ABSTRACT

The Black Ledges, a series of islands, shoals, and ledges in East Penobscot Bay, Maine, was mapped with digital sidescan sonar and shallow marine seismic reflection equipmentA total of 38 km2 of sidescan and 600 km of seismic data was collected during four cruises in 2000–2001. The sidescan sonar reveals a surficial geology dominated by muddy sediments with frequent, patchy outcrops of gravel and minor amounts of bedrock. There are seven large concentrations of pockmarks with populations totaling over 3500 in the areas of muddy sediments. Generally circular, pockmarks range in size from five to 75 meters in diameter and up to eight meters deep. Calculations show over 2 × 106 m3 of muddy sediment and pore water were removed from the system during pockmark formation. Seismic data reveal a simple stratigraphy of modern mud overlying late Pleistocene glaciomarine sediment, till and Paleozoic bedrock. Seismic data indicate areas of gas-rich sediments and gas-enhanced reflectors in close association with pockmarks, suggesting methane seepage as a cause of pockmark formation. Pockmarks are alsorecognized in areas lacking evidence of subsurface methane accumulations adding further validity to the late stage of development for the field. Elliptical pockmarks, found in nearly 40 m of water, show modification by currents and degradation of the pockmark form. This suggests depletion of methane and a late stage of development. A more intensive investigation of the area, including coring and high-resolution geophysics is currently in progress.

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