Acoustic backscatter of the 1995 flood deposit on the Eel shelf

Acoustic swath mapping and sediment box coring conducted on the continental shelf near the mouth of the Eel River revealed regional variations in acou stic backscatter that can be related to the shelf sedimentology. The acoust ic-backscatter variations observed on the shelf were unusually narrow compa red to the response of similar sediment types documented in other areas. Ho wever, the acoustic data revealed four principal bottom types on the shelf that can be related to sedimentologic differences observed in cores. The fo ur areas are: (1) low acoustic backscatter associated with the nearshore-sa nd facies and the prodelta terraces of the Eel and Mad rivers, composed of fine sands and coarse silts with low porosity; (2) high acoustic backscatte r associated with fine silts characterized by high porosity and deposited b y the 1995 flood of the Eel River; (3) intermediate acoustic backscatter in the outer-shelf muds, where clayey silts are accumulating and the 1995 flo od apparently had limited direct effect; and (4) intermediate acoustic back scatter near the fringes of the 1995 flood deposits and in areas where the flood sediments were more disrupted by post-depositional processes. The hig hest acoustic backscatter was identified in areas where the 1995 flood sedi ments remained relatively intact and near the shelf surface into the summer of 1995. Cores collected from these areas contained wavy or lenticular bed ding. The rapid deposition of the high-porosity muddy layers results in bet ter preservation of incorporated ripple forms than in areas less directly i mpacted by the hood deposit. The high-porosity muddy layers allow acoustic penetration into the sediments and result in greater acoustic backscatter f rom incorporated roughness elements. (C) 1999 Published by Elsevier Science B.V. All rights reserved.