A New Stratigraphy 4 the Islands of the Sumatran Forearc - Indonesia

8/3/2019 A New Stratigraphy 4 the Islands of the Sumatran Forearc - Indonesia

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Pergamon

A new stratigraphy for the islands of the SumatranForearc, Indonesia

M. A. Samuel,*t N. A. Ha-bury,* A. Bakri,: F. T. Banner*and L. Hartono$

SE Asia Research Group. Research School of Geological and Geophysical Sciences. BirkbeckCollege and University College London. Cower Street. London, WCI E 6BT UK: $Simon PetroleumTechnology. PT Robertson Utama Indonesia. PT Horizon Nu santara Eksplore. Building IOXC .

Cilandak Comm ercial Estate. Jakarta. Indonesia

Abstract--The Sumatran Forearc of western Indonesia contains a number of islands whet-c extensiveexposures of baement rocks and their sedimentary cover ma> be examined. The islands. such as Niasand the Batu Islands. are located along the outer-edge of the Sumatran Forearc. whilst others.including the Banyak Island Grou p and Pini Island. lie wnithin the forearc basin. Derailedsedimentological. palaeontological and palaeobathymetric data from the Tertiary strata from theforearc region require ;I new stratigraphy. as previous stratigraphic schemes have not explained thevariations across the region adequately. This Ttratigraphy. developed initially from detailed datacollected on Nias and the Banya k Islands, can fully account for the successions of sedimentary rockson the Banyak and Batu Islands and Siberut and explains many of the apparent inconsistenciesbetween previous stratigraphies. A basement complex and six new formations are formally definedin this paper: important sedimentological differences between these formations represent key stagesin the evolution of the outer part of the Suma tran Forearc. Studies of the basement rocks acrossthe forearc area suggest the basement is inhomogeneous: large intact sections of ophiolitic materialoccur in some areas. whilst there is evidence for both oceanic and continental basement in others.Such heterogeneity is to be expected in B long lived obliquely convergent margin. InOligoceneeearliest Miocene times extension of the heterogeneous basement is inferred throughindirect evidence. Palaeobuthymetric data from the Oyo Formation indicates that the initialdeposition in the newly formed extensional sub-basins on Nias was, in most areas. deep marine. inmany cases below the CCD. Detailed biostratigraphic analysts and structural and gcochronologiculstudies indicate ;I mi!jor Early Miocene unconformity in the western (Lah ewa Sub-basin) and partsof central Nias (MUJO ~ Sub-basin). This unconformity was developed as ;I direct result of 21periodof basin inversion that affected western parts of Nias. Whilst sub-aerial erosion occurred in partsof western Nias. conformable deposition of the Gawo and Olodano Formation continued in theGomo and eastern p arts of the Mujoi Sub-basins. The shallow m arine sedimentary rocks of theOlodano Formation tended to accumulate on intra sub-basinal highs whose position w as controlledby active faults that transected the sub-basin\. The sedimentary record reveals that the Lower andMiddle Miocene phases of differential uplift and subsidence had ceased by the Late Miocene. Amassive influx of Himalaya n derived Bengal Fan sediments reached the Sunda T rench in the Sumatraarea in the late Middle Miocene. Continued addition of Bengal Fan material to the accretionarywedge south-west of Nias resulted in steady plate detlection and subsidence through tlexural processesin forearc basin arcas. The flexural consequence of increased load added to the prism. and associatedsubsidence history is documented by the sedimentary record on Nias where the shallow marineOlodano Formation passes up into the neritic to upper bathyal Lahomie Formation. The Plioceneunconformity which is observed over all areas studied in the forearc is well constrained by structural.biostratigraphic and sedimentological studies. The unconformity represents the initiation of ;I majorphase of uplift and deformation that continues to the present day. Rapid uplift of the outer arc ridgeand deformation of the prism during the Pliocene Icd to increased subsidence landward of thedeformation. The rapid subsidence of the forearc basin landwlard of the outer-arc ridge hascontributed greatly to the apparent differences between the forearc basin and the outer-arc ridgeat the present day: two areas with remarkably similar pre-Pliocene histories now have remarkablyditfercnt physiographies. ( 1997 Else\ier Science Ltd

Introduction Subsequent to the advent of plate tectonic theory thesignificance of islands, such as Nias, lying at the outer

The Sumatran Forearc has been the subject of edge of the Sumatra Forearc, (Fig. I) has been realisedconsiderable geological interest for over a century. (Moore and Karig 1980). These islands are commonly

cited as type examples of uplifted accretionaryCurrent address: British Gas Exploration and Production . complexes although recent work has shown that their10 0 Thames Valley Park Drive, Reading. Berkshire RG 6 I PT. geology may be interpreted in significantly different waysUK . (Pubellier CI rrl. 1992; Samuel 1994: Samuel (I rrl. 1995).

33 9


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M A. Samuel et al.

LEGEND

active volcanoes

90 E 1OO E 110 E 120EI I

Fig. 1. Tectonic map of the Sunda Arc with the locations of the Sumatran Forearc Islands. Modified fromMoore et al. ( I980a) and Curray ( 1989).

The disparate models for the evolution of the SumatranForearc are based on differing stratigraphic schemes andthere has been no modern synthesis of the stratigraphyof the forearc islands.The Sumatran Forearc forms part of the Sundasubduction system w hich runs for over 5000 km fromSumba in the east to Burma in the north (Fig. 1, Mooreet al. 1980a; Curray 1989). In the Sumatran area the archas a classic morphology of trench, accretionary prism,outer-arc ridge, forear c and volcanic chain (Karig et al.1979). Simeulue, Nias, the Batu Islands, the MentawaiIslands (Siberut, Pagai and Sipora ) and Enggano formthe north-w est to south-east sub-aerial expression of theSumatran outer-arc ridge (Fig. 1). Sarangbaung and theBanyak Islands lie to the north of Nias, w ithin theforear c basin. Nias is the largest of the forear c islandsbeing 125 km long and about 40 km wide, with a landarea of 4772 km.

Inland areas of the larger forear c islands are typicallyrugged and mountainous with steep slopes and deeplyincised river valleys. Prominent strike ridges haveindividual peaks com monly in exces s of 500 m elevationwith occasional peaks almost 900 m high. Reliefdecreases abruptly to coastal areas. Mangrove swamps

cover large areas of Siberut (Mentawai Islands),Simeulue, Tuangku (Banyak Islands) and parts ofNias. The smaller islands generally lack rivers andsurface fresh water, and have minor relief with theexception of uplifted coral reef s. The Batu Islands lie onthe equato r and the climate experienc ed by all theforear c islands is typical o f equatorial regions ex ceptthat there is no marked difference between the morecommonly dry (January to June) and wet (July toDecember) seasons. Rainfall is generally highthroughout the year with an average total of 240 mmper month.

As part of an integrated study of the SumatranForearc by the University of London Group forGeological Research in Southeast Asia two islandgroup s have been the focus of detailed investigation.These are Nias and the Banyak Group (Fig. 1). Theywere chosen for their contrasting geographic positionswith respect to the subduction system; Nias lies along theouter-edge of the forearc basin whilst the Banyak Islandslie within the central part of the basin. In additionreconnaissance studies hav e been carried out on the BatuIsland Group and Siberut of the Mentaw ai Islands,along strike from Nias.


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Stratigraphy for the islands of the Sumatran Forearc 34 1In this paper w e provide an overview of previous their north-west Sumatra contract area. Burrough and

research on the Sumatran Forearc and discuss the need Neilson (1969) continued the survey and examinedfor an integrated stratigraphic schem e. We illustrate the outcro ps on the Banyak Islands during three days andgreat stratigraphic similarities between Nias and the then spent a further three days in north-eastern Nias.Banyak Islands, based on our extensive data set, and The fieldwor k w as aimed at establishing the stratigraphyshow how our new stratigraphic scheme m ay be applied of the area (Fig. 2). A further trip in 1971 was carriedto all the islands of the Sumatran Forearc. We conclude out over a week by Bradley (I 973). Two traverses werefinally with a discussion on the main implications this com pleted in eastern Nias on the Gaw o and Nalaw onew stratigraphy has for the geological evolution of the rivers and Bradley produced a stratigraphic column forSumatran Forearc and show how the sedimentary rocks Nias based on the geology exposed in these two riversexposed on the Sumatran Forearc Islands can provide (Fig. 2). Verstappen (1973) carried out a geomorpholog-important clues to the nature of the stratigraphic fill of ical reconnaissance study of Sum atra and the forearcbasins within the present-day forear c basin. islands and dedu ced rapid Quaternary up lift.Previous work on the Sumatran Forearc Islands

Nias Island has been the most studied of all theforearc islands as it is the largest and mo st acce ssible.

1850-1950Early studies w ere carried out by Dutch workers

(Verbeek 1876). Schroder (1917) a government officialon Nias between 1905 and 19 11, was also an avid naturalhistorian. He collected rock samples w hich were sent toEuro pe an d studied by wor kers including Douville( 19 12) who worked on foraminifera, Gerth (1925) whostudied samples of modern and Neogene corals, lcke andMartin ( 1907) and van der Veen ( 19 13) who consideredthat Nias consisted of an igneous core overlain by amantle of metamorphic and sedimentary rocks.

Between 1975 and 1978, Moo re (1978) and Karigspent three years researching the structural geology andsedimentology of Nias for Moores PhD researchsupervised by Karig. Karig and Moores field studieswere concentrated in the more accessible areas of Nias.The cross-island road was unnegotiable at this time andfieldwork was centred in an area from the east coast ofthe island near Gunungsitoli inland to the Mujoi area ofcentral Nias (Fig. 3) with further traverses to the southand to the west.

van Bemmelen (1949) reports on the limited workcarried out by Pontoppidan and De Jong on Enggano,by Terpstra on Sipora and Siberut of the MentawaiIslands and by Jansen wh o was a governmen t official onSimeulue. These workers mainly identified Neogenesedimentary rocks although T erpstra discovered pre-Tertiary and Paleogene rocks on Sipora.

During the period 1939 to 1941 the forearc islandswere the subject of considerable surveys by Dutchgeologists of the Nederlandsche Pacific PetroleumMaatschappij (NPP M) and the Bataafsche PacificPetroleum Maatschappij (BPPM ). The NPP M geol-ogist Den Hartog (1940a. b) produced stratigraphies forSiberut and Simeulue, whilst Hopper (1940) spent sixmonths working on western and northern Nias (Fig. 2).Whilst head hunting and slavery w ere still a feature ofNian life, the original Dutch road system was moreextensive than the present network and Hopper (1940)was able to erect a very detailed stratigraphy. Geologistsof the BPP M included Elber (1939a) w ho completedfieldwo rk on the Batu Islands, eastern and southern Niasand the Banyak Islands and Paul ( 1941) who erected hisown stratigraphy for Nias (Fig. 2). Elber (1939b) wasable to study Den H artogs (1940a) report on Simeulueand he produced a further stratigraphic scheme for theisland (Fig. 2).

Whilst the work of Moore and Karig includedsedimentological, palaeobathymetric (Moore et al.1980b) petrographic and provenance studies (Moore1979). the most influential aspects of the work were thestructural observations and interpretations. They wer eable to produ ce a convincing mode l f or the structuralgeology of Nias with its implications for subduction zonetectonics. Two main tectonostratigra phic units wer edefined (Fig. 2). Their lowest unit named the OyoComplex. a melange, was reported as containingangular inclusions imm ersed in a sheare d matrix . Theyestimated that 70 per cent of the inclusions weresandstones. shales and conglom erates. Othe r inclusionswere mainly pillow basalts and cherts with subordinatemafic and ultramafic rocks. Moore and Karig (1980)suggested the Oyo Complex was the tectonic melangeof an accretionary complex comprising deformedtrench-fill turbidites and slices of oceanic crust andsediments accreted to the base of the inner trench slope.Neogene strata. termed the Nias Beds., with anestimated thickness of just over 3000 m are documentedoverlying the Oyo Complex (Fig. 2 and Moore el N/.1980b). The Nias Beds were interpreted as upliftedtrench-slope basin deposits unconformably overlying theOyo Complex . The Nias Bed s were considered tohave been imbricated into an accretionary wed ge duringcontinuous deform ation and uplift (Mo ore and Karig1980).

Wo rk on the forear c islands in the late sixties andearly seventies was again centred on Nias Island andmainly associated with petroleum exploration. In 1968two Union Oil geologists, Burrough and Power (1968),spent a month on Nias and a few days on Pini, the mosteasterly of the Banyak Islands, as part of the survey of

During the 1980s field geolog ists of the IndonesianGeological Research and Development Centre (GRDC ),Bandung mapped Enggano (Amin et (I/. 1986) Pagaiand Sipora (Budhitrisna and Andi Mangga 1986:Budhitrisna 1989). Siberut (Andi Mangga and Burhan1986). the Batu Islands (Nas and Supandjono 1991).Nias (Djamal rt ul. 1991) and Simeulue (Endharto andSukido 1991). The GRD C group produced geologicalmaps at a scale of 1 250,00 0 for all these islands anderected new stratigraphic schemes (Fig. 2). Correlationbetween the different island groups was not attemptedand the Banyak Islands wer e not visited.

Studies by the University of London Research Grouptogether with LEMIGA S (the Indonesian Research andDevelopment Centre for Oil and Gas Technology.


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342 M A. Samuel et al. ----Ijamal et al.1991NiasBurrough &Power 1966AgeHolocene

PleistoceneQz-If4ale

Elber 1939b Moore et al.1960bradley 1973unobewedZ-Z;;$5 $5 1 Q:

Unlt 1Sandstone, daySlOfW,Sik3tOnS

& mnglomaraie

_ AlluviumZapping imeatone,tanda, silts 6 days

Marls, uffa,and-~tonea, IimeStorwS

Unfoldedllmastonaa

Niea bedsMa~alve Sand-Stone, alltStone 8Shale

Blodastlcllmastone1 MarlsUrnsstones,0 CalcareouS

E aandStoneS,tuffS&dawGiaumnitlc sand-

Co n lomeraleBca Am ac8ouasandstone

Baaal$d;~Stone

?$d-SandStcmesslllcil%dclaystoner

NIaS BedrUnit 4

Siltstone.SandStone8mudstone

Calcareous aand-stone and sliMone21llaoeotm antiorca CareouSMhlC&llthlc quartz aand-stones, mudStones.cmglomeraiea.limestones &minor coals

Unlt Bwna~~g$~ amudstones withminor limer4tones :zz:sandstone Serler

Mad & thlnturbldkea

AsforElber (1939b)Base and lowermita UnObseNSd

Unit A IPre-Mlocena

ComplexSadlmantary ods,ard cemented 4 rey,bz?%%Tsmkx, Shale andmndoineratea~. -7

ojo BedsLithlc, micacewa.quartrose, non-calcereous aand-~trmeg8 non-CalcareouSwithSome al!ckensidea

1ComprbxlOyo ComplexStgagl l;zdoiycontaining angular

ZZ%!

Dphlolfte BwdlnaComplex of meta-

kene foramlnifer mn lomeraie&gb ,u rdlnatelndusloiw ofoceanic crust &Sediments 8upper mantle:inta retedaspecton c melange

PrWTWtfSrYOf Sumaauma u ObSSNSdBedmdc atSuma-Sumall angular%ea%$.=quattzites. oph4Ollteerare llmeetmea

Fig. !(a).


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Niasiatbury &Gallagher 199

Reciyslalliidmefal lfnestme(7 Necgsne)

0~0 Comp*xBlocks clmicacewssandstone,congkxnerate.gm acke,K&an ,bfmalt,ched; matrix

rarely obsewedin the field

ubellier et al.992Reefs 8coastal deposits

Coame dasiks,m!!S

Elber 1939bBanvak Islands I Batu Islands I

Tertiarylimestcnes

I I

Elber 1939b

Hard.white o Tuengku Beda

ReczFedlimeelone

/ Tuff.dayetonehenlstonelb Pometlon----.wT mallwwlboutdels2cz2czizs?iakee.micaaInph&s!$!z==%

3ase uripedfiedFig. 2(b).

Fig. 3. Summ ary of past stratigraphic schemes for Nias, the Banyak and Batu Islands and Siberut. Dashedlines represent uncertainties in the stratigraphic position of units.34 3


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M A. Samuel et al.

Fig. 3. Ma p of Nias showing the location of major roads , river and towns including locations referred to inthe text. Roads a re denoted by double lines and river names ar e given in abbreviated Nian language (e.g.I. Gawo = Gawo river). The unfilled circles represent the location of towns and villages. The thick grey linesidentify the locations of traverses complete d on the island. The inset shows the positions of the three sub-basins

and the basement high on the island. The sub-basins are separated by major faults.

Jakarta) commenced in the Sumatran Forearc in 1986with a brief expedition to the island of Simeulue(Situmorang et al. 1987). The following year a moreextensive expedition to Nias, Sarangbaun g, Simeulueand the Banyak Islands w as undertaken (Fig. 2,Kallagher 1989, Harbury and Kallagher 1991). Harburyand Kallagher (1991) published the results of thegeological studies of the University of London Resea rch

Group at the time. Their paper raised questionsregarding the validity of the model of Moore and Karig(1980) because they interpreted much of the Miocenesuccessions expo sed on the island as shallow marine inorigin (Harbury and Kallagher 1991).

In 1991 a group of French geologists examinedsections on Nias during a three w eek trip (Pubellier et al.1992). Their account of the geology of Nias implies a


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Stratigraphy for the islands of the Sum atran Forearc 345

.lN

LEGENDFaults

Sircmb~

Recent AlluviumGunungsitoli Formation

pg!J Tetehosi Formation

Olodano FomationI Comlomeraterwlw hor!zor~ I

17E

Undii~y;~~~l)yo 8 Gawo

Oyo FormationSediment and Melange ComplexBangkaru Ophiilite Complex

97=30E

-1LagundiNt

0 25km

Fig. 4. Geological map of Nias compiled from field traverse data, aerial photographs, SAR and LANDS ATimages. T he map is simplified as detailed structural data cannot be shown at this scale. Sediment and MelangeComplex refers to areas where the late Paleogene and Neogene sedimentary successions are intimatelydeformed with melanges. The contacts between the melanges and the deformed sedimentary sections areintrusive and have complex geometries (Samuel et 01. 1995 ). For this reason it is not possible to differentiatethe melanges and the deformed sedimentary sections at the scale of this map.


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Stratigraphy for the islands of the Sum atran Forearc 34 7structural and stratigraphic evolution remarkablydifferent to that deduced from any previous research(Fig. 2) as they observed:

Aim of studies

a complex belt affected by polyphase tectonics; during theEocene and the Middle Miocene. Sedimentary series are shelfelastics and limestone. The classical N ias Melange alsoappears to be composed of extremely thin myionites andolistostromic scaly clay localised at several decollement levels.

They suggest that an alternative hypothesis for thestructure of the forearc island s may be:the reactivation of an Eocene Tethys zone within the crustalblocks of the Sunda margin.

Recently oil companies have again had interests in thearea. PT Caltex Pacific Indonesia have recently acquireda block that includes the on-shore eastern area of Nias,with acreage off-shore to the east.

The studies sum marised in Fig. 2 were devised for avariety of different reasons. The early studies (1939 to1973) were aimed primarily at elucidating stratigraphy.Later studies (e.g. Moore ct al. 1980b; Pubellier et al.1992) were less concerned with the pure stratigraphicaspects of the studies and more concerned with the tec-tonic implications of the geology. Moo re et al. (1980b)for instance state that they were happy to subdivide thestratigraphy of Nias into two main units (the OyoComplex and the Nias Beds) which originated in twovery different tectonic settings. The studies by GR DCwere designed with the major aim of producinggeological m aps for the islands; geographic coverage w astherefore extensive and formations were chosen withcharacteristics that were the most easily mappab le.

Problems in correlation of previous stratigraphicschemes

There are substa ntial difficulties in correlating thedifferent schem es (Fig. 2) and reaso ns for this aresuggested below:

Studies such as those of Elber (1939b) and Hopper(1940) are based on large amoun ts of field data withgood palaeontological support and the stratigraphieswere devised with minimal interpretation. Other studieshave been more superficial.

Litho.strrrtigrapl~~~ ~~ersus hiostratigraphj~ A new stratigraphy for Nias and the BanyakIslandsBurrough and Power (1968) point out that the earlieststratigraphic schemes were based primarily on palaeon-tological evidence. The biostratigrap hic units often

cross-cut lithological boundaries. All schemes includingand post-dating Burrough and Power (1968) have beenbased. however. on lithostratigraphy.

van Bemmelen (1949) remarked on the problem ofcorrelating the stratigraphy of Nia s according to theNPPM (Hopper 1940) and BPPM schemes (Elber 1939b,see Fig. 2). Both of these schemes were based onfield-program mes covering different geograp hic areas ofNias. The NPPM held a concession consisting of thenorth-western region of Nias whilst the BPPM operatedin the south a nd east of Nias. A reasonable explanationtherefore, for the discrepan cies betwe en the two schem es.could be that the stratigraphy in different areas of theislands is different. Conversely Elber (1939b) w as able toproduce stratigraph ies for three of the island groups(Nias. Banyak and Batu Groups) that are easilycomparable whereas many other research teams havetended to erect new stratigraph ic schem es for each of theisland groups (e.g. fieldwork during the 19 80s bygeologists from GRDC).

Nias and the Banyak Islands have been mapped indetail as part of the study documented here (Figs 4and 5). Field mapping was conducted for several fieldseasons with the total time spent in the field exceedingsix months. LAN DSA T and SAR (Synthetic ApertureRadar) images have been used in conjunction with thefield data to produce the maps, as characteristically areasbetween river courses offer only low levels of exposuredue to the thick tropical vegetation. The mappingexercise highlighted the need to produce a newstratigraphy for the islands as past schemes w ere foundto be either poorly defined. offered insufficient de tails orproved unusable.Preliminary mapping revealed that three mainsub-basins could be identified on Nias; these are namedfrom north-east to south-west the Gomo. Mujoi andLahewa sub-basins (Samuel 1994, Samuel et 01. 1995,Fig. 3). Seismic lines across the easternmost bulge ofNias show ed it to be a long-lived basement high with arelatively thin ( z 2 km thick) sedimentary cover (Samueland Harbu ry. in press). Four distinc t areas are thereforeidentified on Nias. T his is largely reflected in the newstratigraphic scheme for the island (Fig. 6). Thestratigraphy deduced for the Banyak Island Groupcorresponds very closely to that of the Gom o Sub- basin(Fig. 6).

As Rock (1989) pointed out, the mapping of the late1930s and early 1940s was carried out before the adventof plate tectonic theory. Since this time geologicalconcepts of units such as melanges have greatlyadvanced and stratigraphers have been able in someinstances to recognise and separate tectonostratigraphicunits from purely lithostratigraphic units (e.g. And iM angga and Burhan 1986, see Fig. 2).

The new stratigraphy is exceptionally well-controlledby biostratigraphic data. Over 350 samples havebeen dated. The biostratigraphic zonation schemeused for the nannofo ssil and foraminiferal separatio nswas that of Bolli et al. (1985). Some fauna. particularlylarger benthic foraminifera, were identified in thin-section. Age determinations were made for theseforaminifera according to the Far Eastern Fo raminiferaZonation Scheme. T he latter stages follow) the Indo-West Pacific stages of Adams (I 984).


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34 8 M A. Samuel et al.

Palambak

Gawo Fm.

TflL

0 5 10 15 25 30- - - - - 1km

/_!!- Bedding 66 Overturned NNll Age rust & reverse faults , Strike-slip fault,

Recentalluvium

LahomieFormation

OlodanoFormation

.:. :. GawoEl ,: ,.: : , ; Formation OYOFormation

Sediment &melange complex

Bangkaru OphioliteComplex

Fig. 5. Geological map of the Banya k Islands com piled largely from field traverse data and aerial photographs.Six formations and a basement complex have been

identified on Nias and the Banyak Islands (Fig. 6). InThe Bangkaru Ophiolite Complex

this paper we formally define these and provide brief Introduction and .yynony n?Jsedimentary interpretations. The relation o f the newlydefined form ations to units identified by other worker s The majority of previous researchers have reportedis discussed and the stratigraphic subdivisions have been the existence of basic igneous and sedimentary rocks ofclassified accor ding to the guidelines of Whittak er et al. ophiolitic affinity on the forear c islands (Fig. 2). These(199 1). For complete descriptions and interpretations rocks can be grouped into a unit named here thethe reader is referred to Samuel (1994). Bangkaru Ophiolite Complex. The sequences have been


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Stratigraphy for the islands o f the Sumatran Forearc 34 9placed in two main stratigraphic positions by theseworkers: these are either as part of pre-Tertiary of the rock types, such as basaltic rocks, are commonlycomplexes which are dominated or wholly composed of found wherever the Bangkaru Ophiolite Complex isophiolitic material, or as part of melange complexes exposed, whilst other lithologies, such as epidote-actino-(Fig. 2). In both instances the comp lexes containing lite schists form only a subordinate compon ent of thecomplex (Table 1). The proportions of the different rockthese rocks have been interpreted as forming the types comprising the complex vary between differentbasement to the younger stratigraphic successions.although no form of stratigraphic or structural contact areas and exposures. Detailed mapping and examinationwith these successions has been observed by the previous of outcro ps reveals that there is no consistent pattern tothese changes in abundance.workers. Rocks of the Bangkaru Ophiolite Complex are

exposed on Nias. and the Banyak Islands. in two discreteLitldogj- ud ocu.mwm~ structural settings: as intact sections and as blocks and

Seven main groups of rocks have been identified clasts in melanges. The intact sections are map ped aswithin the Bangkaru Ophiolite Complex (Table 1). Some Bangkaru Ophiolite Complex, whereas the melanges aremapped as such (Figs 4 and 5).

Nias-Gomo Nias-MolaBasementHiahStructural

events

I4) Earli,Eocenet-

-____

(3) Extension ofbasement

& developmentof basins

under transtension

I , asement in eastern N&~ I

f lo 1 NP14/ ,, ., , 1= Bangkaru Ophiolite Complex, II = I. Me Group; - IIa = Oyo Formation; - IIb = Gawo Formation, III: Gawo Formation conglomerate marker horizon, V = Olodano Formation, VI = Olodano Formation cclorizon, VII = Lahomie Formation, VIII = Lahomie Formation tuff marker horizon, IX = Lahomie Fnarker horizon, X = Tetehosi Form ation, XI = Gunungsitoli Formation Limestone Marker H orizon,ormation.

of basement(1) Formation &

hydrothermalalteration of

oceanic crustand mantle

= Moi Member, IS>nglomerate markeyorrnation limestoneXII = Gunungsitol

Fig. 6. New stratigraphic scheme for Nias. the Banya k and Batu Islands and Siberut. N and P denoteplanktic foraminiferal zones, NN and NP denote nannofossil zones and T denotes far eastern benthicforaminiferal letter stages. The Roman numerals refer to the relevant formations and marker horizons. Keystructural events are indicated in the right-hand column. For details of the structural evolution of the outerSuma tran arc the reader is referred to Samuel (1994 ) and Samuel rt u/. (1995 ).


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35 0 M A. Samuel et al.Table 1 . Relative abundance of the main lithologies comprising the Bangk aru Ophiolite Com plex

Rock Group Lithologies AbundancePeridotitesPlutonic rocks

DoleritesBasaltic rocksAll generallymetamorphosed toprehnite-actinolite facies

TuffsSedimentary rocks

Metasedimentary rocks

Light green-yellow coloured soft serpentinitesDark coloured hard harzburgitesMeta- gabbro s (prehnite-actinolite facies(Liou et al. 1984) to greenschist)Gabbro pegmatitesDiorites and plagiogranites*Sub-oph itic dolerites, prehnite-actinolite faciesmetamorphismPillow basaltsPeperitesHeterolitho logic pillow fragme nt brecciasIsolated-pillow breccias and hyaloclastitesGarnet amphibolite (Moore and Karig 1980)Epidote-actinolite schistPalagonitePlanktic foraminiferal packstonesBanded chertsOchresMetagreywackesQuartz, pyrite and barroisite schists

Common

CommonRelatively rareVery commonRelatively commonRareVery rareRareRelatively commonRelatively commonRareRareVery rare

Relatively commonCommonRelatively commonVery rare*Common

*Diorites and plagiogranites are more commonly recorded on the Batu Islands.

Intact sectionsOne of the largest and best exposed outcrops of theBangkaru Ophiolite Complex occurs on BangkaruIsland, the westernmost of the Banyak Islands (Fig. 5).There an extensive sequence of basic igneous rocks arethrust over Oligocene and Lower Miocene sedimentary

successions. Similar, b ut less well exposed bodies occuron Nias, along the west coast and on the north-easterncoast, at Sifahandro (Figs 3 and 4). The sequences of theBangkaru Ophiolite Complex exhibit marked internaldeformation, lithologies are often faulted and shearedtogether.Blocks and clasts in melanges

Clay-matrix melanges are well exposed on Nias andon Bangkaru Island and some of the melanges containinclusions (ranging in size from small clasts to largeblocks) of the Bangkaru Ophiolite Complex (Fig. 7a).An understanding of these melanges is crucial to anyaccount of the geology of these islands. Table 2documents the main features of the melanges; furtherdetails can be found in Samuel (1994) Samuel et al.(1995) and Sam uel and Harbury, in press. These workshave shown that the melanges formed and incorporatedrocks from parts of the Com plex, after thick sequence sof sediments were unconformably deposited above theComplex. The melanges do not therefore form basementto the sedimentary successions described in followingsections of this paper. The melange exposures do,how ever, provide an excellent opportun ity to study therange of rock types of the Bangkaru Ophiolite Complex.Type ureus

The type area for the Bangkaru Ophiolite Complex isthe south-west coast of Bangkaru Island (Fig. 5). Thisrepresents the largest intact sequ ence of the Com plexexposed on either Nias or the Banyak Group. Thelithologies exp osed in the area are predom inantly basic

igneous rocks and include intersheared bastite serpen-tinites, gabbros, dolerites and a range of basaltic rocks.A readily accessible section on Nias, where a wide rangeof Bangkaru Ophiolite Complex lithologies are exposedas blocks in a melange, occurs in the Moi river betweenthe cross-island road and the confluence downstreamwith the Oyo river (Fig. 3).Thickness and age range

It is not possible to determine the maximu m thicknessof the Bangkaru Ophiolite Complex as its base is neverexposed.Prior to this study pelagic sedimentary rocks from theComplex remained undated. The age of formation ofsome of the basement complexes as defined by earlierworkers (e.g. Hopper 1940 and M oore et al. 1980b) isconstrained solely by the presence of reworked shallowmarine limestone clasts in siliciclastic conglomerates thatthese workers associated with the complexes. Such clastsfrom conglomerates in south-west Nias were dated byDouville (1912) as Eocene, since they contained severalspecies of Nummulites.Igneous material on Nias and the Banyak Islands istoo altered to yield reliable (and sensibly interpretable)K-Ar ages (S. Baker pers. comm. 1993) however,reliable biostratigraphic ages have been obtained fromtwo sedimentary rocks of the Bangkaru OphioliteComplex collected during this study: a pelagic limestonefrom the Moi river of central Nias is dated asCam panian (Upper Cretaceous) on the basis ofSpiroplecta spp, Globotruncanita cfstuartiformis, Contu-sotruncana fornicata, hedbergellids and globotruncanids.Seven samples of chert (Fig. 7b) and pelagic limestonewere analysed for radiolaria and one dark brown chertsample from Soma Soma on the west coast of Niascontained a good Middle Eocene assemblage includingLithochytris vespertilio, Dictyoprora, Theocotylissa jcusan d Podocyrtis spp. (Y. Ling pers. comm. 1992). TheMiddle Eocene date constrains the lower age ofemplacement of the Complex as Middle Eocene (Fig. 6).


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Stratigraphy for the islands of the Sumatran Forearc 353Table 2. Main features of the melanges of Nias and the Banyak Islands modified after Samuel and Harhury

(in press)Distribution: Extensive development in Mujoi Sub-basin. parts of Lahe wa and Gomo Sub-basins. P.Bangkaru. Banyak Islands.Inclusions: At least 50% and commonly 90% Oligocene and Lower Miocene sedimentary material. Othercomponents recorded in some, but not all. of the melanges include basement clasts and Middle Miocene toPleistocene sediments.Deformation of inclusions: Deformation styles of melange blocks and clasts are the same as those recordedin the deformed sedimentary successions at the margins of the melanges. Some sedimentary inclusions exhibitbrecciation, pinch and swell and web structure (as seen also at the melange margins).Matrix: Mudrock. This mudrock has the same composition and thermal history as the Oligocene and LowerMiocene mudrocks. In some sections it contains microfossils which are identical to those in the sections intowhich the melanges have intruded.Deformation of matrix: Scaly foliation is pervasively developed throughout the melanges. The fabricis generally sub-vertical, although in places it is folded and aligned parallel to the margins of the melanges.Melange-deformed sedimentary section contacts: Contacts are always intrusive with the melanges intrudingsections from the Oligocene up to the youngest formations of the study area. Intrusions propagate alongbedding planes and extensional fractures developed in the deformed sedimentary sections at the margins 01the melanges. The melanges map out as irregular intrusions.Timing of formation: The major, and probably sole. phase of melange formation initiated during the Pliocene.Melange formation is continuing to the present day in the form of mud volcanoes.

The rocks of the Bangkaru Ophiolite Complex aretypical of oceanic rocks studied on land in ophiolites andophiolitic melanges, and recovered from the oceans bydrilling and dredging . A spectrum of ophiolitic rocksfrom mantle lithologies to crustal and pelagic rock s arerepresented . It is the co-association of these rocks thathas led to the naming of the Complex as an ophiolitecomplex. The vast majority of the rocks experiencedhydroth ermal alteration with prehnite-actinolite andgreenschist facies metamorphism but there are rareexamples of higher grade metamorphism (epidoteactinolite schists, garnet amph ibolites and barroisiteschists).

It is difficult to fit the spectrum of rocks from theComplex into one palaeogeographic setting; theinformation available sugges ts that the rocks may havebeen derived from MOR B-like crust, from oceanicislands and seamoun ts, from oceanic fracture zones andfrom supra-subduction zone settings. Structural re-lationships between the different rock types are complexand expo sures are generally too limited to gain acomp rehensive understanding of the structural history ofthe Complex.

The Bangkaru Ophiolite Complex is interpreted ascomprising a set of amalgamated fragments. Theamalgam ation of distinct terranes is not uncommo n atconvergent plate boundaries (e.g. Hawk ins et rrl. 1985;Blake ct trl. 1985). Continued subduction produces theopportunity for accretion and amalgam ation of crustaland mantle fragm ents from different tectonic settings.Oblique sub duction and transcurrent faulting can lead tothe lateral displacement and amalgam ation of diversefragme nts. Oceanic islands. seamou nts, island arcs,transform fault zones and ridges are examples oftopographic highs that have a high chance of beingincorporated into forearc areas as opposed to beingsubducted.

The Bangkaru Ophiolite Complex appears to haveform ed the basement, at least in most central andwestern areas of Nias and on Bangkaru Island. on which

the Late Paleogene and Neogene sequences accumu-lated.

The Idano Me Group (Oyo and Gawoformations)

Two formations of the new stratigraphic scheme(Fig. 6), the Oyo and Gawo Formations have beengrouped together as the ldano Me Group. This hasspecifically been done to highlight the important factthat the two formations form part of a single,stratigraphically continuous succession (Fig. 8). Ofprevious researchers in the area, only Elber (1939b) andPaul (194 1) believed this to be the case (Fig. 2): theirBasal Sandstone Series is equivalent to the OyoFormation and it is shown p assing con formably up intotheir Tuff-M arl Series which is largely equivalent to theGaw o Formation (Fig. 2). Pubellier et ul. (1992) alsoshow a continuous sequence lacking an unconformitybetween the Oligocene and Miocen ce. Their strati-graphic scheme is however markedly different to anyother, as sedimentary units that most workers haveplaced in the Oligocene or Miocene are denoted asEocene (Fig. 2). Howev er, Douville (1912) and thepresent study have shown that many of the Oligoceneand Lower Miocene sedimentary rocks on Nias containreworked Eocene foraminifera. sometimes in greatabundance.

All other researchers placed an unconformity betweenthe Oligocene (Unit A) and the Miocene (Unit B) andprovided two different sets of reasons which arcdiscussed separately below:

I. Researchers such as Burrough and Power (196X).Burrough and Neilson (1969) and Harbury andKallagher (for the Banyak Islands, 1991) followedHoppers (1940) original h ypothesis that:un important angular unconform ity separa tes Units A andB _. .evidences of its presence are: ( a ) the slight metamorphism


14/38

354 M A. Samuel rt al.of Unit A , as shown by phyllitic shales and distor tedripplemarks and the complete lack of metamorphism in UnitB and younger units; (b) the presence o f intrusive bodies inUnit A , and the lack of them in Unit B and you nger units: (c)the greater coarseness of the basal parts of unit B than thehigher parts, believed to indicate a transgressive overlap ofUnit B on Unit A.

These workers never actually ob served a contactbetween the two units, whereas the contact, whereobserved during this study, is always seen to begradational and conformable (for example in the Meriver section, Fig. 8). It is true that the rocks of the OyoFormation (Hoppers 1940 Unit A) have generally beenmore deeply buried than those of the Gawo Formation(Unit B), but this does not necessarily imply the presenceof an unconformity (Sam uel 1994 ). The evidence of (c)also does not imply an unconformity and furthermorethe change in lithological character described above canbe related to a number of factors, including changes ineustatic sea-level, w hich led to changes in provenance.The description of the intrusive b odies mentioned byHopper (1940) fits observations made during this studyof the me langes. It is the varied recognition andinterpretation of these melanges that has led to thesecond set of stratigraph ic interpretations discusse dbelow.2. Djamal et al. (1991), Harbury and Kallagher (forNias 1991) and Pubellier et al. (1992) followed Mooreand K arig (1980) in placing rocks equivalent to the OyoFormation solely in melanges and complexes at the baseof the stratigraphy. These complexes are shownunconforma bly overlain by Mioce ne sedimen tary rockswhich can be correlated with the Gawo Formation.Whilst the rocks of the Oyo Formation form animportant component of the melanges on the islands,coherent Oyo Formation sections are exposed in anum ber of localities (as originally reported by Elber1939b). Furthermore rocks of the Gawo Formation (andall younger formations) are in cases intruded by, and/orincorporated into, the mela nges. It is for this reason thatthe melanges do not appear in the new stratigraphicscheme (Fig. 6).

Lithology and occurrenceThe facies and subfacies of the Oyo and GawoFormations are listed in Table 3. The two formationscan be distinguished in terms of age and can in mostcases be separated in the field; whilst thin bedded faciesof the Oyo and Gawo Formations are not always easilydistinguishable, massive micaceous sandstones onlyoccur within the Oyo Formation (Fig. 7c and Fig. 9).Rocks of the Oyo and Gawo Formations have beenmapped over large areas of Nias and the Banyak Islands(Figs 4 and 5). Coherent sequences of the two formationsoccur also within many areas portrayed as sedimentand melange complex on Figures 4 and 5.Exposu res throughou t the island s are generally verygood (Fig. 7d). All successions have however beensubject to deformation and care must be taken inreconstructing the original stratigraphy within theformations. As with the Bangkaru Ophiolite Complexrocks, lithological information can be obtained bystudying the blocks and clasts of the Oyo and GawoFormations preserved w ithin the melanges.

The name for the Oyo Formation has been adaptedfrom the Ojo B eds of Burrough and Power (1968 ). Atype section w as not strictly defined by these workers butthe name implies that the Oyo river was the type section.This is a fairly suitable choice but, beca use the OyoForm ation does not occur along all parts of the river, itis suggeste d that the type section be limited to the stretchof the Oyo river between the cross-island road a nd theconfluence with the Moi river (Fig. 3). Interbeddedmicaceous sandstones and mudstones and largemelange blocks of massive micaceous sandstones andconglom erates are well exposed along this portion of theriver.The name for the Idano Me Group was taken from theMe river of central Nias where the Oyo Formation hasbeen mapped passing conformably up into the GawoFormation (Fig. 3, Samuel et a/. 1995).The type area for the Gawo Formation has beenchosen as the upper and middle reaches of the Gaworiver (Fig. 10). The river exposes one of the mostcomplete and least deformed sections of the GawoForm ation. Further excellent exposures o ccur also inupstream tributaries to the Gawo (Fig. 10).Two distinctive marker horizons have been recognisedwithin the Idano Me Group. The lower horizon occurstowards the top of the Oyo Formation and comprises aset of volcanic-rich conglom erates, pebbly and epiclasticsandstones (Fig. 8). The horizon is well exposed in theMoi river, 50 -150 m downstream of the cross-islandroad an d is therefore called the Moi Mem ber. Although

the section is folded and intruded by melanges (Figs 3,4 and 9) its maximum thickness can be determined as15m.The second marker horizon, termed the GawoFormation Conglomerate Marker Horizon, occurstowards the top of the Gawo Formation and comprisesbroadly similar lithologies to the Moi Mem ber. Thehorizon can be mapped along much of eastern Nias (Figs4 and 10) and consistently yields a Lower M iocene age(Tfl lower, Fig. 6).Thickness and uge range

It is impo ssible to determine the thickne ss of the OyoForm ation by direct methods becau se of the structuraldisruption of the rocks and the paucity of fauna withinthem. O f 27 samples analysed for microfossils only eightyielded age information. Ages obtained range fromNP21 lower (Early Oligocene) to Te5 (Early Miocene).Reworked fauna are apparent in some of the samples; asample from the Moi river, Nias, with an age of P21-N4(Mid-Oligocene to Early Miocene) and a sample fromBangkaru Island, Banyak Islands, with an age of N4-N 6(Early Miocene) contained reworked fauna of NP15 -NP20 age (Mid-Eocene to Late Eocene). It is possiblealso that the Lower O ligocene fauna recorded in theoldest sample may also be reworked.The earliest age for the deposition of the OyoFormation cannot be categorically determined from thedates obtained during this study and unfortunatelyprevious researchers did not record any dates frommudrocks. The formation cannot however be older thanM id-Eocene for two reasons: firstly conglom erates fromthe formation contain limestone clasts with Eocene


15/38

Stratigraphy for the islands of the Sum atran Forearc 35 5Num mulites (Douville 1912) and secondly the under- The succession exposed in the Gawo river compriseslying Bangkaru Ophiolite Com plex contains material of the thickest section through the Gawo FormationMid-Eocene age. measured in the study area: 3.3 km of latest O ligocene

The ldano Me Group,+, +, .6 km

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Rounded to

0Contemporaneouskli abraded shallowmarine b&lastsand carbonaceous materia

lphiolite and andesite 0 Contemporanoeusplanktic 8 bath albenthic foramini Yraand radiolariaRounded Eocenelimestone lithoclastsand reworked Eocenebioclasts Pulsed input ofrounded to sub-rounded Tuffs. cuspateandesite and minor dacite g*y;Z;t

Fig. 8. Schematic stratigraphy and provenance groups of the ldano Me Group. The thickness on thelithologic column is the maximu m as deduced from direct measurements and structural reconstructions(Samuel and Harbury in press). Detailed field and petrographic studies of sections on Nias. the Banyak andBatu Islands and Siberut have led to the identification of a total of nine distinct provenance groups for theldano Me Group.


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35 6 M A. Samuel e t a l .Table 3. Facies and subfacies of the Idano Me Group

FACIESGawo Formation

SUBFACIESG.I-Stratified non-tuffaceous clast-suppo rted conglom erates, sandstones and siltstonesG.II-Massive calcareous sandstonesG.II-Pebbly sandstones and epiclastic sandstones and volcanic-rich con glom eratesG.IV-Tu ffaceous sandstones, siltstones and claystonesG.V-Silty claystonesG.VI-Biogenic claystones G.VII.I-Detached strataG.VII Contorted/disturbed/chaotic strata

G.VIII-Very coarse brecciaOyo Formation

G.VII.2-Coherent folded strataG.VII.3-B recciated and balled strataG.VII.4-Matrix-supported conglomerates

0.1. I-Massive tnicaceous sandstones an d siltstonesO.I-Very thickly bedded micaceous O.I.2-Stratified very thickly bedd ed micaceoussandstones and siltstones sandstones and siltstonesO.II-Thickly to thinly bedde d micaceous sandstones and siltstonesO.III-Silty claystonesO.IV-Pebbly sandstones, con glomer ates and epiclastic sandstones

and Early Miocene sedimentary rocks are monoclinallytilted to the north-east. The maximum thickness of theformation mu st be greater still as the base of the sectionis missin g; from reconstruction of structural cross-sec-tions the maximum thickness is estimated as 3.8 km(Samuel and Harbury in press).A large number of mudstone and sandstone samplesfrom the Gawo Formation have been analysed forplanktic and benthic foraminifera and for nannofossils.Many of the samples have yielded reliable and tightlydefined ages and excellent biostratigraphic controlthrough the Formation has been obtained over the studyarea. The contact of the Gawo Formation with the OyoFormation is diachronous. In eastern Nias a samplefrom the lowest parts of the exposed Gawo Formationsection yielded a data of Tel-4 upper wh ilst in otherareas of Nias and in the Banyak Islands dating of over50 samples reveals the base as within the earliest EarlyMiocene (N4 lower). The youngest Gawo Formationrocks are of Mid-M iocene age (Fig. 6).

P a l a e ob a t h y m e t tyA large number of mudrocks sampled du ring thisstudy from the Oyo and particularly the GawoFormations have been rigorously analysed forpalaeobathymetric information. Caution is necessarywhen using microfossil palaeobathymetric analysisespecially for deep marin e redeposited sedimen taryrocks. During this study the hemipelagic upper edivisions of turbidites were preferentially samp led(Facies G.VI-biogenic claystones) however in manycases it was not possible to distinguish such divisionsfrom lower e divisions (Facies G.V-silty clay-

stones). In many cases the mudrocks sampled yieldedboth indicative deep marine benthic foraminifera and inaddition shallower (sublittoral) fauna. M any of theGawo Formation sandstones on Nias, whilst clearlyturbiditic and probably deep marine, contain rede-posited shallow m arine fauna and it is therefore areasonable assumption that shallow faunas havebeen redeposited downslope in both sandstones and

mudrocks. In all analyses therefore the palaeodepth hasbeen estima ted from identification of the deepest benthicforaminifera preserved. A similar strategy w as used byBillman (Moore e t a l . 1980b) for a previouspalaeoba thymetric study of the sedimen tary rocks onNias. It must be noted, however, that samples may notnecessarily ha ve contained or preserved the deepestmarine fauna. A further problem is that many of the taxaused for depth determinations are not age diagnostic andtherefore older reworked benthic foraminifera may biasthe data. Reworking of foraminifera has been identifiedin some Gawo Formation mudrocks from analyses ofplanktic foraminifera.Wh ilst most workers use the same indicative fauna intheir depth zonation schemes (Pflum and Frerichs 1976)different depths are assigned to the same assemblagesdepending on geographic location (Fig. 11). The depthsused in this study are as for Simon PetroleumsIndonesian Scheme based on Murray (1973) and arebroadly similar to the Union Oil Indonesian scheme usedby Billman for Nias (Moore e t a l . 1980b, Fig. 11). Thebase for the lower bathyal zone is taken as 4000 maccording to the level of the CC D e stimated for the areaduring the Early Miocene (Berger and Winterer 1974;Van A ndel 1975: Moore e t a l . 1980b).

Moore e t a l . (1980b) suggest that the basalsedimentary rocks of their N ias Beds were depositedbelow the CCD as they lack fauna. The extensive studypresented here largely adds support to this interpretationas many samples placed, from stratigraphic mapping, atthe base of the Gawo Formation (equivalent to thelower Nias beds) were barren. F urthermore, themajority of Oyo Formation sedimentary samples (shownin this study to be overlain conformably by the Gaw oFormation) are barren. Two samples (Localities Moi 3,Ba 6, Fig. 12) of the Oyo Formation however containedboth age diagnostic nannofossils and plankticforaminifera and also depth diagnostic benthicforaminifera. The depths indicated are lower bathyal(Fig. 12).Moore e t a l . (1980b) state that the sedimentary rocksin the East Nias Basin record a shallowing inenvironment of deposition from lower bathyal


17/38

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24/38

36 4 M A. Samuel et al .Table 4. Relative abundance of the facies comprising the two facies associations for the Olodano Formationdifferentiated over Nias and the Banyak IslandsFacies association IOccurrence Much of Nias and the Banyak Islands

0L.I Povites-rich limestones Very rareOL.11 Algal-rich limestones Very rareOL.111 Skeletal and micritic CommonlimestonesOL.IV Impure limestones Common0L.V Hybrid wackestones CommonOL.VI Mangrove-rich Unrecordedmudrocks

IIThe area of the GomoSub-basin to the westof Gunungsitoli andth e southern-centralregion of the Gomo-Sub-basinCommonCommonCommonCommonCommonCommon

suppo rt a deep marine environment of deposition theyreport palaeobathymetric data from foraminifera recov-ered both above and below the limestone sections. Thepresence of bathyal foraminifera in the overlying andunderlying formations does not how ever in any wayimply that the Olodano Formation lithologies them-selves should be interpreted as deep marine. Moore et al.(1980b ) report that the limestone they studied in theMoaw o contained deep water foraminifera within it.This is contrary to the results of previous studies byother workers. In addition a large number of limestonesamples collected from N ias during this study have beenexamined and no deep water foraminifera have beenobserved.Porites-rich limestones and algal-rich limestones arenot recorded in previous literature althoug h theycomm only occur over significant a reas of Nia s (Table 4and Fig. 16). Detailed sedimentological examination ofthese and all the other facies of the Olodano Formationleaves little doubt that they were deposited in shallowmarine, predominantly inner to middle shelf environ-ments (Fig. 16 and Sam uel 1994).

The Lahomie FormationIntroduction and synonymy

The majority of worke rs stud ying the outer part ofthe forearc during the last twenty years did not subdividethe Miocene successions into separate formations.This is largely be cause the upper parts of theMioc ene are poorly exposed and it is not easy tocollect field and biostratigrap hic data. M oore et al.(1980b) and Harbury and Kallagher (1991) recognisedone continuous Miocene succession whilst other workerssuch as Bradley (1973) and Burrough and Power (1968)believed that Upper M iocene rocks were absent andplaced an unconformity between the Middle M ioceneand Pliocene successions (Fig. 2). Two early workers onNias, Elber (1939b) and Hopper (1940) examineddifferent areas of Nia s in great d etail and they were ableto subdivide the Miocene. Correlation between their twostratigraphic schemes is difficult and has led to someconfusion in subsequent literature (e.g. van Bemm elen1949). It has been found during this study that theMiddle Miocene to Lower Pliocene successions in

eastern Nias can be distinguished from those in westernNias (e.g. Fig. 1 7a). The successions are however largelytime correlative and share impo rtant lithologicalcharacteristics that allow them to be treated as a distinctformation; it has therefore been decided to groupthese two successions as one formation, theLahomie Formation, in the new stratigraphic scheme(Fig. 6).Lithology and occurrence

Table 5 lists the facies identified within the Lahom ieFormation and outlines their relative abundance inwestern parts of Nias (Lahewa Sub-basin), eastern partsof Nias (Gomo Sub-basin) and the Banyak Islands.The sedimentary rocks exposed in the Mujoi Sub-basin are generally of formations older than theLahomie Formation (Fig. 4) and it has been difficultto fully determine the exact characteristics of theLahomie Formation in this central part of Nias. Ingeneral howeve r the pattern of facies is gradatio nalbetween that recorded in eastern and western areas ofthe island.It has been possible to map the Lahomie Formationover N ias with confidence as the rocks have a distinctappearance on SA R images. They are distributed overlarge areas of eastern a nd north-western Nias (Fig. 4).Two marker units, the Lahomie Formation tuff andlimestone m arker horizons (Fig. 6) are apparent ineastern Nias where the resistant lithologies comprisingthese units form readily mappable ridges (Fig. 4). TheLahomie Formation has also been discovered on someof the easternmost of the Banyak Islands during thisstudy (Fig. 5).Type areas

The type section for the Lahomie Formation extends8 km inland from the west coast o f Nias, along theLahom ie river (Fig. 3). This section is chosen largelybecause it provides one of the most accessible andreasonably exposed set of exposures through theFormation in western Nias. The lithologies expo sed aretuffs and m arls whilst diatomites occur in a small hill tothe north. An easily accessible succession through theLahomie Formation in eastern Nias occurs along the


25/38

Stratigraphy for the islands of the Suma tran Forearc 36 5Gawo river to the north-east of Olodano village (Fig. 3); Thickness and up rangea thick succession of thin to thickly bedded litharenites,foraminiferal packstones and rare conglomerates is The age range of the Lahomie Formation is very wellmoderately well exposed. constrained by a comprehensive biostratigraphic data

0 xSl imcSand Grawi

l&-upcl&Platy coralsBranching coralsSolitary ad massive coralsRounded coral cliatrRbodolithsA@?

Fg. 14. Sedimentary log of the Olodano Formation type section in the Gawo river. A complete sectionthrough the Formation is well exposed in the river. The Olodano Formation conformably overlies the GawoFormation and is conformably overlain by the Lahomie Formation. The inset provides the key to this log

and also those shown in Figs I5 and 18.


26/38

36 6 M A. Samuel et al.

B. Sisobahilli

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Fig. 15. (a) Sedimentary log through a well exposed fresh road cut near Onowaembo. There can be little doubtthat these Olodano Formation limestones were deposited in a shallow marine environment. The abundanceof Porites with platy morphologies, in particular, suggests deposition within carbonate mud banks. (b)Progression from the Olodano Formation to the Lahomie Formation well exposed at Sisobahilli. (c)Synchronous but contrasting Olodano-Lahom ie Formation transition at Onowaem bo, about 6 km to thenorth-east of Sisobahilli. The geographic location of the logged sections is shown in Fig 3. For the key to

the logs see Fig. 14.

set. The sedimentary rocks range in age from NN 5 lower Formation ranges in age from N 13 (Mid-Miocene) to(latest Early M iocene) to NN 12 (Early Pliocene) in NN 12 (Early Pliocene) (Fig. 6).western Nias and parts of central Nias (Fig. 6). In In most cases the basal contact of the Lahomieeastern Nias and other parts of central Nias the Formation can be readily identified and mapped in the

Fig. 16. (a) In situ Porites with a coralline algal packstone matrix recovered from Sisobahilli. This OlodanoFormation bafflestone (Subfacies OL.I.3) was deposited in a shallow marine environment. Plane polarisedlight, horizontal field of view is 8 mm. (b) Field photograph of Olodano Formation rhodoliths (Subfacies0L.III.4) supported in a calcarenite matrix from the Tumo ri river, eastern Nias. The sediment was depositedin a moderately high energy inner shelf environment. (c) Branching Porites coated with larger benthicencrusting benthic foraminifera and red algae in an Olodano Formation rudstone (Facies Class OL.I.IA)sampled at Sisobahilli, east-central Nias. Note also the smaller benthic foraminifera encrusted onto thePorites. Plane polarised light, horizontal field of view is 8 mm. (d) Olodano Formation benthic foraminiferalpackstone (Facies Class OL.III.IA) from the Gawo river by Olodano village, eastern N ias. The largerforaminifera are predominantly Nephrolepidina oneatensis. The sediment was deposited as a shallow marineshoal. Plane polarised light, horizontal field of view is 8 mm.


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Stratigraphy for the islands of the Sum &an Forearc 36 7


28/38

Stratigraphy for the islands of the Sumatran Forearc 369Table 5. Facies of the Lahomie Formation

FAClESL.I TuffsL.11 Marls:calcilutitesL.111 DiatomitesL.IV Sandstones (litharenites)L. V Foraminiferal packstonesL.VI MudstonesL.VII Conglomerates

EasternNiiISRareUnrecorded

UnrecordedCommonRelati\;elyc o m m o n

RareVer) rare

WesternNias

CommonCommonRareRare

UnrecordedRare

Unrecorded

BanyakIslands

RareUnrecordedUnrecorded

CommonRelativelycommonRareUnrecorded

Table 6. Palaeobathymetric data for samples from A) eastern Nias and the Banya kIslands and B) western Nias. Fo r each area the data are arranged inchronostratigraphic order. The locations of the sections from which the samples w eretaken are shown on Fig. 3. The zonation scheme used is as for the other formations:IN = inner neritic. ON = outer neritic, DM N = deep middle neritic. DO N = deepouter ncritic. SUB = shallow upper bathyal. UB = upper bathyal and MB = Middle

Locality (A) Faciesbathyal

Age IalaeobathymetrqP. BagukI. MasioI. Ds. iauwaLahusa Mas~oI. GidoI. GawoBawogowasaliI. G;,w,I, ~;a\voI. sowuI. DsjauwaI. MajayaP. BalaiI. DsjauwaRd to AlasaRd 10 AlasaI. MoruOnowaemboRd 10 AlasaRd to Alasa

L.VIL.VIL.VIL.IVL.VIL.IVL. 1L.IVL.IVL.IVL.VIL.VIL.VIL.VIL.IVL.IVL.VIL.VIL.IVL.IV

NNl2-Nl5NNl2:Nl8NNlZ.NlX

NI XNNIINIX-I9

NNIIU Nl7-NIXNl6-NIY 20NNII,Nl7

NNI INNII

NNI I Nl6-I7NNII Nl6-I7NNI I Nl6-I7NNIO-I I Nl6-N17NNIO Nl6-I9

NNIOUNN6-Y Nl3-I6NN9 N 12-1XNI-Nl3

NOT N4

ONON-LIBON-UB

ONIJ B

ONON

ON-UBONUB

DON-UBDON-SUBDON-UB UB

UBUBONONU BIN

Locality (B)AfuluI. OyoI. DumulaI. LahomieLahomie

Facies Age PalaeobathymetrqL.ll NNII N17 L: BL.11 Nl7-NIY UBL.IV Nl7-IX DON-UBL.II NNSL NX-9 UBL.III NN5L IN

Table 7. Facies of the Tetehosi and Gunungsitoli FormationsTetehosi FormationFacies T. I Mudstones and claystonesFacie\ T. I I Quartz arenites with skeletal detritus andclast-supported conglomeratesFacies T.111 Matrix-supported conglomerates andchaotic strataFacie\ T. IV Heterolithic quartz arenites and mud-stone\Facles T.V Organic-rich depositsFacles T.VI Tuffs-__Gunungsitoli FormationFacies (3~1.1Facies Gu.11Facies Cu.111-

Coral-rich limestonesMangrove-rich mudrocksSkeletal and micritic limestones

field. In western Nias, Lahom ie Formation lithologiesare readily distinguished from the Gawo Formationlithologies that they unconformably overlie and ineastern Nias and the Banyak and Batu Islands there isa clear change in facies between the limestone-dom inatedfacies of the Olodano Formation and the conformabl>overlying L ahomie Formation lithologies. T he LahomicFormation is itself unconformably overlain by distinc-tive formations (Tetehosi and Gunungsitoli) throug houtthe study area.

The full thickness of the Lahomie Formation can beascertained as 1.5 km along th e relatively undeform edsection in the Gaw o river. eastern Nias. Greater degre esof deformation in other parts of the study area makefurther a ccurate determinations of the thickness of theFormation impossible. The thickness of the Lahomic


29/38

M A. Samuel et al.

ClastsB = basaltT=tuffM = mudstone

45

30

15

0

i:

Coarsens up,-_)detailed section

/ from top\

\, 0! &snt Sand

Gradually coarsens down, /I_ detail from base ,/1.2m

I I

)r

Gradually coarsens up, _detail from base

\,II

Cla& 81 f &d

Fig. 18. Sedimentary log through the Tetehosi Formation type section. Both coarsening-up andrn?r~C.ninn_Alnw 11nitr 5ll.P wrnonix.A The w=dims=nta ~nn~ar +n hawe hwn &nna;td ;n fcan_Adts< anAWVU1., ,6 U I. II Ullll.. Ylw I ~~,Y. 1 L .,U.AL.A.UII.Y uyyuu L .&.&.I II l l . . yY.LY 1.1 lul l U .LUY U.S..storm-dominated shelfal environments.

Formation tuff marker horizon is about 5 m whilst Palaeohathymetry and interpretationthe Lahomie Formation limestone marker horizonranges in thickness along strike from 50 m (Masio Thirty three samples from the Lahomie Formationriver) to 4 m (Gawo river) and less than 1 m (Sowu were analysed for palaeobathymetry and all have yieldedriver). useful information. As with the analyses of the older


30/38

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A New Stratigraphy 4 the Islands of the Sumatran Forearc - Indonesia

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