The accompanying map shows the amount of acoustic penetration achieved in the sediments of the western Gulf of Maine using a 3.5 kHz echo sounding system and illustrates the pronounced control of topography on sediment distribution. Little or no sub-bottom penetration was observed in areas of sand and gravel on the major banks and ledges. Sonic penetration in the silt- and clay-filled basins was to the acoustic basement and in excess of 40 m where the acoustic basement was beyond instrument range. Stratification and unconformities were observed locally in the penetrable sediments. The fine-grained penetrable sediments in the major basins and near the coastline are inferred to originate from two sources: 1) glaciomarine deposits of mostly rock flour deposited during the retreat of the ice from the western Gulf of Maine, and 2) silt and clay winnowed from the banks and ledges and redeposited in the topographic lows during the Holocene lowering and later rise in sea level. The local unconformities, contacts between stratified and non-stratified sediments, and a shallow sub-bottom reflector observed in Cape Cod Bay may have been formed during the Holocene emergence and submergence of the western Gulf of Maine. Changes in sedimentation rates appear to be related to deglaciation, late-glacial submergence, and Holocene sea level changes.
