Sedimentary Texture of Sedimentary Rocks.pptx

Sedimentary Texture

1. Texture of Sedimentary Rocks

2. It deals with the size, shape and arrangement of the component minerals of a rock It emphasizes on grain to grain relationship, like their contacts, orientations, packing etc. Generally concerns with the clastic rocks; in case of chemical precipitates grains (or crystals) remains in contact with each other, showing a interlocking texture like an igneous or a metamorphic rock; chemogenic sedimentary rocks differ in having very restricted mineralogical variation compared to other rock types; mostly monomineralic The texture of sedimentary rocks affects such derived properties of rocks as porosity, permeability, bulk density, electrical conductivity, and sound transmissibility Encompasses three fundamental properties of sedimentary rocks: grain size, grain shape (form, roundness and surface texture [microrelief] of grains) and fabric (grain packing and orientation) Grain size and shape are properties of individual grains Fabric is a property of grain aggregates What is Texture

3. Three principle axes, major, intermediate and minor Nominal Diameter – diameter of the sphere having the same volume as the grain Intercept Diameter – maximum diameter encountered in a thin section and observed through petrographic microscope Settling Diameter –used for silts and clays; based on Stokes’ Law; diameter can be calculated by the equation v = cgR 2 ∆ρ/μ where, v = settling velocity; R = grain diameter; c = 2/9; g= gravitational acceleration; ∆ρ = density contrast between settling grain and fluid; μ = viscosity of the fluid. During free settling frictional force and gravitational pull act oppositely and are equal. So, 6 πRvμ (Frictional Force) = 4/3 πgR 3 ∆ρ (Gravitational Pull) The practical approach (for sand and gravels) is to take a representative sample of the sediment population and run them through a set of sieves to break the sample subset in to size classes and using statistics reconstruct what the population’s size characteristic are. Grain Size

4. Presentation of Grain-size distribution data Udden-Wentworth Grain-size Scale Grain-size are usually measured in mm-scale. Distribution of grain-size within a population presented on a logarithmic scale. In practice, grain-size is represented by the phi (φ) scale (after Folk and Ward, 1957), where, φ = -log 2 d Wentworth’s scale is based on logarithms to the base 2, and after the modification of Udden, this is now the most widely used grain-size scale The simplest way of presenting grain-size distribution data is by means of histograms , taking grain-size classes (in φ scale) in x-axis and percentage (or number) of grains within each class in y-axis. These show the percentage, by weight, of the grains falling within each chosen subdivision of the size range.

5. A cumulative distribution curve can be prepared by plotting grain-size against cumulative frequencies. This kind of curve represent what percentage of a sample is larger or smaller than a particular grain size. In case of probability cumulative curve, probability ordinate is used instead of linear one, to straighten the curve. Histogram Cumulative distribution curve Probability cumulative curve

6. Grain 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 by using a hand lens or microscope and reference to a visual estimating chart More accurate determination of sorting requires mathematical treatment of grain-size data, or grain-size analysis Graphical standard deviation is the best measure of overall sorting The grain-size of sediments, in a broad sense, reflects the hydraulic energy at the time of sedimentation If the energy decreases gradually, sediments tend to segregate according to their size, as deposition of different-sized particles toke place under different hydraulic conditions Such size-wise segregation produces sorting Sorting requires slow rate of sedimentation and longer transportation, as both allow longer time for segregation, and also efficiency of the transporting medium, so that vertical movement within the medium remain effective during transportation

7. A cumulative distribution curve can be prepared by plotting grain-size against cumulative frequencies. This kind of curve represent what percentage of a sample is larger or smaller than a particular grain size. In case of probability cumulative curve, probability ordinate is used instead of linear one, to straighten the curve. Histogram Cumulative distribution curve Probability cumulative curve Grain Size Analysis

8. Statistical parameters to represent grain-size distribution data Statistical parameters used in grain-size analysis includes – Mode – most frequently occurring particle diameter; corresponding to the steepest point (point of inflection) on the cumulative curve and highest point on the distribution curve Median – diameter which divide the population in such a way that 50% by weight of the sample grains are smaller, and 50% are larger; corresponding to the 50% mark on the cumulative frequency curve Graphic (Geometric) Mean –arithmetically calculated average grain-size diameter; best graphic measure for determining overall size; can be calculated from cumulative frequency curve through the simple formula, M z = ( φ 16 + φ 50 + φ 84 )/3 Only in cases of completely symmetrical distribution, the mean diameter (M z ), the mode diameter (Mo) and the median diameter (Md) coincide The mean will otherwise shift further than the median in the direction of the “tail” of the distribution

9. Inclusive Graphic Standard Deviation – best overall measure of sorting; general formula runs, σ I = ( φ 84 - φ 16 )/4 + (φ 95 – φ 5 )/6.6 Statistical analysis of several population reveals very well sorted - under 0.35 φ well sorted - 0.35 to 0.50 φ moderately well sorted - 0.50 to 0.71 φ moderately sorted - 0.71 to 1.0 φ poorly sorted - 1.0 to 2.0 φ very poorly sorted - 2.0 to 4.0 φ extremely poorly sorted - over 4.0 φ Best sorting attained by natural sediments is about 0.20-0.25 φ, observed in eolian dunes and coastal sands Poorest sorting observed in glacier tills ranges from 5 φ to 8 φ, or even 10 φ River sands has a more wider range, from 0.4 φ to 2.5 φ, even 3.5 φ

10. Graphic Kurtosis – an expression for the spread of the extreme ends of a grain-size distribution curve in relation to the central part; measures peakedness of a probability curve, generated by joining the mid-points of each class in a histogram. In case of normal probability curve (Symmetrical, bell-shaped), defined by the gaussian formula, the phi diameter interval between the 5 phi and 95 phi points should be exactly 2.44 times the phi diameter interval between the 25 phi and 75 phi points. Kurtosis actually measures the deviation from the normal curve. General formula runs, K G = ( φ 95 – φ 5 )/2.44 (φ 75 – φ 25 ) Hence, for normal distribution kurtosis is 1.00 If the central portion is better sorted than the tails, the curve is said to be excessively peaked or leptokurtic; if the tails are better sorted than the central portion, the curve is deficiently or flat-peaked and platykurtic. Statistical analysis of several population reveals very platykurtic under 0.67 platykurtic 0.67 to 0.90 mesokurtic 0.90 to 1.11 leptokurtic 1.11 to 1.50 very leptokurtic 1.50 to 3.00 extremely leptokurtic over 3.00 Kurtosis values in natural sediments varies between 0.85 to 1.4

11. Inclusive Graphic Skewness – a measure of asymmetry of the grain-size population; general formula runs, SK I = ( φ 84 + φ 16 – 2φ 50 )/2(φ 84 - φ 16 ) + (φ 95 + φ 5 - 2φ 50 )/2(φ 95 – φ 5 ) Symmetrical curves have skewness 0.00 If the sample has a wide spread (tail) towards the fine grain sizes (larger phi values) and a relatively sharp delimitation at the coarse grain-size end, we say that the sample has positive skewness or fine-tailed Similarly, those with excess coarse material (a tail to the coarser grain-size) have negative skewness or are coarse-tailed More the skewness value departs from 0.00, the greater the degree of asymmetry Statistical analysis of several population reveals strongly fine-skewed +1.00 to +0.30 fine-skewed +0.30 to +0.10 near symmetrical +0.10 to -0.10 coarse-skewed -0.10 to -0.30 strongly coarse skewed -0.30 to -1.00 This is the best skewness measure to use because it determines the skewness of the "tails" of the curve, not just the central portion Furthermore, it is geometrically independent of the sorting of the sample

12. Grain Shape Encompasses all aspects of the external morphology of particles, including form, roundness, and surface texture Form refers to the gross, overall morphology or configuration of particles Roundness is a measure of the sharpness of the corners of a grain, commonly measured in two dimensions only Surface texture refers to microrelief features, such as scratches and pits, that appear on the surfaces of clastic particles These three shape properties of grains are independent parameters Form is a first-order property, inherited by the grain from the precursor rock mass, and undergo little changes during transportation Roundness, on the other hand, is a second-order property superimposed on form; changes along with transportation due to chiefly abration of the corners Surface texture is a third-order property superimposed on both during transportation as well as during diagenesis

13. In practice, form is often approximated by the term Sphericity , referring to the degree to which the shape of grains approaches the shape of a sphere (Wadell, 1932) Basically it is the ratio between the surface area of a sphere having the same volume to that of the grain concerned and the actual surface area of the grain As surface area of irregular grains can be hardly measured, Wadell proposed a practical Operational Sphericity (ψ), represented by ψ = 3 √(Volume of the smallest circumscribing sphere/volume of the grain) The commonest method is to determine Intercept Sphericity (ψ I ) ψ I = 3 √(D S D I /D L 2 ) [Krumbein, 1941] Another method is to determine Maximum Projection Sphericity (ψ P ) ψ P = 3 √(D S 2 /D L D I ) [Sneed & Folk, 1958] where, D L , D I and D S refers to the length of the long, intermediate and short particle axis respectively Form

14. Shape classification of Zingg (1935) and Sneed and Folk (1958) appears to be the best alternative expression than sphericity to describe form Zingg’s shape classification is quite similar to that of sphericity; derived by plotting on a bivariate diagram the ratio of the intermediate to long particle axis versus the ratio of the short to intermediate particle axis Four shape fields or four types of particle shapes are identified: roller or prolate, bladed, oblate and equant Sneed & Folk used a triangular diagram, whose axes are complex Ten basic shape fields have been identified where, V = very, B = bladed, C = compact, E = elongated, P = platy Form of a grain is dependant upon the fracture system and weathering in provenance and indicate nothing about depositional Mechanism

15. Roundness Roundness is more significant than sphericity as a descriptive parameter Roundness was first defined by Wentworth (1919) as "the ratio of the average radius of the sharpest corner to the radius of the largest inscribed circle" This definition was modified by Wadell (1932) who proposed that roundness is the arithmetic mean of the ratio of the radius of curvature of the corners to the radius of the maximum-size circle that can be inscribed within the outline of the grain in the plane of measurement Roundness is a function of the transport history of the sediment During sediment transport individual clasts will repeatedly come into contact with each other and stationary objects: and as a consequence the abrasion smoothened the surface of the clast progressively, starting from the sharp edges first Roundness is normally visually estimated, but may also be calculated from the cross-sectional shape of a clast Roundness (R w ) can be expressed as R w = ∑(r/R) / N where, r is the radius of curvature of individual corners (the radius of a circle that fits within the corner), R is the radius of the maximum inscribed circle and N is the number of corners

16. Usually roundness is represented by five well established broad pictorial categories Visual assignment of any grain to one of these category is sufficient enough for simple purposes Roundness indicates prolonged transportation Well rounded clasts also requires reworking of sediments as such rounding is not possible to develop during a single transportation from provenance to sink

17. Surface Texture The minor or microrelief features of the grain surface, which are independent of its size, shape and roundness Some are visible to naked eyes; others are discernible only under microscope Surface textural studies are boosted after the advent of Scanning Electron Microscope (SEM) Surface textures can be produced during and/or after the deposition of sediment, or even during diagenesis Microreliefs are diverse in nature, but can be grouped under two major categories; one deals with the surface lustre, while other concerns the markings on the surface Surface Lustre Polish and Frost are two major varieties; while polished surface produce regular reflection of light, frosted (or matte) surface produce scattering Polish can be produced mechanically, by gentle attrition or wear, particularly by fine abrasive agents: such as the wind polish over quartzites in desert (ventifacts) Chemical polish is also apparent by the deposition of a vitreous film or glaze: desert varnish is a good example, where hard coatings of silica, iron, and some manganese-oxide forms over the grains High polish is known as gloss, and is an exceptional phenomenon

18. Frosted or matte surfaces are more common in nature It is now well established that, although the coarser-textured features may be produced mechanical attrition, the fine-textured microrelief, in order of 2 microns or less (the typical frosting) is chemically produced by alternation of wetting and drying related to dew formation and evaporation Such chemical frosting affects entire grains, even the recesses in the grain surface Surface Markings Well known surface markings include striations, scratches, percussion marks and indentations or pits Striations are ideally narrow, straight, or nearly stratight scratches clearly cut into the surface Other scratches includes bruises, nailhead scratches (with a definite head) etc, which are rather crude compared to striations These are mainly produced by the action of ice, generally glaciers Percussion marks are the crescentic impact scars probably caused by the blows of high velocity flows (river water) on the surface Indentations or pits are most common and may be produced by etching and differential solution of inhomogeneities of the rockmass Pitted surface posses concavities not related to the texture of the rock or to differential weathering

19. Fabric Fabric refers to the textural characteristics displayed by aggregates of grains Fabric encompasses two properties of grain aggregates: grain packing or arrangement and grain orientation Fabrics can either be produced during sedimentation or later during burial (diagenetic) or tectonic (post-lithification) processes Grain packing is a function of the size and shape of grains and the post- depositional physical and chemical processes that bring about compaction of sediment Grain orientation is mainly a function of the physical processes and conditions operating at the time of deposition However, original grain orientation can be modified after deposition by the activities of organisms (bioturbation) and to some extent by the processes of compaction during diagenesis

20. Grain Packing Packing can be best defined as the manner of arrangement or spacing of the solid clastic particles in a sediment or sedimentary rock, apart from any authigenic grains that may have crystallized between them Packing is regarded to be a function of several variables or properties, including particle size and sorting, particle shape, and particle orientation or arrangement The amount of fine-grained matrix and the matrix–grain relationship affect the packing Where grains in a sediment are in contact, the sediment is grain-supported or clast-supported (term used for conglomerates mainly) Where the grains are not in contact, the sediment is matrix-supported ; the matrix itself can also be well-sorted or poorly sorted

21. In case of grain-supported fabric, contacts between the grains becomes important Major four kinds of contacts are discernible; point or tangential, long, concavo-convex and sutured If grains are separated from each other by intervening matrix, as in case of matrix-supported fabric, then the grains are referred to as ‘floating grains’ Relative abundances of these various types of contacts can be used as a rough measure of the degree of compaction and thus the depth of burial Point or tangential contacts occur only in loosely packed sediments or sedimentary rocks, whereas sutured contacts within a rock body imply considerable compaction during burial Two packing indices are in common use now; contact index (= average number of contacts/grain) and the tight packing index (= average number of long, concavo–convex and sutured contacts/grain)

22. The matrix-grain inter-relationship sometimes are interpretative about the broad depositional conditions In general, grain-supported fabric with no mud indicates reworking by currents and/or waves/wind, or deposition from turbulent flows where suspended sediment (mud) is separated from coarser bed load Packing is important because it directly influence the porosity and in turn the permeability of sediments and sedimentary rocks As a consequence, porosity and permeability changes as a function of burial depth and sediment compaction Packing also indirectly influence the petrophysical properties and the geomechanical behavior of rocks Porosity and permeability are important secondary, or derived, properties of sedimentary rocks Porosity is defined as the ratio of pore space in a sediment or sedimentary rock to the total volume of the rock Porosity can be of two major types ; Primary and Secondary Primary porosity is chiefly controlled by physical properties of the rock, such as grain size, sorting, shape and packing or grain arrangement Packing is most important among them; cubic packing is loosest which produce about 47.6% porosity, whereas rhombohedral packing is tightest with 26% porosity Permeability is defined as the ability of a medium to transmit a fluid

23. Grain Orientation Grain Orientation Platy, flaky, or elongated particles in sedimentary rocks commonly display some degree of orientation that reflects the nature of the depositional process Such orientation can be shown by prolate pebbles in a conglomerate or breccia, and fossils in a limestone, mudrock or sandstone Sandstones also can show a preferred orientation of elongate sand grains but microscopically only Small platy or flaky particles settling from suspension onto a flat bed in the absence of current flow are commonly deposited with their flattened dimensions parallel to bedding surfaces Small elongated grains settling under the same conditions also tend to have their long dimensions oriented approximately parallel to the bedding surface; however, the grains may have any orientation (random arrangement) within the bedding plane Elongated pebbles or fossils deposited under flowing current generally display an orientation that reflects the direction of the depositing current Preferred orientations can be both parallel to, and normal to, the flow direction Orientations of particles arise from interaction with the depositional medium (water, ice, wind)

24. Elongated pebbles (or fossils) carried as traction load, used to roll with long axis perpendicular position and settles over the intermediate axis, hence they show flow perpendicular alignment in bedding plane If carried as suspension load similar pebbles (or fossils) flow making long axis parallel to the flow and whenever settles remain elongated along the flow direction on bedding plane Elongated pebbles or fossils sometimes show imbrication, where elongated pebbles (or fossils) overlap each other (like a pack of cards), dipping in an upstream direction Imbrication resulted from scouring in upcurrent side of the settled pebble by flowing current as well as by the basal friction during time of settling Orientation, especially imbrication, can be a useful texture for deducing the palaeocurrent direction Preferred orientations can also be tectonically induced, but restricted to deformed areas

25. Textural Maturity Textural maturity is a concept which is based upon the basic idea that with longer transportation as well as reworking physical parameters such as roundness and sorting changes to attend most stable end product Nearer to that goal, more matured is the sediment This concept is mostly used for sandstone Folk (1951) suggested that textural maturity of sandstones encompasses three textural properties: (1) the amount of clay-size sediment in the rock, (2) the sorting of the framework grains, and (3) the rounding of the framework grains Four stages of textural maturity: immature, submature, mature, and supermature Any sandstone containing considerable clay, say more than 5 percent, is in the immature stage; the framework grains also are poorly sorted and poorly rounded In the submature stage, sediments are characterized by low clay content but grains are still not well sorted or well rounded The following mature stage is characterized by low clay content along with well sorted but are not yet well rounded framework grains

26. Sediments in the supermature stage are essentially clay-free, and framework grains are both well sorted and well rounded The transformation from immature to supermature stage requires increasing transportation and reworking Although clay within sandstone can be introduced during diagenesis also, the overall maturity concept holds good for majority of cases

27. Folk (1951) also proposes a diagram showing particular environments with sands of a characteristic maturity Though the diagram is not very much popular in recent times as it is now established that no environmental constrains are there over textural maturity, it gives a broad idea about the nature of sandstones present in different broad environments Marine sediments appear to be more matured than fluvial ones Eolian and beach sandstones are the only supermatured end members