Geology Reference
In-Depth Information
to major ports and serve as conduits through which
nutrients are exchanged between lagoons and estuaries
and the coastal ocean. Numerous species of fi nfi sh and
shellfi sh rely on inlet channels for access to backbarrier
regions for feeding, breeding, and nursery grounds of
their young. Sedimentologically, inlet-fi ll sequences
may comprise a substantial portion of the barrier litho-
some (Moslow and Tye 1985) and their presence may
be the only means of identifying the existence of for-
mer barrier island chains on the continental shelf (Hine
and Snyder 1985; Rieu et al.
2005
) or in the rock record
(Tye and Moslow
1993
). Tidal inlets not only serve as
important interruptions in the longshore transport sys-
tem, but their fi lls represent subsurface anomalies that
infl uence groundwater exchange and reservoir perme-
ability when preserved in the rock record.
Most tidal inlets are associated with coastal plain
settings along passive margins where abundant sedi-
ment has formed extensive barrier chains, such as those
along the East and Gulf Coasts of the United States,
the Friesian Islands along the North Sea, and the North
Slope of Alaska. Barrier island coasts and tidal inlets
are most prevalent in the Northern Hemisphere where
sea-level rise during the Holocene fl ooded deltaic and
other coastal plain settings, forming lagoons and bays
behind developing barrier systems. It is the fi lling and
drainage of these open-water backbarrier areas that
produce tidal currents and maintain the inlet channel.
The formation of tidal inlets has been a time-dependent
process. In the early evolution of barrier systems, tidal
inlets developed preferentially in the low areas along
developing barrier systems, coinciding with existing or
former river valleys. In other settings, abundant sand
supplies led to the construction of spit systems across
embayments. Tidal inlets also became stabilized at the
downdrift ends of spits, if tidal prisms were large
enough to keep them open. Subsequent to these initial
phases of inlet development and extending to the pres-
ent time, new tidal inlets have been a product of barrier
breaching during large magnitude storms. If the hyp-
sometry and hydrodynamics of the backbarrier are
conducive to the capture of a suffi ciently large tidal
prism, the new inlet will remain open. Tidal inlets are
much less common in the Southern Hemisphere
because falling sea level (1-4 m; Angulo et al.
2006
)
since the mid-Holocene has resulted in strandplain,
rather than barrier island, development. Exceptions to
this trend occur where abundant sand resources have
built spits and barriers at the mouths of estuaries and in
front of large embayments, such as those that occur
along the coasts of Australia and Brazil.
Tidal inlets are normally accompanied by an ebb-
tidal delta on the seaward side and a fl ood-tidal delta
in the bays. Inlets and their associated tidal deltas
exhibit a wide diversity in morphology, hydrodynamic
character, and sediment transport patterns, which are a
function of the variability in oceanographic, meteoro-
logic, and geologic settings, including parameters
such as tidal range, wave energy, sediment supply,
storm magnitude and frequency, fresh water infl ux,
and antecedent geologic controls.
This chapter examines the general morphodynamics
and hydrodynamics of tidal inlets for the purpose of
illustrating how inlet processes, such as barrier breach-
ing, inlet evolution, channel shifting, and bar migrations,
produce the facies architecture and large-scale bedding
surfaces that defi ne tidal inlet and associated tidal delta
sedimentary deposits. An excellent treatment of textural
characteristics, bedform patterns, and sedimentary
sequences at tidal inlets exists in Boothroyd (
1985
) . In
addition to tidal-delta morphology, this discussion cov-
ers the development of bedforms, their migrational
trends, and resulting sedimentary structures in the vari-
ous tidal inlet sub-environments. Smith (
1991
),
Boothroyd (
1985
) , Reinson (
1984
), and Hubbard et al.
(1979) have also provided stratigraphic models for inlet-
fi ll sequences and the tidal deltas; however, these are
hypothetical models based primarily on surfi cial data.
During the past 25 years, extensive coring efforts and
geophysical studies carried out at numerous tidal inlets
offer new perspectives on inlet dynamics and geological
legacy, and provide empirical data for more detailed
stratigraphic models of inlet-fi ll sequences and tidal del-
tas. The second half of the chapter reviews many of
these fi eld studies and discusses the variability and com-
plexity of inlet systems. Finally, the preservation poten-
tial and an overview of tidal inlet deposits as recognized
in the rock record are presented.
12.2
Morphology and Stability:
General Concepts
A tidal inlet is the narrowed region of water between
two barrier islands or between a single barrier and an
adjacent bedrock or glacial headland (Fig.
12.1
).
