Geology Reference
In-Depth Information
Fig. 8.24
The Holocene
evolution of the Varde Å
Estuary. The relatively recent
coastlines in
f
are drawn from
maps. The three lines in
g
indicate the position of the
coring lines after which the
reconstruction was carried
out. The base of each sketch
is about 10 km long (After
Pedersen et al.
2009
)
the subsequent small sea-level fall and moderate sea-
level rise (Fig.
8.25
), the salt marsh grew outward on
top of the lagoonal mud (Fig.
8.24f, g
). This last part of
the evolution shows that if sediment supply is large
enough, a salt marsh is capable of maintaining a regres-
sive coastline, even if the sea level is rising (see Fig.
8.6
upper left which shows this transition as it looks today).
The reason is that the area is importing large amounts
of fi ne-grained sediment from the North Sea (Bartholdy
and Madsen
1985
; Pedersen and Bartholdy
2006
)
which builds up a huge mudfl at area in front of the
mouth of the estuary. From here, fi ne-grained sediment
is imported during storms and deposited inside the
estuary (Bartholdy
1984
). The accretion rate in the salt
marsh area of the estuary facing the mudfl ats is pres-
ently between 5 and 10 mm year
−1
.
The preservation potential of salt marsh is highest
for the mainland type, where deposits are also poten-
tially thickest. In the geological record, this type of salt
marsh should be found as elongated enclaves between
high laying substrates and consists of a basal peat
overlain by fi ne-grained sediment interbedded with
peat and frequently interrupted by channel-fi ll deposits.
The backbarrier type should be found as interbedded
slaps of salt marsh deposits in washover sand, signal-
izing a slowing down of the relative sea-level rise.
The characteristic salt marsh sediment is associated
with a hierarchy of channels from very small (less than
