Day 2 - Sunday
After breakfast on Sunday we caravanned an hour south from Pepperwood, through Petaluma and along Highway 116 into the southern Sonoma Mountains. As the entire day’s trip would be on private ranch lands, there is no road log or map in the guidebook. Dave Wagner has provided a summary of the geology and tectonics of the area and was leader for the day, introducing at various stops the three graduate students who have worked within the EdMap program. On the east flank of the mountains along the Bennett Valley fault, Jim Allen (MS, San Jose State) presented a long-needed comprehensive study of the Petaluma Formation (8.4 to 4 Ma) and its western, marine equivalent, the Wilson Grove Formation. Argon/argon dating and tephrochronology by the USGS provides the essential framework for Allen’s study and, in a significant breakthrough, includes a dating (9.3 - 9 Ma) of the volcanic rocks beneath the Petaluma Formation in the Petaluma oilfield. The lower Petaluma, 90 to 183 m thick, is shallow-marine to lacustrine mudstone that likely sourced the oil in that small field. Allen has differentiated the middle and upper Petaluma, fluvial to shallow-marine deposits derived from the east and totaling at least 1037 m in thickness, on the basis of clasts in the conglomerate beds. Both units contain Franciscan clasts but the upper Petaluma conglomerates are dominated by clasts of Monterey chert and a unique white sandstone with glauconite and quartz veins. This distinction is carried westward into a belt of interbedded Petaluma and Wilson Grove formations that represents an oscillating Mio-Pliocene shoreline. The Monterey clasts are readily related to the 13 Ma Claremont chert but Allen had to search the East Bay Hills to find the source of the white sandstone in the Briones Formation east of San Jose. This match suggests a right-slip of about 67 km across the Hayward fault – and its northern equivalents – since about 5 – 6 Ma. Regarding the northern extension of the Hayward fault, Jim Allen has dropped a bombshell by finding unequivocal outcrops of upper Petaluma east of the Rodgers Creek fault within and on the east flank of the Sonoma Mountains. This appears to require a hitherto-unrecognized extension of the Hayward fault that veers northerly beneath San Pablo Bay and beneath the alluvial floor of the Sonoma Valley, complicating the right step from the Hayward to the Rodgers Creek fault.
After Allen’s presentation we followed ranch roads onto the crest of the Sonoma Mountains and encountered the Rodgers Creek fault at Lee Lake, certainly the loveliest sag pond this geologist has yet seen.
The Lee Lake Sag Pond
Dave Wagner and Carrie Randolph-Loar leading discussions at Lee Lake
Further discussions at the Lee Lake Sag Pond
Carrie Randolph-Loar led us south along the fault zone to the site of her Gravelly Lake trench, a key element of her recent MS thesis at San Francisco State. The trench site was chosen after detailed air-photo mapping and a ground-penetrating-radar transect that proved to show quite clearly the two fault traces and several key horizons later revealed by trenching. The paleoseismic investigation at this site documented six events over the past 6,800+ years, with three events in the past 3,700 years. Earlier trenching some 8 km to the north (visited by a 1992 NCGS field trip) had found three events over the past 925 – 1,000 years, with a recurrence interval of 230 years. Several explanations were advanced for this discrepancy and further trenching is planned.
Discussions at the Gravelly Lake trench site
Our lunch stop.
The Mt. Burdell discussion lead by Erik Ford, with assistance provided by Dave Wagner and Rolf Erikson
Mt. Burdell in the distance.
We paused near the trench site for lunch and a brief presentation by Eric Ford, (SF State graduate student) on his mapping of the Burdell Mountain Volcanics (13.6 – 11.1 Ma) and Burdell Mountain fault zone, about six miles to the southwest. (Rick will lead a NCGS field trip there in November.) We were joined during lunch by the ranch owners, a pleasant and accommodating couple who accompanied us to the next several stops. These took us south into the trough between the Rodgers Creek and Tolay faults, where we saw an outcrop of Donnell Ranch Volcanics, 10.64 to 9.28 (8.52?) Ma, and discussed the complex faulting in this vicinity that includes east-vergent thrusting of Donnell Ranch Volcanics over Petaluma Formation. Recent major excavations at the Sears Point Raceway provided ephemeral exposures of a zone over 2,000 feet wide of intensely sheared Petaluma Formation and Franciscan rocks where the Tolay fault has been mapped. The Sears Point “anticline”, a knob of Franciscan west of the Tolay fault, is now seen as a diapiric uplift, intermittently raised in late Miocene and subsequently. West of this uplifted knob, beneath the Petaluma River marshes and the Petaluma Valley, an inactive fault located chiefly by gravity data and called the Petaluma Valley fault, is believed to be the ancestral northern extension of the Hayward fault. Our final stop, just north of Highway 121, was a rhyolite outcrop just east of the Rodgers Creek fault, at about 8 Ma the oldest known part of the Sonoma Volcanics.
Overview of the trough between the Rogers Creek and Tolay Faults.
Mt. Tam
Dave Wagner leading discussions at the overview.
Further discussions.
Across the trace of the Roger Creek - Tolay fault trough
For those of us who are fascinated spectators as the interpretation of San Andreas tectonics unfolds, and may participate to the extent of occasionally throwing a loose ball back into the field of play, it is most heartening to see this team of skilled veterans and talented “rookies” working together in the North Bay region. We were left with a sense of great progress, yet much more work still to be done in unraveling a complex picture of late Cenozoic volcanism and evolving fault patterns in a young San Andreas transform zone. Just east of this zone in the Hollister area, the Quien Sabe Volcanics (11.6 – 7.4 Ma) are thought to correlate with the Burdell Mountain Volcanics (13.6 – 11.1 Ma), suggesting 175 km of displacement. Each successive fault block to the east contains a younger sequence of volcanic rocks: the Donnell Ranch Volcanics , 10.64 to 9.28 (8.52?) Ma east of the Petaluma Valley fault; the Sonoma Volcanics , 8.17 (8.65?) to 1.96 Ma within and east of the Rodgers Creek fault zone; and east of the Maacama fault, the Clear Lake Volcanics (2.2 – 0.007 Ma), the subject of a NCGS field trip on August 2. We have the concept of a volcanic hotspot (now the Geysers geothermal area) at the plate margin migrating northwestward in the wake of the Mendocino triple junction, perhaps a slab window behind the subducted Gorda slab. As portions of the resulting volcanic field become attached to the Pacific plate they are sliced off and carried northwest in a series of fault slices. In each slice the volcanic rocks are oldest at their southern end. The four volcanic units now overlap geographically, having been carried along almost as far as the “hotspot” has traveled over the past 12 My. The localized contraction north of San Pablo Bay, including the diapiric Sears Point uplift, may be involved in the deflection of dextral faulting from the Hayward fault north to Allen’s postulated Sonoma Valley fault. This is just one of the complications yet to be resolved in the North Bay region. Bob McLaughlin and Dave Wagner have started to talk about a symposium on North Bay geology and tectonics at the 2005 Pacific Section/Cordilleran meeting in San Jose. I, for one, am looking forward to that.
Photos kindly provided by Mark Detterman.