1999 Mancheral-bar-IAS Sp Publ

Spec. Publs int. Ass. Sediment. (1999) 28, 451–466

Bars are the principal depositional elements withinrivers. Although most rivers show the presence of lateral,upstream and downstream accretion of bars (Bristow &Best, 1993), different channel types are characterized bya dominant bar type (Bluck, 1979; Haszeldine, 1983a,b;Willis, 1993; Miall, 1994). Reconstruction of bars fromancient alluvium, therefore, serves as an important toolfor palaeoenvironmental analysis and reconstruction ofpalaeochannel characteristics.

A variety of bars, differing in scale and morphology,has been documented from ancient alluvium (e.g. Cant& Walker, 1978; Bluck, 1980; Haszeldine, 1983b; Kirk1983; Rust & Jones, 1987; Wizevich, 1992; Roe &

Hermansen, 1993; Willis, 1993; Miall, 1994). Such studieshave become more popular with increasing use of thetechniques of architectural element analysis (sensu Miall,1985). Few of these studies, however, dwell on the detailsof the internal sedimentary structures that can provideimportant palaeohydraulic information other than thedominant migration direction of these features. Thispaper presents a detailed account of inferred bar depositsfrom the Proterozoic alluvial strata of the MancheralQuartzite. It also presents estimates of palaeohydro-logical parameters and attempts to present a generalizedmodel for the reconstruction of shallow braid bars fromancient fluvial deposits.

451

Reconstruction of fluvial bars from the Proterozoic Mancheral Quartzite,Pranhita–Godavari Valley, India

TAPAN CHAKRABORTY

Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Calcutta 700 035, India(Email: [email protected])

ABSTRACT

Reconstruction of bars in ancient fluvial deposits is crucial in interpreting channel pattern and palaeohydraulics.Excellent exposures of the late Proterozoic alluvial deposits of the Mancheral Quartzite around Ramgundam(18˚48′N, 79˚25′E) provide an opportunity for such reconstruction. The Mancheral Quartzite is a pebbly, coarse-grained sandstone and conglomerate succession 30–76 m thick. Several braided fluvial facies and an alluvial-plainaeolian facies are well developed in the Mancheral Quartzite at Ramgundam. The planar cross-bedded bar faciesoverlie major erosional surfaces and is characterized by complex interlayering of planar cross-beds 30–120 cmthick and trough cross-beds 5–15 cm thick.

A number of low-angle, downcurrent-inclined cross-bed bounding surfaces indicate accretion of the bar leesurface. Thickening of the cross-bed sets in a down-palaeoflow direction, the presence of well-developed alter-nately coarse- and fine-grained foresets and counter-current ripple lamination at the toe of the cross-beds, denoteflood-stage accretion of the bedforms into the adjacent pools. Lateral transformation of the larger planar cross-setsinto cosets of smaller cross-beds, truncation by smaller trough cross-beds showing flow at high angle to those of thelarger planar sets and thin muddy sandstones denote modification of the bar form during falling water stage.

Combined palaeocurrent data from all planar cross-beds show a mean south-westerly orientation, which isassumed to be the local channel direction. Palaeocurrent data from the bar deposits (both planar and trough cross-beds combined) indicate downcurrent, oblique or symmetric accretion of the bars with respect to the local channeldirection and are inferred to document lateral or mid-channel braid bar deposits.

The thickness of the bar deposits suggests a shallow depth to the channels (~ 2.5 m). Discharge was character-ized by rapid and pronounced flow fluctuation. Internal organization of the bars is intermediate in characterbetween topographically differentiated large braid bars and simple linguoid dunes, which commonly characterizemany shallow sand-bed braided channels.

INTRODUCTION

FSC31 14/01/2009 05:17PM Page 451

Fluvial Sedimentology VI Edited by N.D. Smith and J. Rogers © 1999 The International Association of Sedimentologists. ISBN: 978-0-632-05354-4

Fig. 1. (A) Generalized geological mapof Pranhita–Godavari Valley. Inset showsProterozoic sedimentary basins of India.(B) Location of sections 1 to 4 andvertical log (Fig. 2) through the barsuccession in the enlarged outcrop mapof Mancheral Quartzite at RamgundamGutta. (C) Geological map of the areaaround Ramgundam.

452 T. Chakraborty

FSC31 14/01/2009 05:17PM Page 452

Reconstruction of fluvial bars 453

GEOLOGICAL SETTING

The Pranhita–Godavari Valley Basin is one of the majorProterozoic basins of India, where two linear belts ofProterozoic rocks, flanking an axial belt of Permo-Jurassic Gondwana rocks are exposed (Fig. 1). TheSullavai Group is the youngest unit of the Proterozoicsuccession exposed in this basin (see Table 1). TheMancheral Quartzite, a constituent of the Sullavai Group,is very well exposed around Mancheral and Ramgundamin the south-western Proterozoic belt (Fig. 1). In theRamgundam section, the Mancheral Quartzite uncon-formably overlies the rocks of the middle ProterozoicPakhal Group and is overlain by the Venkatpur Sandstone(Table 1). In and around Mancheral, the fluvial strata ofthe Mancheral Quartzite erosively overlies the RamgiriFormation of the Sullavai Group and gradationally passesupward through a transition zone of flat-bedded sabkha-playa sediments into the erg deposits of the VenkatpurSandstone (Chakraborty, 1991a, 1994).

The Mancheral Quartzite comprises poorly sorted,coarse-grained pebbly sandstones and minor conglomer-ates. Typically, 1–6 m thick, salmon-red, fine- to veryfine-grained sandstones occur at different stratigraphicallevels and comprise a subordinate part of the succes-sion. The sandstones and conglomerates of the Man-cheral Quartzite are interpreted to have been depositedfrom high-gradient braided streams and the well-sorted,fine-grained sandstone interlayers are the product offluvial–aeolian interactions in the distal part of thesandy braidplain (Chakraborty, 1991b; Chakraborty &Chaudhuri, 1993). The facies recognized within theMancheral Quartzite are summarized in Table 2, andFig. 2 shows the vertical log through a well-exposedsection of the succession. Details of the coarse-grainedplanar cross-bedded sandstone facies (facies 6 in Table2) are discussed below.

RECONSTRUCTION OF BARS IN THEMANCHERAL QUARTZITE

Fluvial bars are defined as depositional highs withinchannels that might become partially exposed during thefalling-river stage (Miall, 1981; Bridge, 1985; Roe &Hermansen, 1993). In modern braided rivers, definitionof bars has been strongly influenced by channel dimen-sions. Both a simple bedform (Collinson, 1970; Smith,1971) and a large periodic to quasi-periodic ‘macroform’with complex erosional and depositional history (Bluck,1976, 1979; Cant & Walker, 1978; Crowley, 1983) have

been designated as bars. The ‘bars’ within braidedstreams include a hierarchy of bed configurations, e.g. (inincreasing scale) individual bedforms (two-dimensionalor three-dimensional), small ‘unit’ bars, bar complexes orsandflats and mature vegetated islands (Walker & Cant,1984).

In view of the morphological complexities involvedand the difficulty of their recognition in the ancientrecord, Miall (1981) proposed a simple classification offluvial bars consisting of three categories:1 gravelly, planar or massive bedded bars;2 sandy, simple foreset bars;3 compound bars of sand or gravel.Recognition of the first two categories may be easier, butrecognition of compound bars requires reconstructionof the palaeomorphology of the bar and involves lateraltracing of sets or cosets of strata, identification of thedifferent order of bounding surfaces and measurementof the palaeocurrent pattern of these structures (Bluck,1976, 1979). The concept has been applied successfullyto the analysis of several ancient fluvial deposits (e.g.Bluck, 1980; Allen, 1983; Haszeldine, 1983a,b; Kirk,1983; Steel & Thompson, 1983; Wizevich, 1992). The‘bars’ inferred in this study are mostly compound typesas defined above.

The bar succession

The inferred bar succession of the Mancheral Quartziteconsists predominantly of cosets of planar cross-beds(facies 6, Fig. 3), with minor amounts of small troughcross-beds (facies 5) and laterally impersistent layers ofmuddy fine-grained sandstone (facies 7). The upward-fining succession has an overall upward-fining trend butmay show local coarsening upwards (Fig. 2).

The succession at Ramgundam is organized into several channel-fill successions. Each channel-fill suc-cession is marked at its base by a major erosional surfacethat is flat to concave-up and can be traced laterally for upto several hundred metres. The surfaces may show localrelief of up to a few metres (Chakraborty, 1991a; see alsoFig. 8 in Chakraborty & Chaudhuri, 1993). The domin-antly planar cross-bedded succession, described below,is separated by a major erosion surface from the under-lying trough cross-bedded sandstone of facies 4 (Fig. 2).Elsewhere in the Ramgundam Gutta (hill) the bar suc-cession overlies the conglomeratic sandstone (facies 2)or may uncomformably overlie the Pakhal Limestone.Details of the primary structures, internal bounding sur-faces, facies architecture and palaeocurrents of the barsuccession are well exposed in several outcrop sectionsdescribed below.

FSC31 14/01/2009 05:17PM Page 453

Tab

le 1

.S

trat

igra

phic

al s

eque

nce

of th

e so

uthw

este

rn P

rote

rozo

ic b

elt,

Pra

nhit

a-G

odav

ari v

alle

y.

For

mat

ion

Sup

ergr

oup/

Age

Gro

up

Per

mia

n –

Ear

ly

Cre

tace

ous

Ang

ular

unc

onfo

rmit

yP r o t e r o z o i c

Ang

ular

unc

onfo

rmit

y

Ang

ular

unc

onfo

rmit

yA

rcha

ean

(?)

Bro

ad li

thol

ogy

and

sedi

men

tary

str

uctu

res

Red

, fine

- to

med

ium

-gra

ined

, wel

l-so

rted

sand

ston

e w

ith

met

re-s

cale

pla

nar c

ross

-bed

san

d fl

at b

eds

Man

cher

al Q

uart

zite

: pur

ple

to re

d, c

oars

e-gr

aine

d pe

bbly

san

dsto

ne a

nd c

ongl

omer

ate;

upw

ard-

fini

ng s

eque

nces

; int

erla

yere

d fi

ne-

grai

ned

sand

ston

e un

its

wit

h ad

hesi

on s

truc

ture

s

Ram

giri

For

mat

ion:

red

cong

lom

erat

e an

dpe

bbly

ark

ose;

con

glom

erat

es re

vers

e gr

aded

;sa

ndst

one

cros

s-be

dded

; lat

eral

ly e

xten

sive

,sh

eet-

like

sed

imen

tati

on u

nits

Dep

osit

iona

len

viro

nmen

t

Erg

and

erg

-mar

gin

depo

sits

(Cha

krab

orty

,19

91a)

Bra

ided

fluv

ial w

ith

inte

rlay

ered

aeo

lian

unit

s (C

hakr

abor

ty &

Cha

udhu

ri, 1

993)

Dis

tal a

lluv

ial-

fan

–br

aide

d-ri

ver

(Cha

krab

orty

, 199

1b,

1994

)

Gon

dwan

a S

uper

grou

p

G o d a v a r i s u p e r g r o u p

Sg

ur

lo

lu

a p

v a i Pg

er

no

gu

ap

n g a

Man

cher

al a

rea

Tal

chir

For

mat

ion

Ven

katp

urS

ands

tone

(4

8m

+)

Man

cher

alQ

uart

zite

(7

6m

)

Ram

giri

For

mat

ion

(250

m+)

Sat

Nal

a S

hale

Cha

nda

Lim

esto

ne

Pra

nhit

aS

ands

tone

Gne

issi

cB

asem

ent

Com

plex

Man

cher

alQ

uart

zite

(23

m)

Ram

giri

For

mat

ion

(456

m+)

Mul

ug S

ubgr

oup

Dis

conf

orm

ity

Mal

lam

pall

i S

ubgr

oup

? ? ? ?

454 T. Chakraborty

Pg

ar

ko

hu

ap

l Gne

issi

c B

asem

ent

Com

plex

Ram

gund

am a

rea

Tal

chir

For

mat

ion

Ven

katp

ur S

ands

tone

FSC31 14/01/2009 05:17PM Page 454

Tab

le 2

.S

umm

ary

of li

thof

acie

s re

cogn

ized

in th

e M

anch

eral

Qua

rtzi

te.

Fac

ies

num

ber

Bri

efde

scri

ptio

n

1M

atri

x- o

r cla

st-s

uppo

rted

, peb

ble

to b

ould

er c

ongl

omer

ate;

bed

s 11

–140

cm th

ick

and

show

upw

ard-

coar

seni

ng m

axim

um p

arti

cle

size

and

upw

ard-

fini

ng tr

end;

MP

S o

f ind

ivid

ual b

eds

vari

es fr

om 5

to 8

4cm

; bed

s sh

ow M

PS

: bed

-thi

ckne

ss p

osit

ive

corr

elat

ion;

few

cla

st-

supp

orte

d be

ds s

how

cla

st im

bric

atio

n an

d cr

ude

hori

zont

al s

trat

ifica

tion

; per

vasi

ve h

emat

itic

cem

ent l

ocal

ly p

rese

nt; g

rade

s up

war

d in

to tr

ough

cro

ss-b

edde

d sa

ndst

one

of fa

cies

4

2P

oorl

y so

rted

peb

bly

sand

ston

e to

san

dy c

ongl

omer

ate;

san

dy c

ongl

omer

ate

mas

sive

; peb

bly

sand

ston

e cr

oss-

bedd

ed; 4

–40

mm

cla

sts

disp

erse

d th

roug

hout

the

cros

s-be

dded

uni

ts;

cros

s-se

ts 8

–60

cm th

ick;

occ

asio

nal s

igm

oida

l for

eset

s; s

oft-

sedi

men

t def

orm

atio

nco

mm

on; f

acie

s oc

curs

nea

r the

bas

e of

the

chan

nel-

fill

seq

uenc

es; i

nter

laye

red

wit

h fa

cies

3an

d gr

ades

upw

ard

into

faci

es 4

3M

ediu

m- t

o co

arse

-gra

ined

san

dsto

ne w

ith

larg

e tr

ough

cro

ss-b

eds;

set

thic

knes

s >

45cm

; >

30m

wid

e in

bed

ding

pla

ne e

xpos

ures

; occ

ur in

terl

ayer

ed w

ith

faci

es 2

4W

hite

to g

rey,

fine

to v

ery

coar

se-g

rain

ed s

ands

tone

wit

h fe

w p

ebbl

es; 2

–15-

cm-t

hick

tr

ough

cro

ss-b

eds

ubiq

uito

us; c

oset

s of

trou

ghs

form

she

et-l

ike

sand

bod

ies

boun

ded

bype

bble

-str

ewn

flat

ero

sion

al s

urfa

ces;

san

d bo

dies

30

–81

cm (a

vera

ge 5

0cm

) thi

ck a

ndtr

acea

ble

in s

trik

e-pa

rall

el d

irec

tion

for 8

0–1

10m

; gra

dati

onal

ly o

verl

ain

by fa

cies

7 o

r 8;

at p

lace

s fa

cies

6 e

rosi

vely

ove

rlie

s th

is fa

cies

uni

t

5R

ed to

redd

ish

brow

n co

arse

-gra

ined

san

dsto

ne lo

call

y w

ith

gran

ules

and

sm

all p

ebbl

es;

2–10

-cm

-thi

ck tr

ough

cro

ss-b

eds;

cos

ets

5–2

5cm

thic

k; o

ccur

alw

ays

as le

ntic

ular

uni

tsin

terl

ayer

ed w

ith

faci

es 6

and

oft

en fi

ll s

hall

ow s

cour

s ov

erly

ing

larg

e pl

anar

set

/cos

et

6D

eep

brow

n to

pur

ple,

coa

rse-

grai

ned

sand

ston

e lo

call

y w

ith

gran

ules

and

sm

all p

ebbl

es;

plan

ar c

ross

-bed

s 10

–130

cm th

ick;

cos

ets

50–1

60cm

thic

k; p

lana

r set

s ta

bula

r to

lent

icul

aran

d of

ten

evol

ve d

ownc

urre

nt in

to c

ompo

und

cros

s-be

ds; d

ownc

urre

nt-i

ncli

ned

set/

cose

tbo

unda

ries

; cou

nter

-cur

rent

ripp

les

and

reac

tiva

tion

sur

face

s co

mm

on; i

nter

laye

red

wit

hfa

cies

5 a

nd 7

7R

ed to

pur

ple

fine

-gra

ined

mud

dy s

ands

tone

/mud

ston

e; u

sual

ly ri

pple

lam

inat

ed; i

nter

fere

nce

ripp

le m

arks

, sha

llow

cha

nnel

s, p

ools

and

des

icca

tion

cra

cks

com

mon

; len

ticu

lar u

nits

up

to15

cm th

ick;

inte

rlay

ered

wit

h or

ove

rlie

faci

es 6

and

4

8T

ypic

ally

sal

mon

red,

ver

y fi

ne to

med

ium

-gra

ined

, wel

l-so

rted

, wel

l-ro

unde

d sa

ndst

one;

abun

dant

adh

esio

n st

ruct

ures

; aeo

lian

str

ata

com

pris

e c.

40%

of t

he fa

cies

suc

cess

ion;

5–2

5cm

thic

k, le

ntic

ular

aqu

eous

uni

ts o

f mas

sive

/fai

ntly

cro

ss-l

amin

ated

san

dsto

ne;

ferr

icre

te la

yers

at p

lace

s; in

Man

cher

al a

rea

poly

gona

l sal

t-ri

dge

stru

ctur

es c

omm

on;

grad

atio

nall

y ov

erli

e/la

tera

lly

inte

rton

gue

wit

h fa

cies

4

Pal

aeoc

urre

nt

Imbr

icat

ion

in fe

w b

eds

show

sout

h-w

este

rly

tran

spor

t

Exp

osur

e m

ean

dire

ctio

n va

ries

from

251

˚ to

296˚

; dis

pers

ion

loca

lly

high

Con

sist

ent m

ean

flow

tow

ards

291˚

At R

amgu

ndam

exp

osur

e m

ean

dire

ctio

n va

ries

from

151

˚ to

244˚

; ove

rall

mea

n 21

5˚; a

tM

anch

eral

mea

n di

rect

ion

006˚

Exp

osur

e m

ean

vari

es fr

om11

8˚ to

275

˚; fl

ow w

as u

sual

lyat

hig

h an

gle

to th

at o

f the

encl

osin

g pl

anar

cr

oss-

beds

of f

acie

s 6

Dis

pers

ion

is lo

call

y hi

gh,

over

all fl

ow c

onsi

sten

tly

sout

h-w

est;

exp

osur

e m

ean

dire

ctio

nfr

om la

rge

plan

ar c

ross

-bed

sva

ries

from

204

˚ to

284˚

Var

iabl

e ri

pple

ori

enta

tion

Hig

hly

vari

able

pal

aeow

ind

dire

ctio

n m

easu

red

from

adhe

sion

cro

ss-l

amin

ae

Inte

rpre

tati

on

Vis

cous

to n

on-c

ohes

ive

debr

is fl

owor

hyp

erco

ncen

trat

ed fl

ood

flow

depo

sit o

n al

luvi

al fa

ns

Rap

id d

epos

itio

n fr

om s

edim

ent-

lade

n, h

igh

velo

city

flow

in s

hall

owch

anne

ls

Dep

osit

ion

from

larg

e th

ree-

dim

ensi

onal

dun

es in

dee

per p

arts

of

the

chan

nels

Dep

osit

ion

from

sm

all t

hree

-di

men

sion

al d

unes

in s

hall

ow, w

ide

chan

nels

occ

urri

ng a

t the

hig

her

topo

grap

hic

leve

ls/p

roxi

mal

floo

dpla

in o

f bra

ided

str

eam

s

Thr

ee-d

imen

sion

al d

unes

in s

mal

l,lo

w-s

tage

cha

nnel

s di

ssec

ting

top

ofla

rger

bed

form

s/ba

rs in

bra

ided

rive

rs

Dep

osit

ion

from

mig

rati

ng b

raid

bar

sth

at e

xper

ienc

ed ra

pid

fluc

tuat

ion

offl

ow d

epth

and

vel

ocit

y

Dep

osit

ion

in p

ools

/slu

ggis

h ch

anne

lson

bar

tops

or h

ighe

r top

ogra

phic

leve

ls o

f bra

ided

str

eam

s

Aeo

lian

/aqu

eous

dep

osit

s or

wea

ther

ing

profi

le m

ater

ial i

n th

ehi

ghes

t top

ogra

phic

leve

ls o

f the

over

bank

are

as o

f bra

ided

rive

rs


FSC31 14/01/2009 05:17PM Page 455

456 T. Chakraborty

Hierarchy of bounding surfaces

The most prominent bounding surfaces observed withinMancheral Quartzite are those marking the base of thechannel-fill successions. These are designated as first-order bounding surfaces. Sections 1, 2 and 3 below showthese surfaces as basal erosion surfaces underlying barsuccessions. Within the overlying planar cross-beddedsuccession, most prominent bounding surfaces form theerosional bases of the larger, solitary or compound planarcross-sets. These surfaces are irregular to flat and areeither subparallel or at a low-angle to the first-orderbounding surfaces. These surfaces are designated assecond-order bounding surfaces and are displayed mostprominently in section 1 (Fig. 4, see foldout). It should benoted, however, that within the limits of outcrop avail-able, these two surfaces have not been found to truncateeach other. First-order surfaces are easily recognized,because they separate distinct facies successions and arelaterally much more extensive than the second-order sur-faces. For example in section 4 (see below), although thefirst-order surfaces are not exposed the erosional lowerbounding surfaces of the larger planar cross-beds in thissection facilitate easy recognition of the second-orderbounding surfaces. Internal reactivation surfaces withinthe cross-sets or the intraset boundaries of compoundcross-sets constitute the third-order bounding surfaceswithin the Mancheral bar successions. The shapes of thethird-order surfaces are highly variable and have beenfound to change from convex-up to concave-up withinthe same set.

Description of the outcrop sections

Section 1 (Fig. 4)

The northern end of the section is marked by a triangularunit, comprising a 70-cm-thick, partly deformed planarcross-set (L) and an overlying thin unit of rippled, fine-grained sandstone (8–25 m, Fig. 4). This triangular unitis overlain to the north by a coset of planar cross-setswith set boundaries dipping to the north. To the southsecond-order bounding surfaces, overlying this deformedcross-bed, are inclined downcurrent. Combined thesecond-order bounding surfaces in this section define aform set geometry (sensu Anatase et al., 1997). Southof set L, three major downcurrent-inclined second-ordersurfaces accommodate three planar cross-sets (marked A,B and C in Fig. 4). These second-order surfaces areinclined 3–8˚ towards the south with respect to the first-order bounding surface at the base of the section. Each ofthe three sets thickens considerably (about 10 times in thecase of set A) as they migrate over the downcurrent-

Fig. 2. Vertical log through a representative section of MancheralQuartzite, Ramgundam Gutta. For location of the log see Fig. 1B.The vertically striped rose shows palaeocurrent data from facies2 and rose with horizontal stripes shows data from facies 4. Forall the rose diagrams N = number of data and C = consistency ratio.

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dipping basal erosional surfaces. The basal erosionalsurfaces of sets A and B, in their downcurrent extremity,however, become flat to slightly concave-up, producing ascoop-like shape. Consequently, the overlying cross-setshave a lenticular set geometry. At the northern end, thelower bounding surface of set A (as well as an internalreactivation surface) descends stepwise as it erodes downinto the underlying succession (between 15 and 27 m,Fig. 4). Note that finer grained, parallel lamina sets lapover the step-like reactivation surface at its upcurrent endand counter-current ripples are present in the thickest partof set A (between 27 and 30 m, Fig. 4). Inclination of theforeset laminae of the set diminishes in the downcur-rent direction and ultimately evolves into concave-up,channel-form layers near its downcurrent termination(34–43 m, Fig. 4). The top of the channel-form layershave been dissected by several small trough cross-beds.The overlying set B also thickens as it migrates over adowncurrent-inclined surface. The upcurrent part of set Bis dominated by avalanche foresets, with few intervalscontaining reactivation surfaces, whereas in the down-current part, it evolves into a compound set (Fig. 5).The reactivation surfaces in the upcurrent part of set B

are convex-up, whereas at the terminal part of the set,the third-order (intraset) bounding surfaces become verygentle, slightly convex-up and subparallel to the underly-ing concave-up (second-order) bounding surface (46–55m, Fig. 4). As in set A, the downstream margin of set B isdissected by small trough cross-beds containing coarse-grained sandstone with dispersed granules. Palaeocurrentmeasurements from the small trough cross-beds indicatepalaeoflow at a high angle to that of the large planar sets.Near its downcurrent termination, set B and overlyingtrough cross-beds are capped by a 15-cm-thick unit ofrippled, muddy sandstone of facies 7 (52–62 m, Fig. 4).In the central part of the section (around 35 m, Fig. 4),smaller planar cross-sets with granules and pebbles over-lie set B. Near the southern end of the section, set B issucceeded by another downcurrent-dipping second-ordersurface overlain by planar cross-beds (set C) that alsothicken in the downcurrent direction.

Section 2 (Fig. 6; exposed north of section 1)

A basal sheet conglomerate separates the planar cross-stratified succession from the underlying succession of

Fig. 3. Coset of planar cross-beds ina bar succession. Hammer handle is34 cm long.

Fig. 5. Compound cross-beds at thedowncurrent end of a large planarcross-bed, section 1, RamgundamGutta (set ‘B’, Fig. 4). Lower left-handcorner of the photograph showsupcurrent-dipping laminae at thesouthern margin of underlying set ‘A’(Fig. 4). The hammer handle is 34 cmlong.

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Section 4 (Fig. 8; located between sections 1 and 3)

The section, about 50 m in the inferred downstream direc-tion of section 1, contains three different orientations ofoutcrop faces and allows a three-dimensional reconstruc-tion of the depositional architecture. Organization offacies in the two parallel north–south trending faces isessentially similar to those in other sections. Second-order bounding surfaces are generally irregular, but someof these surfaces are distinctly dipping to the south-westor south-east. Consequently, some of the planar cross-setsthicken in the downcurrent direction (north and westwall of the exposure). Cosets of smaller cross-sets withoblique palaeoflow directions, interbed with or erodedown into the larger planar sets. Concentrations of gran-ules and pebbles locally define upward-coarsening trends(southern end of eastern wall). The east–west trendingsection shows large planar sets migrating oblique to thestreamwise direction and thickening away from the axialpart of the exposure (Fig. 8).

Interpretation of the sections

Planar cross-beds that dominate the bar succession prob-ably were deposited by migrating two-dimensional dunes(Ashley, 1990). Bedforms were both simple and com-pound types, the latter with superposed smaller bedforms.

facies 4. The section is dominated by superposed setsof large planar cross-beds with a persistent west-south-westerly flow. Planar sets are tabular to wedge-shapedand are characterized by alternating coarse- and fine-grained avalanche foresets with rare to common counter-current ripples. In the downcurrent direction, some of theforesets become finer grained; their inclination decreasesand some evolve into compound cross-beds. Some otherplanar sets pass laterally into ripple-laminated and smalltrough cross-bedded zones. Shallow channels (up to30 cm deep) cut into the top of the planar sets and arefilled with small trough cross-beds. Palaeoflow directionsof these trough cross-beds are at high angles to the planarsets (Fig. 6).

Section 3 (Fig. 7, see foldout; south of section 1)

Compared with sections 1 and 2, coarse-grained, smalltrough cross-bedded units are volumetrically more abun-dant in this section. Large planar cross-sets interbed with,or laterally pinch out within, trough cross-bedded unitsat different stratigraphical levels. The troughs show flowboth to the south-west and east (Fig. 7). Several rippled,muddy sandstone units (up to 23 cm thick) occur in thesequence and often are associated with the low-angleplanar cross-beds having convex-up foresets (near thesouthern margin of the section).

Fig. 8. Fence diagram showing the details of section 4. Note divergent accretion of large planar sets and locally upward-coarseningtrend within small trough cross-beds (indicated by the inverted arrow, east wall). Palaeocurrent arrows are plotted with respect to thenorth direction shown in the top right-hand corner of the fence diagram.

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460 T. Chakraborty

Similar planar cross-bedded successions are commonin many modern and ancient braided fluvial deposits(Collinson, 1970; Smith, 1970; Banks, 1973; Bluck,1976; Cant, 1978; Haszeldine, 1983a,b; Roe, 1987).Paucity of fine-grained sediments and low directionalvariability of the planar cross-beds both within andbetween the exposures (Figs 4, 6, 7 & 8) support deposi-tion in low-sinuosity bedload streams (Willis, 1993).Transition of solitary planar sets downcurrent into com-pound cross-sets (Figs 4 & 6), presumably formed by themigration of smaller bedforms at falling stages and arereported to be common features of the braided PlatteRiver (Blodgett & Stanley, 1980). Reactivation surfacesindicate frequent fluctuation of flow depth and velocity.Change in the shape of the reactivation surfaces fromconvex-up to concave-up (set B, Fig. 4) probably repres-ents a progressive lowering of the flow stage (Jones &McCabe, 1980). Erosional truncation of the set or coset ofplanar cross-beds by lenticular, trough cross-bedded units(Figs 4, 6 & 8) is inferred to represent the divergent,shallow, low-stage channels that dissected the bar tops.Rippled fine sediments (Figs 4 & 7) probably recorddeposition in slough channels on emergent bar tops (cf.Bluck, 1976).

Many of the planar cross-beds of the sections describedabove show a remarkable tendency to thicken downcur-rent as they migrate over the inclined bounding surfaces(sets A, B and C in Fig. 4; sets in the north and west wallof Fig. 8; and also a few sets in Figs 6 & 7). In order toachieve net deposition, migrating bedforms must climb inthe downcurrent direction (Rubin & Hunter, 1982; Rubin,1987). Consequently the 3–8˚ southward dips of the setbounding surfaces within the Mancheral bar successionmust imply the presence of sloping primary depositionalsurfaces. In the overall context of the braided river originof the Mancheral Quartzite, inclined surfaces in thesesections (particularly in section 1, Fig. 4) are inferred torepresent the lee slopes of downcurrent accreting fluvialbars. Straight-crested dunes grew larger as they migrateddown the lee side of the bars into deeper water of adjacentpools (cf. Haszeldine, 1983a,b; Kirk, 1983; Willis 1993).One of the most remarkable features of section 1 is thedowncurrent transition of a thick planar cross-bed (set A)into a concave-up trough-shaped lamina-set (Fig. 4).Similar features have been reported earlier from sandyand gravelly braided rivers and have been interpretedvariously as channel-fill between two bars (Ramos etal., 1986) or as pool-filling structures (Sigenthaler &Huggenberger, 1993). The second-order bounding sur-face at the base of the set defines a scoop-shaped de-pression with a steep, upcurrent margin and slightlyconcave-up downcurrent end. Set A filling the scour

shows systematic variation in the foreset geometry. Theshallow upcurrent part shows parallel laminae (18–23 m,set A, Fig. 4) and the thickest central part shows angularforesets and counter-current ripples. At the downcurrentend it forms trough-like laminae. The variable foresetgeometry is best explained by invoking the depth-ratioconcept (depth of water : depth of basin or scour beingfilled by a bedform (Jopling, 1965; Fig. 9) ). At theupstream margin of the scour pool, the flow was shallow(high depth ratio) and formed upper regime plane beds.As the bedform prograded into deeper waters of thescour pool, increased bedform height (lower depth ratio)ensured formation of angular foresets. The increased bed-form height also produced well-developed flow separa-tion in the lee of the bedform and back-flow eddiesformed regressive ripples at the toe of the foresets. As thebedform approached the other end of the scour, decreas-ing depth of the scour caused the foresets to becomelow-angle and tangential. Eventually it formed laminaesubparallel to the upcurrent-dipping base of the scour,resulting in trough-shaped lamina (Jopling, 1965; Fig. 9).

The above interpretation supports the inferred exis-tence of deeper-water pools in front of the depositionalhigh, represented by the sequence of deformed cross-beds(set L) and overlying fine sandstone in Fig. 4. The second-order surface at the base of the overlying set B has a sim-ilar geometry. The bounding surface at the base of set C,although incompletely exposed, hints at a similar trend.These surfaces are inferred to represent the successivepositions of the advancing pool in front of a bar. It isapparent from Fig. 4 that the scour pools migrated bothlaterally and vertically and the depth of the successivescours varied. Formset-like geometry (cf. Anastase et al.,1997) of the onlapping second-order surfaces across thedeformed cross-set (set L) in Fig. 4, probably indicate thatthe deformed cross-set and the overlying fines acted as a

Fig. 9. Schematic diagram showing the changing foreset shapeof a bedform as it fills a scoop-shaped scour. Note regressiveripples in the thickest, central part of the cross-set (after Jopling,1965).

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depositional high or core of the bar succession preservedin this section. The up-dip migrating planar sets to thenorth of the section probably denote upcurrent accretionof the bar core (Miall, 1985; Bristow, 1987; Bristow &Best, 1993). The scours, defined by the scoop-shapedto down-dipping second-order surfaces in the lee ofthe inferred bar core probably represent positions ofmigrating-pool structures. Similar bar–pool units havebeen reported from modern braided rivers as well aslaboratory models (Ashmore, 1993; Bluck, 1976; Bristowet al., 1993; Clifford, 1993; Ferguson, 1993). Pools prob-ably were formed at the flood stage and dune bedformsencroached upon them as the discharge fell (Clifford,1993). Progradation of the bar probably was achievedthrough the migration of the scour pool and the accretionof the pool-filling cross-stratified units on to the bar lee.As the river stage fell, smaller superposed bedforms over-took the larger dunes, forming compound cross-beds andeventually shallow channels dissected the emergent bars.Less commonly, fine-grained sediments formed on top ofthe exposed bars, to be partly removed during a sub-sequent flood, when a new scour was formed in the lee ofthe existing depositional high. Local concentrations ofcoarser material, within the smaller trough cross-beddedunits, probably reflect the winnowing action of shallowflows over emergent bars (Bluck, 1976). Two fine-grainedsandstone units are preserved in section 1 (13–17 m and52–62 m in Fig. 4). Assuming that the fines accumulatedat the top of the bars, occurrence of these two units,roughly at the same stratigraphical level above the first-order bounding surface, indicates that positive relief ofthe bar (with respect to the base of the channel) was main-tained over several flood and low-stage cycles.

The shapes of successive scoop-shaped second-orderbounding surfaces in Fig. 4 have a remarkable similar-ity to the computer generated scalloped cross-bedding(fig. 17, Rubin, 1987). In the computer generated cross-bed, bounding surfaces cyclically scoop down into under-lying deposits (as in the second-order surfaces in Fig. 4).Rubin (1987) postulated that such structures are producedby migration of bedforms undergoing large and rapidfluctuations of height. Such change in bedform heightalso has been reported from different depositional set-tings, including the Brahmaputra River (Coleman, 1969).When combined with downcurrent accretion, changingheight of the fluvial bars in response to imposed flowcondition is likely to produce features similar to thosegenerated by computer simulation.

It should be noted that the average thicknesses of pla-nar cross-beds in sections 2 and 3 are less than those insections 1 and 4. In sections 2 and 3, lower bounding sur-faces of the planar sets are subparallel to or inclined at

very low-angle to the basal first-order bounding surfaces(Figs 6 & 7). These surfaces lack the well-developedconcave-up scour morphology observed in section 1.Some features in these sections are consistent with theirdeposition on braid bars:1 transformation of larger solitary planar sets into com-pound cross-beds;2 erosional truncation by cosets of smaller trough sets;3 low palaeocurrent dispersion of planar cross-beds ascompared with small trough cross-beds.Smaller bedform size and absence of deep scour pools, asobserved in section 1, however, probably indicate that thecross-stratified successions were formed in shallow chan-nels, probably smaller anabranches within the Mancheralbraided streams. Downstream transition of solitary planarsets into cosets of small cross-beds and higher dispersionof the palaeocurrent azimuths of smaller trough cross-beds (sections 2, 3 and 4) probably represent convergingflow at the lee of larger bedforms during the falling-river stages and is very similar to the type c sand-stones described by Roe & Hermansen (1993) from thePrecambrian fluvial deposits of Norway. These largertwo-dimensional individual bedforms acted as emergentdepositional highs during falling stages and were modi-fied or dissected by low-stage shallow flows.

A variety of scours has been reported from modernrivers (see Slater, 1993). Two of the commonest type ofscours in braided streams are riffle-pools and confluencescours (Keller & Melhorn, 1978; Ashmore, 1993). Flowconvergence can be inferred from sections 3 and 4, butsimilar evidence is absent in section 1 (Fig. 4), which pre-serves the scour structures. Therefore, there is no first-hand evidence to relate these features to confluencescours. Correlation of these features with riffle-pool suc-cessions is problematic, because the relationship betweenriffles and braid bars is not very well understood (E. Wohland N. Clifford, personal communications, 1994). Al-though it is not known whether the riffles evolve intobraid bars the presence of an elongated pool flanking thebraid bar is well documented from modern braided rivers(Ashmore, 1993, Ferguson, 1993). Close association withdepositional highs, low height and evidence of periodicmigration and infilling by unidirectional, downstream-accreting two-dimensional dunes suggest that the scourstructures of Mancheral bar successions probably repres-ent bar–pool units (cf. Ashmore, 1993). Developmentof strong eddy currents, during high flood at the lee ofthe channel bars, possibly provided the driving mechan-ism for the formation of these scours. Data from thetwo-dimensional outcrops of the Mancheral Quartzite,however, are inadequate for precise process-based inter-pretation of these scour structures.

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462 T. Chakraborty

PALAEOCURRENTS AND BAR TYPES

Azimuths of planar cross-beds in each of the exposuresshow a very consistent mean towards the south-west(Figs 4, 6–8) and is inferred to reflect the mean channeldirection (Rust, 1972; Bluck, 1976). The small troughcross-beds, within and between exposures, show muchgreater dispersion (up to 220˚) as compared with that ofthe planar sets (maximum spread of about 100˚ about themean). This palaeocurrent pattern is consistent with theorigin of small trough cross-beds from shallow, divergentflows that dissected the bar top at the low-river stage (cf.Bluck, 1979).

Palaeocurrent patterns in section 1 indicate dominantflood-stage accretion of the bar to the south-west alongthe channels. The smaller cross-beds show a more west-erly flow direction (Fig. 4). This is inferred to indicatedeposition as a mid-channel bar that during falling stagesaccreted more towards the right (west) bank of the localchannel (Bluck, 1976; Hazeldine, 1983b).

Disposition of large planar cross-beds in section 4(Fig. 8) indicates divergent accretion of larger bedformsinto the deeper channels on both flanks of a topographichigh, as is typical of mid-channel braid bars (Bluck, 1980,pp. 33–37; Allen, 1983; Bristow, 1993). Erosively over-lying troughs in this section show a higher spread, buta symmetrical orientation with respect to the inferredmean channel direction. This pattern also is consistentwith the mid-channel bar interpretation of the section.

In section 3 (Fig. 7) some of the larger planar cross-beds initially thicken downcurrent and then pinch out lat-erally into cosets of smaller trough cross-beds. Whereasthe planar cross-beds show consistent flow to the south-west, the trough axes show much greater variation, withstronger modes towards the east or south-west. This prob-ably indicates flow convergence in the lee of the largerbedforms and preferential growth of the bar to the east(left bank of local channel) during low stage. Section 2(Fig. 6) contains similar successions, but palaeocurrentpatterns indicate preferential low-flow-stage accretion tothe west (right bank of local channels).

Summarizing the data from the sections, it can beinferred that most of these bars developed as mid-channeldepositional features with dominant flood-stage accretionin the downstream direction. During low stage, floweither converged at the lee of the larger two-dimensionalbedforms or bars (section 3) or flowed obliquely over thebars, either towards the right (section 1 and 2) or left (sec-tion 4) bank of the stream. Low-stage accretion of the barstowards alternate banks suggests a transformation from mid-channel to lateral bars (Bluck, 1976; Haszeldine, 1983b).

DEPTH OF THE PALAEOCHANNELS

Rippled, fine-grained sandstones in Fig. 4 are inferred tohave formed at the top of the emergent bars. Two fine-grained sandstone units (at 13–17 m and 52–62 m inFig. 4) are about 190 cm above the basal, first-order ero-sion surface (the inferred channel base). Therefore, oneestimate of the relief of the preserved bar in this section is190 cm. The heights of the bars provide the minimumestimate of the bankful channel depth (Allen, 1983).Accordingly the channels were at least 2 m deep.

As already discussed, the formset geometry of thesecond-order bounding surfaces in Fig. 4 (with a shortstoss and a longer lee side) probably reflect the deposi-tional topography of the palaeobar (cf. Anastase et al.,1997). The highest elevation of the brink point of thistopography (above 17 m) is 235 cm above the basalfirst-order bounding surface. This value provides analternative estimate of the bar height and would imply aminimum channel depth of about 2.5 m for this section.

It is difficult to make similar estimates for the succes-sion in Fig. 6, because of the lack of preserved deposi-tional topography. The minimum height of the low-stagechannels, presumed to be flowing over emergent bars,from the basal first-order erosion surface, however, isabout 90 cm (south-western end of Fig. 6). Assumingthis value to reflect the thickness of the emergent bar/bedform in this section, the minimum estimate of thechannel depth should be around 1 m. Lower estimates ofthe palaeochannel depth from the succession in Fig. 6 isconsistent with the lower heights of the preserved cross-bed sets.

Flow depth also can be estimated using the relationship

H = 0.086d1.19 (Allen, 1968) (1)

where H is the mean dune height and d is the flow depth.The calculated value of flow depth (d) can be modifiedfurther by incorporating compensation for (i) compactionof sand as a result burial, c, and (ii) variation in flow depthbetween straight and sinuous reaches, c′. The modifieddepth is given by

dm = cc′d (Khan, 1987) (2)

Values of c range from 1.1 to 1.2 and those of c′ rangefrom 0.585 to 1.0 (Khan, 1987, and references therein).The mean thicknesses of planar cross-beds were calcu-lated for section 1 and section 2 and 117 measurementswere collected throughout the Ramgundam area (Table3). The flow depth calculated by eqns (1) and (2) for thesuccession preserved in section 1 (Fig. 4) ranges between146 and 272 cm and that for section 2 varies between 97

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and 181 cm. The average flow depth estimated for theMancheral braided system (on the basis of facies 6 planarcross-bed thicknesses) ranges between 92 and 145 cm.The calculated values tally closely with the values esti-mated from the bar morphology and thus support thebar dimensions reconstructed on geometric grounds asdiscussed earlier.

TOWARDS A MODEL FOR SHALLOWBRAIDED-RIVER BARS

Bars have been the focus of many sedimentologicalstudies aimed at deciphering the distinctive signature oftheir depositional processes (e.g. Williams & Rust, 1969;Smith, 1971, 1974; Bluck, 1976, 1979; Cant, 1978,Brierley, 1991; Bristow 1993). Slater (1993) and Bristowet al. (1993) recently pointed out that other features ofstream beds, notably scours of different types, havinghigh preservation potential and sedimentological impli-cations, have received little attention from sedimentolo-gists. These scours include both quasi-regular riffle-poolsequences (Keller & Melhorn, 1978; Clifford, 1993) andmore irregular confluence and constriction scours (Mosley,1976; Best, 1986; Ashmore et al., 1992). Recognition ofscour pools within the Mancheral Quartzite underlinesthe importance of these features as an essential elementof ancient braided-river deposits and provides an actual-istic basis for constructing a braided-bar facies model.

The common features that characterize the bar succes-sions in the Mancheral Quartzite are:1 the presence of a succession of planar cross-beds abovea major erosion surface and paucity of fine-grained sedi-ments in the succession;2 downcurrent thickening of some of the larger planarcross-beds as they migrated over downcurrent-dippingsecond-order bounding surfaces;3 transition of the simple planar cross-beds into com-pound cross-beds or into cosets of smaller trough cross-beds in the downcurrent direction;

4 the presence of shallow channels that cut down into thetop of the larger bedforms and are filled with small troughcross-beds — flow in these shallow channels was at a highangle to that forming the larger cross-sets.5 low dispersion of palaeocurrents measured from largeplanar cross-beds both within and between exposures —in contrast, directions from the smaller trough cross-bedsshow higher dispersion.

These features collectively indicate the followinginferences.1 Bars dominantly are comprised of large two-dimen-sional dunes.2 The presence of a ‘depositional high’ or bar and adeeper-water pool in its lee, comparable to the ‘pool-bar’units described from modern braided rivers and scaledlaboratory models (Ashmore, 1993). In comparativelydeeper channels, these scour pools were well developedand in current-parallel sections reveal a ‘spoon-shaped’geometry (cf. Ashmore, 1993, p. 131). In shallower chan-nels, pools had a shallower depth and the lower bound-ing surfaces of the two-dimensional dunes (the inferredbar–pool interface) are either subparallel or subtend asmall angle with the first-order bounding surfaces at thebase of the sand body (the inferred channel base).3 The bar successions in the Mancheral Quartzite wereformed during rapidly fluctuating flow conditions. Dur-ing low-flow stage, smaller bedforms developed initiallyin the lee of the larger bedforms (Blodgett & Stanley,1980) but as water level fell further, bedforms were dis-sected by shallow divergent flows.4 Mancheral river channels had a low sinuosity. Barswere mostly mid-channel type. During low-flow condi-tions, some of the bars accreted either to the left or rightbank of the stream channels.5 The Mancheral fluvial system was characterized by thecontemporaneous existence of both shallow and deeperchannels similar to the different order of coexistingchannels in braided streams (Bristow & Best, 1993).

Bluck (1976, 1979) recognized multiple topographiclevels within braid bars and termed them platform,

Table 3. Estimation of the palaeoflow depth on the basis of thickness of facies 6 planar cross-beds.

Mean height of DepthSection number/location Number of data dunes (H) (cm) dm = cc′ (H/0.086)1/1.19 (cm) Remarks

Section 1 (Fig. 4); 9 54.55 145.7–271.7 Height of Ramgundam Gutta reconstructed bar 235 cm

Section 2 (Fig. 6); 10 33.66 97.01–181.01 Minimum height of Ramgundam Gutta reconstructed bar 90 cm

All data from facies 6; 117 31.65 92.20–143.28 —Ramgundam area

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464 T. Chakraborty

supra-platform bar-head, bar-head lee and bar tail. A hier-archy of bounding surfaces, inferred to have developed asa result of the lateral migration of these topographicallydifferentiated bars was used by Haszeldine (1983a,b) toreconstruct a 10-m-thick bar succession. A hierarchy ofsimple periodic to complex quasi-periodic bedforms isknown to exist within braided channels and consequentlythe scale and internal organization of the bar succes-sion is expected to vary considerably. The scale of bed-forms considered to be a braid bar is probably stronglyinfluenced by the scale of the channels in which theyexist, varying from simple dunes to large channel islands(see Smith, 1978; Miall, 1981). The existence of pool–barunits, convergence of flow in the lee of the bars, dissec-tion of bars or larger bedforms at low stage, coexistenceof multiple orders of channels and the presence of mid-channel bars with flow on both sides of the depositionalhigh, however, are typical of most braided rivers, inde-pendent of their scale or size of the bedload (Smith,1970; Rust, 1972; Bluck, 1976; Cant, 1978; Bristow,1987; Ashmore, 1993; Ferguson. 1993). Recognition ofthese features provides the key to the reconstruction ofthe bars in the Mancheral Quartzite. Simpler internalorganization and the lower thickness of the bar succes-sion in the Mancheral Quartzite appear to reflect thelack of multiple topographic levels and the simpler mor-phology of the bars. The bars of the Mancheral Quartziteare probably intermediate in character between large barswith multiple topographic levels and simple linguoiddunes that occur within braided streams.

In many respects, Mancheral bars are analogous tothe bar sequences described by Roe & Hermansen(1993) from the Precambrian fluvial deposits of northernNorway. As they point out, unstable sandy banks andthe absence of cohesive fines in the overbank areas(see Chakraborty & Chaudhuri (1993) for a descriptionof overbank deposits of the Mancheral Quartzite) ofthese Precambrian rivers favoured channel widening, inresponse to increased flood discharge, resulting in a highwidth : depth ratio. Shallow channels resulted in low barheights (Roe & Hermansen, 1993, their figs 4 & 8). Theinternal organization of these Norwegian bars, notablythe downcurrent thickening of the larger cross-sets, up- ordowncurrent transition of these larger sets into cosets ofsmaller cross-sets, erosional dissection of the top of thelarger sets and low palaeocurrent variability, bears a strik-ing resemblance to the Mancheral bar successions. Thegrain size of these Norwegian rocks, however, is muchfiner, which may account for the abundance of plane bedswith parting lineation and sigmoidal cross-sets in the suc-cession. The coarser grained deposits of the MancheralQuartzite contain very few of these features.

CONCLUSIONS

Well-exposed sections of the planar cross-bedded faciesof the Mancheral Quartzite allow reconstruction of theshallow braid bars that characterized the precursor chan-nels. The exposures reveal:1 sets of cross-beds with downcurrent-inclined boundingsurfaces, which denote accretion of bedforms on the frontof the depositional highs (bars) into the adjacent pools;2 scoop-shaped second-order bounding surfaces andoverlying sets or cosets of planar cross-beds are inferredto be pool-filling successions;3 the bar successions were characterized by fluctuatingflow, flow convergence in the lee of the bars/bedformsand dissection of the bar tops;4 depositional channels had a low sinuosity and barswere mostly of mid-channel types;5 different lines of evidence suggest maximum bar-forming depths of about 2.5 m.

ACKNOWLEDGEMENTS

Part of the data presented here has been extracted fromthe author’s PhD thesis, carried out under the supervisionof Professor A.K. Chaudhuri of the Indian StatisticalInstitute. Very competent field assistance was providedby S.N. Das. Drafting and redrafting of the diagrams werepatiently carried out by A.K. Das. I am grateful to all ofthem. Exchanges with Nicholas Clifford (University ofHull) and Ellen Wohl (Colorada State University) on theriffle-pool sequences were really instructive. I gratefullyacknowledge the constructive reviews of Michael Wizevichand an anonymous reviewer and the editorial efforts ofNorm Smith that greatly improved the manuscript.

REFERENCES

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