Geology 309
Final Daly City Dilemma
M. Ferraz
GEOLOGY 309
DALY CITY DILEMMA
PROBLEM 3 STAGE 1
MICHELLE FERRAZ
5/20/03
Page 1 of 4
Geology 309
Final Daly City Dilemma
M. Ferraz
Mother Nature will relentlessly cause havoc to the residences of Daly City’s Westlake Drive and those who dwell on the West side of Skyline Blvd at Mussel Rock. The problem the homeowners face in this area are, landslides caused by several mitigating factors but mostly from the geologic nature and orientation of the rocks, water saturation, the topographic nature of the area, and seismic activity.
According to Jacobs Associates’ (JA’s), Nov. 5, 1998 report to Daly City’s Engineer, James Porter, the geologic units that make up the landslide area are as follows:
Franciscan Complex Greenstone (KJg) – Altered fine-grained basaltic rocks that are gray to green-gray in color. These rocks are variable weathered, moderately hard to hard, and weak to very strong. Where highly weathered, these rocks are typically light brown to dark yellow-orange.
Merced Formation (QTm) – Thinly interbedded, medium gray to yellow-orange sandstone, siltstone and claystone. These rocks are typically soft to relatively hard, friable to weak and contain occasional gravels.
Marine Terrace Deposits (Qt) – Predominately friable, well-sorted and fine grained sand that is yellow-orange to light gray. Contains local gravely alluvial channel deposits and colluvial clay.
Landslide Deposits (Qls) – Heterogeneous and unconsolidated mixture of sand, silt, clay and gravel. Landslide deposits are primarily derived from bedrock of the Merced Formation and contain man-placed fill.
Fill (Qf) – Crudely stratified and unconsolidated sand, silt and clay, with occasional debris and gravel (riprap).
Franciscan Complex is the oldest of geological make up, formed during Cretaceous time over 67 million years and is shown to be 100 feet thick in the southwest corner of the study area. The Merced Formation unit is middle Pliocene in age, over 1.8 million years and is approximately 600 feet thick in this area. Thus, the Merced Formation is said to be the culprit of the landslide deposits that divides two of the remaining three units above, Marine Terrace Deposits (Qt), Landslide Deposits (Ols) into subunits to form a 100-foot wide zone, Mussel Rock Fault (noted later in this report). Hence the later three units were formed during the Quaterinary time less than a million years ago.
From the above description of the geologic units, we know that the older rocks are, jointed and fractured, highly weathered, thinly folded (layered), and in particular areas, faulted (broken), crumbly and weak. The younger rocks are combined mixture of various materials are also crumbly, not overly cohesive and highly susceptible to landslides.
Furthermore, because the northwest area of Westlake Dr. is comprised of enormous amount of landfill, the clays comprised in this area also factor into the landslide equation. Two broad groups of clays exist in the area, kaolinite and montmonillonite. Kaolinite is the most common in our region and is non-expansive therefore is easily portable through wind and water. Thin seams of Kaolinite that exist in this area are interbedded within the underlying siltstone allowing any loose sediment to factor into the sliding zone. Montmonillonite clay is another culprit to landsliding in this area as well. During rainy periods, Montmonillonite can absorb 10 times its own weight and expand, becoming increasingly weighted. Once it reaches a point where water saturation becomes moot, lift of the sediment occurs causing less friction between the geological materials and hence a landslide transpires. Because of the weight, nature of the material and less friction, the increase of landslides becomes apparent.
Another coefficient to the landslide phenomenon in this area is the topography of the land and its natural slope. The elevated site is approximately 450 feet along the eastern boundary that coincides with Longview Drive north of Westline Drive and has a large amount of vegetation. The face of the cliff is 150 feet above the ocean on Mussel Rock. A curve escarpment slopes moderately to very steep down towards the northwest to the relative level open space at Mussel Rock (JA 1998). The maximum relief between Westline Drive and the open space is approximately 200 feet. At the toe platform resides a pit of riprap that extends 2,200 feet north from Mussel Rock along Mussel Beach, is 25 feet high, about 50 feet wide, and has an average slope of 25 degrees (Smelser 1987). The northeast platform is man-made dirt mounds that form a pond for winter drainage. The ponds’ water is channeled around the main pit, downslope, and onto the beach. Included in this unique topography is a depression created by the 1906 earthquake that left an obvious depression in the toe platform and formed a small gully. At the northern end of the riprap there is a large ravine with displaced blocks of the Merced Formation and running water that indicate fault-controlled springs. Because of this areas hummocky topography and its westward open space, allows the natural forces of gravity to pull any loose sediment further toward the coastline. Loose sediment, prior seismic activity and landslides have formed intermittent v-shaped drainage channels in the landslide deposits about 20 feet deep. These drainage channels promote further slumping as evidence by cracks all over the surface of the slide area.
Also at Mussel Rock, the westward treading waves has its own agenda, to further straighten the coastline of California. Evidence of wave erosion is apparent because of the once famous Tobin tunnel. The tunnel in 1874 was roughly 90 feet long, 11 feet high, and 10.5 feet wide is now a weather sea stacks slowly being eroded by the seasonal tides.
Within the subject area exist several faults, the active right lateral strike slip San Andreas fault, the vertical Mussel Rock Fault (MFR) that is sub-parallel to the San Andreas Fault zone (at about N45ºW) and the Alquist-Priolo Fault that is a split hazard zone trending Northwest through the study area along the San Andrea Fault (JA 1998).
Historically, the seismic activity of these faults has contributed to the demise of this area. The 1906 earthquake left an obvious depression and displaced about 21 feet at Tomales Bay, approximately 30 miles north of Mussel Rock plus a noticeable trace of the fault trends about 48 degrees northwest through the Mussel Rock area. In March 1957 a 5.3 earthquake in the near vicinity contracted between the two subunits of the Merced Formation and parallels the damage created in the 1906 earthquake and thus creating a number of small, low-angle normal and reverse faults in the area. Since the 1989 Loma Prieta earthquake, gas pipes and other utilities have jeopardized the livelihood of those homeowners in the subject area.
Other historical features to the landslide area besides the Tobin Tunnel mentioned earlier, was the once thriving Ocean Shores Railroad line in operation from 1905 to 1920 along the coastline near the toe of the landslide. Until the 1906 earthquake that partially destroyed the rail line, helped trigger numerous landslide that also affect its operation. Following the railroads departure was the opening of the Coastal Highway in 1936 that also had problems with the landslide area until the 1957 earthquake forced its closure as well. From 1963 to 1979 a waste management company utilized the depression left from the 1906 earthquake as a sanitary landfill (Konigsmark), which is allegedly said to be responsible for waste drifting as far north as Stinson Beach in Marine. The significance of these early “modern day” throughways, weaken the stability of the rocks and contributes to the failure of the landslide area.
Additionally, another annual cause contributing to the landslide area is water saturation from rainfall. During the 97-98 El Nino winter, more than 33.55 inches of rain fell in this area, which is more than double the norm of 13.65 inches.
Finally, construction in the area, intermittent shallow earthquakes, water saturation, weak geological material, coastal erosion and the topographic nature of the slope provides the perfect platform for Mother Natures’ reentrant.
Resources:
Geologic Trips: San Francisco and the Bay Area by Ted Konigsmark 1998 Geopress
Geology of Mussel Rock Landslide, San Mateo County by Mark Smelser, March 1987 Environmental Geology
Westline Drive Site Study, Daly City, California by Jacobs Associates, Engineers/ Consultants 1999
A series of articles from Feb 16, 2000 to August 16, 2000 published in the San Francisco Chronicle
Page 1 of 4