Page 35 - Sedimentology and Stratigraphy

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9781405193795_4_0
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ARaju
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Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
Stratigraphy
02
Final
22
26.2.2009
22
page
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Compositor
Name:
and
26.2.2009 8:14pm Compositor Name: ARaju
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Terrigenous Clastic Sediments: Gravel, Sand and Mud
22 Nichols/Sedimentology and Stratigraphy 9781405193795_4_0 02 Final Proof page 22 26.2.2009 8:14pm Compositor Name: ARaju
components. Silt-sized lithic fragments are only abun- ; )< 5 5 ; 1< 5 5
dant in the ‘rock flour’ formed by glacial erosion
5 ; => $?> >
(7.3.4).
In aqueous currents silt remains in suspension until
the flow is very slow and deposition is therefore char-
acteristic of low velocity flows or standing water with
little wave action (4.4). Silt-sized particles can remain
in suspension in air as dust for long periods and may
be carried high into the atmosphere. Strong, persis-
tent winds can carry silt-sized dust thousands of kilo-
metres and deposit it as laterally extensive sheets
(Pye 1987); wind-blown silt forming loess deposits
appears to have been important during glacial periods
(7.6 & 7.7).
$ ; 1< 5 5 / 1< 5 5

5 ; ? ) $>
5 ; > $?
2.4.3 Clay minerals
Clay minerals commonly form as breakdown products
of feldspars and other silicate minerals. They are phyl-
losilicates with a layered crystal structure similar to
that of micas and compositionally they are alumino-
silicates. The crystal layers are made up of silica with
aluminium and magnesium ions, with oxygen atoms
linking the sheets (Fig. 2.14). Two patterns of layer-
ing occur, one with two layers, the kandite group,
and the other with three layers, the smectite group.
Of the many different clay minerals that occur in 5
sedimentary rocks the four most common (Tucker
1991) are considered here (Fig. 2.14). 5
Kaolinite is the commonest member of the

5
kandite group and is generally formed in soil profiles
in warm, humid environments where acidic waters
intensely leach bedrock lithologies such as granite. Fig. 2.14 The crystal lattice structure of some of the more
Clay minerals of the smectite group include the common clay minerals.
expandable or swelling clays such as montmoril-
lonite, which can absorb water within their struc-
ture. Montmorillonite is a product of more moderate 2.4.4 Petrographic analysis of clay minerals
temperature conditions in soils with neutral to alka-
line pH. It also forms under alkaline conditions in arid Identification and interpretation of clay minerals
climates. Another three-layer clay mineral is illite, requires a higher technology approach than is needed
which is related to the mica group and is the most for coarser sediment. There are two principal techni-
common clay mineral in sediments, forming in soils in ques: scanning electron microscopy and X-ray diffrac-
temperate areas where leaching is limited. Chlorite is tion pattern analysis (Tucker 1988). An image from a
a three-layer clay mineral that forms most commonly sample under a scanning electron microscope
in soils with moderate leaching under fairly acidic (SEM) is generated from secondary electrons produced
groundwater conditions and in soils in arid climates. by a fine electron beam that scans the surface of the
Montmorillonite, illite and chlorite all form as a sample. Features only microns across can be imaged
weathering product of volcanic rocks, particularly by this technique, providing much higher resolution
volcanic glass. than is possible under an optical microscope. It is