food if you are like me and must eat all the time to think - no
Figure 2: Regional Map showing
location of Natural Bridges State Beach, (red) Salinian Block
blue arrow) and major active faults (thick black lines). Modified
from Horns & Verosub, 1995.
2. Geologic History (Broadly, of the Area)
Natural Bridges State Beach is located on the Salinian Block, the basement
of which is Cretaceous granite which was transported to its present
position (MBNMS geology website). The granite moved northward along the
San Andreas fault and may be related to granites of the southern Sierras.
During transport, marine sediment was deposited on top of the granite
to form most of the surface of today's Salinian Block.
Since the sediment has been deposited during the long period of transport
on the San Andreas Fault, it has been affected by fault-related
deformation. The sedimentary units exposed along West Cliff Drive have
been uplifted and gently folded (Aiello et al, 2001). Therefore their dip
is not exactly horizontal, as they were laid down.
The sea cliffs of the Santa Cruz area, including Natural Bridges, are
mostly comprised of outcrops of the Santa Cruz Mudstone. The Santa Cruz
mudstone is a local name for the upper few meters of the Monterey
Formation, which outcrops extensively all along the central and southern
California Coast (Aiello et al, 2001). The Santa Cruz Mudstone is
comprised of bedded siliceous mudstones and rarer porcelanites near the
top of the formation. These formed from diatom-rich mud (Aiello et al
2001.) Elsewhere along the West Cliff Drive seacliffs, the Santa Cruz
Mudstone is overlain unconformably by the sandy Purisima Fm., but the
Purisima is all eroded away at Natural Bridges. Overlying both formations
are Quaternary terrace deposits, which are not affected by any of the
4. Active Tectonics
Natural Bridges State Park is located on the Salinian Block (Figure 2)
which moves with the Pacific Plate in a north-northwest direction relative
to the North American Plate. The Salinian Block in this area lies
between the San Andreas Fault (which runs up the Santa Cruz Mountains)
and the San Gregorio Fault (which runs offshore and recombines with the
SAF near San Francisco). The local segment of the San Andreas Fault is a
site of NW-directed compression, due to a left step in the fault
which has uplifted the Santa Cruz Mountains. Exposure of marine sediments
extensively along the coastline is further evidence of this long-term
5. Description of Field Trip Stops including
At Natural Bridges we visited one outcrop, the rocky shelf on the west side
of the beach. The shelf is composed of the Santa Cruz Mudstone, which in
this location is extensively fractured with two sets of joints and a set of
conjugate shear fractures. The dominent joint set is semi-regularly
spaced (4-15cm), roughly vertical and strikes northeast-southwest. The
secondary set is also vertical but perpendicular to the first, striking
northwest-southeast. The second set of joints is irregularly spaced,
varying with the local spacing of the first set. The joint sets are
decorated with plumose structures on exposed faces. Intricate iron staining
along joints and concentric between joints suggests fluid flow through the
joint system at some time in the past.
The joints are cross-cut by sets of conjugate shear fractures, which are
best observed in the cliff face above the terrace. The shear fractures are
listric, small-offset normal faults, as evidenced by abundant slickenlines
which rake almost 90 degrees. The shear fractures clearly offset the joint
The well developed sets of joints and shear fractures, as well as the
observed cross cutting relationships, enable us to calculate the stress
orientations which formed these structures. We plotted the data on stereonets. For the joint
sets, each set gives just the orientation of sigma 3 (it's the pole to the
average joint). However since there are two sets of joints which formed at
about the same time, we can extrapolate how the minimum principal stress
rotated during the deformation.
During the deformation at Natural Bridges, the Santa Cruz Mudstone was
deformed in a stress field where the maximum principal stress was vertical.
The intermediate and minimum principal stresses were roughly equal in
magnitude during the first stage of deformation, the jointing. Jointing was
caused by high fluid pressures (negative sigma 3) which caused
hydrofracturing of the SC Mudstone. Once the joints had developed, water
was able to drain from the formation and fluid pressure dropped (moving the
Mohr circle to the right). In this new stress regime, failure occured as
small normal faults.
biosiliceous (adj): describes a sedimentary rock which contains
biologically produced silica, ie, the diatom tests in the Santa Cruz mudstone.
porcelanite (n): a dense silicious rock which is not as vitreous (glassy)
or as dense as chert. A silica-infused claystone.
orthogonal (adj): at right angles
*willfoxing (v): stealing the TA's lunch
A field trip guide for the ES10 students at UC Santa Cruz, has some info
on the bedrock geology.
Monterey Bay National Marine Sanctuary page for NBSB
Geology of the Monterey Bay Marine Sanctuary - lots of detailed
Horns, Daniel M, and Verosub, Kenneth, L. (1995) Paleomagnetic
investigation of late Neogene vertical axis rotation and remagnetization
in central coastal California, Journal of Geophysical Research, vol. 100,
no. B3, pp 3873-3884
Used this paper for tectonic history of region
Aiello, I. W., Garrison, R. E., Moore, J. C., Kastner, M., and Stakes, D.
S. (2001) Anatomy and origin of carbonate structures on a Miocene
cold-seep field, Geology, v. 29, no. 12, pp. 1111-1114
Used this paper for depositional history of Santa Cruz mudstone