SEDIMENTOLOGIC AND STRATIGRAPHIC STUDIES ON THE NEW HAMPSHIRE INNER SHELF

The inner shelf of New Hampshire is highly complex and is composed of outcropping bedrock, tills, glacial marine sediments, sand and gravel deposits, and muddier sediments further offshore. In addition, there are frequent remnants of glacial features such as eroded drumlins. Based on a synthesis of over 300 bottom samples and several seismic surveys, a surficial sediment map was developed for the area from the shoreline to the 100-meter depth contour. Although the scale of this map (1:100,000) does not allow detailed assessments of bottom environments, the general distribution of sediment types and bedrock outcrops is depicted. The northern New Hampshire inner shelf tends to be dominated by a rocky and gravelly bottom, although a tongue shaped sand body intrudes into Portsmouth Harbor. Bedrock and gravel are still common to the south; however, sand deposits become more extensive towards the Merrimack River and its associated subtidal delta. In order to describe the sedimentologic and shallow stratigraphic characteristics of these sandier, nearshore deposits, a high-resolution geophysical survey was conducted along a 15 km section of the coast north of the Merrimack River. Over 150 km of side scan sonar and subbottom seismics were obtained along thirty-four shore-normal transects (4 to 6 km in length). Within this study area bedrock outcrops and gravel deposits occur. However, the most distinctive features are several large, nearshore sand bodies exceeding 6 to 8 meters in thickness, extending less than 2 km offshore, and 1 to 2 km alongshore. The size and shape of these features indicate they are remnant ebb tidal delta deposits. A high-resolution seismic survey was also conducted further offshore (10 to 15 km) adjacent to a previously described large Holocene sand deposit near the Isles of Shoals. Approximately 80 km of seismic lines were run in an ~15 km2 area in order to describe the complex relationship between the shallow bedrock and overlying sediments. Although muddy sand dominates the seafloor in this area, bedrock outcrops and gravel are common. Here, the antecedent topography (primarily bedrock) has a major influence on the shallow stratigraphy and the morphology of the seafloor.