Facies (2011) 57:331–349

DOI 10.1007/s10347-010-0241-1

ORIGINAL ARTICLE

Slides, soft-sediment deformations, and mass Xows from Proterozoic Lakheri Limestone Formation, Vindhyan Supergroup, central India, and their implications towards basin tectonics

Partha Pratim Chakraborty

Received: 21 May 2010 / Accepted: 26 September 2010 / Published online: 21 October 2010© Springer-Verlag 2010

Abstract The Neoproterozoic Lakheri Limestone (LL)Member of Vindhyan Supergroup, central India, interpretedas a low-gradient homoclinal ramp, contains a wide rangeof signatures indicating syn-sedimentary basinal extensionand compression. Whereas features like intraformationaltruncation (slide) surfaces of varying geometry, creep andbedding translation manifest the phases of extension, thecompressional events are registered in bed-conWned thrustsand outcrop-scale folds. A wide range of outcrop andmicroscopic deformational features are associated with thesliding events, the expressions of which vary based on theirrelative position with respect to the slide surface (over- orunderlying) and the degree of built-up pore water pressure.The detached sediment mass often evolved in the form ofmass Xows with rheology varying between cohesive debrisXow and low-density turbidity current. In particular, opera-tion of reXected turbiditic Xows is suggestive of irregulardepositional substrate, induced by curvilinear syn-sedimen-tary slides in otherwise low-gradient distal shelf platformalsetting. The present study intends to relate the observedextensional and compressional features of LL successionwith the Xexural response of early rifted Vindhyan base-ment under reversing in-plane stress in its post-rift deposi-tional history. Bipolar NE–SW orientation of the slideplanes is well consistent with the proposed rifted conWgura-tion of Vindhyan basement. Centimeter- to decimeter-deepslide detachments and equally thick mass-Xow beds areindicative of relatively deeper level of necking during theearly syn-rift phase of Vindhyan history.

Keywords Neoproterozoic · Lakheri Limestone Formation · Slide · Mass Xow · Flexure

Introduction

Sub-aquatic creep, slide and slump within soft or semi-solid sediments on slopes of various angles (as little as afew degrees), and mass Xows generated from there, arestudied in both siliciclastic and carbonate settings (Prioret al. 1983; Coniglio 1986; Stow 1986; Jones and Preston1987; Gawthorpe and Clemmey 1985; Gawthorpe 1986;Chakraborty 2004). The majority of the studies, however,are from the slope settings of passive margins, rift, andforeland basins. Results from the studies are particularlyimportant in the Welds of (1) geotechnology, (2) neotecton-ics, as soft sediment structures are considered to have acritical link in the chain of reasoning associated withearthquake prediction studies, and (3) carbonate platformgeometry reconstruction, a common target area for explora-tion of hydrocarbon. Though it has been generally agreedfrom the stability analysis that failure at some depth belowthe sediment surface occurs only when shear stress exceedsthe shear strength of the sediment, opinions vary on defor-mation mechanism, causative forcing/s (auto- or alloki-netic), and bearing of penecontemporaneous deformationfeatures in understanding basin conWguration or deposi-tional tectonics in diVerent space–time scale. Leaving asidethe challenge of diVerentiating features originated out ofsyn-sedimentary deformation from those of tectonic origin,the other major goals are to understand the allo-/auto-kinetic controls on sedimentation, paleoslope and thegenetic link between the slides, and gravity Xow processesin sedimentary environments. Paleoseismologists, however,tend to ignore slope-controlled penecontemporaneous

P. P. Chakraborty (&)Department of Geology, University of Delhi, Delhi 110007, Indiae-mail: parthageology@gmail.com

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332 Facies (2011) 57:331–349

deformation structures, lest they blur the distinction betweenthe auto- and allokinetic products (Sims 1975; Vittori et al.1991; Bose et al. 1997, and many others).

In categorizing carbonate platforms, sedimentologistscommonly put an emphasis on geomorphic proWles, pro-ductivity, facies, and evolutionary sequences. Besides theseattributes, the features relied on most in rock record fordiVerentiating a homoclinal carbonate ramp (of gentleslope, generally less than 1°) from a distally steepenedramp or rimmed shelf are the occurrences of slumps, brec-cias, allochthonous lime sands, and intraformational trunca-tion surfaces (Wilson 1975; Read 1982; Tucker and Wright1990; Einsele 2000; Drzewiecki and Simo’ 2002). Suchfeatures are tied to slope physiography and a homoclinalramp is distinguished by their absence. With the under-standing that earthquakes can act as a major trigger behindchanging the physical state of a sedimentary deposit fromsolid-like (with substantial yield strength) to liquid-like(with little or no yield strength) irrespective of any sedi-mentary environment, the reliance on gravity Xow productsfor reconstruction of platform geometry seems to be anoversimpliWcation.

This paper presents: (1) a wide spectrum of intraforma-tional truncation surface geometries, hitherto identiWed asslide planes; (2) signatures indicating penecontemporane-ous disturbances (deformation; compressional and exten-sional including slide failure); and (3) co-genetic gravityXow deposits of widely varying rheology from LakheriLimestone (LL) Member of the Vindhyan Supergroup, cen-tral India (Fig. 1a), modeled as a Proterozoic homoclinalcarbonate ramp lacking shelf-slope break (Sarkar et al.1996; Chakraborty 2004). Several soft sediment deforma-tion (SSD) features belonging to both extensional and com-pressional regimes and diVerent genetic varieties ofcarbonate gravity Xow deposits are documented from»100-m-thick shallow-marine LL succession exposed overa 200-km2 study area in central India. Attempts were madeto explore genetic constraints for diVerent kinds of SSDfeatures, their genetic relationship with the gravity Xows,and further, the bearing of these features in understandingthe tectonic framework of the basin in which the depositiontook place.

Geologic setting

Geophysical investigations have revealed rifting acrossfaults aligned almost E–W that initiated formation of theMeso- Neoproterozoic Vindhyan basin. The rifting wasaccompanied by a dextral shear (Fig. 1b) and the basin wassegmented into several NW–SE elongated sub-basins, theconWgurations of which controlled the formation thicknessand facies disposition in the early part of basin history(Bose et al. 1997, 2001). It is believed that with attenuation

of tectonic control, the basin evolved into a sag basin (Boseet al. 2001). An alternate model envisages initiation andevolution of the basin in a peripheral foreland moat (Chakr-aborty and Bhattacharya 1996) in a southerly dipping sub-duction setting that led to the collision of Bhandara andBundelkhand cratons (Chakraborti et al. 2007).

The Neoproterozoic marine LL Member belongs to theBhander Formation (Bhattacharya 1996), and occupies aposition towards the top of the Vindhyan Supergroup(Venkatachala et al. 1996; Fig. 1c). The limestone unit issandwiched between Wne-grained siliciclastics of theLower Bhander sandstone (LBS) above and the GanurgarhShale (GS) below (Fig. 1c). Both the bounding membersare marginal marine in origin; LBS represents a muddytidal Xat (Chanda and Bhattacharya 1982) and GS repre-sents a muddy chenier plain (Chakraborty et al. 1998).From consistent southwestward slope and depth-relatedfacies organization, Sarkar et al. (1996) supported theepeiric ramp model, proposed by previous workers(Chanda and Bhattacharya 1982; Chaudhuri and Chanda1991) and suggested southwestward opening for the low-gradient LL ramp. Upward gradational transition from theGanurgarh Shale to the Lakheri Limestone (Fig. 1c) isinterpreted as total turn around in depositional conditionfrom siliciclastic to carbonate with minor change inbathymetry. A continuous progradation in the siliciclasticGS gave rise to a very low-gradient surface on which asmall rise in relative sea level resulted large scale inunda-tion (Chakraborty 2004). From process-based faciesanalysis within LL, Sarkar et al. (1996) delineated WvediVerent facies types of wide-ranging paleo-environmentalsigniWcance, viz. (1) planar and cross-stratiWed (PCS) car-bonate of beach-shoreface, (2) stromatolitic (SM) shore-face, (3) limestone-shale heterolithic (HL) inner shelf, (4)plane laminated lime mudstone (PLM) of outer shelf, and(5) shaly (SH) distal shelf (Fig. 2; Table 1). The Lime-stone Member is virtually undeformed, unmetamorphosedand the beds are subhorizontal. Dominant micritic charac-ter of limestone, well-preserved primary features (stromat-olitic laminae, peloidal and intraclastic texture in limewackestone), low Mn/Sr ratio (<1; Ray et al. 2003), unal-tered �18O values, non-luminescent CL and poor correla-tion between �13C and �18O values obtained from mineralseparates (Ray et al. 2003) and bulk carbonate samples(Kumar et al. 2002; Chakraborty et al. 2002) discard thepossibility of large-scale deep burial diagenesis suVeredby the LL Member.

Although devoid of any regional and pervasive post-depositional deformation, the LL Member reveals meso-and microscale features that bear telltale signatures forbasin-scale extension and compression in course of itsdepositional history (Chakraborty 1996, 2004). Whereasthe compressional features are invariably restricted to the

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Facies (2011) 57:331–349 333

Fig. 1 Outcrop map of the Bhander Formation and the bounding for-mations in the Son Valley, central India (a). Inferred tectonic model forVindhyan basement showing its east–west rifted character and sub-basinal conWguration (modiWed after Bose et al. 1997) (b). Broad

stratigraphic subdivision of Vindhyan Supergroup, stratigraphic positionof Lakheri Limestone Member and measured lithology depicting gra-dational transition between Ganurgarh Shale and Lakheri Limestoneare presented in the lower half (c)

Fig. 2 Schematic illustration of the inferred depositional environments in the Lakheri Limestone succession (not to scale)

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334 Facies (2011) 57:331–349

Tab

le1

Lit

hofa

cies

type

s of

Lak

heri

Lim

esto

ne M

embe

r an

d th

eir

infe

rred

dep

osit

iona

l env

iron

men

t (m

odiW

ed a

fter

Cha

krab

orty

200

4)

Fac

ies

type

Des

crip

tion

Fac

ies

asso

ciat

ion

Env

iron

men

t

A. S

trom

atol

ite

(SM

)D

igit

ate

and

colu

mna

r st

rom

atol

ites

; occ

asio

nall

y in

clin

edC

omm

on a

ssoc

iatio

n w

ith

faci

es B

and

C;

rare

ly o

verl

ies

faci

es D

; co

ntac

t wit

h fa

cies

C a

nd D

gra

dati

onal

; co

mm

on in

traf

acie

s tr

ansi

tion

betw

een

dom

al

and

colu

mna

r va

riet

y

Agi

tate

d in

tert

idal

to s

hall

ow s

ubti

dal

Dom

al c

ompo

site

str

omat

olit

e w

ith

mic

ritic

wav

y la

min

ated

hem

isph

eroi

dP

rote

cted

sub

tida

l ext

endi

ng

up to

inne

r sh

elf

Mic

robi

al la

min

ite

with

thin

, Xat

, pla

nar,

cri

nkly

lam

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, oc

casi

onal

pre

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qua

rtz

silt

gra

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to s

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l

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)

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to f

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calc

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L)

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et-

like

bed

geo

met

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alca

reni

tes

with

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sion

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elf

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tern

ally

wav

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lam

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ed a

nd h

umm

ocky

cr

oss-

stra

tiW

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hale

uni

ts m

assi

ve o

r pl

ane

lam

inat

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Gut

ter

cast

s at

the

sole

of

calc

aren

ite b

eds

D. P

lane

-lam

inat

ed

lim

e m

udst

one

(PL

M)

Pla

ne la

min

ated

with

thic

k-th

in a

lter

atio

ns b

etw

een

calc

areo

us la

min

ae,

whi

ch o

ccas

iona

lly s

how

inte

rnal

gra

ding

. Int

erca

lati

on o

f gr

aded

or

ung

rade

d re

lati

vely

thic

ker

carb

onat

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ass-X

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eds

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des

into

fac

ies

C,E

, and

rar

ely

faci

es B

; co

ntac

t wit

h fa

cies

A in

vari

ably

sha

rp, e

rosi

onal

OV

shor

e be

low

sto

rm w

ave

base

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-ang

le to

bed

-par

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l tru

ncat

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plan

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(S

H)

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ne la

min

ated

or

mas

sive

sha

leIn

vari

ably

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rp c

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lyin

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fa

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tal oV

shor

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sion

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surg

ence

of

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rm

Thi

n sa

ndst

one/

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terb

eds

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acie

s er

osio

nal

123

Facies (2011) 57:331–349 335

packstones/grainstones, the extensional features, i.e., thetruncation surfaces and related soft-sediment deformationfeatures are noticed exclusively within the calcareousmudstones (calcilutites) of Plane Laminated Mudstone(PLM) facies (cf. Sarkar et al. 1996) (Fig. 3a). Each of thecarbonate laminae is draped by a very thin (commonlyless than a millimeter) veneer of argillaceous material(‘parted limestone’, cf. Cook and Enos 1977). The car-bonate laminae, at places, also show normal grading. Inthe undisturbed parts, the calcareous laminae (ignoringargillaceous veeners) characteristically show rhythmicthick-thin alternations that are reminiscent of tidal deposi-tion (Fig. 3b; Chakraborty 2004). Often, the lamina setsare also intercalated with coarser (commonly sand andgranule grade) and thicker (average thickness 4 cm) paral-lel-sided beds of mass Xow origin (Fig. 3a; discussedlater). The present study is focused within this faciesbecause of (1) very Wne grained lithology with preservedthin interlaminations that allowed easy recognition oftruncation and deformation features, and (2) distal low-energy depositional setting that excluded possibility ofreworking by shallow-marine agents or storm wave load-ing, and, in turn, enhanced the possibility of identifyingextra-basinal forcing, if any.

Signatures of intrabasinal tectonics

Features indicative of both extension and compressionoccur at diVerent stratigraphic levels within the LL succes-sion. Most prominent among the extensional features aredecimeter- to meter-wide low-angle two-dimensional listric(concave-upward) discontinuity surfaces and centimeters-thick shear zone lacking discrete basal shear plane. Thecompressional features include low-angle bed-conWnedthrusts and outcrop scale folds.

Extensional features

Failures, block movements, and deformations

Failures within the PLM facies of LL are observed broadlyin two diVerent motifs, viz. (1) detachment leading to sepa-ration of allochthonous mass from the autochthonous sedi-ments, and (2) bedding translation without removal oraccumulation of sediment. Whereas the movement of arigid, internally undeformed mass along a discrete shear(detachment) surface in case of the former is referred to as aslide following the deWnitions provided in Nardin et al.(1979), the bedding translation presumably resulted fromdownslope creep along synsedimentary shear zones (cf.Coniglio 1986). The original position of a slide mass isdemarcated by an intraformational detachment surface,where underlying bedding planes intersect the surface withangular disconformity. Transported slide mass, displacedlaterally relative to underlying sediment, may cause localrepletion of the stratigraphy or ‘sediment excess’ (Fair-bridge 1946), whereby the slide mass and the underlying,overridden sediments may or may not be identical litholog-ically.

Detachment surfaces

Numerous local intraformational detachment surfaces areidentiWed either parallel to the bedding (Xats) or cut acrossit (ramps) and commonly with a listric geometry. In the Xatpart, the truncation planes are commonly at 3–4° to the pri-mary laminae, whereas in the ramp part, the planes assumeangles as high as 12°. Intraformational breccias and con-glomerates (types I–V; discussed later) often overlie ortraced laterally in continuation with the slide planes.Though commonly associated with the extensional features,

Fig. 3 Thinly laminated lime mudstone within PLM facies (a). Note intervening mass-Xow unit at the lower part. b Histogram showing thick-thin laminae alternation with PLM facies

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compressional features (discussed later) are also observedassociated with slide planes cutting up-section in the trans-port direction. Within the study area, the slide planes areobserved with varied geometries, viz. stepped, planar,curved: convex or concave up, sinuous and ramifying(Fig. 4a–e) and with dip towards northeast and southwest,respectively (Fig. 5). Deformation features associated withthe slides are discussed here based on their presence withinthe hanging wall (surWcial) and footwall (under-surface)block.

Creep and bedding translation

Packstone with single train of ripple-like asymmetric formsrepresents this zone (Fig. 6). The overlying sediment pack-age is completely devoid of any deformation. Elevation ofbasal surface in harmony with the crests and thrusting ofgentle arms on the stepper arms of the ripple forms are thetell-tale features in this zone.

The elevations of the basal surface of ripple-forms pre-clude the possibility of their bedform origin. Evidence ofthrusting manifests operation of directional shear. A shearzone possibly developed immediately below a truncation

surface or below sediment mass that crept for a short dis-tance downslope without getting detached from the underly-ing sediments. The stress that caused the deformation wasevidently resolved within a thin zone enriched in clay. Thelimited lateral extent of thrusting, however, suggests that themovement had been very temporary, and ceased almost assoon as it commenced. Such a situation is the likely result ofan earthquake, because earthquake shocks are short-livedand once a shear plane develops, subsequent shocks cannotpropagate above the shear plane (Lee et al. 1991, 1993). Thelack of deformation in the overlying strata suggests that theoverlying sedimentary body behaved as an undrained sys-tem denying incorporation or expulsion of water. Deforma-tions in the shear zones suggest minor downslope creep of asediment package relative to a presumably, though not nec-essarily, in situ lower package of sediments.

Deformation features

SurWcial varieties

Type I: growth fault Sediment package constituted of anarray of structures in ascending order, viz. (1) roll-over

Fig. 4 Slides of varying geometry: a stepped, b planar-curved, c concave-up (arrowed), d sinuous (arrowed), and e ramifying (pen length 14 cmand coin diameter 2.5 cm)

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Facies (2011) 57:331–349 337

fold, (2) a set of laminae down buckled and thickenedtowards the fault plane, and (3) a set of laminae graduallysmoothening out the relief (Fig. 7a), represent the fault sys-tem. The displacement commonly is in centimeter scale andlaminae truncation is observed within vertical thickness of3–10 cm only. Average amplitude and wave length of asso-ciated roll-over folds are 3.5 and »9 cm, respectively.

Continuation of detachment with ongoing sedimentationis suggested (growth fault; Elliott and Ladipo 1981;Wignall and Best 2004). Attenuation in the scale of defor-mation from base upward within the package is suggestiveof causative forcing from the base. Sliding along certainargillaceous laminae, common within the PLM facies, mighthave provoked the deformation. Reasonably, the scale of

movement in the hanging wall block was limited that didnot allow disintegration of hanging wall mass.

Type II: rotational slide and In situ block rotation A blockof sediment with internal lamina sets at an angularrelation with both underlying and overlying sediment oronly with its underlying sediment represents this type(Fig. 7b). Whereas in the former type detachment surfacecan be traced both at the base and top of the block, thedetachment in the later case is recorded only at its base.Often, brecciated layer (of average thickness »3 cm) inconformity with the lamination is noticed within thelater. Such blocks are traced for a maximum exposurewidth of 1.6 m and maximum thickness recorded is0.45 m.

While the former is interpreted as the product of in situblock rotation, the later may be the result of rotational slideor slump on an inclined slide plane in which the overridingblock underwent tilting upslope or downslope. In in situblock rotation, ramiWcation of the slide planes causes rela-tive movement of the blocks enclosed between the coupledslide planes not only in downslope direction but may alsoslide upward, overriding the relatively static forwardblocks. The laminae within the overridden substratum maythen be dragged upward and eventually break up from theparent body.

The brecciated layers within the later are presumablythe result of in situ brecciation in course of any previouscatastrophic event. The brecciation indicates that the sedi-ment turned semi-consolidated even before the catastrophicevent. Transport of the layers separated by the zone of brec-ciation indicates that the slide block, as a whole, was wellcemented before emplacement.

Fig. 6 Details of a décollement zone (coin diameter 2.5 cm). Note elevated bases of ripple-like structure (arrowed) in congruence with their crests.Internal thrusting within the structures is evident

Fig. 5 Stereographic projections of poles of slide planes present withinLL Member. Note northeastern and southwestern clusters of poles

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338 Facies (2011) 57:331–349

Fig. 7 SurWcial deformation features: a Syn-sedimentary listricdetachment with overriding roll-over fold (detail discussion in text).b A rotated block. c Multiple beds of contorted laminae. Note imbri-cated load casts at the bottom, relatively larger imbricated convolutes

in the middle, and multiple sets of convolutes with upward-decreasingsize at the top. Note twisting of mud Xame in direction opposite to thedirection of load cast imbrications

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Facies (2011) 57:331–349 339

Type III: convolute lamination, load cast, and water escape channels An array of Xuidization structures, intra- orinter-bedded, is present within the PLM facies. Convolutefolds and load casts, measurable in centimeter-scale, showstrong preferred asymmetry (Fig. 7c) and are concentratedalong selected planes that can be traced over entire outcroplength, often spanning over a kilometer. Convolutes havediameters up to 20 cm. At places, the contacts between suc-cessive deformed beds are disturbed and indistinct; main-taining the consistent sense of asymmetry all throughout.At places, the deformational structures with preferred ori-entation present at the lower part shows gradational upwardtransition to undeformed sediment except for some verticalwater-escape structures. Openings discordant or parallel tobedding and Wlled with massive host sediment (PLM) orgranular mass-Xow (discussed later) sediments representthe pathways of water escape. The structures evidently haveformed at or very near the sediment–water interface. Persis-tent and uniform asymmetry of convolute orientationimplies operation of directional shear all through the depo-sition of bed sequence. Considering the distal oVshoredepositional set up for the PLM facies with slow rate ofdeposition, the possibility of shear generated from overrid-ing current (Brodzikowski and Halusczak 1987) or lique-faction from overloading are largely overruled. Theasymmetry in convolute orientation is probably related toslide on slope (Seth et al. 1990). The paleoslope is indi-cated opposite to the direction of inclination of convolutefold axes. The oppositely twisted Xame structures (Fig. 7c),present between the load casts in the basal part of the bedsequence, imply the tendency of the overlying bed to slide.Seismic shock is considered as the most plausible trigger-ing mechanism. The bipartite intrabed division with convo-lutes at the base and underformed sediments above can beexplained as gradual decrease in directed shear from baseupward within a surWcial slide sheet (Cook et al. 1972).Preferred upslope orientation of axial planes of convolutefolds and water-escape structures at the basal part and verti-cal water-escape structures (Neptunian dykes) at the upperpart of the bed conWrm the contention.

Undersurface varieties

Type I: shraded bed

Chaotic, highly disturbed sediment (Fig. 8a) capped byundisturbed sediment represent this type. Often, the contactbetween the two is marked by the presence of tensioncracks. Thickness of such sediment package varies between12 and 86 cm.

Coniglio (1986) described such chaotically deformedsediment as the product of creep movement at depth. SuchconWned deformation may trigger tensional cracks at the

base of overlying undisturbed sediments. Considering thechaotic and pervasive nature of deformation, it is presumedthat the sediment was in a saturated state and the surWcialsediment behaved as an undrained system. The undisturbedlayer on top evidently acted as a capping, restricting waterescape through it. In an undrained saturated system, thepore-water pressure resists external stress and the stressturns hydrostatic. High-magnitude shocks, as of divergingstress at right angle to the conWning walls are to be created(Pascal’s law) and the resulted deformation within the sedi-ment is likely to be chaotic.

Type II: fractures

Sets of fractures with or without displacement are recordedfrom diVerent stratigraphic intervals of the LL succession.Geometrically, the fractures are either curved, sigmoidal, ortapering, parallel Wssures and veins (Fig. 8b). The curvedones are without any perceptible displacement across them,invariably extend from a well-deWned slide plane and bluraway downward. The hanging wall laminae are upturnedand tend to conform to the fracture planes. The laminaewithin the footwall are minutely folded or massive close tothe fracture planes. Sigmoidal fractures, in contrast, arewithout any associated slide surface and reveal undoubtedsignature of small-scale thrusting against them. Tensionalfractures often form a system of tapering parallel Wssuresand veins arranged in en echelon fashion. Internally, theWssures often reveal blocks of material that slide past oneanother. Restricted within the footwall part of the slidesstylolitic joints are noticed at an orthogonal relationshipwith the tension fractures.

The generation of sigmoidal fracture is expected in a sat-urated drained system. A high rate of water escape possiblycaused the upper layer to be more cohesive compared to thelower part. The upper layer undergoes brittle fracture,whereas the lower part remains virtually intact. Pore-watersaturation possibly inhibited transmission of overridingstress to the lower layer. Terminal deformation of en eche-lon veins with lateral tapering indicates some degree ofductility in course of stretching. The occurrence of stylo-lites suggests compression at a right angle to the exten-sional veins.

Transformational fractures Wlled with anisotropic mineral growth

In some cases, slide surfaces exhibit microscale warping.The relative movement of the two blocks slide past eachother with alternate sectors of tension and compression.Extensional fractures of lenses of dominos form in rowswithin the extensional zones, aligned parallel to the surfaceof the slide. Mineral growth within the tensional fractures

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Fig. 8 Signatures of undersurface deformation: a Highly disturbed shraded bed (exposure width 4.7 m). b Set of sigmoidal fracture (arrowed).c Transformational fracture Wlled with ‘S’- shaped anisotropic mineral growth (bar = 0.125 mm)

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may be synchronous with the progressive coaxial openingin the middle of the crack with centripetal growth of Wbersfrom the wall and acquire an ‘S’-shaped growth pattern(Fig. 8c). Occasionally, lenses of host lithology boundedbetween the ramifying tensional fractures are found to bearshear fractures at an angle of 30°. Narrow zones show evi-dence of extensive recrystallization in the form of sparrycements, blocks of highly stretched crystals outlined by ahigh degree of concentration of iron oxides. Occasionallythere are layer-parallel extension fractures that show evi-dence of intense deformation.

Mass Xows: characters and generative processes

Accommodation space created by the slide in surWcial sedi-ment is Wlled either by co-genetic mass-Xow or by youngersediment. Centimeter- to decimeter-thick bed/s of calcare-nite and calcirudite represent the mass-Xow units; detachedor in lateral transition with severed, terminally tapered orstep-wise truncated beds of PLM facies. The absence ofwave structures, generation of tabular scoops on the imme-diate substrate, dominant bed-parallel orientation of tabularclasts, and shear laminae underneath, all corroborate themass-Xow origin for the carbonate clastic beds. The salientfeatures of the interbeds include (1) tabular to broad lentic-ular (over tens of meters outcrop distance) bed geometry,(2) sharp erosional and/or non-erosional base and grada-tional and/or sharp top, (3) normal grading (size, concentra-tion, or coarse-tail), and (4) Bouma Ta-e sequences (or theirvariants). On the basis of internal organization, the massXow beds are classiWed into Wve types as follows.

Type I

Decimeter-thick wackestone–packstone (micrite content>85%) conglomerate beds characterized by chaoticallyXoated light-colored tabular clasts set within a dark-coloredmatrix (Fig. 9a). Clasts are angular, show wide size distri-bution (long axis varying between 0.75 and 13.5 cm), andgenerally with negligible grading or internal layer structures.Both the maximum clast size (ca. 8 cm) and the bed thick-ness (ca. 18 cm) in this type far exceed those in other types.Beds are laterally persistent at the outcrop scale (up to afew tens of meters) and base of the beds is sharp but devoidof any erosional feature.

These massive conglomerates with wide clast size distri-bution and without any signature of grain sieving or basalerosion reXect deposition from high-density laminar Xow(Lowe 1982; Mulder and Alexander 2001). The clasts pos-sibly remained aXoat during transport owing to matrixstrength. Angularity of the clasts may suggest high viscosityof the Xow, which is also capable of transporting large-sizedclasts (Middleton and Hampton 1976).

Type II

Clast-supported beds (average thickness of 6.5 cm) withcoarse-tail grading. Light buV-colored pebble to silt-sizetabular clasts derived from PLM facies deWne size as wellas concentration grading (Fig. 9b), whereas the clastsderived from SM facies are smaller (silt size), equant inshape, and deWne uniform size distribution. The large tabu-lar clasts (max. recorded length 1.5 cm) are characteristi-cally bed-parallel. The base of the beds is erosional withfrequent presence of scoops and fractures. InWltration ofmass Xow material within the fractures as well as the tabu-lar gaps created by the scooping of the fractured blocks isnoticed. Occasionally, beds are recorded in amalgamationwithout any ordering in terms of thickness or clast-size var-iation. Often thin shear laminae are noticed between theamalgamated beds.

The presence of scoops and fractures at the base of thesebeds suggests strong basal shear. The clasts generated fromthe substrate, in consequence, are typically tabular and pri-marily bed-parallel (Mutti and Normark 1987; Mutti et al.2003; Bera et al. 2008). Notwithstanding this, the combina-tion between the grading and the bed-parallel orientation ofthe clasts is enigmatic. Particularly because, unlike type-IXow, the upper surface of this type of Xow was presumablyopen, except for the ambient water, and virtually free ofshear. Whereas the grading indicates deposition from turbu-lent suspension, the bed-parallel clasts suggest operation oflaminar condition (Enos 1977). Arguably, the clasts, whichhave made the coarse-tail grading apparent, derived fromthe substrate, concentrate locally and did not travel muchafter their entrainment. It is suggested that the observedcoarse-tail grading is not imparted by vertical settling, butprincipally because of incorporation of clasts at the bottomof the Xow. Alternatively, the grading is inherited from apreviously turbulent phase that, prior to deposition, under-went ‘body transformation’ into a laminar Xow induced byrapid decrease in gradient of depositional slope (cf. Fisher1981). Incorporation of clasts in substantial number mightalso have increased the bulk density of the Xow causingsuppression of turbulence and turning the Xow into laminar.With the suppression of turbulence and increase in bed-par-allel shear, the clasts were rotated parallel to the bedding.The tabular shape of the clasts could have facilitated theprocess. The amalgamated beds possibly represent thequick succession of turbidite emplacement in an area proxi-mal to source area.

Type III

Beds of this type have an average bed thickness of 4.5 cm,and internally constituted of a basal graded subdivision(Bouma Ta; 0.8–1.6 cm thick) and an overlying cross-stratiWed

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subdivision (Bouma Tc; average set thickness of 3.2 cm);contact between the two diVuse and irregular (Fig. 9c).Clast: matrix ratio varies widely. Whereas the clasts are oflight grey lime mudstone, tabular, and with varying size(maximum length of 0.25–3 cm), the matrix is dark coloredand micritic. In the basal part of the graded subdivision, thetabular clasts are aligned parallel to the bedding. Upward,the clasts diminish in size and lose preferred bed-parallelorientation, except near the topmost part where theclasts again increase markedly in size and are aligned

parallel to the overlying cross-stratiWcation. In rare cases,the basal-graded sub-division is overlain by a co-set ofripple-drift cross lamination with an erosional, scooped andsharp contact.

Normal grading within the basal Ta subdivision suggestsdeposition from turbulent suspension (Middleton and Hamp-ton 1976; Lowe 1982; Boggs 1995). The high concentrationof pebble-sized clasts within the subdivision, however, doesnot Wt well with this contention. It is well appreciated thatturbulence alone is not capable of transporting clasts in

Fig. 9 Variety of mass-Xow units: a Debris Xow with chaotic orienta-tion of poorly sorted clasts in Type I. b Coarse-tail grading marked bylight-colored tabular clasts within type II. c Multiple-graded subdivi-sion giving way upward to ripple drift cross-stratiWcation in type III.

d Successive graded bed with intervening shear laminae in the basalpart of type IV (coin diameter 2.5 cm). e Multiple-graded subdivisionwithin an overall graded framework within type V

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suspension larger than the medium sand grade (Pantin 1979).Despite the fact that the tabular shape of the clasts must haveimparted extra buoyancy, considering the preferred bed-par-allel orientation a fully sheared character of the Xow isinferred, preferably in a modiWed grain Xow condition (Mid-dleton and Hampton 1976; Lowe 1982; Postma 1986). Fur-ther, discernible decrease in bed-parallel orientation of clastsup the graded division suggests that the basal shear dissipatedupward within the density Xow. The cross-stratiWed (Tc) divi-sion is presumably a product of bedform migration in thelight density phase of the Xow. Marked decrease in size ofthe clasts up the foreset slopes indicates operation of strongbackXows that possibly scooped up the clasts from the under-lying graded division and tended to transport them up theforesets, rendering the contact between the two divisionsdiVused. The diVused contact between the two sub-divisions(Ta and Tc) suggests their origin from a single Xow, trans-formed into two phases (Felix and Peakall 2006; Bera et al.2008). Concentration of larger clasts at the top of Ta sub-divi-sion is interpreted as a result of drag imparted by the overrid-ing current. The drag is also evident in imbrication of clastsat the top of Ta sub-division, conforming the orientation ofcross-stratiWcations within Tc sub-division.

Type: IV

Beds of this type are centimeters thick and constituted oftwo subdivisions. Normally graded laminae set (thicknessranging between 0.8 and 2 cm) stacked in thinning-upwardmotif in an overall upwardly graded framework composethe basal subdivision (Fig. 9d), whereas ripple drift cross-laminae (set thickness ca. 1.6 cm) constitute the uppersubdivision. Within the lower subdivision, preferred bed-parallel orientation of tabular clasts is distinct, which becomesincreasingly less distinct upwards. The clast composition ischaracteristically unimodal, derived exclusively from PLMfacies. Maximum size of clast is 2.8 cm.

Multiple-graded laminae within the overall gradedframework are likely to arise from surge-type turbidity cur-rent (Helmholtz wave, Middleton 1967), each successivesurge having lower maxima and minima of velocity andrelated Xow parameters. With continuing deceleration andconcomitant deposition, the vacillating turbidity currenteventually turned diluted and resulted in the formation ofripple-drift cross lamination. Repetitive graded laminaemay also arise from (a) multiple sources, or (b) currentreXections and ‘sloshing’ (Van Andel and Komar 1969;Pickering and Hiscott 1985) arising out of reXection of anindividual turbidite on ponding. Such possibilities wereoverruled on the basis of (1) uniform clast composition andconsistent stacking tendency (thinning- and Wning-upward)for the graded laminae and (2) absence of any unequivocalevidence (discussed later) for reXected turbidite.

Type V

Like type IV; repetitive grading within an overall gradedframework is the characteristic of this type (Fig. 9e). Thedistinctions with type IV, however, are: (a) higher thickness(»5 cm) for the basal graded laminae, and (b) the succeed-ing laminae, though comparable in thickness (»2.5 cm),are patchy, elongated, and with convex-up geometryinstead of laterally persistent character of their analogues intype IV. The tabular clasts within basal-graded subdivisionsshow edgewise arrangement (cf. Shinn 1983; Tucker andWright 1990) with predominance of imbrication in theopposite direction in successive layers. The preferred clastimbrication, however, is lost upward within the uppergraded subdivisions. Most signiWcant is the occurrence ofthin, dark-colored mud lenticles between the patches ofcoarser clastics. Further, the clastic unit, as a whole, isdraped by the lime mudstone of PLM facies with irregularand diVused contact.

A ‘reXected turbidite’ origin is proposed for this typefrom the reverse imbrication of clasts in successive gradedsubdivisions. Occurrence of oppositely imbricated clastswithin the same bed indicate that the paleoXow azimuths160–180° opposed to one another in course of deposition ofthese beds (Hiscott and Pickering 1984). The homogenizedmud encapsulated between coarse clastic lenticles may,however, suggest deposition from isolated ‘solitons’ (Pan-tin and Leeder 1987; Edwards et al. 1994). An elongatedturbidite (length À height) cannot possibly reverse its Xowdirection as a single entity, but does so in form of isolatedsolitons. Energy dissipates rapidly in front and back of thetrain of solitons as well as of the individual solitons. Depo-sition from a reverse Xow takes place on arrival of a soliton.On passing of a solitons and arrival of the next, depositiontakes place from the residual obverse Xow. At the interven-ing time ponding may result as the reverse and obverse Xownullify each other and permitting the settling of homoge-nous Wnes. The overall grading and deposition of the mudlayer at the top document the gradual waning of the entireXow. The edgewise clasts in the basal-graded subdivisionscan be attributed to interaction between shear generatedby friction at the base and the shear created on the top bycombination of reverse and obverse Xows during return ofthe advanced part of the turbidity current. Bed-parallelorientation of the clasts in the basal layer can be attributedto internal shear.

Compressional features

In the study area, meter-thick compressional structures(low-angle thrusts and slump folds) are recorded in threediscrete stratigraphic intervals within the LL succession

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represented by packstone/grainstones made either of intra-clasts or deformed ooids. Often the surfaces of the compac-tion zones resemble large ripple-like structures (Fig. 10a)with amplitude and wavelength of 6 and 55 cm, respec-tively. In vertical sections, the ripple-like forms reveal lowto moderate angle (<10°) bed-conWned thrusts (Fig. 10b)deWned by overriding of gentler Xank laminae on those ofthe steeper Xanks. Interestingly, the strikes of the thrustplanes or the crests of the ripple-like forms are arranged intwo distinct sets at high angle to each other. Folds (averageamplitude 0.25 m), wherever present, have their axialplanes that dip towards the SW. Often the folds are exten-sively truncated at the upper contact of the compactionzones with the overlying extensional zones.

The conWnement of compressional features within sedi-ments consisting of intraclasts and ooids is particularlyinteresting, as it is hard to attribute these clasts to erosion

and long-distance transportation in absence of any evidencefor emersion. Possibly at the time when the compressionalevent occurred, the depositional substrate was to someextent coherent, and in the Wrst stage of lithiWcation. Thecompression and resulting folds and thrusts created reliefs inthe substrate and prompted the substrate to shed clasts underthe inXuence of shallow-water agents. The truncation of thefolds against the overlying zone of extension implies that theevents of compression alternated with those of extension.

Discussion

Basin tectonics and role of in-plane horizontal stress

The small-scale deformation features distributed at diVerentstratigraphic levels within the LL succession clearly suggests

Fig. 10 Compressional features: a Low-angle bed-conWned thrust (hammer length 27 cm). b Mesoscopic outcrop scale fold (stick length 1.2 m)

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tectonic intervention in course of LL deposition. The slides,folds, and thrusts presented in this paper provide clues forstress-induced extension and compression in the riftedVindhyan basin during its post-rift evolution stage throughthe LL depositional history. This is, however, in contradic-tion to the traditional belief that the tectonic control on sed-imentation essentially abound the lower part of Vindhyansedimentation (Semri Formation); the upper Vindhyanevolved in an essentially tectonic quiescent sag mode(Chanda and Bhattacharya 1982; Chakraborty 2006). Con-sidering the initiation of the Vindhyan basin on a riftedbasement the evolution of the basin seems to be morecomplicated than the simple sag motif. Bose et al. (2001)surmised the possible role of intermittent rift-blockadjustments even in the course of post-rift sag stage evolu-tion of Vindhyan basin. It may be pertinent to mention herethat the 750–700 Ma time frame assigned for the LLMember coincides well with the time period assigned forthe dispersion of Neoproterozoic supercontinent ‘Rodinia’on a global scale (Rogers and Santosh 2002; Li et al. 2008;Reddy and Evans 2009). In an Indian perspective, the750–770-Ma-old Malani felsic suites bear the signature ofextensive continental magmatism associated with the‘Supercontinent’ fragmentation and dispersion (Panditet al. 2001). It is likely that such plate-margin events mightalso have triggered in-plane horizontal stresses. Subsidenceand uplift, though at much slower rates than that at plateboundaries, with associated relative sea-level Xuctuationsunder the inXuence of intraplate stresses, has already beeninvoked for intra-continental basins (Van Balen et al.1998a; Tang and Lerche 1992; Reemst et al. 1994; Park andJaroszewski 1994). For an intracratonic/epicratonic riftedsedimentary basin, as that of the Vindhyan, such changinghorizontal intraplate stress, if generated, would cause Xex-ural subsidence and uplift, as long as the stress level islower than the limit deWned by strength of the lithosphereand its weakness zones (cf. Cloeting et al. 1989; Van Balenet al. 1998b). The nature of such Xexural response is guidedprincipally by the faulted shape of the upper crustal competentlayer, in particular, the large-scale shape of the downward-bent faulted upper crustal Xexural mid-plane. The commonresponse is the basin center subsidence and Xank uplift.

Although diVerential subsidence (Chakraborty 1996;Bose et al. 2001) and signatures of relative sea-levelchanges through the history of upper Vindhyan sedimenta-tion, in general, and the LL succession, in particular, hasbeen amply documented in literature (Bose et al. 2001;Sarkar et al. 2002; Chakraborty 2004, and many others),attempts were rarely made to underpin the causative forc-ings. The present study invokes moment generation at thefault planes of the vertically oVset Vindhyan basement bythe superposition of horizontal loading oVered by in-planestresses. The moment is generated by the concentration of

in-plane stress as the stress is transmitted through the over-lap zone of the two parts of the plate, which are separatedby the fault. Though basin center subsidence and Xankuplift are the general characters for such in-plane forcechange in an early rifted basin, reversing of the sign in thein-plane stress reverses the situation, and can cause Xexuraluplift at the center of the basin. For an elastic stress-induced diVerential subsidence model, a decrease in ten-sional stress has the same eVect as an increase in compres-sional stress (Richardson 1992). The present study intendsto relate the observed extensional (slides) and compres-sional (folds and thrusts) features within the LL successionwith the reversing in-plane stress superimposed on riftedbasement.

NE–SW orientation of the slide planes (Fig. 5) measuredfrom the LL succession commensurate well with the NE–SW extension proposed by Bose et al. (1997) from thebasal Vindhyan succession to account for the developmentof multiple NW–SE-elongated sub-basins at the initialstage of basin development. Signatures of NE–SW exten-sion are also cited from the slump folds and slide planesobserved within the Lower Bhander Sandstone and SirbuShale (Bose et al. 2001; Sarkar et al. 2001), both strati-graphically above the LL. The most plausible explanationfor such pervasive NE–SW extension through the entireVindhyan succession is the readjustment of underlying riftblocks under in-plane stress and consequent Xexural subsi-dence/uplift.

Sediment failure, mass-Xows, and their evolution

Movement of semi-solid/soft sediments such as creep orslide takes place if the shear stress exceeds the shearstrength of the sediment at some depth below the sedimen-tary surface. The shear stress increases both with the slopeangle and depth below the sea Xoor. Deep below the surfaceof a gentle slope, the shear stress can be as high as on asteep slope at shallow depth. For this reason, there is a ten-dency for thick mass movements to develop on gentleslopes, whereas thin ones are characteristic of steep slopes.However, the mass Xow units within the PLM facies ofLakheri succession do not follow such a straightforwardrelationship. Whereas very low dipping beds of PLM facieswith well-preserved millimeter-scale horizontal lamina-tions support a very gently sloping character of Lakheribasin Xoor, the centimeter to decimeter-thick mass Xowunits undoubtedly suggest their near-surface origin. Suchmismatch between thicknesses of mass-Xow units and basinXoor gradient can possibly be explained by either anoma-lous increase in shear stress or decrease in shear strength.In a low-gradient system, earthquake-induced suddenincrease in pore-water pressure of unconsolidated sedi-ments is considered as a major trigger for sediment failure

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(Allen 1986; Pederneiras 1991; Vittori et al. 1991; Audemardand De Santis 1991; Hibsch et al. 1997) in marine or lacus-trine subaquatic settings. The present work intends to relatethe observed slide planes and slide-generated sedimentgravity Xows with seismic seiches in course of Lakheri his-tory triggered by Xexural warping of the basin along thedeep-seated fault planes under in-plane stress.

Sediment gravity Xows on the carbonate platforms maybe initiated by several mechanisms, e.g., erosion, block-fall, and failure of unstable carbonate build-ups. For exam-ple, carbonate debris Xow and other gravity-Xow depositscontaining clasts of shallow water and marginal lithofaciesmay be found basinward of build-up margins and arethought to be the result of platform-edge failures. Hostedwithin the PLM facies of distal oVshore origin, the sedi-ment gravity Xows, described in the present study, contain-ing clasts derived dominantly from the host lithology and inlateral association with slide planes, suggest a genetic linkbetween sliding events and generation of sediment gravityXows through break-up of slide sheets at their leadingedges. Previous works on the relationship between slidesand gravity-Xow processes in sedimentary environmentsalso suggested a genetic link between slides, debris Xows,and turbidites (Crevello and Schlager 1980; Gawthorpe andClemmey 1985; Martins-Neto 1996). The clast-supportedcharacter for the gravity-Xow deposits with presence ofboth angular and rounded clasts indicate derivation of slidesheets from sources comprising both lithiWed and unlith-iWed sediments.

The cohesive, modiWed cohesive, surging, and dilute tur-bulent character for the gravity Xow units may be visual-ized as products of either discrete Xows or as a depositionalcontinuum evolving out of high-density failures. Incorpora-tion of tabular carbonate intraclasts in course of the Xowoften changed the Xow rheology (turbulent to laminar; typeII) and forced freezing of the Xow. Bipartite beds with abasal high-concentration layer and turbulent rider (type III)are interpreted as products of transformed cohesive Xows.A Xow separation resulted in a viscous, non-turbulent, iner-tia-Xow layer below and a turbulent layer above, the inter-face between the two being a physical discontinuity (Felixand Peakall 2006). At the interface, the clasts were elutri-ated (ripped up) by strong turbulence of the upper, normalgraded layer (cf. Bera et al. 2008). At times when the Xowscould maintain turbulent character, two diVerent types ofXow velocity deceleration are interpreted, viz. the continu-ous (type III) and the oscillating (type IV). In type IV Xowdeposits, instead of showing a continuous decrease invelocity, competence, and capacity, an oscillating declineinvolving repetitive abrupt increase in velocity and follow-up gradual deceleration is interpreted as a signature forsurge-type character. Reverse clast imbrication in succes-sive graded subdivisions within type VI gravity Xow units

indicates reXection of primary Xow and generation ofreverse Xow condition, possibly triggered by relatively gen-tle, adverse bottom slope conditions (Pantin and Leeder1987; Chakraborty 1996). In terms of relative motion, Xowsgenerated out of submarine slides may resemble Xows overbumpy topography (Thorpe 1975), and could likewise gen-erate ‘soliton’s under reverse Xow conditions. The recogni-tion of slide surfaces, and tracking their genetic connectionwith the gravity Xow units, allowed establishment of theirallogenic (seismogenic) origin triggered by Xexural adjust-ment of the basin under in-plane stress development. Suchsediment gravity Xows are in no way related to any geo-morphic or biohermal slope generation on a carbonate plat-form and characterization as well as categorization of anyancient carbonate platform on the basis of presence of suchgravity Xow deposits often may turn Xawed. The presentwork warrants reconsideration of sediment gravity-Xowdeposits as the hallmark signature for categorization of anycarbonate platform and process-based study on carbonatesbefore using them.

Conclusions

Despite being the product of very low-gradient distaloVshore setting, the plane laminated lime-mudstones ofLakheri Limestone records a wide variety of syn-sedimen-tary disturbance features bearing signatures of both exten-sion and compression. The signatures of extensional failureare in the form of shallow intraformational truncation sur-faces, translated and rotated beds, and creep sediment witha wide range of syn-depositional deformation features.Expressions of deformation, however, vary both with thebehavior (drained, undrained, etc.) and relative position(overlying or underlying) of sediment column with respectto the detachment surface. The compression signatures arein the form of bed-conWned thrusts and folds with decime-ter-scale amplitude. In absence of any possibility of slopefailure, and taking into consideration the early rift-relatedorigin for the Vindhyan basin, the features are related todiVerential Xexural motion triggered by post-rift in-planehorizontal stresses acting on predeformed lithosphere. TheNE–SW orientation of the slide planes is consistent withthe multiple NW–SE-elongated sub-basinal conWgurationsproposed for the early Vindhyan development. The consis-tent orientation of the slide planes all over the PLM faciesof LL succession indicate that they do not owe their originto local factors, but are manifestations of regional (basin-scale) tectonic eVects. The detachments, however, createdonly small-scale vertical accommodation and thin massXow beds. Such low-order diVerential vertical motion ispossibly indicative of a deeper level of necking during theearly syn-rift phase of Vindhyan history.

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The failed sediments often evolved in the form of centi-meter to decimeter-thick sediment gravity Xows withwidely varying rheology. The cohesive, modiWed cohesive,and dilute turbulent character for these mass-Xow units maybe visualized as products of either discrete Xows or as adepositional continuum evolving out of high-density Xows.Occasional reverse grading at the basal part of the somemass Xow beds and presence of shear laminae in the imme-diate substrate indicate that the basal shear was not alwaysnegligible, but was soon overcome presumably for rapidconsolidation of the Xow on its dilation and consequentextraction of porewater and Xuidization. Opposite clastimbrications within the same turbidite bed are suggestive ofreXection of currents along their Xow path, possibly causedby internal basin highs. In an otherwise low-gradient, low-sedimentation distal oVshore LL setting, generation of suchirregular depositional substrate conWguration was possi-bly triggered by syn-sedimentary slides with curvilineargeometry.

The establishment of a genetic link between the slidingevents, bedding translations, and generation of mass Xowunits during the LL depositional history warrants precau-tion in calling a carbonate platform a distally steepenedramp or rimmed shelf on the basis of presence of massXow units. The present study highlights the importance ofindependent physiographic slope identiWcation beyondthe mere reporting of mass Xow occurrence in course ofreconstruction of platform geometry in ancient carbonatesequences.

Acknowledgments The author is thankful to the Council of ScientiWcand Industrial Research (CSIR) and the Department of Science andTechnology (DST), government of India, for providing necessary fund-ing. The Department of Geology at the University of Delhi providedthe necessary infrastructural facilities. The author is indebted to PradipK. Bose and Subir Sarkar for their continuous encouragement andvaluable scientiWc suggestions in course of this work. Thanks are alsodue to Md. Atif, a research scholar, for his help in drafting the Wgures.

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