Sedimentary Environments and Depositional History of a Paraglacial, Estuarine Embayment and Adjacent Inner Continental Shelf: Portsmouth Harbor, New Hampshire, USA

Kelly Leigh Nifong, University of New Hampshire, Durham

Abstract

Portsmouth Harbor, Hew Hampshire and the adjacent inner continental shelf are typical of paraglacial, bedrock-influenced embayments that characterize large areas of the Western Gulf of Maine. This thesis describes the surficial geology and seismic stratigraphy of Portsmouth Harbor and the adjacent shelf using high-resolution multibeam echosounder (MBES) bathymetry and backscatter, side scan sonar (SSS) backscatter, subbottom seismics, and ground truth data 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 results of this study better define the seafloor characteristics and provides an aid for defining habitats, as well as identifying a possible mineral resource (sand and gravel).

The majority of the seafloor within the inner harbor and harbor mouth is a mixture of sand and gravel. Two sand bodies characterized by large-scale bedforms (sand waves), composed of 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 onto 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 sand resource for future beach nourishment. The inner shelf seafloor is highly variable with the apron of fine sand, sand and gravel mixture dominating the seaward extent, large areas of bedrock outcrops, and a large moraine field (containing ~30 moraines). Another moraine field is located within the harbor mouth (containing 3-5 moraines).Both of the moraine fields are hypothesized to be De Geer Moraines, which are recognized as features of ice front retreat.

The major stratigraphic features characterizing Portsmouth Harbor and the inner shelf include an early Pleistocene paleochannel, a possible estuarine point bar deposit, and an erosional unconformity 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 separated from overlying Holocene sediments by an erosional unconformity. The glaciomarine sediments are likely Pleistocene age muds and sands that make up the Presumpscot Formation (Bloom, 1963) and were deposited during the first marine transgression and the sea-level highstand that occurred 15,000 – 12,500 yr B.P. The erosional unconformity, including a ~3 m v-shaped incision that occurs offshore, is the result of the marine regression, sea-level lowstand, and initial phase of the second marine transgression between 12,500 – 11,500 yr B.P. The overlying Holocene sediments, which at the surface are sands and sandy gravels, were deposited during the second (presently ongoing) marine transgression. The possible estuarine point bar deposit is hypothesized to have been deposited during a period of slower sea-level rise, known as the slowstand, between 11,500 – 8,000 yr B.P.