Download field project

GEOLOGY OF THE NATIONAL PARKS
FIELD PROJECT
NICOLE VELAZCO
GEOLOGY 350
MAY 13, 2015
TABLE OF CONTENTS
Introduction ................................................................................................ 1
Location Map............................................................................................... 3
Site #1, Sedimentary: Graywacke And Shale, Devil’s Slide ........................... 5
Site #2, Igneous Plutonic: Granodiorite, Devil’s Slide ................................ 10
Site #3, Igneous Volcanic: Pillow Basalt, Point Bonita ................................ 14
Bibliography .............................................................................................. 14
Camera: Canon 40d/iPhone 6
Reviewed by__________________________
1
INTRODUCTION
The Bay Area is a region of California that belongs to two different
tectonic plates, the North American Plate and the Pacific Plate. The
boundary separating the regions is the San Andreas fault, a transform fault
formed ~30 million years ago. The transform boundary that exists today
was once a convergent boundary approximately 100 million years ago
(Sloan, 2006, pg. 31). The force of compression was causing the Farallon
Plate to subduct beneath the North American Plate, causing oceanic rocks
from the Farallon Plate, like basalt, to be left on the North American Plate.
When the Farallon Plate was completely subducted, the North American
Plate came into contact with the Pacific Plate. The boundary between the
plates changed from convergent to transform, and the process of subduction
was replaced with a strike-slip motion.
Today, to the west of the San Andreas Fault, is the Salinian Complex,
which is made mostly of the igneous plutonic rock granodiorite. To the east
is the Franciscan Complex, which is made of graywacke, shale, chert, pillow
basalt, schist, and serpentinite.
Devil’s Slide, being a coastal location on the Salinian Complex, is a
great place to see various types of sedimentary and granitic rock. Devil’s
Slide has always been a place of interest for me since my family moved to
Pacifica over 10 years ago. It’s name made it seem like a place of danger,
2
and it was – constant landslides, closures, accidents, and even deaths. The
recent opening of the Tom Lantos tunnels diverted traffic away from Devil’s
Slide, and it has since been converted into a trail. It was too dangerous to
study the rocks along Devil’s Slide when it was still part of Highway 1, as
the road was too narrow, but now it is easily accessible and only open to
foot traffic. Point Bonita has some of the best examples of pillow basalt in
the Bay Area, with outcrops visible on the trail to the lighthouse and a sea
arch that is being carved out by ocean waves. Being able to see outcrops of
pillow basalt along the trail would allow for close up inspection, and the
views of the pillow basalt in the water would allow me to see fresh
exposures of the volcanic rock.
3
LOCATION MAP
4
5
SITE #1, SEDIMENTARY: GRAYWACKE AND SHALE, DEVIL’S
6
SLIDE
7
SITE #1, SEDIMENTARY: GRAYWACKE AND SHALE, DEVIL’S
SLIDE
SITE DESCRIPTION
Along the Devil’s Slide trail, there are several outcrops of the
sedimentary rocks graywacke and shale. The light gray colored,
thicker layers are composed of graywacke, while the darker
gray/black, thinner layers are shale. Graywacke has small, fine
particles that are visible to the eye, and shale also has small, but finer
sized particles. There are deposits of the mineral calcite present in
some layers of the graywacke. The mineral was determined to be
calcite when it fizzed after a test using hydrochloric acid. The orange
colors in the layers come from the oxidation of iron. The chert appears
to have low iron content, as there is more evidence of oxidation in the
layers of graywacke.
The crumbling and fragmented surface of these alternating
layers of graywacke and shale show signs of weathering. These rocks
are constantly exposed to water, through rain and fog, wind, and
interaction with animal and plant life. Between layers, along joints
and faults, and at the top of the outcrop, there is plant and algae
growth. The plants, roots, and soil are also contributing to the
weathering of the rock. This continuous crumbling and weathering of
the rock reveals fresh, darker gray and black surfaces of graywacke
8
and shale. The graywacke breaks in a more angular way than the shale,
which breaks off in smaller chips.
SITE INTERPRETATION & EXPLANATION
The graywacke and shale were lithified in a marine environment
during the Cenozoic Era, approximately 65 million years ago (Sloan,
2006, pg. 174). The fine-grain size of both the graywacke and shale
show that their comprising sediments were deposited in an
environment with relatively low movement, where the small particles
could settle and lithify. This environment was most likely an oceanic
ridge or trench, into which turbidity currents deposited sediment. The
turbidity currents that deposited the sediment were likely triggered
by earthquakes, and each layer of graywacke is evidence of an
earthquake. As seen in Photo #1-a, the layers of graywacke range from
a couple inches to almost one foot in thickness. Different magnitudes
of earthquakes might have caused the varying sizes of layers, with
larger earthquakes causing larger turbidity currents and bigger
depositions of sediment.
This outcrop is inclined and sits at either a ~45° or ~135° angle.
According to the Principle of Original Horizontality, sedimentary
layers are always deposited horizontally, so this outcrop must have
9
been titled after it was lithified, incurring many joints and faults in
the process. This once marine site is also now high above sea level.
This indicates that plate motion and uplift have taken place to move
the layered rock to its current position.
10
SITE #2, IGNEOUS PLUTONIC: GRANODIORITE, DEVIL’S
SLI
DE
11
SITE #2, IGNEOUS PLUTONIC: GRANODIORITE, DEVIL’S
SLIDE
SITE DESCRIPTION
At the southern end of the Devil’s Slide trail, the visible rock
types change from mostly sedimentary to granodiorite of Montara
Mountain.
The large visible crystals in the granodiorite are plagioclase feldspar
(milky white), quartz (grayish), and biotite (black). The characteristic
texture of this rock is phaneritic, which can easily be seen by the
random arrangement of the visible crystals. Throughout the outcrop,
there are also several calcite veins. One of the larger veins, as seen in
the top portion of Photo #2-a, shows faulting along multiple sections
of the vein. Photo #2-b shows a calcite vein up close, and how it is
more resistant to weathering than the granitic rock it intruded. As an
intrusion, the calcite veins are younger than their host rock, according
to the Principle of Cross-cutting relations. There are several
intrusions, joints, and faults all throughout this outcrop and in the
surrounding outcrops.
The granodiorite is not very hard as a knife easily scratches it,
and the bumpy and crumbling surface shows the granodiorite here has
been very weathered. A nearby sign warns pedestrians of falling rock,
an indication that the surface is constantly changing as small to
12
boulder-sized fragments break off. Fresh exposures of granodiorite
show the colors of its crystals clearly, a mixture of whites, grays, and
blacks. This outcrop is tinged with brown and orange from soil,
erosion of the surrounding sedimentary rock, and orange algae
growing on the surface.
SITE INTERPRETATION & EXPLANATION
Granodiorite is the basement rock of the Salinian Complex, and
it formed during the Mesozoic Era ~100 million years ago (California,
2013). The graywacke and shale above and around the granodiorite is
~35 million years younger, so there is a large gap in the geologic
record of these rocks. This indicates that the surface of contact is an
unconformity as millions of years of rock is missing due to erosion.
Granodiorite is originally molten rock that cooled and solidified
deep underground. Igneous plutonic rocks form over 1km under the
Earth’s surface, so this outcrop has been uplifted ≥1km from beneath
the Earth. As a part of the Salinian Complex, this granodiorite must
have also formed as far south as Mexico, and traveled northwest along
the San Andreas Fault for millions of years to its current location.
Uplift and the strike-slip motion of the San Andreas Fault have moved
these rocks thousands of miles, forming many joints and faults in the
process.
13
14
SITE #3, IGNEOUS VOLCANIC: PILLOW BASALT, POINT
BONITA
15
16
17
SITE #3, IGNEOUS VOLCANIC: PILLOW BASALT, POINT
BONITA
SITE DESCRIPTION
Point Bonita is characterized by the abundance of the igneous
volcanic rock basalt, which is found in the form of pillows throughout
the area. The outcrops of basalt along the trail are above sea level and
have been exposed for a much longer period of time than the pillow
basalt still in the ocean. The basalt in and near the ocean is more
newly exposed and darker in color than the pillow basalt higher above
sea level, which is gray and lighter in color. The dark gray/black
colors indicate a high level of iron. A close up view of the pillow basalt
shows many surfaces of the older exposures are turning orange from
the oxidation of iron. Photo #3-a shows a layer of chert in the pillow
basalt, which also has a red-orange color from the oxidation of iron.
The basalt has finer grains than the chert and is much harder.
Photo #3-c shows a cross section of an outcrop of pillow basalt,
and the different sizes of the pillows can be seen. The sizes of the
pillows range from a couple feet to several feet across. Photo #3-c also
shows how weathering occurs more rapidly around the pillows of
basalt at the top of the outcrop, which are older and have been
exposed longer. The gaps that separate the pillows are larger and the
18
pillows are turning a light gray/greenish color from the iron-rich
mineral chlorite.
SITE INTERPRETATION & EXPLANATION
During the Mesozoic Era, as the Farallon Plate was subducting
beneath the North American Plate, basalt, chert, and shale were
scraped off and became part of the North American Plate. This
explains the presence of the pillow basalt in the Franciscan Complex.
The pillow basalt in Point Bonita must have formed ~100 million years
ago when the Farallon Plate was still converging with the North
American Plate (Wahrhaftig & Murchey, 1987, pg. 263).
Basalt is an igneous volcanic rock formed by the solidification of
lava, and because this outcrop of basalt is pillow-shaped and not
columnar, it was formed underwater in a marine environment. The
lava was cooled upon contact with ocean water and solidified into
pillow shapes that piled on top of each other. The presence of chert
shows that these rocks formed in an oceanic ridge. Chert lithifies in
oceanic ridges, and because it is on top of and below pillows of basalt,
the basalt must have also solidified at an oceanic ridge. The layer of
chert can also indicate a period of several million years with little to
no volcanic activity in which microfossils had time to settle and lithify.
19
20
BIBLIOGRAPHY
California Department of Transportation. (2013). Geotechnical Aspects
at
Devil’s Slide. Retrieved from
http://www.dot.ca.gov/dist4/dslide/images/devils%20slide_1u
pdate d.pdf
Sloan, D. (2006). Geology of the San Francisco Bay Region. (pp. 31-32,
174-180) Berkeley and Los Angeles, CA: University of California
Press, Ltd.
Wahrhaftig, C., & Murchey, B. (1987). Marin Headlands, California:
100- million-year record of sea floor transport and accretion. In Hill,
M.
(Ed.), Cordilleran Section of the Geological Society of America (pp.
263-268). Boulder, CO: Geological Society of America, Inc.