UNDERSTANDING THE PRESERVATION OF SUBMARINE MORAINES OFF WELLS, MAINE

In 2005, the seafloor off Wells, ME was mapped with interferometric side scan sonar, yielding bathymetry and acoustic images of 102 km2 of the seafloor between 20 and 50 m depth. New Chirp seismic records were also added to more then 500 km of “boomer” seismic data. Fields of moraines clustering from 25-35 m depth, observed earlier by Miller (1988), show remarkable preservation despite having experienced both late Pleistocene emergence through the surf zone, and later submergence. Though some moraines are substantially wave-eroded, with apparent drowned spits and inlets, many show little effect from wave exposure. One hypothesis to explain the minimal erosion is that both falling and rising sea-level change rates were extreme (0.5-1 cm/yr) when these features experienced wave exposure. At estimated erosion rates of 0.1 m/yr, < 50 m of till erosion would occur. Some moraines appear to have experienced even less erosion than this, however. It is possible that eroded material remained nearby to armor the features from subsequent erosion. Another hypothesis, that paleobathymetry shielded the moraines from waves at lower sea levels was investigated with the SWAN numerical model. When sea level was 25 m below present, a large exposure of land existed north of the moraines and shielded them from wave energy during northeast storms, the most powerful in the region. Though the moraines were still exposed to the common southwesterly swells, these smaller waves had impact only on some of the moraines in the northern part of the study area. Thus, rapid sea-level change coupled with more protective paleobathymetry, probably led to minimal moraine erosion.