Annual Field Trip Number 36, Stops and Discussions June 19 and 20, 2010 Geology and Hydrology in the Eastern San Gabriel Mountains A Journey through the River of Time Saturday, June 19, 2010 (Kenney, Reynolds, Goetz, Harris, and El-Zaynab field presentations) #A1: Lost Lake: A Sag Pond and Terraces along the San Andreas Fault #A2: Cosy Dell Formation (K-T Boundary) #A3: Vaqueros-Cosy Dell Fm. Contacts #A4: Crowder Formation Outcrops #A5: Cajon Valley Formation, Victor Valley Fan & Cajon Fault #A6: Vincent Gap and Grassy Hollow (Lunch Stop) #A7: Slope Stability Project at the Angeles Hwy 2 Bridge Sunday, June 20, 2010 (Nourse, Saint , and Summers field presentations) #B1: Cow Canyon Saddle Overlook #B2: USFS Visitor’s Center #B3: Gneiss-Dikes Intrusions and 1938 Floods #B4: Manker Flats and San Antonio Falls (Lunch Stop) #B5: 1969 Flood Terraces and SAF Outcrop #B6: San Antonio Dam Overlook Introduction to Geology and Hydrology in the Eastern San Gabriel Mountains The eastern San Gabriel Mountains, as illustrated by the oblique aerial view on the cover page and the geological cross-section (Figure 1) constitute a spectacular field laboratory for geological and hydrological processes and their interaction with ecology and human activities. The San Gabriel Mountain block is bounded by the Frontal thrust faults (Sierra Madre-Cucamonga Systems), in the south and the San Andreas Fault System, a right-lateral transform fault system, that now forms the plate boundary between the Pacific and the North American plates, The mountains are dissected by right- lateral and left-lateral strike slip faults, many of which are accentuated by deep canyons, forming the headwaters of our river systems. During this trip we will encounter the San Gabriel River, the San Antonio Creek, the Lytle Creek and the Cajon Creek and examine their response to tectonic shifts, floods, landslides and fires. On the first day we will visit the San Andreas Fault System in the Cajon Pass and in the Wrightwood area, under the leadership of Miles Kenney, Bob Reynolds, Chris Goetz and Chris Harris. On the second day we will visit the San Antonio Creek, from its source to the mouth, examining its response to the San Antonio left-lateral strike slip fault, the landslides, the floods, the fires and the dam, under the leadership of Jon Nourse of CalPoly Pomona, who has done field work and supervised student theses since 1991. We will be camping at the Mt. Baldy Ranch Road R.V. Park in the Cow Canyon Saddle, overlooking the San Gabriel Fault and at the tow of the Cow Canyon Landslide. Figure 1. Geological cross-section across the eastern San Gabriel Mountains, with several of the formations and the faults that we will encounter during the field trip. (from NAGT Field Guide, 2001) ROAD LOG (compiled by Prem Saint, Boris Zaprianoff and Vik Mathur) Figure2. East San Gabriel Mountains with stops for Day 1 and Day2 Day 1, Saturday, June 19, 2010 Highlights: San Andreas Fault Zone in the Cajon Pass, Sedimentation Story of the K-T Boundary, Crowder Formation, Cajon Valley Formation; Cajon Fault, Victor Valley Fan, and Slope Stability Project on the Angeles Highway 2 Bridge 0.0 (0.0) Start at Cow Canyon Saddle at the entrance of Mt. Baldy Ranch R.V. Park, 30601 East Glendora Ridge Road, Mt. Baldy, CA 91759, driving down towards Mt. Baldy Village. 0.8 (0.8) Turn right onto Mt. Baldy Road, passing through the village and the school, with outcrops of Pre Cambrian gneisses and landslide deposits and chaparral vegetation. 5.4 (4.6) Turn left on Shinn Road. Cross the bridge on the San Antonio Creek and San Antonio Fire Station, approaching San Antonio Dam on your right. 7.6 (2.2) Curve right onto Mountain Road and San Antonio Heights Village. 9.6 (2.0) Turn left onto the I-210, Foothill Freeway, East ramp, towards San Bernardino. 18.7 (9.1) Exit on I-15 North towards Barstow. 27.4 (8.7) I-215 joins I-15. 31.0 (2.6) At the top of the rise on I-15, we are crossing the trace of the San Andreas Fault, the site of potential disruption following a major earthquake on the fault. To our right (east), two branches of the fault are very close together. To the left (northwest), the fault zone appears as the large, 0.25 mile-wide shear zone of Lone Pine Canyon with fault-related features like Lost Lake, a sag pond. Figure 3. San Andreas Fault in the Cajon Pass, along with Los Angeles Basin utility lines, railroads, gas pipes, electrical lines which could be severed during future earthquakes, causing severe economic disruptions. 32.8 (1.8) Prepare to exit at Cleghorn Road, 0.5 mile ahead. 33.3 (0.5) Exit at Cleghorn Road. 33.5 (0.2) Stop, TURN LEFT and proceed under the freeway to the junction of Cleghorn Road and Cajon Boulevard. Proceed south toward Blue Cut. 35.3 (1.8) TURN RIGHT onto Swarthout Canyon Road and cross Cajon Creek. 35.8 (0.5) Caution; cross BNSF railroad tracks. 36.3 (0.5) Caution; cross BNSF railroad tracks. Proceed north toward Lost Lak 36.6 (0.3) TURN RIGHT into the parking lot at Lost Lake. Stop A1 STOP A1: LOST LAKE: A SAG POND ALONG THE SAN ANDREAS FAULT, TERRACE DEPOSITS Field Presentations by Miles Kenney and Robert Reynolds The sag pond at the Lost Lake is a product of groundwater seepage in an area of tectonic depression in the San Andreas Rift Zone, a linear valley in the Lone Pine Canyon. Dr. Ray Weldon conducted a detailed study of a series of stream terraces associated with Cajon Creek in the region of Lost Lake (Weldon and Seih, 1985). The work identified progressive offset of radiocarbon dated, late Pleistocene and Holocene deposits and landforms by the San Andreas fault that yielded a set of slip rates spanning the past 14,400 years. Near Lost Lake they determined a slip rate for the San Andreas fault of ~24 mm/yr for the past 14,400 years and inferred that this slip rate had remained relatively constant during that period of time. The primary data utilized was the identification of a series of stream terraces associated with Cajon Creek that had been progressively offset by the San Andreas fault. Their findings provided strong constraints on the local kinematics of the San Andreas Fault System in the area. For example, other work by Weldon and others determined a slip rate on the San Andreas fault approximately 10 miles northwest near Wrightwood of ~34 mm/yr which is north of the juncture between the San Jacinto and San Andreas faults. They postulated that the total slip of 34 mm/year near Wrightwood was portioned between the San Jacinto and San Andreas faults in the area of Cajon Valley. The estimated slip rate for the local San Jacinto fault zone is 10 mm/yr, which once added to the estimated 24 mm/yr totaled 34 mm/yr. Figure 4. Lost Lake Sag Pond: Groundwater seepage in the tectonic depression in the San Andreas fault zone has created the pond, here covered with cattails along the shore. 36.9 (0.3) View south of Blue Cut, its color from glaucophane and actinolite schist. From Blue Cut, it is a quick walk from the Pacific Plate across the San Andreas Fault to the North American Plate, to our next stop at the Cretaceous Cosy Dell Formation. 37.9 (1.0) STOP at Cajon Boulevard. Watch for cross-traffic. Proceed diagonally southeast across Cajon Boulevard to the unused north-bound lane of Rte 66. 38.0 (0.1) Stop A2. STOP A2: COSY DELL FORMATION (K-T BOUNDARY?) Field Presentation by Bob Reynolds Cajon pass contains 70 million years of history in the rock record. Sediments of Cretaceous age are present as is the Earliest Miocene Vaqueros Fm. Continental deposition starts with the Middle Miocene Cajon Valley Beds. The Inface Bluffs on the northern horizon contain the Brunhes-Matuyama reversal (765,000 yr) suggesting a record of deposition thru the middle-late Pleistocene. At this stop, between I-15 and old Route 66, we are examining the Cretaceous Cosy Dell Formation (Morton and Miller, 2003), formerly called the Paleocene–Eocene? San Franscisquito Formation (Dibblee, 1967; Woodburne and Golz, 1972). The basal conglomerate is overlain by limey sandstones and dark, silty limestone. Its Cretaceous age is determined by the presence of elasmosaur vertebrae in the outcrop to the west (Kooser, 1985). Articulated vertebrae in an outcrop to the northwest (Whale Mountain) suggest that this large marine reptile died in place and was not reworked from other sediments (Lucas and Reynolds, 1991). The section may have potential to contain the K/T boundary, but none of the abundant fossil fish scales, crustaceans, gastropods, pelecypods, or plants from this outcrop provide additional age control relating to that transition. Paleocene Coccoplithus pelagicus, a nanofossil, Apectodinum plexus, a dinoflagellate, and Turritella pachecoensis, a gastropod, were reported (Fred Berry p. c. to Kooser, 1985; Weaver 1951-56) from apparently similar sediments somewhere in Cajon Pass, but by the 1970s the specimens and locality data could not be found. Return to vehicles. 38.1(0.1) STOP at Cajon Boulevard. Watch for cross traffic. DRIVE NORTH (right) toward the Cleghorn onramp. 39.3 (1.2) Pull right onto the unused northbound lane of Route 66. Stop A3 STOP A3: VAQUEROS –COSY DELL CONTACTS Field Presentation by Bob Reynolds To the northwest is Whale Mountain, where the Vaqueros (?) Formation crops out and is in fault contact with the Cretaceous Cosy Dell Formation.