Chapter III GRAIN-SIZn; ANALYSIS OF SANDSTONES ...shodhganga.inflibnet.ac.in/bitstream/10603/57626/7/07_chapter 3.pdfVisher's (1969) outstanding work on grain-size distribu tion in

Chapter III

GRAIN-SIZn; ANALYSIS OF SANDSTONES

INTRODUCTION

This chapter deals with grain-size characteristics of

the terrigenous clastic sediments of the Jaisalmer Formation,

The grain-size study was undertaken with a view to interjsretinq

the depositional processes and environments in the Jaisalmer

Basin.

The genetic interpretation of grain-size 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

(1966), Visher (1969) and i''riedman (1979) published excellent

reviews of grain-size parameters and their relationship

with depositional processes.

in general grain-size studies have followed two main

genetic approaches : (i) relating the grain-size 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 grain-size distribution.

92

Many gra in-s ize d i s t r i b u t i o n s are mixtures of two or

more sub-populations which are products of d i f fe ren t modes

of sedimentary t ranspor t (Doeglas^ 1946; Harr is 1958;

Moss, 1962, 1963, 1972; Visher, 1969). Opinions d i f fe r on

the nature of sub-population - tha t i s , whether they are

log-normal truncated (Sindowski, 1957; Visher, 1969) or

log-normal overlapping (Tanner, 1964; Pu l l e r , 1961; Walger,

1965; Spencer, 1963; Middleton and Southard, 1977).

V i she r ' s (1969) outstanding work on gra in-s ize d i s t r i b u ­

t ion in r e l a t i o n to deposi t ional processes was based on

extensive t ex tu ra l study o± both modern and ancient sands and

i t provided the bas is for a genetic i n t e rp re t a t ion of

g r a in - s i ze , Visher ' s work was extended and developed by h i s

coworkers (Visher and Howard, 1974; Freeman and Visher,

1975; Sagoe and Visher, 1977). Visher and h i s coworkers

concluded tha t each log-normal g ra in-s ize d i s t r i bu t i on curve

comprised a number of s t r a i g h t - l i n e segments of d i f fe ren t

slope separated by sharp ' b r e a k s ' . The s t r a i g h t - l i n e

segments represented truncated log-normal sub-populations

generated by d i f ferent modes of sediment t ranspor t and

deposi t ion. The number, amount, s ize-range, mixing, and

sor t ing of these sub-populations varied systematical ly

in r e l a t ion to provenance, sedimentary processes , and

sedimentary dynamics. The nature of the s t r a i g h t - l i n e

segments and 'breaks ' were accordingly r e l a t ed to the

importance of the various t ranspor t mechanisms operating

93

during deposition, presumed distinctive in different

depositional environments. Several workers used Visher's

approach in the recognition of ancient depositional

environment (Holmes and Oliver, 1973; Glaister and Nelson,

1974; Amaraland Pryor, 1977, Moshrif, 1980).

Many workers attempted direct interpretation of

depositional environments on the basis of statistical measures

of grain-size, such as mean, standard deviation, skewness

and kurtosis (folk and Ward, 1957; Mason and i'olk, 1958;

Harris, 1959, Friedman, 1961, 1962, 1967; Sahu, 1964;

Moiola and Weiser, 1968; Duane, 1964; Solohub and Klovan,

1970; Buller and Mc Manus, 1972; Reed_et_al,, 1975;

Staper and Tanner, 1975; Valia and Cameron, 1977). 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 (1957) for distinguishing the various

depositional processes.

The approach ot directly interpreting the agent and/or

environment of deposition on the basis of grain-size

distribution is not wholly valid. It is doubtful that

a particular environment is characterized by a completely

unique pattern of grain-size distribution because similar

sediment transport and depositional mechanisms may operate

in different environments ( R e e d ^ al,, 1975; Ruzyla, 1977;

94

Steidtmann, 1977; Jackson, 1978; Sedimentation Seminar, 1981).

Moreover/ grain-size distribution is also affected by provenance

and sediment generative processes as well as by diagenetic

modifications. Despite these ambiguities, most sedimentologists

agree that grain-size 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 grain-size 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 grain-size 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 (1964) 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, 16 to 230

(size from - 0.25 to 4.0 <li or 1.19 mm to 0.0625 mm) were used.

95

Each fraction of the samples obtained on the sieves was weighed

to the 2nd place of decimal by chemical balance, and its weight

percentage was calculated (Appendix I). Arithmetic probability

graph paper was employed for constructing log-probability

curves of the grain-size data. The log-probability 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.

Grain-size analysis in the present study was based on

approaches described by Visher (1969), Folk and Ward (1957),

Passega (1957, 1964) and Stewart (1958).

Visher's (1969) method was employed to recognise the

various sub-populations (surface creep, saltation, suspension)

represented by straight-line segments on the log-probability

curve of a grain-size distribution. Percentages of each

sub-population were determined from the curve. Sorting of

each sub-population was evaluated on the basis of slope

of the straight-line segment representing the sub-population.

The following arbitrary angular limits of steepness of the

curve suggested by Visher (1969) were adopted here with some

modifications as a measure of sorting

' < 50* Poorly sorted / Poor sorting

SO^-eO" Moderately sorted / Fair sorting

60°-70* Moderately well sorted / Good sorting

> 70° Well sorted / Excellent sorting

96

Truncation poin ts a t the coarser and f ine r ends of each sub-population

segment were determined. All the above determined c h a r a c t e r i s t i c s of

gra in-s ize d i s t r i bu t i on are summarised in Table 2.

TABLE 2. Charac te r i s t i c s of Type I , Type I I , Type I I I and Type IV gra in - s ize curves of sandstones, Jaisalmer Formation.

T = Coarse t runcat ion point of t r a c t i o n ; T„ = Coarse t runcat ion point of s a l t a t i o n ; T^ = Pine t runcat ion poin t of s a l t a t i o n .

Traction Sa l t a t ion Suspension

Dominant Subordinate S ample

No. r'er- Angle Per- Angle Per Angle Per- Angle cent (deg- cent (deg- cent (deg- cent (deg­

ree) ree) ree) ree)

T.

Type

6 8 9 10 76

127 131 191 200 222 233 257 268 330 331 334

I Curves

0,03 0.10 1.20 0.10 0.05 0.50 0.60 0.10 5.00 0.70 0.30 0.05 0.07 6.00 0.20 0.50

22 28 33 61 16 70 42 35 38 45 40 9 39 58 28 55

63.97 74.00 58.80 85.85 56.00 12.50 79.40 79.90 76.00 75.30 54.00 87.00 67.93 59.00 67.00 83.50

64 56 61 63 57 62 67 72 73 63 64 64 63 75 78 79

31.50 9.90 31.00 0.05 34.95 3.50

15.50 17.50 16.00 21.00 37.70 7.95

30.20 28.00 17.80 6.00

48 51 52 7

70 33 42 49 49 48 75 43 54 48 55 42

4.50 16.00 9.00 14.00 9.00 3.50 4.50 2.50 3.00 3o00 8.00 5.00 1.80 7.00 15o00 10.00

29 24 18 19 30 13 16 17 17 20 15 26 12 27 36 22

0.50 0.00 0.25 0.25 0.50 0.25 0.00 0.75 0.25 0.00 1.25 0.00 0.25 0.75 0.50 1.00

1.00 0.75 1.00 0.50 1.55 0.05 1.00 1.45 1.70 0.85 1.85 0.75 0.55 1.75 1.50 1.60

3.25 3.00 2.65 2.60 3.20 2.05 2.80 3.10 2.95 2.80 3.00 3.40 2.80 2.85 2.80 2.90

Type II Curves

3 5 7 71 72 73 79 124 130 139

2.00 0.40 0.10 0.10 0.10 0.40 0.20 0.07 0.05 0.05

41 28 32 18 15 29 35 16 41 20

76o00 94c00 95.40 93.40 94.40 92.10 85.80 90.03 97.25 96.45

68 61 55 65 58 62 60 67 56 65

2 2.00 5.50 4.50 6.50 5.50 7.50

14.00 9.00 2.25 3.50

24 23 26 20 20 36 29 25 18 27

0.60 0.00 0.25 0.00 0.00 0.50 0.50 0.50 0.00 0.50

2.00 1.25 1.05 1.25 1.25 1.50 1.25 1.85 0.75 1.40

2.85 2.95 3.50 2.75 3.40 3.10 2.90 3.25 3.05 3.15

97 TABLE 2 . ( C o n t d . )

Sam­ple No.

Type

189 220 269 289 294 311 313

Type

11 192 196 230 239 290 357

Type

193 197 226 328 352

Traction

Per- Angle cent (deg­

ree)

II Curves (Ci

3.00 35 0.30 22 0.15 41 0.06 18 0.20 29 6.00 36 1.40 39

III Curves

_ _

- -- -- -• - -_ _ - —

IV Curves

5.00 50 - -

2.50 44 3.00 32 1.80 33

Saltation

Dominant

Per­cent

ontd)

57.00 94.70 87.85 90.94 88.80 91.50 93.00

76.00 82.00 95.00 80.00 91.00 95.00 70.00

45.00 2.00 19.50 11.00 6.20

Angle (deg­ree)

79 66 71 67 67 65 64

55 50 67 62 61 67 56

62 67 32 68 69

Subordinate

Per­cent

—

-----—

^

-----—

_

---

Angle (deg­ree)

— -----—

_

-----—

—

---

Suspension

Per­cent

40.00 5.00

12.00 9.00 11.00 2.50 7.00

24.00 18.00 5.00

20.00 9.00 5.00

30.00

50.00 98.00 78.00 86.00 92.00

Angle (deg­ree)

47 28 19 23 30 13 16

30 40 24 36 26 14 31

40 55 57 53 53

^1

1.25 0.50 1.25 0.50 1.25 0.00 0.25

_

-— ---—

1.25 -

0.00 1.00 1.00

^2

3.00 1.45 1.75 1.20 2.40 1.70 1.50

0.25 0.25 1.25 1.75 1.25 1.25 1.00

1.70 1.75 1.50 2.50 2.50

^3

3.30 2.95 2.80 2.70 3.70 2.90 2.85

2.60 3.15 2.70 3.10 3.20 2.70 3.05

2.80 2.25 2.90 2.75 2.70

98

The second approach to study grain-size distribution

employed the parameters described by Folk and Ward (1957) .

The grain diameter in phi units represented by $ 5, 516,

425, $50, 575, 584 and 595 percentiles were computed

(Appendix II). The following statistical graphic parameters

were calculated (Table 3) .

Graphic Mean (M ) z

516 + 550 + 584

Inclusive Graphic 584 -516 595 -55 standard Deviation (CT-r) = +

6.6

Inclusive Graphic 516 + 584 - 2550 55 +595 - 2550 Skewness (SK^) = +

Graphic Kurtosis (K )

2(584-516) 2(595-55)

595 - 55

2.44(575 - 525)

The third approach to studying grain-size distribution

employed the methods of bivariate analysis described by

Passega (1957, 1964) and Stewart (1958). Following Passega,

values of C ( one percentile ) and M (median grain-size

at 50 percentile ) in microns were computed from log-probability

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. Passega (1964) considered 20 to 30 sanples as

optimum number. Values were determined on 5 scale and then

99

TABLE 3 . S t a t i s t i c a l parameters of g r a i n - s i z e d i s t r i b u t i o n of sands tones , J a i s a l m e r Formation

M = Graphic Mean; cr = I n c l u s i v e Graphic S tandard

D e v i a t i o n ; SK = I n c l u s i v e Graphic Skewness; K_ = Graphic

K u r t o s i s .

Sam- M ° I Verbal SK Verbal K Verbal pie ^ limits of limits of limits of No. ( ) i^) sorting skewness kurtosis

3 2.75 0.51 Moderately +0.40 Strongly 1.20 Leptokurtic well sorted fine-skewed

5 2.30 0.43 Well sorted +0.08 Near-symme- 1.15 Leptokurtic trical

6 2.28 0.45 Well s o r t e d +0.32 S t r o n g l y 1.15 L e p t o k u r t i c f ine-skewed

7 2.63 0o57 Moderately +0.02 Near-symme- 0.89 Platykurtic well sorted trical

8 2.57 0o66 Moderately +0.22 Fine-skewed 1.07 Mesokurt ic wel l s o r t e d

9 1.90 0.54 Moderately +0.28 Fine-skewed 1.32 Leptokurtic well sorted

10 2.17 0.55 Moderately +0.24 Fine-skewed 1.84 Very l e p t o -well sorted kurtic

11 2.35 0,74 Moderately +0.20 Fine-skewed 1.39 Leptokurtic sorted

71 2.23 0.40 Well sorted +0.24 Fine-skewed 1.54 Very leptokurtic

72 2.70 0,52 Moderately +0.07 Near-symme- 0.90 Platykurtic well sorted trical

73 2.65 0o45 Well sorted +0.12 Fine-skewed 0.77 Platykurtic 76 2,63 0.47 Well sorted +0.31 Strongly 0,91 Mesokurtic

fine-skewed 79 2.50 0.57 Moderately +0.30 Fine-skewed 1.45 Leptokurtic

well sorted 124 2.85 0.36 Well sorted +0.05 Near-symme- 1.46 Leptokurtic

trical 127 1.50 0.31 Very well +0.02 Near-symme- 1.08 Mesokurtic

sorted trical 130 1.98 0.53 Moderately +0.17 Fine-r skewed 1.17 Leptokurtic

well sorted 131 1.83 0.43 Well sorted +0.28 Fine-skewed 1.55 Very

leptokurtic 139 2.62 0.41 Well sorted +0.15 Fine-skewed 0.70 Platykurtic 189 3.40 0.36 Well sorted +0.61 Strongly 1.18 Leptokurtic

fine-skewed 191 2.22 0.33 Very well +0.24 Fine-skewed 1.52 Very

sorted leptokurtic 192 2.58 0.67 Moderately +0.00 Symmetrical 1.39 Leptokurtic

well sorted 193 3.05 0.70 Moderately -0.14 Coarse- 1.16 Leptokurtic

well sorted skewed

TABLE 3. (Contd.) 100

Sam­p l e Nos ,

196 197

200

220

222 226

230

233

239 257

268 269

289 290 294 311

313

328 330

331

334

352

357

M z w

2.22 3 .62

2 . 4 7

2 .47

1.80 3 .25

2 .88

2 .60

2 .67 2 .88

1.78 2 . 5 7

2 . 3 3 2 . 1 3 3 .31 2 .22

2 . 2 8

3 .33 2 . 4 8

2 . 6 3

2 . 7 1

3 . 5 0

2 .92

Ci

W

0 . 3 8 0 . 4 6

0 . 3 3

0 .47

0 .46 0 .60

0 .49

0 . 4 1

0 .49 0 .42

0o41 0 . 4 3

0 .40 0 .39 0 .37 0 . 4 0

0 .42

0 .50 0 .39

0 .29

0 . 2 5

0 .52

0 . 6 7

V e r b a l l i m i t s of s o :

Well Well

Very

r t i n g

s o r t e d s o r t e d

w e l l s o r t e d Well

Well

s o r t e d

s o r t e d M o d e r a t e l y w e l l

Well

Well

Wel l Well

Well Well

Well Well Well Well

Well

Wel l Well

Very

s o r t e d

s o r t e d

s o r t e d

s o r t e d s o r t e d

s o r t e d s o r t e d

s o r t e d s o r t e d s o r t e d s o r t e d

s o r t e d

s o r t e d s o r t e d

w e l l s o r t e d Very w e l l s o r t e d M o d e r a t e l y w e l l s o r t e d M o d e r a t e l y w e l l s o r t e d

SK^

+ 0 . 2 0 - 0 . 2 4

- 0 . 2 5

+ 0 .36

+0 .22 - 0 . 3 1

- 0 . 2 2

+0 .40

+ 0 . 1 1 - 0 . 3 1

+0 .17 +0 .40

+0 .20 +0 .23 +0 .15 +0 .09

+ 0.08

+0 .17 - 0 . 3 0

+0 .08

+ 0 .29

- 0 . 0 3

+0 .36

V e r b a l l i m i t s of skewness

F i n e skewed Co arse-T skewed C o a r s e -skewed S t r o n g l y f i n e skewed F i n e - s k e w e d S t r o n g l y c o a r s e -skewed C o a r s e -skewed S t r o n g l y f i n e - s k e w e d F i n e - s k e w e d S t r o n g l y c o a r s e -skewed F i n e - s k e w e d S t r o n g l y f i n e - s k e w e d F i n e - s k e w e d F i n e - s k e w e d F i n e - s k e w e d Near-symme­t r i c a l Near-symme­t r i c a l F i n e - s k e w e d C o a r s e -skewed Near-symme­t r i c a l F i n e - s k e w e d

Near-symme­t r i c a l S t r o n g l y f i n e - s k e w e d

^G

1.49 0 . 7 9

1.64

1.27

1.35 1.60

1.17

1.43

0 . 9 3 1.06

1.04 1.80

1.37 1.32 1.46 1.41

1.15

1.00 1.44

2 .50

2 . 5 6

0 .86

1.19

V e r b a l l i m i t s of k u r t o s i s

L e p t o k u r t i c P l a t y k u r t i c

Very l e p t o k u r t i c L e p t o k u r t i c

L e p t o k u r t i c Very l e p t o k u r t i c

L e p t o k u r t i c

L e p t o k u r t i c

M e s o k u r t i c M e s o k u r t i c

M e s o k u r t i c Very l e p t o k u r t i c L e p t o k u r t i c L e p t o k u r t i c L e p t o k u r t i c L e p t o k u r t i c

L e p t o k u r t i c

M e s o k u r t i c L e p t o k u r t i c

Very l e p t o k u r t i c Very l e p t o k u r t i c P l a t y k u r t i c

L e p t o k u r t i c

converted to micron scale with the help of conversion graph given

by Folk (1968). The second bivariate plot was constructed

following Stewart's (1958) method by plotting Median diameter against

101

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.

LOG-PROBABILITY CURVES

The log-probab"ility curves of grain-size distribution

of the studied samples were analysed by Visher's (1969)

methodo The curves were broadly divided into four types

(Types I-IV) on the basis of number of populations and their

relative development in a single distribution. Figs. 14-16

show the various types of grain-size distribution curves of

the studied samples.

Type I Curves

16 out of 45 samples show Type I grain-size distribution

which comprises four populations (Fig. 14A,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 1,0 percent but some

samples show relatively higher percentages (upto 6 percent).

The population is predominantly poorly sorted but in a few

samples it is moderately sorted to moderately well sorted.

The coarse truncation point of the population lies at

-0.25t5 to 1.255. However, in most distributions this point

89(1

-4 «9«

- i M M

-?»»«

Phi S c « l .

«««•

^(tiUHf U fvp* I >«• ^r«t>'b<*'lr ^'A*^ a'^* l*'* ^Ml' « (.u'*«« • A. a C t 0 ) 0f •«n(t*l*A««

102

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 1.85S.

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 sorted but its sorting ranges from

moderately sorted to well sorted. The percentages of

subordinate saltation population range from 0.05 to 37.7

percent. This population is commonly poorly sorted but

occasionally moderately sorted to well sorted. The truncation

point of saltation population and surface creep population

is quite variable and lies at 0.05 S to 1.85 S>. The break

between saltation population and suspension population occurs

at 2.05 4 to 3.40 4. 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 1,8 to 10 percent but few samples show higher

values upto 16 percent. The suspension population is poorly

sorted. The overall size range of suspension population is

from 2,O50to 4 S.

103

The Type I Curves of the study area resemble the curves

of modern beach sands described by Visher (1969/ Fig. 6 & 7).

The high percentage of saltation (both dominant and subordinate

together) in the studied samples (83«9 - 98.1 percent) matches

the high percentage of saltation population of modern beach

samples (50-99 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 (1.8 to 10 percent) also match the

percentages of suspension population reported from modem beach

sands (0 - 10 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,

1969), 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

sorted. The other saltation population is subordinate and

poorly sorted. This may be explained by the unequal strength

of the opposing onshore and offshore currents.

104

Type II Curves

The Type II distribution comprises three populations

surface creep, saltation, and suspension. 17 samples show

Type II distribution (Fig. 15A,B,C,D).

The surface creep population is generally very poorly

developed constituting less than 1,0 percent in majority

of samples. The percentage of the population in some samples

is relatively higher, ranging upto 6 percent. The population

is poorly sorted. The coarse truncation point of the

population falls at 0.0 S to 1.25 S but commonly at 0,5 S.

The grain-size 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 sorted with the exception of few samples

which show moderately sorted and well sorted saltation

population. The coarse truncation point of the population

lies at 1.05 5 to 2.4 5 and the fine truncation point at

2.7 cB to 3.7 Sc The grain-size of the population ranges

from 1.05 a to 3.7 5.

The suspension population is mostly poorly developed.

The percentages of the population generally range from

2.3 to 14 percent but in two samples higher percentages

(22 and 40 percent) are met. The population is invariably

poorly sorted. The grain-size of the population ranges

from 2.7 a to 4 4.

T

0.

fr

98 >9

- ' M l

QOl

T 1 r -I r «*M

Phi Seal*

9998

Phi Sc«l«

=

^ £

^ a.

1 t j ^

^ 1 J i

« i -^ ^ o

- 1 f 1

0

-

/ /

- J - - ^ 1.

/ /

•

1 1 1 1 1

y - * 5 ^ ./?.-/•, ' ' •?• ' ' • • / y ''*' / / J

1 •• 7 • /

/ // / / 1 *; 7 /

••' / V / / / /

i / .-/ . / /

y / y /

^ r

V .. .J. . 1_ . _ i , 1

^' ..r-

1 ' <iS'8a

J

•' ' <^

-

~ ~ "

" •

-

-1

99 »

98

90

70

SO

JO

10

I

OS

01

0 01

Phi S e l l .

flOURE IS ' y p « a locj ptub.biUty 91 «ui iU» iMilnUilMii iutv»» ( A. 6 C t [) ) ol v*in)»luii«».

Jfci*»tmtr ^ocm»^lon i aMm Vii h^t, I9ti9 I

105

The characteristics of Type II Curves closely match

the characteristics of modern shallow marine wave zone sands

described by Visher (1969). 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 grain-size distribution, shown by 7 samples,

comprises only two populations which were identified as

saltation and suspension populations (Fig. 15A,B,C). The

break between the two populations occurs at 2.6 4 to 3.2 (i

which corresponds to the breaK between saltation and

suspension populations observed in Type I and Type 11

distributions.

The saltation population is well developed constituting

70 to 95 percent. The population is moderately well sorted

to moderately sorted. At its coarser end, the saltation

population is truncated at 0.25 S to 1.75 4. The grain-size

of the population ranges from 0.25 cB to 3.2 4.

The percentage of suspension population ranges from

5 to 30 percent. The suspension population is poorly

sorted. The coarse truncation point of the population lies

at 2.6 S to 3.2 cb. The grain-size of the population ranges

from 2.6 4 to 4 (6.

The characteristics and curve shapes of the Type III

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106

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 111 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. 16D) .

The surface creep population is present in four samples

and forms upto 5 percent. The population is poorly sorted.

The coarse tiruncation point of the population lies at 0.0 S

to 1.25 S. The grain-size of the population ranges from

0.0 S> to 2.5 a.

The saltation population constitues 2 to 19.5 percent

in four samples and 4 5 percent in one sample. The population

is moderately well sorted in majority of the samples and

poorly sorted in one sample. The saltation population is

truncated at its coarser e-id at 1,5 3 - 2.50 5 and its

fine truncation point lies at 2.25 cB - 2.9 c6. The grain-size

of the population rancies fcrom 1,5 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 sorted but poorly sorted in one sample. The

107

grain-size 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 grain-size distribution

of the sandstone samples from the study area were determined

with the help of formulae given by Folk and Ward (1957).

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 (1957)' verbal

limits for sorting, skewness and kurtosis.

Mean Size

M values of the studied samples range from 1.5 S to

3.62 5 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 (1) the size range of available

sediment and (2) amount of energy imparted to the sediment

which depends on current velocity or turbulence of the

108

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, Therefoj-o the narrow range of mean grain-size

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 grain-size,

several sandstone units of the Hamira Member, the Joyan

Member and the Fort Member show a coarsening-upwards of the

mean grain-size. 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 (1958),

Friedman (1961, 1962, 1967), Duane (1964), Tanner (1966),

and Valia and Cameron (1977) .

Sorting

O" Values of the studied samples range from 0,25 $

to 0.74 4. According to the verbal classification scale

for sorting, the sandstones are mostly moderately well sorted

to well sorted. However, few samples are either moderately

sorted or very well sorted.

109

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 (1968)

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

1 a to 3 4 sand class generally range from 0.25 5 to 0.50 d)

for beach sands and from 0,35 i> to 1.0 S> for river (or shallow

marine) sands (Folk, 1968). 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 (1968). About 70 percent of the

studied samples show sorting values between 0o25 $ and 0,5 iS

matching the sorting of modern beach sands. The remaining

30 percent samples show sorting values ranging from 0.51 S

to 0,74 5. These sandstones perhaps represent nearshore

marine sands which, according to Folk (1968), are somewhat

more poorly sorted 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 -0,31 to

+0.61. 82 percent of the samples have positive skewness

and the remaining 18 percent are negatively skewed. Among

the positively skewed samples, 20 percent are near-symmetrical,

44 percent are fine-skewed and the remaining 18 percent are

strongly fine-skewed. 13 percent of the negatively skewed

110

samples are coarse-skewed, and 5 percent are strongly

coarse-skewed.

K^ values of the studied sandstones range from 0.70

to 2.56. Majority of the samples are leptokurtic (51 percent)

followed in descending order by very leptokurtic (20 percent),

mesokurtic (16 percent) and platykurtic (13 percent).

SkewnesG and i^urtosis wetu teferx-ed to as indicators

of selective action of the transporting agent by Krumbein

and Pettijohn (1938). Since then the parameters have been

employed by various investigators but have been little studied

from a geological standpoint. Polk and Ward (1957) considered

that skewness and kurtosis reflect bimodality of grain-size

even though modes are not readily apparent. Mason and Folk

(1958) 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 (1961) 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 (1964) 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 wave-base surge are not effective, the sands have

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 near-symme­

trical or negative skewness which suggests winnowing action

of currents and waves. The remaining 50 percent of the samples

show positive skewness and range from fine-skewed to strongly

fine-skewed. 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 (1957), or

relatively deeper water conditions where wave-base surge

and bottom currents were not effective.

BIVARIATE PLOTS

Bivariate plots were constructed by combining two

statistival grain-size parameters of the studied sandstones.

The sample points were plotted on two different combinations

which included (1) 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 (1957, 1964) methods. Figure 17 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

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112

to limit C=M^ and hence the sediments are generally well

sorted. Passega (1957) 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 (1958)

to distinguish between«»ve and river processes. The bivariate

plot of the studied sandstones (Fig, 18) 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.