Nivedita Chakraorty,Suir Sarkar

aDepartment of Geology,Kabi Jagadram Roy Govt.General Degree College,Bankura-722143,India

bDepartment of Geological Sciences,Jadavpur University,Kolkata-700032,India

Abstract The Cretaceous(Albian—Cenomanian)Dalmiapuram Formation is one of the economically significant constituents in the hydrocarbon-producing Cauvery rift basin,SE India that opened up during the Late Jurassic—Early Cretaceous Gondwanaland fragmentation.The fossil-rich Dalmiapuram Formation,exposed at Ariyalur within the Pondicherry sub-basin of Cauvery Basin,rests in most places directly on the Archean basement and locally on the Lower Cretaceous(Barremian—Aptian)Basal Siliciclastic Formation.In the Dalmiapuram Formation,a facies association of tectonically-disturbed phase is sandwiched between two drastically quieter phases.The early syn-rift facies association(FA 1),records the first carbonate marine transgression within the basin,comprising a bar—lagoon system with occasionally storms affecting along the shore and a sheet-like non-recurrent biomicritic limestone bed on the shallow shelf that laterally grades into pyrite—glauconite-bearing dark-colored shale in the deeper shelf.Spectacular breccias together with varied kinds of mass- flow products comprise the syn-rift facies association(FA 2).While the breccias occur at the basin margin area,the latter extend in the deeper inland sea.Clast composition of the coarse clastics includes large,even block-sized limestone fragments and small fragments of granite and sandstone from the basement.Marl beds of quieter intervals between tectonic pulses occur in alternation with them.Faulted basal contact of the formation,and small grabens filled by multiple mass- flow packages bear the clear signature of the syntectonic activity localized contortions,slump folds,and pillow beds associated with mega slump/slide planes and joints,which corroborates this contention further.This phase of tectonic intervention is followed by another relatively quieter phase and accommodates the late syn-rift facies association(FA 3).A tidal bar—interbar shelf depositional system allowed a transgressive systems tract motif to grow eventually passing upwards into the Karai Shale Formation,whose contact with the Dalmiapuram Formation is gradational.

Keywords Syn-sedimentary tectonics,Facies analysis,Carbonate shelf,Cretaceous,Dalmiapuram Formation,Cauvery Basin

1. Introduction

Carbonate rocks deposited along rifted continental margins display a wide variety of sedimentary facies and geometrical relationships(Basilone et al.,2014).Tectonics has often been overlooked during carbonate sedimentation because of its preferred association with passive margin settings.Hence,detailed studies relating facies and tectonics are rare,except for a few(e.g.,Basilone,2009;Basilone and Sulli,2016;Bosence et al.,1998;Burchette,1988;Gawthorpe and Leeder,2000;Grotzinger,1986; Santantonio, 1993). The hydrocarbonproducing Cretaceous(Albian—Cenomanian)Dalmiapuram Formation,exposed in and around Ariyalur,within the Pondicherry sub-basin of the Cauvery Basin,SE India(Fig.1),formed during a critical juncture of the Earth's history,but has yet not been studied in this light.A sedimentologic account of the Dalmiapuram Formation is important because of:(1)its deposition during the first marine transgression along the east coast passive margin of the Indian subcontinent,in relation to the Late Jurassic—Early Cretaceous Gondwanaland fragmentation;(2)it being entirely a carbonate succession comprising a syn-rift facies association sandwiched between early syn-rift and late syn-rift facies associations,on a Cretaceous shallow shelf;and(3)preservation of tectonically-induced and/or re-deposited sediments at multiple phases within the disturbed interval.

The contact between the basement(Archean basement/Barremian—Aptian non-marine siliciclastics)and the marine carbonate Dalmiapuram Formation is invariably faulted everywhere within the basin;however,carbonate sedimentation was initiated prior to this faulting.The presence of carbonate clasts along the basin margin,many as huge blocks(～1.5 m),attests to this claim.The present study recognizes a facies package bearing an early syn-tectonic record of carbonate deposition during a rapid transgression.Tectonic activity,following that,triggered conspicuous carbonate rock-fall/mass- flow deposits in multiple phases.Carbonate deposition,though,continued during the syn-rift phase and even after the tectonics dissipated.

Fig.1 A—Map of South India showing the locations of Cauvery Basin(purple oval),and the study area(blue rectangle)in the Pondicherry sub-basin;B—Location and lithological map of the study area(the studied region of Dalmiapuram Formation is marked by solid rectangles);C—Tectonic background of the Pondicherry sub-basin.

This paper attempts a detailed sedimentary facies analysis in a rift setting,assessing the distribution of facies in space and time and its tectonic control over carbonate sedimentation to frame the tectonosedimentary evolution of the Cretaceous Dalmiapuram Formation of the Cauvery Basin.Development of a palaeogeographic model,long awaited for this otherwise well-known fossiliferous Formation,provides a meaningful analogue for similar less well studied carbonate shelves at the passive continental margins of India's east and west coasts,which are commonly targeted for hydrocarbon exploration.

1.1. Geological background

The Late Jurassic—earliest Cretaceous time witnessed rifting between India(with Madagascar)and Australia/Antarctica with the opening up of the Indian Ocean during the breakup of East Gondwanaland(Powell et al.,1988;Rangaraju et al.,1993).The Cauvery Basin is the southernmost representative,among a number of NE—SW trending basins that formed as a consequence of rifting,along the eastern passive margin of India(Fig.1A;Nagendra et al.,2011;Sundaram and Rao,1979;Watkinson et al.,2007).The basin is located in the southeastern part of Peninsular India between latitudes 8°30′N and 12°30′N and longitudes 78°30′E and 80°30′E(Fig.1C;Narasimha Chari et al.,1995).It is a block-faulted pericratonic basin comprising horst—graben basin architecture(Nagendra and Nallapa Reddy,2017).The basin incorporates several depressions/sub-basins separated from each other by subsurface(basement)ridges,with the Pondicherry sub-basin (Ariyalur—Pondicherry Depression)in the north(Chakraborty et al.,2017;Sastri et al.,1979)(Fig.1C).The present study concentrates on the Dalmiapuram Formation,exposed in the southern part of the Pondicherry sub-basin in and around Ariyalur(Fig.1B)and this formation is the time-equivalent of the hydrocarbon-producing Andimadam,Sattapadi and Bhuvanagiri Formations in the subsurface(Fig.2;Govindan et al.,2000).

1.2. Stratigraphy

Fig.2 Stratigraphic sub-division of the Lower—Upper Cretaceous Uttatur Group in the study area in outcrop and sub-surface.GS=Garudamangalam Sandstone;BS=Basal Siliciclastic Formation.

The Lower—Upper Cretaceous Uttatur Group,within the Cauvery Basin,consists of the Dalmiapuram Formation(ca.300 m thick),the Karai Shale Formation(ca.410 m thick),and the Garudamangalam Sandstone Formation(ca.264 m thick),from base to top,with gradational transitions in between bounded by an unconformity at its top(Fig.2;Banerjee et al.,2016;Ramkumaret al.,2003;Sarkaret al.,2014;Sundaram et al.,2001;Tewari et al.,1996a).The Dalmiapuram Formation is an entirely limestone formation directly overlying the crystalline basement in most places.Pre-rift basement is formed of the Archean(～2.5 Ga)rocks typical of this part of southern India(Ghosh et al.,2004;Peucat et al.,2013).The limestone formation locally rests on the Basal Siliciclastic Formation(Gondwana sediments)of the Lower Cretaceous(Chakraborty et al.,2017)(Figs.2 and 3).The basal fluvial deposits are almost invariably faulted against the Archean basement during the initial phase of rifting and possibly represents lowstand wedge products.A marine transgression event in the earliest Albian deposited limestones of the Dalmiapuram Formation.Carbonate sedimentation commenced on a shallow shelf environment,punctuated thereafter with rift-related tectonics(Fig.4).Basin restriction and ongoing relative deepening within the basin led to the gradual establishment of deep-water open-marine conditions and deposition of Karai Shale Formation.The upward gradational transition from the Dalmiapuram Formation to the overlying Karai Shale Formation is a product of a transgressive systems tract(Figs.3 and 4;Nagendra et al.,2011;Watkinson et al.,2007).The marine origin of the fossil-rich Dalmiapuram Formation is obvious from the records of the macro-and micro-fauna(Banerji et al.,1996;Hart et al.,2001;Mishra et al.,2004;Tewari et al.,1996b).The carbon and oxygen isotope data(±2)overall corroborate marine depositional conditions(Table 1).However a few samples(from the bar systems and was hover deposits)are more depleted in δ18Oboth(“＊” marked in Table1),may be diagenetically affected(Fig.5).

Fig.3 Dalmiapuram Formation onlapping the Archean basement(A)and the Basal Siliciclastic Formation(B),and gradationally overlain by Karai Shale Formation(C).Human=1.74 m(A),1.70 m(B);Car height=1.88 m(C).

2. Methods

The present study involves sedimentary facies analysis,based on field geology supplemented by petrographic observations,in addition to carbon(inorganic)and oxygen isotope analysis of twenty-four samples.Powdered samples of carbonate were prepared for carbon-isotope analysis to react with～100%phosphoric acid at 80°C using GEO 20-20 stable isotope ratio mass spectrometer(CF-IRMS)at the National Stable Isotope Facility of IIT,Kharagpur,India.All the values are reported in per mil(‰)relative to the VPDB standard assigning by standard NBS19 carbonate utilizing BDH (University College of London)and Z-Carrara(PRL/Cambridge University)δ13C(2.1‰)and a δ13C value of-5.0‰ to NBS18,Calcite(IAEA),respectively,in those laboratories.Experimental precision of±0.1‰ was maintained.

Fig.4 Representative section of Uttatur Group showing systems tract(modified after Sarkar et al.,2014;upper left part).Vertical log showing facies and their associations within the Dalmiapuram Formation.Note the content of fossils,sedimentary structures and lithology within the respective facies.UC=Unconformity.

3. Facies analysis

A systematic and detailed sedimentary facies analysis was performed for a better understanding of the palaeoenvironmental settings and syn-tectonic control over sedimentation.The main compositional and textual characteristics and sedimentary features of distinguished facies with palaeogeographic interpretation are summarized in Table 2 and displayed in Figs.6—11.The facies are grouped into three facies associations,in relation to rift-related tectonics.

Table 1 The carbon and oxygen isotope data measured in the Dalmiapuram Formation,Cauvery Basin,SE India.Note,the “＊”marked oxygen isotope values may indicate the samples were diagenetically affected.

3.1. Facies association 1(FA 1)

FA 1 consists of four distinguished facies(Table 2)that are organized in a shallowing-upward trend.

Fig.5 δ13C vs. δ18O distribution within Dalmiapuram Formation,Cauvery Basin,SE India.

Facies 1A is composed of large-scale(ca.20 cm high),locally chevron cross-stratified,convex-up-topped and flat-based lenticular bio-intrasparite beds,with large shell fragments,alternating with sheet-like smallscale cross-stratified to rippled beds of biosparite,intrasparite and biomicrite with a thickening-upward trend(Figs.6A,7A—B).This facies is interpreted as a shallow marine wave-affected,at least locally,bar—inter bar system(Mandalet al.,2016;Sarkar et al.,2014).Facies 1B is composed of fine-grained,pinkcolored,massive biomicrite alternating with coarse grained yellowish massive orgraded biomicrite(Fig.6B).The first lithotype comprising an abundance of fenestrae with “bird's eye” structures(Fig.7C),local presence of algal fragments and poor skeletal fragments with micritic rims.It is interpreted as sediments of a lagoon-like waterbody,whereas the lithotype of facies 1B contains numerous broken shell fragments indicating occasional storm was hover events(Fig.7D)(Sarkar et al.,2014).Presence of bar—interbar system(Facies 1A)in immediate association also supports the view and no cyclicity has been observed within the sediment of Facies 1B.The non-recurrent,thin,sheet-like,massive biomicritic limestone facies(Facies 1C;Figs.6C and 7E)occurs draping the immediately underlying Archean basement/Basal Siliciclastic Formation with a very sharp but planar basal contact,displaying small shells and abundant silt-sized siliciclastic particles “ floating”within a micritic matrix.This facies thus indicates a single basin-wide marine transgressive event during the Albian(Nagendra et al.,2011;Watkinson et al.,2007)deposited on the marine shelf below storm wave-base.The biomicritic limestone laterally grades basinwards into non-recurrent,dark-colored shale interspersed with paper-thin planar stringers of calcarenite(Facies 1D)containing abundant glauconite and pyrite(Figs.6D and 7F).This facies represents basinal fines in a lowenergy restricted shelf under reducing conditions,possibly where the depositional basin achieved its maximum depth below storm wave-base(Schieber et al.,2007;Seidler and Steel,2001).

3.2. Facies association 2(FA 2)

FA 2 comprises five characteristic facies(Table 2,Figs.8 and 9).Facies 2A,immediately overlying the basement with extremely irregular and stepped boundary,is distinctly wedge-shaped unsorted boulder megabreccias with a polymodal angular clast population with variable composition(including Archean granite gneiss,amphibolite basement,basal siliciclastics and carbonates)(Fig.8A).The facies is interpreted as a product of rock-fall at the base of steep fault scarp(Bose et al.,2008;Shanmugam,2015).Facies 2B,occurring in association with the preceding facies,as well as in contact with the basement,is characterized by wedge-shaped,poorly-sorted,clastsupported,pebbly breccias(Fig.8B).Clast composition and characteristics are similar to the preceding facies.This facies is interpreted as being deposited as slope aprons spreading over the earlier facies,as well as on the fault scarps(Bose et al.,2008;Burchette,1988).Facies 2C is made up of matrix-supported,ungraded,massive limestone conglomerate,having lenticular geometry,with flat base and convex-up top,with clast composition similar to the preceding facies,and displaying basement clasts “ floating” within a micritic groundmass.It has been interpreted as a product of debris flow(Fig.8C)(Bose and Sarkar,1991;Chakraborty et al.,2017).Facies 2D is composed of locally graded sheet-like calcarenite beds attaining variable thickness(20 cm—1.9 m),very sharp bases bearing sole marks( flute casts,gutters in places and rarely groove casts)and relatively less sharp tops(Fig.8D).The mega beds(thickness＞70 cm)possess a partly developed Bouma sequence(graded/massive bedding followed by ripples).Under the microscope,this facies appears as intra-biomicrite(Fig.10A)and has been interpreted as carbonate turbidites.Facies 2E,characterized by thin sheet-like laterally persistent,homogeneous or slightly planar laminated marls in between the beds of Facies 2C and 2D with a sharper top than their bases,has been interpreted as shelf indigenous mud deposited during interludes between mass- flow events(Figs.8E and 10B).

Table 2 Summary of facies associations,facies description and palaeogeographic interpretation of each facies.

Fig.6 Facies association(FA)1.A—Large-scale cross-stratified bio-intrasparite alternating with sheet-like small-scale cross-stratified bed sets of biosparite,intrasparite and biomicrite within Facies 1A.Human height=1.74 m;B—Heterolithic limestone comprising pink-colored massive biomicrite alternating with significantly coarse-grained yellowish massive or graded biomicrite(dashed lines)(Facies 1B).Length of the Swiss army knife=8.5 cm;C—Non-recurrent massive biometric limestone having laterally uniform thickness overlying basal siliciclastics(yellow arrows)(Facies 1C);D—Dark-colored shale interspersed with paper-thin planar stringers of calcarenite(Facies 1D).

3.3. Facies association 3(FA 3)

FA 3 comprises two facies(Table 2).Facies 3A is characterized by medium-scale(6—7 cm high),cross-stratified calcarenite with flat base and convexup top,comprising large broken shell fragments and micritic rims around some shells(Fig.10C and D).The pattern of cross-stratification within the beds changes laterally,from steep to gentle,showing high-angle directional variation in successive beds and,locally,herringbone cross-strata are also present,exhibiting a bipolar-bimodal palaeocurrent pattern(NNW—SSE)(Fig.11A).The gentle cross-strata are often draped by mud(Fig.11A).Deposition of this facies is interpreted as occurring on a shallow tidal shelf.Facies 3B is composed of comparatively thin sheet-like beds of small-scale cross-stratified calcarenite and possibly deposited in a comparatively deep tidal shelf(Fig.11B).This facies association,overlying the preceding syn-rift facies association(FA 2)with a slightly undulatory contact,has been interpreted as a tidal bar—interbar complex(Bose et al.,1997;Sarkar et al.,2014).

4. Areal distribution of Dalmiapuram Formation

Fig.7 Photomicrographs from different facies of the early syn-rift facies association(FA 1).Q=Quartz;F=Feldspar;G=Glauconite.A—Bryozoan-coating brachiopod shell in micrite in Facies 1A;B—Syntaxial overgrowth around echinoid shell in Facies 1A;C—Fenestrae within micritic limestone in Facies 1B;D—Large algal clast with broken shells of echinoid and bivalve in Facies 1B;E—Biomicrite with siliciclastic grains in Facies 1C;F—Glauconite within the siliciclastic shale in Facies 1D.

Fig.8 Facies association 2(FA 2).Hammer length(circled)=34 cm.A—Facies 2A comprising unsorted boulder megabreccias with haphazardly oriented clasts;B—Facies 2B is composed of pebbly conglomerate overlying the basement rocks(marked by the letters“B” as well as yellow arrows).Note the difference in clast size and better roundness with respect to the preceding Facies 2A;C—Matrix-supported large pebbly limestone conglomerate with chaotically arranged clasts within Facies 2C;D—Graded matrix-supported small pebble limestone conglomerate or calcarenite with sharp bases and relatively less sharp tops comprising Facies 2D;E—Sheet-like marl beds(white arrows)alternating with the mass- flow units of Facies 2C and 2D.

The Cretaceous succession of Dalmiapuram Formation is subdivided into three facies associations representing episodes when distinctly different tectonic regimes were in operation.Early syn-rift association(FA 1)comprises all the indigenous facies,except the tidal association occurring at the top of the formation(Fig.4).A panel with fourteen litho-sections extending over ca.55 km along the southern sector of the study area in KVK mines,Dalmiapuram,the southwestern sector at Melarasur,and the northeastern sector in abundant quarry sections at Veppur,has been constructed to record the spatial distribution of facies within the study area and to correlate with the sea-level(Fig.12).The panel shows that there is variation in facies distribution pattern within the carbonate platform.Dominance of bar—interbar complexes of FA 1 and FA 3,wave-and tide-dominated respectively,at Veppur and Dalmiapuram sections is evident,while it is restricted in occurrence at Melarasur.In addition,the overall presence of syn-rift assemblage comprising breccias/mass- flow facies(FA 2)is dominant at Dalmiapuram and considerable at Veppur.The early syn-rift facies association(FA 1)is limited in thickness,with all of its members present only at the Dalmiapuram sections.The Archean basement is characterized by structural highs and lows,evidenced by strong tectonic activity affecting the basin(Nagendra and Nallapa Reddy,2017).The panel diagram reveals the irregular topography of the basement as well as variation in accommodation space,enhancing variability in distribution of carbonate facies(Fig.12).The Basal Siliciclastic Formation occurs only in pockets,mostly at Dalmiapuram and rarely at Melarasur area.

5. Syn-sedimentary tectonics

The deposition of shallow-marine carbonate rocks started on a rifted continental margin,affected since the Late Jurassic/Early Cretaceous(Scotese,1997;Chand et al.,2001)by extensional tectonics and followed by a progressive rift that seems to have continued until the end of the Turonian(Watkinson et al.,2007).In this carbonate platform,the tectonic event manifested as a taphrogenic rifting and associated block movement along the dominant NE—SW trend,resulting in morphotectonic humps and steep slopes(Nagendra and Nallapa Reddy,2017).The effects of syn-tectonic activities are recorded in terms of the following phenomena:

Fig.9 A—Schematic diagram illustrating the relationship between flow mechanism,deposition and interpretation of the constituents of synrift facies association(FA 2)modified after Porten et al.,2016;B—Cartoon showing distribution of breccias and mass- flow units within the synrift facies association.Note occurrence of boulder-rich breccias and pebbly conglomerate at basin-margin and turbidites in the basin interior with debris flow forming in between these two(Schematic,not to scale)(adopted from Chakraborty,2016).

Fig.10 Photomicrographs from different facies of syn-rift(FA 2)and late syn-rift(FA 3)associations.A—Bored intraclast along with bioclasts in intra-biomicrite within Facies 2D;B—Dissolved shells with mouldic spar and micritic rim around them within biomicrite in Facies 2E;C—Various fossils and intraclast in biomicrite within Facies 3A;D—Drusy spar within dissolved shell showing micritic rim around them in Facies 3B.

Fig.11 Facies association 3(FA 3).A—Medium-scale cross-stratified calcarenite within Facies 3A.Note bipolar orientation of cross-strata.Also note mud drape along the foresets(yellow arrows);B—Small-scale cross-stratified calcarenite beds within Facies 3B.

Fig.12 Panel diagram showing litho-sections constructed at the studied locations of Dalmiapuram,Melarasur and Veppur within Dalmiapuram Formation.

5.1. Faulted basal contact

The marine sequence,within the basin,starts with the Cretaceous(Albian—Cenomanian)Dalmiapuram Formation directly overlying the Archean crystalline basement mostly,or Barremian—Aptian non-marine siliciclastics (Gondwana sediments)locally(Chakraborty et al.,2017).The present study found that the basal contact of the Dalmiapuram Formation is characteristically irregular and/or stepped in nature(Fig.3A and B).The contact is obviously faulted everywhere against the Archean basement as well as the Basal Siliciclastic Formation(Barremian—Aptian)and often spotted with small grabens(Figs.3,9B and 12).The Archean crystalline basement subsided and the Karai Shale Formation,stratigraphically younger than the Dalmiapuram Formation,rests directly on the basement at places.Possibly it happened at the time of the maximum flooding during the deposition of KaraiShale Formation (Chakrabortyet al.,2018;Nagendra et al.,2011;Watkinson et al.,2007).The continual faulting triggered the ongoing syn-rift phase and created accommodation space for deposition of ca.300-m-thick(Sundaram et al.,2001)carbonate succession.

5.2. Multiphase tectonics

Watkinson et al.(2007)stated that deposition of the Dalmiapuram Formation took place during a synrift phase.However,the present study reveals that the entire formation did not experience the effect of rifting in a similar fashion but only at certain intervals(Fig.13).A tectonically-disturbed assemblage(FA 2)is sandwiched between two undeformed packages(FA 1 and FA 3).The rifting was reactivated at least twice,within the disturbed interval(FA 2)alternating with FA 1(Fig.13).The boundaries are very sharp and irregular everywhere(Fig.13A,B,C),however there is a broadly lenticular geometry,pinching out within the undisturbed host sediments at places(Fig.13D).Seismicity is the probable reason for the tectonic trigger at multiple stages.

5.3. Instability structures

The instability structures,preserved within the syn-rift interval,are restricted to a number of beds or even within a single bed.These structures present strong evidence of syn-sedimentary tectonics during carbonate sedimentation.

a) Fault megabreccias/gravity-slide breccias.These consist of unsorted megabreccias(Facies 2A)comprising boulder-sized clasts of granite/amphibolite,basal siliciclastics or carbonates directly overlying the basement(Fig.8A).The limestones of Dalmiapuram Formation were dissected by normal faults and fractures during the time of its deposition implying early lithification of the carbonate sediment(Watkinson et al.,2007).As a result,spectacular breccias were formed along the steep fault scraps and deposited at their bases as rock-fall units at the basin margin part,which may be related to a buttress unconformity(Davis and Reynolds,1996;Basilone,2009)(Fig.9).The clasts were obviously derived from the shedding off the platform margin and tumbled down along the steep slopes of fault plane in response to gravitationalaction, thereafter re-deposited as chaotic breccias bodies.

b)Graben fillings .These comprise several mass flow units filling small-scale grabens exposed along the upper part of the southwestern and northeastern wall of Veppur mines,illustrating syn-tectonic sedimentation.(Fig.14A—D).Within the small grabens,massive or internally laminated lagoonal,indigenous sediments are locally interbedded with successive mass- flow packages dominated by clast-to matrixsupported debris- flow products and/or breccias.Such mass- flow units stack one upon another making a number of coarsening-upward successions(Fig.14E)overlain by the stratigraphically younger Karai Shale Formation.In the basement slope direction,the limestone thickens rapidly,while in the upslope part,the basement is directly followed by the shale.The top surface of the Dalmiapuram Formation dips much less(max.7°)with respect to the wall of the grabens(max.50°).The wall of the half-grabens exhibits variation in slope,as manifested by the dip amounts(12°—50°)recorded at some measured sections(Fig.14A—D).

c)Contortions and slump folds.A few beds of indigenous facies of 30 cm average thickness display contortion of internal laminae as well as bedding planes(Figs.14F and 15A).Unfortunately,poor exposures inhibit the tracing of the lateral persistence of the contorted laminae.The bed-confined contortions canbedesignated as soft-sediment deformation structures(SSDS;Shanmugam,2017 and references therein).In addition,a～3.5-m-thick interval comprises metre-scale slump folds with sub-vertical axes bearing signatures of soft-sediment deformation due to instability on a slope(Allen,1982;Alsop et al.,2017;Basilone,2017)(Fig.14F).The fold wavelength is variable,from 0.5 m to 1.8 m.Bed thickness variation from fold hinge to limb is difficult to measure due to the presence of closely spaced sub-vertical joints(Fig.15A).The tightness of the anticlines becomes less towards the top within individual folds.Deformation also gradually decreases upwards into un-deformed beds within the field of view(Fig.14F).These SSDS developed after deposition but before complete lithification,as soft-sediment deformation develops in unconsolidated sediments through the loss of shear resistance(Basilone et al.,2016;Mastrogiacomo et al.,2012;Owen et al.,2011).

Fig.13 A—Panoramic view showing multiphase tectonic disturbance hosted within undisturbed host sediments;B—C—Enlarged view of two sectors from A,illustrating alternating deformed—undeformed packages.Note the very sharp and irregular nature of the basal contact of synrift interval;D—Syn-rift package laterally pinching out within the host undisturbed sediments.The yellow arrows indicate the base of the lensoidal syn-rift package within the host undisturbed sediments.Human=1.70 m(B)and 1.60 m(D).

d)Pillow structures.The pillows are strongly imbricated tight folds(Bose et al.,1997).These are the only bed-confined instability structures observed within the syn-rift facies association of the studied formation.Pillow beds have been noticed at the upslope tip of large slump/slide planes within the disturbed zone(Fig.15B,C).The maximum recorded amplitude of pillows is 27 cm(Fig.15C).Internally,they are massive in character,while their bases are very sharp due to projection of bulbous pillows.The base of the pillows bears slickensides showing a high angular relationship to the strike of the beds(Fig.15C).The pillow structures are interpreted to reflect liquefaction processes,the temporary loss of shear strength in granular sediments,which may be induced by seismic shocks(Basilone et al.,2014;Mariotti et al.,2002).

e)Joints .Sets of joint plains are recorded penetrating a number of beds locally within the tectonic interval(Fig.15A).These nearly vertical joint plains are restricted within a sigmoidal segment with a maximum vertical thickness of 5.1 m and overlain by nearly horizontal unaffected beds.The sediments must have been consolidated enough to suffer brittle deformation,suggesting formation of joints at a certain depth(Bose et al.,1997).

6. Tectono-stratigraphic evolution

Fig.14 Mass- flow units( fill and sketch)exposed in Veppur Mines.A and B—Mass- flow units filling small-scale grabens illustrating syn-rift sedimentation;C and D—Contortion of internal laminae as well as bedding planes.Note the variation in dip amount along the graben walls;E-Coarsening-upward trend within the mass- flow units stacked one upon another;F—Meter-scale slump folds with sub-vertical axes bearing signatures of soft-sediment deformation. Note the gradual decrease in deformation upwards into undeformed beds.Human=1.60 m(A and B)and 1.74 m(C and D).

The Archean basement forms the exposed hinterland that borders the Ariyalur—Pondicherry sub-basin where the present study was carried out(Watkinson et al.,2007).The occurrence of the shelf limestone on top of the crystalline basement or localized nonmarine siliciclastics atop it(Fig.3)clearly documents deepening and widening of the rift valley and consequent inundation by the sea.Within the limestone formation a tectonically disturbed facies association is sandwiched between two facies associations of distinctly quieter phases.FA 1 representing the early syn-rift phase,is meagre in thickness,only locally preserved (Figs.4 and 12).Nevertheless,itis stratigraphically significant because it records the onset of a marine transgression within the basin perhaps in concert with the global sea-level rise during the Albian time(Chakraborty,2016;Hancock and Kauffman,1979;Nagendra et al.,2011;Watkinson et al.,2007).In the inland sea,a bar-lagoon system was generated at the sea-margin and thin finely laminated pink-colored biomicritic limemud alternating with distinctly coarse-grained massive or graded yellowish biomicritic limemud,fringed by large-and small-scale biosparite and biomicrite.On the shelf was deposited a biomicritic lime sediment that graded into a gray and then a dark-colored siliciclastic mud as the water depth increased(Fig.16B).Diagenetic glauconite at a comparatively shallower level and pyrite at a deeper level formed within the siliciclastic mud.

This initial quieter phase of carbonate sedimentation was replaced by the syn-rift sedimentation system.Spectacular breccias together with varied kinds of mass- flow products intervened by marl comprise this FA 2.FA 2 is inordinately coarser at the hinge zone and rests directly on the basement.The base of this association is stepped because of block faulting.Basinwards,FA 2 rests on FA 1.Gradually the basin was

filled up. Presence of huge carbonate blocks possibly signifies widening of the basin to a significant extent.

In the late syn-rift phase,tidal shelf sediment constituting the FA 3 was deposited.Tidal bars intervened by inter bar sediments comprise this association,accumulated therein.On further deepening and opening of the basin,tidal shelf gave way to mixed carbonate—siliciclastic fines of offshore origin comprising the overlying formation,the Karai Shale Formation(Fig.16).

Tectonic imprints over the spatial and temporal variation of sedimentary facies are frequently overlooked for the construction of carbonate depositional models.Carbonate sedimentation within a rift basin displays wide variation in sedimentary facies and their geometrical as well as architectural relationships(Basilone et al.,2014).Despite this variation a common thread in development is noted on carbonate rift basin- fills studied worldwide.Bosence et al.(1998)reported a lagoon—tidal flat deposit at the landward fringe of the basin,as reported from the Dalmiapuram Basin. Leeder and Gawthorpe (1987) likewise described coastal/shelf carbonate facies within a half graben system,emphasizing the fundamental control of characteristic asymmetrical subsidence vectors developing across the graben upon facies distributions.

7. Conclusions

The Cretaceous Dalmiapuram(Limestone)Formation exposed in and around Ariyalur in the Cauvery Basin,SE India,portrays an excellent illustration of tectono-sedimentation history of a shallow carbonate platform at a passive continental margin.Being a part of the transgressive systems tract,the entire carbonate marine fossil-bearing formation onlaps the Archean basement in most places and locally the Basal Siliciclastic Formation of the Barremian—Aptian age,while it is overlain by the Karai Shale Formation of offshore origin(Chakraborty et al.,2018).The contact between the Dalmiapuram Formation and the basement is always faulted.However,circumstances indicate carbonate sedimentation was initiated prior to tectonic disturbances.Moreover,a detailed facies analysis reveals an early syn-rift association present only locally,but being a product of rapid marine transgression,bears stratigraphic significance.Multiphase syn-rift activities due to local/intra-basinal tectonics guided deposition of major breccias(rock fall)at the base of fault planes along the basin margin steepened during rifting,and mass- flow assemblages at relatively deeper part of depositional regime.Different types of instability structures,viz.gravity slide breccias,mass- flow units filling small grabens,contortions of laminae and slump folds,pillow structures,and joint planes bear signatures of synsedimentary tectonics. Carbonate sedimentation continued even in the late syn-rift phase comprising a tidal system,and passed up to the overlying Karai Shale Formation in accordance with the ongoing transgression.

Acknowledgements

Subir Sarkar acknowledges the Centre of Advance Study(CAS Phase V)and University with Potential for Excellence(UPE II)programmes of Jadavpur University.Geochemical work was carried out at the National Stable Isotope Facility of IIT,Kharagpur.The authors are indebted to their respective departments for infrastructural facilities.The authors are thankful to Prof.G.Shanmugam,Prof.Ian D.Somerville,Prof.Luca Basilone,Prof.Santanu Banerjee for their critical comments during revision.The authors deeply appreciate Prof.Zeng-Zhao Feng(Editor-in-Chief),Dr.Xiu-Fang Hu(Editor)and Dr.Yuan Wang(Editor)for their meticulous editing and Prof.Franz Fürsich for his valuable remarks.