I have to learn

1. TEXTURES
OF
SEDIMENTARY ROCKS

2. SEMINAR PRESENTATION
Kaijirsong Rongpi
Roll No: 377
B.Sc 3rd Semester
Department Of Geology
Arya Vidyapeeth College,
Guwahati

3. 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 )
2. Particle Shape
* Particle Form
* Particle Roundness
* Surface Texture
3. Fabrics
* Grain Orientation
* Grain packing, Grain-Grain Relations & Porosity
 Conclusion
 Bibliography
 Thank You

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

5. CONCEPT ON SEDIMENTARY ROCKS
 Sedimentary rocks are types of rocks that are formed by the deposition and subsequent
cementation of mineral or organic particles on the floor of oceans or water bodies at the
Earth’s Surface.
 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.

6. TEXTURE OF SEDIMENTARY ROCKS
 Sediment texture refers to the Shape, Size 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.

7.  FIGURES SHOWING THE PRIMARY AND SECONDARY TEXTURES

8. 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 0r 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.

9. GRAIN SIZE SCALES
 Particles in sediments and sedimentary rocks range in size from a few
microns 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 almost 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 large, 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

10. PARTICLE SIZE SCALE (UDDEN & WENTWORTH)

11. 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-diffracter size analyser
4. Electro resistance size analyser
5. Image analysis

13. 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: It is essentially a histogram in which a smooth curve takes
the place of a discontinuous bar graph. Connecting the midpoints of each size
class in a histogram with a smooth curves gives the approximate shape of the
frequency curve. An accurate frequency curves can by derived from cumulative
curves by graphical method.
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.

15. 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.

17. 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.

18. VERBAL TERMS FOR SORTING CORRESPONDING TO VARIOUS VALUES OF STANDARD DEVIATION

19. SKEWNESS AND KURTOSIS
 The frequency curves of such non-normal populations are not perfect bell-
shaped curves or they shows some degree of asymmetry or called
skewness. The mode, mean and median in a skewed population of grain
size population are all different. Skewness reflects sorting in the ‘tails’ of grain
size population.

20.  Populations have a tail of excess fine particles are said to be positively
skewed or fine skewed that is skewed toward positive phi values.
 Populations with a tail of excess coarse particles are negatively skewed or
coarse skewed.

21. VERBAL SKEWNESS IS RELATED TO CALCULATED VALUES OF SKEWNESS AS FOLLOWS:

22. KURTOSIS
 Grain size frequency curves can show various degrees of Sharpness of
peakedness. The degree of peakedness is called Kurtosis. Although kurtosis
is commonly along with other grain size parameters, the geological
significance of kurtosis is unknown, and it appears to have little value in
interpretative grain size studies.

24. 2. Particle Shape
 Particle Shape is defined by three related but different aspects of grains.
They are the following:-
1. Form: It is refers to the gross, overall configuration of particles and reflects
variations in their proportions. The form of some particles resembles that of a sphere
and other particles may have a platy or rod-like form.
2. Roundness: It is refers to the sharpness of the corners. Well-rounded grains have
smooth corners and edges and poorly-rounded grains have sharp or angular corners
and edges.
3. Surface Texture: It is refers to Small-Scale, micro-relief markings such as pits,
scratches and ridges that occur on the surface of the grains.

26.  The Shape of minerals and clasts in sedimentary rocks are determined by a
variety of factors
1. The original shape of mineral grains in the source rocks
2. The orientation and the spacing of fractures in bedrock that influence the
shapes that clasts take on when they weather from exposed rock.
3. The nature and intensity of sediment transport, which can abrade grains
and
change original shapes.
4. Sediment burial processes such as compaction which can also change
original
shapes.

27. 3. FABRIC
 The fabric of sedimentary rocks is a function of grain orientation and packing
and is thus a property of grain aggregate. Orientation and grain packing in
turn control such physical properties of sedimentary rocks as bulk density,
porosity and permeability.
 Grain Orientation : Particles in sedimentary rocks that have a platy shape or
an elongated shape commonly show some degree of preferred orientation.
Platy particles tend to be aligned in planes that are roughly parallel to the
bedding surfaces of the deposits. Elongated particles show a further
tendency to be oriented with their long axes pointing roughly in the same
direction. The preferred orientation of these particles is caused by the
transport and depositional processes and is related particularly to flow
velocities and other hydraulic conditions at the depositional site.

29. GRAIN PACKING, GRAIN-GRAIN RELATIONS AND POROSITY
 Grain Packing: It is refers to the spacing or density patterns of grain size,
shape and the degree of compaction of the sediment. Packing strongly
affects the bulk density of the rocks as well as their porosity and permeability.
The effects of packing on porosity can be illustrated by considering the
change in porosity that takes place when even size spheres are rearranged
from loosest packing to the tightest packing.

30. CUBIC PACKING
The arrangement in space of uniform spheres ( atoms and molecules in mineral crystals, or
grains in clastic sedimentary rocks ) that results in a cubic material structure. Cubic Packing is
mechanically unstable, but it is the most porous packing arrangement, with 47% porosity in the
ideal situation. Most sediments are not uniform spheres of the same size, nor can they be
arranged in a cubic structure naturally, so most sediments have much less than 47% porosity.
RHOMBOHEDRAL PACKKING
The most compact arrangement in space of uniform spheres ( atoms and molecules in mineral
crystals, or grains in sedimentary rocks ) that results in a structure having no more than 26%
porosity. Rhombohedral packing is more stable mechanically than cubic packing. Cubic
packing is the most porous packing, with about 47% porosity in the ideal solution. Most
sediments, however are not uniform spheres of the same size nor can they be arranged in a
cubic structure naturally, so most sediments have much less than 47% porosity of ideal
rhombohedral packing

31.  Grain-Grain relations: Taylor identified four types of contacts between grains that
can be observed in thin sections.
 Tangential Contacts
 Long Contacts
 Concavo-Convex Contacts
 Sutured Contacts
Tangential contacts occur only in loosely packed sediments or sedimentary rocks,
whereas Concavo-Convex contacts and sutured contacts occur in rocks that have
undergone considerable compaction during burial.

33.  Porosity:
Poorly sorted sediments tend to have lower porosities and permeability's than
well-sorted sediments because grains are packed more tightly in these sediments
owing to finer sediment filling pore spaces among larger grains. Petroleum and
groundwater geologists are especially concerned with the porosity of sedimentary
rocks because porosity determines the volume of fluids that can be held within a
particular reservoir rock. Compaction causes major reduction in porosity.
The percentage of pore volume or void space or that volume within rock that
can contain fluids. Porosity can be a relic of deposition ( primary porosity , such
as space between grains that were not compacted together completely ) or can
develop through alteration of the rock ( Secondary porosity, such as when
feldspar grains or fossils are preferentially dissolved from sandstones ). Porosity
can be generated by the development of fractures in which case it is called
Fracture porosity. Effective porosity is the interconnected pore volume in a rock
that contributes to fluid flow in a reservoir, It excludes isolated pores. Total
porosity is the total void space in the rock whether or not it contributes to fluid
flow. Thus, effective porosity is typically less than total porosity. Shale gas
reservoir tend to have high relatively porosity, but the alignment of platy grains
such as clays makes their permeability very low.

34.  Permeability :
It is the property of the rocks that is an indication of the ability for fluids ( gas or liquid ) to
flow through rocks. High permeability will allow fluids to move rapidly through rocks.
Permeability is affected by pressure in a rock. A rock with 25% porosity and a permeability
of 1md will not yield significant flow of water. Such tight rocks are usually artificially
stimulated to create a permeability and yield a flow.
It can also be described as the measure in which the ease with which a fluid can move
through
a porous rock.

35. 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.

36. BIBLIOGRAPHY
https://www.slideshare.net/textureofsedimentaryrocks
https://www.wikipedia.org/textureofsedimentaryrocks
Principles of sedimentology and stratigraphy by SAM BOGGS JR

37. THANK YOU