Jharia Basin Structure and Tectonics

C H A P T E R

6 Jharia Basin Structure and Tectonics Abhijit Mukhopadhyay Geological Survey of India, Kolkata, India

O U T L I N E

Introduction

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6.1 Structure

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6.2 Depositional and Tectonic History of the Jharia Basin

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INTRODUCTION The “Sickle-shaped Jharia Basin” (Fig. 6.1; Fox, 1930; Mehta and Murthy, 1957; Mukhopadhyay, 2018; Ghosh and Mukhopadhyay, 1985), the second basin of the Damodar Koel Valley belt from the east is roughly arcuate in shape and is surrounded on all sides by the Proterozoic Chotanagpur Granite Gneiss basement. At the base is the Talchir Formation (0–225 m; early Permian, Ghosh and Mitra, 1975), overlain by 2700 m of the conformable freshwater fluviatile strata of the Damuda Group also of the Permian age. Fox (1930) subdivided the Damuda Group into three “series,” namely, the Barakar “Series” or the Lower Coal Measures (60–1130 m), the Raniganj “Series” or the Upper Coal Measures (500–800 m), and the intervening middle strata, the Barren Measures (625–800 m). The Gondwana strata were later intruded by a phase of lamprophyric sills and dykes, and later, by dolerite dykes. Although the Jharia Basin has an oval outcrop, the overall synclinal morphology is asymmetric in cross-section (Fig. 6.2). The axial trace with local deviations stretches in an E-W direction. Except for a few areas of relatively steep dip (15–30 degrees), which can be attributed to faulting, the strata on an average dip lie at angles of 5–10 degrees in inward direction. Thus, the interlimb angle is generally less than 160 degrees, and the broad fold structure is of an elongate structural basin with a predominant E-W axis. Within this broad synclinal geometry, there are several anticlinal or dome-like features (the Parbatpur anticline, the Dugda anticline) and the corresponding synclinal features (the Bhojudih-Mahal syncline, the main syncline, and the Dugda syncline). All these intrabasin anticlinal and synclinal features generally run in the E-W direction, and die out laterally. There are horst-like bodies of gneisses near Pathardih in the East, Dugda in the West, and Dumra in the Northwest. In all such cases, the basement/sediment contact is faulted. Subsurface data indicates that the Pathardih “High” continues as the Parbatpur brachy-anticline under the sedimentary cover (Figs. 6.1 and 6.2). In the Chandrapura Outlier, strata have similar E-W to NW-SE strikes with steeper dips varying from 15 to 40 degrees.

6.1 STRUCTURE Normal gravity faults are the primary structural element of the basin. Most of these faults are high angle faults (>45-degree dip). However, there are a few faults which dip at low angles. The faults in the basin have developed in all scales ranging from microfaults with a few centimeters’ strike length and a few millimeters’ throw to the boundary fault stretching for several kilometers with several hundred meters’ throw. A large number (466) of normal faults

Developments in Structural Geology and Tectonics, Volume 4 https://doi.org/10.1016/B978-0-12-815218-8.00006-6

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6. JHARIA BASIN STRUCTURE AND TECTONICS

FIG. 6.1 Geological map of the Jharia Basin. BF, Bhelatanr fault; BHF, Bamangora-Hariharpur fault; JF, Jamadoba fault; JKF, Jarma-Kapuria fault; MLF, Madhuban-Lewabad fault; PF, Pathardih fault; RKF, Ramkanali fault; TF, Tisra fault.

(A)

Dugda

Jamunia R.

XVIII

IX

V/VI/VII

Dugda high Damodar R

RXII

Lohipati

RXIII

Madhuban

XVIII

IX

V/VI/VII

Dumra

(B) Scale 0

2 km.

1

Damodar R

(C) Parbatpur dome Damodar R. XVIII

IX

V/VI/VIII IX

VIII

(E)

Tisra V/VI/VIII

Pathardih high

(D)

Damodar R.

V/VI/VIII

1

2

3

4

5

6

7

8

9

10

11

12

13

14

FIG. 6.2 (A–E) Transverse geological sections across Jharia Basin. Index: (1) basement, (2) Talchir Fm. (3) Lr. Barakar, (4) combined V-IX interseam sediments, (5) IX-XV interseam sediments, (6) XV-XVIII interseam sediments, (7) supra XVIII sediments, (8) Barren measures, (9) Raniganj Fm., (10) coal seams, (11) Mica lamprophyre, (12) Dolerite, (13) faults, (14) bore hole.

6.1 STRUCTURE

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have been traced and mapped (Fig. 6.1). These are distributed all over the basin and have affected part or the entirety of the Lower Gondwana strata. The paucity of faults shown in the exposed Talchir Formation is mostly due to the difficulty in their identification, in absence of marker horizons. Although parts of the northern boundary of the basin are faulted at either end, the foremost fault of the basin is the Southern Boundary Fault. Generally, the fault runs in a WNW-ESE direction all along the southern margin of the basin (Fig. 6.1). The Southern Boundary Fault, a zone varying in width from 25 to 40 m, is seldom a single plane of dislocation. Generally, the boundary fault is comprised of a number of subparallel planes of dislocation. Each of these dislocations is individually a fault of large throw. They are connected with each other by an anastomosing network of oblique faults and give the boundary fault its total displacement. The main boundary fault has terminated southeast of Bhojudih. Further east, another boundary fault has developed with an ENE-WSW trend, delineating the southeastern corner of the basin. A short transverse fault (Fig. 6.1) connects these two boundary faults. In all likelihood, these minor deviations are governed by the presence of preexistent structural or lithological characteristics of the Precambrian basement (cf. Gilbert, 1928, pp. 20–22). Subsurface data indicate that the abrupt discontinuity of the main boundary fault and formation of a new boundary fault were probably governed by the presence of a topographic high on the basin floor, which also owes its existence to the resistant nature of its constituent lithology (Mukhopadhyay, 1981, 1984). The fault is relatively steep dipping (72–78 degrees) as measured at the outcrops. Moreover, subsurface data indicate that the fault does not show significant flattening at depth. Fox (1930) estimated the throw of the Southern Boundary Fault as “at least 5000 feet (1500 m) and probably very much more,” which was based on the cumulative thickness (the thickness of strata now present and the thickness that might have been eroded) of the formations developed in the basin (i.e., on downthrown side of the fault). However, the present study shows that there exists only 300–1600 m of strata adjacent to the fault. Hence, the minimum throw that can be attributed to the fault might have been 1600 m. However, based on the maturity of coal (Ghosh et al., 1969), the thickness of the eroded strata can be projected as 700–800 m (Mukhopadhyay, 1981). If the above is also taken into consideration, the maximum throw of the fault could be around 2400 m. Another important fact revealed from the study is that the throw is not uniform all along the fault. While the throw, on average, ranges between 1200 and 1600 m in the central part of the fault trace, it gradually decreases to 300 and 650 m toward the eastern and the western end of the fault, respectively. There are rare instances of crushing along the fault zone, and the fault-line scarps (Billings, 1954) are also rare, while silicification along the individual planes is quite common, with occasional lensoid zones of brecciation. Slickenlines present along the fault planes, within the basement and in the neighboring sediments, generally show a rake of 75 degrees and more, indicating a predominantly downdip movement. Available subsurface data indicate that majority of the faults (80%) are cover faults and die within the sedimentary prism. The rest are basement faults that continue through the sedimentary cover and penetrate the basement. Generally, these latter faults are of large strike lengths, being more than 3 km in some cases. The major basement faults with large strike lengths can be grouped into a number of fault systems, every system consisting of a set of parallel or subparallel faults, with individual faults comprising subparallel branches of bifurcating faults. The principal fault systems are as follows: 1. The arcuate fault system comprising the south dipping faults running through the central part of the Jharia basin from east to west. It consists of (i) the Pathardih and the Sudamdih faults, (ii) the Jamadoba and the Bhelatanr faults, (iii) the Tisra Fault, (iv) the Madhuban-Lewabad Fault, (v) the Jarma-Kapuria Fault, (vi) the Bamangora-Hariharpur Fault, (vii) the Pipratanr Fault, and (viii), the Dugda Fault. 2. The north dipping fault system, north of the first arcuate fault system, consists of the following individual faults: (i) the Jharia Fault, (ii) the Kirkend Fault, (iii) the Kusunda Fault, (iv) the Sijua Fault, and (v) the Ramkanali Fault. 3a. The system of transverse faults running across the Parbatpur area in the southcentral part of the basin. 3b. A similar system of transverse faults running across the Dugda area in the southwestern part of the basin. 3c. A system of transverse faults near the northwestern margin of the basin, which dip toward the west and are not spatially related to any basement dome, such as the Salanpur Fault and the Sahnidih Fault. 3d. The system of oblique faults including the Bhojudih Fault, the Mahal Fault, the Dumarda Fault, the Nutandih Fault, and the Lohipiti faults from east to west, have coalesced or have abutted against the Southern Boundary Fault, being in all likelihood related with it in origin and development. The rest of the intrabasin faults, that is, the cover faults, are spatially associated with these basement faults and are either antithetic or synthetic (Cloos, 1928) with reference to these basement faults. The faults are aligned in strike directions of 114, 84, 166, 18, and 38 degrees. Of these, the largest concentration of fault plane poles occurs with a strike of 114 degrees. The second preferred direction is 84 degrees and the third is 166 degrees, normal to the second preferred direction. There are local concentrations of strike of intrabasin faults at 18 and

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6. JHARIA BASIN STRUCTURE AND TECTONICS

38 degrees strike directions. The intrabasin faults with 114 or 166 degrees strike alignment, dip on either direction, the faults with 84 degrees strike alignment generally head toward the south, being antithetic to the Southern Boundary fault. The basement faults are generally aligned along 88, 116, and 174 degrees, of which 88 degrees is the most predominant direction. A few basement faults also strike at 116 degrees, while the N-S trending basement faults occur only in a few localized areas. Strike of the cover faults makes a significant but small angle of 15–20 degrees with the strike of the basement faults. In the Chandrapura Outlier, at least five gravity faults trending 110 and 150 degrees are traced dipping in either direction. Pascoe (1959) and Chatterjee and Ghosh (1970) have stated that the fault alignment of the Gondwana basins was controlled preferably by the regional grain of the basement. However, in the Jharia Basin, there are three preferred strike directions, namely 90, 116, and 174 degrees. While the E-W strike alignment of the basement faults shows a marked subparallelism with the axial trend of the Jharia Basin, as well as with the elongation direction of the Damodar Valley basins, the second preferred alignment of 116 degrees is subparallel to the Southern Boundary Fault and the regional strike of the Precambrian basement, the ternary direction of 174 degrees is perpendicular to the primary direction. In other words, the majority of faults are aligned perpendicular to main strain field. Although the total range of dip of both the basement and the cover faults is between 29 and 81 degrees, the majority of the faults dip 61 degrees, which compares favorably with Anderson (1951). However, subsurface data show that all the faults flatten with depth. It is estimated that while the basement faults flatten at a rate of 5–8 degrees per 1000 m, the cover faults flatten more than 15 degrees for similar depths. The throw (Billings, 1954, p. 129) of the majority of cover faults (70%) ranges between 10 and 15 m. Of the basement faults with relatively shorter strike length, 60% possess throw ranging between 75 and 100 m. However, throws of the major basement faults show a maximum of 300 m. Thus, there is a general tendency for increase of throw with the increase in the strike length of the fault. The cover faults and the basement faults with relatively shorter strike lengths do not show any significant variation of throw along the major part of their strike length and die out rapidly at either ends. However, the major basement faults show significant variations of throw along their strike. Thus, the plotted throw of the Madhuban-Loyabad Fault shows three locales of larger throw interspersed with areas of shorter throw. The major basement faults occur as systems of closely spaced parallel faults that often branch out in two or three forks and occasionally coalesce. It was noted that large basement faults do not have same throw all along the strike. But these faults are so arranged within the fault system that the maximum throw of one fault is matched by the minimum throw of the next fault of the same fault system either in the updip or downdip direction (Mukhopadhyay, 1986). Usually, the faults show a slight refraction in the sense that when the fault plane crosses an interfering surface between a sandstone bed and a bed of coal or shale, there are changes in dip. Invariably, the fault plane flattens in a shale or coal bed; generally, such flattening does not exceed 5 degrees. However, this parameter becomes an important criterion for fault-plane angle where the strata dip at a steeper angle. In those cases, the fault plane flattens perceptibly after crossing from a sandstone bed into a thick coal bed, but instead of straightening, the flattening continues. With this process repeating, the occurrence of several low angle faults in the steeply dipping areas, such as in Sudamdih, can be explained. The local dip of the basin floor and/or of the bedding in the sedimentary cover often shows steepening in the vicinity of a fault, with the displaced surface and the fault plane generally tilted in the same direction. While small cover faults show drag effects along the fault planes, the major basement faults do not show such drag effects. Although the steepening is local in nature, the increase in dip varies from 10 to 25 degrees, mainly along a belt about 60–100 m wide from the fault plane. However, in several instances, the strata along the major basement faults show steepening in dip attaining regional status, and there are variations of pattern in such steepening. The example of the Pathardih fault flanking the northern border of the Pathardih “High” illustrates one such variation. In this instance (Figs. 6.1 and 6.2) although there is a small displacement, the fault is flanked by a flexure in its downthrown side. While the strata away from the fault dip normally ranging from 6 to 10 degrees, the strata about 300–500 m wide and adjacent to the faulting become subparallel to the fault plane with a dip of 65 degrees. While the basement faults do not show significant drag effect, the throw and displacement along the fault are the same, occasionally a fault with small displacement has a large flexure of normal drag with a total large displacement. Even in a few instances, both the variations can be traced along the same fault. In other instances, where there are two closely spaced faults, both the flanks may have small displacements along the fault with the region between the two faults, either flexured or tilted into a tilt block. This is best illustrated in the western part of the basin, where the strata normally dip 10 degrees toward the south and are disturbed by two parallel faults, namely, the Mahespur fault, and the Madhuban-Loyabad fault (Figs. 6.1 and 6.2). In the eastern part of this faultsystem, the region between the two faults forms a tilted-fault block with the strata uniformly dipping at 18 degrees toward the south, while in the western part, south of Phularitanr, a 400 m wide zone of flexure occurs between the two faults, with the beds dipping at an angle of 40 degrees or more at the contact of Mahespur fault.

6.2 DEPOSITIONAL AND TECTONIC HISTORY OF THE JHARIA BASIN

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Reverse drag structures (Hamblin, 1965; Cloos, 1968) are exposed in Jharia Basin (Mukhopadhyay, 1981, 1986) only in a few places, namely, (a) along a zone, nine kilometers long near the central part of the Southern Boundary Fault; (b) on the downthrown side of the Jamadoba Fault; and (c) in a small segment of the Madhuban-Loyabad fault, south of Loyabad. In the first instance width of reverse drag feature is 400–500 m. Here the regional dip is 8 degrees northerly, while the maximum amount of dip of bed against the fault is 15 degrees southerly. In the second instance, the width of the flexure is 1200 m. The regional dip here is 6 degrees westerly, while the beds dip at an angle of 25 degrees easterly near the fault. In the third instance, the zone of flexure is 600 m wide. The regional dip of the strata is 14 degrees southwesterly and the maximum dip toward the fault plane is 12 degrees northwesterly. Thus, along each of these fault zones, the reverse drags along the faults have caused the beds to arch up into gentle anticlines trending parallel to the fault. Moreover, in each of these cases, these anticlinal flexures terminate against other oblique inclined faults. The comparative rarity of reverse drag flexures in the Jharia basin may perhaps be explained by a large number of antithetic faults (Hamblin, 1965; Cloos, 1968). Several authors are of the opinion that the overall synclinal geometry of the basin and all the local anticlinal and synclinal structures had been the result of ductile folding movement, while the faulting had been the result of later tensional force.

6.2 DEPOSITIONAL AND TECTONIC HISTORY OF THE JHARIA BASIN Environment of deposition, depositional and tectonic history of the Jharia Basin as well as of the Damodar basins, comprising different basins of the Damodar Valley and the subsidiary belt of Giridih-Deoghar Basin, is complex. An attempt has been made to unravel it from the spatial and trend of development of different lithopacks in this basin. The role played by the regional grain and earlier fracture systems toward the development of the Jharia Basin needs to be discussed. The foliation direction of the basement is E-W to NW-SE, dipping moderately toward the north. The last generation of folds also have E-W to WNW-ESE fold axes with moderate to steep northerly dipping axial planes. The E-W linearity of the Damodar Valley basins suggests that they were subjected to a N-S trending extensional regime (Sarkar, 1988). Under this condition, regional grain, transverse to the extensional trend, facilitated the development of many new primarily E-W trending gravity faults. Moreover, most of the existing fractures, transverse or nearly transverse to the extensional trend, reactivated as normal faults. The magnitude of such reactivation depended on their location in space and time and the strength of the tensional field at the time of reactivation. But those earlier fractures making a small angle with the extensional trend should have a very large strike slip component. The WNW-ESE trend for the major part of the boundary fault with an E-W trend in the eastern part can be explained if it is assumed that a large fault with WNW-ESE trend reactivated during the Gondwana time (cf. Fox, 1930; Chatterjee and Ghosh, 1970). In the Jharia Basin, deposition of the Talchir strata commenced on the granitic basement. The Talchir is exposed all along the northern periphery with maximum development in the northwestern part. Subsurface data indicate that the Talchir strata pinch out about four kilometers downdip from the present Talchir/Barakar contact, and for the rest of the basin, the Barakar strata directly overlie the basement. It is, on average, 50–75 m thick over most of the area and 200–225 m thick in the northwestern part (Fig. 6.3A). As suggested by Ghosh and Mitra (1975), during the onset of the Talchir sedimentation, the depositional area was, in all probability, restricted to erosional valleys. Deposition of thick Talchir sequence in the northwestern corner of the basin suggests the existence of such a valley. However, the sediment distribution pattern suggests that, in all probability, the glaciers were the architects of such valleys. The presence of thick basal tillites, partly faulted, may be considered end moraines, and hence, the locations where the glaciers melted. The clast-supported conglomerate at the base followed by matrix-supported conglomerate also points toward a similar inference. The northern paleoslope for the Talchir tillite (Ghosh and Mitra, 1975) also suggests that the main gathering ground was located north of the present Jharia Basin. Withdrawal of ice during deglaciation gave rise to fluvioglacial environment, and the outwash sandstone conglomerate was laid down by a braided melt water stream. With the rise of water level, the depositional area widened, and the northern fringe of the basin areas became part of the depositional area. Over the course of time, the fluvial condition gave way to a lacustrine environment, and shale was deposited. The deposition of turbidities was the result of frequent slumping of the unstable pile of sediments near the margin (Ghosh and Mitra, 1975; Mukhopadhyay, 1984; Ghosh and Mukhopadhyay, 1985). Frequent slumping in the turbidite facies on the upper part of the Talchir sequence were, in all probability, caused due to microseism and can be taken as an indication of the onset of tectonic movement, which caused the subsidence of the depositional area from the last phase of the Talchir period (Mukhopadhyay, 1984). Widening of the Talchir basinal area at the time of deposition of top shale facies might be either due to rise of sea level or due to subsidence, or a combination of both.

50 A

D

6. JHARIA BASIN STRUCTURE AND TECTONICS

Talchir time

Middle Barakar II time

B

E

Lower Barakar time

C

Middle Barakar-I time

Upper Barakar time

F

Barren Measures time

FIG.

6.3 Paleotectonic maps of Jharia Basin during different periods (A–F).

Areas of relative rapid subsidence Areas of relative lesser subsidence Inselbergs Contemporaneous fault Areas of relative uplift

The Damuda Group overlies the Talchir with a gradational contact. Consistent lithology, sand-body geometry, primary structures, and overall lithic character show that the bulk of the 2700 m of Damuda sediments in the Jharia Basin were deposited in rivers and associated swamps under a shallow water environment (Fox, 1930; Pascoe, 1959; Mukhopadhyay, 1984). It is made up of two lithological assemblages, namely the gray to milky white, compact, subarkosic to arkosic wacke of granule to fine-grained sand size and an assemblage of laminated micaceous sandy shale, micaceous shale, carbonaceous shale, and coal seams representing a “fining upward” megacycle (Casshyap, 1970a,b). In addition, the grain size markedly varies from granule to coarse in the lower member of the Barakar to medium to coarse-grained in the middle and upper members. In the Barren Measures and Raniganj Formation, sediments progressively become finer from medium to fine-grained. Sediments are never free from cement. While the lower and middle Barakar sandstones contain siliceous cement, upper Barakar and Barren Measures sandstones are ferruginous. Sandstones from the Raniganj Formation show a prevalence for calcareous cement. In addition, while lower Barakar sandstones are arkosic, the rest of the Damuda sandstones are either arkosicwacke or subarkose. In the fine-grained lithoassemblage, the quantum of shale, carbonaceous shale and coal seams increase rapidly from 4% in lower Barakar to more than 50% in the upper Barakar member. However, the lower and middle Barakar contain all the thick seams. Thick gray shales of lower Barren Measures are somewhat different lithofacies from rest of the Damuda sediments. Although lower Raniganj strata have a very high component of finer sediments, the total coal seam thickness is relatively modest. Within the fining upward megacycle (Casshyap, 1979) occur repeated “fining upward sequences of intermediate scale, generally 20–30 m thick and culminating with top-stratum deposits and coal seams. Many of these sequences are, however, truncated toward the top as indicated by scoured contact. While the coarse-grained lower member of the fining upward sequences can be attributed to deposition in river channels, fine clastics forming the upper part represents interchannel overbank facies of the flood plain. In most of the instances, there were luxuriant growths of vegetal matters on the floodplains, which also received drifted vegetal matters and gave rise to development of peatland and ultimately giving rise to formation of coal seams. Due to fluvial nature of sedimentation, these coal seams are relatively thin, local in nature, and pinch out laterally (Banerjee, 1960; Roy Choudhury et al., 1975). But Horne et al. (1978) showed that even in a fluvial regime, subsidence is the controlling factor regarding the thickness of a coal unit. Therefore, in the Jharia Basin, development of relatively thicker regional seams suggest a relatively slower subsidence rate. Simultaneously, it is probable there was a smaller sediment supply. In the Jharia Basin, there is a unique uniformity in spatial distribution and thickness of the Barakar and Raniganj coal seams development. This might be due to the fact that we are studying the seam development within the Jharia basin, (covering an area of about 500 km2) a relatively small part of the main Damodar depositional basin. If we consider the analogy of the Rewa Basin, the areal spread of

6.2 DEPOSITIONAL AND TECTONIC HISTORY OF THE JHARIA BASIN

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the Jharia is even less than 5%, and uniformity is naturally expected. However, coalescence and splitting of seams are common. Moreover, there is a pattern in such coalescence and splitting of the coal seams, and in all likelihood, the normal fluvial processes (such as breaching of flood plains, scouring of interseams, or intersplitseam parting, as well as spatial variation of subsidence) were causal factors of above phenomenon. This huge thickness of relatively shallow water sediments could only accumulate if the floor of the basin had also subsided, keeping pace with sedimentation. In other words, the process of sedimentation remained almost constant, and with variation in subsidence and climate, mainly controlled the development of coal seams or their absence. For in-depth analysis of such subsidence vis-à-vis variation of the lithopack, it is proposed to divide the Damuda sediments into a few intervals. It is observed that the trend of tectonic evolution of the basin will be best revealed by the isopach maps between some index horizons, namely certain persistent regional Barakar and Raniganj seams. Accordingly, the entire Barakar sediments have been divided into four intervals, namely, (a) the Lower Barakar Interval, considering the sediments occurring between the Talchir/Barakar contact to the floor of V/VI/VII seam; (b) the Middle Barakar Interval I occurring between the floors of V/VI/VII and IX/X seams; (c) the Middle Barakar Interval II occurring between the floors of IX/X and XV seams, and (d), the Upper Barakar Interval occurring between the floor of XV seam and XVIII seam. In the absence of any regional index, the Barren Measures interval has been considered in its totality. Likewise, the Raniganj sediments have been divided into six intervals separated by seven regional seams: R O, R II, R III, R V, R VII, R IX, and R XI (Mukhopadhyay, 1981; Ghosh and Mukhopadhyay, 1985). The widespread occurrence of very coarse detritus at the basal part of the Barakar is indicative of a “protracted epirogenic impulse in the source area” (GSI, 1977). It may be pointed out that in deep basement boreholes, the lowermost portion of the basal conglomeratic sandstones with dark gray shale matrix intersected are the weathered product of granitic basements. Moreover, similar weathered, coarse detritus (scree material) constitutes the major part of the basal Barakar sediments. Relative to the upliftment of the positive areas, subsidence was not very rapid, and in most cases, it happened in fits and starts. The isopach map of the Lower Barakar interval (Fig. 6.3B) shows a usual thickness variation from 100 to 175 m over most of the basin area except in the Dugda area, where subsurface data indicate no Lower Barakar sediment. Relative to the Lower Barakar interval, thickness variation is relatively small, indicating a uniform subsidence of the floor of the depocenter. In addition, as the Lower Barakar strata rests unconformably over the basement over most of the basin area, the variations in the pattern of isopach contours probably indicate the morphology of paleo-valleys. Because the thickness does not show any change toward the boundaries on all the side, it can be assumed that the depositional basin extended beyond the present limits of the basin (Mukhopadhyay, 1981). The area around Dugda remained a positive area during this period. The Southern Boundary Fault also did not develop at that time. There were two small inselbergs near the present southern boundary, which also remained as positive areas during Lower Barakar interval. Incidentally, of the total Barakar seam thickness; about 40% come from the Lower Barakar Member. Although a combined I/II seam occurs throughout almost the entire basin, their development as individual seams goes side by side with local splitting. One explanation of such a complex pattern of coalescence and splitting is as follows: after the development of lower seam, with accelerated subsidence, the seam was overlain by coarse clastic sediments. Then, during the period of relatively slower subsidence, the area again converted back into paleo peatland, but in some localized areas, at the initial phase of slower subsidence, some areas became positive areas with the erosion of interseam-parting sediment, and overlying paleo peatland developed directly over the underlying coal seam, and the manifestation was coalescence of the overlying seam with the underlying seam. As the parting sediment either within interseam parting or within intersplit seam parting in Lower Barakar is mainly coarse to granular sandstone, it is difficult to delineate such scoured contacts. However, in some instances, very thin sandstone or shaly sandstone band separates two seams. Only local seam IV A has development in the northeastern and central part of the basin, indicating the development of local peatland in that area during that time. The development of the combined V/VI/VII seams is unique. The seam structure shows that the seam remained a combined seam over most of the dip side area, while over most of the outcrop area, it exited as three separate seams with the presence of thin intraseam parting in Dugda and in Bhagabandh-Kendwadih areas. All these intraseam partings might be the remnants of paleo distributory channels. To sustain the development of such a thick seam, generally more than 30 m thick, the paleo peatland remained in existence for a prolonged period. This was possible as the period of relatively lesser subsidence or tectonic quiescence prevailed over sustained period. During the Talchir and Lower Barakar interval, in the absence of contemporaneous faulting, the subsidence was effected through regional downwarping of the depositional basin floor. From the middle Barakar interval, the pattern of subsidence by regional downwarping was modified by synsedimentational displacements along the normal faults (Mukhopadhyay, 1981; Ghosh and Mukhopadhyay, 1985). Two such normal faults developed during the Middle Barakar Interval I. But these two faults became inactive during the Middle Barakar II interval (Fig. 6.3C and D).

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6. JHARIA BASIN STRUCTURE AND TECTONICS

However, a number of other contemporaneous faults developed during this period. Indeed, most of the intrabasinal contemporaneous basement faults (16 in numbers) were initiated during the Middle Barakar II interval. They were aligned in an ESE-WSW direction and were located in the neighborhood of maximum subsidence of depositional basin. Because the pattern of thickness distribution does not show any relation to the Southern Boundary Fault, and because the isopach contours met the southern boundary at high angles, it is justified to conclude that the Southern Boundary Fault had not yet developed and the depositional basin extended beyond the present boundary of the basin. One of the inselbergs located near Parbatpur continued to remain a positive area. As a result, the basin was differentiated into a few connected depocenters with relatively slower sinking in the intervening zones. (Fig. 6.3D). The maximum sinking during the middle Barakar interval was in the Jamadoba-Talgaria area, followed by the Madhuban-Kharkharee area. Sedimentation processes remained almost similar during Middle Barakar interval. Periods of relatively slower subsidence alternated with the periods of faster subsidence (Bott, 1976). Moreover, due to differential sinking within the depositional area, there were localized developments of paleo peatland with relatively slower subsidence over prolonged period of time giving rise to the development of local seams. The Southern Boundary Fault—one of the most prominent structural features of the Jharia Basin—was initiated during the Upper Barakar Interval (Mukhopadhyay, 1981; Fig. 6.3E). The above fault influenced pattern of subsidence and deposition of the Jharia Basin. However, the effect of this contemporaneous fault was restricted in the central segment of the fault between Mahal and Dumarda. Separate depocenters developed during Middle Barakar interval lost their separate identities and merged together to form a single major depocenter with the center of maximum subsidence located around the Jamadoba-Talgaria area. Most of the intrabasinal contemporaneous faults which came into existence during Middle Barakar II interval continued to remain active during this period. They greatly increased their strikewise extent. A few contemporaneous faults were initiated during this interval. It may be noted that most of the intrabasinal faults were located in the area of maximum subsidence. The inselberg located south of Parbatpur, which had so long remained a positive area, was covered with sediment from the Upper Barakar time. Although the depositional processes remained the same, there is a perceptible increase of finer sediments indicating maturity of the fluvial system. But the period of tectonic quiescence became shorter, as indicated by lesser thickness of coal seams developed during this period. Unlike earlier intervals, there are a number of local seams which came into existence in Upper Barakar. Their localized development suggests that, although there was a single major depocenter during the Upper Barakar interval, as suggested from sedimentation and structural analysis, there were unequal instances of subsidence within the major subsidence. Repeatedly, two separate subdepocenters came into existence, in which, due to relative tectonic quiescence, local paleo peatlands could develop surrounded by areas of relatively rapid subsidence. The development of sideritic lenses and bands from Seam XVII is suggestive of sea connection. However, it is not reflected in S (dmf ) values. The predominant fluvial milieu that set in and continued throughout the deposition of Barakar sediments gave way to lacustrine and fluviolacustrine environment during the subsequent Barren Measures time. In the Jharia Basin, there is a thick gray shale lithoassemblage (60–125 m thick) at the basal part. It comprises gray to dark gray micaceous shale with sideritic matrix with coarse siltstones, interlamination of shale, siltstone and fine-grained sandstones, carbonaceous shales, and thin medium-grained sandstones. The thick shale facies were formed under prodelta lacustrine environment (Sengupta et al., 1979) in conjunction with sea level rise. The rest of the Barren Measures strata are represented by coarse to medium-grained feldspathic sandstone to subarkosic wacke with interlamination of shale and sandstones, and gray micaceous shale, often carbonaceous, indicating the return of fluvial milieu. In a later phase, the fluvial milieu gave way to a lacustrine environment for a small interval when the Hariharpur Shale beds were deposited. This was effected through the reactivation of intrabasinal contemporaneous faults (Mukhopadhyay, 1981; Ghosh and Mukhopadhyay, 1985). The pattern of subsidence of the Jharia depositional areas underwent a sharp change. A main feature of the change is the elimination of all but separate depocenters and the development of an E-W trending, single, elongated basin stretching from south of Dugda in the west to Jamadoba in the east, with the center of maximum subsidence located south of Mahuda. However, the eastern depocenter located around the Bhojudih-Mahal area in the southeastern corner retained its separate identity. The region in between this depocenter and the major depositional area was a zone of relatively slower subsidence than the two areas of depressions on either side; this elongated zone of slower subsidence thus appears in the form of an elongated ridge on the basement floor (Fig. 6.3F; Mukhopadhyay, 1986). This was first appearance of the Parbatpur-Pathardih ridge in the evolutionary history of the Jharia Basin. In the earlier intervals, this zone was a part of the area of maximum subsidence in the Jharia Basin. Fig. 6.3F shows that the pattern of subsidence of the depositional area was mainly controlled by a northerly dipping Southern Boundary Fault and a south-dipping intrabasinal system of faults, both contemporaneous and acting in conjunction with each other. However, the zone of maximum subsidence was subparallel and followed along the Southern Boundary Fault, which by this time, extended all along the present southern boundary of the basin.

6.2 DEPOSITIONAL AND TECTONIC HISTORY OF THE JHARIA BASIN

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Except for a few local thin coal seams at the basal part, the Barren Measures sediments are free from the development of coal seams, although the fluvial milieu of sedimentation remained active for the major part of the same interval. It can be postulated that, due to the hot and dry climate, the environment suitable for development of coal seams ceased altogether. Climatic conditions again changed in the Raniganj time into hot and humid conditions, and paleo peatlands leading to the deposition of coal seams came into existence. However, the period of tectonic quiescence was relatively shorter in comparison to periods of rapid subsidence, and the coal seams developed are generally thin. Moreover, due to synsedimentational movements along the Southern Boundary Fault, there were continuous sediment supplies from the south into the paleo peatlands leading to splitting of all the Raniganj seams adjacent to and along the Southern Boundary Fault. Because the Raniganj Formation occurs in the central part of the structural basin, it occupies a small fraction of the total basinal area. Because a part of the Raniganj strata has been eroded away, the above figures show only a part of the E-W trending intrabasinal graben; the preserved portion of the graben indicates that the faulting along the boundaries of the graben continued along with the Raniganj sedimentation. The zone of maximum sedimentation shifted further westward with respect to Barren Measures interval. If we assume that there was some sort of periodicity in between two alternating rates of subsidence or tectonic movement, namely, a phase of relative tectonic quiescence and a phase of rapid subsidence (Bott, 1976), we can have a rough idea of the subsidence rate during the rapid subsidence phases from the thickness of the parting sediments between two major coal seams, as suggested by Mackowsky (1968) for the Rhine coals. When major coal formation began in the Lower Barakar time, the parting thickness between two major seams was about 25–40 m. In the Middle Barakar period, the parting thickness marginally increased to 30–40 m. In the succeeding period, it rose to 55–60 m. The parting thickness between two shales of fining upward cycles is comparable to that of the Upper Barakar time. The average paring thickness for the Raniganj seams, however, ranges from 30 to 40 m. Thus, if there were any periodicity in tectonic movement, it would imply that the maximum rate of subsidence prevailed from the Upper Barakar time (Mukhopadhyay, 1984). It may be noted that, while the cummulative thickness of Early Permian sediments (thickness of Talchir and Barakar Formations) is around 1400 m, it is 1600 m for Late Permian time (thickness of Barren Measures and Raniganj strata). As the duration of the Early Permian is twice that of the late Permian, the rate of deposition must have also accelerated from the Upper Barakar time in the late phase of Early Permian time. The record of deposition of the Jharia Basin ends with the Raniganj strata. However, there is no doubt that a certain thickness of post Raniganj Gondwana sediments was deposited (about 800 m) and later eroded away (Ghosh et al., 1969). Tectonic evolution of the Jharia Basin continued after the deposition of the Raniganj strata. This is indicated by the occurrence of large numbers of postdepositional faults restricted entirely within the basin itself. Some of these faults formed by reactivation of preexisting contemporaneous faults, while a large number of postdepositional faults had newly developed. Fig. 6.3E shows that many of the major contemporaneous faults, including the Southern Boundary Fault, developed in short segments and extended their strike lengths in the subsequent time. In certain instances, two separate faults became connected as they extended along their strike directions or a single fault forked out into two branches. Evidence of post-Raniganj movements is also furnished by three basement highs, namely, the Pathardih High in the East, and the Dugda High and the Dumra Inlier in the West (Fig. 6.3F). Although one of these three, the Pathardih High, had been initiated as a buried basement ridge since the Barren Measures interval, none of them became positive areas during the deposition of Raniganj beds. Hence, the upward movement, which finally raised the basement to the present level and caused a sharp steepening of the flanking sedimentary cover, must have taken place in post Raniganj time. Dykes and sills of basic and ultrabasic rocks have intruded Gondwana beds of the Jharia Basin. The ultrabasic rocks of the Early Cretaceous Rajmahal Trap affinity and the affected seams are found to be displaced by many of the faults that came into existence in the postdepositional period. In a few instances, as in the northeast of Mohuda, the dykes show evidence of shearing where they have been emplaced along the fault planes. In rare instances, ultrabasic dykes cut across preexisting faults. Hence, the time of emplacement of lamprophyres overlapped with that of late stage faulting in the Jharia Basin. It may be noted that the ultrabasic rocks are restricted to the basin area alone. While they occur in profusion within the Gondwana sediments, they hardly occur in the exposed Precambrian basement except for a single instance of a lamprophyre dyke in the Dugda High. Moreover, the subsurface data show that the maximum concentration of the lamprophyre intrusives coincides with the zones of maximum subsidence in the central part of the Jharia Basin, and the volume of lamprophyres increases with depth. This is also true for other basins of the Damodar Valley. The volume of the ultrabasic intrusives within the Gondwana sediments is largest in Jharia, Raniganj, and East Bokaro basins in the eastern part of the Damodar lineaments; the volume decreases as we move toward the western basins and is at its minimum in the North Karanpura Basin. It may be noted that the total thickness of Lower Gondwana

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6. JHARIA BASIN STRUCTURE AND TECTONICS

sediments is maximum in these three basins, and decreases gradually westward, and is much less in North Karanpura basin (Laskar, 1977). Thus, there is correspondence between the amount of subsidence of the basement and the volume of the ultrabasic intrusives within the sedimentary prism (Ghosh and Mukhopadhyay, 1985). Although the emplacement of the lamprophyres is later than the development of the Jharia Basin, the spatial correspondence between the amount of intrusives and the amount of subsidence suggests a close genetic relation between the development of the basin and the igneous activity at deeper levels. Close spatial association of the basin and lamprophyre emplacement and the separation of two events in time may be explained in the following way: Epirogenic movement followed by development of the contemporaneous faults and intrabasinal graben no doubt indicates a crustal tension during the development of the basin. Normal faulting and crustal tension associated with it continued even after the deposition of Raniganj beds, the youngest strata in the Jharia Basin. It is suggested that only toward the late phase of such crustal tension—when the Jharia Basin was filled up with Gondwana sediments—did the resulting fractures reach the mantle and result in the development of a lamprophyric melt due to sudden release of pressure? Increased thermal gradient, either from superincumbent load or from geothermal activity, is the causal factor for coal metamorphism. Isovol or isorank lines are the indicators of the grade of coal metamorphism. In the Jharia Basin, the Barakar seams have attained their highest regional metamorphism. They show an uncommon picture. Although the isovol/isorank lines and structural contours show apparent parallelism, there is a cross-cutting relationship for the total basin, as well as in localized areas. It is best illustrated if we draw isovol of the XV seam over the structural map of same seam. The XV seam outcrop at Dharmabandh-Kessurgarah area has 28% volatile matter, which increases to 30% in Jamadoba-Bhowra area and to 34% in Chasnala-Bhojudih area. The fall in volatile matter with depth is also steeper in the western part (drop of 1% of VM per 100 m of overburden thickness) than in the eastern part (drop of 1% of VM per 125 m of overburden thickness), while the beds are dipping 10–12 degrees southerly in the western part to 8–10 degrees in the eastern side. Ghose et al. (1969) postulated that the thermal energy attained due to maximum depth of deposition is the causal factor of thermal metamorphism. Following Karweil’s (1956) formula with geothermal gradient, of 1°C/25 m, Ghose estimated an overburden thickness of around 3000 m to the requisite thermal metamorphism of the Barakar seams. They estimated that there was deposition of 600 m of Panchet strata, which was later eroded away. They also suggested that, while some of the faults are postcoalification in age, the main coal metamorphism was syntectonic with the faulting when the present structural morphology was attained. According to them, the presence of intrusive bodies within the basement (Verma et al., 1973) does not have any effect on thermal metamorphism. However, they were silent regarding the cross-cutting relationship between the structural contours and isovol/isorank lines. Sen (1996) suggested that such inconsistencies are due to latter modification of the basin morphology and igneous activity. He is of the opinion that the basin attained its present morphology when coal metamorphism was nearing completion. Differential sinking, coupled with the development of anticlinal-synclinal structure, was the main causal factor for such a cross-cutting relationship. He is also of the opinion that the presence of ultramafic body within the basement (Sen, 1996), as indicated by the magnetic anomalies (Verma et al., 1973) might also be the source of thermal energy for such modification. But, as the ultramafics are directly related to the mantle for their origin, and as these K-rich melts had a very small time span from its generation to emplacement, development of a magma chamber for the melts can be ruled out. On the other hand, at the end phase of basin development, dolerite dykes that might be related to the Rajmahal Trap (Early Cretaceous) intruded the Gondwana sediments. If we consider the coal maturation of Barakar seams across the Damodar Valley and its subsidiary belt of basins, a unique relationship possibly developed. The rank of the Barakar seams is highest in the western part of the Jharia Basin and in the adjoining eastern part of East Bokaro Basin. The rank gradually decreases as one moves away either toward the east or toward the west. Giridih Basin, located about 50 km toward the north, also has a very high rank of Barakar seams which goes down as one moves away in either direction. Moreover, magnetic anomalies suggest the presence of two magmatic bodies within the basement below the Jharia Basin, one along the central axis and other in the northwestern part of the basin, mainly beyond the present boundary. It is quite feasible that magmatic bodies as depicted by the magnetic anomalies supplied the thermal energy for local improvement of coal rank. Although the radiometric age dating suggests almost same age, that is, Early Cretaceous, for the ultramafics and the dolerites, it can be assumed that magma chambers came into existance after the intrusion of the ultramafics because dolerites always postdate the ultramafics. Recent studies also indicate that the Salma dolerite dyke in the Raniganj Basin although proximal to Rajmahal flood basalt, are contemporaneous to Deccan volcanism (Lala et al., 2014), the dolerite dykes in Jharia Basin may also be related to the Deccan magmatism. The local modification of coal rank might have taken place by the dissipation of thermal energy from the magmatic body within the basement. If we assume 950°C as the melting temperature and a thermal gradient of 45–60°C/km, then the magmatic body responsible for thermal energy was located about 15–18 km below the sedimentaries.