1

CYCLIC FI�E-GRAI�ED DEPOSITS WITH POLYMICT BOULDERS I�

OLAIPADI MEMBER OF THE DALMIAPURAM FORMATIO�, CAUVERY

BASI�, SOUTH I�DIA: PLAUSIBLE CAUSES A�D SEDIME�TATIO� MODEL

Mu.RAMKUMAR

Department of Geology, Periyar University, Salem-636 011, India

E-mail: muramkumar@yahoo.co.in

ABSTRACT

The fine-grained cyclic deposits of Olaipadi member of Dalmiapuram Formation, Cauvery

basin, contain large boulder sized lithoclasts of gneissic basement rocks and older

sedimentary rocks. Earlier these were interpreted as glacial drop deposits and siliciclastic-

coral reef complex, etc. Occurrence of these clasts in typical basinal sediments that show a

general fining upward nature, repetitive occurrences of classic Bouma sequences,

sedimentary structures in the bases of sandy and silty layers indicative of fluidized flow

and post depositional dewatering, general reduction of thicknesses of sandy layers from

basal beds to top and coeval increase in thickness of claystone layers, occurrences of

conformable bedding planes along the margins of the large extraformational and

extrabasinal boulders have all not been documented or explained by previous studies.

Availability of newer exposures helped documentation of sedimentary textural, structural

and lithofacies characteristics, contact relationships, facies association and tectonic

structures. With these, it is inferred that these sedimentation of these deposits was initiated

by hyperpycnal flow following faulting and upliftment of former shelf and coastal regions

that led to, gravity flow of fractured and unstable fault scarp blocks and to turbiditic

deposition into adjacent deeper regions of sea, maturation of depositional topography from

rugged fault controlled to gentle slope concomitant with progradation of fan into deeper

basinal regions along with sea level oscillations.

1. I�TRODUCTIO�

The sedimentary terrain of

erstwhile Tiruchirapalli district, South

India, located in the Ariyalur-Pondicherry

depression of the Cauvery basin (Fig.1)

expose a more or less complete Barremian-

Danian succession (Sastry & Rao, 1964).

Followed by the pioneering studies of Kay

(1840), Blanford (1862) and Stolizca

(1861-1873), many hundreds of papers

were published owing to the lithological

and faunal diversities of the succession and

the possibility of locating huge

hydrocarbon reserves. Among these

publications, the works on foraminifera (Govindan et al., 1996), ostracoda (Bhatia, 1984),

ammonite (Ayyasamy, 1990), nannofossil (Jafer & Ray, 1989; Kale & Phansalkhar, 1992;

Fig. 1

2

Kale et al., 2000), bryozoa (Guha, 1987; Guha & Senthilnathan, 1990, 1996),

lithostratigraphy (Ramanadhan, 1968; Banerji, 1972; Sundaram & Rao, 1986; Ramasamy

& Banerji, 1991; Tewari et al., 1996) and tectonics, (Kumar, 1983; Prabakar & Zutchi,

1993) present comprehensive accounts. However, owing to the poor and scarce exposures

and comparatively less fossiliferous nature, the cyclic, regular bedded, fining upward

sandstone-shale deposits of the Dalmiapuram Formation (Albian-Cenomanian) have not

been thoroughly understood. Presence of huge limestone blocks (~10 m dia) of shallow

marine origin with basement rocks and lithoclastic boulders of similar size embedded

conformably in basinal sediments added many genetic controversies namely, glacial

dropstones (Sundaram and Rao, 1986) and mixed coastal siliciclastic-carbonate reef

systems. Furthermore, these models have not explained many characteristics of these

deposits namely; absence of major glacial activity during deposition and occurrence of

lithoclasts of conglomerates that were not recorded in any of the older deposits and large

basement rock blocks and thus the sequence remained less understood. Access provided by

exposures from newer mine sections, expansion of old mine sections, trenches dug for

construction of new bridges and road sections and traverses at closer intervals helped

document detailed micro-scale lithological, sedimentary structural and facies

interpretations, thereby improving our understanding on the cyclic deposits.

2. GEOLOGY OF THE STUDY AREA

Present study is confined to the Olaipadi member of Dalmiapuram Formation

exposed along NE-SW trending basin margin faultlines of the Cauvery basin (Fig.1). Table

1 presents the general stratigraphy of the Dalmiapuram Formation. Geographic distribution

of the formations and members of the Cauvery basin exposed in the study area are

presented in figure 2.

Table 1. Lithostratigraphy of the study area (after Ramkumar et al., 2004)

Age Formation Member Thickness(m)

Karai -----------------------------------Unconformity----------------------------------

Cenomanian Kallakkudi calcareous sandstone 60

Olaipadi sandy argillite 85 Dalmiapuram Dalmiya biohermal limestone 15

Albian Varagupadi biostromal limestone 23

Grey shale 7 -----------------------------------Unconformity----------------------------------

Sivaganga

An angular erosional unconformity separates the Sivaganga and Dalmiapuram

formations. The basal deposits of the Dalmiapuram Formation, the Grey shale member

3

Fig

.2 D

istr

ibu

tion

of

lith

ost

rati

gra

ph

ic u

nit

s of

the

Cau

ver

y b

asi

n (

aft

er Ram

kumar et al., 2004)

4

consists of alternate beds of calcareous grey shale and upward thickening bioclastic grey

limestones. The next younger lithostratigraphic unit, the Varagupadi biostromal limestone

member contains pale yellow colored, thin to thick, even and parallel bedded bioclastic

limestone deposits interbedded with arenaceous limestones, calcareous sandstones and thin

gypsum layers. The Dalmia biohermal member is of well-cemented pure algal and coral

limestones and displays typical reef core forms. It is 15 m thick and contains massive pink

to greyish white limestone deposits with abundant vugs and cavity fillings. Govindan et al.

(1996) assigned Middle Albian age to this member. This Dalmia biohermal member is

typically exposed in Kallakkudi mine. Isolated and highly weathered counter parts are

observed in northwestern region of this basin. Its very limited occurrence and that too

above the biostromal limestone deposits (Varagupadi member), is interpreted as a result of

reef growth along the fault scarp (Steinhoff & Bandel, 2000). Post depositional reactivation

of the pre-existing faults might had also resulted in large scale erosion of this member that

allowed preservation of only little portion of it. This member is capped by a type I

sequence boundary represented by forced regression surface created by faulting, sealevel

fall, subaerial exposure, meteoric diagenesis and subaerial erosion.

The deposits of Olaipadi conglomerate member are predominantly basinal sands,

argillaceous siltstones, silty claystones and claystones (Plate 1, Fig.1). These basinal

sediments contain large boulders of lithified coralline limestones (similar to the underlying

biohermal limestones - angular and sub-rounded boulders that some times reach >10 m in

diameter; Plate 1, Fig. 2 and Fig. 3), basement rocks (Plate 1,Fig. 3), claystones (similar to

that of Terani clay member), pure algal bindstone boulders (Plate 1,Fig. 4 – these are also

not recorded as a distinct lithology in any of older deposits of this region) lithoclasts of

older conglomerates that have not been recognized/mapped anywhere in this part of basin -

Plate 2, Fig.1). Tewari et al. (1996) interpreted this member to have been deposited from

rapid submarine debris flows/talus deposits at the foot of eroding fault scarps. Occurrences

of lithified claystone conglomerate boulder clasts and algal bindstone boulder clasts, which

were not present elsewhere in the exposed area as distinct lithologies in addition to

complete absence of biohermal counterparts of Varagupadi biostromal member and

presence of >10 m large basement clasts, supports the interpretation of existence of such

lithologies further west before faulting and complete erosion of them due to faulting.

General fining upward nature of the beds from bottom to top, recurrent Bouma sequences,

load casts (Plate 2, Fig. 2 and Fig. 3), dish structures (Plate 2, Fig. 4), reworked fauna

namely, belemnites, rudist corals and serpulids are very common in this member.

5

Fig. 1. Basal pebbly s.st-claystone facies exposed in a mine section

located in Tirupattur village. Arrows indicate individual bouma

sequences. Photograph measures 4.5 m X 2.5 m.

Fig. 2. A large coralline limestone boulder embedded in fine

grained cyclic deposits. Note the conformable bedding that

encircles the clast. Mine section in Tirupattur village.

Fig. 3. Basement rock boulders (a),

coralline limestone boulders (b)

embedded in fine grained cyclic deposits. Mine section located near

Tirupattur village.

Fig. 4. Close-up view of typical algal

bindstone with fenestral porosities indicated by arrows. Note that this

particular lithology has not been

recorded anywhere in the older deposits in and around the study area. One rupee

coin indicated by bold arrow is placed

for scale. This boulder is found embedded in fine grained deposits

exposed in mine sections of Tirupattur

village.

a b

b

Plate.1

1.1 1.2

1.3

1.4

6

7

Pla

te 2

Fig. 1. Lithoclastic conglomerate em

bedded in fine silty s.st. Pen is placed for scale. M

ine section in

southeast of Perali village.

Fig. 2. Irregular load along bottom surface of pebbly s.st facies. Irregular depositional topography

and differential compaction could have resulted in this irregular load structure. Mine section in

southeast of Olaipadi village.

Fig. 3. Another type of irregular load structure along bottom surface of coarse sandy siltstone facies.

Irregular depositional topography and differential compaction could have resulted in this irregular

load structure. Mine section in southeast of Perali village.

Fig. 4. ??Dish structures developed in fine s.st deposit. These structures are indicative of fludized

flow during deposition and result in latter stage dew

atering concomitant with increase in overburden

and compction. One rupee coin is placed for scale. M

ine section located in Tirupattur village.?

8

The information that this member is described as Middle Nallurian stage by Raju et

al. (1996) and record of LAD of Planomalina Buxtorfi (99.36 Ma.) in the underlying

limestone deposits and record of FAD of Rotalipora Reicheli (95.81 Ma.) by Govindan et al.

(1996) at the base of Kallakudi member suggest deposition of Olaipadi member under a

third order (?) sea level cycle. The Kallakkudi member comprises fine-coarse sandstone

deposits with alternate medium to thick beds of silty claystone, calcareous siltstones,

bioclastic arenaceous limestone and gypsiferous claystone. Tewari et al. (1996) recorded

foraminifera, mid Cretaceous phyloceratid ammonite along with shell debris of exogyra,

alectryonia, echinids and bryozoa. According to them, this member ranges in age from Late

Albian to Late Cenomanian.

3. METHODS A�D MATERIALS

Systematic field mapping in the scale of 1:50 000 was conducted in and around

Ariyalur, Perambalur and Tiruchirapalli districts, (erstwhile Tiruchirapalli district) to

collect data on lithology, sedimentary and tectonic structures, facies characteristics and

fauna from natural exposures, dugwell, bridge, road and mine sections. Special efforts were

also made to document the occurrence, association, morphological characteristics of the

sedimentary lithoclastic and gneissic boulders embedded in the cyclic deposits and contact

relationships of these clasts with host deposits. The stratigraphic information,

interpretations on depositional environments, major geological events such as faulting, sea

level changes, depositional breaks, etc., inferred from the ground truth data, facies

characteristics and published data enabled construction of composite stratigraphic column

of the Olaipadi member depicting lithological succession and the depositional history

(Fig.3). Comparison of the facies characteristics of the Olaipadi member with similar

deposits elsewhere was also attempted to ascertain compatibility of the interpretations made

with world equivalents. The approach of this paper is, elucidation of geological events that

caused deposition of fine-grained and argillaceous sediments with large, hydrodynamically

non-compatible extraformational and extrabasinal boulders and interpretations of facies and

sedimentary characteristics of the Olaipadi member in association with tectonic and

evolutionary history of the study area.

4.LITHOFACIES CHARACTERISTICS A�D DEPOSITIO�AL E�VIRO�ME�TS

Figure 3 shows more or less continuous fining upward sequence of three basal facies

types along with recognizable bedding planes/depositional breaks in between. Among these

three, the 10 m thick, lowermost lithofacies shows recurrent, meter thick fining upward

cycles of coarse-fine pebbly s.st-s.st-siltstone-silty claystone, in which the pebbly s.st and

9

s.st. layers are thickest while the finer layers are very thin. Base of each cycle is a sharp

erosional surface. The beds also contain rounded gravel-boulder sized clasts of algal

limestone and granitic gneiss and gravel-pebbles sized clasts of smocky quartz. Rounded

nature of large clasts and the pebbles in sandstone unit of cyclic deposits indicate either

recycling of older deposits and or longer transportation, brought to the newly created steep

slope by a protochennel established over fault scarp. Occurrence of limestone clasts

indicates erosion of fault scarp itself over which the reefs might have grown earlier and got

destabilized by faulting. Associated granitic gneiss and smocky quartz clasts indicate

erosion of continental region also (detailed in latter section).

Similar to the fault scarp-controlled rocky shoreline to upper continental slope

environment of Neogene Coquimbo Formation, north-central Chile, as described by Roux et

al. (2004), these basal pebbly s.st. deposits also directly abutting the basement wall from

which they originated; have angular as well as rounded clasts of older rocks (sedimentary

and basement) and hence could be termed purely as deposits of gravity debris flow. (Not

clear?) However, well developed Bouma sequences albeit represented by pebbly s.st to silty

clay layers in association with upward fining nature of layers instead of upward coarsening

cycles as commonly found in gravity flow vouch for hyperpycnal flow. The gravel-boulder

sized clasts might have been the result of eroding basement fault scarp, brought to the

depocenter of turbiditic deposits by gravity fall. Occurrence of conformable layers of

Bouma sequences parallel to the clast morphology suggests coeval nature for deposition of

Bouma sequences and occasional but recurrent supply of large clasts from fault scarp into

the adjacently located deeper regions of depositional basin. Piret & Steel (2004) stated that

conversion of catastrophic sediment failures into turbidites is a common geological

phenomenon, which has been most widely reported but not widely documented. These

authors have listed possible causes by which such initiation takes place resulting in

instantaneous sediment failures leading to short-lived surge-type flows that deposit turbidite

beds described as Bouma sequences. According to these authors, they can also be generated

by a variety of mechanisms, important among them are: seismically triggered subaerial

sliding within the drainage, retrogressive slope failure and hyperpycnal flows. Occurrences

of many similarities between pebbly s.st. facies of the study area and Eocene turbiditic

deposits described by Piret & Steel (2004) such as abundance of thick turbidite sandstone

beds, (Ta) sand-prone nature of the turbidite systems, downslope changes of individual

thick, sandy turbidite beds and their collapsed pinch-out segments and low abundance of

associated slumped or debris-flow beds and or absence of it in the study area strongly

10

suggest the sustained flows in the study area were generated by hyperpycnal flows, i.e.,

turbidity currents generated by direct river effluent (Is this really through?) . Thus, this

facies could be termed as coarse turbidite with Ta,b,d,e divisions and deposition of this facies

might had been took place under higher energy turbidity currents after faulting. Comparison

these facies characteristics with fan model of Walker (1979; p.98) indicates proximal part of

the fan.

The next younger unit is about 25 meters thick and rests over the underlying unit

with gradational contact, may be indicative of gradual reduction of energy conditions and

maturation of depositional topography. It comprises cyclic deposits of 10 to 30 cm thick,

typical of Bouma sequences represented by Ta,b,d,e divisions of coarse s.st-siltstone-silty

claystone suggestive of continuation of hyperpycnal flow concomitant with maturation of

depositional topography, denudation of continental region and sealevel rise on a

comparative scale compatible with varying thicknesses of sequences. A progressive

reduction in general grain size in this facies than the underlying facies suggests, increasing

accommodation space in the depositional basin either by sea level rise or by sinking of fault

block (as the depositional basin was controlled by block faulting) or both. It also signifies

retrogradation of the fan from proximal into middle. While the sandy layers are thick, the

claystone layers are thin-very thin laminae. Each individual Bouma sequence starts with a

sharp erosional contact with underlying sequence. Typical normal grading within sandy and

silty layers towards top, dish structures (within beds) and load structures (at the base of

beds) in the bottom portions of sandy layers, parallel lamination in all the individual sandy,

silty and clayey layers could be recognized. However, absence of significant faunal remains

and trace fossils in any of these layers indicate faster rate of deposition. Walker (1979)

stated that middle fan regions always experience higher rates of deposition than outer and

proximal regions. It is further substantiated by the fact that this is the thickest facies than

any other lithofacies recognized in the Olaipadi member. Occurrence of exclusively

coralline limestone boulders of 1-5 m diameter in these sequences and display of

conformable layering of Bouma sequences following the morphology of these boulders

signify coeval simple sediment gravity flow of lithoclasts from fault scarps and turbiditic

deposition from hyperpycnal flow. Frequent occurrence of slump features and dish

structures in this facies indicate strongly that the finer sediments were fluidized during

deposition and the deposits were dewatered during subsequent compaction by overburden, a

character typical of turbiditic deposits, more so wherein faster rate of deposition took place.

11

The next younger unit is also shows a simple gradational contact with the underlying

unit, again suggestive of continued reduction of intensity of hyperpycnal flow and

maturation of depositional topography. This 5m thick unit consists of decimeter thick,

parallel, even bedded cycles of s.st-siltstone-silty claystone show no noticeable erosional

contact in between individual layers. Occurrence of granitic gneiss, lithoclastic

conglomerate and coralline limestone boulders of 3-5 m dia in these cycles indicates

prevalent erosion of continental region as well as extraformational sedimentary lithoclastic

conglomerate too. It is to be noted that the lithoclastic conglomerate and the massive

bioclastic and biohermal limestone boulders were not recorded anywhere in the sedimentary

basin, meaning severe erosion of coastal sedimentary region as well as basement rocks,

associated with advancing torrential channel flow.

A distinct bedding plane separates the 8 m thick next younger facies. This facies

shows a shift towards comparatively coarser grain sizes as evidenced by the occurrence of

30-60 cm thick Bouma sequences with Ta,b,d,e divisions of sandy siltstone-siltstone-silty

claystone beds. It is also observed that among individual divisions, the sandy layer is thicker

than others in the bottom portion of this facies and its thickness gets gradually reduced

towards top of the unit indicative of either a brief fall in sea level followed by a gradual rise

and or increase in carrying capacity of turbidity current at bottom and gradual reduction.

Based on the criteria listed in Walker (1979; p.98), this facies may be interpreted as a part of

middle fan besides prograding nature of the fan concomitant with sealevel fall and or uplift

of fault blocks. It is also to be noted that the limestone boulders in this unit are

comparatively very smaller (~30 cm in dia) and are angular suggesting continuation of

gravity flow of limestone deposits located along the unstable fault scarp, but with a

reduction in magnitude of erosion.

Next younger unit is 6 m thick, rests over the older unit with gradational contact and

contains typical decimeter thick Bouma sequences of sandy silt-ferrugenous silty claystone-

claystone. Individual silty beds show sharp erosional surfaces. This unit has boulder to

cobble sized coralline limestone clasts aligned parallel to the bedding. The next younger,

topmost unit of the Olaipadi member is the 15 meter thick, classic 10-100 cm thick

individual Bouma sequences with with Ta,b,c,d,e layers that show typical normal gradation

from fine pebbly s.st-s.st-siltstone-claystone. Even within a 10-15 cm thick claystone layer,

gradual reduction of silt content towards top could be observed. Bases of pebbly s.st.

divisions (Ta) rest over sharp erosional surfaces of underlying claystone layers (Te). The

claystone bed looks massive and has unrecognizable trace fossils. The dish, load and slump

12

structures are frequently observed in the bottom surface of pebbly s.st. layers. These two 6

and 15 m thick facies types contain few or rare limestone clasts, which, together with

general reduction of clast sizes from basal pebbly s.st. facies indicate reduction of intensity

of fault scarp erosion and stabilization of depositional basin after initial tectonic activity.

The occurrences of typical Ta,b,c,d,e divisions of Bouma sequences, comparatively finer grain

sizes of individual layers than other facies types, thicker Te layers than other dividions,

occurrences of trace fossils and abundance of slump, load and dish structures in these two

facies types indicate relatively distal part of the fan.

On the whole, a gradual reduction of grain sizes concomitant with increase in

thicknesses of finer divisions of Bouma sequences and reduction in lithoclast size from

bottom to top of the Olaipadi member could be visualized from the facies descriptions..

Within this general fining upward trend on a member scale, sub-trends between and within

individual facies types were also observed. The basal three lithofacies types show a gradual

fining, followed by a relatively coarser lithofacies and again followed by two very fine grain

sized lithofacies types, signifying a overall reduction in grain size. These characteristics

suggest rising sea level or establishment and maturation of the fluvial source with time,

complete with seasonal flow changes and or transformation of the fault scarp controlled

steep depositional slope into gently sloping depositional topography.

While the basal pebbly s.st facies is considered as proximal fan deposit, the

following three facies types were interpreted as middle fan deposits and the final two

lithofacies types were established to have characteristics of distal fan. It is brought out that

the middle fan deposit represented a faster rate of deposition compared to other two parts of

the fan. It is also interpreted that following the deposition of 25 m thick middle fan facies,

there might have been increase in intensity of turbidity current either related with advancing

fan and or falling sealevel followed by sealevel rise but continuation of retrogradation of fan

as witnessed by 8 m thick comparatively coarser grain sized lithofacies followed by

deposition of 6 and 15 m thick lithofacies types of distal fan with classic Bouma sequences.

5. DISCUSSIO�

Having established depositional history of Olaipadi member that of a sequence of

events namely, initiation of turbiditic depositional conditions following reactivation of basin

margin faults followed by establishment of proximal, middle and distal fan concomitant

with gradual reduction of sediment gravity flow and persistence of hyperpycnal flow, it is

imperative to analyze the nature of coexistence of gravity flow and hyperpycnal flow as

such cohabitation is scarce in geologic history. In addition to analyses of the prevalent

13

geological conditions that led to their coexistence, comparison of the facies characteristics

of the study area with world equivalents was also attempted.

5.1. Gravity flow of boulder-gravel sized lithoclasts from basin margin fault scarps

Deposition of the Olaipadi member had commenced after reactivation of basin

margin faults at the dawn of Cenomanian as evidenced by the occurrence of contact

between Olaipadi member and immediate and much older sedimentary rocks besides

basement rocks. While the contact between basin margin faults and Olaipadi member could

be traced in mine sections, natural exposures and well sections located along the western

margin of the study area (Fig. 2), occurrence of this faulting movement after deposition of

Dalmia biohermal member is evidenced by the displaced beds of Dalmia member and

Varagupadi member as seen in the Dalmia mines located southwest of the study area and in

mine sections located southeast of Perali village (Fig. 2). Direct contact of Olaipadi member

with basement rocks is pronounced in exposures in and around northwest of the study area

adjacent to Govindarajapatnam and Olaipadi villages. This faulting movement had

destabilized the uplifted former shelf regions and initiated significant mechanical erosion of

fault scarp materials. South and northwest of Kallagam village, the basal member of

Dalmiapuram Formation, Grey calcareous shale is exposed along a fault, which means, the

deposits that were younger to the grey calcareous shale member were all eroded, signifying

the magnitude of erosion that followed reactivation of fault at the dawn of Cenomanian. The

fresh nature and comparatively high angularity of basement rocks, lithoclastic

conglomerates and coralline and algal limestones clasts embedded in fine sediments

suggests their break-down by physical agents, quick transport and burial before any

noticeable weathering which could only be accounted in the event of physical break-down

along fault scarp located adjacent to the deep basin, simple gravity fall and burial. Tectonic

uplift of these areas and associated block faulting and tilting is assumed to be responsible

for this physical erosion. It is similar to the fault controlled mechanical erosion prevalent

during Palaeogene in Rhenodanubian Flysch basin of Austria (Egger et al., 2002).

Occurrences of lithoclastic conglomerate clasts which have been never recorded in this part

of the sedimentary basin together with the basement clasts clearly suggests that the erosional

process followed by faulting event might have been so intense to remove the entire

sedimentary pile deposited over the basement rocks together with basement rocks

themselves.

Although the deposits of Olaipadi member contain large clastic boulders embedded

in finer sediment matrix and thus could be interpreted as diamictites initiated by catastrophic

14

mass failure as a result of faulting as happened in Port Askaig Formation of Scotland

(Arnaud & Eyles, 2002), comparison of the facies characteristics with classic diamictites

and turbidites strongly suggests the Olaipadi member to be of turbiditic origin interspersed

with mechanical erosion of fractured fault scarps located along the basin margin and gravity

flow of the eroded blocks of rocks. McHugh et al. (2002) stated that generally the mass

transport deposits rest upon prominent stratal surfaces and sequence boundaries. The

Olaipadi member is also resting on a forced regression surface (upper contact of the Dalmia

biohermal member) and is capped by a non-depositional-erosional surface, allowing

interpretation of this member as a mass-transport deposit. However, the host sediments of

these clasts are showing conformable relationship between clasts and surrounding individual

layers of Bouma sequences and general fining upward nature within each facies and also

between successive facies types instead of randomly oriented and unsorted clasts, clast and

matrix supported, coarsening upward sequences, tabular bedding, sheet sands that could be

expected from mass-flows (Drzewiecki & Simó, 2002) confirming turbiditic origin for the

finer sediments and simple fault scarp gravity flow for larger polymict clasts. Nakajima and

Kanai (2000) examined the Recent mass flow deposits and turbidite deposits triggered by

earthquakes and enlisted the differentiating criteria for these two types of deposits. While

examining the Olaipadi member based on those criteria, it is observed that this member

shows absence of amalgamated beds, irregular structure sequences, grain-size

breaks/fluctuations, abrupt changes in composition within bed, and variable composition

among beds strongly suggesting turbiditic origin rather than mass flow origin.

Thus, it is safe to interpret that owing to the faulting event, the shelf might have been

very narrow and deeper basinal conditions might have existed very close to the basin margin

due to which, the unstable, uplifted former shelf regions and fractured continental regions

were eroded and deposited within finer deep basinal deposits before inheriting any change

in clast morphology. It is also clear that this sporadic fault scarp erosion and simple gravity

flow of eroded rocks is independent of prevalent transport and deposition of finer host

sediments in which the larger clasts are embedded.

5.2. Deposition of cyclic beds by turbidity currents emanated from hyperpycnal flow

Recent studies have advanced our understanding on turbiditic deposition that helped

recognizing turbidites from modern deposits and ancient rock records and also to

differentiate them into sub-systems (for example, Leverenz, 2000; Baas et al., 2000; Stow &

Mayall, 2000). In this paper, the guidelines on turbiditic modeling envisioned by Walker

(1979) and Bouma (2000) while taking into consideration the cautioning notes of

15

Shanmugam (2000, 2002). It was the statement of Bouma (2000) that prior to developing a

turbiditic model that best represents the deposits under investigation, adequate

understanding of the major external parameters that influenced the transport and deposition

of sediment, tectonic influence on the sediment source area and its overall distance to the

shoreline, shelf width and basin morphology, relative sea-level fluctuations and their impact

on the transport from coast to basin have to be considered had formed the basic guiding

factor in this paper.

Independent of the mechanical erosion described in preceding section, there was

another major sediment source and transporting agent that brought in finer clastics and

argillacous sediments in the study area during the deposition of Olaipadi member. While

studying modern turbiditic deposits of SW Mediterranean Sea, Alonso and Ercilla (2002)

stated that the turbidites were influenced by number of submarine feeding sources, sealevel

fluctuations, basin margin faults and gradients of feeder channels. They have also stated that

relative relief of the continental drainage basin, quantity, types and quality of sediment

entering the sea are also important. Among all these factors, these authors list tectonic

forces as first order factor that controls turbiditic systems. The genesis of turbiditic deposits

in faulted margin basins was examined in detail by Mutti et al. (2003) who have stated that

during periods of tectonically forced lowstands of sealevel, depositional systems commonly

shift basinward to shelfal and slope regions. Triggered by instability along the edges of

sedimentary basins, sand-laden hyperpycnal flows generate immature and coarse-grained

turbidite systems commonly confined within structural depressions. Turbidity currents are

very likely to be mainly triggered by floods, via hyperpycnal flows can carry sediment load

over considerable distances down the basin axis”. These criteria fit amply to the basal

pebbly s.st facies of the Olaipadi member to be termed as turbidite deposited by

hyperpycnal flow, initiated by faulting. It is common for sandstones and conglomerates to

get deposited from high-density turbidity currents and debris flows in major channels

following tectonically more active periods (Dam et al., 2000). While reviewing the

characteristics of turbidites, Shanmugam (2002) stated that turbidity currents generate

normal grading if the deposit was laid down by a single event. Although based on his

descriptions the basal pebbly s.st. facies could be interpreted as result of debris flow, general

fining up of successive facies types culminating with development of classic Bouma

sequences in upper portions of the Olaipadi member clearly supports the interpretation of

initial high-density, higher energy turbidity flows that gradually reduced with maturation of

depositional topography, denudation of catchment area in continental regions, reduction of

16

gradient between baseline in continental and depositional basin regions concomitant with

reduction of mechanical erosion, that are witnessed through general reduction in grain size

in turbidites, chaotic clast sizes and thicknesses of sandy layers and increase of clayey layers

in Bouma sequences from bottom to top.

It is interpreted that individual facies types of the Olaipadi member indicate different

phases of turbidite formation and involvement of varying intensities of geological agents.

The general grain size variations between different facies types of the turbidites of the study

area could be linked to the changes in turbidity flow velocity and relative sealevel. Mulder

et al. (2003) while reviewing the hyperpyncal flow characteristics of turbiditic deposits

stated that a basal coarsening unit followed by upward fining sequence characterizes waxing

and waning periods of turbidity current. Absence of upward coarsening nature of the basal

facies in the study area and the occurrence of general fining upward nature follwed by a

coarse facies and finally a very fine facies could be construed either as the result of

variations in turbidity current velocities or sealevel oscillations or both (Mulder et al.,

2003). Similar to the observations of Wynn et al. (2000) in Northwest African slope apron,

the Olaipadi member also shows grain size variations controlled by depositional topography

during deposition.

The lithofacies section has documented the recurrent occurrence of Bouma

sequences in all the facies types of the Olaipadi member albeit with different bedding and

textural characteristics. The turbidite beds, known as the Bouma sequence is the product of

gradual waning of a turbidity current over a depositional site (Walker, 1979). Shanmugam

(2000; 2002) described that when deposition takes place under suspension mode, it creates

five divisions of the classic Bouma sequence. Each division and each turbiditic bed set

records the complex interaction between the strength of the current, the concentration of

moving sediment, its size distribution and composition, and the configuration of the

underlying bed surface at the time of deposition (Baas et al., 2000). Walker (1979) while

laying down guidelines for modeling turbiditic deposits observed that development of

Bouma sequences with one or few of the divisions is common and owing to the

heterogeneity of natural sedimentary environments, turbidites would not always posses

Bouma sequences with all the divisions (Ta, b, c, d, e) and the same is documented in the study

area wherein even though all the facies types show the development of Bouma sequences,

except the top two 6 and 15 m thick facies types all other facies types show missing of Td

division. Relative frequency of occurrence of Bouma divisions of sandstone beds have been

17

used to establish sub-systems within submarine fan models although their general

applicability is strongly disputed (Leverenz, 2000).

Mattern (2002) discussed in detail the thickness variations of individual divisions, in

Bouma sequences based on which he has attempted to interpret hydraulic differences

between channelized and unchannelized flows of turbidity currents. He has concluded that

the average turbidite layer thickness in channelized successions is markedly greater than in

unchannelized flows. Application of his interpretation to the study area wherein the

thicknesses of sandstone layers are more in basal portion which in turn are gradually

replaced by thick clayish layers towards top implied that the turbidity currents in the study

area were indeed channelized flows whose energy conditions were gradually dissipative

with time as the depositional gradient reduced by continued sedimentation and reduction in

accommodation space owing to initial fault block movement followed by stability. Leverenz

(2000) observed that transport distances within turbidite systems correlate inversely with

grain size. He has stated that Bouma division Ta is an indicator of depositional proximality.

Comparison between relative proportions of Bouma division Ta within each turbidite bed,

their overall assumed proximality based on bed thickness, relative proportion or occurrence

of sandstone or mudstone could help interpret the proximal, intermediate and distal nature

of turbiditic deposits could be recognized. Kneller (2003) stated that increase or decrease in

flow density or thickness, results in variations of grain-size of deposits and their thicknesses

besides erosional to aggradational nature of the fan.

Mattern (2002) also established that dish structures seem to be considerably more

common in midfan than in outer-fan successions. This may indicate a higher sedimentation

rate from individual suspension currents in midfan areas. Application of this observation,

coupled with the abundance of dish structures in 25 meter thick silty facies and the first 15

m thick facies that are separated by 6 meter thick sandy silty facies are interpreted as midfan

of turbiditic deposit. Occurrence of coarse grain sized facies on lower part of this fan

represented by pebbly sandy-clay facies is interpreted as proximal fan while the finest and

thick clay layered facies types that occur on top of the mid fan deposit (6 and 15 meter thick

classic Bouma sequenced beds) are interpreted as distal fan deposit. These interpretations

are supported by the relative abundance of trace fossils in different parts of the inferred fan.

While the basal pebbly s.st. facies lacks significant faunal traces, the middle fan facies

shows scarce occurrence while the distal fan shows abundance, may be due to relative grain

size differences and energy conditions of the turbiditic deposits (Wetzel and Uchman,

2001). Wetzel and Uchman (2001) also stated that in turbidite deposits it is often difficult to

18

recognize trace fossils owing to post depositional compaction-induced deformation and the

lack of contrast with the surrounding sediment. It is often inferred that only the animal

activities that disturb the primary sedimentary structures could get preserved in turbidites. It

is common that proximal fans commonly contain a low-diversity ichnofauna with a

preponderance of shallow-water types; middle fan sub-environments have a mixed

ichnofauna of shallow-water and deep-water types and outer fan environments rarely have

shallow water traces, but deep-water forms are abundant and diverse. In this context,

occurrence and preservation of higher abundance of trace fossils in top two 6 and 15 m thick

facies types in Olaipadi member, scarce occurrence in inferred middle fan deposits (three

25, 5 and 8 m thick facies types) and rarity of trace fossils in coarse 10 m thick basal facies

are in conformity with the observations of Wetzel and Uchman (2001). There have been

extensive studies (for example, Nilsen, 2000; Shiki et al., 2000) on distinguishing criteria

between seismoturbidite and flood generated turbiditic deposits. Comparison of facies and

textural characteristics of the study area with those examples, signify high density, high

gradient hyperpyncal flow origin to the turbidites of the study area. Similar faulting

initiated, fault block controlled depositional topographic sandy turbiditic deposits were

documented by Gregersen and Rasmussen (2000) and Drzewiecki and Simó (2002).

While studying Mesozoic accretionary complex of New Zealand, Leverenz (2000)

established a modified approach to facies analysis to distinguish distinct characteristics of a

turbidite system through criteria such as complexity of the pattern of facies distribution,

position of slumping of oligomict material, and proximal-to-distal distribution of Bouma

sequences and conglomerate fabrics based on which it is claimed by him that it is possible

to distinguish between a trench-depositional environment and submarine fan on a broad

basin plain. Occurrence of less complex facies pattern within inferred individual parts of

fan, abundance of slumping only in inferred distal fan, occurrence of continuous grain size

separation between different facies types and also within individual layers of Bouma

sequences, continuous transition from one facies type to other i.e., from proximal to distal

part of the inferred fan, absence of sudden change of sediment character indicative of

sudden change in slope are all suggestive of the fact that although the turbiditic deposition

started with faulting and over fault controlled depositional topography and the distance

between distal and proximal parts of fan were indeed short and there might have been a

narrow shelf that enforced delivery of hyperpycnal flow laden sediment to get deposited

directly in basinal portion of depositional basin.

19

6. CO�CLUSIO�S

a. The present study indicates that depositional history of the Dalmiapuram Formation

would have been a sealevel cycle dominated biostromal and biohermal limestone

deposition, bounded on either side by normal sequence boundaries had there not been

reactivation of lower Cretaceous fault at the dawn of Cenomanian resulting in elevation

of basin margin and coastal marine regions into topographic and structural highs

followed by initiation of significant continental and fault scarp erosion, episodic supply

of lithified marine sediments and basement rocks into basinal depocenters, creation of

narrow shelf and presumably a major fluvial source onland. The information that,

depositional cycles of Cauvery basin predominantly took place under the influence of

sea level oscillations except Late Barremian-Middle Cenomanian deposits as indicated

by structural, faunal and lithological studies (Guha and Mukhopadhyay, 1996;

Ramkumar et al., 2004a) signifies introduction of Olaipadi member by tectonic,

mechanical erosional and fluvial depositional agents within an otherwise normal

sealevel controlled depositional cycle.

b. The mechanical erosion of fault scarps and supply of the eroded materials to the basinal

depocentres was indicated by fresh unweathered nature of the polymict clasts embedded

in finer sediments that show conformable bedding planes that follow the morphology of

the clasts. Initial severity of the mechanical erosion and its gradual reduction if intensity

are indicated by the gradual reduction in size of clasts towards top and absence of

basement clasts at top of the Olaipadi member.

c. Occurrence of Bouma sequences all through the Olaipadi member, presence of regular

bedding surfaces, occurrence of sedimentary structures characteristic of fluidized flow

of finer sediments at the time of sedimentation are all indicative of sedimentation of

these sediments by processes independent and other than the processes that deposited

lithoclastic boulders. Occurrences of typical grading in individual bed sets and also

between different facies types, absence of unsorted nature of sediment grains and other

characteristics clearly suggestive of non-prevalence of mass wasting mode of

deposition.

d. The fining upward nature of successive facies types that lead to classic Bouma

sequences with thick Te division suggests establishment of turbiditic system at the dawn

of Cenomanian following reactivation of faults along basin margins and persistence and

maturation of hyperpycnal flow for a considerable time, punctuated by gravity fall of

rocks of fault scarp consisting of fractured basement rocks and older sedimentary rocks.

20

Data after Raju et al. (1996) indicate that Olaipadi member deposited presumably

between 99.36 Ma (LAD of Planomalina Buxtorfi) and 95.81 Ma (FAD of Rotalipora

Reicheli) implying prevalence of turbidity currents for a time span of 3.55 Ma.

e. General grading of the beds from bottom to top of the member also suggests

retrogradation of the member from proximal to distal parts of fan indicating either

sinking of the basin or sealevel increase or both. It also indicates general smoothening

of the depositional topography concomitant with denudation of drainage basin and

reduction of feeder channel gradient.

7. ACK�OWLEDGEME�TS

Work on the Cretaceous strata of the Cauvery basin was initiated through financial

assistance from Alexander von Humboldt Foundation, Germany and Council of Scientific

and Industrial Research, New Delhi, India, and currently it is being supported by the

Department of Science and Technology, New Delhi, India. Prof. Dr. Doris Stueben and

Dr.Zsolt Berner, Germany, are thanked for academic support and laboratory facilities. Herr

Nickoloski prepared thin sections for petrographic study. Shri S. Jeevankumar, Research

Scholar, Department of Earth Sciences, IIT-Bombay, helped in drawing diagrams.

Permission to collect samples was accorded by the mines managers and geologists of

Messers. Dalmia Cements Pvt. Ltd, India Cements, TANCEM Mines, Ramco Cements,

Nataraj Ceramics Ltd., Vijay Cements, Fixit Mines, Parveen mines and Minerals Ltd.,

Alagappa cements, Rasi cements, Tan-India Mines, TAMIN mines and Chettiyar mines.

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ABOUT THE AUTHOR

Dr.Mu.Ramkumar has obtained B.Sc. Degree from National College, Tiruchirapalli

and completed M.Sc. at Annamalai University, Chidambaram. He had worked on carbonate

sedimentology for his doctoral degree from Bharathidasan University, Tiruchirapalli and

completed PG Diploma in Personnel Management and Labor Legislation from Alagappa

University. Since completion of Ph.D. degree, he has worked in various research

laboratories viz., IIT-Kharagpur, Andhra University, Karlsruhe University, Germany and

IIT-Bombay before joining Periyar University, Salem. His research interests include

sequence and chemostratigraphy, environments across K/T boundary and Cenomanian-

Turonian boundary, modeling deltaic evolution, coastal zone and natural disaster

management, effluent dissemination capacity of estuaries, etc. He has published more than

100 articles in national and international journals and authored two books on computers. He

is a recipient of Humboldt Fellowship, CSIR Research Associateship, CSIR Pool Officer

Award, Young Scientist Award (twice) and was nominated for S.S.Merh Award. He is a

member and honorary fellow of various scientific bodies.