Geology of the Right Stepover Region Between the Rodgers Creek, Healdsburg and Maacama Faults
Reported by Tom Wright
The view from Pepperwood
Thirty-one members of NCGS enjoyed a wonderful field-trip in the mountains of Sonoma County over the weekend of June 6-8. Our tour was led by Bob McLaughlin and Andrei Sarna-Wojcicki of the USGS and Dave Wagner of the California Geological Survey (CGS). They provided a progress report (including a fine guidebook) on recent studies of the long-neglected North Bay region, where active dextral faults juxtapose hard-to-differentiate sequences of poorly-exposed late Cenozoic volcanic rocks.
Day 1 - Saturday
Our base was Pepperwood Ranch north of Santa Rosa, formerly a Bechtel family property and now a preserve owned by the California Academy of Sciences. Evenings were centered in the main room of Bechtel House, with preliminary geological maps available for discussion. The advance guard on Friday night fed on pizza and on Saturday night the entire group feasted on Tridib Guha’s splendid barbecue of salmon, chicken, roast corn, and beans. The only downsides to the weekend were: the thick, wet fog that enveloped our mountainside lair late each evening and at Saturday’s Stop 1, reduced our “360-degree regional overview” to a few hundred feet; and the rambunctious nanny-goat at Stop 3, the best accessible exposure of the southern Maacama fault but located in a goat pen where the goats browse on poison oak and then nuzzle any trespassers (this goat ate part of John Karachewski’s guidebook while John was trying to photograph the fault!).
The morning hike up to Stop 1!
Stop 1: End of Day Revisit
Ongoing geological studies in the North Bay area, aimed at a better understanding of its earthquake history and seismic hazards, are being funded through elements of the National Geological Mapping Act of 1992. Those elements include: FedMap, that is funding USGS mapping north of Santa Rosa; StateMap, providing 50/50 Federal matching of State funds for work in the southern Sonoma Mountains; and Edmap, supporting several exceptional graduate students in thesis mapping under CGS and USGS guidance. All the mapping is at the 1:24,000 scale. The USGS is providing argon/argon dating and the wonderful tephrochronology of Andrei Sarna-W, who has fingerprinted at least 12 tuffs and flows within the 8.17 (8.65?) to 1.96 Ma span of the Sonoma Volcanics. Results to date from this joint project suggest a very fruitful synergy within this group.
Stop 2: East-dipping folded Plio-Pleistocene fluvial gravels that overlie Sonoma Volcanics
Saturday’s trip ranged north and west of Santa Rosa and was led by McLaughlin and Sarna. Of particular interest was the Maacama fault and the ash beds and fluvial gravels that constrain geological mapping in this area. Six of the nine stops are accessible to the public and the guidebook includes a road-log, map and stop descriptions.
Stop 3: Location identified as the central trace of the Maacama fault within the fault zone with Mel Erskine, Bob McLaughlin, and Andrei Sarna-Wojcicki (left to right)
Stop 3: The infamous goats and part of a field guide!
Stop 3: A wider view of the Maacama fault zone
Lunch was at the Petrified Forest, a vintage tourist stop, where redwoods and other conifers lie entombed in a 3.35 Ma ash bed.
Lunch Stop: Notice the details of preservation in the bark
At stops 1, 3, and 5 we saw the Maacama fault in detail and in the geomorphic features of an active dextral fault zone up to 1 km wide. To the south in the Santa Rosa area, the right-step from the Maacama fault to the Rodgers Creek fault is reflected in the local, north-trending Rincon Valley graben, but elsewhere the new mapping shows a more complex structural pattern. The N-NW-trending Maacama fault zone is superimposed on a series of W-NW-trending, NE-dipping thrust and reverse faults that have shaped the valleys and ridges within and NE of the stepover area. These W-NW-trending faults are thought to be relicts of the Cascadian convergence regime, though some are seismically active (as is the Maacama fault). Offsets of obsidian-pebble lithofacies in Plio-Pleistocene gravels from their very localized sources east of the Maacama fault give a minimum displacement of 13 km on the fault; offset of a 2.85 Ma ash flow tuff section (visited at stops 5 and 7) suggests a displacement of about 24 km.
Stop 5: Ash flow tuff section of Franz Valley - See road log for further details.
Stop 5: Ash flow tuff
Stop 6: Air fall and water lain tuff along Chalk Hill Road (including diatomaceous units).
Stop 6: Load structures
Stop 6: Air fall and graded bedding structures
Stop 6: Current transport features
Our last stop on Saturday was on the subdued scarp of a down-to-the-east fault that cuts the Santa Rosa plain. This broad plain has been built of fluvial sediments derived from the mountains to the east. Beneath it, as shown by Bouguer gravity, are the Windsor and Cotati basins, 2.5 to 3 km deep.
Day 2 - Sunday
Photos kindly provided by Mark Detterman.