DEVELOPMENT OF A LATE QUATERNARY DEPOSITIONAL MODEL OF PORTSMOUTH HARBOR AND ADJACENT SHELF, NEW HAMPSHIRE

The sedimentological and stratigraphic characteristics of Portsmouth Harbor, New Hampshire and adjacent inner continental shelf were described using high resolution multibeam echosounder (MBES) bathymetry and backscatter, side scan sonar (SSS), subbottom seismics, videography, and bottom sediment samples in order to develop a depositional model and assess the late Quaternary geologic history. Several major depositional environments were identified within Portsmouth Harbor including coarse channel lags, sand wave fields, and extensive bedrock outcrops that strongly influence the channel configuration. The inner harbor shows mainly high intensity backscatter, with the sand wave fields identified by less intense backscatter than the surrounding channel lag deposits. At the mouth of the harbor, an apron of low intensity backscatter, a fine sand, extends offshore onto the inner shelf. The inner shelf shows mixed high and low intensity backscatter reflecting bedrock outcrops interspersed with fine to coarse sand and gravel fields. The seismic characteristics of the major depositional environments of Portsmouth Harbor and inner shelf were characterized based on reflector intensity, internal structure, and external shape. The subbottom seismics associated with the sand wave fields were highly reflective with no internal structure, and the sand waves clearly visible. The main river channel deposits were a mix of stratified glaciomarine sediments filling carved channels, mounded stratified units, and beds sloping towards the river mouth. Offshore, paleochannels apparently associated with the extension of the Piscataqua River during the sea-level lowstand, are filled by glaciomarine sediments, the largest containing several stratified units, both glaciomarine and recent sediments. Determination of the surficial geology based on the high resolution bathymetry, SSS and direct sampling, coupled with an assessment of the underlying seismic stratigraphy enhances the development of a 3D depositional model.