Page 34 - Sedimentology and Stratigraphy

Nichols/Sedimentology
Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
and
Stratigraphy
Stratigraphy
and
Name:
Compositor
8:14pm
8:14pm
Compositor
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ARaju
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9781405193795_4_0
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page
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26.2.2009
26.2.2009 8:14pm Compositor Name: ARaju
26.2.2009
Proof
02
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9781405193795_4_0
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Final
Final
Clay, Silt and Mudrock 21
information the rock can then be named using an 2.4.1 Definitions of terms in mudrocks
appropriate classification scheme (e.g. the Pettijohn
classification, Fig. 2.11). Silt is defined as the grain size of material between
4 and 62 microns in diameter (Fig. 2.2). This size
range is subdivided into coarse, medium, fine and
Point counting
very fine. The coarser grains of silt are just visible to
To make a quantitative analysis of the components of the naked eye or with a hand lens. Finer silt is most
a sedimentary rock some form of systematic determi- readily distinguished from clay by touch, as it will feel
nation of the proportions of the different clast types, ‘gritty’ if a small amount is ground between teeth,
matrix and cement is required. The commonest tech- whereas clay feels smooth. Clay is a textural term
nique is to attach a point counting mechanism on to to define the finest grade of clastic sedimentary parti-
the stage of the microscope: this is a device that holds cles, those less than 4 microns in diameter. Individual
the thin-section slide and shifts the position of the particles are not discernible to the naked eye and can
slide to the side in a series of small increments. It is only just be resolved with a high power optical micro-
attached to a mechanical counter or to a computer scope. Clay minerals are a group of phyllosilicate
such that each time a button or key is pressed, the minerals that are the main constituents of clay-sized
slide moves sideways. The operator determines the particles.
clast type under the cross-wires at each step by press- When clay- and silt-sized particles are mixed in
ing different buttons or keys. A series of transects unknown proportions as the main constituents in
across the slide is made until a sufficient number of unconsolidated sediment we would call this material
points have been counted – typically not less than mud. The general term mudrock can be applied to
300. The number of counts of each grain type, matrix any indurated sediment made up of silt and/or clay.
and/or cement is then converted into a percentage. If it can be determined that most of the particles (over
The size of the step, the magnification used and the two-thirds) are clay-sized the rock may then be called
number of categories of clast will be determined by the a claystone and if silt is the dominant size a silt-
operator at the outset, depending on the grain-size stone; mixtures of more than one-third of each com-
range and clast types recognised in a preliminary ponent are referred to as mudstone (Folk 1974; Blatt
examination of the thin-section. et al. 1980). The term shale is sometimes applied to
any mudrock (e.g. by drilling engineers) but it is best
to use this term only for mudrocks that show a fissi-
2.4 CLAY, SILT AND MUDROCK lity, which is a strong tendency to break in one
direction, parallel to the bedding. (Note the distinction
Fine-grained terrigenous clastic sedimentary rocks tend between shale and slate: the latter is a term used for
to receive less attention than any other group of deposits fine-grained metamorphic rocks that break along one
despite the fact that they are volumetrically the most or more cleavage planes.)
common of all sedimentary rocks types (2.1). The grain
size is generally too small for optical techniques of
mineral determination and until scanning electron 2.4.2 Silt and siltstone
microscopes and X-ray diffraction analysis techniques
(2.4.4) were developed little was known about the The mineralogy and textural parameters of silt are
constituents of these sediments. In the field mudrocks more difficult to determine than for sandstone
do not often show the clear sedimentary and biogenic because of the small particle size. Only coarser silt
structures seen in coarser clastic rocks and limestone. grains can be easily analysed using optical microscope
Exposure is commonly poor because they do not gen- techniques. Resistant minerals are most common at
erally form steep cliffs and soils support vegetation this size because other minerals will often have been
that covers the outcrop. This group of sediments broken down chemically before they are physically
therefore tends to be overlooked but, as will be seen broken down to this size. Quartz is the most common
in later sections concerning depositional environ- mineral seen in silt deposits. Other minerals occurring
ments and stratigraphy, they can provide as much in this grade of sediment include feldspars, muscovite,
information as any other sedimentary rock type. calcite and iron oxides amongst many other minor