Geology Reference
In-Depth Information
vicinity of salt marsh creeks, (2) sedimentation
associated with sheet fl ow over vegetated salt marsh
surfaces, and (3) sedimentation associated with exposed
salt marsh edges. A comparison between key factors
dominating the fl uvial and those prevailing in the tidal
salt marsh system is presented in order to highlight the
special conditions related to salt marsh sedimentary
environments. Import of fi ne-grained material to salt
marsh areas are analysed in relation to salt marsh creek
dynamics, including a discussion of shared morphody-
namic features between the creek and the salt marsh
system like natural levees and crevasse splays. The
nature of fi ne-grained sediment transport over salt
marsh platforms is discussed and related to the head-
ward migration of salt marsh creeks and morphologi-
cal features like salt pans, piping, ice rafting and salt
marsh edge morphodynamics.
Salt marsh formation is reported to start at a level
close to the high water level from a variety of different
climate zones. An example of plant zones across a salt
marsh platform is given from the Skallingen backbar-
rier, Denmark. Effects of vegetation are illustrated by
means of recent examples of direct measurements of
dynamics in the salt marsh vegetation canopy. Even if
vegetation can be regarded as a factor that enhances
sedimentation, it is also discussed how vegetation in its
outset can cause channel erosion in tidal landscapes
when a patchy vegetation cover enhances the ability of
fl ow to concentrate and erode.
The textural composition of salt marsh sediments
are described and related to a measure of the environ-
mental exposure of the depositional environment via a
ratio between the content of material in the range 5 F
to 7 F (31-8 mm) and that of 5 F to 9 F (31-2 m m).
Autocompaction is discussed in relation to a procedure
enabling a description of the bulk dry density variation
with level under the salt marsh surface by means of the
bulk dry density of the upper 5 cm. Furthermore, the
role of autocompaction is discussed and shown to be of
crucial importance for a correct interpretation of dif-
ferent types of salt marsh accretion measurements.
Existing dynamic salt marsh accretion models are
discussed, explained and related to the salt marsh accre-
tion continuity equation. One of the discussed models is
used to give examples of salt marsh stability in relation
to different tidal conditions and sea-level rise scenarios,
and to discuss the concept of salt marshes being able to
reach dynamic equilibrium in a rising tidal frame.
The chapter is concluded by a description of salt
marshes in the geological record with distinctions
made between mainland coast salt marshes and back-
barrier salt marsh formations. The preservation poten-
tial of salt marsh is highest for the mainland type,
where deposits are also potentially thickest. In the geo-
logical 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
sediments interbedded with peat and frequently inter-
rupted by channel-fi ll deposits. The backbarrier type
should be found as interbedded slaps of salt marsh
deposits in washover sand, signalizing a slowing down
of the relative sea-level rise. The characteristic salt
marsh sediment is associated with salt marsh creeks
with cut banks, point bars, channel lags and natural
levees. Sedimentological and ichnologically centred
facies analysis for identifying salt marsh deposits are
described and discussed at the end of the chapter.
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