Geology Reference
In-Depth Information
CLAY
<8
M
m (flocs & aggregates)
a
b
5
5
90
90
25
25
75
75
50
50
50
50
37
75
25
75
25
10
10
95
95
8 - 63
M
m
(sortable silt)
SAND
5
25
50
75
95 SILT
SAND
5
25
50
75
95
Fig. 10.9
Ternary diagram of sand/silt/clay ratios (
a
) and sand/
sortable silt/flocs & aggregates (
b
) observed in a back-barrier
tidal basin of the German Wadden Sea. Note the 50/50 partitioning
of sortable silt and aggregates in the latter case, as opposed to a
63/37 partitioning of silt and clay in the former case (Based on
Chang et al.
2007
; subdivisions after Flemming
2000
)
mixed sedimentary environments (Flemming
2000
).
At the same time the progression reveals energy gradi-
ents from sand to mud, on one hand, and between silt
and clay, on the other (cf. Pejrup
1988
; Molinaroli
et al.
2009
). In diagram a, the position of the data band
between the silt and clay endmembers suggests a rela-
tively exposed depositional environment, whereas in
diagram b it occupies a more intermediate energy
position. Which of the diagrams is hydraulically more
relevant in this context requires further investigation as
comparative data for the case b are currently lacking.
Nevertheless, in both cases the grain-size composition
allows a relative energy classification of the environ-
ment (Flemming
2000
; Molinaroli et al.
2009
).
parameters in this context are wet and dry bulk densities,
porosity, water content, and organic matter content.
Mass balancing exercises are particularly important
in disciplines such as sedimentology, geochemistry,
biology, microbiology, and biochemistry. Good exam-
ples can be found in Bartholomä et al. (
2000
) for the
import and export of sand and mud, and in Delafontaine
et al. (
2000
) for organic matter.
In tidal flat environments, both wet and dry bulk
density have been found to be highly correlated with
mud content and average values of the former can thus
be calculated from the latter on the basis of regression
analyses. Examples from the Wadden Sea are shown
in Fig.
10.10
. From the calibration curves it can be
seen that pure sand has an average wet bulk density
(BD
w
) of ~2.0 g cm
−3
and a corresponding dry bulk
density (BD
d
) of ~1.6 g cm
−3
. At the other end, the wet
and dry bulk densities of pure mud are ~1.2 and
~0.3 g cm
−3
, respectively. More precise average values
can be calculated on the basis of the regression equa-
tions. Although the Wadden Sea trends should be gen-
erally valid for many other tidal flat systems composed
of terrigenous material (quartz, feldspar, rock frag-
ments, carbonate, clay minerals), it is nevertheless
advisable to establish separate calibration curves for
other areas, especially if organic matter contents are
high (Delafontaine et al.
2004
).
Wet and dry bulk densities can also be determined
from the water content (Wc) of intertidal sediments.
An example for dry bulk density is illustrated in Fig.
10.11
.
10.3.3 Mass Physical Sediment Properties
Sedimentary environments such as back-barrier tidal
flats are highly dynamic systems that constantly change
their outward appearance in response to energy fluc-
tuations, on a regular basis in the course of the spring-
neap tidal cycle and episodically by sediment reworking
during storms. To quantify such changes, the import
and/or export of material to or from a tidal flat area is
commonly achieved by repeated elevation surveys with
subsequent calculation of volume changes between
surveys. The volume changes then need to be converted
into material masses. This is achieved by determining
critical mass physical sediment properties. Important
