BukuTomini Basin.pdf

8/19/2019 BukuTomini Basin.pdf

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Deep sea sediment gravity core

GRT-05-03 with long of 145 cm

was obtained from the seafloor

of Tomini Basin at the water

depth of slightly below 2400 m

at coordinates 000°31.699’ S

and 120°51.979’ E at the south-

western slope of the TominiBasin.

http://www.mnhn.fr/mnhn/geo/Collection_Marine

/moyens mer/Engins_de_prelevements_eng.htm


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D. Kusnida

I.R. Silalahi

T. Naibaho

Subarsyah


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MARINE GEOLOGICAL INVESTIGATIONSIN THE TOMINI BASIN, CENTRAL SULAWESI

Editor in chief : Agus Setiya Budhi

Editor : Susilohadi

Prijantono Astjario

Managing editor : Asep Makmur

Andi Sianipar


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Research Vessel Geomarin III


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M ARINE GEOLOGICAL INVESTIGATIONS IN THE T OMINI B ASIN i


PPaaggee

PPrreef f aaccee ................................................................................................................................................................................................ ii

SSuummmmaarr y y ........................................................................................................................................................................................ iiiiii

Introduction …………………………………..………..……...…………………….. 1

Seismic Stratigraphy............................................................................ 5

Basement Configuration ……………………………..….………………..…….. 11

Seismic 2D and Gravity Anomaly...................................................... 17

General Mineralogy ………………………………….………………….……....… 23

CCoonncclluussiioonn ................................................................................................................................................................................ 2299

R R eef f eerreenncceess .................................................................................................................................................................................. 3311

BBiiooggrraapphh y y


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M ARINE GEOLOGICAL INVESTIGATIONS IN THE T OMINI B ASIN ii

Preface

This small book presentsthe composite and general

picture of limited articles

related to the marine

geological and geophysical

surveys in Tomini Basin-

Central Sulawesi that have

been published recently.

Marine geological andgeophysical investigations

carried out by Marine

Geological Institute of Indonesia (MGI) in 2005 using RV Baruna Jaya

VIII was to obtain geological and geophysical information related to

potencies on geological resources which could possibly discovered.

Additional revisited geophysical surveys concentrated in Tomini Basin

using RV Geomarin III in 2010 were directed to obtain a denser and a

better quality of seismic records.

The surveys were financed by Systematic Marine Geological Mapping

Project of Marine Geological Institute of Indonesia. The authors wish

to thanks Mr. Susilohadi as a director of MGI for his supports to

publish this book. Thanks are also directed to Mr. Joni Widodo as a

team leader and Mr. Kristanto as a chief scientist during data

acquisition using RV. Baruna Jaya VIII in 2005, and for allowing us to

work on the material and use the data for the articles. Thanks are alsodirected to Mr. M. Hanafi as a team leader during data aquisition

using RV. Geomarine III in 2010. High apretiations are directed to

Mrs. Yudhicara for onboard sample preparation, and to the Crews and

Technicians of RV. Baruna Jaya VIII and RV. Geomarine III for their

help and patient during data acquisitions.

MMMGGGIII ((( 222000111000 )))


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M ARINE GEOLOGICAL INVESTIGATIONS IN THE T OMINI B ASIN iii

Summary

Tomini Basin is a deep sea frontier sedimentary basin lies at the innerpart of the Gulf of Tomini where the northern and eastern arms of

Sulawesi flank the basin. The gulf is characterized by a bathymetric low

of slightly below 2400 meters in Tomini Basin in the west, and a

bathymetric low of a slightly below 4000 meters in Gorontalo Basin to

the east. The Tomini Basin can be considered as a complex back arc

basin of nearly elongated-shaped depocenter, which is oriented east-

west. The islands group of Togian characterizing the NE-SW traversed

highs together with the Una-Una islands where the Colo volcano issituated separates the Tomini Basin from the Gorontalo Basin.

Studies on offshore multi-channel seismic reflection data

complemented by published on-land geological data indicate a series of

tectonic events that influenced the depositional system in Tomini

Basin. Seismic data confirmed that the lower sediment sequence in

Tomini Basin is characterized by synrift-sagging-postrift-syninversion

sequences typical of the Sunda tectonic system. Subsequently, during

the late Neogene, alternating pulses of terigenous sediment were

deposited in the basins in the form of deep-sea slump-turbidite-pelagic

sediments. A sediment gravity flow deposits system at the slope and

base of slope of the basin changed gradually into a deep-sea pelagic fill

system toward the center of the basin. Three tectono-stratigraphy

sequences ( A , B and C) separated by unconformities indicating the late

Neogene history and development of the basins were identified. These

tectonic processes imply that the earlier sediments in Tomini Basin are

accomplished by differential subsidence, which allows a thickening of

basins infill. The Pliocene-Quaternary basins fill marks the onset of

sediment gravity flow deposits dominated deposition system.

Based on marine magnetic modeling, the main structural and

geological elements of the basement of Tomini Basin are identified.


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M ARINE GEOLOGICAL INVESTIGATIONS IN THE T OMINI B ASIN iv

At the center of the basin, the up-doming feature is lead to elevated

magnetic susceptibility values. Magnetic and geological model indicate

that the entire basement of Tomini Basin is characterized by anoceanic-like crust with a basin axis at the center nearly an east-west

direction and presents rift-related basement graben. However, on the

basis of gravity map and seismic data, the Tomini Basin in fact is

subdivided into the north and south sub-basinal structures.

The general mineralogy aspect analyzed from a single core GRT-05-03

of surficial sediments of Tomini Basin with the thickness of 145 cm,

reveals the concentrations and shallow vertical distributions of minorelements (Au, Ag, Cu, Pb, Zn, Cr, Mn, Ni, Fe and Co), major elements

(SiO2, Al2O3, Fe2O3, CaO, MgO, K 2O, Na2O, TiO2 and LiO), and trace

elements (Th, Zr, Ba, Nb, Ce and Sr). However, the highest

concentration of minor elements is dominated by manganese (2865-

3211 ppm) and trace elements is dominated by barium (245-289 ppm).

High content of Mn within the surficial sediment column in Tomini

Basin indicates anoxic environment where Mn solubed and burried within anoxic sediment, which then slowly migrated and accumulated

in oxidized sediment layers above formed MgO.

Published articles show that the conspicuous occurrence of barium in

Tomini Basin can possibly be controlled largely by the biogenic matter,

although the detritus fraction in Tomini Basin was dominant in the

sediments. Likewise, it can also be explained that the authigenic barium

(Baex ) correlates with gradual change in sedimentation environmentduring glacial ages. The Baex may relate to calcareous organisms besides

siliceous ones. Baex was reduced to sulfide and dissolved away in a

strongly anoxic environment during biologically productive period.


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M ARINE GEOLOGICAL INVESTIGATIONS IN THE T OMINI B ASIN 1

Introduction

The Indonesian archipelago (Figure 1) is a group of micro tectonicplates, which are united by the convergence of three main tectonic

plates; those are Indo-Australian Plate from the southeast; Eurasian

Continental Plate from the northwest, and Pacific Oceanic Plate from

the northeast. The predominant convergent plate margins in Indonesia

are the active deep-sea trench-island arc systems that called the Sunda-

Banda Arcs. The arc displays the classic morphology of outer rise,

trench, forearc ridge, forearc basin, volcanic inner arc and backarc

basin (Hamilton, 1988).

Figure1. Major tectonic elements of Indonesian archipelago

Recently, Koesoemadinata (2006) proposed that the Indonesian

archipelago tectonically could be divided into three plate tectonic

regions those are west, central and east Indonesia. West Indonesia,

where the Sunda Platform acts as a continental core, is characterized by

subduction tectonics between the Indian Ocean and the Eurasian

Plates, with frontal subduction south of Java, and oblique subduction


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west of Sumatra. Central Indonesia which is ornamented by many

micro continents and micro oceanic plates is separated from West

Indonesia by Makassar Strait in the west, and trenches of Nusatenggara-Banda Arcs in the south, Halmahera-Sangihe Islands in the east and

Sulawesi Sea in the north. East Indonesia with Arafura Platform acts

as continental core and so called as the Banda Arc Complex is

dominated by collision between the Eurasian, the Pacific and the Indo -

Australian Plates. The study area (Figure 2) is situated at the poorly

understood region, where it precisely lays in the transition zone

separating the north arm of Sulawesi Volcanic Arc to the north from

the Banggai-Sula micro-continent collision-Molluca Sea Platesubduction systems to the east and Palu-Koro Transcurent Fault to the

west.

Figure 2. Map shows topographic and tectonic elements of the study area.

Map modified from Silver et al (1983), SRTM and DEM of NASA (2000).

Line D and line B indicate seismic profiles produced in Figure 3 and Figure 4


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The tectonic complexity displayed in Central Indonesia is the results of

a series of Neogene collision events between island arcs, continental

fragments, and the Australian continent. The Gulf of Tomini ischaracterized by a bathymetric low of slightly below 2400 meters in

Tomini Basin, and a bathymetric low of a slightly below 4000 meters

depth in Gorontalo Basin to the east. The islands group of Togian

characterizing the NE-SW outer traversed highs together with the Una-

Una islands where the Colo volcano is situated, separate the Tomini

Basin in the west from the Gorontalo Basin in the east. The present of

dunite in Colo volcanic products may indicate that the magma source

had through an oceanic material that possibly is part of East SulawesiOphiolite Complex (Silver et al, 1983).

On the basis of geophysical and geological expeditions in SW Molucca

Sea, NW Banda Sea, and in the eastern arms of Sulawesi, Silver et al

(1983) indicate that the Gulf of Tomini is underlain by oceanic crust,

and its south edge is uplifted against the thrust. The Sulawesi ophiolites

that can be traced offshore indicate its origin as basement of the Gulfof Tomini. Permana et al (2002) indicate that the Gulf of Tomini is

dominated by an east-west direction of steep graben-like structure. To

the north, the gulf is bordered by the north arm of Sulawesi and to the

south is bordered by East Sulawesi Ophiolite and Old Mélange

Complexes. These may suggest that the gulf to have been formed as the

result of opening and rotating of northern Sulawesi in Neogene about

5 Ma (Walpersdorf et al, 1998).

Seismological data (Permana et al 2002) show two different patternsbeneath the Colo volcano. The first pattern, at 100 to 200 km depths

related to the southeast subduction of Sulawesi Sea Plate and the

second, directly beneath the Una-Una Island at 70 to 100 Km depths is

related with the northwest collision of Banggai-Sula micro-continent to

the Eastern Arm of Sulawesi where in some places show imbricated

thrusts.


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According to Karig et al (1980), tectonic evolution of convergent plate

margins makes the back and forearc basins very sensitive to study

because of the complex interaction between tectonics andsedimentation processes. The basins form an important element of

convergent plate boundaries. They represent large basin as major sites

of sediment accumulation with a volcanic/plutonic arc represents the

major source of basin fill.

Offshore sedimentary basin situated around the west and southeastern

regions of the Sunda Shelf have been explored and have been

considerably well informed. Oil exploration and geological studies ofNorth Sumatra Forearc Basin (Izart et al, 1994), Middle Sumatra Basin

(Matson and Moore, 1992), West Sumatra Basin (Beaudry and Moore,

1985), East Java Basin (Letouzey et al, 1990; Brensden et al, 1992;

Koesoemadinata et al, 1999; Basden et al, 2000; Sribudiyani et al,

2003) and forearc basin off southwest Sumatra and southwest Java

(Susilohadi et al, 2005) gave valuable information on the geological

evolution of this region. Studies of central Indonesian basins such as:

Flores and Savu-Lombok Forearc Basins (Silver et al, 1986; Van Weering et al, 1989; Van der Werff et al, 1994) and Bali backarac basin

(Kusnida, 2001) provided a broad outline of the geometry and

sedimentary sequences of this active margin system and portrays the

Cenozoic evolution of the basin.

According to the Indonesian offshore basin status map (Dirjen Migas,

2003), the Gulf of Tomini where the Tomini and Gorontalo

physiographic basins situated are still unexplored, and so far never havebeen studied and discussed considerably in detailed and it is remains

poorly understood. For this reasons, it was decided to study the basins

within Gulf of Tomini particularly the Tomini Basin to determine

stratigraphic succession, structures and depositional characteristics with

considerable interest for scientific purposes.


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Seismic Stratigraphy

The seismic reflection profiles show the development of three seismicsequences A, B and C (Figure 3 and Figure 4). Seismic sequence A

extends into the center of the basins, and gradually thins to a few

reflectors above the flanks of the basins with thickness of less than 100

mSec. The sequence pinches out against the top of the basement high

especially in the southern flank of the basins. As recognized in the Bali

backarc basin (Kusnida, 2001), seismic facies in the Tomini Basin can

also be subdivided into two sub-sequences with different reflection

characters. In Tomini Basin (Figure 4.), the lower part of sequence A has a thickness of 600 mSec TWT in average and it composed of

alternating transparent to weakly reflective beds of a limited continuity.

At the base, minor base of slope mound deposits is found and lap onto

seismic sequence C at both flanks of the basin. The upper part is 400

mSec TWT thick and consists of a band of continuous alternated by a

chaotic, low amplitude and low frequency reflectors. The geometry and

seismic facies of both subsequences indicate an active lower slope

progradation similar to seismic sequence B underneath. Both the weak

reflectivity and the low seismic coherence of the lower seismic facies

unit indicate slump deposits. The high amplitude reflections of the

upper sub-sequence suggest an alternation of turbidites and pelagic

sediments. Following Mc. Caffrey and Silver (1981), this sequence

possibly indicates an alternating of Quaternary turbidite-pelagic

deposits.

Seismic Sequence B is characterized by semi-transparent mostly chaotic

and medium amplitude at the base-of-slope of the basin, and stratified,

divergence with parallel reflectors toward the center part of the basin.

This difference presumably reflects differentiation of sedimentary facies

of the basin fill. This sequence has been slightly deformed, especially

along the margins of the basin, and laps onto sequence C underneath.


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In the center of the basins, the sequence has a maximum thickness of

400-600 mSec TWT and is faulted.

Figure 3. Seismic line across Tomini Basin indicates sediment gravity

flow deposits in the center of the basin derived from both the NE and

SW directions. For location see Figure 2.

Figure 4. Seismic profile across Tomini Basin indicates sediment gravityflow deposits in the center of the basin derived from both the SE and

NW directions. For location see Figure 2.

Toward the flanks of the basin, it rapidly thins to a few hundred

meters, and extends northward across graben and basement high in the

center of the basin, as a parallel bedded depositional unit. Seismic


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sequence B is laps onto seismic sequence C at both flanks of the basin

beneath seismic sequence A . The seismic facies is composed of

relatively steep north-dipping oblique progradational reflectors, whichdownlap seismic sequence C and have an interval velocity of 2000 m/s.

The reflection pattern is characterized by semi transparent to chaotic,

even bedded, high amplitude, low frequency reflectors. The upper

boundary is erosive and characterized by a high amplitude reflector.

The depositional setting and seismic facies are typical for active slope

mass-flow progradation of submarine fan complex (Vail et al, 1977).

The same reflector is also recognized in the entire Lombok forearc

basin (Van der Werff et al, 1994) and Bali backarc basin (Kusnida,2001). This reflection character and configuration suggests an

alternation of turbidite and thin bedded pelagic sediments possibly of

lower Pliocene.

At the base-of-slope of the basin, seismic sequence C acts as the basin

floor for the seismic sequence A and B, which fill the depressions

between its basement highs underneath. The lower sequence boundary

is formed by seismic reflection terminations which lap on thebasement. In the center of the basin, the sequence boundary is slightly

erosional to paraconformable and is covered in the south by the

downlapping reflectors of sequence B. The seismic facies is

characterized by continuous, even bedded, high amplitude, low

frequency reflectors and has an interval velocity of 2600 m/s. On the

basis of its depositional setting and reflection configuration (Vail et al,

1977), the lithofacies is interpreted as siliclastic sequence, which was

deposited in deep water environment. Following Mc. Caffrey andSilver (1981), we have assumed an Upper Miocene age for the

unconformity on top of the block faulted, moderately reflective seismic

basement, thus assigning that this sequence is considered as a Miocene

age. The unconformity on top may mark the regional important of late

Miocene tectonic phase.


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Slope Basin Sediments

At base of slope of the basin, seismic sequence C is overlain bysediments with an age that overlaps seismic sequence B but have

characteristics that are similar to seismic sequence A. Seismically,

seismic sequence C from this zone are relatively lithified and faulted,

reflecting exposure of acoustic basement for seismic sequence A and B.

Figure 3 shows that seismic sequence B is thick deposit. The lithologic

unit corresponding to seismic sequence B is differed from seismic

sequence A by an increase in lumpy block elements and a decrease inthe pelagic material. It is also characterized by rapid deposition and

high frequency of locally derived turbidites generated by rapid changes

in seafloor topography, which resulted in slope instability. Blocky and

lumpy slumps deposits are characterizing this zone.

Distal-Plain Sediments

Two main seismic sequences (seismic sequence A and B) occur above

the acoustic basement (seismic sequence C), which is well imaged onFigure 3 and 4. In ascending order, these seismic sequences consist of

sediment with short, irregular reflectors that passes up into sediment

with strong, irregular, and laterally discontinuous reflectors (seismic

sequence C). Seismic sequence B comprises ± 600 mSec TWT of

irregularly reflecting sediment, with stronger, chaotic reflections that

concentrated at base-of-slope. Seismic sequence A comprises an upper

unit of acoustically intercalated sediment with markedly a weakly semi

parallel reflectors in longer segments and an upper unit of stronger,

more continuous reflectors, many displaying diffraction hyperbolae

especially at base-of-slope of the Tomini Basin (Figure 3.).

At base-of-slope of the basin, seismic sequence A represents younger

deep-sea fan sediments and sequence B represents older deep-sea fan

sediments. At base-of-slope of the basins, seismic sequence B is


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characterized by a high degree of deformation and fracturing, consistent

with its long history of movement, burial and deformation. Internal

structures in these units are poorly preserved. Seismic sequence A isalso composed of pelagic interspersed with thick turbidites. The zones

of harder reflectors in the upper parts of seismic sequences A and B are

made up of large numbers of short, concave-downward segments, some

of which represent refractions from hard zones lying within the

sediments, possibly thin slumps-turbidite intercalated with pelagic

sediments.

StructuresOur seismic reflection profiles indicate that the southeastern part of the

Gulf of Tomini exhibits a prominent tectonic features of a series of

buried faulted basement block that trend slightly NE-SW characterizing

the submarine basement ridges which reached its depth greater than

4200 meters and its associated Togian Islands Complex (Figures 3 and

4). The southern part of Gulf of Tomini has been uplifted along the

Togian Islands, which in turn is partially overthrust that developed

possibly since Pliocene. Seismic profiles also indicate that the largest ofthese blocks divide the Gulf of Tomini into the Tomini Basin in the

west and the Gorontalo Basin to the east.

The flat-lying, undeformed sediments fill in the center of the basins

within the basin and lack of the faults underneath suggests that these

normal faults have not been active prior to the opening of the Gulf of

Tomini. The east-west widening of the basin dimension and a smaller

morphology can also be explained probably related with an east-westpull-apart basin formation. Based on the seismic profile interpretation,

four general tectonic phases can be portraying in Tomini Basin:

a. Normal faults were active during the earliest phase of tectonism and

formed late Miocene extensional basin and filled continuously by

late Miocene sediment deposits.


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b. Thrust structural style recognized in the Tomini Basin indicates

inversion tectonic related to the Pliocene collision of Eastern Arm of

Sulawesi with the Banggai-Sula micro continent, where thecompression movement has modified the previously extensional

tectonic environment within the gulf. It was involving basement

differential subsidence of the basins and the formation of Tomini

Basin.

c. The back arc thrust zone has been formed since Pliocene (indicated

by uplifted basement, see Figure 3).

d. Eastward widening of the basin indicates a north-south increase in

the total amount of shortening.


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Basement Configuration

In the framework of deep-sea basin exploration in the frontier areaparticularly in the central and eastern Indonesia, Asikin and Safei

(2008) proposed the application of polyhistory basin concept, which

deals with basin classification formerly introduced by Kingston et al

(1983). However, there are three important parameters recognized in

polyhistory basin concept that have to be considered in sedimentary

basin analyses, those are type of basement underlies the basin

(continental or oceanic), tectonic environment and type of plate

boundaries.

The Tomini Basin in Central Sulawesi so far is still controversy,

remains relatively unknown and less studied in detail in this tectonic

setting. On the basis of seismic reflection studies, two different

opinions related with sediment fill in Tomini Basin have been raised.

Wijaya et al (2007) defined that the sediment fill in Tomini Basin

consist of shallow marine deposits such as sandstones and reefs form a

petroleum system. In contrast, Kusnida and Subarsyah (2008) indicate

alternating pulses of terigenous sediment in the form of deep-sea slump-

turbidite-pelagic sediments that changed gradually into a deep-sea

pelagic fill system toward the center of the basins. These different

opinions may lead to the assumption that the Tomini Basin can

possibly underlain by continental or oceanic-like crusts.

Different with on-land geology; offshore geology cannot directly be

examined as most information related with sub-seafloor geology is

resulted from marine geophysical investigations such as from marine

magnetic surveys. According to Christopher et al (1995), possible

causes for strong magnetic highs are the presence of rock masses

contains magnetite mineral such as gabbros, diorite, basalt and other

mafic igneous rocks. In contrast, felsic igneous rocks, like granite or

rhyolite, and most sedimentary rocks are notably non-magnetic may


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show up as distinct magnetic lows are also mapable. Advanced

techniques and detail analyses of magnetic data have been developed

which can predict the depth, shape, and orientation of basement belowthe seafloor as one of the important parameters required in polyhistory

basin concept as proposed by Kingston et al (1983).

Marine magnetic interpretation from Tomini Basin is aimed to portray

the physical characteristics of the basement underlies the basin more

clearly and hope can give a better understanding for scientific and

academic purposes. In this book, the study is limited on delineation of

the lateral and a vertical variation of rocks underlies the Tomini Basinrepresented by magnetic susceptibilities distribution.

Four lines of nearly 450 km long of magnetic survey technique were

resulted from the Tomini Basin. However, here only two magnetic lines

are performed and modeled (Figure 5and 6). The positive and negative

anomalies values portray magnetic basement lineation and represent

the highs and the lows. Magnetic profiles show the up-doming like-

feature in the center of the basin where the emerge anomalies variesfrom -284.0 to 171.1 nT in Line-B and from -68.8 to 149.3 nT in Line-

D. Total magnetic anomaly at the southeastern flank of the basin is

more complex due to the present of Togian Islands where the Colo

volcanism activity is located.

Profile Line-B and Line-D (Figure 5 and 6), both indicate the possibly

sedimentary cover of the basin as characterized by susceptibility value

range from -0.005 to 0.001 cgs units. Magnetic model Line-B (Figure 5)shows that the high total magnetic anomaly in general occupy the

center of Tomini Basin, and in fact is characterized by -0.11 cgs units.

The southeastern flank of the basement is characterized by

susceptibility values range from 0.04 to 0.1 cgs units, while the

northwestern flank is characterized by susceptibility values range from

0.01 to 0.04 cgs units. Line-D (Figure 6) indicates that the center of the


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basin is characterized by -0.12 cgs units. To the northwest, the

basement is characterized by -0.05 cgs units, while to the southeastern

from the center of the basin, the basement is characterized bysusceptibility value of -0.013 cgs units.

Figure 5. Magnetic and geological models of Line-B.

Magnetic susceptibility in cgs unit.

Figure 6. Magnetic and geological models of Line-D.

Magnetic susceptibility in cgs unit.


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Magnetic model Line-D (Figure 6) shows that the depth of basement

rocks of the Tomini Basin laterally vary from 2500 meters to 3400

meters below sea level, where each block of rock mass is bounded at itssided by a series of graben like-structures.

Regionaly the lower total magnetic anomalies values dominating the

flanks of the Tomini Basin, except the center of the basin where the

anomaly tend to be high. Most of the elongation of negative values

occupy zone which indicate the occurrence of magnetic basement

characterising the flanks area of the basin and seems to be related with

the magnetic basement setting which is indicating the presence of therift-related basement.

From the center toward the northern flank of the basin, magnetic

models indicate a major graben, and the total magnetic anomalies

range between -284 to 171.1 nT. On model Line-B the anomalies at

the southeastern part of the basin, varies with several lows and highs

and it is possibly caused by the present of Togian Islands represent

imbricates zone where the Colo volcano is also present. It can beassumed that even though the general trend of the total magnetic

anomaly is slightly north-south, but the occurrence of high closures

toward the center of the basin indicate the underwent and controlled

of the east-west structural lineation formed horsts and grabens.

Magnetic models portray that the center of Tomini Basin possibly is

underlain by basaltic rocks (peridotites ?) with susceptibility values

range from 0.11 to 0.12 cgs units and by (diorites ?) with susceptibility

value of 0.013 cgs units toward the Togian Island where the Colo volcano is situated (see Table 1) .

A negative (-) symbol behind susceptibility values in the models possibly

denote a susceptibility contrast that is the relative susceptibility between

two or more rocks composed the basement (Huang and Fraser, 2001),


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or it can also be caused by “magnetic reversal” during basement rock

formation (Christopher et al, 1995).

Table 1. Magnetic susceptibility of selected rocks

(Christopher et al, 1995)

Rock/Mineral Density

(103 kg

m-3)

Volume k (10-6

SI) = cgs unit

Mass λ (10-8m3kg-1

gneous

Rocks

Andesite 2.61 170,000 6,500

Basalt 2.99 250 -180,000 8,4 - 6,100

Diabase 2.91 1,000 - 160,000 35

Diorite 2.85 630 -130,000 22 - 4,400

Gabbro 3.03 1,000 - 90,000 26 - 3,000

Granite 2.64 0 - 50,000 0 - 1,900

Peridotite 3.15 96,000-200,000 3,000-6,200

Porphyry 2.74 250 - 210,000 9,.2 - 7,700

Pyroxenite 3.17 130,000 4,200

Rhyolite 2.52 250 - 38,000 10 - 1,500

Igneous rocks 2.69 2,700 - 270,000 100- 10,000

Average acidic

ignous rocks 2.61 38 - 82,000 1,4 - 3,100

Average basic

ignous rocks 2.79 550 - 120,000 20

Sedimentary

Rocks

Clay 1.70 170 - 250 10 - 15

Coal 1.35 25 1,9Dolomite 2.30 10 -- 940 1 - 41

Limestone 2.11 2 - 25,000 0.1- 1,200

Red sediments 2.24 10 - 100 0.5 - 5

Sandstone 2.24 0 - 20,900 0 - 931

Shale 2.10 63 - 18,600 3-

Average

sedmntry rocks 2.19 0 - 50,000 0- 2,000


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Geologically, the high magnetic susceptibility of -0.11 to -0.12 cgs units

characterizes the interior under the center of the basin, where thegraben-like structures are in it. In contrast, the low magnetic intensity

anomaly characterizing the flanks of the basin, which is in general

characterized by -0.01 – 0.05 cgs unit at both flanks.

Based on the susceptibility values, the basement of Tomini Basin is

predicted to be equivalent to crystalline basement (Kusnida et al, 2009).

Shallower susceptibility contrasts which is occur toward Colo volcanic

is in contrast with host rocks, may mask or complicate theinterpretation of magnetic basement, therefore susceptibility variations

within magnetic basement in Tomini Basin are common.


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Seismic 2D and Gravity Anomaly

A careful examination of the primary and secondary marine geophysicaldata acquired by MGI in 2005 and 2010 indicate the present of the

subasinal structures in the Tomini Basin. The occurrence of high

gravity anomalies that extend from the northeast (Togian Islands) to the

southwest indicate the possibility of basement high that separates the

Tomini Basin into the southern and northern subasinal structures.

Identification of the occurrence of subasinal structures in Tomini Basin

was implemented by using gravity anomalies published by Centre forGeological Survey of Indonesia - CGS (Pusat Survey Geologi) in 2008

(Figure 7), and seismic 2D lines acquired by the Marine Geological of

Indonesia (MGI) in 2010, those are seismic lines 28, 30 and 32

(Figures 8, 9 and 10).

Gravity anomalies indicate the occurrence of two areas with negative

anomalies separated by the northeast-southwest elongation of positive

anomaly in between. Low gravity anomalies of 0 to -80 mgal indicated

by light to dark blue colors represent the northern and southern

subasinal structures in Tomini Basin (Figure 7).

Seismic line 28 (Figure 8), indicates the uplift of basement rocks causes

high gravity anomalies in this area. These basement rocks separated the

northern and southern sediment infill in Tomini Basin. The blue line

within the seismic profile indicates the top of basement rocks. Line 30

(Figure 9) shows the submarine exposure of basement rocks causing the

gravity anomaly of this line is high. This basement rocks suggest being

composed of ophiolites (Parr and Hananto, 2002). In the southern

part of this seismic profile clearly show the faults caused by the uplift of

basement rocks. Seismic profile also indicates that on the upper part of

the basement rocks there are still unfaulted sedimentary layers which


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lead to the interpretation that basement uplift underwent later after

sedimentary filling.

Plotting of the peaks of the basement rocks indicated from seismic lines

28, 30 and 32 marked by red dots (Figure 11), show the shifting of the

basement rocks boundary relatively northward. In seismic line 28, the

peak of basement rocks lies at ± CDP 6378 or 40 km northward from

the southern end of the seismic line. In seismic line 30, the peak of

basement rocks lies at ± CDP 5430 or 34 km southward from the

northern end of the seismic line. In seismic line 32, the peak of

basement rocks lies at ± CDP 8250 or 54 km northward from thesouthern end of the seismic line.

Yellow area (Figure 12), is a boundary line connecting the three peaks

of basement rocks, whereas the lower boundary is assumed to be a

rough zone as there is no seismic data, but it is only based on gravity

data. From this zone clearly seen that the value of gravity anomaly not

necessary identical with the basement rocks margin.

The present of the subasinal structures in Tomini Basin clearly seen

from the negative gravity anomaly pattern and from three seismic

profiles. The maximum sediment thickness can be seen from seismic

line 30, and suggest to be the central part of this subasinal structures.

The southern subasinal structure has a lower gravity anomaly value

compared to the north. It indicates that the sediment fill in the

southern subasinal structure is thicker than the northern.


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Figure 7. Gravity anomalies map of Tomini Basin (CGS, 2008)

and location of seismic lines produced in figures 8, 9 and 10


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Figure 8. Seismic line 28 (Subarsyah and Sahudin, 2010)

Figure 9. Seismic line 30 (Subarsyah and Sahudin, 2010)


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Figure10. Seismic line 32 (Subarsyah and Sahudin, 2010)


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M ARINE GEOLOGICAL INVESTIGATION IN THE T OMINI B ASIN

22

Figure 11. Southern subasinal structure of Tomini Basin based on gravity

anomaly (blue zone) and based on seismic profiles (yellow zone)


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23

General Mineralogy

Regional deep sea marine geological survey in Tomini Basin has beenexecuted in the framework of deep sea geological thematic mapping

conducted by Marine Geological Institute of Indonesia in 2005. The

Tomini Basin is geologically considered less studied and relatively

unknown particularly on seafloor mineralogy. To invent and determine

the vertical distribution of the trace, minor and major elements in

surface sediments of the basin, core sample GRT-05-03 from

coordinates 000°31.699’ S and 120°51.979’ E at site on the sea floor of

the basin at a water depth more than 2400 meters was studied.

Seafloor sediment core was taken by using geological and geophysical

instruments installed on RV Baruna Jaya VIII that complemented by

depth sonar, single-beam echosounder 10.000 m (EA500). The

navigation in the surveyed area was carried out by means of Global

Positioning System (GPS) using EIVA A/S NAVIpac software.

Sediment sampling method was gravity corer, sub-sampling 1-15 corer

made of PVC with diameter of 10 cm and height of 15 cm. Sampling

and handling procedures for all trace elements such as Th, Zr, Ba, Nb,

Ce and Sr were those used in general analytical practice using X-ray

fluorescence (XRF) method, whereas for minor and major elements

using AAS Flame analyses and gravimetry methods.

Table 1 and Table 2 show the results of AAS Flame and gravimetric

analyses for minor and major elements, whereas trace elements from

XRF analyses are shown in Table 3. Table 1 indicates the domination

of Manganese (Mn) with concentration increase downward from 2876

to 3211 ppm. Other elements indicate a decrease concentration

downward such as Au (0.0130-0.0055 ppm), Ag (40-30 ppm), Cu (95-

90 ppm) and Co (22-18 ppm). However, Pb (122-130 ppm), Cr (45-70

ppm), Ni (33-44 ppm) and Fe (1.11-1.55 %), indicates the increasing


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24

concentration downward, whereas Zn (± 100 ppm) seems to show a

stable concentration.

Table 2 indicates that the major elements is dominated by SiO2 with a

relatively constant concentration that is ± 67-69 ppm, followed by

Al2O3 (7-10 ppm), Fe2O3 (1.5-2.5 ppm), CaO (± 2 ppm), MgO (4-8

ppm), K 2O 1-3 ppm) and NaO (± 3.5 ppm). In contrast, TiO2, can be

considered as a major elements with a very small concentration that is

0.1-0.7 ppm

Table 3 indicates that barium (Ba) seems to increase from 276 ppm atthe top to 278 ppm at the bottom. This trend phenomenon is followed

by thorium (Th) from 34 to 32 ppm and strontium (Sr) from 65 ppm at

the top to 67 ppm at the bottom. Cerium (Ce) concentration along the

core seems to be constant that is less than10 ppm. In contrast, zircon

(Zr) and niobium (Nb) show a different trend compared to Ba, Th and

Sr. These two trace elements seem to have their maximum

concentration in sub-sample GRT-05-03F and GRT-05-03H as

indicated by concentration of ± 44-45 ppm. Zr has concentration of 41ppm at the top and 44ppm at the bottom of the sample. Likewise, Nb

seems to have the same trend with Zr, where it has a constant

concentration that is 45 ppm at the whole core length.


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M ARINE GEOLOGICAL INVESTIGATION IN THE T OMINI B ASIN 25

Table 1. Result of minor elements analyses from sediment

sample GRT-05-03

The conspicuous occurrence of barium in Tomini Basin (Figure 13)

possibly can be explained by study of Leong (2001) which stated that

barium has a good correlation with organic matter. Therefore,

sedimentation of barium can possibly be controlled largely by thebiogenic matter, although the detritus fraction in Tomini Basin was

dominant in the sediments. Likewise, Masayasu et al (2002) explained

that the authigenic barium (Baex ) correlates with gradual change in

sedimentation environment during glacial ages. The Baex may relate to

calcareous organisms besides siliceous ones. Further, Masayasu et al

(2002) explained that the Baex was reduced to sulfide and dissolved

away in a strongly anoxic environment during biologically productive

period.


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Table 2. Result of major elements analyses from sediment

sample GRT-05-03

Paytan et al (2007) observed the increasing of Barite burial at

continental margin and shelf of Peru and indicate the occurrence of

suboxic conditions, leading to Ba release into the water column. These

authors also stated that sediments from the Peru shelf are lack of any

barium enrichment, but this element is significantly enriched in slope

and basinal deposits in water columns deeper than 2000 m. If this

nature seems to be compatible with Tomini Basin, then the barium

distribution in sedimentary oxic and suboxic environments at deep

water depositional sites in Tomini Basin can also probably has a high

potential as a palaeoproductivity indicator.

The occurrence and vertical distribution of barium in surface sedimentsin Tomini Basin can be explained by considering of two opposite

opinions related to the origin and formation of barium as represented

by Masayasu et al (2002) and Pirrung et al (2008). The downward

increase of barium concentration in Tomini Basin suggests that the

particulate barium uptake and flux is enhanced by higher barium

concentration in the deep waters of the Tomini Basin.


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Table 3. Result of trace elements analyses from sediment

sample GRT-05-03

According to Masayasu et al (2002), barium fluxes indicate arelationship between upper ocean biological processes and barium flux

to the seafloor; hence the ratio of organic carbon to barium decreases

systematically with water depth. Consequently, the systematic upward

decrease of barium with decreasing core depth in Tomini Basin can

possibly synthesized as the results of simultaneous decomposition of

organic matter and uptake of barium in settling particles


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Figure 12. Core samples of GRT-05-03 used in this study.

Letter A, B; C etc indicate analyzed sub-sample locations.


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Figure 13. Content of trace elements in 15 sub-samples of GRT-05-03


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Conclusion

Three tectono-stratigraphy sequences separated by unconformities in

Gulf of Tomini indicating the Late Neogene history and development

of the basins were identified. During the late Miocene, the basins

within the Gulf of Tomini seem to have been stable. However, here we

examine the emplacement of a large sediment gravity flow deposits,

resulting from the late Neogene collision between the East Sulawesi

Ophiolite Belt and a micro continent, the Banggai-Sula platform.

Collision system between Eastern Arm of Sulawesi and Banggai-Sula

micro continent since Pliocene forms a Gulf of Tomini and the basins

in it. Submarine sediment gravity flow deposits are the major

mechanism of sediment transportation and transfer from the slope to

deep-sea environments in Tomini and Gorontalo Basins.

Marine magnetic method applied in the Tomini Basin provides a data

information on magnetic intensity anomalies. The Tomini Basin is

underlain by oceanic-like crust and shows a nearly NE-SW symmetriclateral lineation of susceptibilities values. The up-doming of slightly SE-

NW structural style in the center of the basin with susceptibility value

of -0.11 to -0.12 cgs units possibly indicate a suspect thermal stretching

and active tectonism is in commencing. It implies that the earlier

basement in the basin is undergoing a thinning and differential

subsidence. The occurrences of relative susceptibility between several

rocks within the basement create susceptibility contrasts which are

either positive or negative. Geological model indicates that the entirebasement of Tomini Basin is characterized by an oceanic-like crust with

a basin axis at the center nearly an east-west direction and presents rift-

related basement graben. However, on the basis of gravity map and 2D

seismic data, the Tomini Basin in fact is subdivided into the north and

south sub-basinal structures.


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Within all sub-samples of Core GRT-05-03 were found opaque

minerals and micas possibly originated from metamorphic rocks (gneiss

and schist) derived from the terrenes serounding the Tomini Basin.The general trace elements occurrence also indicates that sediments

were mostly originated from the eastern and the southern terrenes that

are composed mainly by ophiolite rocks. Barium concentration in deep-

sea sediments in Tomini Basin might be expected in areas of high

paleoproductivity and associated high barium. High content of Mn

within the surficial sediment coloum in Tomini Basin indicates anoxic

environment where Mn solubed and burried within anoxic sediment,

which then slowly migrated and accumulated in oxidized sedimentlayers above.


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Research Vessel Geomarin III


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BBiiooggrra a pphh y y

Dida Kusnida , born in Bandung, 15 September

1957. Graduated from Geological Engineering,

Bandung Institute of Technology in 1984 and

Master of Scince in Marine Geology from the Free

University of Amsterdam in 1989. Senior Scientist

in sedimentary gelogy at the Marine GeologicalInstitute of Indonesia. Works and involved in

marine geological and geophysical investigations

since 1984 until now.

Imelda Rosalia Silalahi, born in Jayapura, 19

October 1967. Graduated from Geological

Engineering, University of Pembangunan Nasional

(UPN) “Veteran”, Yogyakarta in 1992. Works in

Marine Geological Institute of Indonesia since

1993 until now as a marine geologist. Involved in

marine geological and geophysical investigations inIndonesian waters and coastals since 1994 until

now.


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Tommy Naibaho, born in Pematangsiantar, 26

August 1958. Graduated from Geological

Engineering, University of Pembangunan Nasional

(UPN) “Veteran”, Yogyakarta in 1987. Works in

Marine Geological Institute of Indonesia since

1993 until now as a sedimentologist. Involved inmarine geological and geophysical investigations in

I ndonesian waters and coastals since 1994 until

now.

Subarsyah, born in Garut, 20 April 1977.

Graduated from Geophysical and Meteorological

Engineering, Bandung Institute of Technology in

2000. Work at Geothermal Division Directorate of

Mineral Inventory 2001-2003. Works in Marine

Geological Institute of Indonesia since 2004 until

now as a geophysicist. Involved in marine geological

and geophysical investigations in Indonesian waters

and coastals since 2005 until now.

Documents

BukuTomini Basin.pdf