Geomorphology 116 (2010) 175–188

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Geomorphology

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Glacioisostatic influences on 's late 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 in southeastern Virginia Received in revised form 18 October 2009 and two formations east of the bay on the southern 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 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. (1982) Sedgefield Member Uranium-series Vertebra 78.5±3.5 Mixon et al. (1982) Wachapreague Fmtn Uranium-series Quahog 128±1a Mixon (1985) Wachapreague Fmtn Radiocarbon M. lateralis >33 a Mixon (1985) Kent Island Fmtn Radiocarbon Peat >40 Mirecki et al. (1995) Sedgefield Member Uranium-series Coral 71±5 Mirecki et al. (1995) Sedgefield Member Uranium-series Coral 79±5 Mirecki et al. (1995) Sedgefield Member Uranium-series Coral 69±4 Mirecki et al. (1995) Sedgefield Member Uranium-series Coral 67±4 Mirecki et al. (1995) Sedgefield Member Amino-acid Mercenaria 93 Mirecki et al. (1995) Sedgefield Member Amino-acid Mercenaria 101 Mirecki et al. (1995) Sedgefield Member Amino-acid Mercenaria 125 Mirecki et al. (1995) Sedgefield Member Amino-acid Mercenaria 97 Mirecki et al. (1995) Sedgefield Member Amino-acid Mercenaria 86 Oaks and Coch (1963) Poquoson Member Radiocarbon Wood in peat >40 Oaks and Coch (1963) Sedgefield Member Radiocarbon Driftwood >40 Oaks et al. (1974) Sedgefield Member Uranium-series Astrangia sp. 62–86 Owens and Denny (1979) Kent Island Fmtn Radiocarbon Peat 30±1 Pavich et al. (2006) Kent Island Fmtn OSL Sand 37±6.1 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 73.9±.8 Wehmiller et al. (2004) Sedgefield Member Uranium-series Septastrea 72.6±1.3 Wehmiller et al. (2004) Sedgefield Member Uranium-series Septastrea 75.6±6.8 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 63.9±.7 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 65.6±.8 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 67±.7 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 69.2±.7 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 72 ±1.5 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 69.6±.9 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 72.7±1.3 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 73.6±1.2 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 75.2±1.3 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 74.4±1.1 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 62.5±1.2 Wehmiller et al. (2004) Sedgefield Member Uranium-series Astrangia sp. 63.8±1.2

Units from Oaks and Coch (1963), Oaks et al. (1974), Cronin et al. (1981), Mixon et al. (1982) and Szabo (1985) have been reassigned to the Tabb Formation. a Ages reported with concern from authors as explained in Section 4.2.

et al. (1987) and Mixon (1985). Exposed stratigraphy was compared to 30 cm from a stratigraphic boundary (Aitken, 1998). Post-depositional sections previously published; in areas where stratigraphy was not weathering can affect the luminescence signal by the breakdown of exposed (Wachapreague, Poquoson) a hand auger was used to sample minerals in the sediment followed by the translocation of smaller the sediment in order to see if the area was suitable for OSL analysis prior fragments (Rosholt, 1982). Over time, this can significantly alter to excavation. Ground penetrating radar (GPR) was used in these two radionuclide concentrations near the site of the OSL sample. The dose areas to extend stratigraphic interpretations laterally from exposures rate of sediment sampled close to a stratigraphic break is difficult to assess and borehole sites. For the Wachapreague Formation, Swift et al. (2003) accurately because sediment within 30 cm of the sample site may be very provided a west-to-east 145 m GPR transect in Oyster, Virginia. A new different above the stratigraphic boundary than below it (Aitken, 1998). GPR transect was obtained for the Poquoson Member in Munden Point Before taking samples for OSL analysis, we removed 20 cm of Park, Virginia Beach, Virginia. The GPR survey was conducted along a sediment from the face of the exposure prior to inserting two 197 m west-to-east track across what appeared to be a natural drop in schedule 80 PVC pipes 6 in. in length with a threaded cap sealed with elevation of 1.86 m. A pulseEKKO IV was used to take readings every Teflon tape on one end. These pipe sections were hammered into the 0.5 meters using a radar wave propagation velocity of 0.1 m/ns. The sediment at an angle approximately +45° to the horizontal using a antenna frequency was set to 100 MHz and the paddles were kept one 5 lb sledgehammer. The pipe was then dug out of the sediment and, meter apart. A small pit was then excavated on each GPR transect to once exposed, the distal end was covered with black opaque plastic expose the stratigraphy mapped. secured with UV-resistant duct tape. This was a fast and effective OSL sample locations within each unit were selected based on sampling method that minimized exposure of sediments in the ends proximity to sequence boundaries, amount of staining and weathering, of the pipes to the sun. Care was taken to ensure that the pipes were and sediment type and size. An ideal sample location consisted of fine-to- completely full of sediment and, if necessary, sediment from the coarse sand with minimal staining and weathering and at least sampling site was added to the end of the pipe. This is necessary to 178 T.W. Scott et al. / Geomorphology 116 (2010) 175–188

3.2. Laboratory procedures

Sediment samples were prepared and handled for OSL dating in the laboratory under controlled red-light conditions. About 5 cm of sediment was removed from each end of the sample tubes for dose rate estimation. Luminescence measurements were made on the central section of the sediment cylinder that was least likely to have been exposed to sunlight during sampling. Sediments for OSL analysis were washed with water and then treated

with 10% HCl and 30% H2O2 to remove carbonates and organic material. Sieving isolated the 120–150 μm-size fraction and then density separation using Na-polytungstate (ρ=2.58 g/cm3) was used to separate quartz from feldspar minerals. The quartz fraction was etched with 48% HF for 80 min followed by 36% HCl for 40 min to remove the alpha skin. Quartz grains were mounted on stainless steel discs with the help of Silkospray™. Light stimulation of quartz mineral extracts was undertaken with a RISØ array of combined blue LEDs centered at 470 nm. Detection optics comprised two Hoya 2.5 mm thick U340 filters and a 3 mm thick Schott GG420 filter coupled to an EMI 9635 QA photomultiplier tube. Measurements were taken with a RISØ TL-DA-15 reader. A 25-mCi 90Sr/90Y built-in source was used for sample irradiation. A thick source Daybreak alpha counting system was used to estimate U and Th for dose rate calculation. Potassium was measured by ICP90, with Fig. 2. Graphic illustrating the relationship among the δ18O SPECMAP composite, sea- a detection limit of 0.01%, using the sodium peroxide fusion technique at level curves from the Huon Peninsula in Papua New Guinea, and range of deposition the SGS Laboratory in Toronto, Canada. Water content was assumed to dates from previous workers detailed in Table 1. be 10±5% for all samples and the cosmic-ray dose was estimated at 150 μGy/a as recommended for sediments located below an altitude of prevent movement of sand grains during transportation that could 1000 m between latitudes 0 and 40° (Prescott and Hutton, 1994). result in contamination of the central section of the sediment cylinder The SAR protocol (Murray and Wintle, 2000)wasusedto that we used for dating. determine paleodose. From 14 to 15, 4.0 mm diameter aliquots from

Fig. 3. Generalized geology of Virginia's Eastern Shore modified from Mixon et al. (1989). Areas studied were in Oyster (Qw) and Kiptopeke (Qnb). T.W. Scott et al. / Geomorphology 116 (2010) 175–188 179

Fig. 4. Generalized geology of Virginia's southside modified from Mixon et al. (1989). Areas studied were at Crittenden Pit (Qts) and Munden Point Park (Qtp).

each sample were analyzed. A five-point regenerative dose strategy et al., 2006). However, a major danger in using minimum ages is that was adopted with three dose points to bracket the paleodose, a fourth they can represent a period of sediment bioturbation substantially zero dose to test for recuperation effects, and a fifth repeat dose, more recent than the period of sediment deposition. The absence of usually of the smallest regenerative dose. The OSL response to the pronounced bimodality in our aliquot age distributions, the lack of repeat dose was measured to monitor whether the sensitivity change prominent tails either old or young, and the possibility of some correction incorporated in the SAR protocol was successful. All (and possibly significant) bioturbation in the study area, suggest measurements were made at 125 °C for 100 s after a pre-heat to that weighted mean OSL ages are the best estimates of the time of 220 °C for 60 s. For all aliquots the recycling ratio between the first deposition and burial of Virginia Coastal Plain sediments. and fifth point ranged within 0.95–1.05. Data were analyzed using the The distributions of the measured equivalent doses (De) for each ANALYST program of Duller (1999). sample indicate that the aliquots we examined were not saturated nor mixed significantly. Each sample did display a minor tail of older 4. Results aliquots, suggesting some unbleached grains but this was minimal (Fig. 6). Furthermore, within the range of uncertainties the duplicate 4.1. Age data samples from each site gave statistically similar ages and measured K contents (% by weight) were comparable. U and Th activities varied Age data for the eight samples analyzed, ranging from 33.4±4.3 ka slightly between the sample pairs with higher values accompanied by to 69.7±8.4 ka, are presented in Table 2. OSL dating relies on the higher paleodose estimates, thus resulting in similar age determina- removal of all luminescence from the sediment being dated by ex- tions. These differences show that dose rates vary within the sediment posure to sunlight prior to its deposition and burial (see Aitken, 1985, units, probably depending on heavy mineral and clay content, and 1998). It has been argued that some river-transported and coastal these variations influence final paleodose values but not age estimates. sediments contain a proportion of unbleached grains (e.g. Roberts However, the two samples from the Sedgefield Member had ex- et al., 1998, 1999; Wallinga et al., 2001; Wallinga, 2002; Olley et al., ceptionally high U and Th (U=4.0±0.9 and 5.2±0.8 ppm; Th=7.1± 2004), in which case minimum OSL ages are a more appropriate es- 3.4 and 8.2±2.6 ppm) and slightly higher K contents (0.98 and 0.89%) timate of true sediment age (see also Srivastava et al., 2006; Brook compared to the other samples, thus giving some cause for concern. In

Fig. 5. Correlation among late Pleistocene stratigraphic units located on the Eastern Shore and southside Virginia modified from Mixon et al. (1982) and Mixon (1985). 180 T.W. Scott et al. / Geomorphology 116 (2010) 175–188

Table 2 counting elemental concentrations to dose rates assumes that the 238U OSL results from samples obtained from the Butlers Bluff Member (Qnb), Wachaprea- and 232Th decay chains have been in equilibrium since sediment burial. gue Formation (Qw), Sedgefield Member (Qts) and Poquoson Member (Qtp). In fact, disequilibrium in the 232Th chain is rare in naturally occurring Sample Method U (ppm) Th (ppm) K % Dose rate Paleodose Age (ka) sediments because of the short half-lives of the 232Th daughters, but it is (Gy/ka) (Gy) relatively common for the 238Uchain(Olley et al., 1996). Qnb-1 SAR 0.3±0.1 1.0±0.2 0.48 0.7±0.1 48.5±3.0 69.3±7.6 Disequilibrium can be induced by addition or subtraction of U and Qnb-2 SAR 0.3±0.2 0.4±0.0 0.48 0.6±0.1 43.7±2.3 69.7±8.4 Th after initial sediment accumulation. In the case of the Sedgefield Qw-1 SAR 1.1±0.2 1.6±0.7 0.54 1.0±0.1 45.1±4.7 46.9±6.9 Member, addition of U and Th is more likely given the high con- Qw-2 SAR 1.5±0.3 2.7±0.9 0.53 1.1±0.1 43.2±3.9 38.9±5.5 Qts-1 SAR 4.0±0.9 7.1±3.4 0.98 2.2±0.3 77.6±5.5 35.6±5.8 centrations of these elements. If addition occurred thousands of years Qts-2 SAR 5.2±0.8 8.2±2.6 0.89 2.6±0.3 85.6±4.6 33.4±4.3 after sediment deposition, or if it was intermittent, dose rate esti- Qtp-1 SAR 1.7±0.3 2.2±0.9 0.80 1.4±0.1 53.6±6.9 39.6±6.6 mates based on present U, Th, and K will greatly overestimate the Qtp-2 SAR 0.9±0.2 1.7±0.7 0.72 1.1±0.1 48.1±3.9 44.4±5.2 average annual dose that the sample has been subjected to since deposition. The result would be the estimation of a young age for the sample. addition, they gave the youngest ages of all the samples despite being The Sedgefield sediments probably had high U and Th at the time older stratigraphically than the Poquoson Member and Wachapreague of deposition, but clay laminae above the sampling sites and Fe Formation. Furthermore, the ages are much younger than previous ages staining attest to post-depositional changes in clay and iron after for the Sedgefield Member (see Table 1 and Mallinson et al., 2008). As a sediment deposition with the likely addition of Th and possibly also result, additional sediment was collected from the Sedgefield Member unsupported U. The most likely explanation of the young ages for the at two sites around the perimeter of Crittendon Pit to assess variations MIS-5 Sedgefield Member is that the sediments at the sample site in U and Th content. Measurements showed U and Th at one site to be have not been in equilibrium since their deposition due to post- 2.5±0.4 and 3.6±1.3 ppm and at the other 2.5±0.5 and 6.1±1.6 ppm, depositional, differential migration of 234U in solution, in colloids or respectively; values still significantly higher than at any of the other with humic and fulvic acids. 234U and Th also migrate within the sites examined. The considerable spatial variations in U and Th at the sediment profile attached to migrating clays (Osmond and Cowart, Crittenden Pit, and high U/Th ratios, suggest disequilibrium in the U and 1982; Rosholt, 1982; Szabo and Rosholt, 1982; Short et al., 1988; Olley possibly also the Th decay chains (Table 2). They also indicate that et al., 1996; Prescott and Robertson, 1997). In any sediment horizon, a estimation of annual dose rates may be difficult because of extreme decrease in 234U and/or an increase in Th would reduce the apparent spatial variability in U and Th isotope concentrations. Disequilibrium age of the sediment. In our view the very high U and Th values make affects age calculations because conversion of thick source alpha the ages for the Sedgefield Member shown in Table 2 extremely

Fig. 6. Histograms of equivalent dose (Grays) for one sample from each of the sediment units studied. Histograms show frequencies between 32 and 56 Grays for three samples and between 60 and 90 Grays for the Sedgefield sample. T.W. Scott et al. / Geomorphology 116 (2010) 175–188 181 questionable and for this reason we have not used them in developing the estimate unreliable based on the shell material being too old to be the sedimentation history for the area under investigation. The other accurately dated by the radiocarbon method. Mixon (1985) obtained samples listed in Table 2 were taken from locations with none of these an age of 128±1 ka with an amino acid analysis of a shell fragment problems and so their ages are considered reliable. In addition, the washed from a Bell Neck borehole but emphasized that the reliability ages we obtained are in reasonably good agreement with previous of this age could not be evaluated. The stratigraphic equivalent of the ages obtained for the sediment units using other methods of dating Wachapreague Formation on the bay side of the peninsula, the Kent (Table 1). This said, we cannot totally rule out that the high U and Th Island Formation, has been dated to 30±1 ka (radiocarbon; Owens levels in the Sedgefield Member at the Crittendon Pit (Fig. 4) are due and Denny, 1979), >40 ka (radiocarbon; Mixon, 1985) and to 37±6.1 to some other cause, either human or natural, that greatly increased (OSL; Pavich et al., 2006). The youngest and probably most the level of radioactive isotopes at this site but not at the other sites controversial ages associated with the Wachapreague Formation we investigated. (23.34–33.94 ka; Finkelstein and Kearney, 1988) were obtained from peat beds contained within the lagoonal muds of Mockhorn Island, a 4.2. Geomorphology stratigraphically correlated deposit that is most likely a regressive strandplain. With the exception of the unreliable age of 128±1 ka 4.2.1. Butlers Bluff Member reported by Mixon (1985), other ages reported from, or correlated to, The Butlers Bluff Member of the Nassawadox Formation has a this unit suggest deposition during MIS-3. maximum thickness of 13 m and reaches 12 m amsl. Stratigraphic Sediment for OSL dating was obtained from the Wachapreague analysis of the unit in the region just north of Kiptopeke State Park, VA, Formation at a pit excavated along the GPR track line of Swift et al. (Fig. 3) shows numerous well-to-moderately sorted, fine-to-coarse (2003). The excavation at the Oyster, VA site (3.5 m amsl) went to a cross-bedded beds with apparent heavy mineral laminae and gravels. depth of 1.65 m where the water table was reached. Samples were The lower part of the member not exposed in Kiptopeke consists of a taken at a depth of 1.6 m (1.9 m amsl) in quartz sand mainly of 1.5ϕ to diverse assemblage of marine fauna consisting of Marginella apicina, 0.5ϕ diameter. Few heavy minerals were present and there was no Mulinia lateralis, Nassarius trivittatus, Gemma gemma, Pleuromeris apparent sedimentary structure. The two samples yielded OSL ages of tridentate, Busycon sp. and Olivella sp. (Mixon et al., 1989). These 46.9±6.9 ka and 38.9±5.5 ka, which places deposition sometime in in situ marine fauna, particularly G. gemma and Busycon sp., indicate the period 54–33 ka or during MIS-3 (Table 2). shallow (less than 10 m) near-shore depositional environments (Rehder, 1981; Mixon, 1985). The Butlers Bluff is a southward-building 4.2.3. Sedgefield Member complex of spit-platform sands and very shallow shoals deposited South of Chesapeake Bay, the Sedgefield Member (Fig. 4) of the during high-stand events throughout MIS-5 (Mixon, 1985; Colman and Tabb Formation rises to approximately 9 m amsl and has a maximum Mixon, 1988; Colman et al., 1990; Foyle and Oertel, 1992; Oertel and thickness of 15 m (Johnson et al., 1987; Mixon et al., 1989). The Foyle, 1995; Foyle and Oertel, 1997; Scott, 2006). MIS-5 stadial periods lithology of this unit varies throughout its range. Its easternmost interrupted spit growth as indicated by the various paleochannels filled deposits are described as barrier beach deposits with source sands and buried by the member. As far as we are aware, there are no from the continental shelf while its westernmost deposits near the published ages for this unit. Suffolk Scarp (Fig. 4) are described as protected bay beach deposits Butlers Bluff sediment for OSL analysis was taken at 3.25 m amsl (Peebles et al., 1984; Johnson et al., 1987; Darby and Evans, 1992). within a bed ranging from 2.5 to 3.6 m amsl. This bed consisted of sub- Generally, this unit consists of sediment ranging from clays to boulders parallel laminae at a 2° southeast dip with moderately sorted 1.5–1ϕ with molluscs such as Mercenaria mercenaria, Anadara transversa, (3–0.5ϕ) gray sand (5y 7/1) and <2% feldspar and 1–3% heavy Rangia cuneata, Eupleura caudate, Olivella mutica, Crassostrea virginica, minerals (Fig. 7). Overall elevation of the Butlers Bluff in this region is Busycon sp. and Ensis directus (Peebles et al., 1984; Johnson et al., 1987; 10 m amsl. The Butlers Bluff samples yielded ages of 69.3±7.6 ka and Wehmiller et al., 2004). These in situ marine fauna all suggest a 69.7±8.4 ka indicating an age in the range 78–62 ka based on shallow near-shore depositional environment ranging from the uncertainties (Table 2). This most likely places the end of spit intertidal zone to up to 15 m in depth (Rehder, 1981). Peat and in progradation to the regression from MIS-5a to MIS-4. If progradation situ tree stumps are also found within the unit. Due to the abundant began from the Ames Ridge (Fig. 3) at the beginning of MIS-5e and biota, numerous amino acid and U-series ages have been obtained was interrupted by stadial periods MIS-5d and MIS-5b, then spit ranging from >45 to 124±9 ka (Table 1). Mixon et al. (1982) obtained growth averaged 1.76 m/yr during the approximately 30 kyr of the oldest age of 124±9 ka but disregarded it because it was obtained accumulated interstadial time during MIS-5. from a highly leached quahog specimen. The majority of Sedgefield ages, which average 76 ka, were obtained from coral (Astrangia sp., 4.2.2. Wachapreague Formation Septastrea sp.) and molluscs taken from the now closed Gomez Pit To the east of the Butlers Bluff Member and the Mappsburg Scarp (GP; Fig. 4) in Virginia Beach, VA (Szabo, 1985; Mirecki et al., 1995; lies the Wachapreague Formation (Fig. 3) with a maximum thickness Wehmiller et al., 2004). Stratigraphic sections for the Sedgefield of 12 m reaching up to 4.5 m amsl. Its coarsening-upward sequence Member within the GP area, including information from the Womack consists of interbedded clay-silt and silty sand overlain by medium to and New Light pits (Oaks and Coch, 1963; Cronin et al., 1981; Mixon coarse gravelly sand (Mixon et al., 1989). The in situ marine molluscs et al., 1982; Johnson et al., 1987) show two separate sequence Mesodesma arctatum and Siliqua costata indicate shallow near-shore boundaries within aminozone IIa (c. 80–130 ka; Mirecki et al., 1995). depositional environments of no greater than 12 m in depth (Rehder, These boundaries may mark stadial periods MIS-5d and MIS-5b and 1981; Mixon, 1985). Cooler-than-present climates existed during correlate to the Belle Haven and Eastville paleochannels buried by the deposition of this formation; these molluscs and the ostracod progradation of the Butlers Bluff Member, a unit also assigned to assemblages (predominantly Elfosonella concinna and Muellerina aminozone IIa (Fig. 1) on the Eastern Shore (Mirecki et al., 1995). This canadensis) indicate cooling ocean temperatures while the pollen would suggest that both the Sedgefield Member and Butlers Bluff (mostly pine, spruce, birch, and alder) suggest cool to cold-temperate Member were deposited throughout MIS-5. conditions on adjacent land (Mixon, 1985). GPR (Fig. 8)and Stratigraphic analysis (Fig. 10) of exposures within Crittenden Pit stratigraphic (Fig. 9) analyses at Oyster, VA (Fig. 3), suggest deposition in Suffolk, VA (Fig. 4), reveals a meter-thick clayey, heavily Fe-stained during a sea-level transgression. The stratigraphy further east on the paleosol with mottling, root casts and horizontal bedding overlying Bell Neck strandplain records a later regression (Mixon, 1985; Swift the Yorktown Formation. The base of the paleosol is 3.05 m et al., 2003). Mixon et al. (1982) dated this unit to >33 ka, but deemed below the surface (2.45 m amsl). Above the paleosol is a basal lag with 182 T.W. Scott et al. / Geomorphology 116 (2010) 175–188

Fig. 7. Stratigraphy of the Butlers Bluff Member of the Nassawadox Formation as mapped in Kiptopeke, Virginia. Samples (Qnb-1, Qnb-2) were taken at approximately 3.25 m amsl. Total land elevation is approximately 10 m amsl. pebbles ranging in size from 5 mm to 20 cm followed by a 1.72 m- collected from the Sedgefield between the soil laminae at the surface thick unit consisting of clayey (4.0ϕ–3.5ϕ) grains coarsening upward and the blue staining in moderately Fe-stained well-sorted quartz to well-sorted quartz sand (2.5ϕ–1.5ϕ) capped with thin soil laminae sand (2.5ϕ–1.5ϕ; Fig. 10). The two samples yielded OSL ages of 35.6± near the surface. The lower 1.37 m of this unit is heavily stained blue, 5.8 ka and 33.4±4.3 ka (Table 2). As discussed above, these ages are suggesting a former anoxic environment. Sediment samples were younger than our ages for the stratigraphically younger Poquoson T.W. Scott et al. / Geomorphology 116 (2010) 175–188 183

Fig. 8. Interpreted GPR data from Oyster, Virginia, modified from Swift et al. (2003) illustrating the relationship among the Wachapreague Formation, Joynes Neck Sand and Butlers Bluff Members. GPR was not adjusted for elevation changes.

Member (Table 2; Fig. 4) and because of elevated U and Th values and 1989). Oaks and Coch (1963) report a radiocarbon age from wood in possible disequilibrium in the U and Th decay chains we consider peat of >40 ka and Finkelstein and Kearney (1988) suggest a them very questionable. As previous studies have reported ages geomorphic similarity with Mockhorn Island (Fig. 3), which they suggesting deposition during MIS-5, we assume the Sedgefield determined to be 23 to 33 ka in age. Based on elevations of the Member is of this general age and so equivalent to the Butlers Bluff Poquoson, Johnson et al. (1987) argue the late MIS-3 assignment and Member (Table 1). suggest that this unit was deposited pre-Wisconsinan (>50 ka) as a series of regressive beach ridges along coastal areas and as point-bar 4.2.4. Poquoson Member deposits along rivers. Johnson and Hobbs (1990) further suggest this The Poquoson Member of the Tabb Formation (Fig. 4) is the youngest unit was deposited 65 ka. Pleistocene deposit in southeastern Virginia. It has a maximum thickness and altitude of 4.5 m and 3.3 m amsl respectively (Mixon et al., 1989). Its lithology is comprised of coarse- to medium-grained basal sand that grades upward into a light-to-medium gray clayey, fine sand and silt (Mixon et al., 1982; Johnson et al., 1987; Mixon et al.,

Fig. 9. Stratigraphy of the upper portion of the Wachapreague Member as mapped in Fig. 10. Stratigraphy of the Sedgefield Member of the Tabb Formation as mapped in Oyster, Virginia. Samples (Qw-1, Qw-2) were taken just above the water table at a Crittenden Pit, Suffolk, Virginia. Samples (Qts-1, Qts-2) were taken at 48 cm and depth of approximately 1.6 m. Total land elevation is 3.5 m amsl. 63.5 cm in depth. Total land elevation is 5.5 m amsl. 184 T.W. Scott et al. / Geomorphology 116 (2010) 175–188

A GPR survey (Fig. 11) and excavation (Fig. 12) of this unit at Munden Point Park, Virginia Beach, Virginia (Fig. 4) show this member to be a regressive deposit with its upper portion consisting of moderately sorted medium quartz sand (2ϕ–1.5ϕ; 2.5YR 6/4; sampled) to poorly-sorted clayey medium-grained sand (2ϕ–1.5ϕ; 7.5YR 5/7). Excavation started at 2 m amsl and went to a depth of 1.6 m (0.4 m amsl) where the water table was encountered. OSL samples were taken at a depth of 1.2 m and yielded ages of 39.6±6.6 ka and 44.4±5.2 ka indicating deposition sometime in the period 50–33 ka or during MIS-3 (Table 2).

5. Discussion

With the exception of those from the Sedgefield Member, all of the new OSL ages (Table 2) agree reasonably well with previously reported MIS-5 and MIS-3 periods of deposition. The new OSL ages for the Butlers Bluff Member (Eastern Shore; 69 ka) correlate with MIS-5a ages reported for portions of the Sedgefield Member (southside) suggesting the Butlers Bluff and the Sedgefield members were deposited during the same transgressive and regressive events during MIS-5. The new OSL ages from the Poquoson Member (southside; 40–44 ka) are statistically identical with the new dates from the Wachapreague Formation (Eastern Shore; 39–47 ka), which suggests deposition of these two units occurred during the same transgressive and regressive cycle in the middle of MIS-3. Between these units lie the Joynes Neck Sand (Eastern Shore) and Lynnhaven Member (southside) which appear to have been deposited early during MIS-3. Stratigraphically and geomorphologically distinct, Fig. 12. Stratigraphy of the upper portion of the Poquoson Member of the Tabb their deposition occurred after MIS-5 (Butlers Bluff and Sedgefield Formation as mapped in Munden Point Park, Virginia Beach, Virginia. Samples (Qtp-1, ages) and before mid-MIS-3 (Wachapreague and Poquoson ages). Qtp-2) were taken at 1.2 m in depth. Total land elevation is 2 m amsl. This early MIS-3 assignment agrees with ages derived from Lynnha- ven Member ridges in northeastern North Carolina (Burdette and curves, significant land elevation changes must have occurred within Mallinson, 2008; Mallinson et al., 2008). Sands from the Land of the Chesapeake Bay region. The most likely cause for such change is Promise Ridge yielded OSL ages of 65.3±10.2 ka, 59.6±10.2 ka and the uplift and subsequent subsidence related to glacial forebulges 54.3±6.7. The younger Powells Point Ridge sediments were dated (Potter and Lambeck, 2003; Scott, 2006; Mallinson et al., 2008). between 43 ka and 54 ka (Mallinson et al., 2008); this ridge is equiv- alent to Pungo Ridge, the most prominent Lynnhaven-age shoreline 5.2. Glacioisostatic adjustments and eustatic sea-level curves feature in Virginia (Fig. 4). Histories of sea-level fluctuations provide critical data needed to 5.1. Relative sea-level curve quantify the locations, magnitudes and rates of glacioisostatic ad- justments through time. GIA models explain patterns of subsidence, The age assignments associated with these shoreline features and caused by loading of glacial ice or rising seas that flood continental deposits, each reaching 3 to 12 m amsl, suggest deposition occurred shelves, and forebulge uplift, caused by displacement of mantle rock when eustatic sea levels were much lower than today (Fig. 2). Even from beneath the loaded zones. Model results depend upon estimates given uncertainties inherent in age estimates and in eustatic sea-level of crustal thickness, viscosity of the lower and upper mantle, ice sheet

Fig. 11. Interpreted GPR from Munden Point Park, Virginia Beach, Virginia illustrating the relationship between the Poquoson Member and the underlying Chowan River Formation. Two separate excavations along the GPR transect verified the water table depth. T.W. Scott et al. / Geomorphology 116 (2010) 175–188 185 thickness, ages of ice sheet margins, location of ice dome centers, and shoreline and shelf configurations. Sea-level histories derived from tide-gauge and geologic records permit calibration of the models. (e.g. Peltier, 1987, 1990; Davis and Mitrovica, 1996; Potter and Lambeck, 2003). Field data and GIS model results from near-field settings around the Laurentide ice sheet indicate that peripheral forebulge flexures generated measurable geologic effects (e.g. Bhiry et al., 2000; Hetherington et al., 2004; Bell et al., 2005; Lajeunesse and Hanson, 2008), some of which continue to the present (e.g. Latychev et al., 2005). GIA are often complex and non-linear over space and time; interpreta- tions of field data and model results suggest that forebulges develop more strongly in oceanic crust than on continents, and that they can migrate laterally as the ice sheet shape evolves (e.g. Peltier, 1987, 1990; Barnhardt et al., 1995; Davis and Mitrovica, 1996; Potter and Lambeck, 2003; Bell et al., 2005). Forebulge dimensions and response rates estimated from field studies and calibrated model results vary by geologic conditions and the timing of ice sheet growth and decay. Hetherington et al. (2004) reported 100 m of uplift related to forebulge growth west of the Cordilleran ice sheet. Barnhardt et al. (1995) estimated a forebulge 20–25 m in amplitude migrated through the western Gulf of Maine c. Fig. 13. Estimated glacioisostatic curve for the study area in relation to the δ18O 11–10 ka. Fjeldskaar (1994), using dated tilted shorelines for model SPECMAP composite, and the eustatic sea-level curves from the Huon Peninsula in calibration, suggested the Fennoscandian ice sheet generated a Papua New Guinea. Forebulge subsidence throughout MIS-5 and into MIS-3 would have allowed for shoreline deposition to occur in Virginia during MIS-3. forebulge height of 60 m approximately 100 km from the ice margin during the last glacial maximum. Some forebulges developed quickly during few millennia (e.g. Hetherington et al., 2004; Bell et al., 2005), subsidence of these elevated shorelines may continue into the distant a shorter time than needed for near-complete relaxation of the future. forebulge in response to deglaciation. In one model calibrated to sea Earlier, forebulge uplift along the U.S. east coast during MIS-6 level responses in far-field locations, Potter and Lambeck (2003) generated long-lasting effects during forebulge collapse. Potter and suggest forebulge collapse begins quickly following rapid deglacia- Lambeck (2003) described MIS-5a altitude discrepancies of over 20 m tion, but several tens of thousands of years may be needed to acquire between Barbados and the U.S. Atlantic Coast and attribute these complete isostatic equilibrium. Their model also suggests a relaxation discrepancies to these glacioisostatic adjustments. From Delaware to time in excess of 40 ka for the Laurentide ice sheet. North Carolina, MIS-5 and MIS-3 shorelines identified by this study Additional problems of interpretation arise from the differences in and previous workers also vary in altitude (Fig. 14). Aside from the eustatic sea-level curves produced by a variety of methods. Cabioch units detailed in this study, these shorelines include the Parsonsburg and Ayliffe (2001) compared sea-level curves developed by Shackle- Sand in Delaware and , a stratigraphic equivalent to the Kent ton (1987, 2000) and Chappell (2002) for the past 80 ka that all Island Formation with radiocarbon ages ranging from 13–30 ka and a indicate global sea levels fluctuated rapidly during MIS-5 and MIS-3 maximum altitude of 9 m amsl (Denny et al., 1979); the Sinepuxent with a range that spanned a few tens of meters between successive Formation of Delaware and Maryland, also a stratigraphic equivalent high- and low-stands. Bloom and Yonekura (1990), Bradley (1999), to the Kent Island Formation, reaches 6 m amsl with radiocarbon ages Lambeck et al. (2002a,b), and Siddall et al. (2003) also developed ranging from 28–31 ka (Owens and Denny, 1979); the Ironshire curves that indicate similar patterns. Sea-level peaks on these curves Formation of Delaware and Maryland, another unit stratigraphically for MIS-3 may vary by as much as 60 m elevation and their timing and equivalent to the Kent Island Formation reaches 9 m amsl (Owens and number differ markedly. The sea-level histories shown in Fig. 13 Denny, 1979); the Sedgefield Member in North Carolina reaches 9 m represent the patterns of change shown on most of these curves. amsl with OSL ages of 68.6±7.2 and 79.8±4.5 (Suffolk Shoreline ages Despite the differences in these sea-level curves, it seems clear that from Mallinson et al., 2008); the Land of Promise Ridge of North sea levels experienced sizable oscillations during MIS-5 and MIS-3 but Carolina, a stratigraphic equivalent to the Pungo Ridge, which we after 80 ka the sea-level peaks that occurred crested no higher than 20 correlate with the Lynnhaven Member, reaches 5 m amsl with OSL or 30 m below present sea level. ages of 65.3 ±10.2 ka, 59.7±10.2 ka and 54.3±6.7 (Burdette, 2005; Mallinson et al., 2008); the Powells Point Ridge in NC (also correlated with the Lynnhaven Member) reaches 5 m amsl with eleven OSL ages 5.3. Virginia's glacioisostatic adjustments ranging from 31.2±2.7 ka to 54.9±6.3 ka (Burdette, 2005; Mallinson et al., 2008); and the Hickory Shoreline in North Carolina reaches 4 m In eastern Virginia both streams and shorelines would have amsl with three OSL ages ranging from 39.6±11.6 to 48.9±10.9 responded to the growth and collapse of peripheral glacial bulges. The (Mallinson et al., 2008). Further south, the trend of MIS-5 altitude growth of the Laurentide ice sheet from 30 ka to 19 ka (Lambeck et al., discrepancies continue with the Berkeley and Rifle Range Pits in South 2002a,b) created uplift in the area, possibly recorded as the rapid Carolina having coral-bearing units that reach ~5 m amsl with five U- incisions of the Susquehanna River (Holtwood Gorge) and Potomac series ages ranging from 75.5±9.8 ka to 85.8±10.8 ka (Wehmiller River (Mather Gorge) into underlying bedrock that began approxi- et al., 2004); the Wando Formation of South Carolina reaches 8 m mately 35 ka and ended around 13 to 14 ka (Fig. 13; Reusser et al., amsl with U-series ages ranging from 87 ka to 139 ka (Cronin et al., 2004; Pavich et al., 2006). Subsidence of the forebulge due to glacial 1981; McCartan et al., 1982; Szabo, 1985); and the Jones Pit in Georgia retreat likely began after maximum glacial expansion during MIS-2 has a coral-bearing unit reaching ~3 m amsl with five U-series ages and continued through MIS-1, thus attaining present-day elevations ranging from 82.1±4.7 ka to 84.7±0.7 ka (Wehmiller et al., 2004). but not isostatic equilibrium (e.g. Potter and Lambeck, 2003; Sella Based on the preceding data, it appears that MIS-3 shoreline deposits et al., 2007; Fig. 13). Because of this present-day lack of equilibrium, disappear south of North Carolina and both MIS-5 and MIS-3 deposits 186 T.W. Scott et al. / Geomorphology 116 (2010) 175–188

advances and retreats, we suggest that stratigraphic correlations between the Eastern Shore and southside and discrepancies between the ages and altitudes of these units can be resolved (Fig. 15). During MIS-6, a peripheral forebulge of geomorphically significant size, both in height and extent, was created in this region by the advance of a North American ice sheet (Bradley, 1999). This forebulge would have elevated the region and then began to subside near the beginning of MIS-5e. Continued subsidence throughout MIS-5 would have led to the deposition of the Stumptown Member (Exmore paleochannel fill) and Butlers Bluff Member of the Nassawadox Formation on the Eastern Shore and the Sedgefield Member of the Tabb Formation on Virginia's southside. The stadial periods of MIS-5d and MIS-5b would be marked by the Belle Haven and Eastville paleochannels on the Eastern Shore and two sequence boundaries contained within the stratigraphy of the Sedgefield Member on Virginia's southside. The deposition of the Butlers Bluff and Sedgefield Members would have ended around 70 ka. We suggest that forebulge collapse continued to lower regional land elevations while sea level regressed during MIS-4 and then transgressed at the beginning of MIS-3, which could have led to the deposition of the Joynes Neck Sand and Occohannock Members on the Eastern Shore and the Lynnhaven Member on the southside. Another sea-level oscillation occurred during the middle of MIS-3, which possibly led to the deposition of the Wachapreague Formation on the Eastern Shore and the Poquoson Member on the southside. Evidence from previous studies suggests that after approximately 36 ka, during late MIS-3 and most of MIS-2, the western North Atlantic area experienced additional forebulge uplift due to the most recent ice sheet advance. This uplift could have been the cause of bedrock incision along major rivers such as the Potomac, where incision ended around 13–14 ka (Reusser et al., 2004; Pavich et al., 2006). The isostatic effects of the MIS-2 forebulge can be seen as far south as Barbados based on the altitude discrepancies of MIS-5a and Fig. 14. Approximate locations and elevations of dated MIS-5 and MIS-3 shorelines from MIS-5e deposits between Virginia and Barbados (Potter and Lambeck, this project and previous studies (Denny et al., 1979; Owens and Denny, 1979; Cronin 2003) and MIS-5a and MIS-3 deposits between Delaware and North et al., 1981; McCartan et al., 1982; Mixon, 1985; Szabo, 1985; Wehmiller et al., 1995, Carolina (Scott, 2006; Mallinson et al., 2008; Parham et al., 2008). 1998, 2004; Burdette, 2005; Mallinson et al., 2008). We also suggest that throughout the present post-glacial period (MIS-1), the coastal plain of eastern Virginia continued to experience drop in altitude from north to south (Fig. 14). MIS-5 deposits in forebulge subsidence, but it still remains elevated above its level of Maryland and on the Virginia Eastern Shore reach 12 m amsl; they drop isostatic equilibrium, perhaps by several tens of meters. to 9 m amsl on Virginia's southside and then to 8 m amsl, 5 m amsl and In the future, stratigraphic studies using OSL and even more precise 3 m amsl from North Carolina to Georgia. Early MIS-3 deposits drop dating techniques should be combined with GIA model analyses to from 7.9 m amsl (Joynes Neck Sand; Eastern Shore) to 5.4 m amsl resolve relative and eustatic sea-level histories, glacial-isostatic adjust- (Lynnhaven Member; Southside) to 5 m amsl (Land of Promise and ments and hydro-isostatic unloading effects during “glaciated” time Powells Point Ridges; North Carolina). Mid-MIS-3 shorelines in periods with generally lowered sea levels. Existing models (e.g. Potter Delaware reach 9 m amsl, while those in Virginia reach 4.5 m amsl (Wachapreague; Eastern Shore) and 3 m amsl (Poquoson; Southside). In addition to these unexpectedly elevated shoreline features, submerged MIS-3 shorelines have been reported along the eastern U.S. coast (e.g. Wellner et al., 1993; Carey et al., 2005). Mallinson et al. (2008) reviewed these reports and suggested these shoreline features were not formed at sea-level maxima. MIS-3 repeatedly experienced dramatic temperature changes (e.g. Bond et al., 1993; Dansgaard et al., 1993) that may have translated to significant sea-level oscillations (e.g. Lambeck et al., 2002a,b).

6. Conclusion

According to established eustatic sea-level curves, the late Pleisto- cene coast of Virginia's eastern coastal plain differs from other low-lying coasts in that deposition apparently occurred above present sea level. The stratigraphy consists of units reaching 3 to 12 m amsl, yet deposition seems to have occurred during MIS-5 and MIS-3 when sea levels were as much as 40 m below the present level. However, by incorporating glacioisostatic effects attributed to the rise and collapse of Fig. 15. Revised correlation between late Pleistocene deposits on the Eastern Shore and forebulges created during the last two major North American ice sheet southside Virginia based on ages, altitudes and stratigraphy of units. T.W. Scott et al. / Geomorphology 116 (2010) 175–188 187 and Lambeck, 2003) that focus on MIS-5e and 5a might be refined and Cronin, T.M., Szabo, B.J., Ager, T.A., Hazel, J.E., Owens, J.P., 1981. Quaternary climates and sea levels of the U.S. Atlantic coastal plain. 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