Geology Reference
In-Depth Information
using high-resolution shallow-seismic refl ection pro-
fi ling over water and ground-penetrating radar (GPR)
on land, the latter offering an order of magnitude fi ner
resolution than seismic-refl ection data. The refl ectors
produced by these systems coincide with large-scale
erosional and accretionary surfaces, thereby providing
a means of documenting the sedimentation history of
tidal-inlet fi ll sequences and tidal deltas in great detail.
Cores taken in conjunction with the geophysical data
provide a means of ground-truthing the interpretation
of the various refl ectors and produce a detail character-
ization of individual tidal facies. The results of several
studies dealing with active tidal inlets and tidal-delta
deposits as well as paleo-inlet locations are presented
in this section to illustrate the types of facies architec-
ture associated with inlet sequences including their
geophysical characterization, when available.
2.0 km-wide tidal inlet. The northern spit (13 km long) is
pinned to drumlins and its length is riddled with numer-
ous tidal-inlet scars having widths varying from 60 to
285 m. GPR transects revealed at least 18 former inlet
channels that have breached the Duxbury barrier, none of
which are open today. As depicted in one of these GPR
transects, one of the larger paleo-inlets shoaled against a
till headland (Fig.
12.8
). The refl ector geometry and
sediment cores taken in this region suggest that inlet fi ll-
ing occurred in pulses whereby high energy events were
responsible for transporting pebbly, cobble-rich sand
into the channel, forming the strong refl ectors. The inter-
vening, more transparent refl ectors correspond to peri-
ods of lower energy conditions when sand units were
deposited (FitzGerald et al.
2001b
) .
The position of tidal inlets along a coast is commonly
stratigraphically-controlled in coastal plain settings and
bedrock- or topographically- controlled along glaciated
and rocky coasts (i.e., coasts of Oregon, Washington,
NSW Australia, and in New Zealand). Many inlets coin-
cide with Pleistocene or younger river valleys as reported
along the central East Coast of U.S. (Morton and
Donaldson 1973; Halsey
1979
; Tye
1984
) , New England
(FitzGerald
1996
), and the Friesian Islands (FitzGerald
and Penland
1987
). Presumably, tidal inlet channels
stabilize at these sites due to structural controls or the
relative ease in which tidal currents can erode former
riverine deposits. Thus, tidal inlet fi ll sequences may cut
through, or be nested within, fl uvial sequences; the two
types of deposits can be differentiated on the basis of
grain size and/or fossil content, or through geophysical
imaging. For example, a GPR profi le along central Plum
Island in northern Massachusetts reveals a former chan-
nel cut that is more than 100 m wide, extends from −6 to
−13 m in depth, and is overlain by a 6 m-thick tidal inlet
fi ll sequence. This channel aligns perfectly in a land-
ward direction with the Parker River and seaward with a
channel system that has been imaged in offshore shal-
low seismic refl ection data (Fig.
12.9
, Hein et al.
2007
) .
This deep channel that was subsequently occupied by a
tidal inlet has been interpreted to be part of the paleo-
drainage formed during the transgression following
deglaciation of this region (Hein et al.
2011
) .
12.6.1 Occurrence of Tidal Inlet Deposits
Tidal-inlet fi ll sequences are formed at inlets that close
or migrate for some distances along shore. At large tidal
inlets, they also accumulate where the thalweg shifts
laterally within the main inlet channel, such as the inlets
along the Friesian Islands (FitzGerald et al.
1984
) .
Complete sections are preserved within regressive
deposits, but partial fi ll deposits are also often pre-
served during transgressions because inlets are deep,
particularly at the throat section (tidal ravinement), and
usually erode far below the adjacent barrier lithosome
and deeper than the transgressive unconformity pro-
duced during shoreface retreat.
As shown by Hayes (
1979
) and Davis and Hayes
(
1984
), tidal inlets are more numerous and comprise
greater stretches of shoreline along mixed-energy coasts
and coasts having large bay tidal prisms. Thus, it would
be expected that as tidal range and/or bay tidal prism
increase, tidal inlet deposits will comprise a greater pro-
portion of the Holocene lithosome. However, as pointed
out by several investigators (Moslow and Heron
1978
;
Heron et al.
1984
; Moslow and Tye
1985
; Tye and
Moslow
1993
) wave-dominated barrier coasts can have
extensive tidal inlets deposits (30-50% of the barrier
length; Moslow and Tye
1985
) due to the opening and
closing of inlets and channel migration along the coast.
A case in point occurs along the central coast of
Massachusetts where the small reentrant of Plymouth
Bay is fronted by two long spit systems separated by a
12.6.2 Inlet Fill Sequences
The size, geometry, and facies characteristics of tidal-
inlet fi lls are dependent on a number of factors that
