Fluvial architecture of Early Permian Barakar rocks of Korba Gondwana basin, eastern-central India

Journal of Asian Earth Sciences 52 (2012) 43–52

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Fluvial architecture of Early Permian Barakar rocks of Korba Gondwana basin, eastern-central India Ram Chandra Tewari a,⇑, Rabindra Nath Hota b, Wataru Maejima c a

Department of Geology, Sri J. N. P. G. College, Lucknow 226 001, U.P., India Department of Geology, Utkal University, Bhubaneswar, Odisha 751 004, India c Department of Geosciences, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan b

a r t i c l e

i n f o

Article history: Received 19 May 2011 Received in revised form 20 February 2012 Accepted 27 February 2012 Available online 29 March 2012 Keywords: Lithofacies Paleocurrent Paleohydrology Barakar Gondwana India Master Basin

a b s t r a c t The Early Permian Barakar Formation of the Korba basin represents repeated deposition of fining upward sequences of coarse to medium grained sandstone, fine grained sandstone-shale, shale and coal. The sandstones are channel, sheet like, multistory, and profusely cross-bedded. The shale beds are lens as well sheet like and laminated; coal facies is thin to moderately thick and shows splitting. Paleocurrent analysis suggests a northwesterly paleoslope during Barakar sedimentation. However, the deflection of paleoslope towards northeast in the eastern part of the basin supports the existence of a watershed in the depositional area. The Barakar paleochannel were 4.05 m deep and 176 m wide (single channel) with an average sinuosity of 1.27. The average flow velocity and sediment load during flood stage are in the order of 1.77 m/s and 4.15. These results indicate bed-load (braided) to mixed load nature of the Barakar streams of the Korba basin. The study suggests that the Early Permian braided Barakar streams deposited the coal measure sequence subsequent to deglaciation in a northwesterly slopping paleovalley. The basin floor was highly uneven marked by the presence of a basement high in the northwestern part that bifurcates the paleostreams into northwestern and northeastern branches. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The Early Permian Barakar Formation of the Peninsular India Gondwana basins has been widely studied and interpreted as of fluvial origin (Casshyap, 1979; Veevers and Tewari, 1995). Generalised fluvial models of these sediments are based on data mostly from type area within the Koel-Damodar valley in eastern India. Precise fluvial facies models of these rocks and their bearing on Gondwana basin evolution are almost lacking and the basin floor topography during early Permian times is largely unknown. Limited studies were conducted on these rocks in other Gondwana basins of eastern-central India. The Son-Mahanadi valley basin is one of such critical areas where the Barakar rocks are extensively exposed. However, only for its southern part, the Talchir basin a sedimentation model has been proposed for the Barakar deposition (Hota et al., 2003). The Korba basin in the south-central part of Son-Mahanadi valley is a potential coal bearing area, but has not been adequately studied so far. Field sedimentological studies of Early Permian Gondwana rocks of this area are restricted to the revision of the geological set-up (Raja Rao, 1983) and comments ⇑ Corresponding author. Tel.: +91 9412877214. E-mail addresses: [email protected] (R.C. Tewari), [email protected] (R.N. Hota), [email protected] (W. Maejima). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.02.009

on the subsidence history based statistical analysis of borehole logs (Tewari, 1997, 2008). Indeed, the Early Permian Barakar rocks of Gondwana master basins of Peninsular India represent the first phase of fluvial sedimentation following the retreat of Late Paleozoic glacial deposits. The sedimentological study of these rocks should be quite useful in interpreting not only the fluvial architecture at the onset of Gondwana sedimentation but also the basin floor morphology and basin evolution. The detailed study of lithofacies characters, paleocurrent, and paleochannel morphology of the Barakar sediments of the Korba basin is therefore essential (1) to develop a depositional facies model of these Early Permian rocks and (2) to conclude on basin floor configuration of Gondwana paleovalleys during Early Permian sedimentation. 2. Geological setting The Gondwana master basins of Peninsular India occupy the present day river valleys of Koel-Damodar in eastern, Son-Mahanadi in eastern-central, and Pranhita-Godavari and Satpura in southcentral parts (Fig. 1). The underlying basement of the Gondwana basins is constituted by the Early Proterozoic Mobile Belt and the Middle Proterozoic Mobile Belt that were formed due to collision of various Protocontinents (Naqvi and Roger, 1987). These basins

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Fig. 1. Sketch map showing major basement domains and Gondwana master basins of Peninsular India and location of study area (modified after Veevers and Tewari, 1995; Dev, 2004).

lie almost parallel to basement fabrics through most parts, except the southern part of Pranhita-Godavari valley which runs transverse to Eastern Ghats trend (Veevers and Tewari, 1995). The Gondwana master basins of Peninsular India have been interpreted as a sandwich between Early Permian and Late Jurassic–Early Cretaceous rift episodes (Tewari and Maejima, 2010). The Korba basin is located in the south central part of the SonMahanadi valley covering an area of about 520 sq km in the Bilaspur district of Chattisgarh. The basin runs in east–west direction extending for about 64 km; its width varies from 5 to 16 km. It resembles a half graben structure bounded by a northerly dipping normal fault along the southern margin. The unclassified Late Archean/Precambrian forming the basement is composed of various types of gneisses, schists, quartzite and ambhibolite, which crop out along the northern and southern margins. Small inliers of basement rocks also occur in the northwestern part of the area. The basement is overlain by the Late Paleozoic clastic Lower Gondwana succession comprising Talchir, Barakar and Kamthi formations

(Table 1). The lowermost Talchir Formation crop out as narrow strip along the western and northern margins, and covers a larger area to the north of the Korba basin. The overlying Barakar Formation is most widespread throughout the area with a maximum estimated thickness of about 900 m. A large part of the Barakar outcrops is covered by alluvial deposits, though small outcrops occur along the banks of Hasdeo River and several tributaries. The post-Barakar rocks, known as unclassified Kamthi Formation throughout the Son-Mahanadi valley area, are correlated with Barren Measures and Raniganj formations of Koel-Damodar valley Gondwana basins due to lithological similarity. These formations have no separate identity yet in the geological map of this basin and are largely exposed in the eastern part of the area (Fig. 2). The Upper Gondwana formations which occur in the northern and southern parts of Son-Mahanadi valley basin, are absent in the area. Large scale post-Gondwana erosion is suggested on the basis of geological–structural and coal maturation studies to account for the absence of Upper Gondwana formations (Casshyap

R.C. Tewari et al. / Journal of Asian Earth Sciences 52 (2012) 43–52

45

Table 1 Stratigraphy and lithologic characters of Gondwana rocks of Korba basin of eastern India (based on Raja Rao, 1983). Age

Korba coalfield

Lithologic characters

Middle to Late Permian

Kamthi Formation

Early Permian

Barakar Formation (750– 900 M) Talchir Formation (200 m) Unconformity

Very coarse to coarse and medium grained ferruginous sandstone and thin shale beds. Conglomerate beds in the lower part Fining upward cycles of coarse to medium grained sandstone interbedded with fine-grained sandstone or siltstone, carbonaceous shale and coal Stratified tillite, conglomerate, cross-bedded sandstone, interbedded with rhythmite with or without dropstones and greenish shale xxxxxxxxxxxxxxxxxxxxxxxx Gneisses, schists, quartzites and ambhibolites

Late Carboniferous xxxxxxxxxxxxxxxxxxxx Archean/Precambrian

Fig. 2. Geological map of Korba basin, Son-Mahanadi valley, eastern-central India. Outcrop level paleocurrent directions are also shown.

and Tewari, 1984; Mishra et al., 1990). Regional strike of Gondwana rocks of this area is E–W. The rocks are low dipping (5–10°) towards south. The east–west running normal fault along the southern boundary of this basin is a prominent structural feature of the area, beside a number of small intrabasinal faults. 3. Lithofacies This study incorporates thirtyfive 2–4 m thick outcrop sections covering the whole area (Fig. 2). Relatively thick sections occur in the eastern part along the Hasdeo River and its tributaries. The Barakar Formation of this area is composed of fining upward cycles of coarse to medium grained sandstone, shale and coal similar to those in other parts of Peninsular India (Tewari, 1997). Litho-fill composition computed from bore logs suggests about 65% of sandstone, 15% shale and 20% coal. Fig. 3 displays the Barakar stratigraphy compiled from subsurface borehole logs and outcrop sections. Sandstone occurs as channel to sheet like and multistory bodies. It is very coarse (gritty), coarse to medium grained and poorly to moderately sorted. Occasionally, it is fine grained in association with thin shale beds. Besides, massive to horizontal bedded, a large part of sandstone bodies is cross bedded into cosets of trough and planar cross beds. Shale beds occurring as thin lenses or moderately thick sheets are laminated. Various lithofacies are recognized in the Barakar Formation on the basis of grain size, lithology and sedimentary structure as: (1) coarse to medium grained, massive and horizontal sandstone bedded facies; (2) coarse to medium grained trough cross bedded sandstone facies; (3) coarse to medium grained tabular cross bedded sandstone facies; (4) ripple cross laminated fine grained sandstone facies; (5) laminated shale facies; and (6) coal facies. 3.1. Coarse to medium grained, massive and horizontal bedded sandstone facies This facies forms an essential part of many channels to sheet like bodies (Fig. 4c). It is massive to crudely horizontal bedded, and occurs as discontinuous lenses or beds, a few centimeters to

Fig. 3. Generalized Barakar stratigraphy of Korba basin reproduced from borehole logs.

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decimeters thick. The sandstone is very coarse grained and poorly sorted. Rounded to well rounded pebbles and cobbles of quartzite are embedded within this facies. In addition, pebble-sized feldspar is found at places. It commonly occurs above the erosional surfaces and vertically grades into trough or tabular cross-bedded sandstone facies. The lack of sedimentary structures in the massive sandstone facies suggests gravity flow or rapid deposition during falling flow (Maizels, 1989; Miall, 1996). The horizontal bedded sandstone of very coarse to coarse and medium sand commonly occurs in the lower and upper flow regime (Allen, 1984). The facies resembles longitudinal bars developed during high flood stage and is suggestive of channel aggradations in upper flow regime during flash floods (Miall, 1996). 3.2. Coarse to medium grained, trough cross-bedded sandstone facies Trough cross-bedded cosets are the most abundant features of the Barakar sandstone (Fig. 4a). Individual occurrences are 70 cm to 2 m thick and laterally traceable along the length of the outcrops for several tens of meters. Troughs vary in depth from 25 to 85 cm and width from about 1 to 4.5 m. In general, there is a gradual decline in the size of successive troughs in cosets at a given outcrop. In larger troughs, pebble size quartzite and feldspar particles occur along the bottom layer. In places, the troughs exhibit soft sediment deformation. The facies overlies massive to horizontal beds and vertically grades to tabular cross-bedded and/or ripple cross-laminated fine grained sandstone facies in most occurrences. The trough cross-bedded cosets developed in channel and sheet like bodies correspond to unidirectional migration of sinuous crested dunes in shallow water (Collinson, 1970; Allen, 1984). The abundance of trough cross beds in the given Barakar sandstone implies deposition due to lateral accretion in braid bars/side bars of low sinuous streams (Nichols, 2009). 3.3. Coarse to medium grained tabular cross-bedded sandstone facies Following troughs, the tabular cross-bedded units are the next abundant facies of the Barakar sandstone (Fig. 4b). This facies

occurs commonly in the middle and upper parts of many channels and sheet like bodies; varies in thickness from 30 cm to 1.3 m and extends laterally along the length of sandstone bodies. Individual foresets are 20–60 cm thick and inclined at an angle of 22°. Foreset layers are 2–5 cm apart. Like trough cross-bedded cosets, the facies also show a decline in thickness in successive foreset units in most of the cosets. The tabular cross-bedded units generally occur above the trough cross beds and occasionally overlie massive to horizontal bedded sandstone. Vertically, it passes either into ripple crosslaminated fine-grained sandstone or laminated shale. The tabular cross-bedded cosets in channels and sheet like bodies corresponds unidirectional movement of straight crested sand waves in lower flow regime. It may represent deposition in transverse bed forms which develop at low rates of discharge (Rust, 1972; Smith, 1974; Miall, 1996). The successive decline in the size of foresets of planer and trough in cosets is a common feature of fluvial channel deposits implying aggradations/vertical accretion (Walker and Cant, 1984; Tewari and Gaur, 1991). 3.4. Ripple cross-laminated fine grained sandstone facies The fine-grained sandstone units commonly occurring interbedded with thin shale beds show ripple cross laminations (Fig. 4d). Individual occurrences are 10–30 cm separated by shale beds. Occasionally, the facies is 1 m thick or so forming the top of channels like bodies. The cross laminations are more trough than tabular type varying in thickness from 2.5 to 5 cm. The facies overlie tabular and/or trough cross-bedded sandstone and passes vertically into shale lithotype in most occurrences. The cross-laminated fine-grained sandstone suggests deposition by migration of small scale ripples in lower flow regime (Miall, 1996). 3.5. Laminated shale facies This facies is gray, micaceous and carbonaceous in places. It occurs in association with fine-grained sandstone as thin beds (5–20 cm) as well as between successive channels and sheet like bodies. In places, shale facies is as thick as 1–2.5 m and is

Fig. 4. Photographs showing characteristic lithofacies in the Barakar Formation, Korba basin: (a) coarse to medium grained trough cross bedded sandstone facies; (b) coarse to medium grained tabular cross bedded sandstone facies; (c) horizontal bedded sandstone facies; and (d) interbedded fine grained ripple cross laminated sandstone and laminated shale facies.

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associated with coal beds. Parallel laminations are conspicuous in shale facies through most parts. The laminated shale occurring as thin interbeds/and (or) moderately thick beds with fine grained sandstone correspond deposition through suspension in overbank and flood plain areas (Casshyap and Tewari, 1984). The lens like thin shale beds in between successive channels represent deposition on the top of channel bars through suspension during low flow (Collinson, 1970; Rust, 1972).

3.6. Coal facies Coal exploration has proved 29 coal beds in this area varying in thickness from less than a meter to about 30 m, amounting to about 20% by volume. The thin coal beds are generally confined to the lower part, whereas thicker coal seams occur in the upper part of the Barakar sequence of the Korba basin (Raja Rao, 1983). The former are non-workable and laterally impersistent extending up to few tens of meters and the latter are workable, quite persistent for a couple of kilometers and are under mining operations. The thicker seams contain several bands of carbonaceous shale and occasionally exhibit splitting with intervening coarse to medium grained channel sandstone bodies (Casshyap and Tewari, 1984). Coal facies is underlain by carbonaceous/laminated shale and succeeded by massive to horizontal bedded and cross bedded sandstone or occasionally by shale. The coal-forming environment is not a common feature of alluvial flood plains and the swamp or marsh may develop locally or occupy the entire basin overlying various subenvironments (Strahler, 1963). Recent quantitative studies of Early Permian Barakar coal measures in this area (Tewari, 2008) and other Gondwana basins have suggested the formation of coal swamps in distal flood plains and abandoned channels (Khan and Tewari, 2010a, 2010b). To sum up, the lithofacies and sedimentary character indicate longitudinal and transverse bars for the deposition of coarse to medium grained channel and sheet like sandstone bodies. The thin lens like and moderately thick sheets of shale may have formed by vertical accretion on the top of channel bars during low discharge. The subsurface lithofacies analysis of borehole logs using Markov process and entropy function in a number of Permian Gondwana

coal basins of Peninsular India including Korba basin revealed fining upward cycles where coal facies occupies the top (Tewari, 1997; Khan and Tewari, 2007).

4. Paleocurrent analysis Due to the thick alluvial cover over a greater part of the study area, paleocurrent and paleohydrologic data were collected mostly from the sandstones exposed along the stream sections (Fig. 2). Most of the data comes from the trough and tabular cross beddings of sandstones as these are more reliable paleocurrent indicators (Dott, 1973). In the present study, a total number of 443 directional (direction of inclination of the foreset/trough axis) and 331 hydrologic (cross-bedding thickness) data were collected from 35 locations. The classical hierarchical sampling plan is followed to analyze paleocurrent at three sampling levels of outcrop, sector and formation levels. The area is, therefore, subdivided into four arbitrary sectors to estimate variability associated with different levels of sampling each of which contributes to the total variance (Potter and Pettijohn, 1977). The paleocurrent data were grouped at 30° class intervals to construct rose diagrams in the nonlinear frequency scale both at sector and formation levels. The directions of the resultant vectors were computed at outcrop, sector and formation levels. In addition, 95% confidence limit of the mean, vector magnitude, circular standard deviation and probability of randomness were computed at all levels to estimate the degree of dispersion. The chi-square test was applied to ascertain the uniformity of paleocurrent distribution. Further, the equality of paleocurrent populations of corresponding sector pairs was tested by pooping paleocurrent data of the pairs of sectors and using F-test. The computed values of vector strength (vector magnitude) for the resultant paleocurrent vector at locality, sector and formation levels exceed the critical value for Raleigh’s test for the presence of a preferred trend at 1% significance level (Davis, 2002) and the probabilities of randomness are less than 102. The outcrop level mean paleocurrent vectors show a wide variation (120°) ranging from nearly WNW (289°) to NE (49°) (Table 2, Fig. 2). Further, the computed values of chi-squares in all cases are greater than the critical value (24.73) for 11 degree of freedom at the 0.01 sig-

Table 2 Statistical parameters of paleocurrent data for outcrops, four sectors and the entire area of the Barakar Formation of Korba basin. Level of measurement

Latitude (N) longitude (E)

Location-1

22°220 300 0 N 82°140 290 0 E 23°230 000 0 N 82°150 000 0 E 21°410 150 0 N 82°150 360 0 E 22°220 290 0 N 82°150 510 0 E 22°210 150 0 N 82°160 130 0 E 20°210 000 0 N 82°160 530 0 E 22°230 130 0 N 82°170 240 0 E – 22°120 000 0 N 82°200 150 0 E 22°200 120 0 N 82°210 160 0 E 22°180 230 0 N 82°200 460 0 E 22°180 340 0 N

Location-2 Location-3 Location-4 Location-5 Location-6 Location-7 Sector-I Location-8 Location-9 Location-10 Location-11

Number of data (n) 13 15

mean (hv )

Vector magnitude (L) in (%)

Circular standard deviation (S)

Probability of randomness (p)

Computed value of v2

Nature of distribution

316° (±3°)a

81.85

34°

<103

26.70b

Unimodal

28°

4

37.00

Unimodal

4

Vector

333° (±2°)

87.05

<10

13

350° (±2°)

89.32

25°

<10

34.09

Unimodal

16

348° (±3°)

83.09

32°

<104

35.01

Unimodal

11

296° (±4°)

81.49

34°

<103

64.25

Unimodal

11°

5

69.98

Unimodal

3

14

329° (±0°)

96.99

<10

9

301° (±1°)

97.34

10°

<10

51.00

Unimodal

91 10

328° (±7°) 2° (±1°)

83.10 94.15

30° 18°

<1028 <103

176.14 67.68

Unimodal Unimodal

10

336° (±5°)

79.38

36°

<102

37.92

Unimodal

4

15

354° (±3°)

84.15

31°

<10

32.20

Unimodal

14

328° (±2°)

86.69

28°

<104

32.28

Unimodal (continued on next page)

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Table 2 (continued) Level of measurement Location-12 Location-13 Location-14 Sector-II Location-15 Location-16 Location-17 Location-18 Location-19 Location-20 Location-21 Location-22 Location-23 Location-24 Sector-III Location-25 Location-26 Location-27 Location-28 Location-29 Location-30 Location-31 Location-32 Location-33 Location-34 Location-35 Sector-IV Formation a b

Latitude (N) longitude (E) 82°210 480 0 E 22°180 450 0 N 82°230 000 0 E 22°210 580 0 N 82°230 000 0 E 22°240 390 0 N 82°230 260 0 E – 22°220 510 0 N 82°240 020 0 E 22°180 130 0 N 82°240 070 0 E 22°200 400 0 N 82°240 330 0 E 22°210 580 0 N 82°240 360 0 E 22°180 450 0 N 82°250 050 0 E 22°210 360 0 N 82°250 510 0 E 22°180 130 0 N 82°250 560 0 E 22°210 160 0 N 82°260 480 0 E 22°180 340 0 N 82°260 560 0 E 22°170 410 0 N 82°270 060 0 E – 22°180 340 0 N 82°270 290 0 E 22°180 130 0 N 82°280 210 0 E 22°200 210 0 N 82°280 260 0 E 22°180 560 0 N 82°280 300 0 E 22°220 090 0 N 82°280 410 0 E 22°200 370 0 N 82°290 530 0 E 22°190 390 0 N 82°300 050 0 E 22°220 140 0 N 82°300 260 0 E 22°200 540 0 N 82°300 460 0 E 22°180 450 0 N 82°300 560 0 E 22°190 370 0 N 82°310 220 0 E – –

Number of data (n) 13

Vector mean (hv )

Vector magnitude (L) in (%)

Circular standard deviation (S)

Probability of randomness (p)

Computed value of v2

Nature of distribution

289° (±1°)

95.03

16°

<105

50.71

Unimodal

4

14

347° (±3°)

83.40

32°

<10

44.27

Unimodal

17

334° (±2°)

86.64

28°

<105

38.76

Unimodal

93 14

335° (±7°) 327° (±3°)

81.78 83.44

34° 32°

<1027 <104

171.13 35.70

Unimodal Unimodal

13

337° (±3°)

86.35

29°

<104

30.39

Unimodal

12

347° (±4°)

82.10

33°

<103

38.00

Unimodal

13

346° (±2°)

90.70

23°

<104

34.09

Unimodal

4

12

340° (±1°)

94.12

18°

<10

48.00

Unimodal

13

335° (±3°)

85.98

29°

<104

30.39

Unimodal

11

341° (±3°)

88.44

26°

<103

51.16

Bimodal

38°

3

37.42

Bimodal

4

14

342° (±4°)

77.11

<10

11

322° (±1°)

93.69

19°

<10

42.44

Unimodal

13

338° (±2°)

82.79

33°

<103

26.70

Unimodal

126 14

338° (±5°) 388° (±2°)

86.97 85.71

28° 30°

<1041 <104

299.33 32.28

Unimodal Unimodal

16

26° (±3°)

81.04

34°

<104

38.01

Unimodal

11

25° (±0°)

97.81

8°

<104

59.89

Unimodal

12

44° (±1°)

96.28

13°

<104

50.00

Unimodal

4

11

15° (±1°)

96.64

12°

<10

42.44

Unimodal

13

23° (±2°)

88.77

26°

<104

30.39

Unimodal

11

11° (±1°)

93.65

19°

<104

46.80

Unimodal

23°

3

35.61

Unimodal

4

10

49° (±2°)

91.18

<10

12

34° (±2°)

91.01

23°

<10

38.00

Unimodal

11

25° (±1°)

93.54

19°

<104

38.08

Unimodal

12

41° (±3°)

84.06

31°

<104

26.00

Unimodal

343.22 650.73

Unimodal Unimodal

133 443

29° (±5°) 351° (±4°)

89.39 77.54

25° 38°

46

<10 <10113

95% confidence limits of vector means are given in brackets. Critical value of v2 for 11 degree of freedom at 0.01 significance level is 24.73.

Table 3 Test of equality of paleocurrent populations of different pairs of sectors of the Barakar Formation of the Korba basin.

ª

Sector pairs

Total number of data (N)

Pooled vector resultant (RT)

Concentration parameter (k)

Computed value of ‘F’

Sectors Sectors Sectors Sectors Sectors

184 219 224 226 259

151.34 185.59 169.02 175.25 206.49

3.14262 3.68041 2.36930 2.64613 2.87129

2.14ª 0.36ª 222.26ª 162.26ª 209.19ª

I and II II and III I and IV II and IV III and IV

Critical values of ‘F’ for (1,120) and (1, 1) degrees of freedom at 0.05 significance level are 3.92 and 3.85 respectively.

nificance level (Table 3). This leads to the rejection of the null hypothesis (H0 = data are drawn from uniformly distributed population) and acceptance of the alternative hypothesis (H1 = Data are drawn from non-uniformly distributed population). These observa-

tions suggest the statistical significance of the resultant paleocurrent vectors at outcrop, sector and formation levels. The mean paleocurrents for the sectors I, II and III are nearly towards NNW, while that of the sector IV is N29°E (Table 2 and Fig. 5). The vector

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49

Fig. 5. Sector and formation level paleocurrent roses of the Barakar Formation, Korba basin.

magnitudes in all the cases are uniformly high and range from 81.78% to 89.39%. The paleocurrent data for the sectors III and IV show a more uniform clustering in comparison with sectors II and I. It is evident by low values of circular standard deviation and the probability of randomness as well as higher values of chi-square probabilities. It concludes that the mean paleocurrent of the Barakar Formation of the Korba basin is statistically significant, unimodal and northerly (N9°W) (Fig. 5). A lower value of vector strength and higher circular standard deviation possibly suggest dispersal of sediments by multidirectional currents, which might be in response to tributary streams, or multi-channel systems as well as variation in the direction of sediment transport within meander bars (Sengupta, 2007). The paleocurrents are slope controlled in a fluvial environment (Reddy and Prasad, 1988). Though the overall paleoslope during Barakar sedimentation in Korba basin was northerly, detailed paleocurrent analysis suggests that the depositional paleoslope for sectors I–III was north northwesterly while that for sector IV was east northeasterly. The tests of equality of paleocurrent populations at the sector level also corroborate the above statement. The computed values of ‘F’ for sector I and II as well as II and III are less than the critical value at 0.05 significance level (Table 3). This leads to the acceptance of the null hypothesis (H0) that samples came from population with the same mean direction. By contrast, the computed values of ‘F’ between all these sectors (I, II and III) and sector IV are more than the critical value at 0.05 significance level (Table 3). It suggests the acceptance

of the alternative hypothesis (H1) that samples came from populations with different mean directions (Davis, 2002). Thus, the paleocurrent samples of sectors I, II and III seem to represent single population while the paleocurrent sample of sector IV comes from a different population. This may imply bifurcation of stream channels due to a paleo watershed between sectors III and IV. 5. Paleohydrology The cross-bedding thickness data were averaged both at sector and formation levels to estimate river parameters like channel sinuosity, width, meander wavelength, slope, mean and bankful water depths, sediment load parameter, flow velocities and discharges during Barakar sedimentation. Different empirical equations used to compute the above-mentioned paleocurrent and paleohydrologic parameters are listed in Appendix A. Despite certain limitations (Ethridge and Schumm, 1978) these equations provide useful to deduce the paleochannel morphology and flow parameters of ancient fluvial channels, including those of Gondwana streams (Tewari, 2005; Hota et al., 2007). The mean value of channel sinuosities estimated from the vector magnitudes and sediment-load parameters for different sectors vary from 1.19 to 1.25 and that for the Barakar Formation is 1.27 (Table 4). These low values of sinuosity may be due to braided nature of Barakar streams or braided within meandering pattern as visualized by Hota et al. (2003). The mean cross-bed thicknesses

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Table 4 Estimates of paleohydrologic parameters of the streams of four sectors and the entire area of the Barakar Formation of Korba basin. Parameters

Sector-I

Sector-II

Sector-III

Sector-IV

Barakar formation

Channel sinuosity (P) Mean water depth (ds) Bankful water depth (db) Channel width (w) Width/depth ratio (F) Sediment load parameter (M) Meander wavelength (Lm) Mean annual discharge (Qm) Mean annual flood discharge (Qma) Channel slope (Sc) Flow velocity (v) Flood stage velocity (vf) Froude number (Fr)

1.24 3.87 m 4.22 m 188 m 48.74 4.12 2192 m 270 m3/s 1403 m3/s 0.00035 0.37 m/s 1.76 m/s 0.060

1.25 3.56 m 4.00 m 172 m 48.30 4.16 1998 m 221 m3/s 1222 m3/s 0.00038 0.36 m/s 1.78 m/s 0.061

1.21 3.72 m 4.11 m 180 m 48.52 4.14 2094 m 245 m3/s 1312 m3/s 0.00036 0.37 m/s 1.77 m/s 0.061

1.19 3.41 m 3.88 m 164 m 48.06 4.18 1901 m 199 m3/s 1134 m3/s 0.00039 0.36 m/s 1.78 m/s 0.062

1.27 3.64 m 4.05 m 176 m 48.41 4.15 2046 m 233 m3/s 1266 m3/s 0.00037 0.36 m/s 1.77 m/s 0.061

measured in field for sectors I–IV are 43, 39, 41 and 37 cm respectively and that for the total Barakar Formation is computed to be 40 cm. These values yield mean water depths of 3.41–3.87 m for different sectors and 3.64 m for the Barakar Formation. The computed channel widths vary from 164 to 188 m (average 176 m). These estimated values of channel widths correspond to the width of single channel. However, the actual channel width should be much greater due to multichannel behavior of low to moderate sinuous streams (Casshyap and Tewari, 1984). The width/depth ratios (>40) and the sediment-load parameters ranging from 4.12 to 4.18 are suggestive of bed load nature of the Barakar streams. The average water depth in the stream channel which was 3.64 m that rose to 4.05 m during periodic flood. Consequently, the mean annual discharge that was on average 233 m3/s during normal period rose to 1266 m3/s during periodic flood. The average water velocity of 0.36 m/s during normal period increased to 1.77 m/s during flood season. The meander wavelengths of individual channels were 1901–2192 m and the rivers flowed over a depositional surface that was sloping at the rate of 35–39 cm/km in a northerly direction. The Froude numbers ranging from 0.060 to 0.062 suggests a tranquil and low-flow regime in a stream channel, which accounts for the profuse development of cross-bedded units in the sandstones. The various paleohydrologic parameters computed in the present work agree well with those deduced by Casshyap and Khan (1982), Casshyap and Tewari (1984), Sengupta et al. (1988), Tewari (1993) and Hota et al. (2007) in their studies on the Gondwana successions elsewhere.

medium grained with abundant cosets of tabular and trough cross beds. Such sheet-like fluvial sandstone bodies are suggestive of low subsidence rates than sedimentation (Bridge and Leeder, 1979). The unidirectional migration of sand waves and sand dunes in transverse to linguoid channel bars should account for these tabular and trough cross bedded sandstones. The longitudinal and transverse channel bars are, therefore, the chief architectural elements in this area (Fig. 6). The estimated paleochannel parameters indicate that the Barakar streams were 4.05 m deep and 176 m wide; the channel sinuosity is in order of 1.27. The channel depth deduced here is slightly less than the measured thickness of channel sandstone bodies (6.0 m) in this area and elsewhere in other Gondwana basins of eastern-central India (Tewari, 1998). The actual channel parameters, therefore, should be much higher than the computed values. The high width to depth ratio and low sinuosity values imply bed load streams depositing the bulk of Barakar sediments in this area following Schumm (1972). These Barakar channels flowed over a paleoslope of 37 cm/km with an average flow velocity of 36 cm/s. The maximum threshold velocity of these streams is up to 78 cm/s, which is capable of transporting embedded clasts. The computed value of sediment load parameters is low (4.15) suggesting less silt and clay in the channel perimeter of the Barakar Rivers, corroborating lithofacies composition. These fine clastics facies comprise an interbedded assemblage of laminated shale and ripple cross laminated fine grained sandstone. It represents deposition by vertical accretion on top of channel bars during

6. Fluvial architecture and paleogeography Paleocurrent analysis revealed that the Early Permian Barakar sedimentation of the Korba basin resulted from rejuvenated braided streams subsequent to deglaciation, which flowed consistently from south–southeast to north–northwest through space and time in the western part (sectors I–III) and south–southwest to north–northeast in the eastern part (sector IV). The successive overlapping of Talchir and Barakar sediments upon basement rocks along the northwestern margin implies an expansion of the basin area through time, as in other Gondwana basins of Peninsular India (Tewari, 2005). The coarse to medium grained and occasionally pebbly massive to horizontal bedded channel to sheet like and multistory sandstone bodies resemble longitudinal channel bars that may have formed when the rate of migration within the aggrading channel belt was large enough to cause superposition of channel bars, before the channel belt was abandoned (Gordon and Bridge, 1987; Bridge and Mackey, 1993). Elsewhere, in eastern and central Indian Gondwana basins, these channel bodies are evidently elongated in the direction of paleoslope (Tewari, 1998). The overlying channels and sheet like sandstone bodies are coarse to

Fig. 6. Depositional facies model of Early Permian Barakar sediments of Korba basin.

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a low water stage or as overbank facies during periods of overflow. The marshy conditions may have developed in low-lying areas away from channel and overbank areas and resulted in the development of coal. In addition, lacustrine conditions of stagnant water may have also developed in areas of abandoned channels and distal flood plains to account for similar peat forming environments. The majority of coal seams in this area are sandwiched between sandstone bodies and show frequent splitting. Such interruption and termination of coal forming swamps in this area refers to the lateral shift of channel bars/crevasse splays (Casshyap and Tewari, 1984). Integrated evidences from lithofacies characters, paleocurrent pattern, paleochannel morphology and paleohydrology suggest bed-load to mixed-load moderately sinuous streams during Barakar sedimentation in this area. The northwesterly paleoslope deduced here is in close conformity with the Permian Gondwana sedimentation throughout Peninsular India (Tewari, 2005). However, significant deflection in mean paleocurrent towards northeast in sector IV (Figs. 5 and 6) in the eastern part of basin should represent watershed between sector III and IV. Indeed, the occurrence of exposed basement rocks between sector III and IV (Fig. 1) testifies the uneven basement topography prior to Gondwana sedimentation in Korba basin of Son-Mahanadi valley basins. 7. Conclusions

Table A1 Formulae used for computation of the paleocurrent and paleohydrologic parameters of Barakar streams. Formula

Reference

 Pn  Sin hi Pni¼1 hv ¼ tan Cos hi i¼1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi hP i 2 P n R¼ þ ð ni¼1 Cos hi Þ2 i¼1 Sin hi 1

Davis (2002) Davis (2002)

r ¼ Rn

Davis (2002)

1 ffi pffiffiffiffiffi 95% confidence interval ¼  180 p  1:96  nrk pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ð1  1:0115  rÞ S ¼ 180 p

Davis (2002)

2

p ¼ er n P ðOj Ej Þ2 v2 ¼ 12 j¼1 Ej   3 ðN2ÞðR1 þR2 Rp Þ F 1;N2 ¼ 1 þ 8k ðNR1 R2 Þ 1 r

Batschelet (1981) Curray (1956) Davis (2002) Davis (2002)

P ¼ 0:94M 0:25

Barrett and Fitzegerald (1985) Schumm (1963)

lnðPÞ ¼ 2:49  0:0475L þ 0:000234L2

Ghosh (2000)

P¼

1:19

H ¼ 0:086ds

Allen (1968)

w ¼ 42ds

Allen (1968)

F ¼ 225M 1:08 Lm ¼ 10:9w1:01

Schumm (1963)

1:11

0:46 Lm ¼ 106:1Q m

Leopold et al. (1964) Carlston (1965)

Sc ¼ 60M 0:38 Q 0:32 m

Schumm (1968)

0:29 db ¼ 0:6M0:34 Q m

Schumm (1969)

0:42 db ¼ 0:09M 0:35 Q ma

Schumm (1969)

Q v ¼ wd v f ¼ Qwd m

s

Following deglaciation, the Early Permian Barakar sedimentation began in the Korba basin by braided streams in a northwesterly sloping Son-Mahanadi paleovalley. There are evidences of lateral expansion of the Gondwana master basins in this and other paleovalleys of eastern and south-central India. The resultant Barakar sediments are made up of fining upward cycles of coarse to medium grained sandstone (65%), shale (15%) and coal (20%). The channel to sheet like and multistory massive sandstone bodies showing abundant scoured surfaces, and some horizontal beds are interpreted as aggradations of longitudinal bar. The succeeding channel and sheet like sandstone bodies with abundant tabular and trough cross-bedded cosets imply lateral accretion of transverse and linguoid bars. Quantitative estimates of channel form and paleohydrology are comparable with the bed load channel patterns. The thin lens like or moderately thick sheet like and laminated shale beds represent vertical accretion on top of channel bars. The coal facies is, likewise, thin to moderately thick; the later shows frequent splitting and suggest marshy conditions in abandoned channels and distal flood planes. Paleocurrent analysis interprets northwesterly and northeasterly paleoslope through space, respectively, in the western and eastern parts of the Korba basin. The two populations are statistically different and indicate bifurcation in the flow direction of depositing stream system. The lithofacies characters, paleocurrent, channel form and paleohydrologic parameters together interpret the deposition of Early Permian Barakar rocks of Korba basin as deposits of low to moderate channel sinuosity streams. However, the bifurcation of Barakar River in the northeastern part of the basin is a significant feature. It would be indicative of a watershed in the eastern part of the Korba basin during Early Permian sedimentation. It is, therefore, suggested that the basin floor of the Gondwana Master Basin of Peninsular India represents uneven topography during Early Permian times. Acknowledgements Our sincere appreciation is due to Dr. Z.A. Khan, Directorate of Geology and Mining, Lucknow, U.P. for reviewing the manuscript and making useful suggestions. One of us (RCT) is grateful to Dr.

ma

Schumm (1972)

b

F r ¼ pvffiffiffiffiffi gds

Sengupta (2007)

S.D. Sharma, Principal, Dr. Anshumali Sharma, Head, Geology Department and the Management of Sri J. N. P. G. College, Lucknow for working facilities. We are grateful to the reviewer of Journal Asian Earth Science for many constructive suggestions.

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