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CHAPTER 9
CLASSIFICATION
9.1. INTRODUCTION:
Classification of sandstone is an attempt to categorise the
sediment. Sedimentologists depends solely on composition to classify the
sedimentary rocks. With recent advances of sedimentological studies, many
different classification for sandstone have been proposed in different times.
The basic problem is the chioce of characters for such classification. It seems
best to select those features which gives more insight into the rock genesis for
classification. Therefore, the descriptive classification based on observable
and measurable parameters are suitable for the general purpose.
The major impetus of sandstone classification came from the
proposals of Kiynine (194B) and Pettijohn (1957), both of which recognised
the importance of mineralogy as a clue to source rock composition and source
area composition.
At present most of the sedimentologists made their
classification on the basis of textural and mineralogieal composition. The
classification based on chemical composition has also been proposed recently
by many workers. The mineralogieal composition has proved to be more
meaningful. The character of the source rock is the principal factor for
determining the mineral composition. The mineralogieal composition reflects
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the source areas composition. It may therefore, be related to tectonism also
(Pettijohn, 1975).
The chemical classification mainly differentiates mature and
immature character of the sandstones as these terms are also used in both
mineralogical and textural classification. The mature quartz rich sandstone
have high Si02/Al203 ratios (Pettijohn, 1963).
Folk (1954 and 1980) proposed a classification based on textural
data of sediments. For this purpose, the sand, silt and clay particles of the
sandstones were considered. The term clay is used lor any particle between
0.0039 to 0.0156 mm in size, silt includes the perticles which ranges in size
between 0.0156mm and 0.0625 mm and sand perticles ranges between 0.0625
to 2mm.
9.2. METHOD OF STUDY:
The data obtained from the size analysis of loose sandstone
samples of the Sylhet sandstone of the present study area has been grouped
into sand, silt and clay classes. These three groups were then plotted in a
triangular diagram of sand, silt and clay as suggested by Folk (1954). He
proposed two types of classification . In one he classified gravel, sand and
mud, while in the oilier he showed the classification of sand , silt and clay.
The classification of gavel -sand-mud is not applicable for the present Sylhet
sandstone, as it has been seen from the size analysis data that the gravel
portion is absent in the sediment of the present sandstones as seen from the
size analysis data. Therefore,the sand-silt-clay ratio has been considered for
this classification.
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The size range of the perticles used for this classification are as follows Materials between 0.0625 to 2.0 mm
Sand.
Materials between 0.0156 to 0.0.0625mm
Silt.
Materials between 0.0039 to 0.0156mm
Clay.
Accordingly, sand-silt-clay were calculated to find percentages of sand , silt
and clay from the size analysis data (Table-?) and plotted in a traingular
diagram of Folk (1954) (Fig- is). Most of the samples are found to fall in “L”
division and only few samples are in “M” division. The nomenclature of the
sediment shows that the sediments are “Sand” with very little “Silt”.
The QFL diagram of Dott ( From Pettijohn, 1987) , has been
used extensively for mineralogical classification of sandstone. Therefore, for
mineralogical classification, the QFL are considered following Dott (1964). In
“Q” division, quartz and cherts are considered, for “F” division ,feldsper and
igneous rock fragments and for “L” division , mica and inetamorphic rock
fragments are considered. The QFL are calculated to find out the actual
percentages (Table-1^) and plotted in a composition triangle (Fig-14). Here, the
samples are mostly fall in orthoquartzite division.
The chemical composition is probably the simplest way to
categorise sandstone for supporting the mineralogical and textural
composition. The general geochemical behaviour of the elements are
(Pellijohn, Potter, Siever, 1972) as shown below -High Si02/Ah03
Quartz arenite
(Mature)
Low Si02/Ah03 Na20>K20 Feldspathic graywacke
(Immature)
Na20<K20 Arkose,lithic graywacke
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It shows elemental composition, mineralogy and rock types. The
classification differentaite mature and immature sediments.
9.3. OBSERVATION:
The sand, silt clay percentages of Sylhet sandstone are plotted in a
triangular diagram of Folk(1954). The points are concentrated in the field of
“L”. Only few points fall in the “Ml” field. The sand percentages vary from
67.72 to 98.57, silt from 1.37 to 35.24 and clay from 0.01 to 1.28 percent.
The QFL percentages, when plotted in the QFL triangular diagram shows that
the points concentrated in the field of quartz nrenite. The values of QFL
components varies between 85.90 to 96.61; 2.20 to 11.0 and 0.33 to 8.40
respectively.
The ratio of the chemical composition of Si02 ranges between
86.28 to 97.60 and A1203 ranges between 0.50 to 5.20.
9.4. INTERPRETATION AND CONCLUSION:
From the textural analysis and triangular plot of Sylhet sandstone,
it has been found that the present sandstone belongs to “Sand” and “Silty
sand” group which indicates by their position in “L” and “M l” field. Minor
amount of silt size grains are present in the dominant sand population. Due to
the presence of minor amount of silt size grains to the dominant sand
population, the sand becomes silty sand (Folk, 1954). The clay amount in the
sediments of the present sandstones are very nominal to affect the bulk
constituents. From this, it may be infercd that the present sandstone consists
of sand and silt size materials belonging to mature stage and moderately well
sorted in character. It suggests that the sediments might have travelled long
distance to reach the final site or travelled through a rugged topography for
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short distance or the kinetic energy of the depositing media might be high, for
which the sediment attained maturity and sorting become improved,
The
mineralogical
classification
of
the
present
Sylhet
sandstones indicates that the sandstones belong to quartz arenite group
represented by quartz rich varities of the sandstone , where the quartz
percentages goes above 95 % , It also indicate maturity of the sandstones.
The chemical classification shows that the Si02 percent of the
present sandstone is veiy high, which is above 95 %, also indicate maturity of
the sediments,
J
From the above studies, it may be concluded that the Sylhet
sandstone of the present study area are matured one. They are constituted by
sand size grains with minor amount of silt and clay. The clay portion is very
less. The amount of silt portion is higher than the clay portion.
Mineralogically, the sandstone falls under quartz arenite group. Texturally,
the sandstone fails under sand and silty sand group. The sediments of the
present sandstone belong to mature stage and shows improved sorting, which
may be due to long distance transport of the sediments by the transporting
media.
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