Weed Removal Leads to Discovery of New Fault at CNM

Don Vaughn
Geotechnical Exploration Inc
dvaughn@gei-sd.com

As part of the upcoming National Park Service Centennial Celebration, rangers at Cabrillo National Monument initiated a policy last year to be “weed free” by 2016, the year of the centennial. Cabrillo National Monument, situated at the southern tip of the Point Loma Peninsula is the only National Park Service unit within San Diego County. [www.nps.gov/cabr/index.htm]. The Monument was founded by proclamation signed by President Woodrow Wilson in 1913 and was incorporated into the National Park System in the 1930s. The Monument therefore is celebrating its own centennial this year with a three-day event planned for October 12-14.

The “weed free” policy in the Monument has resulted in the physical removal of various plants deemed to be “weeds” (what is a “weed” can be a subject of surprising complexity) and in the instance described herein, in the exposure of a “new” fault previously covered by ice plant! See Figure 1.

An alert visitor (mwh) in July of this year observed an outcrop in a road cut that appeared to be faulted. The outcrop is located on the south side of the southwestern parking lot at Cabrillo; the first parking area on the right that one drives by, just a short distance from the park entry (see Figure 2). This area was recently exposed by the removal of ice plant draping over the outcrop as part of the “weed free” program. The author and an associate contacted the park’s Head of Natural Resources, Dr. Keith Lombardo, and obtained permission for a reconnaissance level trek on and over the fault and along its apparent trend. (Natural, cultural and historical resources are protected in all National Parks, but are accessible to researchers with permission.)

The Point Loma peninsula as depicted on the geologic map of the Point Loma Quadrangle (see Figure 3) is comprised primarily of Cretaceous and Pleistocene rocks (Kennedy, 1975). Well exposed along the coast at the base of the peninsula within the Monument are thinly layered rocks of the Point Loma Formation (Kp); interbedded fine-grained dusky-yellow sandstone and olive-gray shale and mudstone. These rocks are conformably overlain by rocks of the Cabrillo Formation (Kcc and Kcs), medium-grained sandstone and cross-bedded cobble conglomerate. The Point Loma Formation represents outer to middle submarine fan deposits; the overlying Cabrillo Formation represents coarser middle fan deposits. The Cabrillo Formation is well exposed in natural outcrops and in road cuts within the upper part of the Monument.

The Cabrillo Formation is locally and unconformably overlain by the Pleistocene Lindavista Formation (more currently referred to as Quaternary very old paralic deposits, Qvop, per Kennedy and Tan, 2008) on the upper part of the peninsula within the Monument. See Figure 3. These attractive iron-oxide stained rocks underlie a small mesa where the old lighthouse on the Monument is located. This mesa was also locally excavated to emplace coastal observation bunkers for big gun targeting during WWII just south of the southwestern parking lot (and just west of the Monument’s Coast Defense Exhibit, including an old radio shack). This mesa is faulted and the fault is readily seen in the large high cut slope on the south side of the southwestern parking lot.

The fault is a normal fault and juxtaposes the Lindavista Formation on the east with the Cabrillo Formation sandstone on the west. It strikes generally N25W, and dips 65 to 80 degrees to the northeast. Up top, near the WWII observation bunker, the unconformable depositional contact between the Pleistocene Lindavista Formation and Cretaceous Cabrillo Formation is apparent on the slope, west of the fault. The Lindavista forms a prominent red ledge, or cemented hammer-ringing cap (marker bed) in this area and along the mesa boundaries. This cap rock marker has been downthrown and these hammer ringing rocks are apparent near the base of the slope, east of the fault. We were able to trace the fault laterally and even measure the throw (vertical component of net slip). Preliminary hand-level measurements suggest about 30-35 feet vertical displacement of this marker horizon.

The fault is apparent over several hundred feet, though there may be several parallel strands. It appears to be coincident with or parallel to another mapped fault (Kennedy, Tan, Chapman and Chase, 1975) just to the north. We also found the fault in the cut/excavated slopes behind the radio shack. There are also several small, filled joints parallel to the fault on either side at the type exposure, but especially the west side. The cracking and filling most likely occurred over the period of active faulting. Interestingly, a soil horizon appears to have developed over the downthrown Lindavista Formation. This soil horizon does not appear to be present on the mesa. The fault is most likely part of the Point Loma Fault system that is partly responsible for the formation of the peninsula and San Diego Bay. Some estimates have the Lindavista Formation offset about 150 meters between the peninsula and SD Bay.

This fault is an obvious feature and the perfect example to show your friends who are always asking you “What does a fault look like?” Take a trip out to the Monument in your car or on your bike and show them! Several other mapped faults are also visible within the Monument, notably along the Bay Side Trail (see Kern, 1983).

The discovery of this fault has really excited the Monument rangers who are all dedicated to husbanding their precious resources, including the geologic resources, in keeping with their Mission. Plans include a wayside sign in the parking lot depicting the fault and providing interpretative information for the almost 900,000 annual visitors from across the country and around the world! This feature is also ripe for research. Should any students or other researchers feel so inclined to do additional work on this fault please contact Dr. Keith Lombardo (619-557-5450 x4581, keith_lombardo@nps.gov), Head of Natural Resources at Cabrillo National Monument.

References

  • Abbott, Patrick L.; 1999; The Rise and Fall of San Diego; Sunbelt Publications.
  • Brown, Joseph E.; 1981; Cabrillo National Monument; Cabrillo National Monument Foundation.
  • Houk, Rose, Editor; 2004; Understanding the Life of Point Loma, Cabrillo National Monument Foundation, (CNMF)
  • Kennedy, M.P., 1975; Geology of the Point Loma Quadrangle in Geology of the San Diego Metropolitan Area, California; Ca. Division Mines & Geology, Bulletin 200.
  • Kennedy, M.P., Sean Sieng Tan, Rodger H. Chapman and Gordon W. Chase; 1975; Character and Recency of Faulting, San Diego Metropolitan Area, California; Ca. Division of Mines & Geology Special Report 123.
  • Kennedy, M.P. and SS. Tan, 2008, Geologic Map of the San Diego 30’x60’ Quadrangle; Ca. Geological Survey and the USGS.
  • Kern, John Philip; 1983; Earthquakes and Faults in San Diego; Pickle Press.
New Fault
Figure 1 Recent ice plant removal has resulted in an exposure of a fault at Cabrillo National Monument. Darker Pleistocene Lindavista Formation on the left is juxtaposed against lighter Cretaceous Cabrillo Formation on the right. View south. See text and Figures 2 and 3 for the location.
Map of New Fault
Figure 3 The new fault exposure is near the mapped Kcc/Qln contact.
Map of New Fault
Figure 2 Cabrillo National Monument features. The new fault is visible in the small parking area near the labeled “Ocean View.”

Last revised 24-Jan-16