Geology Reference
In-Depth Information
Fig. 8.20 The distribution
of major coastal depositional
features (After Boyd et al.
1992 )
wave-dominated transgressive barrier coast as well as
local wave-exposed areas in tidal dominated estuaries.
Even sheltered areas in wave-dominated estuaries are
possible. The reason for mentioning salt marsh deposi-
tion in relation to these coastal types is solely to point
out that the facies associations related to the base of
salt marsh deposits can vary signifi cantly and should
be evaluated in a broader sequence stratigraphic per-
spective. Deposition and stratigraphy in estuaries, tide-
dominated deltas and open coast tidal fl ats are treated
elsewhere in this topic and will therefore not be further
discussed in the present chapter. Tide-dominated estu-
aries will briefl y be mentioned at the end of this
section.
relationship is twofold. As a rule, the groundwater
supply (and thus, peat formation) on a mainland coast
is substantial compared to that of barrier islands.
Furthermore, the substrate on a mainland coast can
consist of a variety of surface types, while the sub-
strate of transgressive barrier islands is either wash-
over sand or tidal fl at deposits formed in the rim of the
lagoon. There are of course a number of variations
over this theme, for example missing peat in some
subtropical barrier sequences like those of Georgia,
USA (Howard and Frey 1985 ) , and peat underlying
also the barriers in New Jersey, USA (Pusty 1980 ) .
See Davis ( 1994 ) for an overview of different types of
barrier island systems. The following generalized
stratigraphic model of a transgressive barrier system,
with regressive episodes (Fig. 8.21 ), is based on per-
sonal observations in the Danish Wadden Sea and
results published in Davis et al. ( 2001 ) , Bartholdy
et al. ( 2004 ) , Gehrelds et al. ( 2006 ) and Pedersen et al.
( 2009 ) .
The evolution illustrated in Fig. 8.21 shows two
important anomalies in relation to the general sche-
matic model of Galloway and Hobday ( 1983 ) . First of
all, most transgressions start with peat formation at the
mainland coast (often just regarded as part of the salt
marsh formation) and secondly, even if the preserva-
tion potential in some cases is small, salt marshes form
8.6.1
Mainland and Backbarrier
Salt Marsh Deposits
On a transgressive barrier coast, there are two focus
points for salt marsh formation in a profi le perpendicu-
lar to the coast. One is located on the mainland coast
where the salt marsh in general forms on top of basal
peat, and another is located on the back of barrier
islands/spits, where salt marsh formation as a rule
is formed on top of washover or tidal fl at deposits.
The reason for this general pattern in the stratigraphic
Search WWH ::




Custom Search