tidal currents as a contributing mechanism to the Maine features. 

 Calculation of required waves for these megaripples is empirical, 

 but based on Harms et al. (1982, Figure 2-10), winter storms of 

 >8 sec period and >3 m wave height (Figure 3) are capable of 

 producing the required 40 cm/sec near-bottom orbital velocities 

 in 30 m water depth. Forbes and Boyd (1987) have reported on 

 similar gravel ripples on the Scotian Shelf. They suggest that 

 the ripples demonstrate a linear scaling to wave orbitals, and 

 that published models may overestimate critical velocities for 

 formation by a factor of two. 



The deep paleovalley axes are sites of mud accumulation. 

 Submersible observations reveal abundant molluscan, decapod and 

 fish burrowing. In the upper 5 cm a "turf" of hydroids and 

 amphipod tubes partially stabilizes the sediment surface. 

 Lobsters and other decapods disturb the surface, both in 

 individual burrows and in large shallow pits up to 5 m diameter 

 and 0.5 m depth. We speculate, however, that natural gas seeps 

 may initiate the larger depressions. These pockmarks show up 

 prominently on some side-scan sonar lines, occurring only over 

 gas-rich muddy basins. We have seen gas bubbles in the water 

 column on fathometer traces taken while the side-scan sonar 

 revealed the pits (Figure 12) . Scanlon and Knebel (1985) have 

 observed much deeper and broader pits in northern Penobscot Bay 

 which might be a later stage of evolution of the postulated gas 

 seeps. Similar large-scale features are seen on the Scotian 

 Shelf (King and MacLean, 1970) and the North Sea (comparison of 

 the two by: Hovland et al . , 1984). Vilks et al. (1974) have 

 discussed methane in sediments from similar settings off the 

 Labrador shelf. Sedimentation in the deep basins is by pelagic 

 settling and probably by periodic low-density turbidity currents. 

 Bedrock ridges and the paleodelta are swept relatively clean of 

 fine sediment by tides and waves. In addition, slumping occurs 

 along channel margins, at least within the estuaries (Belknap et 

 al., 1986; Kelley et al . , 1986; 1987b). 



Shipp et al. (in press) have demonstrated the presence of 

 lowstand shorelines on the Maine coast based on four lines of 

 evidence. These are: 1) location of prominent terraces in a zone 

 from 50 to 65 m depth, 2) distinctly thinner Quaternary sediments 

 above -65 m, interpreted as removal during lower sea levels by 

 subaerial and littoral processes, 3) truncation of shelf valleys 

 (lowstand paleovalleys) at -65 m, and 4) the depth of erosional 

 unconformity to conformity transition in shelf basins (60-90 m, 

 lowered sea level plus paleo-wave base) . In submersible 

 traverses across the postulated lowstand position in Machias Bay 

 (NURP-JSL Dive 1985-15: Shipp et al . , in press), we have observed 

 coarser and better sorted materials in a narrow zone centered on 

 a terrace feature at 55 m depth. Similar transitions occur off 

 the Kennebec Paleodelta at the toe of foreset beds (Belknap et 

 al., 1986). We searched for a similar transition in Saco Bay 

 (NURP-JSL Dive 1985-19; Kelley et al, 1987a), but the terrace was 

 draped with a sheet of mud. 



162 



