Detailed investigation of continental shelf morphology using a high-resolution swath sonar survey: the Eel margin, northern California

The Eel shelf, northern California, lies within an active compressional tectonic margin subject to abundant terrigenous sediment input from the Eel River. A recent high-resolution swath sonar survey provides us with the opportunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region. Our analysis of the statistical character of bathymetry demonstrates a clear separation into large- and small-scale morphologies at a ∼3–10 km scale, with smaller-scale morphology heavily damped relative to large-scale morphology. The Eel shelf bathymetry is subtle, but several structures can be readily discerned in a residual bathymetry formed by removal of the downslope gradient. Some shelf structures are evidently related to depositional processes (as evidenced by correlation with 100-yr sediment accumulation rates), whereas others appear related to tectonic processes (as evidenced by correlation with subsurface synclines and anticlines). The sidescan structure of the shelf is dominated by the low backscatter over the Eel and Mad River subaqueous deltas, evidently associated with the sand-to-mud transition. However, contrary to usual correlations between backscatter and grain size, in this situation higher backscatter is associated with the muddy sediments. In addition, we observe a series of shore-perpendicular striations, or `ribbons\u27, spaced ∼0.2–1.0 km apart, which extend northward from the Eel River subaqueous delta and lie at or near the sand-to-mud transition. Some aspects of ribbon morphology suggest that they may be associated with down-slope flows