Geology Reference
In-Depth Information
However, sediment cores and age-dates to corroborate
these seismic interpretations have yet to be taken.
A more detailed study involving a kilometer-size grid
of high-resolution shallow-seismic profi les (475 km)
along with 80 sediment cores was performed off the
western Netherlands coast (Rieu et al.
2005
) . This study
led to the mapping of several tidal-inlet and backbarrier
drainage systems and the inferred position of a paleo-
barrier island chain (Fig.
12.19
). Furthermore, the geom-
etry and migration trends of the partially preserved tidal
network suggested that the barrier system and tidal inlets
did not migrate landward with the transgression, but
rather the barrier system must have been destroyed by
rising sea level (Reiu et al.
2005
) . This study illustrates
the type of information that can be gained from the iden-
tifi cation and interpretation of former tidal inlet systems.
However, as the authors also demonstrated, the preserva-
tion of these systems requires deep initial tidal-inlet
channels. Using Zoutkamperlaag Inlet as an analogue, it
is clear that little of the present channel network is pre-
served when shoreface erosion removes 6 m of the
coastal lithosome (Fig.
12.20
). Belknap and Kraft (
1981,
1985
) showed for the Delaware coast that all but the
deepest Holocene systems (>10 m) would be removed by
the present rate of sea-level rise and the studies of the
Louisiana coast suggest at least a similar magnitude of
shoreface reworking (Miner et al.
2009
) .
12.7
Preservation Potential
The preservation potential of tidal-inlet fi lls and tidal-
delta deposits is relatively high in regressive sequences,
as indicated by the extent of inlet deposits comprising
barrier lithosomes and the common occurrence of
marsh-covered paleo-deltas behind barrier islands. For
example, a series of fi ve stacked fl ood-tidal deltas was
identifi ed in the lagoon behind Mustang Island, Texas
(Simms et al.
2006
) .
In contrast, the relatively thin nature of tidal deltas
(commonly < 6 m) and moderate depth of most inlet
channels and inlet fi lls (mostly < 10 m), particularly
when compared to the depth of shoreface erosion, indi-
cate that inlet-associated deposits are rarely preserved
during a transgression. For instance, a detailed study of
Onslow Bay, North Carolina showed that although the
onshore barrier lithosomes contain numerous tidal inlet
deposits, there is no evidence on the inner shelf of for-
mer tidal-inlet deposits, and the channels that do exist
are of Pleistocene age (Hine and Snyder
1985
) . These
authors suggested that the landward translation of the
shoreface during the Holocene transgression eroded all
expression of even the deepest channels. In fact, there
are few reported tidal-inlet deposits on continental
shelves throughout the world. Possible exceptions
occur offshore of Barataria Bay in Louisiana where a
channel cut-and-fi ll is attributed to tidal-inlet migration
(Tye and Moslow
1993
) . Tidal-inlet fi lls have also been
recognized in shallow-seismic transects collected on
the inner shelf along the southern Delmarva Peninsula
(Foyle and Oertel
1997
) . These channel fi lls, which are
as much as 25 m deep and extend 25 km offshore from
today's coast, are theorized to have developed as the
barriers and associated tidal inlets migrated onshore
during the Holocene transgression (Foyle and Oertel
1997
). The present inlet systems along southern
Delmarva are deep (e.g., Wachapreague Inlet > 18 m;
Quinby Inlet > 23 m; Great Machipongo Inlet > 20 m)
and thus, the paleo-inlets may very well have produced
deep inlet scars during the transgression. A similar set
of clinoforms in shallow-seismic profi les taken off the
West Friesian Islands has also been interpreted to be
tidal-inlet and tidal-delta deposits (Sha
1990b
) .
He bases this interpretation on the fact the inlets
are deeper (> 30 m) than the depth of shore-
face erosion (15 m) during the transgression.
12.8
Examples From the Rock Record
Aside from recent inlet systems, several researchers
have identifi ed and described inlet-fi ll sequences and
associated tidal deltas from the rock record. For exam-
ple, Bridges (
1976
) identifi ed a tidal inlet/ebb-tidal
delta complex within Lower Silurian transgressive
barrier island facies, southwest Wales. A fl ood-tidal
delta, though not exposed, is proposed have existed in
the paleo-lagoon behind the barrier-inlet sequence
exposed at Anvil Bay and Maroles Sands (Bridges
1976
, Fig.
12.9
). In his study of a Carboniferous trans-
gressive succession on a shallow wave-dominated
shelf in the South Munster Basin of southern Ireland,
MacCarthy (
1987
) does not refer to inlet facies, but
suggests their presence in a paleogeographic interpre-
tation (MacCarthy
1987
, Fig.
12.20
). It is possible that
the inlets were few and their preservation limited on
this wave-dominated Carboniferous shelf.
