Sedimentary Environments and Depositional History of a Paraglacial, Estuarine Embayment and Adjacent Inner Continental Shelf: Portsmouth Harbor, New Hampshire,
|Title||Sedimentary Environments and Depositional History of a Paraglacial, Estuarine Embayment and Adjacent Inner Continental Shelf: Portsmouth Harbor, New Hampshire,|
|Degree and Program||Master of Science|
|Degree||Earth Sciences/Ocean Mapping|
|Number of Pages||188|
|University||University of New Hampshire|
|Keywords||Earth sciences; Depositional history; Estuarine; Paraglacial; Portsmouth harbor; Sedimentary environments|
Portsmouth Harbor, New Hampshire and the adjacent inner continental shelf are typical of paraglacial, bedrock influenced embayments that characterize large areas of the Western Gulf of Maine. The surficial geology and seismic stratigraphy were described using high-resolution multibeam echosounder (MBES) bathymetry and backscatter, side scan sonar (SSS) backscatter, subbottom seismics, and ground truth in order to develop high-resolution seafloor maps and better understand the influence of Quaternary glaciations, sea-level fluctuations, and marine processes on the sedimentary environments.
The majority of the seafloor within the inner harbor and harbor mouth is coarse sand to fine gravel. Two sand bodies characterized by bedforms composed of a medium to coarse sand, are located north and south of a channel bend within the inner harbor. An apron of fine sand that begins in the harbor mouth expands into the inner shelf and connects to another region of fine sand at the southwestern corner of the study area. This apron of fine sand corresponds to a mounded seismic unit that may represent a mineral resource of sand for future beach nourishment. A small moraine field is located within the harbor mouth (containing 3-5 features). The seafloor of the inner shelf is highly variable with the apron of fine sand, coarse sand to fine gravel dominating the seaward extent, large bedrock outcrop areas, and a large moraine field (containing ~30 features). Both of the moraine fields are hypothesized to be De Greer Moraines.
The major stratigraphic features characterizing Portsmouth Harbor and the inner shelf include an early Pleistocene paleochannel, a possible point bar deposit, an erosional unconformity (Un) at an average depth of ~25 m below present sea level. The channel fill sequence for the entire study area can be generalized as glaciomarine sediments (Gm) separated from overlying Holocene sediments (Hc) by an erosional unconformity (Un).
The channel fill sequence at the channel bend contains a hypothesized point bar deposit overlying the erosional unconformity (Un). The paleochannel fill sequence of the inner shelf shows the only evidence of possible fluvial erosion of the glaciomarine sediment by the Piscataqua River during the sea-level lowstand.