation at lower right and the
Carmel Valley gravels in the
center and upper left. The
contact between the two is
depositional, but is an
unconformity, which means that
erosion must have taken place
between the time of the
deposition of the Monterey fm
and the time of the deposition of
the Carmel Valley gravels. The
Carmel Valley gravels were
deposited by a river thatrepresent the ancestral Carmel River.
Monastery Beach A nearby canyon causes deep water to occur very close to shore here.
Also, good views of marine terraces.
What differences can you see in grain
size of the beach?
Grain size of the sand here is bigger
than at many other beaches.
What is distinctive about the slope
angle of the beach as it reaches the
water?
The slope angle of the beach is steeper
than at many other beaches. It probably
is caused by the way that the waves
crash on a very steep offshore area.
There is a canyon just offshore.
Garrapata Beach
Two main things to see here:
the distal turbidites and the
San Gregario Fault. The
sedimentary rocks here are
distal turbidites. That means
that they are deposited by
turbidity currents, but far out
on the submarine fan.
Remember these rocks and
compare their grain size to
the rocks that we’ll see at Pt
Lobos later.
This image of the shelf-canyonslope-rise system shows the
depositional setting of the
Garrapata Beach turbidites.
What is the evidence that the turbidites here were deposited far from shore on the
continental slope?
Relatively fine grain size.
Coarse sand is the largest
grain size. Evidence for
turbidites is the graded
bedding.
At left are the thinner
turbidites. Graded
bedding is difficult to see
in the photograph. You
can see however, that the
rock consists of
interbedded fine sands
and shale.
At left are some of the
thicker turbidite
deposits. Note the
thicker sandstone layers.
Shale layers are fairly
thin here. A close
examination of the
sandstone layers
revealed that they are
coarser grained to the
lower right. Thus,
stratigraphic up is to the
upper left.
Also at Garrapata Beach, we’ll see the San Gregario Fault. The San Gregario Fault is part
of the San Andreas Fault system and is one of the faults that separates the Pacific plate
from the North American Plate. It is the same kind of fault as the San Andreas Fault.
What kind of fault is the San Gregario Fault?
It is a right-lateral strike-slip fault, just like the San Andreas. Evidence for this kind of
slip motion comes from the slikensides just beneath the old stairs.
This is the location of the San Gregario
Fault.
How were you able to locate the San Gregario Fault on Garrapata Beach? Where is it?
I hope everybody was able to locate it. The fault is located between the granite and the
turbidites, just beneath the old stairs.
Here’s the granite near the stairs. See the evidence of springs, or groundwater, both of
which are very common near fault outcrops.
Here’s some very disrupted mudstone/shale near
the fault. Movement along the fault has probably
removed any coherency from these fine-grained
sediments.
Here are some slickensides, or
striations caused by movement along
the fault, in granite very close to the
stairs. The horizontal orientation of
these striations indicates that the
SanGregorio fault is a strike-slip fault.
Regional studies indicate it is a rightlateral fault.
Hurricane Point Excellent view of
the Big Sur coastline, one of the
worlds best examples of an
emergent coastline. It just doesn’t
get any better than this! The rock
here is a marble, which is a
metamorphosed limestone.
Limestone is deposited in warm
shallow seas where coral and
tropical fish make it nice to snorkel.
Then the limestone gets
metamorphosed into marble.
What is the difference between a
marine terrace and a wave-cut
platform?
A wave-cut platform is at sealevel, whereas the marine terrace
is above sea level.
Image at left is a great example of
a wave-cut platform. It is located
near the town of Bolinas, north of
San Francisco.
This image shows a wave-cut
platform in the making at
Hurricane Point. In this image, the
waves are breaking on the wavecut platform.
Note the house on the cliff built
on landslide deposits.
Andrew Molera State
Park We’ll walk to the
mouth of the Big Sur
River here and see the
results of volcanism and
sedimentation at a midocean ridge. Note the
pillow basalts and chert.
We’ll also note wave
refraction as the waves
move in towards the
beach. Can you find any
pink sand? If you can I
think it is composed of
the chert that crops out
nearby.
What is the evidence
that this group of
rocks were
deposited near a
mid-ocean ridge or
on a seamount?
The occurrence of
pillow basalts and
chert, a deep sea
sediment.
In this image, I’m bending down
to get a good look at some pillow
basalts.
The purple sand is somewhat of a
mystery. Upon close inspection it
looks crystalline and translucent.
It could be garnet eroded from
rocks washed down the Big Sur
River from the crystalline interior
of the Santa Lucia Range, or it
might be chert, from the local
outcrops fo chert. Or it could be
something else, that we haven’t
thought of yet. The translucent
nature favors the garnet explanation, but there just isn’t that much garnet coming down
the river. Chert would probably be darker in grains that small.