Geosciences JournalVol. 6, No. 3, p. 227�236, September 2002

Analysis of sedimentary facies and depositional environments of the Permian Gondwana sequence in borehole GDH-45, Khalaspir Basin, Bangladesh

ABSTRACT: Lithofacies analysis of the Permian Gondwana sequencein borehole GDH-45 of the Khalaspir Basin was performed with aview to deduce the nature of depositional environments. On thebasis of dominant lithofacies association, the sequence is dividedinto six lithostratigraphic units (units A to F). Five lithofacies (con-glomerate, sandstone, siltstone, mudstone/shale and coal) are iden-tified within these units. Several sub-lithofacies, such as massive,crudely stratified, cross-stratified, ripple and parallel laminatedsandstones are also identified within these lithofacies. The sequenceforms a fining-upward trend with a rare coarsening-upward unit.The generalised Gondwana sequence is characterised mainly bychannel lags, pebbly massive to crudely cross-stratified sandstone,trough and planar cross-stratified sandstone, ripple laminatedsandstone/siltstone, massive to parallel laminated siltstone, mud-stone/shale and coal in ascending order. The facies associationsrepresent several repeated fining-upward units and cycles, indi-cating various sub-environments (channel, floodplain, flood basin/backswamp) in fluvial regime. The conglomerates might have beendeposited as debris flow or channel lag deposits. The sandstoneswere deposited mainly as multistoried channel and lateral bars inmoderately braided and sinuous streams. The siltstone and mud-stone lithofacies indicate bar top, natural levee or floodplain toflood basin environments. The coal lithofacies suggests depositionin low-lying, short to long persistent, moderately to well drainedand sparse to densely vegetated backswamps in fluvial channel-flood-plain complex. The overall succession of the Gondwana borehole sed-iments suggests that the depositional basin became, with time, gentlerin slope gradient, resulting in a more sinuous stream setting.

Key words: lithofacies, Gondwana, Khalaspir, depositional environments

1. INTRODUCTION

The Khalaspir Basin is located in the Pirganj upazilla ofRangpur district in the northwestern part of Bangladesh(Fig. 1). Total coal bearing area of the basin covers about12.30 km2. The borehole GDH-45 lies in the center of thebasin (Fig. 1) in which more than 825 m Gondwana sedi-ments was encountered. The basal part of the sediments isunknown because the drilling was not reached. It overliesthe Basement Complex with basal conglomerate/tillite (Islam etal., 1992). Detailed sedimentological study on the Gondwanasediments of Bangladesh is very limited (Uddin and Islam,

1992; Islam, 1993, 1994). In present study a detailed litho-facies analysis was aimed in order to understand deposi-tional environments of the Gondwana sequence in the basinas it has never been done before since its discovery in 1989.

2. GEOLOGY OF THE BASIN

The Khalaspir Basin is situated in the Rangpur Saddle ofthe Stable Shelf zone of the Bengal Basin (Khan and Rah-man, 1992; Reimann, 1993; Samsuddin and Abdullah,1997). The Gondwana rocks in this basin developed in amore or less NW−SE elongated fault bounded half graben,intracratonic basin, within crystalline basement (Islam etal., 1992). The embryonic paleotopographic depressiongradually became large basin to accommodate the largevolumes of sediments deposited during the Permian. Thepresent structures are due to tectonic activity during theCretaceous, the Gondwana break up, and the TertiaryHimalayan upliftment.

The Khalaspir Basin is a more or less covered withRecent Alluvium and Pleistocene Barind Clay Formation.The sedimentary rocks of the borehole consist of Gond-wana Group, Surma Group, Dupi Tila Formation, BarindClay Formation and Alluvium of Permian, Miocene,Pliocene, Pleistocene and Recent ages respectively (Table1). On the basis of dominant lithology the Gondwanasequence has been sub-divided into six lithostratigraphicunits (Table 1).

3. LITHOFACIES

Lithology, texture, internal sedimentary structures, boundaryconditions of the upper part of the Gondwana sequence arestudied in detailed to identify the individual lithofacies fol-lowing many previous works (Allen, 1968; Williams andRust, 1969; Cant and Walker 1970; Miall, 1978; Rust,1978; Casshyap and Tewari, 1984; Walker, 1984; Reading,1986; Islam, 2002). Individual lithofacies are coded follow-ing the scheme of Miall (1978) and Rust (1978).

Five sedimentary facies and their sub-lithofacies are iden-tified in the Gondwana sequence in the borehole GDH-45.

H.M. Zakir HossainM. Sultan-Ul-Islam*Syed Samsuddin AhmedIsmail Hossain

} Department of Geology and Mining, University of Rajshahi, Rajshahi-6205, Bangladesh

*Corresponding author: sultan_ul_islam@yahoo.com

228 H.M. Zakir Hossain, M. Sultan-Ul-Islam, Syed Samsuddin Ahmed and Ismail Hossain

3.1. Conglomerate Lithofacies

It is developed sporadically at the bottom part of the stud-ied portion of the Gondwana sequence (Fig. 2a). Generallythe lithofacies overlies the erosional surfaces. It constitutesabout 14% of the Gondwana sequence. Generally conglom-erates are grey to dark grey, yellowish and reddish brown,poorly sorted and clast- and matrix-supported (Gm, Gms).In most cases, the matrix comprises medium to coarse-grained sands, argillaceous and carbonaceous materials.The conglomerates are polymictic, texturally immature andcrudely stratified or massive (Fig. 3a, b).

The clast-supported massive conglomerate units weredeposited most probably as small lensoid bodies of channellag or longitudinal braided bars of low sinuous streams(Rust, 1972,1978; Miall, 1977; Reineck and Singh, 1980).The matrix-supported and poorly sorted thin conglomerateunit might have been deposited as debris flow deposits(Carlos et al., 1997). The crudely developed trough cross-

stratifications signify an aggradational filling of frequentlyshifted smaller channels (Miall, 1977) and vertical aggra-dation in multi-channel braided streams (Casshyap andTewari, 1984). These conglomerates might have resultedfrom fan accumulation in response to uplift along faultedmargin (Miall, 1981; Islam, 2002).

3.2. Sandstone Lithofacies

The sandstone (pebbly, coarse to fine-grained) lithofaciesis mainly composed of massive sandstone (Sm), horizon-tally stratified sandstone (Sh/Sl), trough cross-stratified sand-stone (St), planar cross-stratified sandstone (Sp) and ripplecross-laminated sandstone (Sr). This lithofacies constitutesabout 55% of the studied Gondwana sequence in the bore-hole. The sandstones show mostly fining-upward sequences(Fig. 2a, b, c, d). These are grey to dark grey, dirty white,black and yellowish brown in colour, sub-angular to sub-rounded, moderately to poorly sorted, composed of quartz,

Fig. 1. Location of the borehole GDH-45 in the Khalaspir Basin, Rangpur, Bangladesh.

Sedimentary facies analysis of the Gondwana sequence in Bangladesh 229

feldspar, mica, dark minerals and rock fragments. Thematrix/cementing materials are argillaceous, carbonaceousand at places, ferruginous. The contacts between differentsedimentation units and cycles are sharp, erosional and grada-tional (Fig. 3c). Burrows, water-escape and deformed struc-tures are common.

Pebbly sandstone is poorly developed sub-lithofacies thatgenerally overlies the conglomerate and grades verticallyinto coarse to fine-grained sandstones. The lithofacies ismassive to crudely cross-stratified (Figs. 2a; 3b, c). It con-stitutes about 8% of the sandstone lithofacies and about 4%of the Gondwana sequence. Individual bodies are about 1 to2 m thick and commonly occur as multistoried channel-likesandstone bodies. Thin strata of mudstone and siltstone sep-arate the successive bodies.

Coarse-grained sandstone sub-lithofacies constitutes about28% of the sandstone lithofacies and 14% of the studiedGondwana sequence. It occurs as 2 to 3.5 m thick individ-ual channel to sheet-like sandstone bodies that are massiveor trough and planar cross-stratified (Figs. 2a, b, c, d; 3d).

Medium-grained sandstone sub-lithofacies constitutesabout 39% of the sandstone lithofacies and 22% of the stud-ied Gondwana sequence. The fining-upward sequence var-ies from 2 to 3.5 m in thickness (Fig. 2a, b, c, d). Thinbedded (8−16 cm) mudstone or shale is generally presentbetween two sandstone bodies. At places, coarsening-upward sequences are present (Fig. 2e, f). This sub-litho-facies occurs as sandstone bodies with erosional or transi-tional base and a flat top (Figs. 2a; 3e, f). It constitutes

multistoried sandstone bodies having abundant successivesets of large-scale trough, planar and parallel or ripple strat-ifications (Fig. 2a, b, c, d). Burrows, water-escape struc-tures, load casts and other soft sediment deformationstructures are also common.

Fine-grained sandstone sub-lithofacies comprises inter-bedded siltstone and mudstone in the fining-upwardsequence (Figs. 2a, b, c, d; 3e, f). The coarsening-upwardsequence is also present. It constitutes about 28% of thesandstone lithofacies and 15% of the studied Gondwanasequence. Individual bodies vary from 1 to 4 m in thick-ness. Parallel or ripple cross-laminations are dominant withfew trough or planar cross-stratifications (Fig. 3e, f). Bur-rows, rootlets, water-escape structures and ferruginousooids are present. The contacts of the individual lithofaciesare sharp or gradational.

The visual general trend of transition in fining-upwardsequences from base to top is channel lags, pebbly massiveto crudely cross-stratified sandstones, trough and planarcross-stratified sandstones and ripple laminated fine-grainedsandstone/siltstone (Fig. 2a, b, c, d). The pebbly sandstonelithofacies occurs as channel-like bodies which may corre-spond to deposition by longitudinal channel bars of low sin-uous braided streams (Miall, 1977). Crudely stratified,pebbly sandstone indicates channel lag deposits within aheavy loaded fluvial system. The coarse to medium andlocally, fine-grained sandstone bodies with erosional basesand flat top form channel-like deposits. The sequenceshows a profuse development of trough and planar cross-

Table 1. Generalised stratigraphic succession of borehole GDH-45 in the Khalaspir Basin, Rangpur district, Bangladesh (modified fromIslam et al., 1992).

Age Group/ Formation / Unit Lithology

Recent Alluvium (<0.25 m) Sandy and silty clay, organic matter

Pleistocene Barind Clay (6 m) Yellowish to reddish brown silty clay, massive, sticky, contains ferrugenous nodules, lowerpart sandy.

Pliocene Dupi Tila Formation (99 m) Grey to yellowish grey, fine to medium-grained sandstones, pebbly sandstone and bluishgrey mudstone.

Miocene Surma Group (179 m) Grey mudstone and sandstone, apparently massive to laminated.

Permian Gondwana Group

Unit F (68 m) Alternation of feldspathic sandstone, pebbly sandstone, carbonaceous mudstone with mainlythick coal seam and sandy mudstone.

Unit E (168 m) Alternation of feldspathic sandstone, carbonaceous sandstone, carbonaceous mudstone, silt-stone with thin to moderately thick coal seams.

Unit D (112 m) Alternation of feldspathic sandstone, carbonaceous sandstone, siltstone, conglomerate andcarbonaceous mudstone.

Unit C (28 m) Grey, yellowish Grey, clast- and matrix-supported and poorly sorted conglomerates. Unit B (17 m) Alternation of feldspathic sandstone and conglomerate

Unit A (56 m+) Greenish, grey to dirty white, medium to fine-grained and moderately sorted sandstone with few silty shale. Base not seen.

Unconformity

Unconformity

Unconformity

Unconformity

230 H.M. Zakir Hossain, M. Sultan-Ul-Islam, Syed Samsuddin Ahmed and Ismail Hossain

Fig. 2. (a) Summarized lithofacies log of the Gondwana sequence in the borehole GDH-45, Khalaspir Basin, Rangpur; (b), (c), (d), (e)and (f) are the enlargement of lithofacies logs in the Fig. 2a, within the depth range of 348 to 403 m, 467 to 489 m, 575 to 593 m, 610to 616, and 689 to 694 m, respectively. (C=Coal, Sh=Shale/mudstone, St=Siltstone, FS=Fine-grained sandstone, MS=Medium-grained sand-stone, CS=Coarse-grained sandstone, VS=Very coarse-grained sandstone/pebbly sandstone and Cong=Conglomerate).

Sedimentary facies analysis of the Gondwana sequence in Bangladesh 231

stratifications with occasional ripple lamination. Abundantlarge-scale trough and planar cross-stratified co-sets in mul-tistoried sandstone sequence may be attributed to down-cur-rent migration of sand dunes, sand waves and transversebars in shallow water stream channels (Collinson, 1970;Smith, 1972; Walker and Cant, 1984; Miall, 1992, 1996;Cadle and Cairncross, 1993; Islam, 2002). Sheet-like planarcross-stratified sandstone indicates lateral accretion in sin-uous stream channels (Moody−Stuart, 1966; Allen, 1970;Collinson, 1978). Fine-grained sandstone represents depo-sition by vertical accretion on the top of channel sand barsduring lower flow conditions (Rust, 1972; Cant, 1978).Massive or planar cross-stratified, fine-grained, mica-richsandstone indicates crevasse-splay deposits or bar topsequence. Alternation of fine to coarse-grained sandstoneswith dispersed organic matters and coal lenses indicate arapid fluctuation in the hydrodynamic condition of thestream. A typical fining-upward channel-fill sequence sug-gests a progressive decline in current competency fromlower to upper parts of the sequence. The successive over-lapping and multistoried channel bodies might haveresulted from the repeated recurrence and lateral coales-cence of multiple channels (Tewari, 1995). Burrows, waterescaped, load and other soft-sediment deformation struc-tures indicate that the sediments were deposited in a chan-nel-floodplain regime under water-saturated condition.

3.3. Siltstone Lithofacies

Siltstone lithofacies comprises about 8% of the studiedGondwana sequence. It generally occurs as either individualstrata or alternated with very fine-grained sandstones andshales in the upper part of the individual depositional units.Individual siltstone strata are about 0.3 to 1.5 m and evenless than a centimeter thick having both sharp and grada-tional contacts with top and bottom rocks. It forms the fin-ing-upward sequence with siltstone and/or very fine-grainedsandstones at the base and mudstone at the top (Figs. 2a, b,c; 3e, f). Siltstones are dirty white, grey to black and yel-lowish brown in colour with argillaceous or carbonaceousmatrix. These are massive, parallel and cross-laminated.Burrows, rootlets, loads and other soft-sediment deforma-tion structures are present.

The parallel and cross-laminated siltstones suggest dep-osition on top of the bars in abandoned channel and flood-plain conditions during the periods of reduced discharge byvertical accretion or shallow water flow conditions (Walkerand Cant, 1984; Collinson, 1986; Collinson and Thompson,1989). Thin and lenticular siltstone at bar top sequence sug-gests its deposition from suspension during falling stage offlow and due to rapid shifting of channel bars. Thick andpersistent siltstone in association with floodplain sequenceis attributed to deposition through extensive and long per-sistent overflow of stream channel. Siltstones with carbon-

aceous materials indicate that the channel was flanked byvegetated overbank. The presence of load and other soft-sediment deformation structures indicate rapid depositionand poor packing under water-saturated condition.

3.4. Mudstone/Shale Lithofacies

This lithofacies is infrequently developed in the middleand upper parts of the studied Gondwana sequence. It iscommonly present in association with coal, carbonaceoussiltstone and fine-grained sandstone (Figs. 2a, b, c; 3e, f).Individual lithofacies unit ranges from 1.5 to 2.5 m in thick-ness and constitutes about 9% of the studied Gondwanasequence. It is mainly massive to parallel laminated (Fm,Fl) and contains interbedded dark grey, reddish brown orcarbonaceous mudstones and light grey, silty sandstone.Rootlets and impressions of glossopteris leaves are com-monly present. The top and bottom contacts of the lithofa-cies are sharp and gradational. It mainly overlies thesandstone and siltstone lithofacies.

Parallel laminated shale suggests that the sediments weredeposited in floodplain environments with frequent varia-tion of energy condition. Thin mudstone at top of the chan-nel sandstones indicates deposition from suspension duringchannel abandonment. The thick dark mudstone associatewith coal suggests deposition by vertical accretion in back-swamp or flood basin environments (Flores, 1981; Walkerand Cant, 1984; Guion et al., 1995). Bands of sandstoneand siltstone within it were introduced during periodicfloods. The presence of carbonaceous mudstone indicates amoderate growth of vegetation in and around the basin.

3.5. Coal Lithofacies

Nine coal zones are identified within the borehole depthinterval between 284 to 615 m. The maximum and mini-mum thickness of individual coal beds is 15 m and less than0.3 m, respectively with a composite thickness of about 61m. This coal lithofacies (C) constitutes almost 13% of thestudied Gondwana sequence. Coal lithofacies is well devel-oped in the upper part of the sequence, which graduallybecomes thicker towards top. It commonly overlies thealternated sequences of sandstone, siltstone and mudstone(Figs. 2a, b, c; 3f). The coal is dull black to shiny black incolour and vitreous to sub-vitreous in lustre. Splitting andpinching out of coal beds are common in this lithofacies. Atplaces, the coal grades into shaly coal to carbonaceousshale. Interseam partings of sandstone and mudstone arealso commonly present. Frequency of partings reducestowards the top of the Gondwana succession. Leaf impres-sions are present in the shaly coal beds. The coal seams areeither thick, moderately thick or thin bedded. Thick andmoderately thick seams commonly have wide lateral exten-sion.

232 H.M. Zakir Hossain, M. Sultan-Ul-Islam, Syed Samsuddin Ahmed and Ismail Hossain

The coal seam sequence with associated lithofacies sug-gests a moderately drained backswamp with enormous veg-etation that was periodically inundated by floodwater. Thethick coal seam indicates a long persistent, slowly subsid-ing, moderately drained and densely vegetated backswamp(Diessel, 1992; Uddin and Islam, 1992; Seeland, 1993;Islam, 1993, 2002). The thin coal seams contain abundantsplits of carbonaceous mudstone that indicates a short-livedflooding during that period. The coal seams with alternatedbeds of sandstone and siltstone indicate crevasse-splaydeposits resulted from periodic flooding. Although the thinseams form easily in short-lived backswamps, it is very dif-ficult to explain the origin of thick coal seams in inherentlyunstable floodplain/basin environments. The thick coalseams are thought to resulted from the combined interactionof various factors, like localized aggradation of channels,slow and steady subsidence of the basin area, abundantrainfall to grow luxuriant vegetation, abundant supply ofpaleoflora, comparatively higher water table and long period oftime (Flores, 1981; Islam, 1993, 2002).

4. FACIES SEQUENCE

Lithofacies sequence of the Gondwana succession in theKhalaspir Basin is broadly divided into two sedimentarysequences based on their cycle of sedimentation andupward transition of texture and sedimentary structures.These are fining-upward (2a, b, and d) and coarsening-upward sequences (Fig. 2a, e, f).

The fining-upward sequence is present in almost all theunits of the Gondwana Group (Fig. 2a, b, c, d). Thicknessof the sequence ranges from 2.3 to 37 m, which generallydecreases toward the top of the group. On the basis of thick-ness and upward variation of grain size, the fining-upwardsequence is sub-divided into large-scale (>15 m), medium-scale (5 to 15 m) and small-scale (<5 m) sub-sequences.The large-scale sub-sequence is observed in the lower partof the group. Thickness of this sub-sequence varies from 19to 37 m. These consist of conglomerate/pebbly sandstone,coarse to fine-grained sandstones with siltstone and mud-stone/shale. Conglomerates or pebbly sandstones arepresent at the basal part of the sequence that graduallybecomes fine-grained sandstone to silty mudstone at top.Medium-scale sub-sequences occur throughout the groupand consist of coarse to fine-grained sandstone with silt-stone, mudstone and occasional coal beds. Pebbly tocoarse-grained sandstones occupy the bases, whereas siltymudstone/shale or coals lie on top of the sequence (Fig. 2a).Small-scale sub-sequence also occurs throughout the suc-cession except in the unit C (Fig. 2a). This sub-sequence con-sists of coarse to fine-grained sandstones with siltstone,mudstone and coal. But fine-grained rocks with carbonaceousmatter mainly dominate the sub-sequence. Base of the sub-sequence is pebbly or sandy and top is mudstone or coal.

On the other hand, the coarsening-upward sequence israrely present in the succession (Fig. 2e, f). Thicknessranges from 0.5 to 1.5 m. It consists of fine to coarse-grained sandstones at top with siltstone and mudstone/shaleat base. Coarsening- and fining-upward sequences indicatean increase and decrease in current velocity, respectivelytowards top of the sequence (Reading, 1986).

5. FACIES RELATIONSHIP

First order embedded Markov model is used to identifythe cyclical characteristics in the sequence. A five stateMarkov model is structured taking five lithofacies as dis-tinct states (Fig. 4). Vertical transition from one lithofaciesto another and their boundary conditions are taken into con-

Fig. 3. Photographs show (a) clast-supported conglomerate, (b)matrix-supported conglomerate, (c) very coarse to coarse-grainedfeldspathic sandstone with erosional contacts, (d) trough and pla-nar cross-stratified sandstone, (e) parallel laminated fine-grainedsandstone, siltstone and mudstone and (f) fining-upward sequencewith planar cross-stratified sandstone at the base and carbon-aceous shale and coal at top.

Sedimentary facies analysis of the Gondwana sequence in Bangladesh 233

sideration during construction. The results of the first ordertransition count and probability matrices (Table 2) are sum-marized in the facies relationship diagram (Fig. 4). The dia-gram shows the interrelationship of lithofacies in theGondwana Group of the borehole GDH-45. It also demon-strates the presence of fining-upward cycles reflecting thepresence of Markovian dependency of the different litho-facies. The generalized lithofacies transition is channel lagsconglomerate, pebbly sandstone, coarse-grained sandstone,medium-grained sandstone, fine-grained sandstone, silt-stone, mudstone/shale and/or coal from bottom to top of thesequence (Figs. 2a, b, c, d; 4). This transition is most obvi-ous for the comparatively coarser sediments of the lowerportion of the group. Fine-grained small-scale sequencewith medium to fine-grained sandstone at base and mud-stone and coal at the top is well developed in the upper por-tion. Asymmetric transition of facies is common in thelower portion, where mudstone and/or coal is directly over-lained by coarser sediments (Fig. 2a, b, c, d). But the sym-metric transition with a gradual change of coal, mudstoneand coarser sediments into one another are present in theupper portion of the group (Fig. 2c).

Overall lithofacies relationship shows a fining-upwardtendency, which suggest a progressive decline in currentcompetency from the lower to upper part of the sequence.It also shows irregular upward transitions where onephase is interrupted by others lithofacies. The lithofaciesnature, sequence, association and their interrelationshipindicate periodic encroachment of different sub-environ-ments (stream channel, floodplain, swamp etc.) of fluvialregime.

6. DEPOSITIONAL MODELS

In the present study, a conceptual model has been con-structed to provide an idea about the paleogeography andenvironments of deposition of the Gondwana Group ofKhalaspir Basin (Fig. 5). Internal sedimentary structures,boundary conditions lithofacies, their interrelationship,sequence and association are taken into consideration forthe interpretation of depositional environments. Sedimenta-tion history of the lower part of the group is not knownowing to lack of information. Like other Gondwana basinsin Bangladesh (Uddin and Islam, 1992; Bakr et al., 1996;Islam, 1993, 1994, 1996, 2002) and India (Basu and Srivas-tava, 1981; Mitra and Rao, 1987), the deposition in thebasin was most probably started with the onset of glacialand fluvio-glacial sedimentation on the basement. In thestudied portion of the sequence the sedimentation wasstarted with the deposition of trough to planar cross-strati-fied sandstones in moderately sinuous fluvial regime. Thissequence is overlained by a thick conglomerate and alter-nated sequence of conglomerates and trough to planarcross-stratified sandstones. All these suggest depositionunder channelized condition in moderately sinuous streamin an alluvial fan-fluvial setting (Fig. 5a). Consequently, thegradient of the stream reduced to form comparatively fine-grained and small-scale sequences with several horizons ofthin coal seams. The peat-forming swamps were very shortlived and most probably were moderate to well drained,which is indicated by the presence of repeated sequence ofcoarse to fine clastics with intervening thin coal seams.Gradually, this situation changed to more peneplain condi-

Fig. 4. Lithofacies relationship dia-gram of Gondwana sequence in the borehole GDH-45, Khalaspir Basin, Rangpur, Bangladesh.

Table 2. Transition count (a) and probability matrix (b) of the Gondwana Group.(a) (b)

A B C D E A B C D EA 0 29 2 1 0 32 A 0 0.91 0.06 0.03 0B 20 0 74 52 16 162 B 0.12 0 0.46 0.32 0.10C 6 46 0 33 9 94 C 0.06 0.49 0 0.35 0.10D 4 66 10 0 23 103 D 0.04 0.64 0.10 0 0.22E 1 26 5 16 0 48 E 0.02 0.54 0.11 0.33 0

31 167 91 102 48 439

234 H.M. Zakir Hossain, M. Sultan-Ul-Islam, Syed Samsuddin Ahmed and Ismail Hossain

Fig. 5. Schematic block diagrams with sedimentary sequence showing the environments of deposition of the Gondwana sequence in theKhalaspir Basin, Rangpur, Bangladesh.

Sedimentary facies analysis of the Gondwana sequence in Bangladesh 235

tion to deposit thick coal seams in a comparatively longpersistent moderately drained to poorly drained and denselyvegetated peat-forming backswamps along a comparativelymore sinuous stream. The mudstone/shale and siltstonewere deposited in natural levee, flood basin or on bar top.But the occurrence of carbonaceous mudstone indicatesmore stagnant condition in backswamp and abandonedchannel conditions with sparse vegetation (Fig. 5b).

The sedimentary facies, sequence and their vertical changesin facies association indicate a progressive decrease incoarse clastics and an increase in fine clastics and coal fromthe unit A to F (Fig. 2a). This in turn suggests a progressivedecline of stream gradient and current competency frombottom to top of the sequence. The lithofacies of the Gond-wana succession of this basin form several repeated fining-upward sequences, which might have been deposited inseveral sub-environments of the fluvial regime (Fig. 5).Both the allocyclic and autocyclic shiftment of fluvial chan-nels and regimes played an important role in the formationof these cyclic fining-upward sedimentation units with coalin the upper part and an overall fining-upward sequence inthe Gondwana succession of the basin.

7. CONCLUSIONS

The Khalaspir Basin is a NW−SE elongated and faultbounded asymmetric half-graben within the BasementComplex. Lithofacies analysis of the studied part of the Per-mian Gondwana Group in the basin is carried out to inter-pret its depositional environments. On the basis of dominantlithologic associations the group is broadly divided into sixlithostratigraphic units (Units A, B, C, D, E and F). Theseunits consist of five broad lithofacies, i.e., conglomerate,sandstone, siltstone, mudstone/shale and coal lithofacies.Each lithofacies was deposited in different sub-environ-ments (channel, floodplain, flood basin/backswamp) withinthe fluvial regime. The conglomerate lithofacies includesboth clast-and matrix-supported conglomerates, which con-stitutes about 14% of the sequence. It occurs in the lowerpart of the sequence. The conglomerate lithofacies indicatestheir deposition under either gravity flow or concentratedfluid flow conditions along the basin margin and as channellag in the heavy loaded channels. The sandstone lithofaciesconstitutes about 55% of the total sequence. In most cases,it forms typical fining-upward sequence with rare coarsen-ing-upward units. The sandstone lithofacies might havebeen deposited as multistoried and multilateral channel orchannel bar deposits in braided and moderately sinuousstreams. The siltstone lithofacies constitutes about 8% ofthe sequence, which reflects bar top, natural levee andfloodplain deposits. Generally, mudstone/shale lithofaciesoccurs in the middle to upper part of the sequence that con-stitutes about 9% of the sequence. The nature of this litho-facies suggests an overbank, floodplain and flood basin

environments. Coal lithofacies is well developed in theupper part of the sequence, which constitutes about 13% ofthe sequence. This lithofacies indicates deposition in low-lying backswamp flanked by densely vegetated overbank.Thick coal seams indicate long persistent and moderatelydrained backswamps, whereas, thin seams indicate a shortlived and well drained backswamp condition. Increasedproportion of fine clastics and coal seam number and thick-ness in the upper part suggests gradual abandonment ofactive fluvial channel and formation of densely vegetatedbackswamps for the formation of thick coal seams.

ACKNOWLEDGMENTS: The Director General of the GeologicalSurvey of Bangladesh (GSB) kindly permitted the authors to study thecore samples. Colleagues in the GSB and University of Rajshahi pro-vided help and suggestions to improve the manuscript. Authors grate-fully acknowledge all of them.

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Manuscript received April 21, 2002Manuscript accepted August 26, 2002