THE SURFICIAL SEDIMENTS OF COBSCOOK BAY, MAINE: A ROCK-FRAMED, GLACIATED ESTUARY THAT DEPARTS FROM STANDARD MODELS

Cobscook Bay, ME, the easternmost estuary in the US, is the most important aquaculture and scallop fishing site in Maine. It appears typical of many rock-framed, formerly glaciated estuaries in northeast North America, but departs from traditional models of estuarine facies distribution. To characterize this macrotidal bay, earlier intertidal habitat maps and numerical circulation models were augmented by more than 100 km of seismic reflection and side scan sonar records.

Models of estuarine sediment and facies distribution generally depict a gradational landward decrease in wave energy and increase in tidal/riverine influence within a rectangular box model. Surficial sediment is presumed to become finer grained away from the sea, with coarser sediment and migrating bedforms at the estuary mouth.

Lacking significant quantities of sandy sediment, the outer area of Cobscook Bay is erosional, with truncated reflectors of glacial-marine sediment and bedrock cropping out extensively. The bedrock structure, a plunging anticline, provides numerous constrictions to water movement, generating modeled current velocities of 2m/sec and leaving the seafloor bare rock in many places. Near the entrance to the largest constriction, large gyres occur as tides change, and a thick deposit of Holocene mud has accumulated beneath them. Blocks of this mud appear to slump into the channel thalweg as they become oversteepened. Bedrock crops out in random locations throughout the bay, causing many scour holes, abrupt facies shifts and creating localized concentrations of carbonate sediment. Most of the remainder of the subtidal region is a gravel/sand lag deposit resting over glacial-marine mud. Tidal current lineations are the dominant sedimentary structures, with fishing drag marks also common. Fine sediment is most common on tidal flats, which abruptly transition to the coarse-grained subtidal areas. Lack of significant river contributions and sandy sediment, large tidal range and complex bedrock structure cause sediment distribution patterns in Cobscook Bay to differ from that of standard models.