Geology Reference
In-Depth Information
3. Channel Reorientation: Tidal inlets of this type are
characterized by a defl ection of the main ebb chan-
nel due to the preferential accumulation of sand on
the ebb-tidal delta caused the dominant longshore
transport system, such that the channel becomes
skewed along the downdrift shoreline. The inlet
channel can erode deeply into the adjacent barrier
as depicted in Fig.
12.7
at Capers and Price Inlets,
SC. In this confi guration, the low hydraulic gradient
of the channel and ineffi cient tidal exchange even-
tually cause a breaching of the ebb-tidal delta to
effect a more direct pathway for tidal fl ow (see sec-
tion on ebb-delta breaching, 12.5.2.2). Initially
active tidal deposition characterizes the channel fi ll
until a collapse of ebb-tidal delta and the ensuing
landward-migrating bar complex closes the former
inlet mouth, resulting in lower energy currents and
fi ner-grained sedimentation (Tye
1984
; Moslow
and Tye
1985
; Tye and Moslow
1993
) . The inlet fi ll
consists of a fi ning upward sequence that can be
dominated by mud, if the former inlet channel
closes completely and there is ample mud in the
backbarrier system. This type of succession is com-
monly capped by marsh deposition (Tye
1984
) .
In light of the above models and the results of numer-
ous fi eld investigations discussed below, it has been
shown that tidal-inlet deposits can be identifi ed within
barrier lithosomes, in backbarrier deposits, on conti-
nental shelves, and in the rock record by one or more of
the following characteristics (Moslow and Heron
1978
;
Tye
1984
; Moslow and Tye
1985
; Hine and Snyder
1985
; Imperato et al. 1988; Israel et al.
1987
; Siringan
and Anderson
1993
; Tye and Moslow
1993
; FitzGerald
et al.
2001b
; Rieu et al.
2005
; Simms et al.
2006
) :
1. A sharp contact between the base of the inlet chan-
nel and underlying strata, which in deep inlets is
commonly a scoured Pleistocene surface. In shallow
inlets, the base of the inlet is cut into the barrier lith-
osome and into the shoreface in a seaward direction
or into lagoonal deposits in a landward direction.
2. Strike sections of inlet fi lls imaged in shallow-seis-
mic and GPR profi les exhibit a range of refl ector
geometries that include various types of single and
multiple channel cut-and-fi lls and/or repetitive sig-
moidal-oblique refl ectors dipping downdrift that
are formed by a migrating inlet and spit system.
3. A general fi ning-upward sequence that is in contrast
to the coarsening-upward grain size trend of most
barrier lithosomes (due to their being underlain by
either lagoon or nearshore fi ned-grained sediments).
Considerable variability (fi ning and coarsening trends)
can exist within units comprising inlet fi lls, including
layers of mud.
4. The base of the inlet sequence usually consists of a
lag deposit and is composed of poorly-sorted shell
hash, whole and broken shells, heavy-mineral con-
centrations, rip-up clasts, coarse sand, and/or fi ne
gravel.
5. Tidal-inlet fi ll exhibits a wide variety of sedimen-
tary structures, including planar- and trough- cross
bedding, graded beds, mud laminations and mud
drapes, and shell hash layers. Bioturbation is rare in
active sandy channel fi ll sequences but is common
in abandoned inlet fi lls.
12.6.2.1 Field Studies of Tidal-Inlet Sequences
As presented above, there are several criteria to recog-
nize tidal-inlet sequences in the sedimentologic record,
but it should be emphasized that although inlet fi lls do
exhibit some commonality, such as basal sharp con-
tacts, lag deposits, and fi ning-upward trend, there is
also a great deal of variability in the tidal-inlet sequences
as will be illustrated in the following discussion.
Texas Coast
A number of stratigraphic studies along the Texas
coast shed light on the sedimentological and geo-
physical facies typifying tidal inlets and tidal deltas
along a microtidal sandy barrier island chain having
intervening muddy bays and drowned river valleys
(Israel et al.
1987
; Siringan and Anderson
1993
;
Simms et al.
2006
) . Stratigraphic information, includ-
ing borehole descriptions, sediment cores, grab sam-
ples, and high-resolution shallow-seismic data were
used to construct stratigraphic sections for San Luis
Pass at the southwest end of Galveston Island (Israel
et al.
1987
, Fig
12.10
) and Bolivar Pass at the north-
east end of the Galveston Island (Siringan and
Anderson
1993
, Fig.
12.11
). Both of these inlet fi lls
incise into estuarine/bay sediments and are a product
of southwesterly migration of the inlet channel, cou-
pled with spit progradation of the updrift barrier. Inlet
sequences exhibit sharp basal contacts consisting of
shell lags. Inlet fi lls are composed of fi ne shelly sand
to sandy clay, and generally have graded beds and
some degree of bioturbation. Shell hash layers are
common and have normal grading. At San Luis Pass,
shell and sand content in abandoned inlet fi ll deposits
