Page 26 - Sedimentology and Stratigraphy

Final Proof page 13
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02
02
Final
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Proof
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and
and
Stratigraphy
Stratigraphy
Nichols/Sedimentology
9781405193795_4_0
9781405193795_4_0
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
Nichols/Sedimentology
26.2.2009
26.2.2009
26.2.2009 8:14pm Compositor Name: ARaju
8:14pm
8:14pm
Compositor
Compositor
Name:
ARaju
ARaju
13
13
Name:
Sand and Sandstone 13

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Fig. 2.11 The Pettijohn classification of *2
sandstones, often referred to as a 2
‘Toblerone plot’ (Pettijohn 1975). " # $
formal nomenclature is used. This is usually the Pet- terms are interchangeable when referring to sand-
tijohn et al. (1987) classification scheme (Fig. 2.11). stone rich in feldspar grains). By the same token,
The Pettijohn sandstone classification combines 25% of lithic fragments in a sandstone make it a
textural criteria, the proportion of muddy matrix, lithic arenite by this scheme. Over 95% of quartz
with compositional criteria, the percentages of the must be present for a rock to be classified as a quartz
three commonest components of sandstone: quartz, arenite; sandstone with intermediate percentages of
feldspar and lithic fragments. The triangular plot has feldspar or lithic grains is called subarkosic arenite
these three components as the end members to form a and sublithic arenite. Wackes are similarly divided
‘Q, F, L’ triangle, which is commonly used in clastic into quartz wacke, feldspathic (arkosic) wacke
sedimentology. To use this scheme for sandstone clas- and lithic wacke, but without the subdivisions. If a
sification, the relative proportions of quartz, feldspar grain type other than the three main components is
and lithic fragments must first be determined by present in significant quantities (at least 5% or 10%),
visual estimation or by counting grains under a a prefix may be used such as ‘micaceous quartz aren-
microscope: other components, such as mica or bio- ite’: note that such a rock would not necessarily con-
genic fragments, are disregarded. The third dimension tain 95% quartz as a proportion of all the grains
of the classification diagram is used to display the present, but 95% of the quartz, feldspar and lithic
texture of the rock, the relative proportions of clasts fragments when they are added together.
and matrix. In a sandstone the matrix is the silt and The term greywacke has been used in the past for a
clay material that was deposited with the sand grains. sandstone that might also be called a feldspathic or
The second stage is therefore to measure or estimate lithic wacke. They are typically mixtures of rock frag-
the amount of muddy matrix: if the amount of matrix ments, quartz and feldspar grains with a matrix of
present is less than 15% the rock is called an arenite, clay and silt-sized particles.
between 15% and 75% it is a wacke and if most of the
volume of the rock is fine-grained matrix it is classified
as a mudstone (2.4.1). 2.3.4 Petrographic analysis of sands
Quartz is the most common grain type present in and sandstones
most sandstones so this classification emphasises the
presence of other grains. Only 25% feldspar need be In sand-grade rocks, the nature of the individual grains
present for the rock to be called a feldspathic and the relationship between these grains and the
arenite, arkosic arenite or arkose (these three material between them is best seen in a thin-section