Geology Reference
In-Depth Information
deep channel is typifi ed by a shell lag underlying a
thick subtidal unit. In sandy channels herringbone
cross-stratifi cation may occur with some low level of
bioturbation. The main channel may exhibit unidirec-
tional dunes and ripples oriented with the dominant
tide, due to segregation of the fl ood and ebb fl ows either
side of the pointbar. In a muddier regime, the sub-tidal
and low-intertidal regions are typifi ed by planar, hori-
zontal bedding (Pearson and Gingras
2006
) . Moving up
the tidal range, the middle of the intertidal zone exhib-
its predominantly low-angle, planar-bedded and poten-
tially IHS. In sandy environments the presence of small
dunes and ripples may result in cross-bedding with
increasing mud content as the pointbar emerges into the
intertidal zone. The high intertidal zone reverts back to
planar, horizontal bedding, highly bioturbated and the
deposits may exhibits desiccation marks (Pearson and
Gingras
2006
). The unit is fi nally topped by marsh
sediment as the channel moves laterally and ulti-
mately, the marsh follows on. Where the bar is detached
from the channel bank, mud deposits are seen in the
blind-ended, subordinate barb channel that crosses
the surface of the bar. If enough sand is present for
bedforms, ripples and dune in this region will be
oriented in the opposite direction to the main channel.
Barwis (
1978
) identifi ed distinct vertical succession
associated with each of the four tidal pointbar mor-
phologies that he observed. Each form has subtle dif-
ferences in the distribution of fl ows, sedimentation and
biota. Steep apical bars are the only morphology that
would create a continuous, unbroken succession with a
thickness equal to the channel depth. This is because
these features are fully welded to the inner channel
bank up to the elevation of the marsh itself. Additionally,
this type of bar is steeper, with less suitable habitat for
infauna and has no sheltered tidal barb behind the bar
crest. Thus, bioturbation is comparatively low com-
pared to sedimentation. Detailed descriptions of each
are presented in Barwis (
1978
) .
While both the planform morphology and vertical
facies in tidal pointbars have been described in the
literature, a full three-dimensional description is still
missing to fully document the internal structure and
horizontal variations, which result from the heteroge-
neity of the physical (and biological) processes both
across and along the forms.
In salt marsh systems, deposits in lower-order
creeks are steep-sided and narrow. They consist of pre-
dominantly massive mud units, which lack the high
level of organics that would be seen in the surrounding
marsh platform deposits. In marsh sediments (specifi -
cally on high marshes which sit at the high-water
elevation), standing pools of water (pannes or ponds)
may produce similar muddy facies, devoid of rhi-
zomes. They can be distinguished from creek deposits
by the presence of
Ruppia maritime
, a submerged veg-
etation, which is commonly seen in ponds but not in
channels (Wilson et al.
2009
) . A notable levee of coarser
sediment may be present along a channel edge due to
the baffl ing of fl ow speeds by vegetation (Allen
2000
) .
There is no clear relationship between network plan-
form and the sedimentary structures observed in tidal
channels (Eisma
1998
), beyond the obvious infl uence of
meanders on pointbar geometry. Terwindt (
1988
) sug-
gests that the number and the dimensions of drainage
channels could be used give an indication of the tidal
regime: a low tidal range producing a low number of
small channels, indicating micro- or mesotidal condi-
tions; a large number of deep channels indicating mac-
rotidal conditions. This seems unlikely based on the
wide variability seen between sub-basins within inter-
tidal systems such as Venice Lagoon (Marani et al.
2002
). Shallow channels are observed over exposed fl ats
in macrotidal environments (Eisma
1998
) . Likewise,
deep channels can be found in microtidal regions, such
as the back-barrier areas in New Jersey, where the
through-fl owing channels of Ashley and Zeff (
1988
) are
5-100 m wide and 2-5 m deep. In general, there are few
differences between the faces generated in macro- and
mesotidal environments (Terwindt
1988
) with the excep-
tions, however, where current velocities are exception-
ally high and parallel laminated sand-rich facies may be
deposited across bars during upper sheet fl ow (e.g.,
Cobequid Bay-Salmon River estuary, Bay of Fundy;
Dalrymple et al.
1991,
1992
) .
11.8
Summary
Channels provide the pathway for the tidal wave to
propagate and are a primary control on the sedimenta-
tion and ecology of coastal environments. Defi ned by
the alternating fl ow of ebb and fl ood currents, tidal
channels occur across a range of scales within macro-,
meso- and microtidal environments. They often form
dendritic networks, which, despite being described by
some studies as fractal, exhibit a great deal of variation
