SUB-BOTTOM SONAR STRATIGRAPHY AND QUATERNARY GEOLOGIC HISTORY OF MIDDLE GROUND, SALEM SOUND, MASSACHUSETTS

The purpose of this study is to characterize the sub-bottom stratigraphy and the late Quaternary depositional history of a portion of Salem Sound, MA. The study area is Middle Ground, a bathymetric high with a relief of ~5 m from the sea floor, an area of 490 ha, and a tidal range of ~3 m. This region experienced glaciation in the late Pleistocene, and was deglaciated between 17.2 and 15.8 cal ka BP (Ridge, 2008). Georeferenced sub-bottom data were obtained with a Stratabox 3510 along 15 lines; nine lines running N-S spaced ~175 m, and six running E-W spaced ~300 m. Data were post processed with Sonarwiz 5 and reflectors were identified. These data illustrate the sediment/water interface, sub bottom reflectors within the sediment, and the underlying bedrock. Shallow regions of Middle Ground tend to be dominated by bedrock outcrops with minimal sediment accumulation, and bedrock troughs, which are infilled with up to 7 m of sediment. At least three distinct seismic facies were identified within the sub-bottom record. At the base of the record is Unit 1, characterized by a strong reflector without further penetration, and interpreted as bedrock. Unit 2 is characterized by numerous internal reflectors and is up to 20 m thick on the margins of the field area. This unit is found to be draping over most of the region, and the upper portion of the unit has been eroded, as interpreted from the truncation of internal reflectors at the upper contact. Unit 3 is an acoustically transparent unit, which is ~2 m thick and generally not present in shallow areas. The sub-bottom record supports an interpretation of a glaciomarine environment immediately after deglaciation in the late Pleistocene, resulting in the deposition of Unit 2 over bedrock. Subsequent isostatic rebound and relative marine regression ceased the deposition of this unit and resulted in an erosional unconformity. Marine transgression in the middle-late Holocene resulted in the deposition of the acoustically transparent estuarine unit that unconformably overlies Pleistocene-aged glaciomarine sediments. This interpretation is consistent with the current state of knowledge regarding the geologic history of northeastern Massachusetts.