Geology Reference
In-Depth Information
Fig. 12.9
Distribution of former tidal inlets along Duxbury Beach, Massachusetts. A GPR profi le illustrates the manner in which
an inlet channel gradually fi lled against a glacial headland (From FitzGerald et al.
2001b
)
on the tidal prism and how easily the backbarrier tidal
prism is accessed through time.
The geometry and facies characteristics of inlet
deposits are also dictated by the dynamics of the inlet
channel. Three scenarios of inlet behavior are presented
here that build on the pioneering work of many previ-
ous authors (Bruun
1966
; Kumar and Sanders
1974
;
Moslow and Heron
1978
; FitzGerald et al.
1984
,
1988
;
Tye
1984
; Moslow and Tye
1985
; Tye and Moslow
1993
). These dynamic models provide a framework for
developing stratigraphic models for inlet fi lls and iden-
tifying them in the sedimentologic record:
1. Migrating inlets: As the downdrift side of the inlet
erodes, the channel fi lls with sand due to prograda-
tion of the updrift spit and thus the strike section of
the deposit is dependent on the distance that the
inlet migrates along the coast and the depth of the
channel. In GPR and shallow-seismic refl ection
profi les, these deposits are characterized by large-
scale, steep- to moderately- dipping, accretionary
surfaces oriented in a downdrift direction. In dip
sections the deposit is thickest at the inlet throat and
thins in both a seaward and landward direction.
2. Breaches: Inlets that are cut during storms (called
hurricane passes in the Gulf of Mexico and breaches
elsewhere) and later close, produce a fi ll sequence
that is formed through spit accretion from one or
both sides of the inlet. Filling may also involve the
landward migration of a large bar that closes off the
inlet mouth. Subsequent overwash activity fi lls the
channel and/or tidal or wave-induced circulation in
the lagoon contributes fi ne-grained sediment. Thus,
the channel fi ll may consist predominantly of sand,
or sand can be mixed or inter-layered with mud. In
certain instances, a clay plug may comprise the
inner abandoned channel sequence if the infl ux of
sand is prevented due to barrier reconstruction in
front of the breach. GPR and shallow-seismic strike
sections of these deposits exhibit a variety of chan-
nel geometries- including simply conformable, pro-
graded, accretionary, and complex (terminology
after Hine et al.
1979
; Mitchum et al.
1977
) .
