Evolution of the California Margin

by Laurel Goodwin and Basil Tikoff

We left Wisconsin early on May 26, 2009, bound for San Francisco. That is, most of us left then. Co-leader Alan Carroll left a day early in his own plane. Co-leader Shanan Peters joined us on the more traditional route via commercial airline. Traditional, but not boring; we were surrounded by a wonderfully diverse and good-natured group of ten graduate students, nearly evenly divided among structural geologists, sedimentologists, and geophysicists, with a geomorphologist thrown in for good measure. Following a semester-long seminar, we were off to explore the tectonic, structural, and sedimentologic evolution of the west coast.

Alumnus Eric Horsman (then at the USGS, now on the faculty at Eastern Carolina University) met us in the Santa Cruz Mountains on that first day, jump-starting the trip with a remarkably clear overview of the tectonic history-—including important faults and sedimentary basins—of the San Francisco Peninsula. We camped that night among the spectacular trees of Big Basin Redwoods State Park. Up the next morning before dawn (with nary a complaint from our stellar group of graduate students), we rolled down to the coast to catch the low tide exposures of the San Gregorio fault zone at Moss Beach. In a pattern to be repeated in following days, we had animated discussions about sedimentary rocks and deformation on first fault outcrops then spectacular examples of Franciscan mélange to the north.

Shanan Peters
Shanan makes a point about well deformed rocks

For the next eight days, we rambled south down the coast, then looped inland and north again, examining the rock record of both the San Andreas transform fault system and the older Mesozoic convergent tectonic system on which it was imposed. Our coastal route took us first to the petroleum seeps and deformation bands in sand intrusions at Panther Beach south of Santa Cruz, then south of Monterey Bay to Point Lobos with its amazing exposures of a submarine channel, spectacular soft-sediment deformation (see the picture of Shanan appreciating well deformed rocks), uplifted terraces, marine life, and a fair bit of poison oak. After our coastal foray, we were joined by alumnus Mike Smith (a faculty member at Sonoma State University) and his colleague Matty Mookerjee at the more pedestrian King City Campground. These two provided local insight and (especially Mike) entertainment. Astute graduate students picked up on key mannerisms that Alan and Mike share—but that’s another story.

With Mike and Matty in tow we cruised up Salinas valley (Steinbeck country) on dusty outcrops of folds in the Monterey formation next to the Rinconada fault and thought about strain partitioning, then visited sandstones exposed in the Santa Lucia Range. Following the San Andreas fault to the south, we had to stop at Parkfield (attempting to “be there when it happens”, or at least get the group photograph) and the famous Wallace Creek. Our most southerly stops were in the Ridge Basin—interpersed with roadside investigations of the battered core of the San Andreas fault.

The Wisconsin group at the Parkfield Bridge
The Wisconsin group at the Parkfield Bridge

Having had enough of a brush with southern California, we retreated north to Pacheco Pass to explore blueschists and the older record of subduction processes. We also followed a transect through the Great Valley sequence to see the Del Puerto ophiolite. On the last day, taking it a little easier, we saw the offset sidewalks of Hollister (including meeting a very funny inhabitant of one of the houses directly over the fault), the old San Benito winery (also built directly on the fault—hmmm, there seems to be a pattern here), and took a lovely hike to the top of the Pinnacles to think about the cumulative 315 km of dextral displacement on the San Andreas fault. After a nice meal on the Santa Cruz pier, we decided to head home to the stable craton.

Overall, it was a great trip, and the perfect way to end the academic year.

Many thanks to the alumni supporters of the Field Experience Fund who made the trip possible for our graduate students.

Originally published in The Outcrop for 2009, UW-Madison Department of Geoscience, p. 20.