Braided Meandering Coastal India, Ray_chakraborty 2002

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Lower Gondwana fluvial succession of the PenchKanhan valley,

India: stratigraphic architecture and depositional controls

Sanghamitra Ray1, Tapan Chakraborty*

Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Calcutta 700 035, India

Received 11 September 2000; accepted 9 November 2001

Abstract

The Lower Permian Barakar and overlying Motur Formations, in the southeastern part of the Satpura Gondwana basin,

India, reveal contrasting lithology and alluvial architecture. Barakar Formation (f225 m thick) consists of laterally extensive 5-

to 20-m-thick multistoreyed, multilateral coarse-grained sandstone bodies. In the upper part of the formation, a few 1.5- to 11-

m-thick coal carbonaceous shale units alternate with the thick sandstone bodies. In contrast, the Motur Formation is

characterised by a thick (f500 m thick) succession of red mudstone with usually isolated, comparatively thinner (115 m

thick) sandstone bodies. The multistoreyed Barakar sandstone bodies are inferred to represent deposition in sandy braided

streams. Earlier workers inferred development of the associated coal/carbonaceous shale units in contemporaneous floodplains.

The present study, on the other hand, indicates that the coalcarbonaceous shale units accumulated in an extensive vegetated

marshland with small channels and lakes, and were temporally and spatially unrelated to the Barakar braided rivers.Sedimentologic and stratigraphic data suggest that during periods of active subsidence of the basin floor, the braided alluvial

plain was transformed to an extensive, low-gradient wetland, and at times of tectonic quiescence, elevated source regions forced

the braided system to prograde over the coal-forming marshland. Thicker (115 m) sandstone bodies embedded in the red

mudstones of the Motur Formation are inferred as channel fills. Whereas the thinner (0.22.0 m) sandstone sheets, at places

occurring as wings of the channel sandstones, represent leveesplay complexes of the Motur channels. The red mudstone

intervals perhaps represent the alluvial floodplain environment. Abundant calcareous nodules within mudstones are inferred to

record development of calcareous paleosols on the floodplain deposits. Dominance of mudrocks, the smaller dimension as well

as isolated nature of the channel fills and well-developed levee deposits in the Motur Formation, are suggestive of deposition in

an anastomosed fluvial system characterised by multiple, laterally stable channel levee complexes flanked by extensive

floodplains. Occurrence of coal in the Barakar Formation and red mudstone with calcareous paleosols in the Motur Formation

indicates a change of paleoclimate from humid (in Barakar) to semi-arid type (in Motur) during the Lower Permian time in the

Satpura Gondwana basin. There is no independent evidence of major tectonic reorganisation (stratigraphic discordance, changeof paleocurrent pattern) of the basin during the transition from Barakar to Motur Formation. It is inferred that the change from

the thick multistoreyed, multilateral sandstone sheets of Barakar Formation to that of the isolated, thinner sandstone bodies

0037-0738/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.P I I : S 0 0 3 7 - 0 7 3 8 ( 0 1 ) 0 0 2 6 0 - 3

* Corresponding author. Fax: +91-33-5776680.

E-mail address: [email protected] (T. Chakraborty).1 Present address: South African Museum, Cape Town, South Africa.

www.elsevier.com/locate/sedgeo

Sedimentary Geology 151 (2002) 243271


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within thick mudstones of the Motur Formation reflects response of the alluvial system to increasing climatic aridity rather than

to increasing rate of basin subsidence. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Gondwana; Satpura Basin; Permian; Fluvial deposits; Alluvial architecture; Climate

1. Introduction

Comparison of modern stream processes and their

deposits provides powerful tools for the analysis and

interpretation of ancient alluvial facies (cf. Allen,

1965; Walker and Cant, 1984; Smith et al., 1989;

Khan et al., 1997). Because of the limitation of time

span of human observation and record, controls on the

large-scale architecture of alluvial successions are less

clearly understood (Blum and Tornqvist, 2000). Com-

puter simulations, laboratory experiments and deduc-

tions from ancient fluvial deposits suggest that

tectonism and climate exert major controls in shaping

the stratigraphic architecture of alluvial successions

(Mackey and Bridge, 1995; Heller and Paola, 1996;

Olsen et al., 1995; Martinsen et al., 1999). Earlier

works highlighted the role of tectonism in controlling

the architecture of the alluvial sandstone bodies (Bla-

key and Gubitosa, 1984; Kraus and Middleton, 1987).

The role of climate in controlling the sand body

architecture is, however, increasingly emphasised inrecent times (Smith, 1994; Fielding and Webb, 1996;

Pedley and Frostick, 1999; Blum and Tornqvist, 2000).

It is, however, difficult to desegregate the climatic

signals from that produced by tectonism in an alluvial

succession (Pedley and Frostick, 1999).

Indian Gondwana sediments comprise a thick suc-

cession of fluvial deposits (Veevers and Tewari, 1995)

and evidences for several major climatic shifts have

been documented independently from the succession

on the basis of the palynological studies (Kar, 1976;

Tiwari, 1996; Veevers and Tewari, 1995). Transitionfrom Lower Permian coal-bearing Barakar Formation

to the overlying red mudstone-dominated Motur For-

mation in the eastern part of the Satpura Gondwana

Basin (Fig. 1, Table 1) is believed to coincide with

such a climatic shift (Veevers and Tewari, 1995). The

upper part of the Barakar Formation shows thick

sandstone bodies alternating with coalcarbonaceous

shale units, whereas the overlying Motur Formation is

dominated by thick succession of red mudstone inter-

layered with thinner sandstone units. In an earlier

study, Casshyap and Qidwai (1971) interpreted Bar-

akar sandstones as deposits of low-sinuosity braided

rivers, Barakar coal seams as floodplain sediments of

these braided streams and Motur Formation as mean-

dering river deposits on the basis of broad lithology

and detailed paleocurrent analysis. We undertook a re-

examination of these two formations with an aim to

understand the possible controls of changing facies

and alluvial architecture across these two units.

The purpose of this paper is to present a detailed

facies analysis of the Barakar and Motur Formations.

The analysis shows that simple braided and mean-

dering river facies models are inconsistent with the

internal characteristics of the Barakar and Motur

Formations, respectively. We present an alternative

interpretation for the deposition of the Motur and

Barakar sediments and argue that the remarkable

changes in the lithology and architecture from Barakar

to Motur Formations were dominantly driven by a

climatic shift across the Barakar Motur transition

rather than changes in the tectonic regime.

2. Geological background

Gondwana sedimentary successions occur in sev-

eral disparate basins in Peninsular India (Robinson,

1967; Fig. 1a) of which the Satpura Gondwana Basin is

the westernmost. Crookshank (1936) first published a

detailed geological account of the basin and subdivided

the sedimentary fill into seven major stratigraphic units

(Table 1). Traditionally, Gondwana succession in Indiahas been divided into Lower and Upper subdivisions

based on the floral content and presence or absence of

coal-bearing strata. In the Satpura Basin, contact be-

tween Bijori and Pachmarhi Formations marks the

boundary between the two subdivisions (Table 1).

In order to assess the regional climatic regime, it is

necessary to correlate the age and stratigraphic posi-

tion of the Barakar and Motur Formations of the

Satpura Basin with the sedimentary successions of

the other Gondwana basins. Although independent

S. Ray, T. Chakraborty / Sedimentary Geology 151 (2002) 243271244


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Fig. 1. (a) Different Gondwana basins of Peninsular India (after Robinson, 1967). (b) Details of the study area. (c) Geological map of the s

dispersion of the paleocurrent directions. The circled numbers beside rose diagrams denote the number of observations. Bold lines marked A, B

sections in Motur Formation (see Fig. 9); bold lines denoted by BK-I and BK-II mark the locations of measured logs in the upper part of the


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age data including reports of fossils from the Barakar

and Motur Formations of the Satpura Basin are vir-

tually absent, in the following, we attempt to summa-

rise the available information. Palynological studies

indicate that top of the Barakar Formation in other

Gondwana basins marks the transition from Lower to

Table 1

The stratigraphic succession of the Satpura Gondwana Basin (modified after Raja Rao, 1983; Bandyopadhyay and Sengupta, 1999)



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Friend et al., 1979, 2001) and lower boundaries of the

stories are designated as fifth-order bounding surfaces

(Fig. 2). The fifth-order surfaces are flat to distinctly

concave upward and are strewn with coarser sandstone

and claystone clasts. These surfaces lap on to or are

truncated by the sixth-order surfaces and in some cases

are truncated by other fifth-order surfaces (Fig. 2).

Coset of planar or trough cross-strata characterises

the sandstone storeys. In some cases, set or coset

bounding surfaces that are inclined to the fifth-order

surfaces can be recognised within the storeys. Theseinclined set or coset boundaries are designated third-

order surfaces in the Barakar and Motur Formations

and represent either lateral or downcurrent macroform

accretion surfaces. The fourth-order surfaces of Mialls

(1988) scheme, representing the preserved top of the

macroforms, are uncommon in the studied sections.

Isolated plano-concave as well as convexo-planar

sandstone bodies encased within mudstone are com-

mon in the Motur Formation and their bounding

surfaces are also designated as fifth-order surfaces

(Fig. 2). Set and coset bounding surfaces assigned firstand second orders in Mialls (1988) scheme are recog-

nisable in all the sections but have not been marked in

the architectural drawings.

4.2. Barakar Formation

Poor exposures of the Barakar Formation do not

allow detailed reconstruction of vertical and lateral

profiles but plan exposures allow collection of paleo-

current data presented in Fig. 1c. Details of the facies

were observed and sedimentological profiles were

constructed from open cast coal mines in the study

area. Two major facies associations can be recognised:

thick multistorey sandstone association (BFA-I) and

carbonaceous shalecoal association (BFA-II).

4.2.1. Thick multistorey sandstone association (BFA-I)

4.2.1.1. Description. The BFA-I sandstone bodies

delimited from the over- and underlying coalshale

succession by the sixth-order bounding surfaces aretypically sheet-like and vary in thickness from 5 to 20

m (Fig. 3). In quarry faces oriented nearly transverse

to flow, the thicker sandstone bodies can be traced

laterally over the entire length of the quarry ( > 1 km).

Examination of the adjacent quarries and bore-hole

data suggests that many of them are several kilometres

wide and alternate with subregionally extensive coal

carbonaceous shale units (Rai and Shukla, 1977;

Western Coalfields Limited, unpublished data). The

lower bounding surfaces of the sandstone bodies are

erosional in nature, whereas upper contacts are sharpto gradational over short distances. The sandstone

bodies show slight upward fining of the grain size.

The BFA-I sandstone bodies are multistoreyed in

nature. The storey bounding (fifth order) surfaces are

flat to concave-up (Fig. 3b) and are marked by granule-

rich sandstone and intraformational shale fragments.

Storeys are 1.5 to more than 5 m thick, and in sections

oriented transverse to flow directions, can be traced for

a few hundred metres (Fig. 3a,b). The width/thickness

ratios of the storeys are usually >50 and they may

Fig. 2. Schematic drawing showing the hierarchy of bounding surfaces observed within Barakar and Motur Formations of the study area.



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show slight coarsening or fining upward trend. Al-

though most storeys are in direct contact with others, at

a few places, they are separated by laterally impersis-

tent, up to 20-cm-thick, grey mudstones (Fig. 4a).

The storeys are characterised dominantly by cosetsof decimetre- to centimetre-scale planar and trough

cross-strata. Isolated large clay clasts in places mark

coset-bounding surfaces. Basal parts of the storeys

show large downcurrent-dipping compound cross-

strata (Fig. 5) that are overlain by cosets of planar

and trough cross-strata. In some flow parallel sections,

coset-bounding (3rd order) surfaces display a small

downcurrent inclination with respect to the fifth-order

surfaces. In a single isolated case, lateral-accretion

macroforms could be recognised, where the third-

order surfaces dip westward against the north

north west mean paleocurrent direction measured

from the associated cross-strata. Paleocurrents meas-

ured from Barakar Formation show a unimodal pat-

tern and at the level of individual exposures arecharacterised by low dispersion with consistency ratio

(sensu Rao and Sengupta, 1972) varying between

89.0% and 98.9% (for paleocurrent roses, see Fig. 1c).

4.2.1.2. Interpretation. Individual storeys, in places

with concave-up erosional lower boundaries, and

internally consisting of unidirectionally oriented dec-

imetre-scale planar and trough cross-strata with

locally developed fining-upward grain-size trend, sug-

gest deposition in fluvial channel (Collinson, 1996;

Fig. 3. (a) Photomosaic of Barakar sediments in the Tuti open cast quarry. Note the coal seam (No. III), IHS and the gleyed paleosol unit in the

lower right corner of the photo. (b) Line drawing prepared from the photomosaic showing the major bounding surfaces within Barakar sand

body. Note the concave-upward geometry of many fifth-order surfaces.

S. Ray, T. Chakraborty / Sedimentary Geology 151 (2002) 243271 249


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Khan et al., 1997). Compound cross-beds are inferred

to represent in-channel macroforms that grew through

the accretion of smaller bedforms on its lee (Bank,

1973; Chakraborty, 1999). Low-angle downcurrentinclination of the third-order surfaces observed at a

few places probably indicates presence of large-scale

frontally accreting macroforms (Haszeldine, 1983).

Sheet-like geometry of the storeys, dominantly coarse

sand size of the deposits, absence or presence of thin,

impersistent veneers of mudstone, locally developed

DA-elements and low dispersion of the paleocurrent

data collectively suggest a low-sinuosity braided pat-

tern of the Barakar river (Bristow and Best, 1993;

Miall, 1988; Chakraborty et al., 2000). In this context,

lateral accretion surfaces noted in a single exposure of

the BFA-I probably denote sidewise accretion of the

braid bars in zones of local flow expansion (Bridge et

al., 1986; Bristow, 1993). Thin mudstone units are

interpreted as bar-top fines or small floodplain depos-

its in the braided alluvial plain. The major sandstone

bodies of BFA-I produced by the superposition of

individual storeys represent the channel belts of the

Barakar river system. High stacking density and in-

terconnectedness of the channel-fill sandstone units

(storeys) and paucity of mudstone probably indicate

either (i) a high avulsion frequency within a multiple-

channel braided system with poorly developed flood-plain or (ii) a low subsidence rate or (iii) a suitable

combination of these two factors (Mackey and Bridge,

1995; Heller and Paola, 1996). Amalgamated nature

of the coarse-grained BFA-I sandstone bodies, their

thickness on the order of tens of metres and lateral

extent on the scale of kilometres probably imply that

the supply of the coarse clastic far exceeded the

accommodation space created by the basin subsidence

resulting in sandy braided channels wandering back

and forth across the entire alluvial plain.

4.2.2. Coalcarbonaceous shale association (BFA-II)

4.2.2.1. Description. This association alternates with

thick sandstone bodies of BFA-I and comprises an

interlayed succession of coal, carbonaceous shale,

sandshale heterolithic units, sheet-like thin beds of

medium- to fine-grained sandstone and relatively

uncommon lenticular sandstone bodies. The individual

BFA-II successions varies in thickness from 1.5 to

>11 m. Persistent thin laminae characterise the shales,



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whereas parallel laminae, wave or combined flowripple (Fig. 6) and small cross-strata are typical of

heterolithic or fine sandstone units. The thicker units

of BFA-II are sheet-like and can be laterally traced for

many kilometres across the PenchKanhan coalfield

area (Rai and Shukla, 1977; Western Coalfields

Limited, unpublished data). The thinner units of

BFA-II, however, pinch out within several tens to

few hundreds of metres.

The BFA-II succession associated with the topmost

coal seam (seam number III of PenchKanhan coal-

field area, sensu Rai and Shukla, 1977) exposed intwo quarry sections have been shown in Fig. 4a (for

location of the sections, refer to Fig. 1c). The basal

part of the BFA-II succession is marked by the coal

seam that is overlain by a set of inclined heterolithic

strata (IHS, Thomas et al., 1987). In the Tuti quarry to

the east (Fig. 3a), IHS set is overlain by a succession

of structureless, hardened, greenish grey mudstone

containing iron-oxide-lined fractures and poorly deve-

loped iron oxide nodules. This is followed upward by

an alternation of wave/combined flow rippled fine

Fig. 4. (a) Log through the upper part of the Barakar Formation exposed at the two open cast quarries. Position of the logs (BK-I and BK-II

shown in Fig. 1c). For explanation of the symbols, see Fig. 8B. (b) Explanation of symbols used in the log diagrams of this paper.



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sandstone sheets and carbonaceous shale (log BK-I,

Fig. 4a). In the Panara quarry to the west (log BK-II,

Fig. 4a), the IHS unit is erosively overlain by BFA-I

sandstone. In spite of local variation in the BFA-II

successions, many of the adjacent quarries show coal

seam number III to be persistently overlain by inc-

lined heterolithic strata thereby constituting a coarsen-

ing-upward trend in the lower part of the BFA-II

succession.

The relative inclination of the individual stratum

of the IHS sets, with respect to the generalised dip of

the succession, varies between 15j and 2j (Fig. 7).

The IHS set consists of gradationally alternating very

fine sand (110 mm) and carbonaceous shale (340

mm). Well-developed wave and combine flow ripples

and climbing ripple lamination (Fig. 5) are typical of

the sandy strata and are separated by shale laminae

with rare burrows. The inclined heterolithic strata

Fig. 5. Large downdipping compound cross-strata within BFA-I; Panara open cast quarry, Kanhan valley.

Fig. 6. Wave-ripple lamination in the heterolithic facies of the BFA-II; Datla open cast quarry, Kanhan Valley.



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show a non-erosive, downlapping relationship with

the underlying coal seam (Figs. 3a and 7) and are

marked by distinct fining of grain size in the downdip

direction.

4.2.2.2. Interpretation. Sedimentary structures andlithology indicate accumulation of BFA-II rocks in

shallow, ponded water environment. The laminated

carbonaceous shales are inferred to represent deposi-

tion in lakes that covered part of the low-lying

marshland environment and the sheet-like thin sand-

stone units might represent either distal crevasse

splay or storm-emplaced sediments within the lakes.

Lack of in situ tree trunks or extensive root horizons

indicates that much of the plant detritus was allochth-

onous (cf. Rai and Shukla, 1977). High water table in

the poorly drained marshes locally produced anaero-bic reducing environment favouring formation of peat

(Duchaufour, 1982). High ash content of the coal

(Raja Rao, 1983) and the intercalated sandstone and

shale partings imply formation in the low-lying

marshland rather than in raised peat bogs (McCabe,

1984; Collinson, 1996; Jorgensen and Fielding,

1996). Small lenticular sandstone beds are inferred

as small channel-fill deposits. The depositional sce-

nario envisioned for BFA-II succession comprises

extensive, poorly drained swamps with a mosaic of

lakes and sluggish drainage channels such as that

occurring in Mississippi delta plain (Tye and Cole-

man, 1989; Aslan and Autin, 1999) or Cumberland

marshes (Smith et al., 1989).

In the absence of erosional lower bounding surfa-

ces and unidirectional current-generated bedforms inthe IHS sets, it seems unlikely that they formed from

the lateral migration of point bars of sinuous rivers (cf.

Fielding et al., 1993). A coarsening upward trend in

the lower part of the BFA-II succession coupled with

the dominance of wave-generated structures, down-

lapping nature of the heterolithic strata and abrupt

fining of the IHS set in the downdip direction suggest

that the IHS set probably formed through the pro-

gradation of a subaqueous levee complex or small

lake delta (cf. Perez-Arlucea and Smith, 1999).

As the crevasse splays or deltas filled up the smallfloodplain lakes, vegetation encroached upon them

forming incipient soil profiles. Destruction of stratifi-

cation, inclined fracture planes and iron oxide nodules

in greenish mudstone overlying IHS in the Tuti quarry

(Fig. 3a) probably represent such incipient gleyed

paleosols. Wave-rippled fine-grained sandstones with-

in carbonaceous shale at the top of the BFA-II succes-

sion of the Tuti quarry probably represent another

episode of subaqueous progradation of crevasse sheets

in the Barakar wetland.

Fig. 7. Photo showing low-angle inclined heterolithic strata of the Panara quarry. Note downlapping nature of the IHS set on the underlying coal

seam. BFA-I sandstone erosively overlies the IHS set (see log BK-II, Fig. 4a). Human figure for scale.



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4.2.3. Barakar depositional system

The alternation of thick sandstone and coalcar-

bonaceous shale units in many Gondwana basins has

been previously attributed to deposition in fluvialchannel and associated interchannel floodplain envi-

ronments (Banerjee, 1960; Casshyap and Qidwai,

1971; Casshyap, 1979). Our inference of braided

stream deposition for the BFA-I sandstone bodies is

in agreement with that of Casshyap and Qidwai

(1971), but it is difficult to reconcile a floodplain

origin for the BFA-II rocks for the following reasons.

(1) Observation of modern braided rivers suggests

that the floodplain in braided river alluvial plain is

usually small and discontinuous (Brierly, 1991; Rein-

fields and Nanson, 1993) and their deposit have a low

preservation potential (Walker and Cant, 1984). Com-

pacted thickness of up to 15 m and high lateral extent

of the coal-bearing fine-grained sediments of BFA-II

is inconsistent with the above observations from the

modern braided stream systems. In contrast, our

interpretation of thin mudstone units interlayered with

channel-fill storeys of BFA-I as floodplain or bar-top

fines is consistent with the observations from the

modern braided river systems.

(2) Over the entire studied area, the coal and as-

sociated shale facies lack any demonstrable lateral in-

tertonguing relationship with the sandstone of BFA-I.(3) Presence of lacustrine features (persistent lam-

inations in silt, wave ripples, etc.) over a large area

occupied by the BFA-II succession indicates rapid

regional rise of water table rather than existence of

small ponds that are common in alluvial floodplains.

(4) Instead of fining upwards succession, common

in floodplain deposits (Collinson, 1996), the lower

part of the BFA-II is coarsening upward with coal at

the base and heterolithic or sandstone beds occurring

upward.

These features suggest that the depositional milieurepresented by these two associations was spatially

unrelated and was temporally separated. We infer that

vertical transition of BFA-I to BFA-II indicates a

major reorganisation of the alluvial plain when brai-

ded channel system was replaced by an extensive,

vegetated wetland.

Such sharp temporal changes between sandy braid

plain and low-gradient wetland can be brought about

by tectonism (Haszeldine and Anderton, 1980) or by

climatic changes (Fielding and Webb, 1996). On the

basis of remarkable regularity of alternating sandstone

and coalshale intervals occurring over several hun-

dred metres of stratigraphic thickness, Fielding and

Webb (1996) inferred a Milankovich climatic forcingfor such changes in the Bainmedart Coal Measures of

Antarctica. In the Barakar Formation, which is older

than Bainmedart Coal Measures of Antarctica (Vee-

vers and Tewari, 1995, their Fig. 45), BFA-I and BFA-

II alternations are limited only in the upper 100 m of

the succession and lack such regularity. Sandstone

bodies occurring in-between two coal shale succes-

sions vary in thickness from 3 to 18 m (Rai and

Shukla, 1977; Western Coalfields, unpublished data).

We believe such irregular nature of alternation of BFA-

I and BFA-II is more consistent with episodic tectonic

movements than regular periodicity of climatic fluctu-

ations driven by orbital forcing mechanism (cf. Has-

zeldine and Anderton, 1980; Fielding and Webb,

1996). At times of increased tectonic activity, the basin

floor subsided rapidly transforming the entire alluvial

plain into a low-gradient, waterlogged marshland that

favoured accumulation of peat and development of

gleyed paleosols at places. Sluggish channels, lakes

and muddy lake deltas characterised the extensive

vegetated marshland milieu (cf. Smith et al., 1989;

Aslan and Autin, 1999). During periods of tectonic

quiescence, the elevated source region forced thebraided fluvial system to prograde over the peat-

accumulating wetland (Alexander and Leeder, 1987;

Blair and Bilodeau, 1988).

4.3. Motur Formation

Red mudstone-dominated Motur Formation can be

subdivided into three broad facies associations: thick

sandstone association (MFA-I), thin sandstone asso-

ciation (MFA-II) and red mudstone association (MFA-

III). Lack of exposure precludes continuous loggingthrough the entire succession, but composite logs (Fig.

8) spread across the study area display the vertical

succession of these facies associations.

4.3.1. Thick sandstone association (MFA-I)

4.3.1.1. Description. The sand bodies vary from 0.8

to >15 m, but are mostly 12 m thick and are made up

of cross-bedded very coarse- to medium-grained sand-

stone. The sand bodies are sheet-like to lenticular in



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geometry with flat to concave-upward fifth-order

lower bounding surfaces (Fig. 9), usually single

storeyed and encased within the mudstone of MFA-

III (described later). In a few cases where flow normal

dimension is measurable in the exposure, the sand

bodies have width/depth ratio around 20. The sand-

stone units usually show a fining-upward grain size

trend together with upward decreasing scale of sedi-

Fig. 8. Lithologs through the different well-exposed transects through the Motur Formation of the study area. For the location of the individual

sections, see Fig. 1c. For symbols, refer to Fig. 4b.



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mentary structures, and eventually grade into the

overlying red mudstone. Lateral accretion surfaces

characterise few of the fining-upward sandstone units

(Fig. 10a, the lower sandstone body). Paleocurrent

direction measured from trough cross-strata of these

thicker sandstone bodies is usually unimodal and

vector mean directions of paleocurrent data measured

from different outcrops vary from 291j to 22j (total

number of observations 109; Figs. 1c and 9).A single sandstone body (the upper sandstone body

in Fig. 10a,b) occurring in the Pench River section has

features somewhat different from those described

above. The sandstone unit is about 3.5 m thick and

shows a little change in thickness over few hundred

metres across the entire outcrop. This sandstone body,

coarser than most other MFA-I units, is made up of

pebbly, very coarse-grained sandstone, has an ero-

sional, undulating base and lacks well-developed

fining-upward trend. Internally, the unit shows few

laterally extensive subhorizontal erosion surfaces

(Fig. 10a,b) and each of the lithosomes bounded by

these surfaces consists mostly of small (up to 25 cm)trough cross-strata that at places are abruptly overlain

by thin (


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4.3.1.2. Interpretation. The erosively based, lentic-

ular sandstones of MFA-I with their fining-upward

trend and unimodally oriented cross-beds are inferred

to represent fluvial channels fills. Occurrence ofsingle-storeyed sand bodies with limited lateral and

vertical dimension (when compared to storey dimen-

sion of BFA-I) with enveloping red mudstone implies

that the channels were smaller, as compared to BFA-I

channels, and were mostly confined within the mud-

depositing environment of MFA-III (see below). Pres-

ence of lateral accretion surfaces with well-developed

fining-upward trend within some of the sand bodies

(lower sand body in Fig. 10a,b) indicates point bar

accretion in these channels.

However, the coarse-grained sandstone sheet of the

Pench River section (upper sand body in Fig. 10b,c)

that lacks well-defined fining-upward grain size or

thinning-upward trend of the sedimentary structures

and contains impersistent mudstone lenses, resembles

sheet-braided stream deposits dominated by the verti-

cal accretion of the smaller bedforms (Williams, 1971;

Fedo and Cooper, 1990; McCormick and Grotzinger,

1993). Subhorizontal internal bounding surfaces

within this sheet sandstone body imply dominantly

vertical aggradation of shallow, wide channels. Thin,

grey mudstone lenses probably indicate rapid flow

stage fluctuations. This sandstone body probably rep-resents deposition from high-energy flood event that

swept across a large tract of the Motur alluvial plain

and closely resembles the ephemeral sheet-flood

deposits (McKee et al., 1967; Williams, 1971; Tun-

bridge, 1981). Rarity of such coarse-grained, extensive

sheet sandstone bodies within the Motur Formation

indicates the rarity of intense flood events that couldtransport the huge amount of coarse clastics.

4.3.2. Thin sandstone association (MFA-II)

4.3.2.1. Description. The sediments of this associa-

tion are medium- to muddy, fine-grained sandstone

and are thinner than sandstone bodies of MFA-I, and

range in thickness between 0.20 and 2.0 m. The

sandstone usually occurs as isolated units but may

be connected laterally to thicker MFA-I sandston e

bodies. At a flat lower bounding surface, much greater

lateral extent relative to their thickness is typical of

these sandstone units. The upper contact of the sand-

stone unit is usually sharp, but may show a gradational

passage to overlying mudstone facies. Paleocurrents

measured from these sandstone units show an east or

westward divergence from the northward paleocurrent

direction revealed by the MFA-I sandstone bodies

(Figs. 1c and 12a). Depending on the geometry and

the internal features, three distinctive types of sand-

stone units can be recognised within MFA-II.

(A) This is the most common type of sandstone

unit and shows a sharply defined sheet-like or wedge-shaped geometry. At places, the upper surface of the

sandstones units is convex upward (Fig. 11). It varies

in thickness from 50 cm to about 160 cm. The lower

Fig. 11. Photograph showing the sheet-like MFA-II sandstone units inferred to represent crevasse splay deposits. Note slightly convex-up shape

of the two thinner sandstone units and overlying thick MFA-I sandstone body. Section in Pench River near Richhora village.



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Fig. 12. (a) Line drawing showing geometry of a levee complex (type B sandstone, MFA-II) exposed in a section east of Fig. 9. Coarse-grained M

side of the section progressively fines away from the channel sand body. Note also the low-angle clinoform internal bounding surfaces within the

paleocurrent data from main sand body and the finer grained levee sandstone. A unit of grey mudstone siltstone embedded with thin sandst

Photomosaic depicting a slightly oblique view of the levee complex detailed in (a).


18/29

part of the sandstone unit contains small- to medium-

scale trough cross-beds overlain by parallel lamination

or ripple cross lamination. Tops of the sandstone units

at places are calcareous and show colour mottling.(B) These sandstone units are laterally linked with

thicker channel sand bodies of MFA-I and resemble

winged sand bodies described by others (cf. Stear,

1983; Mjos et al., 1993). The type B sandstone units

may be up to 3 m thick. The sandstone beds are

typically wedge shaped and are characterised by

clinoform geometry and a low-angle downlapping

relationship of the internal bounding surfaces with

the underlying mudstone. The units show a fining of

grain size away from the main MFA-I sandstone body(Fig. 12a,b). Parts of the exposure proximal to the

MFA-I sand bodies are coarse grained with medium-

scale trough cross-strata, whereas parts further away

from the MFA-I sandstone bodies are characterised by

muddy fine-grained sandstone with small cross-strata

or ripple cross-lamination (Fig. 12a,b). At places,

Fig. 13. (a) MFA-II sheet sandstone units exposed in the Richhora section. Lower part of the exposure (marked Y) comprises greenish grey

mudstonesandstone alternation. Features of gleyed paleosol are common in this part. Note undulating top (arrows) of the overlying sheet

sandstone body (marked X) and grey mudstone that fills in the depressions resulting from the bed-top irregularities. The view represents about

3 m of Motur succession, near Richhora. (b) Details of the internal features of the sheet sandstone (marked X) in (a). Note undulating basal

surface, form-discordant and bi-directional nature of the foreset laminae, transition of dipping foreset laminae into low-angle laminae and

abundant mud flasers within the sandstone bed.



19/29

desiccation cracks occur near top of the sandstone

unit. The paleoflow directions measured from the

distal part of the clinoform unit show a divergence

with those measured from the laterally linked MFA-Isandstone body (Fig. 12a).

(C) These sandstone units are characterised by flat

to undulating base, wavy top and internal wave-ge-

nerated sedimentary structures (Fig. 13a,b). The sandy

beds are sharply overlain by grey or molted greenish

red claystone that drapes over the undulating bed top

(Fig. 13a). Internal cross-strata show lensoid and

pinch-and-swell geometry, variable foreset dip direc-

tion, form discordance, common mud flasers and

locally grade into low-angle undulating laminae

(Fig. 13b).

4.3.2.2. Interpretation. Reduced thickness, greater

lateral extent, smaller internal structures, finer grain

size and flat base of these sandstone units are inferred to

indicate deposition from shallow, wide flows outside

the deeper channels represented by MFA-I sandstones.

Flat base and convex top of the type A sandstone

resemble the depositional geometry of crevasse lobes

(Collinson, 1996; Mjos et al., 1993). Colour mottling

and calcareous nature of the top of these sandstone

units are inferred to indicate incipient pedogenesis of

the exposed crevasse lobes (Collinson, 1996; Bownand Kraus, 1987). Type B sandstone resembles the

wing (cf. Stear, 1983) of the channel sandstones and

is inferred to represent fluvial levee (Mjos et al., 1993;

Nadon, 1994). The clinoform geometry of these sand-

stones, their lateral fining and divergence of paleocur-

rent with that of the related MFA-I channel sandstone

bodies are consistent with a levee interpretation (cf.

Fielding et al., 1993; Michaelson et al., 2000). Desic-

cation features indicate subaerial to near-emergent

condition for these units. The clinoform geometry

and the downlapping relationship are inferred to denotethe relief of the successive channel overbank interface

close to the channel margin (cf. Nadon, 1994).

Tabular to pinch-and-swell sand bodies and inter-

nal wave-generated sedimentary structures of type C

sandstone units provide evidences for their deposition

under oscillatory flow (cf. Brenchley et al., 1993;

Midtgaard, 1996). Dark grey to greenish grey colour

of the claystone enclosing type C sandstone units

and burrows in this succession probably indicate their

deposition in water-logged low-lying areas of the

flood basin. Small intrafloodplain channels or cre-

vasse channels supplied the sand to the localised

floodplain ponds that were subsequently reworked

by waves (cf. Smith et al., 1989). Large (metre-scale)bed-top irregularity produced by these wave-gener-

ated bedforms was at places preserved during subse-

quent periods of rapid rise of the lake-water level (Fig.

13a, cf. Browne and Plint, 1994). Top of these type C

sandstones were marked by periods of slow deposi-

tion between two floods and favoured infestations by

burrowing organisms.

4.3.3. Red mudstone association (MFA-III)

4.3.3.1. Description. Red mudstone with interlay-

ered thin (less than a cm to 10 cm) sandstone and

siltstone beds comprise the bulk lithology. At a few

places, the mudstone is green, greenish or dark grey.

Claystone/mudstone in some exposures show well-

developed thin laminae but are mostly massive. Cal-

careous concretions (Fig. 14), fossil woods (Fig. 15)

and organic traces (Fig. 16) are common.

Occurrence of calcareous concretions is a hallmark

of the Motur red mudstone. The nodules are 1 to >15

cm in diameter and vary in shape from highly irreg-

ular to vertically elongated and tubular (Fig. 14).

Some of the vertically elongated nodules taper down-ward. Reworked calcareous nodules occur as dis-

persed pebbles at the base of MFA-I and MFA-II

sandstones (Fig. 17). The cores of larger vertically

elongated nodules at places show tubular holes and

radiating cracks filled with sparry calcite cement (Fig.

18). Nodular zones are often associated with carbo-

nate-filled, inclined cracks that are up to 60 cm long in

the sections.

The nodule-rich mudstone develops a distinctive

vertical succession that is typically about 1 m thick but

may be up to 3 m thick (Fig. 19). The base of thesuccession is marked by green or red mudstone and

followed upward by a zone of dispersed nodules.

Dispersed nodules become vertically elongated, and

then coalesce to develop larger and more equant-

shaped nodules higher in the profile (Fig. 19). At

places near the top of the nodule-rich succession,

coalesced nodules form limestone beds that are sub-

parallel to primary stratification. Erosively emplaced

sheet-like sandstone unit (Fig. 19) usually overlies the

succession. However, in some sections, nodular zones



20/29

are thinner (V20 cm) and lack well-developed sequen-

tial arrangement of the different units described above.

Microscopic examinations show that the nodular

mudstones are characterised by pervasive micritic

cement as well as micritic nodules/glaebules of a

variety of shape and internal fabric. At many places,

micritic nodules show circumgranular and radiating

cracks filled with micritic and sparry calcite (Fig. 20).

Detrital siliciclastic grains frequently have corroded

margins and are coated with thin films of isopachous

clay or micrite or micro-spar cement (Fig. 21). The

greenish grey mudstones associated with type C

Fig. 14. Photograph showing elongated to irregular-shaped calcareous nodules with Motur red mudstone. Pench River section.

Fig. 15. Petrified wood fossil within Motur sandstone. South of Datla.



21/29

MFA-II sandstones at places show destruction of

primary layered fabric, corroded quartz grains and

small iron oxide concretions.

4.3.3.2. Interpretation. Red mudstone encasing most

of the MFA-I and MFA-II sandstone bodies is inter-

preted to represent deposition in the overbank areas of

the Motur alluvial plain. Laminated red mudstones

probably represent undisturbed suspension settlement

in the overbank areas. However, activities of burrow-

ing organism, plant roots (inferred from the presence

of both tubular calcareous nodules as well as abundant

Fig. 17. Photograph showing the contact between the top of the calcareous paleosol profile and overlying cross-bedded MFA-II sandstone unit.

Note coalesced nodules at the top of the soil profile and abundance of calcareous nodules (arrows) in the sandstone derived from the underlying

soil profile. Near village Richhora.

Fig. 16. Bedding plane view of burrows at the sandstonemudstone interface within Motur Formation at Richhora Section (Fig. 13).



22/29

silicified woods), and diagenetic changes related to the

development of soil profiles probably rendered bulk of

the Motur mudrocks structureless (cf. Nadon, 1994).

High proportion of the clays in the Motur Formation

implies that extensive, low-energy floodplain environ-

ments surrounded fluvial channels. Pervasive red pig-

mentation of the mudstone denotes oxidising envir-

onment and a generally low water table.

Reworked calcareous nodules within MFA-I and

MFA-II sand bodies imply their syndepositional ori-

gin. Calcareous nodules and its vertical succession areinferred to be related to the soil profiles that developed

over the subaerial floodplain sediments. Pervasive

micritic cement, corrosion and isopachous envelop of

micrite or micro-spar around detrital grains, micro-

spar-filled cracks of different orders are commonly

associated with paleosol deposits (Brewer and Slee-

man, 1964; Nagtegaal, 1969; Esteban and Klappa,

1983). Tubular shape of the calcareous nodules typi-

cally resembles calcified root tubes or rhizocretion in

the soil profile. We infer that the elongate calcareous

nodules that grew around the plant roots and the

central voids, created by the subsequent decay of the

vegetal matter, were later filled-up by sparry calcite.

Bedding-parallel limestone horizons formed of coa-lesced nodules probably resulted from the combined

effect of increasing density of roots up the profile as

well as increased precipitation of calcium carbonate in

the upper part of the soil horizon. Micritic glaebules

with circumgranular and radiating cracks are com-

monly attributed to shrinkswell cycles operative in

the solum (Nagtegaal, 1969; Goudie, 1983; Esteban

and Klappa, 1983). The green-coloured mudstones at

the base of the nodular zones probably represent the

groundwater tables near the base of the soil profiles

where intergranular pore spaces were perennially satu-

rated preventing oxidation of the iron (Buurman,

1980). Thin calcareous mudstone units that lack dis-

tinctive vertical succession probably represent incipi-

Fig. 19. A field sketch showing paleosol profile developed within

Motur red mudstone (MFA-III). Note upward increase in the size

and density of calcareous nodules in red mudstone.

Fig. 18. A view of a large calcareous nodule within Motur mudstone

(MFA-III). Note circular holes (arrowed) filled with a mixture of

clay and micritic carbonate and well-developed radiating cracks

around the holes. The tubular features are inferred to represent relict

root pores in Motur calcareous paleosols. Exposure near Barkuhi.

Lens cap for scale.



23/29

ent soil profile that could not develop into thick soil

horizon due to comparatively higher rate of sedimen-

tation.

Locally developed grey mudstone (those associ-

ated type C sandstone of MFA-II) and occurrence of

corroded quartz grains and iron oxide nodules within

it probably indicate development of incipient gleyed

paleosols (Buurman, 1980). The occurrence of these

grey mudstone-gleyed paleosol successions with type

C sandstones of MFA-II is indicative of their associ-

Fig. 20. Photomicrograph shows a glaebule within Motur mudstone. Note well-developed circumgranular and radiating cracks filled with sparry

calcite cement. X-nicols. Bar scale=1 mm.

Fig. 21. Photomicrograph showing an isopachous rim of sparry cement around a corroded quartz grain within mudstone. X-nicols. Bar scale=

1 mm.



24/29

ation with small floodplain lakes (cf. Zaleha, 1997b;

Kraus, 1999).

Development of pedogenic calcrete is typical of

semi-arid to arid climate, with precipitation exceedingseveral hundreds of millimetres per year and a sea-

sonal distribution of the rainfall pattern (Goudie,

1983; Retallack, 1990; Tandon and Gibling, 1994).

Significant thickness of some of the nodular calcare-

ous zones (possible Bk horizon) indicates precipita-

tion on the higher side of the range and low water

table allowing seepage to some depth below the

exposed surface. In the overall stratigraphic context

of the Motur Formation, thinner calcretes or gleyed

paleosols are probably attributable to changing geo-

morphic setting rather than to change in the climate

within the Motur alluvial plain.

4.3.4. Motur depositional system

The Motur depositional system was characterised

by a mosaic of main channels (thicker sand bodies of

MFA-I) intimately associated with numerous crevasse

splay/levee deposits (types A and B sandstones of

MFA-II) and surrounded by extensive mud-depositing

flood basin (MFA-III). Occurrences of lateral accre-

tion surfaces within MFA-I sand bodies denoting

lateral migration of the sinuous channels are few in

the study area. General paucity of lateral accretionsurfaces, limited lateral extent and flanking levee

deposits of many of the MFA-I sandstone bodies

indicate that the channels were not very mobile and

in many of the cases probably were confined and

stable within the floodplain fines.

General character of the Motur Formation of the

study area marked by the dominance of flood basin

mudstone encasing numerous crevasse sheets and fewer

isolated channel sandstone bodies,bears striking resem-

blance to the anastomosing fluvial deposits (Tornqvist,

1993; Nadon, 1994; Morozova and Smith, 1999;Makaske, 2001). Although contemporaneity among

these channel sandstone bodies cannot be demonstra-

ted, as is the case for the most ancient anastomosing

fluvial deposits (cf. Makaske, 2001), the abovemen-

tioned similarities are strongly suggestive of deposition

of the Motur Formation from a mud-dominated anasto-

mosing fluvial system.

The exposed areas of the floodplain were subjected

to soil-forming processes resulting in formation of

caliche profiles of varying thickness. Variable thick-

ness of the caliche deposits was plausibly controlled by

the time available for soil-forming processes, which in

turn was related to geomorphic stability and rate of

sedimentation in that particular area (cf. Bown andKraus, 1987; Kraus, 1999). The red coloration of the

floodplain fines and calcareous soil profiles indicate a

semi-arid climate with seasonal rainfall pattern during

Motur sedimentation. Localised, ponding of water in

the floodbasin resulted in small lakes, in which some of

the splay sand bodies were subaqueously emplaced and

were subsequently reworked by wave action. Gleyed

paleosols developed sporadically near these floodplain

depressions.

It should be noted that Casshyap and Qidwai

(1971) inferred a meandering pattern for the Motur

Formation from the evidence of higher variance of

paleocurrent azimuth (compared to those of under-

lying Barakar Formation) and overall mudstone-

dominated lithology. The paucity of well-developed

lateral accretion surfaces in the channel sand bodies

on the contrary demonstrates rarity of typical mean-

dering channels. Since the crevasse channels develop

oblique to the main channels (Smith et al., 1989;

Perez-Arlucea and Smith, 1999), the paleocurrent

direction measured from smaller sandstone lenses or

sheets are expected to show slightly divergent direc-

tion and higher dispersion (cf. Figs. 1c and 12).Amalgamation of paleocurrent data from all the sand-

stone units of both MFA-I and MFA-II will therefore,

tend to increase the dispersion value, as observed by

Casshyap and Qidwai (1971) leading them to a

meandering channel interpretation for Motur Forma-

tion.

The coarse-grained, sheet sandstone body of the

Pench River section (upper sand body in Fig. 10a,b)

probably is not consistent with the inferred deposition

from anastomosed channels. Coarse grain size, lack of

fining-upward internal organisation and amalgamationof sheet-like lithosomes across horizontal bounding

surfaces probably represent vertically stacked sand-

stone units deposited by shallow, wide braided chan-

nels. We infer the causative mechanism to be high

magnitude flood that resulted in high-velocity shal-

low, wide flow carrying a heavy load of pebbly

coarse-grained sand. Rarity of such sandstone units

within the Motur Formation of the study area indicates

a low frequency of catastrophic flood events in the

Motur catchment area.



25/29

5. Discussion

Both the Barakar and Motur Formations of the

eastern part of the Satpura Basin are alluvial depositsthough they differ markedly in facies and alluvial

architecture. The alluvial successions of the Barakar

Formation are characteristically mud poor and are

made up of thick (f20 m) multistoreyed, multilateral

sandstone bodies. Individual channel-fill units are up

to 5 m thick and more than few hundred metres wide

in flow transverse sections. In contrast, Motur chan-

nel-fill sandstone units are thinner (usually 12 m),

and occur as isolated sand bodies enveloped by red

mudstone. Decreasing interconnectedness of the chan-

nel-fill sandstone bodies along with an increasing

proportion of floodplain fines, as displayed by Bar-

akar and Motur Formations, have often been

explained in terms of increased rate of basin subsi-

dence (Blakey and Gubitosa, 1984; Kraus and Mid-

dleton, 1987; Bristow and Best, 1993). However,

climatic changes can also produce remarkable effects

on river channel patterns and are thus capable of

effecting major changes in the architectural pattern

of the resultant deposits (Pedley and Frostick, 1999;

Blum and Tornqvist, 2000). In the following, we shall

examine the evidences of allogenic factors that might

have influenced changing facies and architectureacross the Barakar and Motur Formations.

Since there has been no report of marine strata in

the study area, we assume that eustatic base level did

not exert any tangible control on the depositional

pattern of the sedimentary succession under investi-

gation. Therefore, the major factors that might have

controlled the large-scale architecture of the alluvial

deposits were climate and tectonism.

Coal and calcretes form in exclusive climatic con-

ditions. Coal formation is favoured by a humid

climate with rainfall distributed throughout the yearand a waterlogged reducing environment. Calcretes,

on the other hand, are typical of semi-arid climate

characterised by a net moisture deficit with precipita-

tion in the range of 400600 mm/year, and strongly

seasonal pattern of the rainfall (Goudie, 1983; Tandon

and Gibling, 1994). Thus, occurrence of grey mud-

stone and coal in the Barakar Formation indicates

climatic regime quite different from that prevailing

during the deposition of red mudstone and calcretes of

the Motur Formation. It should be noted that Barakar

Formation gradationally overlies glaciogenic deposits

of Talchir Formation and is often inferred as perigla-

cial fluvial system. Palynological studies from differ-

ent Gondwana basins of India also indicate a cold andhumid climate for Barakar Formation and warmer

climate for Motur and equivalent stratigraphic units

(Tiwari, 1996; Veevers and Tewari, 1995).

Semi-arid climate, as compared to arid or humid

ones, is known to enhance the sediment supply

probably through increasing rate of chemical weath-

ering and decreasing vegetation cover (Schumm,

1993). Also the modern day dryland rivers are known

for their higher concentration of suspended load,

about 20 times more than that of the perennial systems

with comparable size of the drainage basin (Reid and

Frostick, 1987). A higher proportion of fine-grained

sediments in the Motur Formation and its inferred

semi-arid climate is consistent with the above two

observations from the modern fluvial systems. It has

also been documented from the historical records of

modern rivers of Arizona that one of the principal

ways in which rivers adjust to decreased precipitation

is by decreasing their depth and width and an asso-

ciated increase in the rate of floodplain aggradation

(Hereford, 1984). We invoke the same cause and

effect relationship to account for the relative decrease

in the sandstone body dimension and increase in theproportion of preserved floodplain fines in the Motur

Formation. Similar transition from sandy/gravelly

deposits of braided streams to sandmud alternation

of meandering streams recorded from many Quater-

nary alluvial deposits has been attributed to climatic

changes (Blum and Tornqvist, 2000).

It is difficult to assess rate of sedimentation (the

proxy for rate of tectonic subsidence) in the absence

of well-constrained age data. Examination of the

large-scale sedimentary packages in seismic profile

often allows correlation of the sedimentation withepisodes of tectonic movements (e.g., Ruffel and

Shelton, 1999). In the absence of these data, we

depend on the evidence observable from outcrop-scale

exposures and stratigraphic relationships. The studied

sections, however, do not show any features that may

indicate increased rate of tectonic subsidence at the

Barakar Motur transition. On the contrary, lack of

any stratigraphic discordance between the formations

and very similar northerly paleocurrent pattern of both

the formations argues against any major tectonic event



26/29

in the Satpura Gondwana Basin during Barakar

Motur transition. There is no tangible increase in the

occurrence of soft sediment deformation features in

the Motur Formation, which otherwise might haveimplied more frequent tectonic movement.

Variation in the proportion and interconnectedness

of the channel-belt sandstones, occurring over a thick-

ness of several hundred metres, and broadly similar to

those observed at BarakarMotur transition, has also

been reported from the Miocene Siwalik deposits of

northern Pakistan. On the basis of available palyno-

logical data that suggest persistence of tropical mon-

soonal climate throughout the deposition of the

Siwalik Group, Zaleha (1997a) attributed the changes

in alluvial architecture to the megafan lobe switching

driven by tectonic and autocyclic processes. In a later

study, Friend et al. (2001), however, suggested that the

climatic control for such changes could not be ruled

out altogether from the field evidences. More impor-

tantly and in contradiction to the prevalent notion, they

pointed out that decreasing proportion of mud and

increasing interconnectedness of the sandstone storeys

at the ChinjiNagri transition coincide with the in-

creasing (not decreasing) rate of sedimentation (a

proxy to basin subsidence) as calculated from well-

constrained magnetic reversal chronology of the Siwa-

lik succession. Similar changes of alluvial architectureand associated changing sand/mud ratio from many

Neogene or Quaternary successions, where allogenic

controls are better constrained, have been attributed to

climatic and not tectonic changes (Smith, 1994; Blum

and Tornqvist, 2000). The case studies discussed here

and many more suggest that the hypothesis that assigns

tectonic processes as the sole allogenic control for

changing alluvial architecture is not supported by

current research.

In the Satpura Gondwana basin, sedimentological

evidences suggest marked climatic shift at the Bara-karMotur transition. This interpretation is strongly

supported by regional palynological studies. On the

other hand, there is no independent evidence of in-

creased rate of tectonic subsidence at this time. Al-

though definitive correlation of the architectural

changes to specific allogenic forcing mechanisms

would require more data, our investigation strongly

suggests that the changing facies and alluvial architec-

ture across Barakar Motur Formations were driven by

climatic changes.

6. Conclusions

(a) The upper part of the Barakar Formation in the

eastern part of the Satpura Gondwana Basin is char-acterised by alternating thick, coarse-grained, multi-

storeyed channel sandstones and laterally extensive

coalcarbonaceous shale units. In contrast, overlying

Motur Formation is characterised by thinner, isolated

channel sandstones embedded in a thick succession of

red mudstone.

(b) Available sedimentological evidence suggests

episodic reorganisation of the Barakar alluvial plain to

an extensive peat-accumulating swamp. Tectonic

movement of the basin floor probably controlled al-

ternation of braidplain and muddy wetlands. Periods

of higher subsidence favoured development of peat

swamps, whereas periods of quiescence coincided

with the progradation of the braided alluvial system

across the basin.

(c) Presence of coal in the Barakar Formation and

presence of calcretes in the Motur Formation indicate

marked shift in climate from humid to semi-arid at the

BarakarMotur transition.

(d) By analogy to modern fluvial systems, increased

proportion of mudstone, decreased thickness of the

channel sandstone bodies and their isolated nature in

the Motur Formation, are attributed to increasing cli-matic aridity and adjustment of the fluvial system to

such changes.

(e) Lack of independent evidence of changing rate

of tectonic subsidence during BarakarMotur transi-

tion and evidence of climatic shift during the same

period are construed to indicate climatic influence in

establishing contrasting fluvial style and markedly

different alluvial architectural pattern across the Bar-

akarMotur transition.

Acknowledgements

We gratefully acknowledge the General Managers

of Kanhan and Pench Valley Coalfields of Western

Coalfield Limited for permission to work in the dif-

ferent open cast mines in this area. We are thankful to S.

Bandyopadhyay who encouraged us to take up this

work. We are grateful to P. Ghosh for his invaluable

assistance during fieldwork and help in identifying the

paleosols in the Motur Formation. We also thank S.N.



27/29

Sarkar, S. Chakraborty and Sanjoy Ghosh for help

during the field work. A.K. Das drafted the line draw-

ings. We gratefully acknowledge the infrastructural

facilities provided by Indian Statistical Institute, andfinancial assistance received from the Department of

Science and Technology, New Delhi (Grant no. ESS/

23/VES/072/99) for the research work. The earlier

version of the manuscript benefited from the critical

comments from C. Chakraborty, P. Ghosh and S.N.

Sarkar. Thoughtful comments by Timothy Cross and

Chris Fielding and Chief Editor Andrew Miall helped

improve the manuscript considerably.

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Braided Meandering Coastal India, Ray_chakraborty 2002