Geology Reference
In-Depth Information
Fig. 12.17
Stratigraphic
section across the ebb-tidal
delta of Breaches Inlet, SC.
The delta sequence is a
product of the southerly
migration and subsequent
abandonment of the main ebb
channel (After Nelligan
1983
)
dominated by channel cut-and-fi lls, large-scale landward-
dipping foresets produced by onshore swash bar migra-
tions and shallow dipping strata (FitzGerald
1976
;
FitzGerald and Nummedal
1977
; Hubbard et al. 1979;
Imperato et al. 1988; Sha
1990a
; Sha and de Boer
1991
;
Smith 1991). Generally, these sandy ebb deltas are sub-
jected to more energetic waves and tides and, therefore,
there is little opportunity for mud deposition.
horizontal to cross-bedded sand with shell hash layers.
Individual lobes are 0.5-1.8 m thick and the entire
delta sequence is slightly more than 3 m thick
(Boothroyd et al.
1985
) .
Similar to the Rhode Island examples, along
Florida's Gulf Coast fl ood-tidal deltas are a few meters
thick, have a sharp basal contact with muddy lagoon
deposits, are often multi-lobate, and are composed of
quartz sand with shelly layers, particularly concen-
trated in channelized regions (Davis et al.
2003
) . The
fl ood delta associated with Shinnecock Inlet contains a
lower unit consisting of a muddy fi ne sand that grades
upward to a proximal delta facies composed of cross-
bedded to massive medium sand with numerous shell
layers (Hennessy and Zarrillo
1987
) . The coarsening-
upward sequence is a product of increasing tidal energy
associated with the opening of Shinnecock during the
1938 Hurricane.
These fi eld investigations show that fl ood deltas are
single- or multi-lobate and can be stacked depending
upon the accommodation space. Typically, they are
bedded and composed of medium to fi ne sand with
shell layers, having a sharp to gradational contact with
muddy lagoonal sediments.
Low Energy Coasts
Flood-tidal deltas are common along microtidal and
mesotidal coasts in which there is suffi cient open-
water area for sand to accumulate landward of a tidal
inlet. Intertidal exposure and thickness of fl ood deltas
depend upon sedimentation rates and accommodation
space. The Rhode Island coast contains a series of
lagoons having multi-lobate deltas that began forming
circa 2.5 ka in response to rising sea level and tidal
current generation (Boothroyd et al.
1985
) . Boothroyd
et al. (
1985
) show that Ninigret Pond contains a
stacked sequence of landward accreting fl ood delta
lobes separated by silty organic layers, including a
sharp basal contact with low-energy lagoon sediments
(Fig.
12.18
). Delta lobes consist of medium to coarse,
