Quantifying Late Quaternary deformation in the Santa Maria Basin: A OSL, GPS and soil chronosequence based model for determining strath terrace deformation in the Zaca Creek drainage, Santa Barbara County

Andrew C. Farris, & Nate W. Onderdonk

Published March 14, 2017, SCEC Contribution #6571, 2017 SCEC Annual Meeting Poster #016

The Santa Maria Basin is located on the Central Coast of California and comprises the area bound by the Santa Ynez Fault to the south, and the Little Pine-Foxen Canyon Fault Zone to the north. The Santa Maria Basin is a zone of convergence between the obliquely rotating Western Transverse Ranges and the non-rotated Southern Coast Ranges. Although early Quaternary shortening due to crustal convergence with the Western Transverse Ranges is well documented in the Santa Maria Basin, the style and amount of Late Quaternary deformation is unknown in the region. 7 levels of fluvial terraces in the Zaca Creek drainage offer significant insight into Late Quaternary deformation. Soil test pits were used for correlative purposes in a setting with a blind terrace stratigraphy and potentially offset terrace structures. Each terrace tread’s soil morphology was described according to standardized United States Department of Agriculture soil survey methods. The oldest strath terrace soils correlated with generally high levels of soil genesis, thick illuvial horizons, and localized iron oxide induced hardpans due to low soil pH. The youngest fill terraces correlated with generally lower levels of soil genesis with localized lithologic discontinuities.

Detailed field mapping and surveying with a differential GPS unit suggests significant deformation in the oldest terraces (terrace levels Qt4 to Qt6), which are likely older than at least 40 ka. Terrace warping occurs in two separate locations due to Quaternary folding and convergence related regional shortening. Select strath terrace treads were sampled for Optically Stimulated Luminescence analysis for age control. After the OSL based terrace chronology is complete, the style and rate of deformation and regional uplift in the Zaca Creek area will be able to be quantified. Results from this study will have significant implications for seismic risk, regional tectonics, and soil-landscape relationships in clay soil and agricultural settings.

Key Words
active tectonics, santa maria basin, chronosequence, Zaca Creek

Citation
Farris, A. C., & Onderdonk, N. W. (2017, 03). Quantifying Late Quaternary deformation in the Santa Maria Basin: A OSL, GPS and soil chronosequence based model for determining strath terrace deformation in the Zaca Creek drainage, Santa Barbara County. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)