Download Geologic Evolution of Point Lobos

The Geologic Evolution of Point Lobos
The rocks at Point Lobos originated far away, under
very different conditions from those that prevail here today.
Their origin and translocation involves the interaction of great
plates of the earth's crust. Plate tectonics, the term commonly
used to describe the effects of this interaction, derives from
heat variations within the earth's mantle. These variations are
thought to drive huge convection cells within the mantle (Fig.
1). Upward-rising mantle creates large cracks in the earth's
lithosphere, oceanic ridges that are the site of volcanism and
new crust formation (Figs. 1 and 2). The new crust moves
away from the oceanic ridge and forms a plate which extends
across the ocean floor until it encounters another plate (Fig.
3). The oceanic ridges are major features of the earth’s
surface and can span entire oceans (Fig. 4). The ridges are
broken by large fractures in the crust called transform faults
(Fig. 4), across which the ridges step sideways and newly
created crust moves laterally (Fig. 5).
LITHOSPHERE
MANTLE
EARTH'S CORE
Clifton, 2010
Figure 1. Cross section of the earth showing the nickel-iron core,
the extremely dense plastic mantle and the rigid outer shell, the
lithosphere. Convection cells in the mantle cause the lithosphere to
crack and spread apart.
Figure 2. Cross-section of the outer part of the earth where
mantle material wells up in a crack on the earth’s surface. The
cracks occur predominantly in the ocean and form extensive
oceanic ridges. Newly formed crust spreads away from the
crack, giving rise to the term “sea-floor spreading”.
1
OCEANIC PLATE
CONTINENTAL PLATE
OCEANIC RIDGE
CONTINENTAL CRUST
(CRATON)
OCEANIC CRUST AND LITHOSPHERE
Eurasian Plate
Figure 3. Idealized oceanic plate
extending from an oceanic ridge to an
adjacent continental plate.
Eurasian Plate
Plates global
Indian
Plate
North American Plate
Arabian
Plate
Pacific Plate
Cocos
Plate
Phillipine
Plate
Indo-Australia Plate
South American
Nazca
Plate
Plate
OCEANIC
RIDGE
African Plate
Antarctic Plate
Figure 4. Major lithospheric plates of the world. Inactive (passive) continental margins shown in blue; active margins in red. Active margins consist of relatively
young rocks and are subject to earthquakes and volcanoes. Dark red lines denote oceanic ridges, which are broken by fracture zones and transform faults (dark
heavy lines). Modified from Tilling, Heller and Wright (1987) and Hamilton (1976).
2
Transform fault FRACTURE
FRACTURE
ZONE
The geologic history at a continent’s margin depends
greatly its position relative to a plate boundary (Fig. 4).
Many continental margins, such as those on the east side of
North and South America, most of Africa and all of Australia
lie well within a plate. Such margins are geologically “quiet”
sites of sediment accumulation. Other continental margins
coincide with the boundary between crustal plates. Rock
here is continually being created and destroyed.
Earthquakes and volcanoes are common features. The
geologic history on an active margin is further controlled by
the nature of the interaction. Where the boundary between
the plates parallels the transform faults, the plates slide
sideways relative to each other. Such a margin is called a
“transform margin” (Fig. 7).
TRANSFORM
ZONE
TRANSFORM
FAULT FAULT
OCEANIC
RIDGE
OCEANIC
OCEANICRIDGE
RIDGE
!
FRACTURE
ZONE
Oceanic Crust
Mantle
FRACTURE
ZONE
OCEANIC RIDGE
Figure 5. “Transform faults” and “fracture zones are giant cracks in the
earth’s crust. In the ocean basins, oceanic ridges are commonly displaced by
transform faults. The ridges are not necessarily moving apart, but the crust
on opposite sides of the fracture may be moving in opposite directions or, in
fracture zones, in the same direction, but at different rates and elevations.
Figure 6. Oceanic crust, composed of heavier elements,
differs significantly in thickness from continental crust,
which consists of less dense minerals. Part of the reason
for this difference in thickness is that oceanic crust is
continually being formed and is destroyed before it can
accumulate to great thickness. The oldest oceanic crust
known is about 270 million years old, much younger than
continental crust, which can be billions of years old.
100 km
(60 mi)
3
Where the plates collide head-on a different process
occurs. If the other plate consists of continental crust, the
thinner, more dense oceanic plate tends to slide beneath
the continental plate in a process called subduction. The
force of the collision generates heat and pressure; water
carried into the system reduces the melting point of the
rocks; five to ten miles below the surface of the earth, they
can become a fluid magma (Fig. 8).
Figure 7 shows the distribution transform and subducting
margins on the western sides of North and South American
continents. Central and southern California lie on a
transform margin on which the Pacific Plate is sliding to the
northwest relative to the North American Plate. Much of
Northern California and the Pacific Northwest lies on a
subducting margin. It is no accident that volcanoes in the
“Ring of Fire” around the Pacific Ocean lie along the
subducting margins: the Andes, Central America, the
Pacific Northwest and the Aleutian Chain (Fig. 7). While
damaging earthquakes can occur anywhere on an active
margin, areas of subduction are most likely to suffer giant,
tsunami-generating earthquakes.
SUBDUCTION
TRANSFORM
(SLIDING)
MARGIN
SUBDUCTING
(COLLISIONAL)
MARGIN
Figure 7. Plate boundaries on the eastern side of the Pacific Ocean. In some areas
the plates are sliding past one another (“transform margin”). In others the plates are
colliding head-on (“subducting margin”). The nature of the boundary controls the
geological processes and history.
CONTINENTAL
CRUST
Figure 8. Subduction results where oceanic crustal plates collide head-on with
continental crustal plates. The denser oceanic crust is driven beneath the lighter
continental crust. Water carried by this process into the depths of the earth
lowers the melting point of the rocks. The resulting molten rock (magma) can
vent to the surface in the form of volcanoes.
.
OCEANIC CRUST
4
Most geologist today agree that Salinia is a southern
extension of the Sierra Nevada that was rifted north
by movement on the San Andreas Fault. The
processes whereby this happened are shown in the
following sections.
The geology of the Central California Coastal area long
mystified geologists because it consists of a large elongate
block of continental crust (the Salinia Terrane) (Fig. 9)
encased in oceanic crust. In the 1980’s a group of
geologists thought that remnant magnetism in this rock
indicated it originated from near the equator. Their
subsequent analyses, however, did not support this
hypothesis, and it was abandoned.
SN
SIERRA NEVADA BATHOLITH
SA
SALINIA TERRAIN
TERRANE
SFB
OCEANIC CRUST
M - Monterey
M
SN
SA
SB - Santa Barbara
SFB - San Francisco Bay
SAF - San Andreas Fault
- Monterey Peninsula
Faults
PACIFIC OCEAN
SAF
SB
Figure 9. A large block of continental crust, called the “Salinia Terrane” (SA on this figure) long puzzled geologists
because it lay sandwiched between masses of oceanic crust.
5
The western margin of North America has long been the
battleground between the continental plate and a series of large
oceanic plates. For many tens of millions of years, a large
oceanic plate, the Farallon plate, collided with the North
American continent, generating a linear chain of large magma
chambers (masses of molten rock). These masses cooled and
were later uplifted to become the rocks that form the presentday Sierra Nevada Range. Eighty million years ago (80 Ma), the
North American continent was split in two by a large northsouth seaway (Fig. 10). In the area of present-day Southern
California and northern Mexico, magma chambers created by
subduction of the Farallon plate were cooling, forming an
igneous rock. Somewhere, probably in northwestern Mexico,
granodiorite was crystallizing 5 miles or more below the surface
of the earth (Fig. 11). This rock would later be exposed as the
granodiorite at Point Lobos and the Monterey Peninsula.
Figure 10. Inferred paleogeography of North America and the North Pacific in
the Late Cretaceous Period, around 85 million years ago. A large seaway
splits the continent. On the western continental margin, the large Farallon
oceanic plate collides with the North American plate, creating subduction all
along the margin (redrawn from Orr and Orr, 2002). Dashed line: current
outline of North America.
Figure 11. Eighty million years ago, a magma of molten rock material
created by subduction was cooling in what probably today is northwestern
Mexico. The newly crystallized solid rock will be called “granodiorite”
because of its mineral composition. It is this rock that underlies Point Lobos
and the entire Monterey Peninsula today. Dashed white lines: current
western state boundaries. Red “X” indicated the possible location of what
would become Point Lobos. The compilation of this and other paleogeographic maps show here is discussed briefly at the end of this paper.
6
How do we know when the granodiorite crystallized? The
systematic decay of trace amounts of radioactive potassium in
the feldspar provides a clock for dating the time of
crystallization. Analyses indicate that this occurred during the
Late Cretaceous period, nearly 80 million years ago (Kistler, R.
W., and Champion, D. E., 2001, U. S. Geological Survey OpenFile Report 01-453). Volcanoes may have erupted above the
chamber, but their existence has since been lost to erosion.
upward along a large thrust fault. In any event, lying deep
beneath the ocean’s surface, the granodiorite was covered by
mud and rocky debris that slid down the ancient undersea
slope. The resulting deposits are reflected by the shale and
limestone breccia beds exposed at Gibson Beach. Rare, poorly
preserved fossils in this rock indicate a late Paleocene age
(Bowen, 1965).
The next geological event that is recorded at Point Lobos
probably occurred in the late Paleogene Epoch, some 20-25
million years after the granodiorite crystallized (Fig. 12). By
this time the granodiorite had shifted from its position miles
below the earth’s surface and now lay exposed on the
continental margin (Fig. 13). How this happened is still debated
among geologists. Some think it was pushed westward and
granodiorite
25-30 my
Figure 13. In the late Paleocene Epoch, 55-60 million years ago, mud
accumulated on top of granodiorite exposed on the continental (or basin)
slope. Consolidated long ago into rock, this material is exposed today in the
cliff behind Gibson Beach. Red “X” indicated the possible location of what
would become Point Lobos.
Figure 12. After the granodiorite crystallized nearly 80 million years ago, the
next geological event recorded at Point Lobos appears to be in the late
Paleocene Epoch, 55-60 million years ago. Dinosaurs no longer existed,
having died out 65-66 million years ago at the end of the Cretaceous Period.
7
the early Eocene
granodiorite
25-30 my
Figure 14. Most of the Carmelo Formation at Point
Lobos consists of submarine canyon fill. The few
microfossils found in this sediment indicates that it
accumulated in the early Eocene, some 50-55 million
years ago.
Although the sedimentary rocks at Gibson Beach are
considered part of the Carmelo Formation (a term given to
deepwater sandstone, shale and conglomerate deposited in
this area during the Eocene and Paleocene epochs), they
differ markedly from the rest of the Carmelo Formation at
Point Lobos, which is mostly conglomerate and sandstone.
Fossils are very scarce in this latter rock, but microfossils
collected from mudstone at Hidden Beach indicate deep (600
to 2000 feet) marine deposition during the early Eocene
(Clark, 1997) (Fig. 14). Sedimentary features in this rock
similarly support a deepwater origin. Since the sandstone
and conglomerate fill a valley cut into the granodiorite, the
deposit is considered to represent the filling of a submarine
canyon cut into an Eocene continental slope that existed in
what is now northwestern Mexico (Fig. 15).
Continued sedimentation buried the sand and gravel that
accumulated within the submarine canyon. Under the
weight of this overburden, the sand, gravel and mud
gradually converted to rock: sandstone, conglomerate and
mudstone, the material we see today at Point Lobos.
Figure 15. During the early Eocene, gravel, sand and mud accumulated in the
submarine canyon that existed on the ancient continental margin in what today is
Southern California. Red “X” indicated the possible location of what would
become Point Lobos.
8
So far in our reconstruction, the interaction of the North
American continental plate and the oceanic plate on its western
margin has consistently been one of collision; subduction has
reigned all along the west coast of North America. That,
however, was about to change in the late Paleogene period.
Another oceanic plate, the Pacific Plate, was approaching the
coast from the south and the Farallon Plate was being
consumed by subduction. As the oceanic ridge that separates
the Farallon and Pacific plates (known today as the “East Pacific
Rise”) approached the continent, Fig. 16).
At some time during the Oligocene Epoch, between 23 and
34 million years ago, the Pacific Plate encountered the North
American Plate. The Farallon Plate split into two segments: the
Juan de Fuca Plate to the north and the Cocos Plate to the
south. As the Pacific Plate moved north, it dragged against the
continental plate. As a consequence the East Pacific Rise (our
term today for the oceanic ridge on the eastern side of the
Pacific Plate) and associated transform faults rotated in a
clockwise direction along the continental margin. The right-
lateral displacement (the right side moves toward you) that
characterizes the boundary between the Pacific and North American
plates today was initiated.
Figure 16. Around 35 million years ago, in the early Oligocene Epoch, the
Farallon Plate was being consumed by the process of subduction and the
northeastern edge of the Pacific Plate approached the North American
continent.
Figure 17. By the late Oligocene, the Pacific Plate had encountered the North
American continent and the Farallon Plate split the smaller plates Juan de Fuca
and Cocos Plates. Transform faults become parallel to the boundary between
the two plates and a transform margin begins to develop.
9
At some point during this initial collision between the
Pacific and North American Plates, a volcano spewed lava
(“basaltic andesite”) over what would become the Monterey
Peninsula. Radiometric dating places the time of eruption at
27 million years ago, during the Oligocene Epoch (Fig. 18). At
that time, the Eocene canyon fill and the Cretaceous
granodiorite had been uplifted and were exposed at the earth’s
surface. The basaltic andesite is not exposed at Point Lobos,
but it does crop out at short distance away at either end of
Carmel Beach (Fig. 19). It probably covered the Carmelo
Formation and the granodiorite at what would later become
Point Lobos, but has since been eroded away.
Oligocene Volcanism (27 my)
25-30
Carmelo
Fm
granodiorite
Figure 18. Twenty-seven million years ago, this area was the site of volcanic
activity
Figure 19. Basaltic andesite outcrop at the north end of Carmel Beach.
Figure 20. Twenty-seven million years ago, volcanic rock lie at the earth’s
surface. It is unclear when the volcanism occurred relative to the collision
between the North American and Pacific plates. The cartoon here shows it
happening well after the initial encounter, but this is pure speculation. Red
“X” indicated the possible location of what would become Point Lobos.
Photograph by John Lyons.
10
As the boundary between Pacific and the Farallon Plates
shifted northward, the transform margin separating the Pacific
Plate from the North American Plate expanded. The timing of
the next event that is recorded in the rocks of this area is very
poorly constrained. Across Carmel Bay from Point Lobos, a
striking granitic boulder conglomerate lines the northeastern
shore of Stillwater Cove. Fragments of basaltic andesite in the
conglomerate document its age to be less than 27 million years,
but how much less is uncertain. It is older than the middle
MIocene sandstone that underlies Carmel Beach. In this
discussion, I refer to it only as “early Miocene?” (Fig. 21).
Early Miocene? alluvial fan
alluvial
fan
volcano
Carmelo
Fm
granodiorite
Figure 21. At some time following the Oligocene volcanic eruption, but before
the middle Miocene shallow water sands accumulated, a coarse alluvial fan
composed of granitic boulders existed in this area.
The poorly-sorted, coarse character of the conglomerate
resembles that of gravel in modern alluvial fans, and the size of
the clasts (some more than a meter across) indicates a nearbyl
source. A crude imbrication (shingling) of the clasts suggests
that the highlands lay to the north.
Figure 23. At some time following the Oligocene volcanic eruption, but before
the middle Miocene shallow water sands accumulated, a coarse alluvial fan
composed of granitic boulders existed in this area. Red “X” indicated the
possible location of what would become Point Lobos.
Figure 22. At some time following the Oligocene volcanic eruption, but before
the middle Miocene shallow water sands accumulated, a coarse alluvial fan
composed of granitic boulders existed in this area.
11
As the transform margin continued to expand into the middle
Miocene (11.6-16 Ma), segments of the North American Plate
became detached and began to move north with the Pacific
Plate. The rocks that were to compose Point Lobos were
probably incorporated into this movement.
Middle Miocene shoreline
coastal
bay
During the earlier part of the mIddle Miocene (Fig. 24), a
shoreline passed across this area (Fig. 25). Sandstone and
mudstone that crop out on Carmel Beach (Fig 26) contain
physical features and trace fossils characteristic of a tidedominated coastal bay. Such bays are typical of subsiding
coasts where the sea is encroaching over the land.
fan
volcano
Carmelo
Fm
granodiorite
Figure 24. The next event that is recorded in the vicinity of Point Lobos is the
passage of a shoreline across this area in the earlier part of the middle
Miocene Epoch
Figure 26. Exposure of sandstone on Carmel Beach after a storm. The
freshly exposed rocks on the beach contain trace fossils typical of a shallow
marine environment, evidence of reversals of current flow typical of tides,
and mudstone that contains tiny fossil rhizomes typical of a salt marsh.
These features in aggregate indicate deposition in a tide-dominated coastal
bay.
Figure 25. During the middle Miocene Epoch, the Point Lobos area lay on
or near a shoreline formed as the sea encroached over the land. The
rocks that underlie Carmel Beach contain evidence of tidal currents and
shallow marine organisms typical of a coastal embayment. Red “X”
indicated the possible location of what would become Point Lobos. Photo
by J. M. Sullivan
12
This “marine transgression” may have been the beginning of
the development of a fairly deep marine basin in this area.
Around 10-13 million years ago, the margin of Southern
California and northwestern-most Mexico consisted of a series of
marine basins (Figs. 27 and 28). The middle Miocene was a time of
Middle Miocene marine basin
marine
basin
extraordinary biologic productivity in the waters off this part of North
America. Basins along the margin (Fig. 28) were floored by
accumulations of the tests of microscopic marine organisms,
particularly diatoms, whose siliceous tests accumulated in great
thickness on the sea floor. The rocks exposed today at Point Lobos lay
beneath on of these basins. Under the weight of accumulating
sediment, the siliceous mud transformed into a siliceous mudstone that
we today call the “Monterey Formation.
shore
fan volcano
Carmelo
Fm
granodiorite
Figure 27. Some 10-13 million years ago, the area of Point Lobos lay beneath
the floor of a biologically highly productive marine basin.
This rock once blanketed the whole area here. It no longer crops
out at Point Lobos, but isolated small boulders on the Reserves
shoreline and pebbles in Pleistocene gravel document its former
presence here. Light-colored outcrops in the hills and road cuts
around Monterey and Carmel document its continued presence in the
area. If appropriately cemented, it forms a building stone (“Carmel
stone”) that is much utilized on the Monterey Peninsula (Fig. 29).
Figure 28. Later in the middle Miocene, the margin of southern California and
northern Mexico consisted of a series of marine basin in which great
thicknesses of diatomaceous mud accumulated. Red “X” indicated the
possible location of what would become Point Lobos. Note that it is shown
here in what today is southern California
Figure 29. “Carmel stone” (in the wall of a structure in Monterey) is a
building stone composed of especially well-cemented siliceous mudstone
of the Monterey Formation.
13
A discussion of the evolution of Point Lobos would
not be complete without discussion of the San Andreas
Fault. This huge fracture forms the boundary between
the Pacific and the North American Plates in most of
California today (Fig. 30). At its northern end, it merges
with the Mendocino Fracture Zone, which separates the
Pacific Plate from the Juan de Fuca Plate. In Southern
California the San Andreas extends to an area east of
Los Angeles where it splits into several faults that trend
toward the Gulf of California. Central California
contains several large faults that are part of the San
Andreas system; the one closest to Point Lobos is the
San Gregorio Fault which lies a few miles offshore from
the Reserve.
San Andreas Fault
Subsidary faults
Mendocino Fracture Zone
Location of 23 million-year-old
volcanic rocks displaced by the
San Andreas Fault
The distance of offset of rocks along the San
Andreas Fault in central California is generally accepted
as 315 km (195 mi), based on the correlation of 23
million-year-old rocks at the Pinnacles National Park
with similar rock of that age on the opposite side of the
fault near Lancaster, Ca. (Fig. 30). Assuming a rate of
northward movement of the Pacific Plate of 2 inches/
year (a typical estimate), an offset of 195 miles would
be accomplished in about 6 million years.
This implies that prior to ± 6 million years, stresses
inherent in the oblique collision of the North American
and Pacific plates generated a northerly transport of
fragments of the continental plate via a network of
shorter (and now largely unidentifiable) faults. The
activation of the San Andreas fault largely consolidated
that transport into a single great block lying between it
and the ocean.
Figure 30. Location of the San Andreas Fault (red) and subsidiary faults that currently
define the boundary between the Pacific and North American Plates. (Source: USGS,
Shultz and Wallace, 1997)
14
Late Miocene shallow marine
shallow
fan
marine
basin
shore
volcano
Carmelo
Fm
granodiorite
Figure 31. Six million years ago, in the late Miocene, the rocks that would
form Point Lobos lay beneath of a shallow sandy sea floor.
Figure 32. Six million years ago, the rocks that would form Point Lobos lay
beneath of a shallow sandy sea floor 180-200 miles southeast of their present
location.
In the late Miocene, 6 or so million years ago, some of the
marine basins had ceased to subside and were filling with
sediment (Fig. 31). Our Point Lobos rocks were still far to the
south-southeast of their present location. The San Andreas Fault
is a newly-formed boundary between the Pacific and North
American Plates, and the Gulf of California is just beginning to
open (Fig. 32). During the Late Miocene, the San Gregorio Fault,
a subsidiary fault to the San Andreas, began to move the rocks
just seaward of Point Lobos to the northwest. Conglomerate at
Point Reyes has been correlated with the Carmelo Formation
Burnahm, 2005)
Figure 33. Two million years ago, the rocks that would form Point Lobos
remained buried and lay some 60-80 miles south of the Reserve. Strata on
the west side of the San Gregorio Fault (SGF) was being transposed to the
north-northwest
By the early Pleistocene, 2 million years ago, the rocks that
will be exposed at Point Lobos were deeply buried about 60-80
miles south of their current exposure (Fig. 33).
15
The Pacific Plate continues today to move north-northwest
relative to the North American Plate. The stresses imposed by
the grinding of these two great plates against one another
create “wrinkles” (think of putting your hands palm down on a
table cloth and simultaneously moving one toward you and the
other away from you). The resulting “wrinkles” are manifested
in the mountain ranges and basins that lie along the coast (Fig.
34), most of which have developed in the last few million years,
and continue to actively grow today.
Coast
Range
PACIFIC
OCEAN
Ben Lomond
Mt. Madonna
UPLIFT
Santa Cruz
Watsonville
DOWNWARP
LAST FEW HUNDRED
THOUSAND YEARS
Sedimentary deposits indicate that the area along the coast
north of Santa Cruz and south of Monterey has been rising
during the past several hundred thousand years, in contrast to
the Monterey Bay plain, which mostly has been subsiding. I
estimate that the rocks at Point Lobos are currently being
elevated at a rate of 6-7 inches per 1000 years, based on the
elevation of the lowest marine terrace at the Reserve (see
following discussion).
Moss
Landing
Salinas
Fremont
Peak
Gabilan
Range
UPLIFT
Mt. Toro
If this rate of uplift remained fairly constant through time, a
million years ago the rocks here would have been some 500-600
feet lower, and the coastline would likely have been formed by
the Monterey Formation. As uplift progressed the Monterey was
removed by erosion and the Carmelo Formation and the
granodiorite emerged. It is probable that the physical shoreline
feature that we know as Point Lobos came into existence several
hundred thousand years ago.
0
25 MILES
0
40 KILOMETERS
Santa Lucia
Range
Figure 34. Forces associated with the grinding of the Pacific and North
American Plates have created huge “wrinkles” that result in the coastal
mountain ranges and lowlands. The upward and downward movements that
respectively produced the ranges and lowlands are still active today.
The Pleistocene (“ice ages”)
MIOCENE
EPOCH
The Pleistocene Epoch (Fig. 35) was a time of repeated
glaciation. Massive ice sheets, 1-2 miles thick, several times
covered then retreated from northern North America and
Europe. The water that composed these great glaciers came
from sustained snowfalls that were ultimately fed by the ocean.
The transfer of this water out of the ocean caused a global fall
of sea level. The transferral of water back to the oceans when
the glaciers melted coincided with a rise in sea level.
Marine
basin
bay
0
OLIGOCENE
volcanism
EOCENE
EPOCH
PALEOCENE
Submarine
canyon
50
MILLIONS OF YEARS AGO
LATE CRETACEOUS
granodiorite
100
Figure 35. The Pleistocene Epoch extends from the present day back to about
2.6 million years ago. It was the time of glaciation (ice ages) on the earth.
16
As a result, Pleistocene sea levels fluctuated between
lowstands during glacial episodes and highstands during
interglacial times (Fig. 36). At the last lowstand (about 17,000
years ago), sea level lay about 130 m (more than 400 feet)
below its present level. These fluctuations left their mark on
the coast of Central California, especially in areas of uplift
(Fig. 34).
LATE PLEISTOCENE SEA LEVEL
SEA LEVEL
(METERS)
0
50
When sea level remains constant for a long period of
time, the waves cut a very gently-sloping platform that
extends from the shoreline to a depth of about 30 feet (Fig.
37). The long periods of sea-level highstand shown in Figure
36 are sufficient to produce wave-cut platforms into the rocks
along this coast.
100
500
400
300
200
100
today
THOUSANDS OF YEARS AGO
While the late Pleistocene sea level was rising and falling
in response to alternating glacial and interglacial conditions,
the land at Point Lobos was slowly rising in response to
stresses imposed by the plate motions. As it rose the old
wave-cut platforms became elevated above the sea in the
form of marine terraces (Fig. 38). In an area of continuing
uplift, the oldest terraces are the highest and generally the
most poorly preserved. Figure 39 shows the uplifted marine
terraces seen from the southeastern side of Moss Cove. The
shoreline scar associated with the third terrace is visible as a
faint horizontal trace on a distant hillside.
INTERGLACIAL EPISODE
GLACIAL EPISODE
Figure 36. During the late Pleistocene Epoch, sea level rose and fell
dramatically as the world alternated between glacial and interglacial
episodes.
Figure 37. If sea level remains more or less constant for a
long (thousands of years) period of time, the sea cuts a
platform along the shoreline.
WAVE-CUT PLATFORM
17
1. EROSION OF A WAVE-CUT PLATFORM
UPLIFT
2. SEA-LEVEL FALL
3. SEA-LEVEL RISE, NEW PLATFORM CUT
UPLIFT
UPLIFT
4. SEA-LEVEL FALL
ELEVATED MARINE TERRACES
Figure 38. Elevated marine terraces result where sea level fluctuates significantly on at uplifting coast. When sea level remains more or less constant during a
high stand of the sea, the waves cut a platform into the shoreline. If sea level then falls this platform is exposed. By the time sea level returns to it approximate
previous position, the wave-cut terrace has been elevated to a position above the sea. Repetition of these processed create a set of elevated marine terraces that
form “stair steps” away from the sea
18
HIGHEST (OLDEST)
MIDDLE
LOWEST (YOUNGEST)
SHORELINE SCAR ASSOCIATED
WITH THE HIGHEST TERRACE
Figure 39. Elevated marine terraces on the northwest side of Point Lobos (Moss Cove in the foreground). Three terraces are visible.
The lowest (and youngest) terrace is very well developed at the margin of Moss Cove. A second terrace is visible near the highway;
Hudson House is built on it. A third terrace can be seen in the distance. The shoreline scar associated with this terraces is visible on the
distant hillside.
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When did Point Lobos materialize as a physical
feature? The configuration of the Pleistocene terraces
give us a clue. Figure 40 shows the likely shoreline
associated with the middle terrace which formed
between 120,000 and 200,000 years ago. The headland
of Point Lobos did not exist during the development of
the middle terrace , although the higher parts of the
Reserve might have projected as islands off a small
headland developed at the present site of Rat Hill. In a
reconstruction of the Reserve’s shoreline during the last
high-stand of the sea some 100,000 years ago, based
on the configuration of the lowest terrace, the headland
of Point Lobos is there and an older, larger version of
Whalers Cove exists (Fig. 41). Terrace distributions
drawn from (Clark et al., 1997).
Rat
Hill
PACIFIC OCEAN
APPROXIMATE
SHORELINE
of the
middle terrace
Dashed line:
present-day
shoreline
0.5 mi
1 km
Figure 40. Inferred shoreline at the time of the middle marine terrace
development (between 120,000 and 200,000 years ago). Shoreline is
based on water depths inferred from the terrace position and current
elevations.
APPROXIMATE SHORELINE
of the lowest terrace
Dashed line: present-day
Point Lobos shoreline
PACIFIC OCEAN
0.5 mi
1 km
Figure 41. Inferred shoreline at the time of the lowest marine terrace
development about 100,000 years ago.
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The shoreline continues to change. Large winter
storm waves actively erode the coast along the south
shore of the Reserve (Fig. 42). It seems likely that a
few thousand years hence the coast there will be
markedly different.
Figure 43 shows the broader geological aspects of
the central California coast geology today. The
Salinia terrane is interpreted as a southern extension
of the Sierra Nevada batholith that was torn off the
North American continent and driven north with the
Pacific Plate. The movement continues today. Large
active faults continue to reflect the movement along
the boundary separating the Pacific and North
American Plates (Fig. 44).
Figure 42. Currently eroding shoreline, Weston Beach.
?
SAF
SN
SIERRA NEVADA BATHOLITH
SA
SALINIA TERRANE
SA
OCEANIC CRUST
SN
SAF - San Andreas Fault
M
M - Monterey
SA
SB - Santa Barbara
- Monterey Peninsula
Faults
SB
SAF
PACIFIC OCEAN
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Figure 43. Major geologic elements of California’s Central
Coast. The Salinia terrane is considered by most geologists
today to represent a southern extension of the Sierra
Nevada Batholith, that because it lay on the seaward side of
the San Andreas Fault, was transported to the northwest as
part of the Pacific Plate.
The great faults of Central California will continue to displace
these rocks. Point Lobos will continue to shift to the northwest.
Some geologists feel that rocks originally connected to those at
Point Lobos have been carried as far north as Point Reyes by
the combined slippage along the San Gregorio and San Andreas
Faults.
The vertical forces associated with stresses of the plate
boundary will continue to warp the coastline. Powerful storm
waves will continue to assault the shoreline, continuously
remolding it. In the geologic future, Point Lobos, as we know it
today will cease to exist. We are most fortunate to experience
its present-day magnificence; like all things on earth, it will not
last forever.
NORTH
AMERICAN
PLATE
PACIFIC PLATE
Figure 44. Major faults in the Monterey Bay area. Their displacements reflect the interaction of the great
crustal plates that created them.
22
About the Paleogeographic Reconstructions
The Paleogeographic reconstructions shown here are based on
a combination of several sources: Atwaterʼs illuminating animations
of the tectonic development of the California Coast
(http://emvc.geol.ucsb.edu/forteachers/CA_geology.htm),
Blakelyʼs beautiful reconstructions of the western half of the
continent and the Colorado Plateau through the past hundred
million years (http:://www2.nau.edu), and a host of regional
reconstructions, e. g., M. S. Clark, San Joaquin Valley Geology
(http://www.sjvgeology.com), all influenced by 50+ years of study of
selected sedimentary rocks in the central and southern California
Coast Ranges. However, the detailed configurations of ancient
shorelines shown here are mostly fanciful and should be considered
only as possible examples of how this region might have looked at
different times in the geologic past.
References
Burnham, K., 2005, Point Lobos to Point Reyes: Evidence of ± 180 km offset on the San
Gregorio and northern San Andreas Faults: in C. Stevens and J. Cooper, eds.,
Cenozoic Deformation in the central Coast Ranges, California, Field Trip
Guidebook, Pacific Section SEPM (Society for Sedimentary Geology), p. 1-29.
Clark, J. C., 1997, Neotectonics of the San Gregorio fault zone: age dating controls on
offset history and slip rates: U. S. Geological Survey Final Technical Report, 30 p.
Clark. J. C., Dupré, W.R. and Rosenberg, L.I., 1997, Geologic map of the Monterey and
Seaside 7.5−minute quadrangles, Monterey County, California: a digital database:
plot derived from U. S. Geological Survey Open−file report 97−30.
Bowen, O. E., 1965, Point Lobos, a Geologic Guide: California Division of Mines and
Geology, Mineral Information Service, v. 18, No. 4, p. 60-67.
Kistler, R. W., and Champion, D. E., 2001, U. S. Geological Survey Open-File Report
01-453).
Nili-Esfahani, A., 1965, Investigation of Paleocene strata, Point Lobos, Monterey
County, California [M.A. thesis]: Los Angeles, University of California, 228 p.
Compiled by Ed Clifton, August, 2016
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