Australian Journal of Earth Sciences

(2003)

50

,

571–583

Geology of southeast Bohol, central Philippines: accretion and sedimentation in a marginal basin

D. V. FAUSTINO,

1,2

G. P. YUMUL Jr,

1,3

* J. V. DE JESUS,

1

C. B. DIMALANTA,

1

J. C. AITCHISON,

4

M-F. ZHOU,

4

R. A. TAMAYO Jr

1

AND M. M. DE LEON

1

1

National Institute of Geological Sciences, College of Science, University of the Philippines Diliman, Quezon City, Philippines.

2

Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan.

3

Philippine Council for Industry and Energy Research and Development, Department of Science and Technology, Bicutan, Taguig, Metro Manila, Philippines.

4

Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China.

Recent field mapping has refined our understanding of the stratigraphy and geology of southeasternBohol, which is composed of a Cretaceous basement complex subdivided into three distinctformations. The basal unit, a metamorphic complex named the Alicia Schist, is overthrust by theCansiwang mélange, which is, in turn, structurally overlain by the Southeast Bohol Ophiolite Complex.The entire basement complex is overlain unconformably by a

~

2000 m thick sequence of LowerMiocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units. Newly identifiedlithostratigraphic units in the area include the Cansiwang mélange, a tectonic mélange interpretedas an accretionary prism, and the Lumbog Volcaniclastic Member of the Lower Miocene CarmenFormation. The Cansiwang mélange is sandwiched between the ophiolite and the metamorphiccomplex, suggesting that the Alicia Schist was not formed in response to emplacement of theSoutheast Bohol Ophiolite Complex. The accretionary prism beneath the ophiolite complex and thepresence of boninites suggest that the Southeast Bohol Ophiolite Complex was emplaced in a forearcsetting. The Southeast Bohol Ophiolite Complex formed during the Early Cretaceous in a supra-subduction zone environment related to a southeast-facing arc (using present-day geographicalreferences). The accretion of this ophiolite complex was followed by a period of erosion and then laterby extensive clastic and carbonate rock deposition (Carmen Formation, Sierra Bullones Limestone andMaribojoc Limestone). The Lumbog Volcaniclastic Member and Jagna Andesite document inter-mittent Tertiary volcanism in southeastern Bohol.

KEY WORDS: accretion, Bohol, marginal basin, ophiolite, Philippines, stratigraphy.

INTRODUCTION

The Philippine island-arc system is an intricate amalgamof various terranes and

in situ

volcanic and sedimentarypackages with still mostly unknown spatial and temporalrelationships (Yumul

et al

. 1997a, 2001a). Subduction zonesof opposing polarities bound the archipelago on eitherside.

The

East

Luzon

Trough–Philippine

Trench

systemis the site of oblique northwestward subduction of thePhilippine Sea Plate with slip rates ranging from 6.5 to13 cm/year (Aurelio

et al

. 1994). The Manila–Negros–Sulu–Cotabato Trenches mark the eastward subduction of theSouth China Sea – Southeast Sulu Sea – Celebes Sea Platesat the western margin of the archipelago. The left-lateralstrike-slip Philippine Fault Zone traverses the entirelength of the archipelago from Luzon to Mindanao. Shearpartitioning along the Philippine Fault Zone occurredconsequent to the archipelago’s interaction with thePhilippine Sea Plate and its collision with the thinnedSundaland margin (Fitch 1972). The relative movementbetween the Philippine Sea Plate and Sundaland is accom-modated in this region by two components. One com-ponent is parallel to the Philippine Fault Zone (Barrier

et al

. 1991) whereas the other component is being

accommodated by subduction perpendicular to thePhilippine Trench at a rate varying from 6 to 8 cm/year(Aurelio 2000). Resulting geometric and kinematic solu-tions from these studies have shown a mean displacementrate of 2.48

±

0.1 cm/year for the Philippine Fault Zone atits central section (Aurelio

et al

. 1994). Excluding thePalawan block, Mindoro, the Sulu Sea, and theZamboanga–Sulu volcanic chain, the archipelago isdefined as the Philippine Mobile Belt separated by thePhilippine Fault Zone into western and eastern com-ponents (Gervasio 1971).

The central Philippines is heterogeneous in lithologicalcharacter

and

structural–tectonic

fabric,

and

is

made upof several islands: Masbate, Samar, Leyte, Cebu, Bohol,Negros and Panay (Figure 1). Bohol is the southeastern-most, and is separated from Mindanao by the deep, north-east–southwest-trending Bohol Sea. Tectonic grain andfault traces suggest that a proto-trench in the Bohol Seaserves as a tectonic boundary between the islands of thecentral Philippines and Mindanao (Yumul

et al

. 1995,

*Corresponding author: rwg@i-next.net

572

D. V. Faustino

et al

.

1997b). Recently acquired offshore gravity and bathymetricdata (Sandwell & Smith 1997), which show anomalieswithin the Bohol Sea similar to those observed alongsubduction regions, are consistent with this hypothesis.

A major part of Bohol, specifically southeast Bohol,grew through collision-related processes similar to thoseobserved in other parts of southeast Asia (Lallemand

et al

.2001; Yumul

et al

. 2001b). The evolution of this part of thePhilippine island-arc system illustrates how young mobilebelts can evolve through space and time. Here, we criticallyexamine relationships between different rock formationsin southeast Bohol and offer alternative interpretationsconsistent with available data. As an example, it will beargued that the Alicia Schist, made up of amphibolites andrelated metamorphic rocks, was not formed as a result ofthe emplacement of the Southeast Bohol Ophiolite Com-plex. Moreover, the geophysical characteristics of theCansiwang mélange, sandwiched between the Alicia Schistand the Southeast Bohol Ophiolite Complex, are consistent

with the suggestion that it is not a diapiric intrusion(Barretto 1997). This implies that the mélange was notemplaced in the same manner as that of the serpentinite-mud volcanoes observed in the Marianas (Fryer

et al

. 1999).The Cansiwang mélange is similar to that observed in

Figure 1

(a) Schematic map of the Philippines showing the major tectonic features that influence the area: the Manila–Negros–Suluand Philippine Trenches, and the left-lateral Philippine Fault Zone cutting through almost the entire length of the archipelago. (b)Bohol is at the southeastern boundary of the Visayan Sea Basin, which also includes the islands of Cebu, Leyte and Negros. See text fordetails. (c) The highlighted portion of Bohol island is the mapped area which covers seven municipalities in southeast Bohol.

Figure 2

(a) Geological map of the study area. The Lower Creta-ceous basement complex is made up of three units: Alicia Schist,Cansiwang mélange and Southeast Bohol Ophiolite Complex(SEBOC). The Alicia Schist is composed of amphibolite schist,quartz–sericite schist and chlorite schist. The SEBOC iscomposed of harzburgite, high-level (non-layered) and layeredgabbro, sheeted dykes and pillow basalt. Inset shows the threemassifs (Alicia, Guindulman, Duero) where the different ophio-lite units are exposed. The Cansiwang mélange is composed ofophiolite-derived clasts in a serpentinite matrix. See Figure 4 forsection lines A–A

�

and B–B

�

. Geological mapping was done by theUP-NIGS 1997 Geology 170 and 215 (Bohol Group) students. (b)Structural map of southeast Bohol. See text for discussion.

Geology of southeast Bohol, Philippines

573

other collision and suture zones and is a thrust packetwithin a subduction-related accretionary prism that

effected amagmatic crustal growth (Aitchison

et al

. 2000;Dimalanta

et al

. 2002; Huot & Maury 2002).

574

D. V. Faustino

et al

.

REGIONAL GEOLOGY AND TECTONIC SETTING OF THE CENTRAL PHILIPPINES

Bohol lies within the north-northeast–south-southwestVisayan Sea Basin and is part of the western zone of thePhilippine Mobile Belt. This basin, with an almost 4000 mthick sedimentary fill, is flanked by two active trench

systems of opposing polarities. The Negros Trench is azone of eastward subduction along the western edge of theVisayan Sea Basin. East of the basin is the active left-lateral strike-slip Philippine Fault Zone and, further eastof Samar, is the 5 million year old Philippine Trench(Cardwell

et al

. 1980; Hamburger

et al

. 1983), along whichthe Philippine Sea Plate is subducted (Figure 1). The

Figure 3

Composite stratigraphic column for southeast Bohol showing that it is underlain by seven formations with the basementcomplex divided into three units that include the ophiolite complex. A Paleocene to Lower Miocene unconformity separates thisbasement complex from the overlying clastic, carbonate and volcaniclastic formations. See text for discussion. QAL, Quaternaryalluvium; SE, southeast.

Geology of southeast Bohol, Philippines

575

Fig

ure

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576

D. V. Faustino

et al

.

Visayan Sea Basin is considered as a single depositionalentity; however, it is subdivided into the Negros–Cebu andnorthwest Leyte sub-basins as a result of uplift along amajor anticline in Cebu during the Late Miocene toPliocene. The basin is underlain by Middle to UpperOligocene limestone–clastic sequences that are uncon-formably overlain by Plio-Pleistocene volcaniclastic andcarbonate rocks (Porth

et al

. 1989). The geological historyof this basin is discussed elsewhere (Rangin

et al

. 1989).Basement units in most of the islands in the Visayan

Sea Basin are composed of Cretaceous to Paleocene meta-volcanic and metasedimentary sequences with interveningcarbonate units. Magmatism during the Paleocene toOligocene is indicated by the intrusion of dioritic stocksalong structural weaknesses (Japan International Cooper-ation Agency – Metal Mining Agency of Japan 1990).Metamorphic and ultramafic rocks that define probablecrust–mantle sequences are exposed in Leyte, Cebu,Antique and in southeastern Bohol. Interestingly, the unitsin Cebu, Antique and southeast Bohol trend roughly north-east. A similar trend is also observed from Zamboanga toSulu and northeast Borneo that led previous workers(Balce

et al

. 1979; Mitchell

et al

. 1986) to infer that a prob-able northeast-trending trench existed parallel to theseislands. Younger formations in the Visayan Sea Basininclude shale, sandstone, limestone and several volcani-clastic units. Carbonate units range from Eocene to Pleis-tocene (Bureau of Mines and Geosciences 1982). Thedistribution of ophiolite complexes and capping sedi-mentary units in the central Philippines can be attributedto progressive accretion of various terranes during severalsubduction events (Yumul

et al

. 1997b; Tamayo

et al

. 2001).Exposure of the basement complex in Bohol reveals its

complexity and its stratigraphic relationships with theoverlying sedimentary rocks and younger intrusive bodies.The proximity of Bohol to the eastern limits of the VisayanSea Basin, which is considered as possibly petroleum-bearing, and the inference of the existence of a palaeo-trench

along

the

southeastern

margin

of

the

basin

makesit an appealing area to study.

GEOLOGY OF SOUTHEAST BOHOL

The island of Bohol is roughly oval-shaped, approximately90 km along its east–west axis and approximately 60 kmfrom north to south, covering an area of 4117 km

2

. The left-lateral strike-slip Philippine Fault Zone lies approximately50 km east of the island (Figure 1). At its centre, the topo-graphy of Bohol is gently rolling to flat. Its eastern andwestern coasts are bordered by mountain ranges thattrend northeast–southwest, reach elevations of up to 800 mand drop steeply into the sea.

The basement complex of Bohol is an agglomeration ofophiolitic units, metamorphic rocks, various volcanicrocks and intrusive rocks made up mainly of dioriticplutons (Bureau of Mines & Geosciences 1982). The over-lying sedimentary units include carbonate and clasticrocks that were deposited in deep to shallow environments.Palaeontological and field evidence on different formationsin the entire island show that almost all of the lithologicalcontacts among the sedimentary units are unconformable.

These lithological suites with the corresponding contactsare also encountered in southeast Bohol (Figure 2a, b).

Seven stratigraphic units are recognised in southeastBohol: three comprise the basement complex, and the restare Miocene to Pleistocene sedimentary rocks with associ-ated volcaniclastic and intrusive bodies (Figure 3). Thebasement complex consists of the Alicia Schist, Cansiwangmélange and the Southeast Bohol Ophiolite Complex thatare separated from one another and from the overlyingyounger formations by thrust fault contacts that generallytrend northeast (Figure 4a, b). Intertonguing rocks ofclastic, carbonate and volcanic origin compose the lowerCarmen Formation, the oldest of the overlying units. Ahornblende andesite body with a whole-rock K–Ar age ofMiddle Miocene (Japan International Cooperation Agency– Metal Mining Agency of Japan 1990) intrudes this unit.An erosional boundary separates the Carmen Formationfrom the overlying sublittoral to bathyal Sierra BullonesLimestone that grades into the younger shallow-marinePleistocene Maribojoc Limestone. Quaternary deposits arefluvial gravel and alluvium.

The Lumbog Volcaniclastic Member of the CarmenFormation, which to our knowledge has not been thor-oughly described previously is defined in Appendix I. Thefollowing sections characterise the stratigraphic units ofsoutheast Bohol.

BASEMENT

Alicia Schist

Chlorite schist, quartz–sericite schist and amphiboliteschist make up the Alicia Schist. The grade of meta-morphism ranges from greenschist to amphibolite facies.At least 650 m thick, the metamorphic complex is bestexposed in the municipality of Alicia. The schists areinterbanded with a general distribution from east to westof chlorite schist, quartz–sericite schist, amphiboliteschist and quartz–sericite schist again (Figures 2a, 4b).Intercalation of these metamorphic units observed in thiscomplex is consistent with the metamorphism of inter-bedded volcaniclastic sediments (Yumul

et al

. 2001b). Anexposure mapped at 9

�

55

�

N, 124

�

28

�

E shows the relation-ship of the three lithologic types wherein amphiboliteschist is sandwiched between quartz–sericite schist andchlorite schist (Figure 4b). In the area north of section lineB–B

�

(Figure 2), the quartz–sericite schist is in directcontact with the overthrust Cansiwang mélange. Foliationin the chlorite schist strikes 040–060

�

with variable dips of20–40

�

NW and 30

�

SE. The amphibolite schist has foliationranging from 010

�

strike, 20

�

SE dip to 142

�

strike, 24

�

SW dipwhereas intercalations of quartz–sericite schist andamphibolite schist are orientated 045–070

�

strike, 20–60

�

SEdip to 110–130

�

strike, 10–40

�

SW dip. The thrust faultcontact between the Alicia Schist with the Cansiwangmélange

is

generally

north–south

trending

with

a

dip

of30

�

to the west.The metamorphic event that produced the Alicia Schist

was interpreted to be due either to a regional event orrelated to emplacement of the Southeast Bohol OphioliteComplex (Mitchell

et al

. 1986; Yumul

et al

. 1995). The Alicia

Geology of southeast Bohol, Philippines

577

Schist was also considered part of metamorphosed contin-ental lithosphere underthrust beneath the oceanic litho-sphere represented by the Southeast Bohol OphioliteComplex (Diegor

et al

. 1996). Yumul

et al

. (2001b) haveshown that the Alicia Schist has a backarc basin geo-chemical signature with a subtle subduction affinity. Thisled them to propose that the Alicia Schist might be afragment of a subduction-related marginal basin that hadundergone ocean-floor regional metamorphism. Unfortun-ately, no radiometric age constraints are available foreither the protoliths or metamorphism in the Alicia Schist.If this unit were part of the Southeast Bohol OphioliteComplex-related marginal basin, then the minimum agefor the Alicia Schist protolith should be Early Cretaceous.

Cansiwang mélange

The Cansiwang mélange was previously mapped as part ofthe Boctol Serpentinite (Japan International CooperationAgency – Metal Mining Agency of Japan 1990; Yumul

et al

.1995). It crops out in three separate areas: La Hacienda,Mayana and Cansiwang (Figures 2a, 5). This mélange,which tectonically underlies the Southeast Bohol Ophio-lite Complex, consists of sheared ophiolite-derived blocksthat include harzburgite, microgabbro, basalt and chert ina dominantly serpentinite matrix. Field evidence and thetransitional island-arc tholeiite–MORB geochemistry of

volcanic rocks found in the mélange suggest its develop-ment in a forearc region. Boninite clasts have also beencollected from this mélange unit. A detailed discussion onthe characteristics and origin of this formation can befound in de Jesus

et al

. (2000) and Barretto

et al

. (2000).

Southeast Bohol Ophiolite Complex

The Southeast Bohol Ophiolite Complex, with a minimumthickness of >2000 m, is a complete mantle–crust sequence(as defined in Penrose Conference Participants 1972)consisting of residual harzburgite, layered ultramaficcumulate rocks that are predominantly clinopyroxenite,massive and layered gabbro, sheeted dykes, and pillowbasalt (Yumul

et al

. 1995). It is exposed in three massifs:Alicia, Guindulman and Duero (Figure 2a). A complete butdisrupted ophiolite sequence crops out only in the Dueromassif whereas harzburgite dominates the Alicia andGuindulman massifs. Geophysical modelling shows thatthe ophiolite complex is thin and not rooted in the mantle(Barretto 1997).

The base of the Southeast Bohol Ophiolite Complexconsists of fractured and mostly serpentinised harzburgitepreviously mapped as part of the Boctol Serpentinite(Japan International Cooperation Agency – Metal MiningAgency of Japan 1985; Yumul

et al

. 1995). This rock is themost widely distributed, and in some areas is the only

Figure 5 Schematic section logsshowing the relationships ofthe Southeast Bohol OphioliteComplex (SEBOC), Cansiwangmélange and the younger sedi-mentary rocks. See Figure 2b forlocation of sections; see text fordetails.

578

D. V. Faustino

et al.

exposed member of the ophiolite. In some harzburgiteoutcrops, dolerite dykes are intruded along fault planes.Serpentinised clinopyroxenite and dunite are limited tosmall lenses within the much more voluminous harzbur-gite. Clinopyroxenite is altered to serpentinite and bastite.Some harzburgites are thrust (fault attitude: 070� strike,25�NW dip) over the gabbros as exposed in the AlejawanRiver (Figure 2b). The layered gabbro (layering attitude:

120–150� strike, 37�NE to vertical dip) is characterised by2 cm thick alternating bands of fine-grained crystals offelsic and mafic minerals. Xenoliths of dolerite are con-tained in the massive gabbro suggesting proximity to theroot of the dyke complex. Parallel dykes of microgabbro,basalt and dolerite, ranging from 10 to 20 cm wide, crop outin Duero and represent the sheeted dyke section. Asidefrom the dominantly parallel dykes, lensoid and pod-likedykes are generally orientated 040–045�, and dip 55–75�NW (Yumul et al. 1995). The dykes are sheeted with nooriginal country rock preserved. Chloritisation andepidotisation of the dolerite and basalt might be due to low-temperature, greenschist-facies ocean-floor metamorph-ism. No field evidence has been encountered to support thepossibility that the observed metamorphism could berelated to ophiolite emplacement.

The massive lava flows to pillow basalts of the Dueromassif are intercalated with chert and manganiferoussediment. Some exposures are capped by bedded pelagicchert containing Lower Cretaceous, upper Albian radio-larians and silicified foraminifers (Tables 1, 2). In contrastto the pillow basalts from the Duero massif that are cappedby cherts and manganiferous sediments, those found in theCansiwang–Labo area are associated with intercalatedradiolarian chert and tuff. The volcanic materials are light-coloured, poorly indurated silt- to ash-sized particles. The

Table 1 Lower Cretaceous (upper Albian) radiolarians andforaminifers from a chert sample.

RadiolariansQuinquecapsularia irregularia (Squinabol)?Pessagnobrachia fabianii (Squinabol)Crucella ? messinae PessagnoSavaryella novalensis (Squinabol)Thanarla sp.Crolanium pythiae (Schaaf)Archaeospongoprunum? cortinaensis Pessagno

ForaminifersSchakoina sp.Hedbergella sp.

The radiolarians allow correlation with Unitary Associations12–14 of O’Dogherty (1994). The original calcareous tests of theforaminifers have been replaced by silica.

Table 2 Results of palaeontological studies done on carbonate samples gathered from the different formations in the study area.

Sample Location Faunal list Palaeoenvironment Age

Anda Limestone Member, Carmen FormationBO6-8-3 Bulawan Miogypsina thecidaeformis

Miogypsina sp.Lepidocyclina (Nephrolepidina) parvaLepidocyclina sp.Amphistegina sp.

– Early to Middle Miocene, probably Middle Miocene

BO0-8-7 Alihan Road, Alicia Planktonics (Orbulina universa)Lepidocyclina (Nephrolepidina) sp.Lepidocyclina (Nephrolepidina) ferreroiMiogypsina thecidaeformisMiogypsina sp.Miliolids

Shallow marine Early to Middle Miocene, probably Middle Miocene

BO-11–2 Guindulman Cycloclypeus sp.Operculina sp.

Outer littoral Probably Early Miocene

BO8-10–1 Mabini Amphistegina sp.Miogypsinopides sp.Lithothamnium?

Middle to inner littoral Early (?) Miocene

BOD-15–00 Talisay–Badiong Echinoid spinesNummulitesAmphisteginaMiogypsina sp. (?)

Middle to inner littoral Early to Middle Miocene

BOD-16–1 Talisay–Badiong Miogypsina sp. (?)Cycloclypeus sp. (?)

– Early Miocene

BOD-15–4 Talisay–Badiong Miogypsina sp. (?) – Early to Middle MiocenePansol Clastic Member, Carmen Formation

BO2-1-1A Alejawan River – Bathyal Middle to Late MioceneBO4-4-13 Candijay (along

National Road)– Bathyal Middle Miocene

BO9-8-8 Napo Road – Bathyal Middle MioceneSierra Bullones Limestone

BO4-6-1A Pansol, Guindulman – Bathyal Middle Miocene to PlioceneBO6-8-9 Pagahat, Alicia – Outer neritic to bathyal Middle Miocene to PlioceneBO5-4-5 Cabungahan – Open marine, probably

outer neritic to bathyalLate Miocene

Geology of southeast Bohol, Philippines 579

Southeast Bohol Ophiolite Complex was emplaced duringthe Late Cretaceous to Paleocene (Yumul et al. 2001b). How-ever, the thrust relationship between the complex and theLower to Middle Miocene Carmen Formation indicatesongoing tectonic activity (Figure 5).

MID-TERTIARY TO QUATERNARY SEQUENCES

Carmen Formation

Three distinct members comprise the Carmen Formation:the Anda Limestone Member, Pansol Clastic Member andLumbog Volcaniclastic Member (new unit: Appendix I).The formation was originally assigned to the MiddleMiocene and characterised as a shallow shelfal, shale–sandstone sequence (Javellosa 1994). Palaeontological datafrom the carbonate section shows that the formationranges from Lower to Middle Miocene (Table 2).

The Anda Peninsula, type locality for the Anda Lime-stone Member, was previously mapped as part of theyounger Sierra Bullones Limestone (Bureau of Mines &Geosciences 1982). Our examination of sublittoral tobathyal faunal assemblages, however, reveals Lower toMiddle Miocene faunas (Table 2). This makes the AndaLimestone Member part of the older Carmen Formationand not of the Sierra Bullones Limestone. Following theclassification of Dunham (1962), the grey to cream lime-stone from this member ranges in composition fromboundstone through packstone and grainstone to crystal-line carbonate rocks. The Anda Limestone Member inter-fingers with siltstone and mudstone of the Pansol ClasticMember of the Carmen Formation.

The Pansol Clastic Member, with a thickness of approxi-mately 1000 m, is widely distributed throughout the studyarea as a sequence of sandstone, siltstone, shale andconglomerate. Palaeontological analysis of clastic rocksmostly yielded Middle Miocene deep-marine fossils(Table 2). The lithologies vary from thinly bedded, calc-areous sandstone and siltstone, to pebbly sandstone thatcontains quartz pebbles, shale lenses, and lithic, corallineand shell fragments. Turbidites consisting of 30–50 cmthick sandstone layers succeeded by thin siltstone andshale layers occur within the sequence. Harzburgite occursamong the clasts, confirming the erosional contact betweenthe Southeast Bohol Ophiolite Complex and the CarmenFormation. Harzburgite has also been overthrusted ontothe Pansol Clastic Member (Figure 5).

A newly defined member of the Carmen Formationrecognised during our study is the Lumbog VolcaniclasticMember (Appendix I), which is approximately 180 m thickand covers an area of 20 km2 (Figure 2a). The LumbogVolcaniclastic Member is dominantly made up of pebble- toboulder-sized basalt and andesite clasts set in an epiclasticandesitic matrix. A few outcrops have felsic matrices. Rareclasts of harzburgite, dacite, gabbro and carbonate toclastic rocks occur in some exposures. The member alsogrades upward into thinly bedded fine-grained volcani-clastic deposits. Although commonly observed as valleyfills in the Pansol Clastic Member, some exposures ofthis unit intertongue with the clastic member of theformation.

Jagna Andesite

The Jagna Andesite was earlier described as andesitebreccia (Javellosa 1994). On the basis of field evidence,the Jagna Andesite was emplaced during the lateMiddle Miocene. The unit, where it is exposed in Jagnaand Mabini, is plug-like. On the Anda Peninsula, it hasintruded the Middle Miocene Lumbog VolcaniclasticMember. Its limited, plug-like topographic expression anda lack of flow structure suggest intrusive emplacement ina hypabyssal environment. Prismatic green hornblendephenocrysts are embedded in glassy groundmass ofmicrolitic plagioclase. Some hornblende crystals areophacitised.

Sierra Bullones Limestone

The westernmost portion of the study area, includingMabini, is underlain by the Upper Miocene to LowerPliocene Sierra Bullones Limestone. This 1350 m-thickcoralline formation is thin-bedded to massive (Bureau ofMines & Geosciences 1982). Marly limestone and grain-stone are rare. Massive and recrystallised sections occurnear Duero and Lonoy. Normally, these are cream to lightbrown and contain calcitised fossil fragments of corals,foraminifers and gastropods. At Lumbog, these carbonaterocks overlie the Lumbog Volcaniclastic Member. TheSierra Bullones Limestone unconformably overlies theSoutheast Bohol Ophiolite Complex (Figure 5). Previousworkers have mapped the limestones in Mabini as part ofthe younger Maribojoc Limestone (Bureau of Mines &Geosciences 1982). However, samples indicate a MiddleMiocene to Pliocene age consistent with the age range ofthe Sierra Bullones Limestone (Table 2).

Maribojoc Limestone

The Upper Pliocene to Pleistocene Maribojoc Limestonecaps most of the western half of Bohol Island, but in thestudy area, exposure is limited to a small hill in Jagna.This formation is composed of cream-coloured, massive tothinly bedded grainstone. Fragments of corals and molluscshells are present. The Maribojoc and the Sierra BullonesLimestones are stratigraphically separated, because of agedifference, although their actual contact is still unclear.

DISCUSSION

Alicia amphibolites: metamorphic sole of the Southeast Bohol Ophiolite Complex?

When a crustal sliver is emplaced, the residual heatcoming from the peridotite results in the development ofdecreasing metamorphic gradient in the metamorphicrocks with increasing distance from the peridotite heatsource. Generally, these metamorphic soles are fromamphibolite facies grading through the greenschist faciesrocks to the original protoliths of igneous and sedimentaryrocks (Malpas 1979; Ghent & Stout 1981).

The serpentinite-rich Cansiwang mélange, and not theAlicia Schist amphibolite, is juxtaposed against the South-

580 D. V. Faustino et al.

east Bohol Ophiolite Complex, similar to relationshipsobserved in the Vourinos and Josephine ophiolites (Harperet al. 1996) (Figure 2a). This suggests that the SoutheastBohol Ophiolite Complex was emplaced as a cold litho-sphere fragment. In addition, the absence of Alicia Schistfragments in the Cansiwang mélange suggests that theAlicia Schist was not in any way contributing material tothe accretionary prism when the mélange was beingformed. This could be interpreted as non-involvement ofthe Alicia Schist, due to its absence during the emplace-ment of the Southeast Bohol Ophiolite Complex and gener-ation of the Cansiwang mélange. The formation of theamphibolite in the Alicia Schist cannot also be directlyattributed to the residual heat coming from the SoutheastBohol Ophiolite Complex peridotites. The temperatureneeded to form the amphibolite (~1000�C) would be so highas to have destroyed the intervening serpentinite sole,which is only stable up to around 550�C. Furthermore, thequartz–sericite schist rather than the amphibolite isdirectly in contact with the Cansiwang mélange. Amongmetamorphic soles found in other ophiolites, foliationsmimic the thrust plane at the base of the overlying perido-tite. The thrust fault that separates the Cansiwang mélangeand the Alicia Schist is generally north–south trending anddips towards the west. Foliation exhibited by the AliciaSchist is also north–south orientated, but dips range fromeast to west. All of these lines of field evidence indicate thatthe Alicia Schist could not have formed as a result of theSoutheast Bohol Ophiolite Complex emplacement. Theinferred relationships can be further tested once ages areavailable for both the Cansiwang mélange and the AliciaSchist.

Southeast Bohol Ophiolite Complex: a marginal basin ocean floor

Two varieties of sedimentary rocks intercalated with thelava flows and pillow basalts of the Southeast BoholOphiolite Complex indicate their formation under variedconditions. Felsic, tuffaceous silts intercalated with thepillow lavas indicate nearby volcanism penecontempor-aneous with extrusion of volcanic rocks on the ocean floor.The presence of umber and radiolarian chert intercalatedwithin the same volcanic section of the ophiolite implies adeep basin (Figure 5). Similar relationships between thepillow basalts and intercalated sedimentary rocks arereported from the Coast Range Ophiolite (Giaramita et al.1998) and other marginal basin ophiolites (Shervais &Kimbrough 1985). The field relationships of the volcanicand sedimentary rocks suggest the formation of theSoutheast Bohol Ophiolite Complex in a deep oceanbasin surrounded by landmasses characterised by arcvolcanoes.

Whole-rock analyses of the pillow basalts reveal a rangeof geochemical composition from boninitic rocks throughMORB to high-magnesian andesites (Yumul et al. 1995;Faustino et al. 1998, 1999; Faustino 2000). Major and traceelements were analysed using the Philips PW 1480 XRF ofthe Geological Institute, University of Tokyo, whereasother trace and rare-earth element (REE) analyses wereperformed utilising a VG Element PlasmaQuad 3 ICP-MS atthe Department of Earth Sciences, University of Hong

Kong. The analytical procedure and data evaluation oftrace elements and REEs are described in Zhou et al. (2000).Plotting these data on conventional tectonic-discrimin-ation diagrams shows a spread, indicating varying tectonicaffiliations for the volcanic–hypabyssal rocks (Figure 6).The boninitic rocks exhibit spinifex texture with Zr/Y <1.At the same Ta/Yb values, the high-magnesian andesiteshave higher Th/Yb compared to the Southeast BoholOphiolite Complex samples with N MORB affinity.Almost all of the rocks show elevated Th and negativeNb anomaly when normalised to N MORB (Yumul et al.

Figure 6 Tectonic-discrimination diagrams showing that theSoutheast Bohol Ophiolite Complex volcanic–hypabyssal rocksspan a range of geochemical affinities from island-arc tholeiitesto N-MORB. (a) Ti–Zr–Y diagram (after Pearce & Norry 1979). a,within-plate basalt; b, island arc tholeiite; c, mid-ocean ridgebasalt/island arc tholeiite; d, calc-alkaline basalt. (b) Hf–Th–Tadiagram (after Wood 1980); two samples have no Ta values. (�),Guindulman massif pillow basalt samples; (�), Duero massif pil-low basalt samples; (�), Duero massif sheeted dyke samples.a, destructive plate margin basalt; b, N-MORB; c, P-MORB; d;within-plate basalt.

Geology of southeast Bohol, Philippines 581

2001b). Formation in a subduction zone-related setting isindicated in Figure 6b in which most of the samples plot inthe destructive plate-margin field. Volcanic–hypabyssalrock assemblages with varied geochemical compositionsare commonly observed in forearc ophiolites and modernmarginal basins (Bloomer 1983, 1987; Johnson & Fryer 1990;Offler & Gamble 2002; Takashima et al. 2002; Wang et al.2002). Field relationships and the geochemical character-istics of the volcanic–hypabyssal rocks support the ideathat the Southeast Bohol Ophiolite Complex was generatedin a marginal-basin environment.

Accretion to sedimentation in a forearc setting

The geology of southeast Bohol is a product of accretion,emergence and submergence. The Cansiwang mélange isinterpreted to have formed in a sediment-starved conver-gent margin ultimately defining an accretionary complexbased on the structures and lithologies observed (de Jesuset al. 2000). Previously, it was proposed that a northeast–southwest-trending, northwest-dipping (present-day geo-graphical position) convergent margin existed between thecentral Visayas Sea Basin and Mindanao in the area nowoccupied by the Bohol Sea (Mitchell et al. 1986; Yumul et al.1995, 1997b). Offshore satellite bathymetry and gravitydata show an anomalously deep, northeast–southwest-trending narrow basin with a low gravity anomaly(Sandwell & Smith 1997). This might correspond to theproposed trench southeast of Bohol (Yumul et al. 2000).This configuration is consistent with the Southeast BoholOphiolite Complex being emplaced in a forearc margin(Faustino et al. 1999).

It is inferred that during the Cretaceous, young oceaniccrust was subducting along a sediment-starved trench.Subduction kneading and other related processes resultedin the formation and accretion of the Cansiwang mélange.As subduction progressed, the Alicia Schist, which hadpreviously undergone regional metamorphism, rides as anoceanic bathymetric high on the subducting slab (Mitchellet al. 1986; Diegor et al. 1996). This conclusion is supportedby the observation that no Alicia Schist clasts are found inthe Cansiwang mélange, which suggests that during theformation of the mélange, the Alicia Schist was still farfrom the trench. Upon collision with the sediment-starvedtrench, the oceanic bathymetric high was accreted insteadof being subducted. The partial subduction of the AliciaSchist resulted in uplift of the overriding plate that borethe Southeast Bohol Ophiolite Complex and Cansiwangmélange. Jamming of the trench, due to the accretion ofthe oceanic bathymetric high, caused the trench to jumptowards the east-southeast (present geographical position).Uplift and erosion of the exposed units produced the uncon-formity that separates the basement complex from itssedimentary carapace. A change in tectonic conditions,typical of an active margin, led to the submergence of thearea during the Early Miocene and the deposition of theAnda Limestone Member and turbidite-dominated PansolClastic Member of the Carmen Formation. Protractedvolcanism during this period is indicated by the LumbogVolcaniclastic Member. This was followed by deposition ofthe Sierra Bullones and Maribojoc Limestones from theMiddle to Upper Miocene.

The entire island of Bohol was then uplifted, startingfrom the southeastern side. The very limited exposures ofthe Maribojoc Limestone in southeast Bohol relative to thecentral and western sections of the island suggest greatlyvariable emergence. Most of central Bohol is covered by thePlio-Pleistocene Maribojoc Limestone that is almost non-existent in the study area. This shows that while centraland western Bohol were still underwater and were activelydepositing the Maribojoc Limestone, the southeastern sidewas already largely emergent.

CONCLUSIONS

A Cretaceous basement complex made up of three unitsfloors southeast Bohol. From the base, the Alicia Schistmetamorphic complex underthrusts the Cansiwangmélange that is, in turn, thrust beneath the SoutheastBohol Ophiolite Complex. The Cansiwang mélange occursbetween the ophiolite and the metamorphic complex,indicating that the Alicia Schist is not a metamorphicsole associated with the Southeast Bohol OphioliteComplex. The accretion of the serpentinite mélangebeneath the ophiolite complex, the highly variablevolcanic-rock geochemistry, and the presence of boninitessuggest emplacement of the Southeast Bohol OphioliteComplex as a forearc ophiolite. Having formed in a supra-subduction zone environment, the marginal basin respon-sible for this ophiolite was related to a northeast–south-west-trending, northwest-verging convergent margin. Theemplacement and accretion of the basement complex wasfollowed by clastic and carbonate deposition (CarmenFormation, Sierra Bullones Limestone and MaribojocLimestone) brought about by the emergence and sub-mergence of the island, typical of an active margin setting.Protracted volcanism also occurred as indicated byvolcanic (Jagna Andesite) and volcaniclastic (LumbogVolcaniclastic Member) deposits. An understanding ofwhat has transpired in southeast Bohol allows us to under-stand the geological and tectonic processes responsible forshaping part of the central Philippines.

ACKNOWLEDGEMENTS

Funding for this project came from the Department ofScience and Technology–Grants-in-Aid Program and wasmonitored by the Philippine Council for Industry andEnergy Research and Development. We are also gratefulfor the support extended by the University of the Philip-pines–National Institute of Geological Sciences (UP-NIGS)and Plantation Mining Incorporated during the course ofour study. Geological mapping was done with our 1997Geology 170 and 215 (Bohol Group) students. Discussionswith them helped considerably in the formulation of ideaspresented herein. Additional palaeontological analyseswere provided by M. N. Tan and E. Y. Mula of the Mines andGeosciences Bureau Central Office and by L. Zamoras ofthe UP-NIGS. Comments on an earlier draft of this paperfrom K. Rodolfo, F. Jumawan, E. Marquez, K. Queaño, A. H.G. Mitchell and F. Ego are appreciated. Reviews made

582 D. V. Faustino et al.

by P. A. Cawood and an anonymous reviewer greatlyimproved the manuscript.

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Received 1 December 2001; accepted 7 June 2003

APPENDIX I: DEFINITION OF A NEW STRATIGRAPHIC UNIT

Lumbog volcaniclastic member

The name Lumbog Volcaniclastic Member is here pro-posed to refer to a new member in the upper part of theCarmen Formation.Type locality The best exposure and type locality isfound at the Lumbog River, 4 km northwest of Guindulman(at approximately 9�45�28�N,124�25�30�E).Description The Lumbog Volcaniclastic Member isdominantly made up of pebble- to boulder-sized basalt andandesite clasts set in an epiclastic andesitic matrix. Someoutcrops are characterised by felsic matrices. Rare clastsof harzburgite, dacite, gabbro, and carbonate and clasticrocks occur in some exposures. The member also gradesupward into thinly bedded fine-grained volcaniclasticdeposits.Thickness The unit is estimated to be ~180 m thick.

Stratigraphic relationships The Lumbog Volcaniclas-tic Member typically occurs as valley fills in the PansolClastic Member. However, some exposures of this unitshow an intertonguing relationship with the PansolClastic Member.Distribution In addition to the exposure at the typelocality, the unit has also been observed in the followingareas: Cambane and Tambongan in Candijay; Tubod Mar,Mayuga–Bayong and Tangguhay River in Guindulman;and Talisay in Anda.Age The presence of clasts belonging to the Anda Lime-stone in some exposures and the intertonguing relation-ship with the Pansol Clastic Member suggests a MiddleMiocene age for this unit. The Lumbog VolcaniclasticMember was produced as Southeast Bohol was being sub-merged starting from the Early Miocene time.