725 



observation in previous studies) was that the surface sediments of the Seawolf Mound would 

 eventually be winnowed of some of the silt-clay fraction resulting in a surficial residuum of 

 fine sand mixed with shell and coarse sand (Johnson and Baldwin 1986). 



In many areas the sand may be transported in bedload over silty sediments without 

 erosion of the cohesive silts, resulting in a "sand-over-mud stratigraphy" detectable in 

 REMOTS® images. It is expected that the sand and shell "lag" deposits (large sediment 

 particles that "lag" behind as the finer materials are washed away) would be resistant to 

 further erosion of the scale experienced on a regular basis. This process is called "textural 

 armoring." These sediment transport features have been widely observed in studies of ancient 

 and modern coastal sediment transport patterns (Johnson and Baldwin 1986). The techniques 

 used to investigate sediment transport history in sedimentary geology are analogous to 

 interpretation of REMOTS® sediment profile images. 



The surface sediments of the Seawolf Mound were evaluated relative to these 

 predictions. Characteristics of surface sediment winnowing identified in REMOTS® images 

 include shell lag, disturbed amphipod tube mats, physical boundary roughness, and sand over 

 mud stratigraphy. Shell lag can be seen as exposed bivalve and gastropod shells (Figure 4-8a) 

 or shells mixed with sand. Winnowed surfaces are observed when the surface shows evidence 

 of recently lost material (mud bands on polychaete tubes, lack of bioturbated "fluff layer, 

 irregular surface topography; Figure 4-8b). Amphipod tube mats go through a cyclic process 

 where tubes are abandoned and begin to decompose, in this state they are easily transported. 

 The decomposition and loss of a few amphipod tubes will trigger instability in the mat and 

 cause the mats to roll-up and be transported in pieces. Stages in this process can be seen in 

 REMOTS® images including new mats, adult mats, decaying mats, and persistent fragments of 

 mats with adjacent exposed sediment (Figure 4-9). Physical boundary roughness is evaluated 

 by the difference between the highest and lowest elevation of the sediment surface in an image 

 and subjectively assigned to biological (tubes, mounds, burrow pits) or physical (shell lag, 

 dredged material clumps, mud clasts) causes. 



Three of the characteristics of winnowing were widely distributed at the Seawolf 

 Mound (Figure 4-10). Only two of the REMOTS® stations showed no evidence of small- 

 scale winnowing (CTR and 300W). Station CTR showed persistent clumps of gray clay and 

 300W showed no evidence of dredged material, but some decaying amphipod tubes (Figure 

 4-1 1). The presence of cohesive gray Gardiners glacial clay (from improvement dredging 

 below the estuarine sediments) across the mound had an influence on the surface sediment 

 distribution. The grain size at the inner stations was finer than at the reference areas, which 

 was characterized as very fine sand. A mix of silt-clay and very fine sand characterized most 

 of the sediments of the Seawolf Mound, and surface sand overlying fine-grained sediment 

 (sand over mud stratigraphy) was noted for most images. 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



