Abstract
Paleoenvironmental records from a southern California coastal saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the wetland. Detailed sediment analyses reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Microfossil analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years. Two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, we infer that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of an approximately 5-km2 area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide insight on the evolution of the saltmarsh, coseismic deformation and earthquake recurrence in a wide area of southern California, and sensitive habitat already threatened by eustatic sea level rise.
Highlights
The Seal Beach saltmarsh is the last undeveloped portion of the Anaheim Bay estuary that once covered a large area in Sunset Gap near Seal Beach, California (Figs 1 and 2)
We interpret the lithostratigraphic sequence in the Seal Beach saltmarsh as recording three abrupt paleoenvironmental changes from saltmarsh deposits, to predominantly freshwater influenced silty sand, and a return to a saltmarsh environment
What historic and geologic factors could contribute to the deposition and preservation of these events? During the winter of 1938, a historically large flood blanketed the coastal plain of Los Angeles and Orange County with >1-m thick deposits of silty sand[12], suggesting the possibility that the thick micaceous silty sand deposits in the saltmarsh could be the remains of terrestrial flooding events
Summary
The Seal Beach saltmarsh is the last undeveloped portion of the Anaheim Bay estuary that once covered a large area in Sunset Gap near Seal Beach, California (Figs 1 and 2). Fringing freshwater wetlands, salt flats, and alkali meadows[18,19] (Fig. 2). Reclaimed areas of the estuary proximal to the fault are occupied by military, municipal and industrial infrastructure including the U.S Naval Weapons Station Seal Beach, Huntington Harbor, the 965 acre Seal Beach National Wildlife Refuge, and an active oil-extraction operation (Figs 1 and 2). Paleoseismology and paleoenvironmental analyses of the saltmarsh offer an opportunity to better understand late Holocene earthquake occurrence on an important and poorly understood southern California fault system[20,21]
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