Geology Reference
In-Depth Information
Moslow and Tye (
1985
) report that wave-dominated
barrier island coasts are composed of 30-50% tidal
inlet sediments. In strike section, inlet deposits are len-
ticular to wedge-shaped whereas dip-sections thin in
both a landward and seaward direction, interfi ngering
with fl ood and ebb-tidal delta sediments, respectively.
The size and dimension of these inlet deposits are
largely a product of tidal prism and geological factors
that control channel deepening.
Central South Carolina is a mixed-energy coast
having relatively short, stubby barrier islands, numer-
ous tidal inlets, well-developed ebb deltas and a back-
barrier consisting of marsh and tidal creeks (Hayes
1975,
1979
). Although South Carolina “mixed-
energy” inlets tend to be stable, some of them have
had histories in which the seaward portion of the inlet
channel is defl ected downdrift and then breaches back
to a straighter, more hydraulically effi cient, channel
confi guration (FitzGerald
1988
, Figs.
12.5
c and
12.7
).
Inlet sequences formed by these processes have been
studied by Moslow and Tye (
1985
) and Tye and
Moslow (
1993
) and are presented for Price and
Capers Inlets in Fig.
12.13
. These types of inlet
deposits are more complex than the wave-dominated
examples, because, in addition to active channel-fi ll
sand, they contain inactive channel deposits, welded-
bar and washover facies, and tidal creek sediments.
As depicted in stratigraphic sections in Fig.
12.3
,
when the inlet channel migrates and erodes the down-
drift barrier, active inlet fi lls are deposited from wave
and tidal introduction of sand to the channel. However,
after the breaching event occurs and the former inlet
mouth is closed by a landward-migrating bar com-
plex (Figs.
12.5
c and
12.7
), the strong reduction in
wave and tidal energy lead to the deposition of muddy
sediment (abandoned inlet channel fi ll). In the case of
Price Inlet, this migration and breaching process has
occurred twice, leaving behind two active and inac-
tive fi ll sequences (Tye and Moslow
1993
) . These
units are topped by tidal creek and marsh sediment.
The most obvious difference between the North
Carolina wave-dominated inlet deposits and the
mixed-energy examples from the South Carolina
coast is presence of muddy, inactive channel sedi-
ment. It should also be recognized that some mixed-
energy tidal inlets migrate and produce sedimentary
sequences similar to those of North Carolina, such
the East Friesian Island inlets (Sindowski
1973
;
FitzGerald
1996
) .
Virginia-North Carolina Barrier Coast
Although sedimentological information and, to a lesser
extent, shallow-seismic refl ection data, have been the tra-
ditional means of defi ning the extent and character of
tidal-inlet deposits, GPR is increasingly used on land
because subsurface GPR refl ections are capable of imag-
ing very subtle lithological transitions, such as slight tex-
tural and compositional variations. GPR profi les provide
a means of deciphering the mode of channel fi lling includ-
ing episodes of channel cut and fi ll. Historical Old
Currituck Inlet, (the original coastal boundary between
Virginia and North Carolina) offers an example of a large
channel that migrated south during the sixteenth to early
eighteenth centuries (Fig.
12.14
). The 10-m-deep inlet
eventually closed, primarily as a result of a new channel
opening to the south and capturing the tidal prism of the
Currituck Sound (McBride
1999
) . The geophysical image
of the inlet channel can be used to approximate the chan-
nel cross-section and calculate the tidal prism, which in
this case exceeds 10 million cubic meters and is compa-
rable to modern large oceanic inlets. Based on successive
channel outlines, represented by strong GPR refl ections,
the tidal prism of the Old Currituck Inlet decreased by an
order of magnitude prior to inlet closure.
12.6.2.2 Field Studies of Ebb- and Flood- Tidal
Delta Deposits
The size and geometry of ebb-tidal deltas are a function
of the inlet tidal prism and wave versus tidal energy of
the coast. Although tidal prism also affects the extent of
fl ood-tidal deltas, these interior shoals are primarily a
product of the dimensions of the lagoon and frequency
of major storms, when large quantities of sand are
delivered into the backbarrier due to increased tidal and
wave activity. Several fi eld studies provide insights
concerning the facies relationships of these deposits.
Central South Carolina
Ebb-tidal deltas in South Carolina are wedged shaped
deposits thickening in a landward direction and domi-
nated by sand. Although fi eld investigations of ebb del-
tas are limited, coring studies of deltas at North Edisto
Inlet (Fig.
12.15
, Imperato et al. 1988) and Breach Inlet
(Fig.
12.16
, Nelligan
1983
) as well as historical migra-
tional trends of inlet main channels suggest that these
deposits range in thickness from 5 to > 15 m. Their stra-
tigraphy largely refl ects processes of migration of the
main ebb channel and reworking of the delta, formation
and abandonment of marginal fl ood channels, landward
