Chapter III. GRAIN-SIZn; ANALYSIS OF SANDSTONES INTRODUCTION. This chapter deals with grain-size characteristics of

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1 Chapter III GRAINSIZn; ANALYSIS OF SANDSTONES INTRODUCTION This chapter deals with grainsize characteristics of the terrigenous clastic sediments of the Jaisalmer Formation, The grainsize study was undertaken with a view to interjsretinq the depositional processes and environments in the Jaisalmer Basin. The genetic interpretation of grainsize characteristics of a sediment has proved to be a challenging task over the years. The extended efforts to study this aspect by a large number of workers have produced voluminous literature, Polk (966), Visher (969) and i''riedman (979) published excellent reviews of grainsize parameters and their relationship with depositional processes. in general grainsize studies have followed two main genetic approaches : (i) relating the grainsize to sediment dynamics and depositional processes; (ii) relating it to specific sedimentary environments on the basis of empirical study of sediments from various modern geomorphic environments. Some workers have also emphasised the role of source materials and sediment generative processes in the generation of grainsize distribution.

2 92 Many grainsize distributions are mixtures of two or more subpopulations which are products of different modes of sedimentary transport (Doeglas^ 946; Harris 958; Moss, 962, 963, 972; Visher, 969). Opinions differ on the nature of subpopulation that is, whether they are lognormal truncated (Sindowski, 957; Visher, 969) or lognormal overlapping (Tanner, 964; Puller, 96; Walger, 965; Spencer, 963; Middleton and Southard, 977). Visher's (969) outstanding work on grainsize distribution in relation to depositional processes was based on extensive textural study o± both modern and ancient sands and it provided the basis for a genetic interpretation of grainsize, Visher's work was extended and developed by his coworkers (Visher and Howard, 974; Freeman and Visher, 975; Sagoe and Visher, 977). Visher and his coworkers concluded that each lognormal grainsize distribution curve comprised a number of straightline segments of different slope separated by sharp 'breaks'. The straightline segments represented truncated lognormal subpopulations generated by different modes of sediment transport and deposition. The number, amount, sizerange, mixing, and sorting of these subpopulations varied systematically in relation to provenance, sedimentary processes, and sedimentary dynamics. The nature of the straightline segments and 'breaks' were accordingly related to the importance of the various transport mechanisms operating

3 during deposition, presumed distinctive in different 93 depositional environments. Several workers used Visher's approach in the recognition of ancient depositional environment (Holmes and Oliver, 973; Glaister and Nelson, 974; Amaraland Pryor, 977, Moshrif, 980). Many workers attempted direct interpretation of depositional environments on the basis of statistical measures of grainsize, such as mean, standard deviation, skewness and kurtosis (folk and Ward, 957; Mason and i'olk, 958; Harris, 959, Friedman, 96, 962, 967; Sahu, 964; Moiola and Weiser, 968; Duane, 964; Solohub and Klovan, 970; Buller and Mc Manus, 972; Reed_et_al,, 975; Staper and Tanner, 975; Valia and Cameron, 977). Most of these studies used scatter diagrams involving two variables to distinguish the various environments but some workers employed, more sophisticated techniques of discriminant analysis involving more than two variables at a time. A useful method of biv'ariate analysis, CM plots, was introduced by Passega (957) for distinguishing the various depositional processes. The approach ot directly interpreting the agent and/or environment of deposition on the basis of grainsize distribution is not wholly valid. It is doubtful that a particular environment is characterized by a completely unique pattern of grainsize distribution because similar sediment transport and depositional mechanisms may operate in different environments (Reed^ al,, 975; Ruzyla, 977;

4 94 Steidtmann, 977; Jackson, 978; Sedimentation Seminar, 98). Moreover/ grainsize distribution is also affected by provenance and sediment generative processes as well as by diagenetic modifications. Despite these ambiguities, most sedimentologists agree that grainsize distribution does reflect the depositional processes and enables reconstruction of ancient depositional environments provided this property is studied in combination with other sedimentary properties. MtiTHUDS OF STUDY AND DATA PRESENTATION A total of 4 5 sandstone samples of the Jaisalmer Formation were employed for grainsize analysiso 39 samples from the Fort Member were selected in such a way as to bring about a uniform coverage, Ijoth horizontally and vertically, of the outcrops of the Fort Member. In addition to these samples, 4 samples from the Hamira Men±)er and 2 samples from the Joyan Member were also studied. The seiving technique was employed for grainsize analysis because the sandstones are soft, friable and capable of complete disaggregation. The samples were carefully disaggregated in a morta,r by a rubber padded pestle, and 50 to 200 gms weight of each sample was taken for sieving. Sahu (964) suggested 40 gms as the optimum weight of a sample for sieving. Sieving of weighed samples was carried out following the standard method of sieve analysis with the help of ASTM sieves and automatic sieve shaker. Sieves were arranged at quarter interval and eighteen sieves, ranging from mesh nos, 6 to 230 (size from 0.25 to 4.0 <li or.9 mm to mm) were used.

5 Each fraction of the samples obtained on the sieves was weighed to the 2nd place of decimal by chemical balance, and its weight 95 percentage was calculated (Appendix I). Arithmetic probability graph paper was employed for constructing logprobability curves of the grainsize data. The logprobability plots were preferred to cumulative frequency curves as the former allow for easy comparisons and measurements and are also believed to be meaningful with regard to depositional processes. Grainsize analysis in the present study was based on approaches described by Visher (969), Folk and Ward (957), Passega (957, 964) and Stewart (958). Visher's (969) method was employed to recognise the various subpopulations (surface creep, saltation, suspension) represented by straightline segments on the logprobability curve of a grainsize distribution. Percentages of each subpopulation were determined from the curve. Sorting of each subpopulation was evaluated on the basis of slope of the straightline segment representing the subpopulation. The following arbitrary angular limits of steepness of the curve suggested by Visher (969) were adopted here with some modifications as a measure of sorting ' < 50* Poorly / Poor sorting SO^eO" Moderately / Fair sorting 60 70* Moderately well / Good sorting > 70 / Excellent sorting

6 96 Truncation points at the coarser and finer ends of each subpopulation segment were determined. All the above determined characteristics of grainsize distribution are summarised in Table 2. TABLE 2. Characteristics of Type I, Type II, Type III and Type IV grainsize curves of sandstones, Jaisalmer Formation. T = Coarse truncation point of traction; T = Coarse truncation point of saltation; T^ = Pine truncation point of saltation. Traction Saltation Suspension S ample Dominant Subordinate No. r'er Angle Per Angle Per Angle Per Angle cent (deg cent (deg cent (deg cent (degree) ree) ree) ree) T. Type I Curves , o o Type II Curves o00 94c

7 97 TABLE 2. (Contd.) Sample No. Type Type Type Traction Per Angle cent (degree) II Curves (Ci III Curves IV Curves Saltation Dominant Percent ontd) Angle (degree) Subordinate Percent ^ _ Angle (degree) _ Suspension Percent Angle (degree) ^ _ ^ ^

8 98 The second approach to study grainsize distribution employed the parameters described by Folk and Ward (957). The grain diameter in phi units represented by $ 5, 56, 425, $50, 575, 584 and 595 percentiles were computed (Appendix II). The following statistical graphic parameters were calculated (Table 3). Graphic Mean (M ) z Inclusive Graphic standard Deviation (CTr) = Inclusive Graphic Skewness (SK^) = (58456) 2(59555) Graphic Kurtosis (K ) ( ) The third approach to studying grainsize distribution employed the methods of bivariate analysis described by Passega (957, 964) and Stewart (958). Following Passega, values of C ( one percentile ) and M (median grainsize at 50 percentile ) in microns were computed from logprobability curves for all the samples ( Appendix III ), and the bivariate plot or CM diagram was constructed by plotting all the sample points on a logarithmic graph paper taking M on abscissa and C on ordinate. optimum number. Passega (964) considered 20 to 30 sanples as Values were determined on 5 scale and then

9 99 TABLE 3. Statistical parameters of grainsize distribution of sandstones, Jaisalmer Formation M = Graphic Mean; cr = Inclusive Graphic Standard Deviation; SK = Inclusive Graphic Skewness; K_ = Graphic Kurtosis. Sam M ^I Verbal SK Verbal K Verbal pie ^ limits of limits of limits of No. (^) i^) sorting skewness kurtosis Moderately Strongly.20 Leptokurtic well fineskewed Nearsymme.5 Leptokurtic trical Strongly.5 Leptokurtic fineskewed o57 Moderately Nearsymme 0.89 Platykurtic well trical o66 Moderately Fineskewed.07 Mesokurtic well Moderately Fineskewed.32 Leptokurtic well Moderately Fineskewed.84 Very leptowell kurtic ,74 Moderately Fineskewed.39 Leptokurtic Fineskewed.54 Very leptokurtic ,52 Moderately Nearsymme 0.90 Platykurtic well trical o Fineskewed 0.77 Platykurtic 76 2, Strongly 0,9 Mesokurtic fineskewed Moderately Fineskewed.45 Leptokurtic well Nearsymme.46 Leptokurtic trical Very well Nearsymme.08 Mesokurtic trical Moderately +0.7 Finer skewed.7 Leptokurtic well Fineskewed.55 Very leptokurtic Fineskewed 0.70 Platykurtic Strongly.8 Leptokurtic fineskewed Very well Fineskewed.52 Very leptokurtic Moderately Symmetrical.39 Leptokurtic well Moderately 0.4 Coarse.6 Leptokurtic well skewed

10 TABLE 3. (Contd.) 00 Sample Nos, M z w Ci W Verbal limits of so: rting SK^ Verbal limits of skewness ^G Verbal limits of kurtosis o Very well Moderately well Very well Very well Moderately well Moderately well Fine skewed Co arset skewed Coarseskewed Strongly fine skewed Fineskewed Strongly coarseskewed Coarseskewed Strongly fineskewed Fineskewed Strongly coarseskewed F ineskewed Strongly fineskewed Fineskewed Fineskewed Fineskewed Nearsymmetrical Nearsymmetrical Fineskewed Coarseskewed Nearsymmetrical Fineskewed Nearsymmetrical Strongly fineskewed Leptokurtic Platykurtic Very leptokurtic Leptokurtic Leptokurtic Very leptokurtic Leptokurtic Leptokurtic Mesokurtic Mesokurtic Mesokurtic Very leptokurtic Leptokurtic Leptokurtic Leptokurtic Leptokurtic Leptokurtic Mesokurtic Leptokurtic Very leptokurtic Very leptokurtic Platykurtic Leptokurtic converted to micron scale with the help of conversion graph given by Folk (968). The second bivariate plot was constructed following Stewart's (958) method by plotting Median diameter against

11 0 Inclusive Graphic Standard Deviation, All the sample points were plotted on a simple graph paper taking phi Median diameter on abscissa and phi Inclusive Graphic Standard Deviation on ordinate. LOGPROBABILITY CURVES The logprobab"ility curves of grainsize distribution of the studied samples were analysed by Visher's (969) methodo The curves were broadly divided into four types (Types IIV) on the basis of number of populations and their relative development in a single distribution. Figs. 46 show the various types of grainsize distribution curves of the studied samples. Type I Curves 6 out of 45 samples show Type I grainsize distribution which comprises four populations (Fig. 4A,B,C,D), These populations were identified as surface creep, suspension and two saltation populations. The surface creep population is very poorly developed and constitutes commonly less than,0 percent but some samples show relatively higher percentages (upto 6 percent). The population is predominantly poorly but in a few samples it is moderately to moderately well. The coarse truncation point of the population lies at 0.25t5 to.255. However, in most distributions this point

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13 02 lies between a narrow range of O.OS to 0,5 i5. The grain size of the population ranges from a maximum of 0.25$ to a minimum of.85s. Two saltation populations are characteristically present in Type X Curves. Of the two saltation populations one is invariably well developed and dominant. The dominant saltation population commonly lies adjacent to the suspension population. However/ in some samples, the dominant saltation population adjoins the surface creep population. The percentages of dominant saltation population range from 54 to 92.5 percent. The dominant saltation population is commonly moderately well but its sorting ranges from moderately to well. The percentages of subordinate saltation population range from 0.05 to 37.7 percent. This population is commonly poorly but occasionally moderately to well. The truncation point of saltation population and surface creep population is quite variable and lies at 0.05 S to.85 S>. The break between saltation population and suspension population occurs at to The overall size range of saltation population is from a maximum of 0,05 4 to a minimum of 3,40 $, The percentage of suspension population commonly ranges from,8 to 0 percent but few samples show higher values upto 6 percent. The suspension population is poorly. The overall size range of suspension population is from 2,O50to 4 S.

14 03 The Type I Curves of the study area resemble the curves of modern beach sands described by Visher (969/ Fig. 6 & 7). The high percentage of saltation (both dominant and subordinate together) in the studied samples (83«9 98. percent) matches the high percentage of saltation population of modern beach samples (5099 percent). The sorting or slope of the dominant saltation population of the studied samples (60 70 ) resemble: the slope of saltation population of modern foreshore beach samples (60 70 ). The percentages of suspension population of the studied samples (.8 to 0 percent) also match the percentages of suspension population reported from modem beach sands (0 0 percent). Higher percentages of suspension population in some of the studied samples are indicative of the proximity of the depositional site to a source of fine elastics. The development of two separate saltation populations is believed to be characteristic of foreshore and shoreface zones of the beach and other strandline environments (Visher, 969), In these environments opposing currents of swash and backwash represent two differing transport conditions. The two saltation populations are produced in opposite flow directions. The two saltation populations of the studied samples are unequally developed. One of the saltation population is invariably very well developed and better. The other saltation population is subordinate and poorly. This may be explained by the unequal strength of the opposing onshore and offshore currents.

15 04 Type II Curves The Type II distribution comprises three populations surface creep, saltation, and suspension. 7 samples show Type II distribution (Fig. 5A,B,C,D). The surface creep population is generally very poorly developed constituting less than,0 percent in majority of samples. The percentage of the population in some samples is relatively higher, ranging upto 6 percent. The population is poorly. The coarse truncation point of the population falls at 0.0 S to.25 S but commonly at 0,5 S. The grainsize of the population ranges from 0.0 S to 2.4 S. The saltation population is well developed and its percentages range from 76 to 97.3 percent excepting one sample showing a lesser value of 57 percent. The population is moderately well with the exception of few samples which show moderately and well saltation population. The coarse truncation point of the population lies at.05 5 to and the fine truncation point at 2.7 cb to 3.7 Sc The grainsize of the population ranges from.05 a to The suspension population is mostly poorly developed. The percentages of the population generally range from 2.3 to 4 percent but in two samples higher percentages (22 and 40 percent) are met. The population is invariably poorly. The grainsize of the population ranges from 2.7 a to 4 4.

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17 05 The characteristics of Type II Curves closely match the characteristics of modern shallow marine wave zone sands described by Visher (969). Narrow size ranges of coarse and fine truncation points, and good sorting of saltation population are indicative of currents of more or less constant strength and wave processes that caused winnowing. Type III Curves The Type III grainsize distribution, shown by 7 samples, comprises only two populations which were identified as saltation and suspension populations (Fig. 5A,B,C). The break between the two populations occurs at to 3.2 (i which corresponds to the break between saltation and suspension populations observed in Type I and Type distributions. The saltation population is well developed constituting 70 to 95 percent. The population is moderately well to moderately. At its coarser end, the saltation population is truncated at 0.25 S to The grainsize of the population ranges from 0.25 cb to The percentage of suspension population ranges from 5 to 30 percent. The suspension population is poorly. The coarse truncation point of the population lies at 2.6 S to 3.2 cb. The grainsize of the population ranges from to 4 (6. The characteristics and curve shapes of the Type III

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19 06 distributions resemble those of modern and ancient fluvial and distributary channel sands. Both fluvial deposits and distributary channel deposits are characterised by two major populations, one related to suspension and the other to saltation. In general. Type curves represent deposition by unidirectional currents. Type IV Curves The Type IV distribution, shown by 5 samples, is chancterised by a highly developed suspension population and poorly developed surface creep and saltation populations (Fig. 6D). The surface creep population is present in four samples and forms upto 5 percent. The population is poorly. The coarse tiruncation point of the population lies at 0.0 S to.25 S. The grainsize of the population ranges from 0.0 S> to 2.5 a. The saltation population constitues 2 to 9.5 percent in four samples and 4 5 percent in one sample. The population is moderately well in majority of the samples and poorly in one sample. The saltation population is truncated at its coarser eid at, and its fine truncation point lies at 2.25 cb 2.9 c6. The grainsize of the population rancies fcrom,5 4 to 2,9 S. The suspension population constit\ites generally 78 to 98 percent and 50 percent in one sample. It is generally moderately but poorly in one sample. The

20 07 grainsize of the population ranges from 2.25 $ to 4 5. The characteristics and shapes of the Type IV Curves match those of modern levee deposits associated with fluvial channels and distributary channels. The presence of a predominant suspension population suggests that the Type IV Curves were formed by the rapid fallout of suspended material. UNIVARIATE PARAMETERS The statistical parameters of grainsize distribution of the sandstone samples from the study area were determined with the help of formulae given by Folk and Ward (957). Statistical measures obtained included Graphic Mean (M ), Inclusive Graphic Standard Deviation ( CT ), Inclusive Graphic Skewness (SK^.), and Graphic Kurtosis {K_), The samples were classified according to Folk and Ward's (957)' verbal limits for sorting, skewness and kurtosis. Mean Size M values of the studied samples range from.5 S to indicating that the sandstones are medium to very fine grained. The sandstones are, however, commonly fine grained as M values of most samples lies between 2 5 to 3 5. Mean size is a function of () the size range of available sediment and (2) amount of energy imparted to the sediment which depends on current velocity or turbulence of the

21 08 transporting medixom. The narrow range of mean size of the studied sandstones suggests that either the available sediment had a limited size range or the hydrodynamic conditions were more or less uniform during the deposition of various sandstone units. The vertical sequence of sedimentary structures and bedding types indicated varying energy conditions, Therefojo the narrow range of mean grainsize in all probability reflects a limited size range of available sediment. The fine nrained mean size of most samples is in conformity with a close association of sandstone units with generally micritic limestones. Despite an apparent homogeneity of mean grainsize, several sandstone units of the Hamira Member, the Joyan Member and the Fort Member show a coarseningupwards of the mean grainsize. This is attributed to prograding coastal sands and shoaling waters that resulted in winnowing of finer sediments due to increasing wave and current action in nearshore environments as proposed by Mason and Folk (958), Friedman (96, 962, 967), Duane (964), Tanner (966), and Valia and Cameron (977). Sorting O"^ Values of the studied samples range from 0,25 $ to According to the verbal classification scale for sorting, the sandstones are mostly moderately well to well. However, few samples are either moderately or very well.

22 09 Sorting is a poorly understood measure. It depends on atleast three major factors which are the size range of available sediment, its rate of deposition, and the strength and variation in energy of the depositing agent. Folk (968) believes that sorting of a given source material decreases in a sequence of aeolian, beach, river (or nearshore marine), and offshore marine environments. Sorting values in the a to 3 4 sand class generally range from to 0.50 d) for beach sands and from 0,35 i> to.0 S> for river (or shallow marine) sands (Folk, 968). Since the mean size of the studied sandstones generally lies between 2 S 3 S, their sorting values can be compared with the sorting values of modern sediments described by Folk (968). About 70 percent of the studied samples show sorting values between 0o25 $ and 0,5iS matching the sorting of modern beach sands. The remaining 30 percent samples show sorting values ranging from 0.5 S to 0,74 5. These sandstones perhaps represent nearshore marine sands which, according to Folk (968), are somewhat more poorly than corresponding beaches, and tend to have sorting values roughly similar to river sands. Skewness and Kurtosis SK^ values of the studied sandstones range from 0,3 to percent of the samples have positive skewness and the remaining 8 percent are negatively skewed. Among the positively skewed samples, 20 percent are nearsymmetrical, 44 percent are fineskewed and the remaining 8 percent are strongly fineskewed. 3 percent of the negatively skewed

23 0 samples are coarseskewed, and 5 percent are strongly coarseskewed. K^ values of the studied sandstones range from 0.70 to Majority of the samples are leptokurtic (5 percent) followed in descending order by very leptokurtic (20 percent), mesokurtic (6 percent) and platykurtic (3 percent). SkewnesG and i^urtosis wetu teferxed to as indicators of selective action of the transporting agent by Krumbein and Pettijohn (938). Since then the parameters have been employed by various investigators but have been little studied from a geological standpoint. Polk and Ward (957) considered that skewness and kurtosis reflect bimodality of grainsize even though modes are not readily apparent. Mason and Folk (958) made a comparative textural studies in recent sands of beach, dune, and aeolian flat environments. These studies indicated that beach sands are normal or negative and leptokurtic, that dune sands have positive skewness and are mesokurtic, and tliat "aeolian flot" sands have positive skewness and are leptokurtic. Friedman (96) attempted to distinguish between beach, dune and river sands on the basis of skewness. He demonstrated that modern beach sands generally have negative skewness, but both dune and river sands usually are positively skewed. Duane (964) demonstrated that sands of the littoral, beach, and tidal inlet environments have negative skewness as the result of winnowing action, waves and tidal currents. In sheltered quiet water areas and in reeper water, where bottom currents or wavebase surge are not effective, the sands have

24 Ill positive skewness. Sediments show local variation'in the sign of skewness in areas of fluctuating energy conditions or of intermittent winnowing action. About 40 percent of the studied samples have nearsymmetrical or negative skewness which suggests winnowing action of currents and waves. The remaining 50 percent of the samples show positive skewness and range from fineskewed to strongly fineskewed. The positive skewness and leptokurtic distribution in the studied samples might be due to either nearness to the source of the sand as suggested by Folk and Ward (957), or relatively deeper water conditions where wavebase surge and bottom currents were not effective. BIVARIATE PLOTS Bivariate plots were constructed by combining two statistival grainsize parameters of the studied sandstones. The sample points were plotted on two different combinations which included () C (one percentile) versus M (median diameter), and (2) inclusive graphic standard deviation versus median diameter. The bivariate plots were employed to interpret the depositional processes and environments. The bivariate plot of C versus M (CM diagram) was constructed by Passega's (957, 964) methods. Figure 7 shows the CM diagram for the study area with superimposed patterns representing two types of deposits. Most of the sample points best fit the pattern representing typical beach environment. Sample points are mostly situated where the pattern is close

25 JOOO 00 T f i t T w.. 5* M.Mtdt.n Oi.m.t.r (Micron»> I. ;:i, "o;.:?:*'''''"^''^ ''*''''''> «

26 2 to limit C=M^ and hence the sediments are generally well. Passega (957) demonstrated that CM patterns indicated the depositional agent as each of the different type of deposits, for example, river, beach, quiet water and turbidity current deposits, was characterised by a distinctive pattern. Bivariate plot of inclusive graphic standard deviation ( 0~ ) versus median diameter was employed by Stewart (958) to distinguish betweenve and river processes. The bivariate plot of the studied sandstones (Fig, 8) shows two different fields, each of river and wave processes. Most of the sample points fall in the field of wave process. It appears that wave processes played a dominant role in the deposition of the studied sandstones.