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 tec tonic margin subject to abundant terrigenous sediment input from the Eel Ri ver. A recent high-resolution swath sonar survey provides us with the oppor tunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region. Our analysis of the statistical character of b athymetry demonstrates a clear separation into large- and small-scale morph ologies at a similar to 3-10 km scale, with smaller-scale morphology heavil y damped relative to large-scale morphology. The Eel shelf bathymetry is su btle, but several structures can be readily discerned in a residual bathyme try formed by removal of the downslope gradient. Some shelf structures are evidently related to depositional processes las evidenced by correlation wi th 100-yr sediment accumulation rates), whereas others appear related to te ctonic processes las evidenced by correlation with subsurface synclines and anticlines). The sidescan structure of the shelf is dominated by the low b ackscatter over the Eel and Mad River subaqueous deltas, evidently associat ed with the sand-to-mud transition. However, contrary to usual correlations between backscatter and grain size, in this situation higher backscatter i s associated with the muddy sediments. In addition, we observe a series of shore-perpendicular striations, or 'ribbons', spaced similar to 0.2-1.0 km apart, which extend northward from the Eel River subaqueous delta and lie a t or near the sand-to-mud transition. Some aspects of ribbon morphology sug gest that they may be associated with down-slope flows. (C) 1999 Elsevier S cience B.V. All rights reserved.