lithofacies, depositional environment and age of...

JOUR.GEOL.SOC.INDIA, VOL.72, AUGUST 2008

190 ASHOK K. SRIVASTAVA AND RUPESH S. MANKAR

Lithofacies, Depositional Environment and Age of the UpperGondwana Succession of Salbardi Area, Amravati District,

Maharashtra and Betul, Madhya Pradesh

ASHOK K. SRIVASTAVA and RUPESH S. MANKAR

P.G. Department of Geology, SGB Amravati University, Amravati – 444 602Email: Ashokamt2000@ hotmail.com

Abstract: The Gondwana succession of Salbardi area (lat. 21°25'15" N: long.78°00'00"E) is ca. 128 m thick, dominantlyarenaceous lithounit. The succession is represented by diverse lithological variations and good preservation of sedimentarystructures. Four lithofacies have been identified in the succession on the basis of detailed field and megascopic observationsviz. (i) thickly bedded, cross and parallel bedded sandstone lithofacies (ii) thinly bedded, multistoryed cross and parallelbedded sandstone-clay lithofacies (iii) concretionary sandstone lithofacies, and (iv) pebbliferous lithofacies. The lithofaciesarchitecture of the succession is interpreted to reconstruct the depositional environment which ranges from point bar tochannel floor of braided river system.

The age of succession is suggested to be Early Cretaceous on the basis of field evidences and lithologicalcorrelations.

Keywords: Gondwana, Sedimentology, Early Cretaceous, Lithofacies, Concretions, Maharashtra.

also been made to assign a precise age to the Gondwanasuccession.

GEOLOGICAL SETTING AND AGE

The Gondwana sediments of the Salbardi and adjoiningarea are tectonically exposed due to Salbardi fault, trendingroughly ENE-WSW. The entire area is criss-crossed byseveral minor faults, which makes the stratigraphy of thearea complicated. However, Archaean quartzo-feldspathicgneiss and granitic gneisses make the basement, abovewhich, the upper most part of Gondwana along with theLameta are tectonically exposed, surrounded by thewidespread Deccan Trap (Table 1, Fig. 1). The Gondwanais mainly represented by medium to coarse-grainedarenaceous unit with well developed sedimentary structures.Borehole data of the Salbardi area suggest that, theGondwana extend up to 288m depth (Ravi Shanker, 1991).The Lameta is mostly represented by chertified limestone,medium to coarse grained sandstone and intraformationalbreccia. It is unconformably overlained by basalts of theDeccan Trap.

The age of the Gondwana succession is still a matter ofdiscussion. Pascoe (1959), in general, considered the inliers

INTRODUCTION

A small patch of NE-SW trending Upper Gondwanasuccession along with the Lameta is exposed inSalbardi area (lat. 21°25'15"N:long.78°00'00"E), constitutedtogether by districts Amravati, Maharashtra and Betul,Madhya Pradesh (Fig. 1). The succession, though showinggood preservation of sedimentary structures, variablelithological units and grain size variations have not receivedmuch attention for sedimentological investigations.However, field based preliminary sedimentologicaldetails were documented about one and half centuriesback (Blandford, 1869). Subsequent specific work on thearea is inadequate and mainly focused on regional set-up,of which, the Salbardi is a small patch of Gondwana in thevast Deccan Trap province. Significantly, it includes ahot spring (Saxena, 1987; Ravi Shanker et al. 1991).Recently, petrological and geochemical studies have beenattempted by Rawale (2004).

In the present work, detailed sedimentological study ofthe Gondwana succession has been carried out whichincludes identification of various lithounits, preparation ofdetailed litholog and facies architecture. Emphasis has beengiven to reconstruct the depositional environment on thebasis of field and megascopic observations. Discussion has

JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.72, August 2008, pp.190-198

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LITHOFACIES, DEPOSITIONAL ENVIRONMENT AND AGE OF THE UPPER GONDWANA SUCCESSION 191

of Gondwana, exposed near Ellichpur (Achalpur) asequivalent to Kamthi (?Mahadeva) of Triassic. Among theseinliers, significant and laterally traceable exposures lie atBairam- Belkher area (lat. 21°16'- 21°22'N: long. 73°37'-77° 31'E), and the study area i.e. Salbardi, apart from 2-3minor exposures in nearby areas. Bairam- Belkher area liesabout sixty kilometers west of Salbardi area. The maincriteria to correlate these inliers with Kamthi and Mahadevaby Pascoe (1959) are the lithological similarities. However,in the recent studies, the Gondwana successions of the

Bairam and Belkher area, have been assigned EarlyCretaceous age on the basis of rich and varied megafloralassemblage (Srivastava et al. 1995, 1996, 1999, 2001, 2003and Gawande, 2003).

The Salbardi Gondwanas are more correlatable toBairam-Belkher Gondwanas because of the followingreasons:

i) The Gondwana inliers of Bairam, Belkher and Salbardiarea are tectonically exposed due to a single major fault.

ii) All the three inliers are overlained by the reddish-brownish-greenish sands of the Lameta.

iii) The top most part of Gondwana succession at all thethree places are represented by similar pebbliferoushorizons.

Hence, the Gondwanas of the Salbardi area are mostpossibly Early Cretaceous in age.

LITHOLOGICAL SETTING AND FACIESARCHITECTURE

The Gondwana outcrop of Salbardi area representsca. 128 m thick column of dominantly medium to coarsegrained sandstone followed by clay and pebbliferoushorizons (Fig. 2). The sandstone is whitish, yellowish-brownto brown, medium to coarse grained, compact to friable innature. The whitish sandstone is comparatively hard due tosilica cementation, whereas the brownish and yellowishsandstone with ferruginous cement is mostly friable in

Fig.1. Geological map of the Salbardi area (Ravi Shanker et al. 1991). Inset A and B shows the location and regional geological set-upof area.

Table 1. Stratigraphic succession of Salbardi area

Age Lithounits Rock types

Quaternary Soil and alluvium

- - - - - - - - - - - - - -Unconformity - - - - - - - - - - - - - -

U. Cretaceous to Deccan Trap Non-porphyritic andEocene porphyritic basalt

- - - - - - - - - - - - - -Unconformity - - - - - - - - - - - - - -

U. Cretaceous Lameta Sandstone, shales andlimestone

- - - - - - - - - - - - - -Disconformity - - - - - - - - - - - - - -

U. Cretaceous Upper Gondwana Sandstone, siltstone,conglomerate and clay

- - - - - - - - - - - - - -Unconformity - - - - - - - - - - - - - -

Archaean Quarzo-feldspathic gneisswith dolerite intrusions



convex contacts between the grains, corrosive boundaries,authigenic silica growth, replacement of silica by ferruginouscement and bending of mica flakes.

On the basis of detailed field data, lithological variationsand megascopic characteristic of the rocks, four lithofacieshave been identified, which are as follows:(i) Thickly bedded, cross and parallel bedded sandstone

lithofacies(ii) Thinly bedded, multistoryed, cross and parallel bedded

sandstone-clay lithofacies(iii) Concretionary sandstone lithofacies(iv) Pebbliferous lithofacies.

All the four lithofacies are distinctly identifiable in thefield and are stacked vertically in stratigraphic order. Thedetails of the lithofacies are as follows:

Thickly-bedded, Cross and Parallel-bedded SandstoneLithofacies

This lithofacies is about 55 m thick, predominantlyarenaceous unit showing well developed large scale crossand parallel beddings (Fig. 3A). Its lower boundary is sharp,faulted and rests on Archaean quartzo-feldspathic gneisswhich is locally intruded by dolerite dykes. The boundaryis exposed in the Maru River channel near the hot spring.The upper boundary is gradational with the overlyingsandstone-clay lithofacies. The lithounit is well exposedalong the Maru River channel and also on the pedestal trackto Jungheri village. The beds are 3-7 m thick and mostlydirty white, brownish to yellowish in colour, medium tocoarse grained, compact to friable in nature. Sorting ismoderate and percentage of matrix is low. Occasionally, thegrains show a gradation in size from medium sand up to thesize of gravel in vertical profile, forming thin granularhorizons of sub-angular to sub-rounded quartz andfeldspar.

Thinly-bedded, Multistoryed, Cross and Parallel-beddedSandstone-clay Lithofacies

This lithofacies is about 30 m thick, represented byyellowish, brownish to whitish coloured, medium grainedsandstone (Fig. 3B). Because of poor cementation andalteration of feldspar, the sandstone is friable. The unit alsoshows good preservation of large scale cross and parallelbeddings similar to previous lithofacies; however, the bedthickness is only 1 to 3 m. Sorting is moderate. It is wellexposed in the north of Maru River on pedestal track toGhorpend village and also near the Cave Hill. The lithofaciesshows interbedding of lenticular or pocketed occurrencesof small yellowish-pink, grey to brown clay bodies, which

Fig.2. Detailed litholog of the succession.

nature. The quartz grains are mostly angular to subangular.Clay, restricted to certain horizons in sandstone, showpocketed or lenticular interbedded occurrence. It isyellowish-pink, grey to brown lithounit having a tendencyto break in elongated fashion. The pebbliferous horizonscover the upper part of succession. It is brownish toreddish, matrix supported gritty-conglomeratic, hard unitcontaining subrounded to subangular pebbles of quartz andfeldspar.

Under microscope, the sandstone is represented byquartz, feldspar and minor amount of mica and lithicfragments. The quartz gains are angular to subangular andpredominantly monocrystalline showing poor sorting. Thefeldspars are mainly orthoclase though albite, microclineare also identified. Mica is present in the form of biotiteand muscovite. Quartz-feldspathic schist as lithic fragmentconstitute a very minute quantity. The cement is siliceous toferruginous in nature. Digenetic features include concavo-



Fig.3. (A) Thickly bedded sandstone exposure showing cross and parallel beddings. (B) Thinly bedded, multistoryed sandstone.(C) Sandstone showing rich preservation of ferruginous concretions.



Fig.4. (A) Clustering of ferruginous concretions. (B) Close up of isolated concretions - filled with whitish sand (left), central hollowspace due to weathering (right). (C) Surface view of the mud cracks in concretionary sandstone.



are well exposed near the Cave Hill. Ferruginous concretionsand ripple bedded surfaces are also present.

Concretionary Sandstone Lithofacies

This lithofacies is characterized by 25 m thick columnof medium to coarse grained, angular to subangular dirtywhite sandstone with abundant concretions (Fig. 3C). Theconcretions are mostly spherical to cylindrical, ferruginoussandy structures with a general diameter of 2 to 4cm,whereas, the length may range up to 8 cm in cylindricalforms. The concretions may be hollow or filled with thesame material as of the host rock. Its occurrences are mostlysolitary but sometime give an appearance of group due toclustering of more than two solitary concretions (Figs. 4Aand B). Some spherical concretions show brownish,yellowish or whitish coloured concentric layers in thetransverse sections. The lithounit also shows richpreservation of mud cracks (Fig. 4C) and large scale crossbeddings. A few thin horizons containing gravel size angularquartz and feldspar are also recorded.

Pebbliferous Lithofacies

The topmost 25 m thick lithounit is distinctlyrepresented by matrix supported brownish to reddishcoloured rock, containing pebbles of quartz and feldspar.Its lower boundary is sharp with concretionary sandstonelithofacies (Fig. 5A). The pebbles are rounded to subroundedin shape, which may range up to cobble size. The matrix ismedium to coarse grained, poorly cemented, brownish toreddish-brown arenaceous rock. Because of the poorcementation, the large pebbles are separated from the hostrock. The lithounit also show the tendency of cross bedding(Fig. 5B).

LITHOFACIES ANALYSIS AND DEPOSITIONALENVIRONMENT

A total of four lithofacies have been identified in theUpper Gondwana succession of Salbardi area (Fig. 6). Thelower most thickly bedded, cross and parallel beddedsandstone lithofacies contains medium to coarse, subangularto angular grains and implies little transportation. It alsoshows well preserved parallel and large scale cross beddings.The lithofacies is mostly monotonous in vertical profileexcept for rhythmicity of grain size variations at a fewplaces. The variation of colours from yellow to brownish-yellow seems to be a result of late diagenesis. At a fewplaces, ferruginous concretions have also been recorded.The point bar sequence is normally represented bylarge scale cross beddings at base followed by small

ripple beddings, climbing mega ripple laminations andlastly horizontal laminations (Mill, 1978; Reineck and Singh,1980; Nanson, 1980). Accordingly, the present lithofaciesmay be considered as lower part of point bar sequence.Reineck and Singh (1980) categorized the point bar depositsinto two types i.e. fine grained point bar deposit and coarsegrained point bar deposit. The present lithofacies can beplaced into the coarse-grained category on the basis of grainsize and bedding geometry. It is mostly formed in theupper reaches of the alluvial plain, where river showextensive braided characters (Reineck and Singh, 1980).Pettijohn (1984) also included such cross bedded, mediumto coarse grained sandstone in his model of point barenvironment. Recent point bar deposits showing planerand trough cross bedded sandy facies are reported fromYamuna and Ganga Rivers (Singh and Khan, 2000; Tiwariet al. 2004).

The overlying 30 m succession is thinly bedded,multistoryed, cross and parallel bedded sandstone-claylithofacies. The entire lithofacies is well marked by crossand parallel beddings. The grains are mostly medium tocoarse, subrounded to subangular in shape. The bedthickness is highly reduced compared to the underlyinglithofacies and ranges between 1 m to 3 m, which may bedue to successive overlapping of channel bodies, resultingin the development of multistoryed sandstone unit. The claybodies make interbedded or pocketed occurrence rangingin thickness from 50 cm to 2 m and lateral traceability up to20 m. Cross and parallel bedded natures of sandstone andclay interbeddings indicate a braided channel environmentof deposition for this lithofacies as proposed by Reineckand Singh (1980) and Miall (1996). Similar lithofacies hasalso been reported by Tiwari (1995) from Late TriassicGondwana of the Son Valley.

The concretionary sandstone lithofacies constitute 25msuccession above the multistoryed sandstone-claylithofacies. It is very distinctive in the field showing richpreservation of spherical to cylindrical ferruginousconcretions occurring solitary as well as in clusters. Thegrain size and texture are similar to those of the previouslithofacies. A few medium grained sandstone surfaces showsgood preservation of mud cracks and ripple laminations.However, cross bedding is also present. This lithofacies isinterpreted as a product of channel environment, showingintermittent phases of deposition. Similar facies architecturei.e. sandstone-clay interbeddings, low scale parallellaminations, iron concretions have been reported frombraided fluvial reservoir of Krishna-Godavari Basin (Kotha,2002). The presence of mud cracks at different verticalintervals is an evidence of repetitive aerial exposure of



lithounit. Presence of cross bedding and large scale ripplelaminations and mud crack horizons may indicate a seasonalvariability of water flow. Similarly, the abundance offerruginous concretions and a few thin brownish to reddish-brown, flat ferruginous horizons may also be considered asan indicative of subaerial processes (Krzyoszkosiki, 1993).The brownish to yellowish colouration may be due to theweathering of iron-bearing silicate minerals within the

sandstone during shallow burial diagenesis (Von Houten,1973).

The pebbliferous lithofacies is the top most 25 mlithounit showing a sharp contact with the underlyingconcretionary sandstone lithofacies, whereas, the uppercontact is abruptly terminated by soil. It is represented byflat-bedded pebbly sandstone bodies of 1 to 3 m thicknesses.The pebbles are dominantly of quartz and feldspar and

Fig.5. (A) Contact of concretionary sandstone and pebbliferous bed. (B) Pebbliferous bed showing tendency of cross bedding.



mostly range from 5 to 10 mm in diameter; however, cobblesare also present. Due to poor cementing, loose flat debris ofpebbles and cobbles form a common appearance in thefield. The facies shows the tendency of large scale crossbedding. This lithofacies is also interpreted as a producedof channel floor environment (Pettijohn, 1984; Miall, 1978,1984 and 1996). The provenance was not very far awayfrom the site of deposition, as most of the grains aresubangular to subrounded in shape. Similar pebbliferouslithofacies have been reported from the alluvium of Krishna-Godavari Basin and interpreted as channel floor deposit(Kotha, 2002).

Based on the detail lithofacies analysis, the UpperGondwana succession of Salbardi area is interpreted as pointbar and channel floor deposition of braided river system.Similar lithofacies are also reported from coeval UpperGondwana succession of Bairam and Belkher area withsame interpretation (Srivastava et al. 2004).

Acknowledgement: Financial assistance received inthe form of a major research project (F-31-194/2005/SR)awarded to one of the authors (AKS) by University GrantsCommission, New Delhi is thankfully acknowledged.Fig.6. Lithofacies association in vertical column.

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(Received: 27 August 2007; Revised form accepted: 7 December 2007)

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