Geology Reference
In-Depth Information
Fig. 10.22
Characteristic sedimentary features around the
mean high-tide level. (
a
) Final meander of a salt marsh creek at
low tide before draining onto the upper intertidal flat. Note that
the water in the creek is not completely drained because it is
dammed by a small 'ebb-delta' lobe at the transition to the open
tidal flat; (
b
) Well developed mud cracks in sparsely vegetated
Salicornia
marsh; (
c
) Cut meander bank in
Spartina
marsh
revealing preserved laminae near the surface and complete oblit-
eration of physical structures at depth
can be subdivided further by specific plant associations
comprising different species in different climates
and geographic locations. In contrast to the smooth
intertidal mud flats, the salt marshes are drained by
an intricate network of meandering marsh creeks
(Fig.
10.22a
). Although the creek beds are generally
excavated to depths below the elevation of the adjacent
mud flats, they rarely extend into the latter as they
commonly terminate in small 'ebb-delta' lobes at the
salt marsh boundary where the channel-confined flow
spreads out onto the open tidal flat. These depositional
lobes often prevent the marsh creeks from draining
completely at low tide.
Mud-cracked surfaces are frequently regarded as
good indicators of emergence in tidal environments
(Klein
1977
). This applies in particular to tidal flat
depositional systems with large differences between
the elevations of neap high tide and spring high tide.
The larger this difference, the longer the period of
emergence over neap tide and the more extensive the
mud-cracked surfaces. In regions where this difference
in elevation is small, for example in the Wadden Sea,
such surfaces are narrow and patchy (Fig.
10.22b
).
Mud-cracks and the roots of salt marsh plants tend to
destroy any lamination in the course of time, as can be
seen in Fig.
10.22c
where the lamination is still pre-
served in the upper few centimetres but completely
obliterated below (cf. also Fig.
10.18j
).
In addition to the characteristic salt marsh zones
associated with specific plants, the transition from
upper intertidal to lower supratidal flats is locally char-
acterized by laminated sediments (mats) produced by
microbial activity, especially in places where this
transition is more sandy (Fig.
10.23
). As pointed out
earlier, these microbial mats have to be carefully dis-
tinguished from algal mats produced by green algae
(Gerdes and Krumbein
1987
). Microbial mats com-
mence at the sediment surface with a thin filamentous
