Geology Reference
In-Depth Information
Fig. 8.2
Profi le across the salt marsh creek Store Lo in the backbarrier salt marsh on Skallingen. For location, see Fig.
8.1
(Modifi ed
from Bartholdy
1983
)
low and high waters; the latter being in direct contrast
to fl uvial systems. A comparison between key factors
dominating the fl uvial and those prevailing in the tidal
salt marsh system is presented below in order to high-
light the special conditions of the salt marsh sedimen-
tary environment.
point of complete obliteration (Bartholdy and Madsen
1985
; Reineck and Gerdes
1996
; Chang et al.
2006a
) .
As illustrated in Fig.
8.2
, the gradual transition between
point-bar and salt marsh deposition on top of point
bars is associated with horizontal lamination with
interlayered sand/mud bedding similar to the classical
example described by Reineck and Singh (
1975
) . It
contains sandy deposits related to deposition during
periods of relative high water levels and current
velocities primarily associated with wind setup during
storms, alternating with mud deposits also related to
rough weather conditions but deposited around high
water slack. During storms, fi ne-grained sediment is
typically mobilized by wave action in the surrounding
tidal area resulting in increased concentrations of mud
in the water entering from the adjacent tidal area
during fl ood. This type of 'layer cake' deposition is
also found in natural levees along salt marsh creeks.
In environments with relatively little mud and a poten-
tially high organic production, such beddings can
consist of layers of sand alternating with layers of
organic fi brous material. This type of bedding was
described by Redfi eld (
1972
) as a kind of varve in New
England (USA) marshes. The fi brous material is asso-
ciated with algal mats with subsequent concentrations
of root fi bres formed during the summer. The sand
horizons (usually with added silt) that separate these
8.3.1
Relationship to Salt Marsh Creeks
Sedimentation associated with channel fl ow in salt
marsh creeks includes all the typical sedimentary units
from the fl uvial system, of which point bars and natu-
ral levees are directly related to salt marsh deposits.
Barwis (
1978
) attempted to distinguish between tidal
creek point-bar deposits and their fl uvial counterparts
and found several differences related to the vertical
sequence. Apart from the origin of the organic material
capping the point bar, however, none of these relates
directly to the salt marsh deposits. Bioturbation was
regarded by Barwis as far more intense in the surface
layers of tidal compared to fl uvial point bars. This,
however, is only true for warm climates like the sub-
tropical/tropical zone. Temperate salt marshes are gen-
erally only slightly affected by bioturbation. If affected,
it is primarily as a result of plants with vertical root
structures that rarely destroy the lamination to the
