Panna Shale Formation

Transcription

1 2.1. Introduction The Panna Shale Formation conformably overlies the Dhandraul Quartzite Formation of the Kaimur Group (Table 1.2). It is characterized by very well developed thinly laminated greenish grey, khaki, brown and chocolate coloured shales. Bedding can be compared with rhythmic type laminations (Chakraborty and Chaudhari, 1990). Individual laminae are laterally persistent and can be traced for over tens of meters. Occasionally, it shows thin limestone bands. Evidence of wave and current activity are rarely recorded. Thickness varies from 100 m to 130 m (Kumar and Sharma, 2012). The presence of Chuaria-Tawuia assemblage has been recorded (Srivastava, 2004; Kumar et al., 2005). Near Panna town, the Panna Shale conformably overlies the Dhandraul Quartzite (Baghain Sandstone) and is best developed in the area east of Panna town. It is poorly developed and completely absent westwards. It mainly comprises purple to olive green (khaki), thinly laminated flaggy shale with thin siltstone and fine sandstone. A 2 m thick pink and yellowish brown flaggy argillaceous limestone bed occurs towards its top. The average thickness of the formation is of the order of m. Ripple marks, rill marks, clay-gals and pellets of psilomelane are seen at places (Kumar and Sharma, 2012). Along a traverse from Badanpur to Maihar area, the topmost part of the Kaimur sandstone becomes somewhat shaly and ultimately grades into Rewa shale. They are greenish, grayish, silty shales showing thin lamination. Intercalated within the shale are few centimeter thick fine sand layers. Thicker sand layers invariably show sole markings. Flute marks are very common; besides load structure, minute tool marks and scour marks are also present. In some cases lower surface of sand layer shows wave and current ripples. However, in most cases ripples show modifications typical of shallow water e.g. flattening and rounding of crests. Thick sand layers exhibit a definite sequence of structure where the lower surface shows sole markings followed by parallel bedding in the lower part. The upper part shows ripple bedding and ripples on the top surface (Singh, 1976). Page 14

2 The Rewa shale earlier considered as lagoonal (Singh, 1973, 1976; Banerjee, 1974) has been reinterpreted as a deepwater offshore deposit with features of wave reworking in its upper part (Akhtar, 1996; Chakraborty et al., 1996). Bose et al. (2001) suggest that the Rewa shale is deposited in the shelf. The depositional environment of the Panna Shale is inner shelf with the sand interbeds emplaced by storm flows (Chakraborty and Chaudhuri, 1990; Bose and Chakraborty, 1994; Chakraborty, 2006). According to Chakraborty et al. (2010), the Rewa shale was deposited in the shelf (inner to outer). The measured section, located in Drummondganj ghat area is about 50 m thick (Figs. 2.1 and 2.2). Drummondganj is a township close to the border of Uttar Pradesh and Madhya Pradesh on National Highway-7. The section is located at latitude N and longitude E. This sequence represents overall coarsening upward trend. The lower part of the section, about 10 m from the base is characterized by very well developed thinly laminated interbedded red coloured shales and calcitic layers; within which interbedded marl and gypseous limestone is also present. Between m level in the section is characterized by the presence of interbedded red coloured shales and green coloured gypseous shales. Above 15 m up to about 39.5 m, green coloured gypseous shales are present with few red coloured shales. The green coloured gypseous shales are fossiliferous as well as unfossiliferous. Few black coloured thin shaly layers are also seen. Above 39.5 m up to about 46 m level is characterized by interbedded siltstone layers and red coloured shales with very few green coloured gypseous shales, above it interbedded red coloured shales and green coloured gypseous shales are present. This section shows various sedimentary structures such as ripple marks, parallel lamination, modified ripples, synaeresis cracks, flaser and lenticular bedding, cross-bedding, graded bedding, desiccation cracks, salt pseudomorph, rain prints, scour and fill structures, tepee structures, deformation structures and megafossils (Figs. 2.7, 2.8 and 2.17). Ripples are mostly wave dominated. The detailed litholog of Panna Shale Formation is shown in Fig Page 15

3 Figure 2.1: The Panna Shale Formation followed by the Upper Rewa Sandstone Formation at Drummondganj ghat section. Page 16

4 Figure 2.2: Vertical distribution of lithofacies, primary sedimentary structures and megafossils of the Panna Shale Formation at Drummondganj ghat section. Sample numbers are also shown alongside litholog. Page 17

5 2.2. Lithofacies Association On the basis of lithology, grain-size and sedimentary structures, the total sequence of the Panna Shale Formation is divided into three major lithofacies associations namely A, B and C (Fig. 2.2) and interpreted for their depositional environment. These lithofacies associations consist of various lithofacies and are vertically stacked in the stratigraphic order. These lithofacies associations are: Lithofacies Association A: Calcareous shales Lithofacies Association B: Gypseous shales Lithofacies Association C: Arenaceous shales In the studied section, the Lithofacies Association A is followed by the Lithofacies Association B (Figs. 2.2, 2.9 and 2.18) and it is capped by the Lithofacies Association C (Figs. 2.2 and 2.18) Lithofacies Association A The total thickness of this lithofacies association is about 15 m from the base of the exposed section. The lower part of the section, about 10 m from the base is characterized by interbedded red coloured shales and calcitic layers; within which about 1.7 m is interbedded marl and gypseous limestone. Between m level in the section is characterized by the presence of interbedded red coloured shales and green coloured gypseous shales (Fig. 2.2). Individual beds are laterally persistent and can be traced for over tens of meters. This association represents overall coarsening upward trend. The various sedimentary structures such as parallel lamination, small scale cross-bedding, small scale wave ripples, flaser bedding, lenticular bedding, graded bedding, desiccation cracks, salt pseudomorph, modified ripples, rain prints, synaeresis cracks and gypsum rosettes are present in this association (Figs. 2.7, 2.8a, b, d and e). On the basis of lithology and sedimentary structures, this lithofacies association is again classified into three lithofacies namely Lithofacies A1- Interbedded red coloured shales and calcitic layers Lithofacies A2- Interbedded marl and gypseous limestone Lithofacies A3- Interbedded red coloured shales and green coloured gypseous shales Within the Lithofacies Association A, Lithofacies A1 is followed by Lithofacies A2 (Figs. 2.4 and 2.18) and it is capped by Lithofacies A3 (Fig. 2.18). Page 18

6 Lithofacies A1- Interbedded Red Coloured Shales and Calcitic Layers The average thickness of this lithofacies is about 30 cm in which the red coloured shales are about cm thick with thin laminae of calcite. The calcitic layers are about 1-5 cm thick. Each bed is persisting laterally up to several tens of meters. Each cycle representing red coloured shales at the base followed by calcitic layers shows coarsening upward trend (Fig. 2.3). This lithofacies is characterized by the presence of ripple marks, parallel lamination, modified ripples, synaeresis cracks, flaser and lenticular bedding, cross-bedding, desiccation cracks and salt pseudomorph (Figs. 2.7a and b, 2.8a, b and e). The thickness of cross-bedding is about 5.5 cm. Ripples are mostly wave ripples. Figure 2.3: Lithofacies A1 of the Panna Shale Formation showing coarsening upward cycles of red coloured shales at the base followed by calcitic layers at Drummondganj ghat section Lithofacies A2- Interbedded Marl and Gypseous Limestone The average thickness of this lithofacies is about cm in which the marl layers are about 6-9 cm and gypseous limestone beds are about 2-5 cm thick. The beds of this lithofacies are also laterally persistent up to several tens of meters (Fig. 2.5). Cycles representing marl layers at the base followed by gypseous limestone show coarsening upward character. In this lithofacies, the thickness of beds increases towards the upper part. The sedimentary structures such as parallel lamination and small scale wave ripples are also present (Fig. 2.8d). Page 19

7 Figure 2.4: Contact (marked by black coloured dotted line) between Lithofacies A1 and A2 of the Panna Shale Formation where Lithofacies A1 is characterized by interbedded red coloured shales and calcitic layers and Lithofacies A2 is characterized by interbedded marl and gypseous limestone at Drummondganj ghat section; hammer length is 27 cm. Figure 2.5: Lithofacies A2 of the Panna Shale Formation showing interbedded marl and gypseous limestone at Drummondganj ghat section; pen length is 14 cm. Page 20

8 Lithofacies A3- Interbedded Red Coloured Shales and Green Coloured Gypseous Shales The average thickness of this lithofacies is about cm where the red coloured shales are about 25 cm and green coloured gypseous shales are about 4-5 cm in thickness. Each bed of this lithofacies is also laterally persisting up to several tens of meters (Fig. 2.6). This lithofacies is characterized by the presence of small scale wave ripples, parallel lamination, lenticular and flaser bedding, graded bedding, mudcracks and rain prints (Figs. 2.7c and d). Figure 2.6: Lithofacies A3 of the Panna Shale Formation showing interbedded red coloured shales and green coloured gypseous shales at Drummondganj ghat section; pen length is 14 cm. Page 21

9 Figure 2.7: Field photographs showing sedimentary structures present in the Panna Shale Formation at Drummondganj ghat section. (a) Desiccation cracks in Lithofacies A1; length of the pen is 13.8 cm. (b) Polygonal cracks in Lithofacies A1; pen length is 14 cm. (c) Rain prints in Lithofacies A3; diameter of the coin is 2.5 cm. (d) Flaser and lenticular bedding in Lithofacies A3; diameter of the coin is 2.3 cm. Page 22

10 Figure 2.8: Field photographs showing sedimentary structures present in the Panna Shale Formation at Drummondganj ghat section. (a) Parallel lamination in Lithofacies A1; diameter of the coin is 2.3 cm. (b) Parallel lamination and ripple cross lamination in Lithofacies A1; coin diameter is 2.3 cm. (c) Lenticular bedding, Scour and fill and Tepee structure in Lithofacies C1; length of the pen is 13.8 cm. (d) Wave ripple in Lithofacies A2; diameter of the coin is 2.3 cm. (e) Salt pseudomorph in Lithofacies A1; coin diameter is 2.3 cm. (f) Ripple cross lamination in Lithofacies C1; pen length is 13.8 cm. Page 23

11 Environmental Interpretation of Lithofacies Association A The Lithofacies Association A consists of three lithofacies where the Lithofacies A1 is characterized by rhythmically organized interbedded red coloured shales and calcitic layers with sedimentary structures such as ripple marks, parallel lamination, modified ripples, synaeresis cracks, flaser and lenticular bedding, small scale crossbedding, desiccation cracks and salt pseudomorph indicating that sedimentation took place in upper intertidal to supratidal zone of tidal flat setting (Reineck and Singh, 1980; Longhitano et al., 2012) (Figs. 2.3, 2.7a and b, 2.8a, b and e). The Lithofacies A2 ; interbedded marl and gypseous limestone with rhythmically organized beds and sedimentary structures such as parallel lamination and small scale wave ripples suggests that it was deposited in a shallow marine environment (Makhlouf, 2003) (Figs. 2.5 and 2.8d). The Lithofacies A3 is characterized by rhythmically organized interbedded red coloured shales and green coloured gypseous shales with small scale wave ripples, parallel lamination, lenticular and flaser bedding, graded bedding, mudcracks and rain prints indicates upper intertidal to supratidal zone of sedimentation (Reineck and Singh, 1980; Chen et al., 2010) (Figs. 2.6, 2.7c and d). In the Lithofacies Association A of the studied section, Lithofacies A1 is followed by Lithofacies A2 (Figs. 2.4 and 2.18) and it is capped by Lithofacies A3 (Fig. 2.18). The coarsening upward trend of this association represents overall transgressive event (sea level rise) (Fig. 2.2). Thinly laminated layers are generally inferred to represent either low-energy suspension sedimentation or lower flow regime planar bedding (Simons et al., 1965; Eriksson et al., 1995; Eren and Oner, 2000; Santos Jr. and Rossetti, 2006; Oyanyan et al., 2012). Bedding can be compared with rhythmic type laminations (Chakraborty and Chaudhari, 1990). The rhythmites indicate the tidal environment mainly intertidal flat setting (Makhlouf, 2003; Goemaere and Dejonghe, 2005; Chen et al., 2010; Gingras et al., 2012; Longhitano et al., 2012). The presence of flaser bedding, wavy bedding, lenticular bedding, graded bedding, indicates the mixed-flat environment of deposition (Reineck and Singh, 1980; Chakrabarti, 2005; Chakraborty, 2006; Chen et al., 2010). Flaser bedding develops when mud fills in the ripple troughs and is then covered by successive ripples (Goemaere and Dejonghe, 2005). The lenticular bedding shows the sand lenses between muddy layers or discontinuous mud layer separated by continuous sand Page 24

12 layers. The genesis of these bedding is due to the energy fluctuation of tidal activity, the sandy layer deposited during period of current activity that create ripples and muddy layer during slack water periods. Mud-cracks, indicating desiccation, would also found in an upper tidal flat environment, and indicate the warmer climate. The presence of gypseous shales and salt pseudomorphs represents the excessive evaporation in arid climates which suggests the upper tidal flat environment of deposition (Thompson, 1968; Reineck and Singh, 1980). The reddish colour of the shales suggests subaerial exposure when the ironforming minerals were oxidized between tidal cycles (Khalifa et al., 2006). They contain a small percentage of hematite, a potent rock pigment that imparts the characteristic red colour. The origin of the hematite pigment has been controversial, but it is widely agreed that the iron gets in the sediment at the time of burial detritally as hydrous ferric oxides which later, during early diagenesis, get converted to hematite, provided that there is not so much organic matter to be oxidized that the iron ends up in the ferrous state. Thus, the Lithofacies Association A represents a period of slow sedimentation in a shallow, relatively low-energy marine environment under upper intertidal to supratidal condition, a suggestion supported by the presence of flaser bedding, lenticular bedding, small scale wave ripples, mud-cracks, salt pseudomorph (Thompson, 1975; Weimer et al., 1982; Galloway and Hobday, 1983; Eriksson et al., 1995; Goemaere and Dejonghe, 2005) Lithofacies Association B The total thickness of this lithofacies association is about 24.5 m ranging from 15 m up to 39.5 m level in the section (Fig. 2.2). At the lower level, green coloured gypseous shales are present with few black coloured thin shaly layers. Above it, interbedded red coloured shales and green coloured gypseous shales are present. The green coloured gypseous shales are fossiliferous. Few black coloured thin shaly layers are also seen. Above 26.4 m level, green coloured gypseous fossiliferous shales are present up to 29 m. From m level, it is again interbedded red coloured shales and green coloured gypseous shales and here the green coloured gypseous shales are unfossiliferous. Very few black coloured thin shaly layers are also seen. Above 35 m up to about 37.8 m in the section, green coloured gypseous shales with thin layers of Page 25

13 black shales are present. From 37.8 m up to 39.5 m level is interbedded red coloured shales and green coloured gypseous shales. Individual bed is persistent laterally up to tens of meters. This association represents overall fining upward trend. The various sedimentary structures such as parallel lamination, small scale cross-bedding, small scale wave ripples, lenticular bedding, desiccation cracks, rain prints, graded bedding and megafossils are present in this association (Fig. 2.17). On the basis of sedimentary structures and megafossils, this lithofacies association is again classified into two lithofacies namely Lithofacies B1- Green coloured gypseous unfossiliferous shales Lithofacies B2- Green coloured gypseous fossiliferous shales Within the Lithofacies Association B, Lithofacies B1 is followed by Lithofacies B2 (Fig. 2.18). Figure 2.9: Contact (marked by red coloured dashed line) between Lithofacies Association A and B of the Panna Shale Formation at Drummondganj ghat section where Lithofacies Association A is characterized by calcareous shales and Lithofacies Association B is characterized by gypseous shales; pen length is 14 cm Lithofacies B1- Green Coloured Gypseous Unfossiliferous Shales The average thickness of this lithofacies is about cm. Each bed is persisting laterally up to several tens of meters (Fig. 2.10). The top of this lithofacies Page 26

14 is characterized by interbedded red coloured shales and green coloured gypseous shales in which the average thickness of red coloured shales is about 6-8 cm and green coloured gypseous shales is about 7-8 cm. Few black coloured thin shaly layers are also seen. Mica content is increasing towards upper part of the lithofacies. This lithofacies is characterized by the presence of parallel lamination, lenticular bedding, small scale cross-bedding, small scale ripple marks, desiccation cracks, rain prints and graded bedding. Figure 2.10: Lithofacies B1 of the Panna Shale Formation showing green coloured gypseous unfossiliferous shales at Drummondganj ghat section Lithofacies B2- Green Coloured Gypseous Fossiliferous Shales The average thickness of this lithofacies is about cm. Each bed is persisting laterally up to several tens of meters. The top of this lithofacies is characterized by interbedded red coloured shales and green coloured gypseous shales in which the average thickness of red coloured shales is about 6-8 cm and green coloured gypseous shales is about 7-8 cm. Few black coloured thin shaly layers are also present. This lithofacies is characterized by the presence of parallel lamination, lenticular bedding, small scale ripple marks, graded bedding, small scale crossbedding and megafossils. Chuaria-Tawuia assemblage is recorded from this lithofacies (Figs. 2.11, 2.17a, b, c, d and e). Page 27

15 Figure 2.11: Lithofacies B2 of the Panna Shale Formation showing green coloured gypseous shales with Chuaria megafossils at Drummondganj ghat section, the maximum diameter of Chuaria is 0.80 cm; the coin diameter is 2.3 cm. Environmental Interpretation of Lithofacies Association B The Lithofacies Association B consists of two lithofacies where the Lithofacies B1 is characterized by green coloured gypseous shales, interbedded red coloured shales at the upper part; and sedimentary structures such as parallel lamination, lenticular bedding, small scale cross-bedding, small scale ripple marks, desiccation cracks, rain prints and graded bedding suggests the lagoonal environment (Oboh-Ikuenobe et al., 2005; Patel et al., 2008) (Fig. 2.10). The Lithofacies B2 is characterized by green coloured gypseous fossiliferous shales, interbedded red coloured shales at the upper part; and sedimentary structures such as parallel lamination, lenticular bedding, small scale ripple marks, graded bedding, small scale cross-bedding and Chuaria-Tawuia assemblage indicates the lagoonal environment of deposition (Kumar and Srivastava, 1997; Ahmad and Majid, 2010) (Figs. 2.11, 2.17a, b, c, d and e). In the Lithofacies Association B of the studied section, Lithofacies B1 is followed by Lithofacies B2 (Fig. 2.18). The presence of gypseous shales with horizontal bedding suggests that the sediments were deposited in quite water or low Page 28

16 wave and current energy in protected lagoonal environment (Patel et al., 2008; Ahmad and Majid, 2010; Patel et al., 2012). Wave ripples dominate over current ripples, this also supports lagoonal environment under low-energy condition with little current activity (Reineck and Singh, 1980). Mud-cracks are evidence for very shallow water with periodic subareial exposure. Shales are gypsiferous and lenticular-bedded, thereby reflecting freshwater to brackish water, low-energy conditions in the coastal plain, in proximal lagoon (Oboh-Ikuenobe et al., 2005). The Chuaria-Tawuia bearing Sirbu Shale of the Bhander Group represents low energy lagoonal setting (Kumar and Srivastava, 1997). A lagoon is an inland water body, usually oriented parallel to the coast. It is separated from the ocean by a barrier, connected to the ocean by one or more restricted inlets, and having depths that seldom exceed a couple of meters. A lagoon may or may not be subject to tidal mixing, and salinity can vary from that of a coastal freshwater lake to a hypersaline lagoon, depending on the hydrologic balance. A lagoon can be generally distinguished into two parts: the lagoonal pond, and the surrounding lagoonal margin/intertidal zone (Singh, 1980b). The lagoonal pond provides ideal conditions for the deposition of silty and muddy sediments where often black muds showing fine laminations are deposited (Warme, 1971). In the marginal part of the lagoonal pond, within the muddy sediments, sand layers are intercalated. The lagoonal margin shows features similar to those of an intertidal flat, except for the current formed structures which are uncommon or absent. The gentle waves of the lagoon caused winnowing of sediments near the lagoonal margins to produce silt and sand deposits with wave-formed structures. The lagoonal margins are usually developed as mixed sand-mud facies, and often show features indicating intermittent subaerial exposure of the sedimentation surface (Singh, 1980b). Thus, the Lithofacies Association B reflects deposition in lagoonal margin environment Lithofacies Association C The total thickness of this lithofacies association is about 9.9 m. It is represented from 39.5 m up to 49.4 m level in the section (Fig. 2.2). Above 39.5 m up to about 46 m level is characterized by interbedded siltstone layers and red coloured Page 29

17 shales with very few green coloured gypseous shales, above it interbedded red coloured shales and green coloured gypseous shales are present. Some beds of silty layers are persisting laterally up to tens of meters whereas some are laterally pinching out. Individual bed of red coloured shales and green coloured gypseous shales is extremely persistent and can be traced for over tens to several tens of meters. This association represents overall fining upward trend. Cycles representing the silty layers at the base followed by red coloured shales or interbedded red coloured shales and green coloured gypseous shales show also fining upward trend. The thick silty layer is characterized by presence of calcite draping. This association shows some sedimentary structures such as parallel lamination, ripples, lenticular bedding, scour and fill structures, tepee structures, deformation structures and desiccation cracks (Figs. 2.8c and f). On the basis of lithology, grain-size and sedimentary structures, this lithofacies association is again classified into three lithofacies namely Lithofacies C1- Siltstone lithofacies Lithofacies C2- Red shales lithofacies Lithofacies C3- Interbedded red coloured shales and green coloured gypseous shales lithofacies Within the Lithofacies Association C, Lithofacies C1 is followed by Lithofacies C2 and it is capped by Lithofacies C3 (Fig. 2.18) Lithofacies C1- Siltstone Lithofacies The average thickness of this lithofacies is about cm. Some beds are persisting laterally up to several tens of meters whereas some are laterally pinching out forming lenses (Fig. 2.12). It shows presence of calcite draping as lenticular bedding which is characterized by the presence of alternate micritic and sparitic layers. This lithofacies is characterized by the presence of ripple marks, parallel lamination, lenticular bedding, scour and fill structures, tepee structures and deformation structures. Ripples are mostly wave ripples (Figs. 2.8c and f) Lithofacies C2- Red Shales Lithofacies The average thickness of this lithofacies is about cm. Each bed is persisting laterally up to several tens of meters (Fig. 2.13). This lithofacies is characterized by the presence of ripple marks, parallel lamination and lenticular bedding. Ripples are mostly wave ripples. Page 30

18 Figure 2.12: Lithofacies C1 of the Panna Shale Formation showing siltstone with calcite draping as lenticular bedding at Drummondganj ghat section; hammer length is 27 cm. Figure 2.13: Lithofacies C2 of the Panna Shale Formation showing red coloured shales at Drummondganj ghat section; hammer length is 27 cm. Page 31

19 Lithofacies C3- Interbedded Red Coloured Shales and Green Coloured Gypseous Shales The average thickness of this lithofacies is about 15 cm where the average thickness of red coloured shales is about 10 cm and the average thickness of green coloured gypseous shales is about 4-5 cm. Each bed of this lithofacies is also laterally persisting up to several tens of meters (Fig. 2.14). This lithofacies is characterized by the presence of small scale wave ripples, parallel lamination, lenticular bedding and mud-cracks. Figure 2.14: Lithofacies C3 of the Panna Shale Formation showing interbedded red coloured shales and green coloured gypseous shales at Drummondganj ghat section; length of the pen is 13.8 cm. Environmental Interpretation of Lithofacies Association C The Lithofacies Association C consists of three lithofacies where the Lithofacies C1 is characterized by siltstone with calcite draping and sedimentary structures such as ripple marks, parallel lamination, lenticular bedding, scour and fill structures, tepee structures and deformation structures indicates upper intertidal zone under tidal flat setting (Eriksson et al., 1995; Goemaere and Dejonghe, 2005) (Figs. 2.8c and f, 2.12). Page 32

20 The Lithofacies C2 ; red shales with sedimentary structures such as ripple marks, parallel lamination and lenticular bedding represents upper intertidal to supratidal domain of sedimentation (Chakrabarti, 2005; Parcell and Williams, 2005) (Fig. 2.13). The Lithofacies C3 consists of interbedded red coloured shales and green coloured gypseous shales with sedimentary structures such as small scale wave ripples, parallel lamination, lenticular bedding and mud-cracks indicating that sedimentation took place in upper intertidal to supratidal zone (Thompson, 1968; Bouougri and Porada, 2002) (Fig. 2.14). In the Lithofacies Association C of the studied section, Lithofacies C1 is followed by Lithofacies C2 and it is capped by Lithofacies C3 (Fig. 2.18). The fining upward trend of this association represents overall sea level regressive event (Fig. 2.2). The alternation of silty layers and shales may reflect variations in tidal energy within an overall upper tidal flat setting (Thompson, 1975; Weimer et al., 1982; Galloway and Hobday, 1983; Eriksson et al., 1995; Goemaere and Dejonghe, 2005). The rhythmites also support the tidal environment mainly intertidal flat setting (Makhlouf, 2003; Goemaere and Dejonghe, 2005; Chen et al., 2010; Gingras et al., 2012; Longhitano et al., 2012). Thinly laminated shales are generally inferred to denote the low-energy suspension sedimentation (Eriksson et al., 1995; Eren and Oner, 2000; Santos Jr. and Rossetti, 2006; Oyanyan et al., 2012). Sedimentary structures such as wave-generated oscillation ripples, parallel lamination, mud-cracks, deformation structures, lenticular-bedding, scour and fill structures, tepee structures consistent with deposition in a low-energy, shallow marine environment, likely in the upper intertidal to supratidal environment with periodic subaerial exposure (Bouougri and Porada, 2002; Chakrabarti, 2005; Korngreen and Benjamini, 2010; Nelson et al., 2010). The interbedded gypseous green coloured shales and red coloured shales was deposited during waning evaporative, supratidal conditions. The red coloured shales suggest subaerial exposure (Khalifa et al., 2006). The red colour of the shale is interpreted to have formed through in-situ oxidation of hydrated iron-bearing minerals during a hot, arid climate in supratidal environment Page 33

21 (Parcell and Williams, 2005). Mud-cracks are the characteristic structure in the upper tidal flat environment (Eriksson et al., 1995). Thus, the Lithofacies Association C reflects the upper intertidal to supratidal environment of deposition Petrography of the Selected Samples in the Panna Shale Formation Sparitic Limestone (Sample 1 and 2) The sample-1 is composed of sparry calcite with ferruginous cement and some clastic grains (Figs. 2.15a and b). The clastics are fine-grained. The sample-2 shows a crystallized structure and is made up of coarse fractured calcite and gypsum crystals showing sparitic texture (Figs. 2.15c and d). It is characterized by the presence of secondary calcite veins and devoid of clastic grains. Texturally this microfacies is considered to be a crystalline limestone (grainstone microfacies). This is compatible with the Standard Microfacies Type (SMF) -17 of Flugel (2010). Interpretation The presence of sparry calcite indicates high energy condition. The aggregate grains recrystallized preferentially from micritic mudstone in platform setting with moderate but fluctuating water energy, low sedimentation rate and low terrigenous input. This facies appears to be very rare in ramp carbonate, but is an excellent indicator for platform interior environment (Flugel, 2010). The fine grained clastics may be come from aeolian activity or small streams. In the geologic record such fine-grained sand may occur as concentrates in intertidal beds (Wilson, 1975). During burial, compaction becomes an increasingly significant process as a result of increasing overburden pressure. In the early stage of burial, mechanical compaction is more important and results in a closer packing and fracture of grains (Tucker and Bathurst, 1990) Green Coloured Gypseous Shales (Sample 3) It consists mainly clay-size mineral grains of quartz, gypsum, feldspar, chert and mica. The quartz is predominantly monocrystalline with both straight and Page 34

22 undulose extinction and miner twinned plagioclase feldspar which is little altered to clays. Most quartz is subangular to angular and some are subrounded. Mica consists of mostly muscovite and chlorite. It is devoid of any sedimentary structure (Figs. 2.15e and f). Interpretation Deposition of clayey sediments suggests a low energy depositional environment (Sarkar et al., 2012). The abundant supply of calcium sulphate dissolved in the influx waters resulted in the predominant precipitation of gypsum under moderate evaporative conditions (Alcicek et al., 2007). Lack of sedimentary structures makes it most problematic one to interpret. Lack of sorting could imply continuous rapid sedimentation from suspension (Potter et al., 1980) and absence of reworking by bottom currents Interlayered Micritic and Sparitic Limestone (Sample 4 and 5) These samples are characterized by alternate micrite and sparite layers. The micritic layers are having some clastics. The presence of alternate micritic and sparitic layers is the characteristic of rhythmites (Fig. 2.16). Interpretation The micrite is normally deposited in low-energy environment and the sparite represents high-energy environment. The alternate presence of micrite and sparite layers indicates that the energy was fluctuating at the time of deposition. The rhythmites support the tidal environment mainly inter-tidal flat setting (Chen et al., 2010; Gingras et al., 2012; Longhitano et al., 2012). The micrite results from recrystallization of carbonate mud during diagenesis or from direct precipitation of calcite and cause lithification of the sediment. Page 35

23 Figure 2.15: Photomicrographs of selected samples of the Panna Shale Formation at Drummondganj ghat section; Sample-1 showing sparry calcite and some clastic grains with ferruginous cement. Sample-2 showing fractured calcite and gypsum crystals having sparitic texture. Sample-3 showing clay size grains of quartz, gypsum, feldspar, chert and mica. Page 36

24 Figure 2.16: Photomicrographs of selected samples of the Panna Shale Formation at Drummondganj ghat section; Sample-4 and 5 showing alternate micritic and sparitic layers Fossils Recorded from the Panna Shale Formation Some megafossils such as Chuaria and Tawuia are recorded from the Panna Shale Formation (Fig. 2.17). White (1928) first concluded that C. circularis might represent an alga (Moczydłowska, 2008). Later, this view was supported by many authors, e.g. Vidal (1974, 1976), Hofmann (1971, 1977), and Jux (1977). Ford and Breed (1973) concluded that Chuaria was a plant, most probably being a large leiospherid acritarch (Dutta et al., 2006). Kumar (2001) has suggested that Tawuia and Chuaria represent parts of a multicellular thallophytic plant which was benthic and attached to a substrate. Tawuia could be preserved only where it was growing and in the adjacent or nearby areas where it could be transported. It could not be transported to long distances as was the case with the Chuaria circularis which was Page 37

25 dispersed to for away places by current (Kumar and Srivastava, 2003). Tawuia is ontogenetically related to Chuaria (Xiao and Dong, 2006; Moczydłowska, 2008). The description of Chuaria-Tawuia assemblage present in the Panna Shale Formation is as follows. Class Chuariaphyceae (Gnilovskaya and Ishchenko in Gnilovskaya et al., 1988) Family Chuariaceae (Wenz, 1938 emend. Duan, 1982) Genus Chuaria (Walcott, 1899), emend. Vidal and Ford, 1985) (Type species: Chuaria circularis (Walcott, 1899), Vidal and Ford, 1985) (Figs. 2.17a, b, c, d and e) Description Circular to elliptical in shape, brown in colour, diameter of most of the specimens is less than 0.1 cm. The maximum diameter for Chuaria specimens recorded in the Panna Shale Formation at Drummondganj ghat section is 0.80 cm. They are present in the form of impression due to weathering or in negative relief where the carbonaceous matter is removed. Wrinkles are indistinct. Family Tawuiaceae (Ishchenko in Gnilovskaya et al., 1988) Genus Tawuia (Hofmann, in Hofmann and Aitken, 1979) (Type species: Tawuia dalensis Hofmann and Aitken, 1979) (Figs. 2.17a, b and c) Description Elliptical to oblong in shape with rounded terminals and occurring as smooth impressions only where the carbonaceous matter is removed. Width of the specimens is less than 0.1 cm. Maximum recorded length is 0.3 cm. Page 38

26 Figure 2.17: Photographs of some megafossils recorded from the Panna Shale Formation at Drummondganj ghat section. They are identified in the field only on the basis of their morphology; here (a, b and c) showing Chuaria-Tawuia assemblage; (d and e) showing Chuaria and (f)? Page 39

27 Significance of Fossils in the Panna Shale Formation Megafossils are considered the important biostratigraphic markers for the terminal Precambrian all over the world. Megafossils Chuaria-Tawuia assemblage has already been reported from the Panna Shale Formation by Srivastava (2004). Chuaria occurs in abundance, whereas Tawuia occurs rarely. Tawuia always occurs in association with Chuaria, however Chuaria can also occur in isolation (Srivastava, 2002). Chuaria-Tawuia assemblage is a useful and significant marker for broad intercontinental correlation of Meso-Neoproterozoic sequences (Sun Wligou, 1987; Srivastava, 2002). Ford and Breed (1973) considered Chuaria as a potential index fossil for the time range of 1000 Ma to 570 Ma but Hofmann and Chen (1981) and Du and Tian (1985) reported Chuaria from sedimentary sequence of Palaeoproterozoic age. Chuaria-Tawuia assemblage suggests an age from 1100 Ma to 700 Ma (Hofmann, 1985). Vidal et al., (1993) discussed the significance of the time range of the Chuaria-Tawuia assemblage with emphasis on the Ma interval which generally predates the Varanger glacial event. A similar conclusion was done by Sun (1987) that Chuaria occurrences fall in the time range of Ma (Dutta et al., 2006). Tawuia has been considered as a valid and convenient biostratigraphic index fossil for the global correlation of Ma sediments (Singh et al., 2009) Depositional Environment of the Panna Shale Formation The Panna Shale Formation consists of mainly three lithofacies associations namely Lithofacies Association A (Calcareous shales), B (Gypseous shales) and C (Arenaceous shales) (Fig. 2.2). These lithofacies associations constitute different types of lithofacies. Page 40

28 Figure 2.18: Schematic diagram showing average thickness, grain size and sedimentary structures of vertically stacked lithofacies and lithofacies associations within the Panna Shale Formation. Page 41

29 The Lithofacies Association A includes three lithofacies namely Lithofacies A1 (Interbedded red coloured shales and calcitic layers) (Fig. 2.3), Lithofacies A2 (Interbedded marl and gypseous limestone) (Fig. 2.5) and Lithofacies A3 (Interbedded red coloured shales and green coloured gypseous shales) (Fig. 2.6) with various sedimentary structures such as parallel lamination, small scale cross-bedding, small scale wave ripples, flaser bedding, lenticular bedding, graded bedding, desiccation cracks, salt pseudomorph, modified ripples, rain prints, synaeresis cracks and gypsum rosettes indicating the upper intertidal to supratidal condition (Thompson, 1975; Weimer et al., 1982; Galloway and Hobday, 1983; Eriksson et al., 1995; Goemaere and Dejonghe, 2005) (Figs. 2.2, 2.7, 2.8a, b, d and e). The Lithofacies Association B consists of two lithofacies namely Lithofacies B1 (Green coloured gypseous unfossiliferous shales) (Fig. 2.10) and Lithofacies B2 (Green coloured gypseous fossiliferous shales) (Fig. 2.11) with parallel lamination, small scale cross-bedding, small scale wave ripples, lenticular bedding, desiccation cracks, rain prints, graded bedding and megafossils suggesting the lagoonal margin environment (Patel et al., 2008; Ahmad and Majid, 2010; Patel et al., 2012) (Figs. 2.2 and 2.17). The Lithofacies Association C constitutes three lithofacies namely Lithofacies C1 (Siltstone lithofacies) (Fig. 2.12), Lithofacies C2 (Red shales lithofacies) (Fig. 2.13) and Lithofacies C3 (Interbedded red coloured shales and green coloured gypseous shales lithofacies) (Fig. 2.14) with sedimentary structures such as parallel lamination, ripples, lenticular bedding, scour and fill structures, tepee structures, deformation structures and desiccation cracks reflecting the upper intertidal to supratidal environment of deposition (Thompson, 1968; Eriksson et al., 1995; Chakrabarti, 2005; Nelson et al., 2010) (Figs. 2.2, 2.8c and f). In the succession, the Lithofacies Association A is followed by the Lithofacies Association B (Figs. 2.2, 2.9 and 2.18) and it is capped by the Lithofacies Association C (Figs. 2.2 and 2.18). The Lithofacies Association A and the Lithofacies Association C represent a period of slow sedimentation in a shallow, Page 42

30 relatively low-energy marine environment in tidal flat setting under upper intertidal to supratidal environment (Weimer et al., 1982; Eriksson et al., 1995; Goemaere and Dejonghe, 2005). The coarsening upward trend of the Lithofacies Association A represents the overall transgressive event whereas the fining upward trend of the Lithofacies Association C represents the overall regressive event (Figs 2.2 and 2.18). The Lithofacies Association B reflects deposition in lagoonal margin environment. The fining upward trend of the Lithofacies Association B represents overall transgressive event (Catuneanu, 2002). The predominance of horizontal bedding and fine grain size suggest deposition under low-energy conditions. The green coloured gypseous shales with horizontal layering, ripple marks, small scale cross-bedding, mud-cracks, and carbonaceous megafossils indicate the lagoonal margin environment (Kumar and Srivastava, 1997; Mellere et al., 2005, Desjardins et al., 2009) (Figs. 2.2, 2.10, 2.11 and 2.17). The studied section of the Panna Shale Formation up to 15 m level from the base consists of coarsening upward succession, followed by an upper fining upward succession above 15 m level (Fig. 2.2). The Lithofacies Association A is characterized by coarsening upward trend suggesting that this is deposited during the transgression, forming the TST (Transgressive Systems Tract) (Galloway, 1989; Catuneanu et al., 1998) and the Lithofacies Association C is characterized by the fining upward trend indicating that this is deposited during the regression, forming the RST (Regressive System Tract). The overall coarsening upward trend indicates the regressive event (Catuneanu et al., 1998; Catuneanu, 2002) (Figs. 2.2 and 2.18). On the basis of above mentioned characteristics and petrography (Figs and 2.16), the Panna Shale Formation indicates lagoon-tidal flat depositional environment (Fig. 2.19) whether during sedimentation, only one long lagoon or a number of lagoons existed is not clear. Page 43

31 Figure 2.19: Schematic Depositional model for the Panna Shale Formation at Drummondganj ghat section showing a lagoon surrounded by intertidal and supratidal zone where the Lithofacies Associations A and C are deposited in the upper intertidal to supratidal zone of tidal flat setting and the Lithofacies Association B is deposited in the lagoonal margin zone. Page 44