Speakers Club: Shelby Fredrickson and Michael Bentz

Event Date: 

Thursday, February 25, 2016 - 2:00pm

Event Location: 

  • Webb Hall 1100

Shelby Fredrickson and Michael Bentz, both of whom are graduate students in Earth Science, will each present talks at this week's Speakers Club.  Shelby's talk is entitled A Geomorphic Investigation of the Transition between the Santa Barbara and Ventura Fold Belts near Rincon Point, California and Michael's talk is entitled Modern Foraminifera in Southern California Salt Marshes: The "death" of the southern California tidal mudflat.  Speakers Club meets at 2:00 PM on Thursday in Webb Hall 1100.

Shelby's Abstract:

The Santa Barbara and Ventura fold belts have very different uplift rates based on marine terrace data: ~1-2 m/ky and ~6-8 m/ky, respectively. These differences are likely related to the position of the two belts relative to the Big Bend of the San Andreas fault. We propose that the onshore transition between these fold belts occurs at Rincon Creek, near Carpinteria, where the first emergent marine terrace rises from below sea level to nearly 200 m elevation over less than 10 km distance. We use a combination of geomorphology and geochronology to describe patterns and rates of crustal deformation near Rincon Creek.

Uplift patterns can be inferred from stream profile analysis via digital topography. A statistical comparison of normalized stream steepness values in weak rocks supports higher uplift rates east of Rincon Creek. Preliminary optically stimulated luminescence age dates on the first emergent marine terrace between Carpinteria and Rincon Creek yield a mean age of 32.1±8.9 ka. This age agrees with previous dates of 40 ka east of Rincon Creek. Using the chronology developed here and a local sea level curve by Simms et al. (2015), we calculate an uplift rate of 2.1±0.5 m/ky. The age and vertical offset of marine terrace deposits indicate minimum vertical slip rates of 0.07 m/ky and 0.14 m/ky on the Railroad fault and Carpinteria fault, respectively.

We propose that a tear fault coincides with the downstream, linear stretch of Rincon Creek and a right-lateral strike-slip fault mapped by Minor and Brandt (2015). This fault may explain the juxtaposition of Carpinteria Basin with Rincon Mountain. Recent seismic data from offshore of Rincon Point contains evidence for a bedrock ridge or fold that aligns with the fault proposed here (Levy et al., 2015). This bedrock ridge may be associated with the Red Mountain fault and argues for a structural explanation for the change in topographic character across Rincon Creek. 

Michael's Abstract:
Salt marsh foraminiferal species live in distinct elevation zones relative to tidal and sea level datums. Their strict vertical zonation can be used as a tool in recognizing rates of sea-level rise as well as tectonic activity along the coast. Although foraminifera have been used worldwide to reconstruct past sea levels, no such elevation zonations have been produced for southern California salt marsh foraminifera. We sampled foraminifera from three southern California estuaries: Carpenteria Slough, Mugu Lagoon, and Sweetwater Marsh, for the purpose of establishing a foraminifera zonation for southern California marshes. The results from Carpenteria Slough display no distinct zonation with regards to elevation. The lack of elevation control on foraminiferal species may be the result of fundamentally altered salt marshes in southern California. Are humans impeding the ability to produce a sea-level transfer function in southern California?


Shelby Fredrickson and Michael Bentz