Evidence for Abrupt Subsidence Event in Carpinteria Marsh at 2 ± 0.2 ka

Laura C. Reynolds, Alexander R. Simms, Thomas K. Rockwell, John M. Bentz, & Robert B. Peters

Submitted August 12, 2016, SCEC Contribution #6677, 2016 SCEC Annual Meeting Poster #087

Recent work by Simms et al., 2016 suggests Carpinteria Marsh, Carpinteria, California, has experienced an average of 1.2±0.4 mm/yr of subsidence in the last 7ka. However, the nature (ongoing vs. abrupt) of this subsidence remained unclear. Here we use a detailed stratigraphy of 43 vibracores and 7 Geoprobe cores, along with 62 radiocarbon dates to show evidence for one occurrence of abrupt subsidence at 4 ±0.5 m below MHHW. At this depth, a sharp surface between organic-rich, fine-grained deposits and overlying fine gray sand exists in all cores penetrating below 4 m. The consistency of the elevation of the surface and lack of erosional signatures indicate an abrupt environmental change. The underlying fine-grained deposits contain numerous organic fragments, Cerithidea californica shells (a high-marsh species of gastropod), and low to mid marsh foraminifera species (Jadammina spp., etc.). We interpret these deposits to represent marsh surface elevations (MSL-MHHW). The sand above the surface is devoid of foraminifera, but includes shells of subtidal (below MSL) species such as Ostrea spp., Macoma nasuta, and Saxidomus nuttallii. Radiocarbon dates taken below and above this surface in 6 cores dates it to 2 ± 0.2 ka. The abrupt deposition of subtidal deposits on supratidal deposits requires an increase in relative sea level. Because no dramatic fluctuations in sea level are known for this time period, we attribute this drowned surface to rapid subsidence of the marsh surface.

Key Words
Subsidence, sedimentology

Reynolds, L. C., Simms, A. R., Rockwell, T. K., Bentz, J. M., & Peters, R. B. (2016, 08). Evidence for Abrupt Subsidence Event in Carpinteria Marsh at 2 ± 0.2 ka. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology