THE SEDIMENTARY TEXTURE [ONLY GRAIN SIZES]

HERE IN THIS PPT WE WILL GET TO KNOW THE THE TEXTURE OF SEDIMENTARY ROCK[ONLY GRAIN SIZE]

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

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

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 ) Conclusion Bibliography

4. 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

5. 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.

6. 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.

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 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.

9. 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 U dden-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. 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

11. 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 .

12. CUMULATIVE WEIGHT PERCENT GRAPH

13. 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 50 th 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.

14. CUMULATIVE PERCENT(ARITHMATIC SCALE)

15. 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.

16. Verbal terms for sorting corresponding to various values of standard deviation

17. 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.

18. 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

19. 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

20. 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.

21. 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.

22. bibliography https://www.slideshare.net/textureofsedimentaryrocks https://www.slideserve.com/saul/sedimentary-textures https://slideplayer.com/slide/9479172/ Principles of sedimentology and stratigraphy by SAM BOGGS JR Sedimentology And Stratigraphy by Gary Nicols