Abstract

During this study, the twenty-three vibracores taken in 1984 and 1988 were reexamined, original descriptions verified and significantly expanded, and the cores sampled to provide complete grain size data (i.e. the original sediment grain size analyses were limited). The vibracores were grouped by location with respect to major physiographic features (geoforms) or surficial sediment type including Offshore Marine-Modified Glacial Features (Drumlins and Lodgement Till Deposits), Northern Sand Body, Isles of Shoals, Nearshore Marine-Modified Glacial Features (Eskers and Drumlins), Nearshore Sheet Sand, and Offshore Seafloor Plain.

The Northern Sand Body (NSB), located near the Isles of Shoals ~10 km from shore, is relatively large measuring ~3.2km in length and ~1.3km in width, with a maximum relief of ~7m. Earlier studies estimated the NSB may contain as much as 17 million m3 of sand and gravel, but this has not been verified. One of the vibracores taken at the northern end of the NSB has ~3.6m of medium to coarse sand with varying amounts of fine gravel overlying fine sand. Similarly, a vibracore from near the center of the NSB has ~3.1m of slightly granuley medium sand with shell fragments and scattered pebbles overlying fine sands. However, other vibracores taken at the NSB are largely fine to very fine sand of varying thickness. The NSB likely formed from deposits that were originally either a marine glacial delta, a subaqueous delta, or sandy outwash that was heavily modified by marine processes.

A vibracore taken on top of an offshore drumlin-like feature located ~24km from shore has ~4.7m of medium to coarse sand overlying fine sand and silty very fine sand to silt deposits. The upper sands likely represent a lag deposit formed by wave action during the last sea-level lowstand. However, it is not known if this lag deposit continues over the surface of the entire drumlin. Except for the NSB, and potentially the offshore drumlin, the other sand and gravel deposits examined are relatively small in aerial extent. However, several of the marine-modified glacial deposits have approximately three to five meters of sand and gravel. For example, a vibracore taken near an esker-like feature had ~5.75m of very coarse sand to gravelly sediments composing the matrix (the largest clasts were not measured due to limited sample size). The eskers were exposed during the last sea-level lowstand and were modified by shallow water waves and nearshore process during the Holocene transgression. The esker was likely eroded, the large gravel left as a lag deposit, and the finer sediment deposited as nearby shoals. The Nearshore Sheet Sand deposits located within a few kilometers of the coast are relatively thin (less than ~2.5m), flat-lying layers of sand and gravel unconformably overlying glacial marine sandy mud which were likely formed from reworked glacial marine sediment during the last transgression, especially wave-modified marine deltas or outwash. In addition, the deposits are likely part of the nearshore sand ramp extending from the beaches in southern NH.

Department

Center for Coastal and Ocean Mapping

Publication Date

2021

Digital Object Identifier (DOI)

https://dx.doi.org/10.34051/p/2021.26

Grant/Award Number and Agency

Bureau of Ocean Energy Management Award Number M14AC00010 and University of New Hampshire/National Oceanic and Atmospheric Administration Joint Hydrographic Center Award Number NA10NOS4000073

Document Type

Report

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