THE SEDIMENTARY TEXTURE [ONLY GRAIN SIZES]

SEDIMENTARY TEXTURE
{GRAIN SIZE}
M.SC SEMINAR
BY
BISWARANJAN DEO
Department of Geology
KHALLIKOTE UNITARY UNIVERSITY
BERHAMPUR, GANJAM

SEMINAR PRESENTATION
BY:- BISWARANJAN DEO
REGD. NO:-PG2302GEO006
ROLL NO:- GEOL23-023
PG 1ST
YEAR GEOLOGY

CONTENTS
 Introduction on Petrology
 Concept on Sedimentary Rocks
 Texture of Sedimentary Rocks
1. Grain Size
* Grain size scales
* Measuring Grain Size
* Graphical And Mathematical treatment of Grain-Size data
* Mathematical Measures ( Sorting, Skewness & Kurtosis )
 Conclusion
 Bibliography

INTRODUCTION ON PETROLOGY
 Petrology is the branch of Geology that deals with the
study of Rocks and under which conditions they were
formed.
 These term ‘Petrology” is again sub-categorized into 3
types of Rocks and they are namely given below –
Igneous Rocks
Sedimentary Rocks
Metamorphic Rocks

CONCEPT ON SEDIMENTARY ROCKS
 The rocks which are formed by the action of weathering ,erosion,
transportation and deposition of any pre existing rock by the
action of wind,water,glacier is known as sedimentary rock.
 The particles that form a sedimentary rock are sediment, and
may be composed of geological detritus (minerals) or biological
detritus (organic matter).
 The scientific discipline that studies the properties and origin of
sedimentary rocks is called sedimentology and it is both part of
geology and physical geography.

TEXTURE OF SEDIMENTARY ROCKS
Sediment texture refers to the shape, size,packing,fabric and three
dimensional arrangement of the particles that makeup the sediment or
sedimentary rocks.
 Two main natural textural groupings exist for sedimentary rocks:
Clastic (or fragmental) and Non-Clastic (essentially crystalline).
 Textures are primary, where the grains possess their arrangement that
existed after they came to rest (or after precipitation in the case of
crystals).
 In sedimentary rocks, however, textures are commonly secondary,
because they have been altered in some way from their original
condition. The most common effect is compaction, where the weight of
overlying sediments causes the component grains to rearrange
themselves or even become fractured.

FIGURES SHOWING THE PRIMARY AND SECONDARY
TEXTURES

1. GRAIN SIZE
 Grain size is a fundamental attribute of siliciclastic sedimentary rocks and thus one of the
important descriptive properties of such rocks. The sizes of particles in a particular deposit
processes, which generate particles of various sizes, and the nature of subsequent transport
processes. Grains can range in size from clay-size particles that require a microscope for
clear visualization to boulders several meters in diameter.
 Sedimentologists are particularly concerned with two aspects of particle size.
1.Techniques for measuring grain size data and expressing it in terms of some type of
grain size or grade scale.
2. Methods for summarizing large amounts of grain size data and presenting them in
graphical or statistical form so that they can be more easily evaluated.

GRAIN SIZE SCALES
 Particles in sediments and sedimentary rocks range in size from a few
micro-meter to a few meters. Due to wide range of particles sizes ,
logarithmic or geometric scales are more useful for expressing size than
linear scales.
 The grain-size scale used universally by sedimentologists is ‘The Udden-
Wentworth’. It was proposed by Udden & Wentworth and it is a
geometric scale in which each value in the scale is either twice as large as
the preceding the value or one half as larger, depending upon the sense of
direction.
 A useful modification of the Udden-Wentworth scale is logarithmic Phi
Scale, which allows grain size data to be expressed in units of equal value
for the purpose of graphical plotting and statistical calculations. This scale
is proposed by Krumbein in 1934 and is based on the relationship.
(phi) Φ = -log2 d

THE SEDIMENTARY TEXTURE [ONLY GRAIN SIZES]

Measuring grain size
 Grain size of small, unconsolidated particles can also be measured by
sedimentation techniques on the basis of the settling velocity of the
particles. In these techniques, grains are allowed to settle through a
column of water at a specified temperature in a settling tube and the time
required for the grains to settle is measured. For coarser particles , the
settling time of the particles is related empirically to a standard size
distribution curve to obtain the equivalent millimeters or phi size.
 The Grain of fine slit and clay particles can be determined but by
sedimentation methods based on Stokes’ law. The standard sedimentation
method for measuring the sizes of these small particles is Pipette analysis.
It is a laborious process because of the many operations involved.
 Different other types of size-Analyzers are also determined and they are:
1. Photohydrometer
2. Sedigraph
3. Laser-diffraction-size analyser
4. Electro resistance size analyser
5. Image analysis

GRAPHICAL AND MATHEMATICAL TREATMENT OF
GRAIN-SIZE DATA
Measurement of grain size by the technique described generates large quantities of
data that must be reduced to a more condensed from before they can be used.
Graphical Plots: There are three common graphical methods for presenting grain-
size data.
1. Histogram: a bar diagram in which grain size is plotted along the abscissa of
the graph and individual weight percent along the ordinate. This provides a
quick and easy pictorial method for representing grain size distribution because the
approximate average grain size and the sorting-the spread of grain size values
around the average grain size- can be seen at a glance. Histogram have limited
application, however because of the shape of the histogram is affected by the sieve
interval used. They cannot be obtained mathematical values for statistical
calculations.
2. Frequency Curve: A frequency curve in a grain size distribution graph
represents the cumulative or relative frequency of different grain sizes within a
sample.
3. Cumulative Curve: It is generated by plotting grain size against cumulative
weight percent frequency. The cumulative curve is the most useful of the grain size
plots. A cumulative curve can be plotted on an arithmetic ordinate scale or on a log
probability scale in which the arithmetic ordinate is replaced by a log probability
ordinate.

CUMULATIVE WEIGHT PERCENT GRAPH

Mathematical Measures
 Average grain size: The mathematical measures of average grain size are in
common use. And they are the following:
1. Mode size
2. Median Size
3. Mean Size
 The mode size is the most frequently occurring particle size in population of grains.
The diameter of the modal size corresponds to the diameter of grains represented by
the steepest point on a cumulative curve or the highest point on a frequency curve.
Siliclastic sediments and sedimentary rocks tend to have a single modal size, but
some sediments are bimodal, with one mode in the coarse end of the size
distribution and one in the fine end and other are even polymodal.
 The median size is the midpoint of the grain size distribution. Half of the grains by
weight are larger than the median size and half are smaller. The median size
corresponds to the 50th
percentile diameter on the cumulative curve.
 The mean size is the arithmetic average of all the particles in a sample. The true
arithmetic mean of most sediment samples cannot be determined because we cannot
count the total number of grains in a sample or measure each small grains size.

CUMULATIVE PERCENT(ARITHMATIC SCALE)

SORTING
The sorting of a grain population is a measure of the range of grain sizes present and
the magnitude of the spread or scatter of these sizes around the mean size. Sorting can
be estimated in the field or laboratory by use of a hand-lens or microscope and
reference to a visual estimation chart. The mathematical expression of sorting is
Standard Deviation. The standard deviation calculated is expressed in phi scale values
and is called Phi standard deviation. The phi symbol must always be attached to the
standard deviation value.

VERBAL TERMS FOR SORTING CORRESPONDING TO VARIOUS VALUES OF
STANDARD DEVIATION

SKEWNESS
 Skewness is a statistical measure that describes the asymmetry of a distribution around
its mean. It indicates whether the data points are spread out more to the right or to the
left of the mean. There are three types of skewness:
1. Zero Skewness[Symmetrical Distribution]: The data points are symmetrically distributed
around the mean, meaning the tails on both sides of the mean are balanced and equal in
length.

2. Positive Skewness[Right-Skewed]:The right tail{larger value}is longer or fatter than the left
tail.Most of the data points are concentrated on the left,with a few larger values stretching out
to the right.
3. Negative Skewness[Left-Skewed]: The left tail{smaller values}is longer or fatter than the
right tail.Most of the data points are concentrated on the right, with a few smaller values
stretching out to the left.
-- ve (Coarse)) Skewed + ve (Fine) Skewness

VERBAL SKEWNESS IS RELATED TO CALCULATED
VALUES OF SKEWNESS AS FOLLOWS:
>+0.30 Strongly fine skewed
+0.30 to +0.10 Fine Skewed
+0.10 to –0.10 Near symmetrical
--0.10 to –0.30 Coarse skewed
< --0.30 Strongly coarse skewed

KURTOSIS
 Grain size frequency curves can show various degrees of sharpness of peakness.
The degree of peakness is called Kurtosis. Kurtosis is a measure of wheather the
data are heavy tailed or light tailed relative to a normal distribution.This of three
types:
1. Mesokurtic:This distribution is similar to that of the normal distribution. A
mesokurtic distribution is medium tailed,so outliers are neither highly frequent nor
highly infrequent. And here the K value is 3.
2. Leptokurtic:Any distribution that is leptokurtic displays greater kurtosis than a
mesokurtic distribution.Here the K value is more than 3. The prefix of lepto means
skinny or narrow making the shape of the distibution easier to remember.
3. Platykurtic:This distribution is thin-tailed meaning this outlier are infrequent.
And platykurtic distribution have less kurtosis than a normal distribution .In other
words,platykurtic distribution have K value is less than 3.The term platy in
platykurtosis is comes from the greek word platus.Which means flat so this graph
has a flatted peak.

CONCLUSION
 From the above discussion, we can state the conclusion that
sedimentary textures helps studies about the size, shape and
fabrics that lets us know about the sharpness , roundness of
grain size.
 Also we know about the sorting of grain whether it is well-
sorted, poorly-sorted that happens to have voids space more or
less depending on the sorting habit , it developed.

BIBLIOGRAPHY
1. https://www.slideshare.net/textureofsedimentaryrocks
2. https://www.slideserve.com/saul/sedimentary-textures
3. https://slideplayer.com/slide/9479172/
4. Principles of sedimentology and stratigraphy by SAM BOGGS JR
5. Sedimentology And Stratigraphy by Gary Nicols

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