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Glacioisostatic Influences on Virginia's Late Pleistocene Coastal Plain

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Glacioisostatic Influences on Virginia's Late Pleistocene Coastal Plain Geomorphology 116 (2010) 175–188 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Glacioisostatic influences on Virginia's late Pleistocene coastal plain deposits Timothy W. Scott a,⁎, Donald J.P. Swift a, G. Richard Whittecar a, George A. Brook b a Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA, 23529, USA b Department of Geography, University of Georgia, Athens, GA, 30602, USA article info abstract Article history: The late Pleistocene of Virginia's outer coastal plain consists of sediments dated to marine isotope stages Received 2 February 2009 (MIS) 5 and 3. Two members from the Tabb Formation south of the Chesapeake Bay in southeastern Virginia Received in revised form 18 October 2009 and two formations east of the bay on the southern Delmarva Peninsula were dated using optically Accepted 24 October 2009 stimulated luminescence (OSL). The stratigraphically older Butlers Bluff Member yielded OSL ages of 70 ka Available online 30 October 2009 (62–78 ka) (MIS-5a), and the younger Poquoson Member and Wachapreague Formation, MIS-3 ages of approximately 43 ka (33–50 ka) and 42 ka (33–54 ka), respectively. These shoreface and near-shore geologic Keywords: Mid-Atlantic coastal plain units reached maximum altitudes ranging from 3 to 12 m above present sea level, and were deposited when Glacioisostasy established glacial-eustatic sea-level curves suggest that sea levels were significantly lower than present by Stratigraphy approximately 40 m. If these new ages and the sea-level curves are correct, there must have been regional Pleistocene uplift of more than 40 m, probably due to isostatic adjustments of forebulges peripheral to North American Sea-level change ice sheets when they were at their maxima during MIS-6 and MIS-2. If the late MIS-6 forebulge collapse OSL continued throughout MIS-5 and MIS-4, we propose that regional land elevations may have been low enough for deposition to occur during the lower eustatic sea levels of MIS-3. During late MIS-3, the units experienced renewed uplift followed by subsidence to present-day elevations. If this paraglacial region is not yet in isostatic equilibrium and still requires further forebulge subsidence, this could explain the present-day altitude and age discrepancies associated with these relict marine deposits. © 2009 Elsevier B.V. All rights reserved. 1. Introduction across the paraglacial zone (Davis and Mitrovica, 1996), a region where land surfaces may need to drop tens of meters to reach isostatic Quaternary sea-level changes have been interpreted and docu- equilibrium following the MIS-2 glaciation (Potter and Lambeck, mented by analyzing ice cores, deep ocean sediment and coral terraces 2003). to create eustatic sea-level curves (e.g. Chappell and Shackleton, 1986; This regional disequilibrium results in abnormally high rates of Bloom and Yonekura, 1990; Siddall et al., 2003). These curves, sea-level rise along the U.S. Atlantic coast (e.g. Clark et al., 1978; Davis however, do not accurately reflect past sea levels relative to present and Mitrovica, 1996; Peltier, 1990), particularly in the region of the day for paraglacial regions because of surface deformation caused by Chesapeake Bay, and in subsidence rates up to −3 mm/yr measured at glacioisostatic adjustments (GIA) (e.g. Clark et al., 1978; Peltier, 1987, GPS-instrumented stations in a wide swath across the U.S. (e.g. Park 1990; Mitrovica, 2003; Potter and Lambeck, 2003). Ice sheet growth et al., 2002; Sella et al., 2007). Results of GIA models suggest large and crustal depression displaces mantle material into peripheral portions of the eastern U.S. will subside several tens of meters more regions creating a forebulge, sometimes with many tens of meters of due to forebulge relaxation (Potter and Lambeck, 2003). Potter and uplift (e.g. Hetherington et al., 2004). When ice sheets retreat, Lambeck (2003) note that in order to constrain GIA model inter- forebulge subsidence begins while rebound occurs in the areas pretations, additional data about the height and timing of marine formerly underneath the ice. Water loading of the continental shelves high-stand deposits along the U.S. Atlantic coast are needed. and re-growth of the ice sheets might alter the rate of forebulge In the southeastern U.S., several authors report apparent geomor- collapse dependent upon the shape of the shelf and the response of the phic and stratigraphic anomalies that can be explained by peripheral mantle and crust to renewed weight (e.g. Potter and Lambeck, 2003; forebulge movement (Wehmiller et al., 2004; Reusser et al., 2004; Gehrels et al., 2004). GIA models of the Western North Atlantic region Scott, 2006; Pavich et al., 2006; Parham et al., 2008). Mallinson et al. suggest that elevation changes associated with forebulge growth and (2008) and Burdette and Mallinson (2008) presented new strati- collapse extend for hundreds of kilometers south of the ice margin graphic detail and OSL age estimates for shoreline features deposited in northeastern North Carolina during marine isotope stages (MIS) 5 and 3 and interpreted these data in light of GIA and established ⁎ Corresponding author. Present address: Sciencenter, Ithaca, NY, 14850, USA. Tel.: +1 607 272 0600; fax: +1 607 277 7469. eustatic sea-level curves. In this paper we provide similar analyses E-mail address: [email protected] (T.W. Scott). and a reinterpretation of late Pleistocene marine stratigraphy in a 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.10.017 176 T.W. Scott et al. / Geomorphology 116 (2010) 175–188 region strongly influenced by glacioisostatic effects — the coastal plain MIS-5a and MIS-3 ages have been deemed inaccurate, either by some of eastern Virginia. of the workers themselves or by other colleagues. In an attempt to resolve these discrepancies, we studied the stratigraphically oldest and youngest units from both the Eastern 2. Regional setting Shore and southside and dated these units using the optically stimulated luminescence (OSL) method. By dating the stratigraphically oldest and The deposits found in Virginia along the southern Delmarva youngest units in each area, units stratified between the two are Peninsula (Eastern Shore) and south of the Cheseapeake Bay (south- constrained in time and can be given approximate ages. On the Eastern side) constitute a record of sea-level change over the last 125,000yr Shore, the Butlers Bluff Member (12 mamsl) of the Nassawadox (Fig. 1). The Eastern Shore is characterized as a prograding-spit Formation and the Wachapreague Formation (4.5 mamsl) were complex that buries ancestral river channels created during multiple selected for the study (Fig. 3). Between the two lies the Joynes Neck interstadial and stadial periods. The bay and ocean sides of this Sand (7.9 mamsl). On the southside, we obtained dates from the peninsula contain wave-cut scarps and younger units, which docu- Sedgefield (9 mamsl) and Poquoson (3 mamsl) Members of the Tabb ment corresponding sea-level fluctuations on either side (Fig. 1; Formation; the Lynnhaven Member (5.4 mamsl; Fig. 4) lies positioned Colman and Mixon, 1988; Colman et al., 1990; Oertel and Foyle, 1995; between them. These new ages can be used to test regional correlations Swift et al., 2003; Scott, 2006). Southside Virginia consists of various between the Eastern Shore and southside originally established only by scarps and offshore surfaces buried by coastal ridges also formed their lithostratigraphic similarities and loosely constrained age deter- during stadial and interstadial periods (Fig. 1; Oaks et al., 1974; Peebles minations (Fig. 5; Mixon et al., 1982). et al., 1984; Johnson et al., 1987; Scott, 2006). Previous workers (Table 1) have dated many of these deposits using radiocarbon (peat; wood), uranium-series (coral; molluscs; vertebra) and amino-acid 3. Methods (molluscs) methods. Most of their findings (Fig. 2), however, conflict with established sea-level curves by suggesting that MIS-5a and MIS-3 3.1. Field methods deposition occurred 3 to 12 m above mean sea level (amsl) during times when eustatic sea levels were much lower than today (e.g. Study areas and sampling sites were established using map Wehmiller et al., 2004). Because of these contradictions, many of the information in Mixon et al. (1989) and stratigraphic sections in Johnson Fig. 1. Study area illustrating the various scarps and ridges found on the Eastern Shore and southside Virginia and the ancestral Susquehanna River Channels (oldest to youngest: Exmore to Cape Charles) underlying the Eastern Shore. Inset map illustrates the distance of the study area from the Laurentide ice sheet during the LGM. Location and contours of ice sheet are modified from Peltier (1987) and Andrews (1987). T.W. Scott et al. / Geomorphology 116 (2010) 175–188 177 Table 1 Deposition dates from previous studies. Author(s), date Geologic unit Method Material dated Age (103yr B.P.) Cronin et al. (1981) Sedgefield Member Uranium-series Coral 74±4 Cronin et al. (1981) Sedgefield Member Uranium-series Coral 75±5 Cronin et al. (1981) Sedgefield Member Uranium-series Coral 62±4 Finkelstein and Kearney (1988) Wachapreague Fmtn Radiocarbon Peaty Clays 23.34–33.94 Mixon et al. (1982) Sedgefield Member Uranium-series Astrangia sp. 62±2 Mixon et al. (1982) Sedgefield Member Uranium-series Astrangia sp. 73±4 Mixon et al. (1982) Sedgefield Member Uranium-series Astrangia sp. 79±5 Mixon et al. (1982) Sedgefield Member Uranium-series Quahog 63.5±1.5 Mixon et al. (1982) Sedgefield Member Uranium-series Quahog 51±3 Mixon et al. (1982) Sedgefield Member Uranium-series Quahog >45 Mixon et al. (1982) Sedgefield Member Uranium-series Quahog 124±9a Mixon et al. (1982) Sedgefield Member Uranium-series Oyster 101±9a Mixon et al.
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