GEOLOGIC NOTES ON SELECT FIELD STOPS

Location map of the four STOPS at or near the San Andreas Fault to be visited during the field trip. STOP # 1: Vasquez rocks, # 2: Palmdale, # 3: Devil’s Punchbowl Park, and # 4: Pallet Creek. The linear feature, running NW-SE is the San Andreas Fault.

STOP #1. VASQUEZ ROCKS NATURAL AREA, AGUA DULCE Physical Address: 10700 Escondido Canyon Rd, Agua Dulce, CA 91390 From West Los Angeles College (WLAC), get on I-405 N in Culver City from Overland Ave and Culver Blvd. Follow I-405 N and CA-14 N for a total of about 41.5 miles to Agua Dulce Canyon Road. Take Exit 15 (Vazquez Rocks) and continue on Agua Dulce Canyon Road Drive to Escondido Canyon Road for about 2.2 miles. Total drive time on weekends from WLAC is approximately 45 minutes.

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VASQUEZ ROCKS AT AGUA DULCE Vasquez rocks natural area park is a 932-acre park located in the Sierra Pelona mountains, in northern Los Angeles county. It is located in the town of Agua Dulce between the suburbs of Santa Clarita and Palmdale. The area is also visible from the antelope valley freeway (State Route 14). The rock formations were formed by rapid erosion during uplift about 25 million years ago and later exposed by activity along the San Andreas fault. The Tataviam American Indians were living here when the Spanish arrived. Their language was probably a Takic Uto-Aztecan language. They lived in grass huts within villages. With the coming of the Spanish missions, some members of this people were forced to work there. They eventually began speaking Spanish and inter-marrying with other tribes. The last of the Tataviam died in 1916. In 1873 and 1874, Tiburcio Vásquez, one of California's most notorious bandits, used these rocks to elude capture by law enforcement. His name has since been associated with this geologic feature. Vasquez rocks was added to the national register of historic places in 1972 because of its significance as a prehistoric site for the Shoshone and Tataviam peoples. The Vasquez rocks consist mainly of coarse- grained conglomerate and breccia which were deposited adjacent to active faults during rapid uplift and consequent erosion of the San Gabriel mountains. Approximately 25 million years ago (late Oligocene time), the collision of the north American and Pacific tectonic plates uplifted the area along the Elkhorn Fault. Energetic erosion of the highland along with uplift and volcanism caused debris flow sediments to be distributed in alluvial fans into a rapidly subsiding rift known as the Soledad basin. These sediments were buried and lithified through the Miocene and became exposed more recently via activity along the San Andreas fault system. The strata are now highly tilted, are disconnected from their source area and are deformed and offset by the later fault activity. The distinctive "hogback" ridges of steeply inclined strata serve to graphically demonstrate the significant fault activity in the area. These sedimentary rocks, named the Mint Canyon Formation, were laid down about 8-15 million years ago. In the upper layers, many animal fossils have been found, including camels, horses and rodents.

Http://ngmdb.usgs.gov/prodesc/proddesc_71691.htm Page 2 of 11

“Hogback” Rock formations of the Vasquez Rocks Natural Area in Agua Dulce

QUESTIONS BASED ON THE GEOLOGY TRAIL While walking on the geology trail, answer the following questions: Q1. Note that the detrital sedimentary rocks are made up of large (gravel-sized) rock fragments. Would you classify the rocks as conglomerate or breccia. Explain your answer.

Q2. For your answer in # 1 above, how far do you think were the sediments transported? Long distance or short distance? Explain your answer.

Q3. Identify faults on the rocks and classify them as dip-slip or strike-slip faults. If dip-slip faults, determine whether they are normal or reverse faults; if strike-slip faults, determine whether they are left-lateral or right-lateral strike-slip faults. Explain your answer. [Remember that this area is close to the San Andreas Fault whose effects may well be observed in these rocks!]

Q4. For the detrital rocks made up of gravel sized rock fragments, determine the nature (type) of the rock fragments. What does this indicate about the provenance (source rock) for this sedimentary rocks?

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STOP # 2. AVENUE S, PALMDALE Physical Address: From Stop # 1, drive for approx. 16.5 miles northwards on Highway 14. Take Exit 33 for Avenue S. At the first light make a left; continue under the bridge (Hwy 14), and make a right towards a paved road. See location of STOP # 2 from figure below.

THE SAN ANDREAS FAULT The San Andreas Fault is the sliding boundary (transform fault) between the Pacific Plate and the North American Plate. It slices California in two from Cape Mendocino to the Mexican border. communities like Desert Hot Springs, San Bernardino, Wrightwood, and Palmdale lie squarely on the fault and are sitting ducks. The North American and the Pacific plates are slowly moving past one another at a couple of inches a year - about the same rate that your fingernails grow. But this is not a steady motion, it is the average motion. For years the plates will be locked with no movement at all as they push against one another. Suddenly the built-up strain breaks the rock along the fault and the plates slip a few feet all at once. The breaking rock sends out waves in all directions and it is the waves that we feel as earthquakes. In many places like the Carrizo Plain (San Luis Obispo County) and the Olema Trough (Marin County), the fault is easy to see as a series of scarps and pressure ridges. In other places, it is subtler because the fault hasn’t moved in many years and is covered with alluvium, or overgrown with brush. In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of powdery, crumbled rock (fault gouge) that has been pulverized by the moving plates. The hallmark of the San Andreas Fault is the different rocks on either side of it. Being about 28 million years old, rock from great distances have been juxtaposed against rocks from very different locations and origins. The Salinian block of granite in central and northern California originated in Southern California, and some even say northern Mexico. Pinnacles National Monument in Monterey County is only half of a volcanic complex, the other part being 200 miles southeast in Los Angeles County and is known as the Neenach Volcanics. Page 4 of 11

LAKE PALMDALE SAG POND Where the land between two parallel faults will sag between, a small graben is created. Water may collect there creating a “sag pond”. These ponds are sometimes enlarged to create reservoirs for drinking water. One such sag pond, Lake Palmdale, is a sag pond.

Lake Palmdale

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Mechanism of formation of sag pond

Faults that created the sag pond – Lake Palmdale Page 6 of 11

PALMDALE ROAD CUT This road cut through the San Andreas fault zone is located on the eastern side of the Antelope Valley Freeway (Hwy 14) just north of Avenue S in Palmdale, California. It is oriented roughly perpendicular to the strike of the fault and exposes many features of the zone where the surface rupture occurred during the 1857 Fort Tejon earthquake. At this location the fault zone, which includes the main rupture and many smaller subparallel ruptures, is approximately 1 mile wide. The main trace of the San Andreas fault zone runs between the southern end of the road cut (at right) and the lake Palmdale reservoir (background). The reservoir is a sag pond that was enlarged by man after the 1857 rupture.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< STOP # 3. DEVIL'S PUNCHBOWL NATURAL AREA Physical Address: 28000 Devils Punchbowl Rd., Pearblossom, CA 93553 You will drive for approximately 23 miles (about 30 minutes) from stop # 2.

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THE PUNCHBOWL FAULT in the Devil's Punchbowl County Park, provides an unequaled view of the internal structure of a large-displacement fault of the San Andreas system. The Devil's Punchbowl is an area of moderate relief between the San Gabriel Mountains and Mojave Desert. The area was eroded to low relief in Pleistocene time and covered by piedmont alluvial fans after the Punchbowl Fault became inactive. Recent uplift has rejuvenated the erosion that has dissected the piedmont and exposed the Punchbowl Fault and associated folds. As a result, the Devil's Punchbowl provides an excellent exposure of the Punchbowl Fault and subsidiary faults and folds associated with strike-slip faulting adjacent to uplifted blocks of basement as might occur at the margins of a strike-slip basin. The Punchbowl Fault accommodates more than 25 miles of right-lateral strike-slip displacement. Microstructures and mineral assemblages of the fault rocks from the Punchbowl Fault are consistent with faulting at 1.5 to 3 miles depth. By analogy with nearby active faults, it is assumed that the Punchbowl Fault was seismically active and that the structure of the fault records the passage of numerous earthquake ruptures. In the Devil's Punchbowl, the fault consists of a broad zone of damaged rock along which the Punchbowl Formation sandstone and an igneous and metamorphic basement complex are juxtaposed. A walk through the Devil's Punchbowl Park along the Burkhart Trail provides the opportunity to inspect the internal structure of the large displacement Punchbowl fault.

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Satellite image of the Devil’s Punchbowl Park underlain by sandstone & conglomerate beds. Note the large-scale fold closure with a west plunging axis.

Q5: Look in all directions to the north and west. Most rocks are tilted. Those on the west are tilted to the north, while rock formations in the north are tilted to the west. What kind of fold would this tilted layers form, a syncline or anticline? Explain.

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STOP # 4. THE SAN ANDREAS FAULT AT PALLET CREEK Physical Address directions: Start at Devil’s Punchbowl Nature Park and drive on Tumbleweed Road for approx. 1 mile. Make a right on 131st E Road and drive for approx. 3 miles. On Pallet creek road make a right and drive for about a ½ mile where you will find the sign “San Andreas Fault”. At this STOP, we will be able to see the surface expressions of the San Andreas Fault at Pallet Creek, where trenches dug by geologists and seismologists to study the ancient seismic activity (paleoseismology) are still visible. Most of the primary evidence for faulting are not visible due to weathering for the last 150 years or more.

Location map and driving directions to the Pallet Creek site (from Devil’s Punchbowl)

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The San Andreas Fault at Pallet Creek, near Devil’s Punchbowl Page 11 of 11