Facies characteristics of the basal part of the TalchirFormation, Talchir Basin, India – depositional

history revisited

Prabir Dasgupta∗ and Rishiraj Sahoo

Department of Geology, Presidency College, 86/1 College Street, Kolkata 700 073, India.∗e-mail: pdggeopc@cal3.vsnl.net.in

The lowest unit of the Talchir Formation of Talchir Basin, Orissa, was described by pioneer workersas the ‘basal boulder bed’. In an attempt to explain the co-existence of gravel and clay, materialsof contrasting hydraulic properties, a probable situation resembling the effects of the action ofground-ice enabled boulders to be carried down by sluggish currents resulting in an intermixtureof large boulders and fine mud was conceived. Misinterpretation of this conclusion led to a generaltendency to describe the ‘basal boulder bed’ as ‘glacial tillite’. However, the unit described as‘basal boulder bed’ is actually represented by a matrix rich conglomerate with pockets of normallygraded silty clay. The present study reveals that the depositional imprints preserved in this partof the sedimentary succession indicate emplacement of successive debris flows generated throughremobilization of pre-existing unconsolidated sediments. Small pockets of fine-grained turbiditespresumably deposited from the entrained turbidity currents associated with the debris flows suggestthe composite character of the debris flow deposit.

1. Introduction

The onset of Gondwana sedimentation marked theend of a long period of non-deposition that pre-vailed in peninsular India from late Proterozoic toupper Paleozoic, and hence is considered as oneof the significant geological events of this part ofthe globe. The Talchir Formation, being the lowestunit of this upper Paleozoic sedimentary succes-sion, thus deserves special attention for a properunderstanding of the Paleozoic geological historyof the Indian subcontinent.

Blanford et al (1856) were first to describe theGondwana succession of the Talchir Basin, Orissaand designated its lower part as the Talchir Groupof rocks. Since then its lithological equivalentsencountered at the lowest levels of all Gondwanabasin-fills in India have been identified as Talchirdeposits with the status of ‘formation’ in presentday lithostratigraphic classification (Sastry et al

1977). The Talchir Formation of different Gond-wana basins has been widely studied by generationsof workers and a wide spectrum of views regar-ding the depositional setting have been expressedin the literature. A literature survey reveals thatthe pioneering contribution of Blanford et al (1856)is often cited in favour of the glacial origin of thelower part of Talchir succession (Sastry et al 1977).The Talchir Formation of the Talchir Basin wassubsequently classified into three informal units(Pandya 1990; Maejima et al 1999). A criticalreview of the classification described by Maejimaet al (1999, p. 105) reveals that in the original pro-posal Pandya (1990) either classified the Talchirsuccession excluding the basal glacial tillite or thesame unit (Facies A, Pandya 1990) was inter-preted as of fluvial origin. However, according toMaejima et al (1999), the lowest unit of that pro-posed classification starts with glacial tillite, whichgradually passes upward into fluvial sediments.

Keywords. Gondwana sedimentation; Talchir Formation; Talchir Basin; debris flow; entrained turbidity current.

J. Earth Syst. Sci. 116, No. 1, February 2007, pp. 15–20© Printed in India. 15

16 Prabir Dasgupta and Rishiraj Sahoo

The other two units were attributed to lacustrineregime. Maejima et al (1999) did not furnish anydescriptive parameter for the ‘glacial tillite’ andcited the work of Blanford et al (1856) in sup-port of their interpretation. Maejima et al (2004)described the lowermost part of the Talchir succes-sion as of glacial origin. Surprisingly, no unequivo-cal evidence in favour of such an interpretation wasfurnished. Marked mismatch in thickness betweenthe lithologs (Maejima et al 2004, figures 3, 4and 5) and the radical difference between the gen-eral successions illustrated by figure 4 in Maejimaet al (1999) and figure 3 in Maejima et al (2004)made the whole thing more confusing. In supportof the interpretation it was reiterated (Maejimaet al 2004, p. 341) that the idea of glacial originfor these sediments was introduced by Blanfordet al (1856). But a careful analysis of the inferencesdrawn by Blanford et al (1856) reveals a differentpicture.

Blanford et al (1856) made an attempt to explainthe co-existence of boulder and fine mud within the‘basal boulder bed’ and analyzed different possi-bilities (Blanford et al 1856, p. 49–56) for thesimultaneous deposition of the materials of con-trasting hydraulic properties. Finally, a probablesituation very similar to the effects of the actionof ground-ice that enabled boulders to be car-ried down by sluggish currents and produced anintermixture of large boulders and fine mud waspostulated (Blanford et al 1856, p. 49). Simultane-ously the possibility of transport by true glacierswas strongly ruled out (Blanford et al 1856, p. 50).Hence, the idea of a pure glacial origin of the lowerpart of Talchir succession of Talchir Basin andidentification of the ‘basal boulder bed’ describedby Blanford et al (1856) as tillite do not have anystrong foundation, and the sediments deserve re-examination for the decipherment of the actualdepositional mechanisms involved. This is of greatsignificance to work out the actual time relationbetween Permo-Carboniferous glaciation and ori-gin of intracratonic Gondwana basins, two impor-tant events in Palaeozoic history of peninsularIndia. The present work aims at the detailed studyof the so-called ‘basal boulder bed’ with a view toexpound the depositional history. This understan-ding has an important bearing on the reconstruc-tion of the tectonosedimentary history of the initialphase of Gondwana sedimentation in peninsularIndia.

2. Geological setting

The Talchir Basin is the eastern-most member ofthe Gondwana basins of Mahanadi Valley (fig-ure 1). It is a NW–SE trending elongated basin

amidst Archean rocks and lies between latitudes20◦45′N and 21◦20′N and longitudes 84◦32′E and85◦32′E. In this basin, the lower Gondwana succes-sion, overlying the Archean basement, starts withthe Talchir Formation (lower Permian). This isfollowed successively upward by the Karharbari,Barakar and Kamthi formations (Blanford et al1856; Raja Rao 1982; Pandya 1990; Maejima et al1999) (figure 1). The ‘basal boulder bed’ (asdescribed by Blanford et al 1856) under study cons-titutes the lowest part of the Talchir Formation andis exposed along the Nandir Jhor and its tributarystreams in the south-southeastern part and partlyin the Tikra river in the northern part of the basin(figure 1).

3. The ‘basal boulder bed’

3.1 Description

The lower part of the Talchir Formation is mainlyrepresented by a matrix-rich conglomerate-shalesuccession with relatively minor amount of sand-stone. As mentioned earlier, Blanford et al (1856)described the lowest unit of this succession as the‘basal boulder bed’, which is best exposed alongthe Poipani (a tributary of Nandir Jhor) section(figure 1). The ‘basal boulder bed’, resting on theArchean basement gneiss, attains a thickness ofabout 9 meters and is conformably overlain by 16meters of thick gray shale with a sharp contact.The ‘basal boulder bed’ is represented by matrix-rich conglomerate (matrix content is about 60–70%by volume of the whole rock) with very poorlysorted gravel fraction randomly distributed withinclay-rich matrix material (figure 2A). Gravel frac-tion ranges in size from large boulder to granule,with modal value around coarse pebble to finecobble. The largest boulder measures 67 cm indiameter. Boulders are sub-angular (in the lowerpart) to well rounded (in the middle and the upperpart) and the other members of the gravel fractionare sub-rounded to rounded. Fragments of amphi-bole gneiss and quartzite constitute the main bulkof the gravel population. The presence of uprightcobbles and boulders characterizes this conglome-rate. In the lower part, occasionally, the elongateboulders show an alignment apparently subparallelto the depositional surface (figure 2A). The mostsignificant feature is the presence of a lenticularpocket (about 1 meter thick and 15 meters wide) ofgraded silty clay within this matrix-rich conglome-rate (figure 2A, B). The basal contact of this pocketis gradational with the conglomerate while it showstruncation relation with the same conglomerateat the top. The silty clay layers gradually pinchesout towards north while towards south the bedding

Basal part of Talchir Formation, Talchir Basin, India 17

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18 Prabir Dasgupta and Rishiraj Sahoo

Figure 2. (A) Matrix-rich conglomerate with pockets ofgraded silty clay. Variation in the orientation of the longaxes of the gravels with reference to the depositional sur-face is noteworthy. Some members of the gravel fractionare in upright position (marked ‘U’) while some others areparallel to the depositional surface (marked ‘P’), bendingof the graded silty clay due to the shearing effect of theover-riding conglomerate is apparent (the hammer is 32 cmlong). (B) Close-up view of the graded silty clay. The arrowshows load structure at the clay-silt interface (the scale is8 cm long). (C) Bitmap images of graded silty clay. The rect-angular area is enlarged to show the graded character. Thetwo specimens show variation in the degree of penecontem-poraneous deformation at the silt-clay interface.

planes suffered an upward bending and finally trun-cated against the host conglomerate (figure 2A).In the lower part of the unit, the graded layers areabout 10 to 15 mm thick and are characteristicallysilt-rich. The thickness of these graded-layers gra-dually increases upward and attains a maximum

value of about 4 cm with simultaneous increasein the proportion of clay. In the lower part, theinterface between the silt-rich and the clay-richparts of successive layers suffered penecontempo-raneous deformation, as is evident from the well-developed load structures and layer contortions(figure 2C).

3.2 Interpretation

Absence of any unequivocal signature of glacial ori-gin and dominance of rounded gravel within thismatrix-rich conglomerate go against the interpre-tation of glacial tillite. Remobilization of the pre-existing sediments in bringing about the presentconglomerate appears more plausible. The overallmassive character and chaotic distribution of gravelfraction floating within clay-rich matrix indicatedeposition of this conglomerate possibly from cohe-sive debris flow. Presence of upright cobbles andboulders are suggestive of the thixotropic nature ofthe flow.

Abrupt freezing of a single debris flow is diffi-cult to be conceived as the mechanism to bringa thick deposit into existence. The thick debrisflow deposits are generally considered as the prod-uct of deposition from incremental aggradation ofa number of surges (Davies 1986, 1990; Major1997; Sohn et al 1999). Deposit of the earlier surgedoes not normally consolidate before emplace-ment of the later one and the single massivelayer results through amalgamation of succes-sive surges. Apparent alignment of a few elon-gate clasts parallel to the depositional surface canbe attributed to the high rate of laminar shearstraining of individual surges (Fisher 1971; Enos1977) associated with spreading or extensionaldeformation during emplacement and subsequentsmearing by later surges before the deposit of ear-lier surges consolidated. The cumulative depositthickness thus greatly exceeded the average flowthickness.

In the lensoid pocket, embedded within theconglomerate, distribution type normal gradingobserved within the silty-clay layers indicates in-flow differentiation of grains, likely to take placewithin turbidity currents as demonstrated byMiddleton (1966a, 1966b, 1967). Appearance ofload cast and contorted layers suggests rapid sedi-mentation before minimum compaction of the claymaterial of the earlier deposit. So influx of tur-bulent surges in quick succession can be inferred.Gradual decrease in the proportion of silt andsimultaneous increase in clay content towards thetop of the unit further suggests gradual decreasein the energy condition of successive turbulentsurges. The turbidity currents were probably gene-rated at the interface between the debris flow

Basal part of Talchir Formation, Talchir Basin, India 19

Figure 3. Schematic diagram showing the inferred stagesof development of the pockets of graded silty clay withinconglomerate. (A) and (B) depict gradual progression of adebris flow with entrained turbidity current. (C) The nextinflux of debris flow set in after deposition from the ear-lier phase of debris flow and the associated turbidity cur-rent. (D) During emplacement of the debris flow over theearlier fine-grained turbidites, the latter underwent ductiledeformation due to the shearing effect of the denser debrisflow.

and the ambient water due to frictional dragduring emplacement of the former (entrained tur-bidity current) (Nemec 1990; Dasgupta 2003). Theexperimental observations of Felix and Peakall(2006) also go in favour of such processes ofgeneration of turbidity currents from debris flow.After the deposition from the debris flow, theentrained turbidity current proceeded further anddeposition took place beyond the snout of thedebris flow deposit (figure 3). The upper part ofthe turbidite deposit was dragged during emplace-ment of the next debris flow, which caused bendingand finally truncation of the silty clay layersdue to shearing. Hence, presence of the lensoidturbidite pocket of graded silty clay within thematrix-rich conglomerate goes in favour of theorigin of host sediments through amalgamationof deposits from successive debris flow surges.Mulder and Alexander (2001) described a simi-lar deposit from northwest Mediterranean Sea asspillover turbidite deposit. Talling et al (2004)described co-genetic debrite-turbidite successionsfrom three widespread successions of variable age:Miocene Marnoso Arenacea Formation in theItalian Apenines, Silurian Aberystwyth Grits inWales, and Quaternary deposits of the AgadirBasin, offshore Morocco. Detailed study of theanatomy of turbidites and linked debrites from theMarnoso Arenacea Formation, northern Apenines,Italy led Amy and Talling (2006) to conclude that

a single subaqueous flow event can comprise mul-tiple flow phases and deposit a bed with complexlateral changes between mud-rich and mud-poorsandstone.

4. Discussion

The depositional imprints preserved in the lowerpart of the Talchir Formation of the Talchir Basinindicate that within this basin the Gondwanasedimentation was initiated through emplacementof successive debris flows. Remobilization of thepre-existing unconsolidated sediments possibly ledto the deposition of the debris flow conglo-merate. Small pockets of fine-grained turbiditespresumably deposited from the entrained turbi-dity currents associated with the debris flows sug-gest the composite character of the debris flowdeposit.

Deposition of the lower part of the TalchirFormation through resedimentation processes hasalso been inferred from other Gondwana basinsof peninsular India. After studying the rocks ofTalchir Formation in the Damodar valley basins,Fox (1930) expressed serious reservations regar-ding the identification of these sediments aspurely glacial deposits. Fox (1930) emphasized thatalthough the general character of the gravel frac-tion indicates ultimate derivation by some movingice sheet from a distal source, the character of thedeposits lacks evidence for a primary glacial ori-gin, and instead shows indications of reworking.Ganju and Srivastava (1959) and Niyogi (1961) alsoadvocated the deposition of the Talchir sedimentsthrough reworking of glacial moraines. Dasgupta(2006) made a detailed study of the Talchir suc-cession of the Jharia Basin of Damodar Valley andestablished a time relation between the Permo-Carboniferous glaciation, formation of Gondwanabasins and the onset of Gondwana sedimentation,and evidences from other basins substantiated theview. This analysis also indicates a post-glaciationresedimentation process involved in the initiationof the Gondwana sedimentation. In the presentcontext also remobilization of some pre-existingglacial debris in generation of the debris flowscannot be ruled out.

Hence the depositional mechanisms postulatedby Blanford et al (1856) appear to be very closeto the inferred situation. Their idea of the prob-able situation resembling the effects of the actionof ground-ice that enabled boulders to be carrieddown by sluggish currents and produced an inter-mixture of large boulders and fine mud (Blanfordet al 1856, p. 49) can be regarded as one of thepioneering contributions in the concept of debrisflow.

20 Prabir Dasgupta and Rishiraj Sahoo

Acknowledgements

This work has been carried out with the financialassistance from the University Grants Commission,India. Authors express their gratitude to Dr. RajatMazumder of the University of Munich, Germanyand to the anonymous reviewer for their criticalreview and valuable suggestions.

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MS received 10 April 2006; revised 31 July 2006; accepted 22 August 2006