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Geology of Sonoma County Guidebook.Pdf I l ASSOCIATION OF ENGINEERING GEOLOGISTS \ San Francisco Section 1 I 1 AEG San Francisco Section Field Trip Geology and Tectonics of the San Francisco North Bay Area Saturday June 14, 2008 Leaders: James Allen, CSU East Bay, Earth and Environmental Sciences Peter Holland, Vector Engineering James Wilen Jr., BACE Geotechnical Guidebook by: James Allen Peter Holland James Wilen Jr. l l TABLE OF CONTENTS (Page numbers exclude figure pages) Directions & Road Log. .. I Overview of Stops... ........................ .. ........................... .......................................... I Introduction. .............................................. ................................................ ...... ..... 2 Regional Geology. 3 Franciscan Complex ................................................................................... 3 Great Valley Group.. ...................... .................... ........................................ 3 Salinian Block at Pt. Reyes ........................................................................... 4 Tertiary Rocks East of the San Andreas Fault. ..................................................... 4 Monterey Group ....................................................................................... 4 San Pablo Group ........ ........................................................ ........................ 4 Orinda Formation..................................................................................... 4 Moraga Formation ............................................................................... ..... 4 Siesta Formation ................................................................................ ....... 5 Volcanic Rocks of the Northern San Francisco Bay Area ....................................... 5 Burdell Mountain ....................... ................................................... 5 Tola y Volcanics .... ....... ..................... .................... ............. .......... 5 Sonoma Volcanics .................. .. .................................................... 5 Mullholland Formation ............................................................. ................... 6 Sycamore Formation ..................................................................................... 6 Undifferentiated Contra Costa Group.. ....................... ....................................... 6 Silver Creek Gravels ................................. ................................................... 6 Huichica Formation .......................... .................. .......... .............................. 6 Glen Ellen Formation .......................................................................... ........ 6 Methods ................ ............................................................................................... 6 Radiometric Dating ............................... ...................................................... 6 Mapping and Measuring Sections ............................ ............. ........................... 6 Sandstone Petrography.. 7 Conglomerate Clasts ........... .......................... ... .......... ..................... ........... 7 Tertiary Strata of the Northern San Francisco Bay Area.. ....................................... 7 Description of Sedimentary Units ............................................................. ................... 8 Wilson Grove Formation (STOP 1) ...................................... .. ......... ............ .. 8 Basal Contact and Thickness ......................................................................... 8 Basement Structure .. .................................................................................. 9 Physical Stratigraphy ...................................... ....................................... .... 9 Age ................. ....................................... .............................................. 10 Petaluma Formation (STOP 2) ................................. ..................................... 10 Basal Contact and Thickness ............. ........................................... .. ............... 11 Physical Stratigraphy................................................................................. 11 Age ....................................................................................................... 12 Interbedded Wilson Grove and Petaluma Formations (STOP 3) ............................... 12 Physical Stratigraphy . .. .. .. .. .. .. ... 13 Age ................. ............... ........................................ ............................... 13 Depositional Environment............................ ........................................................... 13 Wilson Grove Formation ...................... ......... .. ........... ................................. 13 Petaluma Formation ................ .................. ................................................. 14 Paleocurrents.............. .. .. .. .. ... 14 Provenance .......................................................................................................... 14 Regional Correlation ............................................................................................... 15 Tectonic Significance (STOPS 4, 5 & 6)... ..................................................... ........... ..... 16 Dextral-Slip Faults in the Northern Bay Area .. ................................................................ 16 Rodgers Creek Fault. ................................................................................. 16 Sonoma Valley Fault. ............................................................. .................... 17 Petaluma Valley Fault.. ........................................................... ..... ............... 18 Model of Late Cenozoic Dextral Slip ............................................................................ 18 Thrust and Normal Faulting ............................................... ......................................... 19 Summary ............................................................................................................. 20 References .............. ........ ................................................. .. .. ...... ........................... 20 Appendix I .......................... .. ............. .................. .................. .............. ................ 28 7 DIRECTIONS 22.9 Mi- The road changes names from Crane Canyon Road to Grange Road. The meeting location is the Caltrans park-and-ride lot STOP 4: Taylor Mountain in the southwest quadrant of the Highway IOI and STOP 5: 23.6 Mi- At the crest of the road there is a Rohnert Park Expressway (Fig. I). We will curve to the left. Park on the shoulder. Bennett consolidate in vehicles because parking will be limited Mountain is to the right at I :00. at the fieldtrip stops. Continue north on Grange Road. 24.7 Mi- Tum Right at the stop sign Bennett Valley MEETING LOCATION- 0 Mi · CAL TRANS Road. PARK & RIDE LOT IN SOUTHWEST 25.4 Mi- Tum right onto Sonoma Mountain Road. QUADRANT OF IDGHW A Y 101 AND 26.7 Mi- Tum left onto Bottasso Court. ', ROHNERT PARK EXPRESSWAY STOP 6 26.8 Mi- Park along road. This is a private I Exit the Park & Ride on southlxmnd Highway 101. driveway, or we may be able to get access into the 0.9 Mi- Take the 181 exit, Gravenstein Highway, tum creek. right (west) onto Gravenstein Highway (Highway 116). OVERVIEW OF STOPS 2.1 Mi- The highway cuts through the interbedded WGFand PFs. Stop 1 • Steinbeck Ranch 2.4 Mi- Turn left (south) at Stony Point Road The Steinbeck Ranch stop provides rare 3.9 Mi- Tum right (west) onto Roblar Road outcrops of the WGF and Roblar Tuff. The base of immediately after the Washoe House. the WGF at Steinbeck Ranch is just west of the ranch house and barns where it overlies Franciscan Complex STOP I 8.3 Mi - Steinbeck Ranch is to the right. sandstones along an angular unconformity. Across Red House with accessory barns and outbuildings Roblar Road, SE of the ranch, is an outcrop of - 9 ma about 100 yards off the road. Leave Steinbeck Ranch Tolay Volcanics which locally underlies the Wilson the same way we came. Tum left leaving the Grove Formation here. These volcanics also underlie driveway onto Roblar Road. the basal Petaluma Formation (PF) at the Stony 12.8 Mi- Tum left onto Stony Point Road. Once onto Point Rock Quarry and between the Tolay and Stony Point Road, a large berm of waste rock is Rodgers Creek faults, providing a - 9 Ma basal age for visible in the distance on the left side of Stony Point the two formations. The Wilson Grove Formation Road. This is the edge of STOP 2- Stony Point Rock exposures at Steinbeck Ranch are fine-grained Quarry. sandstones typical of this formation and have a gentle STOP 2 13.3 Mi- Get into the left turn lane and tum dip to the east. The Wilson Grove Formation often left into the entrance of Stony Point Rock Quarry. appears massive, however the exposures at Steinbeck Leave Stony Point Rock Quarry and tum left onto Ranch show subtle hummocky cross stratification. Stony Point Road. The late Miocene (6.26 ma) Roblar Tuff 14.2 Mi- Gravenstein Highway (Highway 116). Go occurs about 105 m above the base of the Wilson straight, continue on Stony Point Road through the Grove, therefore we are seeing the late Miocene to intersection. Pliocene transition here. Fossils above the bed are 14.4 Mi• Mount St. Helena is straight ahead in the Pliocene in age. Present at the base ofthe tuff is a 5 distance. to 15 cm-thick angular gravel bed clasts ofred to gray STOP 3 14.9 Mi-
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