Geology Reference
In-Depth Information
Fig. 8.12
Beneath: The topography (in relation to Danish
Ordinance Level) in a cross section across the central part back-
barrier of Skallingen. From
right
to
left
, the tidal fl at area meets
the salt marsh at about 1,600 m, the outer marsh - with a ridge
at about 1,400 m - extends to about 1,100 m. Inland of here, the
lower inner marsh is located between the higher laying outer
marsh and the aeolian reinforced beach ridges at about 700 m.
The profi le crosses the peninsular from south/west towards
north/east.
Above
: The grain-size distribution across the profi le
divided into clay (
black
), silt (horizontally striped) and sand in
various size classes (Modifi ed from Bartholdy
1983
) . The topo-
graphical profi le is adopted from Nielsen and Nielsen
(1973)
Table 8.1
The ratio between the content of 5 F -7 F and 5 F -9 F
particles in salt marsh clay in relation to the exposure of the
sedimentary environment of the salt marsh in question. Based on
Bartholdy (
1985
)
Content of 5 F -7 F/content of 5 F -9 F
and one primarily associated with fl occulated grains
with a mean grain size of 9 F to 10 F (2-1 mm) and a
sorting coeffi cient of about 2.5 F. These two fi ne-
grained populations can easily be recognized to the right
of the sand population in Fig.
8.11
. Thus, the proposed
ratio describes a measure of the amount of fi ne-grained
material deposited as single particles in relation to the
combined amount of silt.
Exposure
Above 0.65
High
Between 0.55 and 0.65
Medium
Below 0.55
Low
In energetic environments, fl occulated material
suffers from mechanical dispersion (e.g. Dyer
1989
;
Pejrup and Mikkelsen
2010
) and is therefore less
abundant in the resulting deposit than coarser grains
that can be deposited as single particles. It needs here
to be remembered that particles close to the size of
clay have a very low settling velocity in relation to
just slightly coarser silt particles. A particle of 6 F
(16 mm) in water of 15°C and 30‰ salinity needs
approximately 1.5 h to settle 1 m, whereas a particle
of 8 F (4 mm) under the same conditions needs 23 h to
settle the same distance. Thus, it is practically impos-
sible for the latter to settle out, if it is not part of a
fl occulated larger particle.
The 5 F limit was chosen to avoid any infl uence
from sand populations, and the 9 F limit was chosen
both for reasons of symmetry and because it repre-
sents the lower limit of a pipette grain-size analysis.
The 7 F limit has later been recognized by others as
blown sand from the exposed beach area (to the left of
the shown profi le) again increases the sand content of
the marsh sediment. Thus, even if the sand content will
refl ect the exposure of a given salt marsh, it can also
refl ect other conditions, and will often be source con-
trolled. In order to avoid this effect and relate the com-
position of salt marsh sediments to the environmental
exposure of the depositional environment in question,
Bartholdy (
1985
) suggested the ratio between the con-
tent of material in the range 5 F to 7 F (31-8 m m) and
the content in the range 5 F to 9 F (31-2 mm) as an
adequate indicator of exposure (Table
8.1
). The argu-
ment for this is that the fi ne-grained part of the investi-
gated salt marsh sediments can be divided into two log/
normal distributions: one, primarily associated with
deposition of single grains with the mean grain size
varying between 4.5 and 6 F (63 mm to 16 mm) and a
sorting coeffi cient (standard deviation) of about 1 F ;
