prerift and synrift sedimentation during early fault ...searg.rhul.ac.uk/pubs/wilson_1999 tonasa...

Prerift and synrift sedimentation during early fault segmentationof a Tertiary carbonate platform, Indonesia

Moyra E.J. Wilson*

Department of Geological Sciences, Science Laboratories, South Road, Durham, DH1 3LE, UK

Received 24 November 1998; received in revised form 24 April 1999; accepted 30 April 1999

Abstract

Eocene carbonate deposits of the Barru area, Sulawesi, Indonesia, provide a rare insight into sedimentation prior to andduring propagation of normal faults to the surface. Three main successions; late prerift, latest prerift/earliest synrift and synrift,are characterised by distinctive facies associations and sequence development. Shallow water foraminiferal shoals and intervening

lower energy depositional environments occurred during the late prerift in areas which latter formed footwall highs andhangingwall depocentres, respectively. During the latest prerift/earliest synrift, shallow water shelves deepened laterally intoslope environments in developing hangingwall depocentres. In both these sequences, sections in developing hangingwall areas arethickest, deepen up-section and thin laterally towards growing footwall highs. Active faulting resulted in rapid drowning of

hangingwall depocentres and massive reworking of material derived from collapse of the platform margin and adjacent shallowwater/emergent footwall highs.Di�erential subsidence, controlling water depths and accommodation space, types of carbonate producers and active faulting

were the main factors a�ecting depositional environments and facies distributions. Carbonate producers are extremely sensitiveindicators of depositional water depth and energy, hence rapid lateral and vertical facies variations in the Barru area providequanti®able insight into environmental changes prior to and during active faulting. # 1999 Elsevier Science Ltd. All rights

reserved.

Keywords: Carbonate platform; Facies; Fault segmentation; Subsidence; Prerift; Synrift; Tertiary; Indonesia

1. Introduction

Numerous studies of synrift successions (cf,Lambiase, 1995; Purser & Bosence, 1998; Ravnas etal., 1997) and conceptual models predicting sedimen-tation patterns during faulting exist (Gawthorpe,Fraser & Collier, 1994; Leeder & Gawthorpe, 1987;Ravnas & Steel, 1998). However, there is virtually nodocumentation of late prerift sedimentation changingto earliest synrift successions. This study provides agood example of this transition from the Barru area ofSouth Sulawesi, Indonesia. The extensive TonasaCarbonate Platform was segmented by faulting, in this

area, during the Eocene. These well exposed carbonatedeposits provide an insight into sedimentation prior toand during fault propagation to the surface for a num-ber of reasons: (1) the carbonate producing organismsare extremely sensitive to changes in water depth orenergy, making detailed depositional environmental in-terpretations possible; (2) sedimentation occurred in ashallow to deep marine setting isolated from terrige-nous clastic input, other than that from local faultsources; (3) good three-dimensional exposure througha complex faulted platform margin, in conjunctionwith biostratigraphic dating, allow accurate correlationof sections and recognition of sequences; and (4) rapidlateral and vertical changes in carbonate facies andthickness changes re¯ect environmental changes causedby di�erential subsidence prior to and during surfacefaulting. It is possible to quantify rates of, and vari-

Marine and Petroleum Geology 16 (1999) 825±848

0264-8172/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0264-8172(99 )00019 -7

* Tel.: +44-191-374-2501; fax: +44-191-374-2510.

E-mail address: [email protected] (M.E.J. Wilson).

Fig. 1. Regional tectonic setting of Sulawesi and the location of the Tertiary basinal area in Sulawesi/Borneo (modi®ed after Daly et al., 1991; Hall, 1996; van de Weerd & Armin, 1992). Inset

shows the position of the research area within Sulawesi.

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Fig. 2. Geological map and stratigraphy of South Sulawesi (after Sukamto, 1982; Sukamto & Supriatna, 1982; van Leeuwen, 1981). Time scale after Harland, et al. (1990). The East Indian

Letter Classi®cation (EILC) for the larger benthic foraminifera zonation scheme is after Adams (1970).

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ations in, subsidence for late prerift, latest prerift/ear-liest synrift and synrift sequences. Latest Eocene tomiddle Miocene synrift sedimentation in the Barruarea occurred along a complex, segmented, NW±SEtrending faulted platform margin that has been docu-mented by Wilson and Bosence (1996).

Tropical Cenozoic carbonate production was exten-sive and occurred in a diverse range of environmentsin SE Asia. This region is also tectonically complex,where three major plates and a number of minor plateshave been interacting since the Mesozoic (Fig. 1; Daly,Copper, Wilson, Smith & Hooper, 1991; Hall, 1996).Carbonate platforms developed in a range of tectonicregimes, on microcontinental blocks, around the mar-gins of extensional backarc basins and in fore-arc orintra-arc settings. Their location and development wasoften strongly in¯uenced by tectonic subsidence, activefaulting, antecedent basement structures and topogra-phy. SE Asia is consequently one of the best areas ofthe world to study syntectonic carbonate sedimen-tation. The Eocene to middle Miocene TonasaCarbonate Platform of western South Sulawesi (Figs. 1and 2) was segmented by active normal or transten-sional faults during various periods of its evolutionand in di�erent parts of the platform (Wilson &Bosence, 1996; Wilson, Bosence & Limbong, 2000).This paper concentrates on well-exposed late preriftand early synrift shallow water and slope deposits ofthe northern Barru area of the Tonasa Formation(Fig. 2). Facies models, sedimentary evolution andcontrols on sedimentation of this syntectonic carbon-ate platform were constrained from extensive faciesmapping, logging, petrographic and biostratigraphicwork and more details can be found in Wilson (1995),Wilson and Bosence (1996, 1997) and Wilson et al.(2000).

2. Tectonostratigraphic setting of the Tonasa CarbonatePlatform

Western Sulawesi has been located along the easternmargin of Sundaland, the stable cratonic area ofEurasia, throughout the Cenozoic (Fig. 1). The geo-logical history of this area, is inextricably linked to theaccretion of microcontinental and oceanic materialonto the eastern margin of Sundaland, the resultantdevelopment of volcanic arcs and the formation ofmarginal basins. Western Sulawesi was accreted toSundaland during the late Cretaceous. The formationof a widespread extensional basin centred on theMakassar Straits resulted in the separation of westernSulawesi from mainland SE Asia in the earlyPaleogene (Fig.1; van de Weerd & Armin, 1992,Wilson & Moss, 1998).

South Sulawesi has an almost complete stratigraphic

sequence spanning the late Cretaceous to the presentday, with carbonate and igneous lithologies spanningmuch of the Tertiary (Fig. 2, van Leeuwen, 1981). TheEocene to middle Miocene Tonasa CarbonatePlatform developed on a basement high in westernSouth Sulawesi. The platform was bordered to thewest by the Makassar Straits. A calc-alkaline volcanicarc developed to the east, related to west dipping sub-duction under eastern South Sulawesi (Sukamto, 1982;van Leeuwen, 1981; Yuwono, Maury, Soeria-Atmadja& Bellon, 1987). The location of the basement highwas in¯uenced by variations in the composition of thepre-Tertiary basement and by pre-existing structures.Depending on their orientation, many of these struc-tures were reactivated as normal or transtensionalfaults during the deposition of the Tonasa Formationand resulted in segmentation of the platform. Seismicand structural data, and facies and thickness changesin the upper Cretaceous and lower Paleogene clasticlithologies indicate that many of these structures haveNW±SE or NNW±SSE trends and that some wereactive prior to the deposition of the carbonate succes-sion. The Tonasa Formation transgressively overliesmarginal marine clastics and coals of the MalawaFormation (Fig. 2). The carbonate succession is inturn overlain by volcaniclastic deposits of the CambaFormation. These volcaniclastics were derived from avolcanic arc which developed in western Sulawesiduring the middle to late Miocene (Fig. 2; Sukamto &Supriatna, 1982; Yuwono et al., 1987).

3. The Tonasa Carbonate Platform and syndepositionalfault segmentation

Initial carbonate sedimentation of the TonasaFormation in western South Sulawesi was diachronousand began in the northern Barru and southernJeneponto areas in the early/middle Eocene (Fig. 2;Wilson et al., 2000). Reactivation of pre-Tertiary faultsin the latter part of the late Eocene resulted in theseareas becoming hangingwall depocentres. Di�erentialsubsidence prior to fault break-through and pre-exist-ing topography both probably in¯uenced the localisedinitiation of marine basin formation and carbonateproduction in South Sulawesi. By the earlier part ofthe late Eocene, shallow water carbonate productionhad spread to all other outcrop areas of the TonasaFormation. There was minor interdigitation of theTonasa Formation with underlying, and partially later-ally equivalent formations. The transition to carbonatesedimentation, however, occurs over a few metres andappears to have been rapid.

During the latter part of the late Eocene, faultbreak-through caused segmentation of the TonasaCarbonate Platform and resulted in localised graben

M.E.J. Wilson /Marine and Petroleum Geology 16 (1999) 825±848828

Fig. 3. Geological map of part of the Barru Area with cross-sections. The cross-sections have no vertical exaggeration. The locations of measured

sections illustrated in Fig. 5 are shown. For the measured sections; B, Bangabangae, R, Rala, U, Bulo Bunting, D, Doi-doi, P, Paluda, S,

Salapura, W, Dam Westward/Sadjang, L, Lisu, M, Rumpio, J, Bulo Buntjeng. Measured sections are composite sections along stream cuttings

and tracks.

M.E.J. Wilson /Marine and Petroleum Geology 16 (1999) 825±848 829

Fig. 4. Block diagrams and cross sections showing the evolution of the Barru area during the late prerift (a), the latest prerift to earliest synrift

(b), and the synrift (c).


or half-graben formation and uplift of footwall highs(Wilson et al., 2000). Faulting drastically altered themorphology of the carbonate platform and resulted inthe formation of fault-bounded graben in the northernBarru, western Segeri areas and possibly in theWestern Divide area to the east (Fig. 2; Wilson et al.,2000). Good outcrops in the Barru area allow analysisof the e�ects of di�erential subsidence and tectonic ac-tivity on carbonate facies prior to and during faulting(Fig. 3). Early carbonate lithologies in the Segeri andWestern Divide areas are less well-exposed and it isnot possible to document facies variations during theearly stages of faulting. From the latter part of the lateEocene until the middle Miocene the main TonasaCarbonate Platform in the Pangkajene area had a tilt-block morphology (Wilson et al., 2000). Larger benthicforaminifera dominated the shallow water deposits ofthis platform. Other bioclasts include coralline algae,echinoid fragments, small benthic foraminifera andrare coral debris. At least 600 m of shallow water plat-form deposits in the Pangkajene area were bounded bya steep, but segmented faulted northern platform mar-gin in the Barru area (Fig. 2). Deposits of the gentlydipping southern ramp-type margin crop out in the

Jeneponto area (Fig. 2). Areas of more complex blockfaulting lay to the east (Western Divide MountainsArea) and west (Segeri Area; Wilson et al., 2000).Further phases of faulting a�ected the carbonate plat-form in the early/middle Miocene and possibly theearly/late Oligocene (van Leeuwen, 1981; Wilson &Bosence, 1996; Wilson et al., 2000). This tectonic ac-tivity resulted in the formation of more fault boundedgrabens in the Western Divide area, reactivation ofsome of the pre-existing faults cutting the carbonateplatform and associated footwall uplift and contem-poraneous hangingwall subsidence (Wilson et al.,2000).

4. Geology of the Barru area

The Barru area of this study includes the northern-most outcrops of the Tonasa Formation (Figs. 2 and3). Two inliers of the pre-upper Cretaceous basementcomplex of Sulawesi, the Bantimala and Barru Blocks(Berry & Grady, 1987) crop out in the south and westof the Barru area, respectively (Figs. 2 and 3). Thesebasement complexes are composed of tectonically

Fig. 4 (continued)


Fig. 5. Stratigraphic correlation and facies distribution between the late prerift and latest prerift/earliest synrift sequences the Barru Area. See Fig. 3 for location of sections.

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intersliced schists, ultrabasics, clastics and cherts, andthe overall structure is of relatively rigid fault-blockstilted to the east. The basement blocks are bordered tothe northeast by normal faults, which were activeduring the deposition of the Tonasa Formation. Thesefaults, which were separated by a relay ramp, boundedthe northern margin of the Tonasa CarbonatePlatform during the latest Eocene to middle Miocene,forming a segmented escarpment margin to the maintilt-block platform (Fig. 4; Wilson & Bosence, 1996;Wilson et al., 1999). The carbonate and clastic succes-sions on these faulted highs were eroded and resedi-mented into adjacent hangingwall depocentres to thenorthwest during the deposition of the TonasaFormation. Recent uplift and further erosion hasexposed the basement blocks as seen today (Fig. 3).The eastern or southeastern sides of the basementblocks are bounded by eastward dipping thrusts orsinistral wrench faults (Berry & Grady, 1987;Sukamto, 1982). A number of poorly exposed, butprobably steeply dipping NW±SE or NNW±SSEtrending faults also cut the Tertiary sequence in theBarru area (Fig. 3).

The eastern and southeastern ¯anks of both base-ment blocks are unconformably overlain by an almostcontinuous stratigraphic sequence from the upperCretaceous Balangbaru Formation through to themiddle to upper Miocene Camba Formation (Figs. 2and 3). These formations crop out on the dip slopeof what were the active fault blocks during the latestEocene to middle Miocene. The Tonasa and earlierformations thicken southwards down the dip-slopes,away from the faults bounding the northeastern sidesof the basement blocks.

Between the two basement blocks the Tertiarylithologies are folded into the WSW verging, re-gional-scale NNW±SSE trending Rala anticline (Fig.3; Wilson & Bosence, 1996). This anticline formedduring compression after the deposition of theMiocene Camba Formation. On the northeast limbof the Rala anticline there is a complete, conform-able stratigraphic succession from the MalawaFormation through to the Camba Formation, dip-ping 15±208 towards the northeast. During faultingof the northern margin of the Tonasa CarbonatePlatform, this area comprised the hangingwall depo-centre or lay on the eastern side of the tilted foot-wall highs, hence the conformable succession. Onthe southwestern limb of this anticline Tertiarylithologies dip 20±408 to the WSW and the carbon-ate succession is considerably condensed or absent(Figs. 3 and 5). An angular unconformity separatesthe Camba Formation from the older underlyinglithologies on the WSW limb of the Rala anticline(Figs. 3 and 5). The incomplete carbonate succes-sions and the angular unconformities re¯ect late

Eocene to middle Miocene faulted and tilted foot-wall highs where much of the carbonate successionwas eroded or never deposited.

Diorite/granodiorite and trachyte of probablemiddle to late Miocene age (Sukamto, 1982) intrudethe Tonasa Formation and older lithologies in a num-ber of localities (Fig. 3).

5. Carbonate facies variations and platform evolution inthe Barru area prior to and during faulting

Reconstructions showing the variations in carbonatedepositional environments and palaeogeographic evol-ution of the Barru area, prior to and during faulting,are illustrated on Fig. 4. These reconstructions arebased on detailed facies mapping (Fig. 3) and logging(Fig. 5) throughout the Barru area. The ®eld data wereintegrated with petrographic and biostratigraphic ana-lyses to determine depositional environments of thecarbonate facies (Tables 1 and 2). Three major changesin facies distributions and morphology of the carbon-ate platform occurred in the Barru area (Fig. 5 andTable 2). Distinctive facies associations during threetime periods re¯ect fundamental environmentalchanges in the Barru area related to the early stages offaulting. These are:

1. Late prerift±early/middle Eocene (Ta) to earliestlate Eocene (Tb)ÐA region of moderate to highenergy shoals developed in shallow to moderatedepth within the photic zone (Fig. 4a). These areasof shallow water sedimentation later formed faultedhighs. Intervening areas, which later formed hang-ingwall half-graben, were the sites of lower energysedimentation. A facies association, consisting of arange of shallow water facies charaterises lithologiesof this time period. Facies from the wackestone andgrain-supported facies groups were deposited in lowand moderate to high energy environments, respect-ively (Table 1).

2. Latest prerift/earliest synrift±late Eocene (Tb)ÐAnouter shelf to slope environment deepened intoareas which would later become hangingwall half-graben (Fig. 4b). The appearance of siliciclasticgrains only in areas which became footwall highsjust prior to rapid deepening and deposition ofmarls is inferred to be related to exposure of silici-clastic lithologies during the early stages of fault-ing. Decimetre bedded packages of planktonicforaminifera and (quartzose) bioclastic packstoneswere deposited in slope environments in developinghangingwall depocentres (Tables 1 and 2). Ondeveloping footwall highs, lithologies of the grainsupported facies group were deposited within thephotic zone.


Table 1

Facies descriptions and interpretations of the Tonasa Formation in the Barru area

Facies group andfacies

Lithologiesa Location andoccurrenceb

Bed thicknessc Bed contactsd Lateral continuityFauna Groundmass Structures Environmentalinterpretation

Wackestone faciesgroupMolluscan wacke/packstone facies

MoW, MoP P(r) dms P & S/G ms Mo & GaÐwhole & wellpreserved, Potamididae &Naticacea & oysters. Mi,CA, E & small benthicforam frag & rare

Micrite or marl ± Low energy, muddy,intertidal to subtidalmarginal marineenvironment

Miliolid wackestonefacies

mi bW, mi & GaW, mi, Ga & anW, mi & an W

P (r) dms P/U & G ms Mi, A, Praerhapidionian,Austrotrillina & GaÐabundant. Some lithologieshave pseudomorphs afteranhydrite or gypsum

Micrite Some of groundmassreplaced by dolomiterhombs

Shallow back-barrier orlagoonal area which attimes may have becomehypersaline

Discocyclinawackestone facies

D marl, D W L Rala & BuloBunting sections

cms/dms P/U & G < 2 km D & Oper large, thin ¯atforms v. abundant, PFÐ< 3% & some smallbenthic forams

Micrite or marl Diagenetic calciumcarbonate nodules

Low energy depositionalenvironment towards thebase of the photic zone

Coral marl facies Co Marl PÐBulo Bunting Poorly exposed bed contacts not seen < 4 km Sparse lithi®ed colonialcorals, mi, large & smallbenthic forams

Micrite or marl ± Low energy, shallow partof photic zone

Grain supported faciesgroupMiliolid/alveolinid& coralgal facies

mi/A & Co bP/FI, mi/A, co &CA bP/FI, CoAbP/FI

P (r) cms/dms P & S/G ms Mi, A, Praerhapidionina,AustrotrillinaÐabundant,branching corals. Frag &encrusting CA

Micrite & frag bioclasts ± Low to moderate energy,shallow part of photic zone

Alveolinid/Nummulites facies

A bW/P/G, A &N bW/P/G

P (r) dms/ms P & S ms < 20% alveolinids, whole& frag N, E, Mo, CA, mi& small benthic forams

Frag bioclasts & equantsparry calcite

± Protected back-barriers tomore turbulent back- orfore-barrier areas

Quartzose bioclasticfacies

Q bP/G, Q & NbG, Q, N & DbG, Q & PF bP

LÐDoi-doisection, LÐDoi-doi &Bangabangaesections (r)

dms P/U & S/E ms QuartzÐ15±60%, frag N,CA, E & small benthicforams common


Normal grading and low-angle, planar cross-bedding

Shallow-marine, highenergy shoal with a sourceof siliciclastics

Munnulites/corallinealgal facies

N bP/G, N & CAbP/G/R, CA bP/R

P & L (c) ms/dms P & S 10 s m N & CA abundant. Othercommon bioclasts includeE, D, Mo, encrusting &small benthic forams


ÿ Change from shallow/moderate depths in photiczone to lower energydeeper waters in photiczone

Discocyclina pack/grainstone facies

D bP/G, D & PbP/G, D, P & NbP/G, P bP/G

P & L in S ms/dms P & S ms Abundant Disco (15±25%),Pellat (5±20%), Numm, E& CA. Also Biplan, Astero,& small benthic forams


Normal-grading, low angleplanar & trough cross-bedding

Rather turbulent marineconditions in the shallowerparts of the photic zone

Bioclastic facies bP, bG, GI bP/G L & M (c), DamWestward section(r)

cms/dms P/U & S 10 s m/ms Abundant whole & fragbioclastsÐE, CA, large &small benthic forams &glauconite in some beds


Some beds ®ne upwards Moderate energy, normalmarine environment

Planktonicforaminifera faciesgroupPlanktonicforaminiferabioclastic packstonefacies

L & S (c) cms/dms P & S ms/kms < 20% whole planktonicforaminifera, frag CA,LBF, E & SBF

Micrite Rare ripple lamination and®ning upwards from m. tof. sand

Lower shelf/slope depositswith an open oceanicin¯uence

Marl facies SÐc in N & E cms/dms P/U & S/G ms/dms Abundant wholeplanktonic foraminiferaand nannofossils, rarebenthic bioclasts

Marl Planar mm lamination,often homogenized bybioturbation

Low energy deep marinebasinal setting

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3. Synrift±late Eocene (Tb) to early/middle Miocene

(NN2±6)ÐTwo major NW±SE trending, syndepo-

sitional normal faults separated by an east dipping

relay ramp developed in the Barru area and

formed the northern escarpment margin to the

Tonasa Carbonate Platform (Fig. 4c). Deposition

of thin shallow water successions, considerable

erosion and/or non deposition occurred on faulted

footwall highs. In contrast, in hangingwall areas,

thick successions of lithologies exclusively from

the planktonic foraminifera group facies were

deposited. Texturally and compositionally imma-

ture redeposited carbonate units, derived from

faulted footwall highs, are interbedded with basi-

nal marls (Fig. 5 and Tables 1 and 2). Detailed

analysis of this redeposited material has revealed

that at least two phases of faulting a�ected the

platform margin. These occurred during the late

Eocene to early Oligocene and early to middle

Miocene. The early/late Oligocene may also have

been a period of faulting on the platform margin

(Wilson & Bosence, 1996). A detailed description

of these deposits and the rationale for the recon-

struction and history of the faulted margin is

given in Wilson and Bosence (1996) and is not

repeated here. Evidence for the syndepositional

faulted margin comes from a combination of

facies and structural ®eld mapping, juxtaposition

of di�erent lithologies across the faults and proxi-

mal to distal changes, palaeocurrent and prove-

nance data from the redeposited units. In

addition, data from in situ platform top and

reworked shallow water deposits indicates contem-

poraneous shallow water carbonate production on,

and localised subaerial exposure of, the main plat-

form area to the south (Wilson & Bosence, 1996,

1997; Wilson et al., 2000).

Fig. 6 summarises the outcrop locations, lateral

thickness changes, and vertical and lateral trends

within these three sequences. Throughout the evol-

ution of the Barru area the main local controls on

carbonate facies development and distribution were

water depth and current strength, types of carbonate

producers, and during the latter part of the late

Eocene to middle Miocene, active faulting. Water

depth, depositional energy, and therefore indirectly

types of carbonate producers, in this area were inex-

tricably linked to accommodation space. Di�erential

tectonic subsidence and uplift, prior to and during

faulting, ultimately controlled accommodation space,

caused the progressive drowning of the northern part

of the Barru area, and resulted in morphological

changes to the platform and spatial and temporal

facies variations.Table

1(continued

)

Facies

groupand

facies

Lithologiesa

Locationand

occurrence

bBed

thicknessc

Bed

contactsd

LateralcontinuityFauna

Groundmass

Structures

Environmental

interpretation

Graded

bioclastic

pack/grainstonefacies

SÐ

cin

N&

Ecm

s/dms

P/U

&S/G

10sm

<25%

whole

PF,frag

CA,LBF,E,SBF&

rare

Co.Lithic

carb.,marl,

meta.,&

clastics

Marl,micrite,frag

bioclastsorequantsparry

calcite

Someerosivebases,scours

&¯utesFU

p/csto

fs/silt.

Rare

waterescape&

clast

imbrication.Planarmm

laminationtopofbed

Redepositedcalciturbidites

depositedonopen

marine

slope/basin

Clast-supported

breccia

facies

SÐ

cin

N&

E70cm

±25m

P&

S/G

10sm/100

sm

Angularlithic

carb.,marl,

meta.,&

clastic

clastsupto

4m

across.Some

fragmentedbioclastsand

rare

PF

Marl,micrite,frag

bioclastsorequantsparry

calcite

OccassionalCU

atbase

and/orFU

throughunit.

Rare

clast

imbrication.

Silicanodule

replacement

Probable

highdensity

turbidity¯ow.Deposited

onopen

marineslope/basin

Marl-supported

breccia

facies

SÐ

rupto

2m

U&

Sms

Sub-angularlithic

shallow

watercarb.,marland

redopositedcarb.clastsup

to1m

Marl

1chanellizedunit.Rare

clast

imbrication

Debris¯ow.Depositedon

open

marineslope/basin

Planktonic

foraminiferawacke/packstone

facies

SÐ

inN

&E

cms/dms

P&

S/G

ms

70±90%

whole

planktonic

foraminifera,somefrag

shallow

waterbioclasts

Micrite

ormarl

Some®ningupwardsfrom

c/msto

silt.Silicanodule

replacement

Distalredeposited

calciturbidites?deposited

onopen

marineslope/basin

aTexture:M,Mudstone;

W,Wackestone;

P,Packstone;

G,Grainstone;

Fl,

Floatstone;

R,Rudstone;

Bi,

Bindstone;

brec,

brecciated;m,micrite;b,bioclastic.Bioclasts:

mi,

miliolid;A,

Alveolinid;If,Im

perforate

foraminifera;Co,Coral;L,Lepidocyclina;H,Heterostegina;S,Spiroclypeus;

CoA,Coralgal;CA,Corallinealgae;

li,lithic;N,Nummulites;

D,Discocyclina;Mo,

Mollusc;E,Echinoid;F,Foraminifera;Ha,Halimeda;Ga,Gastropod;Cyclo,Cycloclypeus;

Pellat,

Pellatispira;Oper,Operculina;Frag,fragmented;PF,Planktonic

foraminifera;LBF,Large

benthic

foraminifera(undi�erentiated),GI,Glauconite.

bLocation&

occurrence:N,North;C,Central;S,South;P,late

prerift;L,latest

preriftto

earliest

synrift;S,synrift;c,

commonr,rare;v,very.

cBed

thickness:cm

,centimetre;dm,decim

etre;m,metre.

dBed

contacts:P,Planar;S,Sharp;G,Gradational;U,undulose;E,Erosive.


Table 2

Summary of variations in the late prerift, latest prerift to synrift and synrift sequences in the Barru area

Late prerift sequence Latest prerift/earliest synrift sequence Synrift sequence

Age Early/middle Eocene (Ta) to earliest late

Eocene (Tb)

Middle late Eocene (Tb) Latest late Eocene (Tb) to early/middle Miocene

(NN2±6)

Facies association Lithologies from the wackestone and grain-

supported facies groups

Bioclastic packstones and pack/grainstones

from the grain-supported and planktonic

foraminifera facies groups

Marls and redeposited facies from the

planktonic foraminifera facies group. N.B.

Footwall deposits from the grain supported

facies group

Summary of depositional environments Moderate to high energy shoals in areas that

became footwall highs, and lower energy

deposits in intervening areas that became

hangingwall depocentres. Depositional

environments for lithologies deposited in

areas that became hangingwall depocentres

progressively deepen up section. All

deposition occurred in the photic zone

Moderate depositional energy and moderate

depth within the photic zone for areas that

became footwall highs. In areas that became

hangingwall depocentres, low energy slope

deposits from below the photic zone were

deposited. Depositional environments for

lithologies deposited in areas that became

hangingwall depocentres progressively

deepen up section

Areas of shallow water deposition, subaerial

exposure and/or erosion occurred on faulted

footwall highs. Bathyal marls and coarse,

immature redeposited carbonates were deposited

in hangingwall graben. The coarse carbonates

units were derived from faulted footwall highs

and resedimented into adjacent hangingwall

depocentres

Thicknesses and depositional environments

on footwall highs or areas that developed

into footwall highs. N.B.Bulo BuntjengÐ

208 m. All shallow water, moderate energy

deposits. Di�cult to correlate with prerift

and synrift successions. All late Eocene (Tb)

Paluda/Doi-doi sectionÐ180 m. Bangabangae

sectionÐ40±50 m. High to moderate energy

shallow water larger benthic foraminiferal

shoal. Fluctuations in energy perhaps related

to storms

Paluda/Doi-doi sectionÐ18±22 m.

Bangabangae sectionÐ11 m. Moderate

energy shallow water shelf. Localised in¯ux

of clastic grains related to exposure of

earlier formations during uplift or formation

of fault scarps during ®nal stages of fault

propagation to surface

Most footwall highs subaerially exposed or thin

cover of shallow water carbonates removed by

erosion. Paluda/Doi-doi sectionÐ210 m. Very

coarse proximal redeposited facies,

accommodation space probably provided by

collapse of faulted margin and plunge of whole

northern faulted margin of the Tonasa

Carbonate Platform to the east. Bangabangae

sectionÐ410 m. Bathyal marls interbedded with

distal redeposited carbonates. Accommodation

space because this area was downthrown

relative to the Bantimala Block and the plunge

of platform margin to the east


in hangingwall depocentres or areas that

developed into hangingwall depocentres

Rala sectionÐ260±375 m. Moderate to low

energy foraminiferal shoal deposits overlain

by low energy Discocyclina wackestones.

Overall deepening up section from

deposition in shallower to deeper part of

photic zone. Short term ¯uctuations in

energy during early deposition of succession

perhaps related to storms

Rala sectionÐ95 m. Dam Sadjang and Dam

WestwardÐ87 m. Bioclastic packstones and

pack/grainstones deposited under an outer

shelf/upper slope setting in lower part of

succession. Fluctuations in energy perhaps

related to storms. Planktonic foraminifera

bioclastic packstones in upper part of

sections indicate an up-sequence increase in

depositional depth

Rala sectionÐ635 m. Bathyal marls interbedded

with coarse, proximal redeposited carbonates

derived from adjacent faulted high. Thickest

section in hangingwall depocentre


on dip-slope of tilted fault-blocks or areas

that developed into dip-slopes

Salapura sectionÐ270±290 m thins

northwards to 180 m at Doi-doi (4 km) 260±

375 m (Rala) thins northwards to 40±50 m

(Bangabangae) over 10 km. Back-barrier or

shoal deposits, thinning to north up what

will become the dip-slope of the tilted fault-

blocks

Salapura sectionÐ26 m. Moderate energy,

moderate depth in the photic zone

depositional environment. Slight thinning of

succession to north towards Doi-doi and

decrease in inferred depositional water

depth. Similar scenario between

Bangabangae and Rala, but depositional

environment below photic zone

Salapura sectionÐ60 m. Coarse redeposited

carbonates interbedded with upper slope

planktonic foraminifera bioclastic wacke/

packstones. Thicker in Doi-doi area because of

inferred collapse of faulted margin

M.E.J.Wilso

n/Marin

eandPetro

leum

Geology16(1999)825±848

836

Fig. 6. Simpli®ed geologic map and stratigraphic correlation of the late prerift, latest prerift/earliest synrift and synrift sequences in the Barru

area.


Plate 1


6. Late prerift successionÐearly/middle and late Eoceneinitiation of carbonate production and development ofshallow water shoals

6.1. Stage 1: Initial transition from marginal marineclastics to carbonate deposits

The lower Paleogene Malawa Formation in theBarru area is composed of poorly-exposed interbeddedcoals, claystones, quartzose sandstones and thin lime-stone beds. This formation comprises a transgressivesequence from terrestrial to marginal-marine deposits,with marginal-marine and fully marine claystones andlimestones becoming more abundant towards the topof the formation. The contact with the overlying moremassive carbonates of the Tonasa Formation is a con-formable one. Rare intercalations of quartzose sand-stones in the basal few metres of the carbonatesuccession and thin limestone beds in the upper part ofthe Malawa Formation indicate minor interdigitationof the formations. The basal part of the carbonate suc-cession in most of the Barru area is early/middleEocene in age, based on di�erent species of Fasciolitessp. and stratigraphic correlation. The exception to thisis the isolated Bulo Buntjeng section, which was depos-ited on the northern margin of the main tiltblock plat-form and is late Eocene throughout (Figs. 3 and 5; Tb,based on the occurrence of Pellatispira ).

Molluscan wacke/packstone facies commonly formthe claystone and limestone units in the top of theMalawa Formation and base of the Tonasa Formation(Plate 1a). These contain abundant gastropods, domi-nantly Potamididae and Naticacea, oysters, miliolids,some ¯attened disc-shaped solitary corals, such asCyclolites, wood, leaves and other disseminated plantmatter. A low energy, shallow, muddy marginal-mar-ine to marine depositional environment is inferred forthis facies.

Localised restriction and ¯uctuations in salinityduring early marine basin formation are suggestedfrom beds of imperforate foraminifera wackestonefacies just in the basal tens of metres of the Rala andDoi-doi sections (Fig. 5). Lithologies of this facies con-

tain 50±70% imperforate foraminifera; dominantlymiliolids, and less abundantly alveolinids,Praerhapidionina and Austrotrillina, up to 20% gastro-pods (Plate 1b) and rare crystal pseudomorphs afteranhydrite or gypsum. With the exception of rarerestricted environments, initial carbonate productionoccurred in a fully marine setting as indicated byabundant stenohaline biota, such as larger perforatebenthic foraminifera, in bioclastic pack/grainstonesand coralgal facies. A rapid transition from marginalmarine to fully marine conditions is inferred for theBarru area.

6.2. Stage 2: Di�erentiation into higher energy shoalsand lower energy intershoal areas

As marine sedimentation continued through theearly/middle to earliest late Eocene, carbonate deposi-tional environments varied laterally over quite shortdistances and through time in the Barru Area.Moderate to high energy foraminiferal shoals devel-oped on areas which would become faulted highs inthe later part of the Eocene (Fig. 4). In interveningareas, which would later form hangingwall depocen-tres, carbonate sedimentation occurred under lowerenergy conditions. Deposition of the late prerift succes-sion occurred within the photic zone in a range ofwater depths as evidenced by abundant well-preservedlarger benthic foraminifera, coralline algae and rarecorals.

Stratigraphic thicknesses of sections together withup-sequence and lateral facies changes vary from northto south across the Barru area (Figs. 5 and 6). Thesechanges re¯ect variations in rates of subsidence andlocal depositional conditions which are describedbelow. Due to a lack of outcrops of similar age in aneast-west direction, it is not possible to ascertain thelateral continuity of facies belts outside the BarruArea.

Plate 1. (a) Outcrop photograph of a molluscan bioclastic packstone (SUM4) from the Salo Umpung section, containing gastropods (g) and

bivalves (b). Scale bar is 4 cm. (b) Thin section photomicrograph of a miliolid (m) and gastropod (g) wackestone (SU25) from the Salo Umpung

section, under plane polarised light. Some dolomitization (d) of the gastropod shell (g) and calcite vein in®ll has occurred. Scale bar is 1 mm. (c)

Thin section photomicrograph of a Nummulites sp. and coralline algae packstone (SU57) from the Rala section. Contains laminar Sporolithon sp.

rhodoliths (s) and small robust Nummulites sp. (N). Scale bar is 1 mm. (d) Thin section photomicrograph of a Nummulites sp. and coralline algae

packstone (SU36) from the Rala section. Includes large, ¯at, unencrusted Nummulites sp. (l) and branching rhodoliths (b). Scale bar is 1 mm. (e)

Field photograph of a Discocyclina marl/wackestone (RR1) from the River Rala section. Large ¯at Discocylina and Operculina occur in a marly

matrix. Scale bar is in centimetres. (f) Thin section photomicrograph of Nummulites grainstone (DD46). Scale bar is 1 mm. (g) Thin section

photomicrograph of a planktonic foraminifera bioclastic packstone (BBP9) from the Bangabangae section, under plane polarised light. Bioclasts

include well-preserved planktonic foraminifera (p), small benthic foraminifera (b), coralline algal fragments (a) and echinoid plates (e). Scale bar

is 500 mm. (h) Thin section photomicrograph of a quartzose planktonic foraminifera bioclastic packstone (SP13) from the Bangabangae section,

under plane polarised light. Bioclasts include well-preserved planktonic foraminifera (p), fragmented bioclasts and quartz grains (q). Scale bar is

500 mm.


6.3. Tectonically controlled lateral facies variations

6.3.1. Bangabangae to RumpioÐModerate energy shoaldeposits with evidence for deepening upward trends insome sections

In the northern part of the Barru area, betweenBangabangae and Rumpio, sequences of Nummulites/coralline algal facies are interpreted as having beendeposited in shoal-type environments. The thickness ofthese facies progressively thickens southwards from30 m in the Bangabangae section to 85 m in the Ralasection, and again from 30 m in the Lisu section to55 m in the Rumpio section (Fig. 5 and Table 2).These thickness changes re¯ect variations in waterdepth and accommodation space across the area.Thinner successions occurred on areas which wouldbecome highs during faulting, whereas the thicker sec-tions accumulated in areas which would later form thedip-slope or hangingwall depocentres. Rare cross-bedded units suggest that, in the shoal areas, palaeo-current directions were mainly towards the east. Thethinner successions of the Nummulites/coralline algaefacies in the Bangabangae, Lisu and Rumpio sectionswere all deposited under moderate energy conditionsin shallow to moderate depths in the photic zone(Table 1). There is no evidence for overall up-sequencechanges in depositional depth for these sections.However, metre-scale grainstone beds which containabundant robust larger benthic foraminifera and frag-mented, abraded bioclasts are occasionally interbeddedwithin the sections. These suggest that periods ofincreased energy, perhaps related to storms, a�ecteddeposition in these shoal areas.

In the Rala section, periods of increased current ac-tivity are also thought to have caused increased frag-mentation and abrasion of large bioclasts and to haveresulted in a number of small-scale ®ning-upwardpackages within the Nummulites/coralline algae facies.However, in the Rala section these short term pertur-bations in depositional energy were superimposed on asection which shows a general up-sequence deepeningtrend. The base of the section was deposited underhigh energy, shallow to moderate depth conditions.Up-sequence there is a progressive change to litholo-gies deposited in lower energy, deeper water parts ofthe photic zone. The inferred up-sequence deepening isbased on a range of textural and biotic evidence.There is a general trend from grainstone and pack/grainstone facies in the lower part of the sequence intopackstone facies in the upper part of the sequence indi-cating an overall decrease in energy. SmallNummulites, less than 1 cm across (mostly A forms),are present throughout much of the section, with theexception of the uppermost beds, and become increas-ingly less robust up-section (Plate 1c). Large, ¯attenedNummulites, greater than 1 cm across, not present in

the lowermost beds in the section, occur as fragmentsor are encrusted by coralline algae in the lower part ofthe section. In contrast, they occur as whole, unen-crusted, well preserved thin forms in the upper part ofthe section (Plate 1d). These changes in the types, pres-ervation and morphologies of the Nummulites up sec-tion are inferred to relate to increasing depositionaldepths within the photic zone and a correspondingdecrease in depositional energy. The progressivechange up-sequence from laminar to leafy corallinealgae encrusting the foraminifera, and then intobranching coralline algae (Plate 1d) is also inferred tobe due to a decrease in the depositional energy aswater depth increased (cf Bosence, 1983). The assem-blage of Lithoporella or Mastophora, Lithophyllum orTitanoderma in the middle part of the successionsuggests water depths in the middle of the photic zone,whereas the additional occurrence of Sporolithon (Plate1c) may indicate shallower water depths in the basalpart of the section (cf Adey, 1979; Minnery, Rezak &Bright, 1985).

6.3.2. Bulo Sadjang to Bulo BuntingÐLow energydepositional area

Between Bulo Sadjang and Bulo Bunting, in thearea which would later become a hangingwall depocen-tre, low energy coral marl facies and Discocyclina marlfacies (Plate 1e) are lateral equivalents to theNummulites/coralline algal facies. The coral marl faciesis poorly exposed and occurs only in the southern andeastern parts of this localised area (Fig. 3). This faciesconsists of sparse, well preserved, lithi®ed, unattachedcolonial corals (<40 cm across), miliolids and bothlarge and small benthic foraminifera in an unlithi®edmarly or muddy matrix. Deposition under low energyconditions, with a minor siliciclastic input is inferred.The corals all show adaptations for dwelling in softsediment (Wilson & Rosen, 1998), and together withthe miliolids and robust Nummulites sp., indicate depo-sition in the shallower part of the photic zone.

In contrast, the Discocyclina marl facies (Plate 1e),deposited in the vicinity of Rala village and to thenorth of Bulo Bunting (Fig. 3), is interpreted as thedeeper water lateral equivalent to the coral marl facies.This facies is composed of large, very thin, often extre-mely abundant Discocyclina and Operculina and lesscommonly small benthic foraminifera in a poorly con-solidated marly or muddy matrix. The large, ¯attened,thin forms of the larger benthic foraminifera species,their preservation in a muddy/marly matrix and thelack of other common shallow marine bioclasts is in-dicative of deposition towards the base of the photiczone under low energy conditions (Ghose, 1977;Hallock & Glenn, 1986). In the Rala section,Discocyclina marl facies occur up-section from theNummulites/coralline algal facies, whereas in the Bulo


Bunting section Discocyclina marls overlie lithologiesof coral marl (Fig. 5). In both sections this indicatesan up section increase in depositional water depth.

6.3.3. Doi-doiÐShallow water, high to low energy shoaldeposit

In the Doi-doi section a variety of shallow-waterfacies deposited under low to high depositional energyoccur interbedded with each other. Nummulites bio-clastic pack/grainstones and bioclastic pack/grain-stones facies contain abundant and often abraded,robust forms of Nummulites sp., Discocyclina sp. andother shallow marine bioclasts (Plate 1f). These litholo-gies were deposited under moderate to high energyconditions in an inferred shallow marine shoal or barenvironment (Fig. 4a). Miliolid bioclastic wacke/pack-stones and coral and miliolid bioclastic ¯oatstonesinterbedded within the Doi-doi section suggest thatsheltered lower energy conditions occurred in someparts of the inferred shoal/bar environment. In thelower and middle part of the shallow water sequencesome of the facies contain siliciclastic grains.Palaeocurrent indicators within these facies indicatetransport directions towards the west. This is oppositeto the dominant transport direction on the main tilt-block platform in the Pangkajene area to the south(Wilson & Bosence, 1997; Wilson et al., 1999), andmay represent a local ¯ow direction, perhaps a�ectedby the location of shoals. These facts suggest that sili-ciclastic lithologies were exposed to the east of theDoi-doi section during the early depositional history ofthis sequence. All the lithologies in the basal 180 m ofthe Doi-doi section were deposited in the shallowerpart of the photic zone and di�erences in lithologiesare related to variations in energy rather than waterdepth.

6.3.4. Paluda to SalapuraÐModerate energy shoal/backshoal deposits

In Paluda and Salapura moderate energy and shal-low to moderate depositional depths within the photiczone are inferred from thick sequences of alveolinid/Nummulites facies. These lithologies contain up to20% alveolinids, whole and fragmented robustNummulites sp., echinoid plates, fragmented corallinealgae and shell debris. Primary wacke-, pack- or grain-stone textures all occur within this facies, indicatingdeposition under a range of energy conditions.Protected back-barrier areas to more turbulent back-barrier or shoal areas are the inferred range of deposi-tional settings for this area. A similar environment isinferred to have occurred to the southwest of Doi-doi,since alveolinid/Nummulites facies also occur on thewestern limb of the Rala Anticline. These sections donot contain siliciclastic grains. The alveolinid/Nummulites lithologies are overlain by bioclastic pack-

stones and Nummulites/coralline algal facies in theSalapura section and Nummulites/coralline algal faciesin the Paluda section (Fig. 5). Thinner ¯atterNummulites in the Nummulites/coralline algal faciesand biotic and lithological changes in both sectionssuggest minor up section deepening of the depositionalenvironment. The lower/middle and earliest upperEocene lithologies which formed in a shoal or backshoal environment thicken towards the south from180 m in the Paluda section to 260 m two and a halfkilometres to the south in the Salapura section (Fig.5).

6.3.5. Bulo BuntjengÐModerate energy shallow waterhigh

Carbonates in the Bulo Buntjeng section whichformed the northern margin of the main tilt-blockplatform are all shallow water deposits of late Eoceneage (Figs. 3 and 5). Lithologies throughout this sectionwere deposited in moderate depths within the photiczone. Deposition prior to and during the early stagesof faulting is inferred for this section with little up-sequence change in the depositional environmentduring the late Eocene. However, accurate correlationwith other sections in the Barru area is not possiblebecause the Bulo Buntjeng section crops out as an iso-lated exposure.

6.4. Summary and controls on the late prerift carbonatesuccession

Carbonate sedimentation in the Barru Area beganas a series of moderate to high energy shoals withintervening low energy areas (Fig. 4). Antecedent topo-graphy, current directions and nature of the substratemay have all in¯uenced the original location of shoalareas. Once developed, shoals probably helped to pro-tect intervening areas from the e�ects of storms or cur-rents. However, only the Doi-doi section showsevidence for very high energy depositional conditionsand most sedimentation in the Barru area occurredunder moderate to low energy conditions.

Di�erential subsidence strongly in¯uenced carbonatedevelopment in the Barru area as evidenced by up-sec-tion and lateral facies changes and signi®cant vari-ations in thickness of the late prerift sequence laterallyover short distances (Fig. 6). Complete sectionsthrough the Tonasa Formation, without any evidencefor major breaks in sedimentation or periods or ero-sion, occur on the eastern ¯anks of the basementblocks. Meaningful comparisons of sequence thick-nesses can be made in this area (Figs. 3±6). The great-est thickness of the late prerift sequence occurs in theRala section (260±375 m) followed by the Salapurasection (270±290 m). The area of the Rala sectionduring active faulting in the later part of the late


Eocene became the deepest part of a hangingwalldepocentre where over 600 m of marls and redepositedfacies accumulated (Figs. 5 and 6). The Salapura sec-tion formed in an area which during active faultingwould later comprise the hangingwall dip slope of atilted fault-block. Northwards from the Salapura andRala sections, the thickness of the late prerift sequencethins progressively into areas which would laterbecome the footwall highs of tilted fault blocks. Northof Salapura (270±290 m), the sequence thins succes-sively to about 180 m at Paluda and Doi-doi over adistance of about 4 km (Fig. 5 and Table 2). The lateprerift sequence at Bangabangae is just 40±50 m thickcompared with 260±375 m in the Rala section 10 kmto the south. To the west of the Rala Anticline a simi-lar thinning of the succession occurs on what wouldlater become the dip slope of the tilted fault block ofthe Barru Block. The thickness of lower/middle to ear-liest upper Eocene carbonates in the Rumpio section is109 m whereas those in the Lisu section, 2.5 km to thesouth, total 60 m (Fig. 6). However, erosion duringlatest late Eocene to middle Miocene faulting mayhave reduced the thickness of both these sections.

In the thickest late prerift sections (Rala andSalapura), rates of subsidence outpaced rates of car-bonate accumulation and the successions deepenupwards. In contrast, in laterally equivalent thinnersections there is no evidence for a deepening-upwardtrend and shallow-water carbonates aggrade and keptpace with subsidence (Fig. 6). Increased di�erentialsubsidence therefore occurred in areas which laterbecame the dip-slopes of the tilted fault blocks orhangingwall depocentres, compared with areas whichlater formed faulted highs.

Eustatic sea level changes or autocyclic factors mayhave had a minor in¯uence on carbonate sedimen-tation in the Barru Area. Small-scale ®ning-upwardcycles within the Nummulites/coralline algal facies arethought to result from periods of increased deposi-tional energy. These might be caused by periods ofstorm activity or by lowering of sea level resulting inincreased wave action. However, the major discerniblecontrol on the development of carbonate sedimen-tation in the Barru Area during the late prerift wasdi�erential subsidence.

7. Latest prerift/earliest synrift successionÐupperEocene slope and shelf facies

7.1. Bioclastic packstones and grainstonesÐslope andshelf facies

A variety of packstone and grainstone facies,deposited in shelf and slope environments weredeposited conformably over the late prerift sequence

in the Barru area. These facies are best exposed onthe eastern limb of the Rala Anticline and thickenand then thin towards the north over a distance of15 km (Figs. 4±6). Pack/grainstone facies, some con-taining abundant larger benthic foraminifera, weredeposited in sections south of Rala (Figs. 4 and 5).Laterally equivalent planktonic foraminifera bioclasticpackstones in the northern Rala and Bangabangaesections are interpreted as outer shelf/slope deposits(Plate 1g). These facies changes suggest a moderateto high energy shallow shelf passing northwards intodeeper water slope deposits with an open oceanic in-¯uence (Fig. 6).

The outer shelf and slope deposits of the latestprerift sequence in the Rala and Dam Sadjang/Westward sections are inferred to have been depos-ited under progressively increasing water depth. Thisinterpretation is based on up-sequence changes frombioclastic packstones and grainstones to planktonicforaminifera bioclastic packstones. An increase in thepercentage of planktonic foraminifera and a corre-sponding decrease in the amount of fragmented shal-low water bioclasts also suggest up-sectiondeepening. In addition the packstones ®ne upwardsfrom coarse to ®ne grain sizes in all sections. Small-scale ®ning upward units, perhaps related to stormactivity, and glauconite do not occur in the upperpart of the Dam Sadjang/Westward section. As wellas showing evidence for increasing depositionaldepth up-section, the Rala and Dam Sadjang/Westward sections, at 95 and 87 m, respectively, arealso the thickest latest prerift/earliest synrift succes-sions (Fig. 6).

7.2. Upper Eocene quartzose bioclastic packstonesÐthe®rst signs of faulting?

Upper Eocene quartzose bioclastic packstones (Plate1h) were deposited only in the Doi-doi andBangabangae sections just prior to the deposition ofthe synrift sequence. In both localities, these lithologieshave thicknesses of less than 10 m and are of limitedlateral extent (traceable for a few tens of metres). Theoccurrence of up to 60% ®ne- or coarse-grade, angularsiliciclastic grains (Plate 1h), suggests siliceous litholo-gies from the underlying formations were exposed inthe vicinity of these sections. During active faulting itwas the Doi-doi and Bangabangae sections which wereadjacent to the faults on the footwall crests. The mostlikely explanation for the appearance of siliciclasticgrains in the carbonate succession is that siliciclasticlithologies were exposed at the surface or in faultscarps during uplift and fault propagation to the sur-face.


7.3. Controls on deposition of the latest prerift/earliestsynrift sequence

Patterns in thickness variations, changes in relativedepositional water depth between sections, and evi-dence for up-sequence deepening in some sections forthe latest prerift/earliest synrift succession are similarto those for the late prerift sequence. In both cases thesequences thin over the areas that will become faultedfootwall highs (Doi-doi, Paluda and Bangabangae).Progressive thickening occurs into areas which laterform hangingwall depocentres (Rala) or were locatedon the dip slope of fault blocks (Fig. 6; Salapura). Onareas that become faulted highs, sediments were depos-ited in shallow water settings. Deeper water shelf orslope environments occurred in areas that becamehangingwall depocentres. The change from shallowwater sedimentation during the late prerift to depo-sition below the photic zone during the latest prerift/earliest synrift indicates progressive drowning andincreased di�erential subsidence in areas that becamehangingwall depocentres. Progressive up section dee-pening trends in developing hangingwall areas occur inboth late prerift and latest prerift/earliest synriftsequences indicating increased di�erential subsidencecompared with footwall areas (Fig. 6). The major con-trol on deposition of the latest prerift/earliest synriftsuccession was di�erential subsidence, creating vari-ations in accommodation space across the Barru area.The exposure of underlying formations due to upliftand erosion, or formation of fault scarps, on footwallhighs during the ®nal stages of fault propagation tothe surface resulted in the localised reworking of silici-clastic grains into adjacent areas.

7.4. Change to the synrift sequence

The sharp conformable change from bioclastic pack-stone and grainstones of the latest prerift/earliest syn-rift to marls in all sections north of the Salapurasection indicates a rapid increase in depositional depthin the northern part of the Barru Area. Associatedwith this rapid drowning of shallow water shelf andslope deposits there was a massive in¯ux of redepos-ited carbonate, clastic, metamorphic and igneous clastsand grains into bathyal areas. As stated, a number oflines of evidence indicate that this redeposited materialwas derived from the northern faulted footwall highsof the Bantimala and Barru Blocks and resedimentedinto adjacent hangingwall graben (Wilson & Bosence,1996).

The faulted footwall highs, in comparison, wereeither sites of continued shallow water carbonate pro-duction, and/or areas of subaerial exposure and ero-sion. This is inferred from abundant limestone clastsand shallow water bioclasts redeposited into adjacent

hangingwall depocentres. During the latter part of theCenozoic some of the footwall high successions werecovered by volcaniclastics of the Miocene CambaFormation, or removed during more recent uplift anderosion. The upper Eocene shallow water deposits ofthe Bulo Buntjeng section have a stratigraphic thick-ness of 208 m, show no evidence for breaks in sedi-mentation or major changes in depositionalenvironment (Fig. 5 and Table 2). This section is aremnant of the footwall high succession on the BarruBlock and may span all or part of the late prerift, thelatest prerift/earliest synrift and synrift. The Doi-doi,Paluda and Salapura sections were located on the foot-wall high or on the dip slope of the Bantimala Blockand include a reduced thickness of very coarse rede-posited carbonates. These are inferred to be related tocollapse and tilting of the faulted platform margin,providing extra accommodation space.

8. Summary of the late prerift and early synriftsuccessions

Fig. 4 shows block reconstructions and cross-sec-tions illustrating the evolution of the Barru area justprior to and during fault segmentation. Threesequences have been identi®ed; the late prerift, the lat-est prerift/earliest synrift and the synrift (Figs. 5 and 6,Table 2). Each sequence has characteristic facies as-sociations and sequence thicknesses. Facies varied sys-tematically across the Barru area as shown on Fig. 5and Table 2. Di�erential subsidence, and for the syn-rift successions active faulting, were the overridingcontrol on sequence development and spatial and tem-poral facies variations for this area. Di�erential subsi-dence directly in¯uenced local variations in waterdepth, and hence facies, and available accommodationspace. Variations in subsidence across the Barru areafor the prerift strata, directly mirror those of the syn-rift strata not in amount, but in nature. For example,during active faulting, hangingwall areas accumulatedthe greatest thicknesses of sediment, and it was theseareas which accumulated the greatest thicknesses ofstrata during prerift sedimentation. Progressive up-sequence deepening of the depositional environmentalso occurs in developing hangingwall depocentres. Incomparison, faulted footwall highs accumulated thethinnest prerift and synrift strata or were sites of ero-sion during active faulting. Therefore even beforefaults became active at the surface, di�erential subsi-dence related to movement on pre-existing structuresand/or propagation of faults to the surface was thedominant control on accumulation space and faciesdistribution. Indeed the initial development of shallowwater depositional environments of shoals and lowerenergy intervening areas may have been partly related


to slight variations in antecedent topography associ-ated with earlier faulting. However, it was systematicvariations in subsidence which controlled the sub-sequent evolution of carbonate depositional environ-ments in the Barru area.

The order of magnitude of the di�erent rates of sub-sidence for the three prerift and synrift sequences inthe Barru area are obtainable, and comparisonsbetween relative subsidence rates for individualsequences are possible. However, accurate values forthe variable rates of subsidence across the Barru areaduring the deposition of the three sequences are di�-cult to obtain because the changes from one sequenceto the next occurred within the same biostratigraphiczone: the late Eocene (Tb). During the deposition ofthe late prerift succession, rates of subsidence in areaswhich later formed hangingwall depocentres oroccurred on the dip slopes of tilted fault-blocks werebetween one and a half to six times more than areaswhich later formed footwall highs. For the latest pre-rift/earliest synrift, rates of subsidence for developinghangingwall depocentres were between four to seventimes as high as for footwall areas. In comparison, forthe late Eocene part of the synrift sequence the hang-ingwall depocentre of the Rala area subsided about130 m, whereas adjacent footwalls highs to the southwere uplifted or accumulated thin shallow water suc-cessions. For the synrift sequence, rates of subsidencein hangingwall areas are di�cult to compare with foot-wall areas. This is due to the complex morphology ofthe faulted margin, erosion, non-deposition and col-lapse of faulted footwall highs, and because the wholeof the platform margin in the Barru area is inferred tohave been tilted to the east (Wilson & Bosence, 1996).

Taking into account the relative changes in waterdepth for the di�erent sequences rates of subsidencefor the Rala section have been calculated. The ap-proximate rates are 0.03±0.04 m/ky, 0.3±0.4 m/ky and0.6±0.8 m/ky for the late prerift, the latest prerift/ear-liest synrift and the late Eocene part of the synrift, re-spectively. Therefore for this area which formed ahangingwall depocentre during faulting there is at leastan order of magnitude increase in the rate of subsi-dence between the late prerift and the synrift. Themaximum accumulation rates for the larger benthicforaminifera dominated carbonates of the TonasaFormation have been calculated at between 0.2±0.3 m/ky (Wilson et al., 2000). It is therefore unsurprisingthat carbonates in the Rala section progressively dee-pen up section, during late prerift times and wererapidly drowned during rifting. On areas whichbecame footwall highs, subsidence rates prior to riftingare at least one and a half times less, and often muchless, than areas which later formed hangingwall depo-centres. During rifting, footwall highs were eitheruplifted or underwent limited subsidence depending on

their position relative to other faults and locationwithin the Barru area. Where subsidence of footwallhighs occurred, such as in the Bangabangae section,rates of subsidence were at least half that of adjacenthangingwall areas. In summary, rates of subsidence forareas that became hangingwall depocentres during lateprerift and synrift were always greater than for areasthat became footwall highs. In developing hangingwallareas, subsidence rates increased by at least an orderof magnitude prior to and after fault propagation tothe surface.

9. Comparisons with other rift successions

Documented in the literature are a plethora ofdetailed studies of rift successions (cf Bosence,Nichols, Al-Subbary, Al-Thour & Reeder, 1996;Lambiase, 1995; Purser & Bosence, 1998; Ravnas etal., 1997) and conceptual models of sedimentationduring rifting based on ®eld studies (Gawthorpe et al.,1994; Leeder & Gawthorpe, 1987; Ravnas & Steel,1998). However, in most of these studies late preriftdeposits are either not present, not recognised or notdescribed. In just a few examples, increased di�erentialsubsidence has also been recorded from other areaswhich later became fault bounded grabens (Elmi, 1990;GarcõÂ a MondeÂ jar, 1990; Gawthorpe, 1986; Lemoine etal., 1986; Nolan, 1989; Scott & Govean, 1985; Sinclair,Shannon, Williams, Harker & Moore, 1994). Thisdi�erential subsidence, similar to the Barru area, hasbeen related to the early stages of graben formationprior to normal faults breaking the surface (cfLemoine et al., 1986; Nolan, 1989).

Recent studies in the Gulf of Suez have documented®ne-grained earliest synrift marine clastic deposits thin-ning onto areas that became footwall highs(Gawthorpe, Sharp, Underhill & Gupta, 1997; Khalil,1998). During faulting at the surface, material waseroded from faulted footwall highs and redeposited asa coarse clastic wedge thickening towards the fault inthe adjacent hangingwall depocentre. This geometry ofsequences is similar to the latest prerift/earliest synriftand synrift carbonate sequences in the Barru area (Fig.6). However, blind faults, growth folding and stratathinning towards the developing footwall high havebeen recognised in the Gulf of Suez at the buried tipsof faults, which in their central segments are character-ised by surface faulting (Gawthorpe et al., 1997).Although this probably also occurs in South Sulawesi,the tips of faults are either covered by later synriftstrata or Miocene volcaniclastics. Largely, conformablelatest prerift/earliest synrift and synrift strata occuradjacent to the central portions of faults which havepropagated to the surface in the Gulf of Suez, as inthe Barru area. In contrast, above the buried tips of


faults in the Gulf of Suez, relatively low rates of subsi-dence, together with bed rotation led to enhancementof marine and subaerial exposure surfaces associatedwith relative sea-level fall (Gawthorpe et al., 1997).

Well exposed carbonates in the Barru area provide arare insight into latest prerift/earliest synrift processesand one of the best documented examples of facieschanges related to di�erential subsidence prior to andduring fault propagation to the surface. There are anumber of reasons for deposition and preservation ofthese sequences in the Barru area compared with otherrift settings where early synrift deposits are rare:

. Many rift basins develop in areas which are initiallycontinental. Erosion and non-deposition oftencharacterise these areas and the preservation of pre-rift strata is rare. As extension progresses and faultspropagate through to the surface, lacustrine, mar-ginal marine and fully marine deposits often ac-cumulate in half-graben and sometimes later onfootwall highs. In comparison, rifting took place ina marine setting in the Barru area.

. Rift successions are by their very nature highly seg-mented. This makes correlation of strata, and hencerecognition of vertical and lateral facies changes dif-®cult. This is particularly true for clastic depositswhich often lack good biostratigraphic data. Goodthree dimensional outcrops and biostratigraphic dat-ing in the Barru area enabled correlation betweensections.

. Late prerift strata and early synrift strata are oftenpoorly preserved and/or exposed compared withdeposits in the Barru area. Initial deposits in areaswhich develop into graben are commonly buriedunder thick piles of synrift strata, whereas sedimentsdeposited on developing faulted footwall highs areoften removed by erosion.

. Slight variations in thickness of the prerift strataand recognition of related stratal packages may bedi�cult to identify on seismic lines due to fault seg-mentation, velocity variations due to rapid lateraland vertical facies changes and the small thicknessesof the sequences involved. Good three dimensionalexposure in the Barru area allowed accurate corre-lation and thickness determinations of sequences.

. Extension rates depend on the tectonic regime andwill be at a maximum where rifting is perpendicularto extension direction and is associated with sub-sequent oceanic spreading. The Tonasa CarbonatePlatform developed behind a volcanic arc associatedwith oblique subduction. The development of theNW±SE trending faulted platform margin in theBarru area was related to reactivation of pre-existingstructures in the basement, which were not perpen-dicular to the extension direction. Although oceanicspreading may have occurred in the North

Makassar Straits, there is no evidence to suggestthat it propagated into the South Makassar Straits.All these factors are inferred to have resulted in lowstretching factors (total Beta of 1.01; Wilson et al.,2000) and low extension rates. This probablyresulted in slow propagation of faults to the surfaceand development of good late prerift strata in theBarru area compared with other rift settings.

The odds are stacked against the development, preser-vation and recognition of a late prerift succession fromthe outset. This is particularly true for rifting whichdeveloped in a continental or marginal marine areawhere clastics dominate deposition. The question thenarises whether facies variations related to di�erentialsubsidence have been overlooked in other carbonatesuccessions segmented by faulting.

Bosence, Cross and Hardy (1998), simulated carbon-ate sequence development of platforms developed onfaulted highs using forward computer modeling.Where rates of carbonate production were greater thanrates of fault movement, faults did not propagatethrough to the surface, sequences thickened into areasof increased accommodation space in developing hang-ingwall areas and sequence boundaries were conform-able on the scale of the model. Although Bosence etal. (1996) gave no ®eld examples, this scenario is ana-logous in many ways to carbonate sequence develop-ment in the Barru area during late prerift. In thecomputer model, deepening of the depositional en-vironment was not observed in the hangingwall area.This di�erence arose due to high carbonate productionrates used in the model; maximum of 4 m/ky between1±10 m water depth, compared with 0.2±0.3 m/ky(Wilson et al., 2000) for the foraminiferal carbonatesof the Tonasa Formation.

Some of the best documented synrift carbonate plat-forms developed on faulted highs after initial rifting(Burchette, 1988; Cross, Purser & Bosence, 1998;Purser et al., 1998), as in the conceptual models ofLeeder and Gawthorpe (1987). Consequently, late pre-rift successions did not form. Many marine areasa�ected by rifting are now deeply buried in the centreof rift basins, making any variations in the preriftdeposits di�cult to study. However, marine succes-sions segmented by normal faulting which show thee�ects of di�erential subsidence prior to fault break-through have been described from a number of areas.These include the Carboniferous of the Dublin Basin,Ireland (Nolan, 1989) and the Bowland Basin, NWEngland (Gawthorpe, 1986), the Triassic/Jurassic ofthe western Alps (Elmi, 1990; Lemoine et al., 1986)and the northern Atlantic borderland (Sinclair et al.,1994). Carboniferous, upper Courceyan to upperChadian deposits in the Dublin Basin are some of thebest documented late prerift and earliest synrift succes-


sions. These were deposited over a period of 4±5 Mabefore normal faults were active at the surface (Nolan,1989). Greater thicknesses of late prerift successions,up to 1200 m, occur in areas which later developedinto hangingwall depocentres compared with 300 m orless in areas which later formed footwall highs.Sediments deposited in developing hangingwall depo-centres show evidence for deepening upward of thedepositional environment. In comparison, gradualshallowing upwards and increasing terrigenous inputinto latest prerift to earliest synrift sediments on areaswhich developed as footwall highs may re¯ect initialtectonic movements, culminating in the formation offault-bounded tilt-block platforms (Nolan, 1989). Thispattern of facies and thickness variations related todi�erential subsidence during growth faulting, butprior to active surface faulting is directly comparablewith the Barru area.

10. Conclusions

An exceptional latest prerift/earliest synrift succes-sion occurs in the Barru area of South Sulawesi,Indonesia, because of the marine setting prior to rift-ing. Pre-existing basement structures, oriented obliqueto the extensional direction are inferred to haveresulted in low rates of fault propagation to the sur-face. Carbonate producers are extremely sensitive indi-cators of depositional water depth and energy, hencerapid lateral and vertical Eocene facies variations inthe Barru area provide a unique and quanti®ableinsight into environmental changes during and prior toactive faulting. Detailed mapping, logging and in-terpretation of temporal and spatial carbonate facieschanges has allowed the recognition of threesequences:

1. Late prerift sequence±early/middle Eocene (Ta) toearliest late Eocene (Tb)ÐA region of moderate tohigh energy shoals, deposited in shallow to moder-ate depths within the photic zone, developed onareas which later became footwall highs.Intervening areas were the sites of lower energysedimentation.

2. Latest prerift/earliest synrift sequence±middle lateEocene (Tb)ÐDeposition on areas developing intofootwall highs occurred as shoal and shelf deposits.Lateral deepening to upper slope environmentsoccurred in developing hangingwall half-graben.The localised occurrence of siliciclastic grains inareas of developing footwall highs in the uppermost10 m of the sequence is inferred to be related to ex-posure of siliciclastic lithologies during the earlystages of faulting.

3. Synrift sequence±late Eocene (Tb) to early/middle

Miocene (NN2±6)ÐTwo major NW±SE trending,syndepositional normal faults, separated by an eastdipping relay ramp, bounded the northern marginof the Tonasa Carbonate Platform in the BarruArea. Faulted footwall highs were sites of thin ac-cumulations of shallow water carbonates, subaerialexposure and erosion. In comparison, thick succes-sions of basinal marls and coarse redeposited faciesderived from adjacent footwall highs accumulatedin hangingwall depocentres. Redeposited materialderived from the footwall areas of these faultssuggest two or three periods of active faulting onthe platform margin.

In the developing hangingwall grabens, sequenceboundaries between the three sequences were conform-able and characterised by inferred deepening of thedepositional environment. The upper sequence bound-ary of the late prerift sequence was conformable ondeveloping footwall highs, whereas conformable, dis-conformable and angular unconformable sequenceboundaries all occurred at the base of the synrift suc-cession and synrift deposits may be absent. For thelate prerift to earliest synrift, sections in areas thatdeveloped into hangingwall depocentres are thickestand showed evidence for deepening upwards and pro-gressive drowning. Di�erential subsidence related tomovement on pre-existing structures and/or propa-gation of faults to the surface was the dominant con-trol on accumulation space and facies distribution.

Acknowledgements

BP Exploration, UK are gratefully acknowledgedfor providing the generous ®nancial support for myPhD study, from which this work developed. My PhDsupervisors, Professor Dan Bosence and Dr TonyBarber are thanked for their constructive comments.The SE Asia Research Group, Royal HollowayUniversity of London, and its funding companies:ARCO Indonesia, LASMO Indonesia Ltd, MOBILOil Indonesia, EXXON Inc, Canadian PetroleumIndonesia, Union Texas Petroleum and Unocal arethanked for providing the funding for postdoc researchand for their technical and administrative support. InIndonesia, GRDC, Bandung, particularly AlexanderLimbong, Kanwil, South Sulawesi, BP o�ces inJakarta and Ujung Pandang, LIPI and so many peoplein Sulawesi all provided technical and practical sup-port. Dr Ted Finch and Prof. Fred Banner, both atUniversity College London, and Dr Toine Wonders,Consultant, UK, are thanked for their detailed biostra-tigraphic work. Ed Purdy and an anonymous reviewerare thanked for their constructive comments on thismanuscript.


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