Geoscience Reference
In-Depth Information
Figure 12.12
Sorting and BSI particle size
characteristics of sediments:
(a) three samples shown at
the same scale but not
ascribed to specific
environments; (b) cumulative
percentage curves, showing
typical particle size
distributions, which measure
sorting visually by the relative
horizontal span of each curve
and statistically by standard
deviation.
(a)
Well-sorted
Moderately-sorted
Poorly-sorted
(b)
100
80
Moderately-sorted
fluvial sand
60
Poorly-sorted
glacial till
40
Well-sorted
beach gravel
20
0
0.002
0.006
0.02
0.06
0.2
0.6
2
6
20
60
200 mm
Cobbles
Boulders
Clay
Silt
Sand
Gravel
through still water or air are subject to an anisotropic force
field and elongated particles tend to move either parallel
or transverse (normal) to the flow or slope. This is often
preserved as particle orientation in sediments and,
together with imbrication - the stacking of particles
dipping (sloping) towards or away from the flow direction
- provide palaeocurrent information of former in-transit
material. This extends to include syndepositional
sedimentary structures , formed during or immediately
after deposition. Parcels of sediment are laid down as a
layer or stratum determined by the underlying topo-
graphical surface, sediment properties and the geometry
of the transporting medium. Its lower surface or
bedding plane usually distinguishes it from the previous
(older) parcel by subtle changes in texture or colour
in a conformable sequence , or dramatically so at an
unconformity which marks a pause, erosive event, etc.
( Plate 12.7 and see Figure 23.1 ). Within each stratum, a
series of subsidiary structures reflect the direction and
energy of transport ( Figure 12.13 and Plate 12.8 ).
Sedimentary facies, environments and
tectonic basins
So far we have reviewed some general, descriptive aspects
of sediments. We also appreciate that each parcel is the
product of specific environmental processes in a broader
landsystem and constitutes a sedimentary facies . A facies
may vary as much internally as it does from adjacent
facies. It is said to be a litho facies or a bio facies, depending
on whether minerogenic (inorganic) or biogenic constitu-
ents predominate, and their characterization is the first
step in reconstructing their environmental origins. Each
facies is the stratigraphic equivalent of a sedimentary
landform and may represent a single event. Contemporary
geological or geomorphic processes are present at the
surface, burying older ones in the stratigraphic record
( Plate 12.8 ).
Facies assemblages linked by common genetic origins
constitute a sedimentary environment whose terrestrial
form corresponds very closely with the geomorphic
landsystems described in later chapters ( Figure 12.14 ).
Moreover, the global location of major sedimentary
 
 
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