Geophysical evidence of a late Pleistocene glaciation and paleo-ice stream on the Atlantic Continental Shelf offshore Massachusetts, USA
Highlights
► Seismic interpretations show Pleistocene glaciation 125 km offshore Massachusetts. ► Paleo-ice stream extended from offshore Maine to offshore Massachusetts. ► Glaciation extended farther offshore than the Wisconsin Ice Sheet. ► Significant implications for glacial and hydrogeologic models.
Introduction
Geophysical evidence of Pleistocene ice sheets and ice streams are critical to reconstruct the timing and extent of Pleistocene glaciations (Dyke and Prest, 1987, Anderson et al., 2002, Dyke et al., 2002), to constrain models of Pleistocene ice sheet formation (Denton and Hughes, 2002, Marshall et al., 2002, Boulton and Hagdorn, 2006), and to evaluate the influences of Pleistocene ice sheets on subsurface freshwater distribution (Person et al., 2007, Cohen et al., 2010). The Wisconsin ice sheet has been well documented throughout the northern US and Canadian Atlantic shelf with the use of bathymetric data, sediment cores, and near-surface seismic data (Schlee and Pratt, 1970, Tucholke and Hollister, 1973, Uchupi, et al., 2001, Shaw et al., 2006). Near-surface seismic data, multibeam data, and sediment cores have helped to characterize the distribution of the Wisconsin ice sheet; however, pre-Wisconsinan ice sheet deposits are too deeply buried to be characterized with these shallow-imaging methods (Piper, 1988). In many areas, the evidence of pre-Wisconsinan glaciation has been removed by erosion during the Wisconsinan glaciation (Giosan et al., 2002). Many studies have dated deposits of glacially eroded sediments on the continental slope offshore southeastern Canada to infer the timing of pre-Wisconsinan ice sheets (Alam and Piper, 1977, Piper, 1988, Amos and Miller, 1990, Piper et al., 1994). Although these methods establish the timing of Pleistocene glacial cycles offshore, they do not provide conclusive evidence for the location and extent of ice sheet erosion on the continental shelf.
Offshore Massachusetts, USA, is a prime region to constrain the offshore extent of Pleistocene glaciations as it marks the transition from the repeatedly glaciated Gulf of Maine to the proglacial continental shelf offshore New Jersey, USA. Many studies of shallow, well-preserved glacial features have been used to infer the maximum extent of the Wisconsin ice sheet offshore Massachusetts (Fig. 1) including: (1) major erosion observed in near-surface seismic data (Uchupi, 1966, Oldale et al., 1974); (2) the distribution of gravel in surface sediment samples (Pratt and Schlee, 1969, Schlee and Pratt, 1970); and (3) moraines and glacio-tectonic structures on Martha's Vineyard, MA, USA and Nantucket Island, MA, USA (Oldale and O'Hara, 1984, Uchupi and Mulligan, 2006). These studies did not interpret pre-Wisconsinan ice sheet margins.
In August, 2009, we collected a high-resolution, multi-channel seismic survey offshore Massachusetts as part of an Integrated Ocean Drilling Program (IODP) site survey. Here, we present evidence for the extent of a late Pleistocene glaciation offshore Massachusetts based on our interpretations of the seismic data. The data image a regionally extensive erosion surface that has glacial–geomorphic features consistent with a paleo-ice stream trough. The direction of ice-stream flow was to the south-southwest and indicates that the ice stream originated to the north, near Georges Bank (southeast of the Gulf of Maine). Georges Bank contains similar glacial geomorphic features that we assume are contemporaneous with the glacial features offshore Massachusetts. From this we develop a regional interpretation for a paleo-ice stream that extended from the Gulf of Maine to offshore Massachusetts, and we infer a potential ice sheet margin. The interpreted glacial extent has a geometry similar to the Wisconsin ice sheet. Our interpretation of the seismic data suggests that the late Pleistocene glaciation extended farther south across the Massachusetts continental shelf than previously thought. This has important implications as it can help constrain hydrogeologic models used to predict the emplacement of sub-surface freshwater, and glacial models used to predict the formation of Pleistocene ice sheets.
Section snippets
Continental shelf
Our study region lies on the passive continental margin offshore Massachusetts. The formation of the continental shelf began with rifting of the Atlantic during the late Triassic to early Jurassic. The rifting formed basement rock comprised of a series of normal fault blocks (Hutchinson et al., 1986). As the basement rock subsided, a thick sedimentary wedge accumulated from the Cretaceous through the present. The result was a passive margin with a sedimentary wedge up to 14 km thick (Schlee et
Data and methods
We collected a grid of high-resolution, multi-channel seismic data offshore Massachusetts using the Scripps Institution of Oceanography's portable seismic system. The system employs a 45 in.3/105 in.3, generator–injector (GI) air gun source that produces frequencies up to 200 Hz, and a digital streamer with 48 hydrophone groups spaced at 12.5 m. We collected seven north–south (dip) lines, and 11 east–west (strike) lines (Fig. 2). Seismic processing included outside trace muting, bandpass filtering,
Observations, interpretations, and age estimates
Six stratigraphic units (A, B, C, D, E, and F) and two regional unconformities (U1 and U2) are identified based on their seismic character, amplitude, and bounding surfaces (Fig. 3).
There are no direct well ties to the seismic lines we acquired; thus age estimates rely on correlation of observed stratigraphic architecture offshore Massachusetts with dated stratigraphic architecture interpreted on high-resolution seismic reflection profiles collected offshore New Jersey (Poulsen et al., 1998,
Interpretation of a late Pleistocene glaciation and ice stream
Based on our observations, we suggest the U1 unconformity records an episode of glacial erosion offshore Massachusetts. U1 has characteristics associated with glacial processes including; (1) it is wide-spread; (2) it contains a broad trough with a width-to-depth ratio similar to glacial troughs; (3) it is bounded above by glacigenic sediments (subunit B2); and (4) it contains a network of channels interpreted as sub-glacial drainage channels.
Regional correlation of glacial features
The high-relief, erosional boundary (U1) offshore Massachusetts contains similar stratigraphic features to those observed on the northern side of Georges Bank (Fig. 8) (Uchupi, 1966, Knott and Hoskins, 1968, Uchupi, 1970, Oldale et al., 1974). Oldale et al. (1974) concluded that the erosional surfaces along the northern edge of Georges Bank were the result of glacial erosion in the Pleistocene as an advancing ice sheet reached the topographic high of Georges Bank. Based on the similarity in
Conclusions
We used high-resolution, multi-channel seismic data to characterize a late Pleistocene glaciation that extended 125 km offshore Massachusetts. The age was estimated based on correlations with AMCOR and USGS well data offshore Massachusetts and adjacent seismic data offshore New Jersey. We suggest that the event may be related to oxygen isotope stage 12 when the first extensive Pleistocene glaciation was shown to cross the continental shelf offshore southeastern Canada.
The late Pleistocene
Acknowledgments
This work was funded by NSF-OCE-0824368. Seismic data were collected on the R/V Endeavor (cruise EN465) using Scripps Institution of Oceanography's portable seismic system. We thank the Scripps technicians and the crew of the R/V Endeavor for their assistance in collecting the data. D. Praeg, D. J. W. Piper, and an anonymous reviewer provided insightful comments that improved this manuscript. We also thank J. Anderson for helpful discussions.
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