High resolution seismic study of the Holocene infill of the Elkhorn Slough, central California

The seismic analysis of the sedimentary infill of the Elkhorn Slough, central California, reveals a succession of three main seismic units: U1, U2, U3, with their correspondent discontinuities d2, d3. These units are deposited over a paleorelief representing the channel location at least 8k years ago. The location of that paleochannel has not changed with time, but the geometry of the infilling sedimentary packages has done so through the years. Discontinuities d2 and d3 show a relic island or relative high in the center of the Slough that separated the sedimentation into two main small basins at least 3k years ago. There is evidence of erosion in the last two sedimentary units showing that the present erosive pattern began decades ago at minimum. We have correlated radiocarbon data of selected cores with the high resolution chirp profiles and reconstructed the infill for the Elkhorn Slough.In the most recent unit, the occurrence of numerous lateral accretion surfaces on both ends of the main channel is discussed within their environmental setting, tidal currents and the net ebb flux of the area. We have interpreted the presence of gas in the sediments of the slough, with a gas front located at the tops of units 2 and 3, which are discontinuities that reflect an effective seal for the gas. Our data shows no obvious evidence for seepage, although the shallow presence of some of the fronts points out the fragility of the environment in the present erosive conditions.

► The Elkhorn Slough's sedimentary infill has been imaged for the first time. ► Its three units overlay a channel that has not changed location since 8000 yr B.P. ► Two main basins were created 3k years ago affecting the sedimentation patterns. ► There is evidence of erosion in the last two sedimentary units. ► The gas in the sediments has a shallow front with no seepage evidence found.