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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
AD 1905 (USGS, 1905); dark blue hatching around the Blackwater valley is
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/).
Bottom: Coalesced ponds forming the informal “Lake Blackwater” in satellite
imagery from AD 2007 (http://www.bing.com/maps/). Wetlands are
converting to open water at a rate of 50–150 ha/yr in the field area (Cahoon et
al., 2010). Image locations are identified in Figure 4B. Red outline shows
location of unnamed island for reference.
geochronological framework for the Blackwater National Wildlife
Refuge landforms and estuarine sediments to a depth of ~9 m
(Fig. 5). Eight radiocarbon dates constrain the timing of Holocene
inundation and the beginning of marsh accretion. Detailed
methods are provided in the online GSA Supplemental Data
Repository1.
RESULTS AND INTERPRETATIONS
The Blackwater National Wildlife Refuge is underlain by
Pleistocene deposits that vary in thickness from ~3–55 m (Fig. 5).
Glacial-interglacial climate fluctuations induced major cycles of
localized river incision and aggradation in the Chesapeake Bay
region (Colman et al., 1990), and the subsurface Blackwater
National Wildlife Refuge stratigraphy includes cut-fill deposits
associated with at least three paleochannel systems (Fig. 5). Isochron
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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Susquehanna River paleochannel and 2.06 ± 0.14 Ma for a local
paleochannel system (2�; Fig. 5 and GSA Supplemental Data
cutting and filling commenced in the study area shortly after
the onset of major Northern Hemisphere continental glaciation
(2.4 Ma; Balco and Rovey, 2010). These ages are significantly older
than previous age estimates for paleochannels of the Chesapeake
Bay (ca. 18–450 ka; Colman et al., 1990). The complex Pleistocene
stratigraphic record and age range of material overlying these dated
deposits suggest that fluvio-estuarine processes dominated land-
scape evolution over glacial-interglacial timescales in the field area
(Fig. 5).
LiDAR allows us to identify a variety of landforms on the
Blackwater National Wildlife Refuge surface that form a
continuum with the shallow stratigraphy (<12 m depth; Figs. 4
and 5). A regressive, wave-cut scarp with multiple bifurcations
(beach ridges, Fig. 4B) separates upland areas to the north and
east from the lower terrain in the south and west that is occupied
by an expansive tidal marsh. These shoreline features consist of
an ~3 m fining upward sequence of burrowed, silty fine sand to
massive, medium sand (GSA Supplemental Data Fig. S5 [see
footnote 1]) with an age range of 53–40 ka (n = 6; see Fig. 5 and
GSA Supplemental Data Table S2). Below the scarp, large
subaqueous bars (Fig. 4B) that roughly parallel the paleo-
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