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GSA TODAY | AUGUST 2015  Figure 3. Time series of the Blackwater River valley. Top: Intact marsh surveyed  ages at the base of the Pleistocene section are 1.72 ± 0.08 Ma for a
                         from AD 1902 to AD 1904 and presented in a 7.5˝ USGS topographic map from         Susquehanna River paleochannel and 2.06 ± 0.14 Ma for a local
                         AD 1905 (USGS, 1905); dark blue hatching around the Blackwater valley is          paleochannel system (2�; Fig. 5 and GSA Supplemental Data
                         tidal marsh; light blue pattern is freshwater swamp. Middle: Initiation of major  Table S1 [see footnote 1]). The older age indicates that major
                         ponding seen in an aerial photograph from 1938 (http://www.esrgc.org/).           cutting and filling commenced in the study area shortly after
                         Bottom: Coalesced ponds forming the informal “Lake Blackwater” in satellite       the onset of major Northern Hemisphere continental glaciation
                         imagery from AD 2007 (http://www.bing.com/maps/). Wetlands are                    (2.4 Ma; Balco and Rovey, 2010). These ages are significantly older
                         converting to open water at a rate of 50–150 ha/yr in the field area (Cahoon et   than previous age estimates for paleochannels of the Chesapeake
                         al., 2010). Image locations are identified in Figure 4B. Red outline shows        Bay (ca. 18–450 ka; Colman et al., 1990). The complex Pleistocene
                         location of unnamed island for reference.                                         stratigraphic record and age range of material overlying these dated
                                                                                                           deposits suggest that fluvio-estuarine processes dominated land-
                         geochronological framework for the Blackwater National Wildlife                   scape evolution over glacial-interglacial timescales in the field area
                         Refuge landforms and estuarine sediments to a depth of ~9 m                       (Fig. 5).
                         (Fig. 5). Eight radiocarbon dates constrain the timing of Holocene
                         inundation and the beginning of marsh accretion. Detailed                           LiDAR allows us to identify a variety of landforms on the
                         methods are provided in the online GSA Supplemental Data                          Blackwater National Wildlife Refuge surface that form a
                         Repository1.                                                                      continuum with the shallow stratigraphy (<12 m depth; Figs. 4
                                                                                                           and 5). A regressive, wave-cut scarp with multiple bifurcations
                         RESULTS AND INTERPRETATIONS                                                       (beach ridges, Fig. 4B) separates upland areas to the north and
                                                                                                           east from the lower terrain in the south and west that is occupied
                           The Blackwater National Wildlife Refuge is underlain by                         by an expansive tidal marsh. These shoreline features consist of
                         Pleistocene deposits that vary in thickness from ~3–55 m (Fig. 5).                an ~3 m fining upward sequence of burrowed, silty fine sand to
                         Glacial-interglacial climate fluctuations induced major cycles of                 massive, medium sand (GSA Supplemental Data Fig. S5 [see
                         localized river incision and aggradation in the Chesapeake Bay                    footnote 1]) with an age range of 53–40 ka (n = 6; see Fig. 5 and
                         region (Colman et al., 1990), and the subsurface Blackwater                       GSA Supplemental Data Table S2). Below the scarp, large
                         National Wildlife Refuge stratigraphy includes cut-fill deposits                  subaqueous bars (Fig. 4B) that roughly parallel the paleo-
                         associated with at least three paleochannel systems (Fig. 5). Isochron            shoreline dominate the geomorphology. The bars consist of
                                                                                                           facies ranging from horizontally bedded, alternating sand and
                                                                                                           silt to moderately sorted, fine-to-medium sand interpreted as
                                                                                                           wave-sorted tidal channel deposits and wave-built bars within
                                                                                                           tidal tributaries or bays. OSL ages for surficial landforms below
                                                                                                           the scarp range from 69 to 35 ka (n = 15). The morphology,
                                                                                                           lithology, and ages of these features indicate that estuarine
                                                                                                           conditions prevailed, at least intermittently, during most of
                                                                                                           MIS 3, with active bar migration continuing during regression.
                                                                                                           Locally, unconformities separate multiple, stacked MIS 3
                                                                                                           deposits, and in some locations MIS 3 deposits cut older estua-
                                                                                                           rine units that were dated to both MIS 5a and MIS 5e (Fig. 5;
                                                                                                           GSA Supplemental Data Figs. S4 and S5 [see footnote 1]).

                                                                                                             The MIS 3 estuarine surface is truncated by a north-south–
                                                                                                           trending, meandering channel with scroll bars as well as elliptical
                                                                                                           depressions interpreted as ephemeral basins (Fig. 4B). The rims of
                                                                                                           basins are composed of laminated, silty, fine-to-medium sand
                                                                                                           with ages 30–26 ka (n = 3); the meandering channel must be
                                                                                                           younger than the ca. 35 ka sand bars it cuts. The basins and
                                                                                                           channel are likely relict from periglacial processes that dominated
                                                                                                           this landscape beginning ca. 30 ka and continued through
                                                                                                           the LGM (Denny et al., 1979; Newell and Clark, 2008; French et
                                                                                                           al., 2009; Markewich et al., 2009; Newell and DeJong, 2011;
                                                                                                           Gao, 2014).

                                                                                                             Sediments from the Holocene transgression (yellow, Fig. 5)
                                                                                                           overlap MIS 3 estuarine deposits within incised valleys of the
                                                                                                           Blackwater River and its tributaries. They consist of a lower silt
                                                                                                           (~3–4 m) with locally abundant organic material that transitions
                                                                                                           to an upper, dense, organic peat (~3–4 m). A radiocarbon (14C)
                                                                                                           age from woody material near the base of the silt (−8.5 m) suggests

     1 GSA supplemental data item 2015211, data tables and methodology, is online at www.geosociety.org/pubs/ft2015.htm. You can also request a copy from GSA Today,
     P.O. Box 9140, Boulder, CO 80301-9140, USA; gsatoday@geosociety.org.

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