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CHAPTER III

REGIONAL GEOLOGY

The Sangatta Bungalun area is located in the northern edge of the Kutai Basin.

The Kutai basin is a triangular shaped Tertiary sedimentary basin opening toward

the Makassar Strait in the east, with maximum sedimentary thickness just offshore

the present Mahakam Delta.

III.1 Tectonic Setting of Kalimantan

Regionally Kalimantan is a cored by a triangular-shaped nucleus (Latreille et al,

1971 op.cit Satyana et. al, 1999) pointing to the north, consisting of the Western

Kalimantan Schwarner continental shield as its base in the south, and of accreted

crust, comprising the Central Kalimantan Ranges to the north. On the plate

tectonic framework, Kutai basin was formed on the southeastern edge of

Sundaland which was influenced by the interaction of a few major plates (Fig.

III.1.1). Kalimantan is made up by a variety of amalgamated basement:

continental, oceanic and transitional (Fig.III.1.2).

Toward the east the continental crust of Kalimantan is bordered by the deep sea

floor of the Makassar Straits, where the continental crusts is highly extended

being rifted apart and an oceanic crust developed as a spreading center. This

spreading is the southern extension of the North Celebes basin. However this

interpretation is controversial, but most gravity modeling are more inclined to

interpret is as such. This opening narrows toward the south and disappears toward

south of the South Makassar Basin. The eastern crustal boundary of Kalimantan

can be considered also as a passive continental margin.

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Figure III.1.1 The tectonic framework of West Indonesia (Hall, 2008)

The northern margin of the continental crust is probably defined by a strike-slip

(former transform) on the northern margin of the Mangkalihat peninsula, which

the western extension of the Koro-Palu faults in Sulawesi. However, few authors

consider the Mangkalihat Peninsula as an isolated micro-continent, and the

Sundaland northern boundary is considered the Adang fault which extends toward

the Lupar Zone in Sarawak. North of Mangkalihat Peninsula, the Tarakan Basin is

considered floored by and accreted crust as the oceanic crust, as part of the

Celebes Sea basin, was subducted beneath the Kuching zone.

The Kalimantan continental crust can be considered as a collage of several micro-

continents, with the Sundaland being the main continent, separated by sutures of

or remains of collisions (Fig. III.1.1). The curving Kuching Zone which separates

the Indonesia and Malaysia is a tightly folded and thrusted belt of metamorphic

rocks and deep marine sediments with intercalated ophiolites is interpreted as a

collision zone between the Luconia micro-continent (and its extension toward

northeast) and the Sundaland continent in the southeast (Fig.III1.2). The N-S

trending Meratus Range in southeastern Kalimantan can be also considered as a

suture amalgamating the Paternoster micro-continent on its east side with the

Study area

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Sundaland on its western side. The Meratus suture can be extended southward

into East Java Sea, where subsurface evidence is available offshore NW Madura

(Mudjiono, 2002 op.cit Satyana et. al, 1999).

Figure III.1.2 Crustal composition of Kalimantan

(Koesoemadinata et.al, 2007)

III.2 Kutai Basin

Kutai basin is the largest (165.000 km2) and the deepest (12.000-14.000 m)

Tertiary sedimentary basin in Indonesia. The basin is bounded to the north by the

Mangkalihat High, to the south the basin hinges on the Adang-Flexure (Adang-

Paternoster Fault), to the west it is terminated by the Kuching High, part of the

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Kalimantan Central Ranges, and to the east the opens into the Strait of Makassar.

Kutei basin can be separated into Inner (Upper) Kutei Basin and Outer (Lower)

Kutei Basin. Both basins are separated by zone of a basement high characterized

by the presence of Eocene to Oligocene volcanics and an adjacent gravity high

(Satyana et. al, 1999) (Fig.III.2.1).

Figure III.2.1 Kutai Basin area and its surrounding. It is divided into inner

(upper) and outer (lower) Kutai Basin (Satyana et.al, 1999)

The Upper Kutei Basin consist mainly of Paleogene sediments which most

authors consider as the exposure as the Paleogene strata underlying the Lower

Kutei Basin. Some other authors consider the Upper Kutei Basin as a separate

basin, presumably a remnant of a Paleogene foreland basin, that was largely

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destroyed during a collision of the Luconia micro-continent and the Sunda

continent.

The Lower Kutei Basin consists mainly of Neogene (Early Miocene to Recent)

strata deposited in a great prograding delta system toward the Makassar Strait

with sediments provided by the uplift of the Pre-Tertiary to Paleocene strata of the

Kuching Zone, which a suture zone as the result of the Luconia micro-continent

and other parts of the southeastern portion of the break-away South China Sea

continent.

From tectono-stratigraphic point of view, Tertiary strata southeast of the Kuching

zone, can be subdivided into two stages of basin development in:

1. The Paleogene stage; The Paleogene stage is often considered as the

rifting stage in Eastern Kalimantan. The Paleogene rifting stage began

Eocene or even Paleocene time and ended at the end of Early Oligocene,

as the topography were denudated and carbonate platforms developed in

the Barito basin and Mangkalihat High.

2. The Transition stage; A transitional stage developed in some parts of east

Kalimantan, presumably a tectonic quiescence with the development of

carbonate platforms in the high areas of East Kalimantan, such as the

BaritoPasirAsem-asem basins, the Paternoster and Mangkalihat

Platforms and also in the Upper Kutei basin

3. The Neogene stage; During the Neogene two uplifted zones played an

important role in the development of the Kuching Zone, also called the

Kuching Uplift, and it is a dominating factor in during the Neogene stage,

as it provided sediments for the Lower Kutei and Tarakan basins.

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III.3 Regional Structure of Kutai Basin

Kutai basin is bounded by Adang Fault in the southern and Mangkalihat Fault in

the northern area. These large faults extend into the Makassar Strait and may be

the land extension of the transform faults related to the spreading of the Makassar

Strait or offshoots from the Palu-Koro Fault. These WNW-ESE trending dextral

strike-slip faults originated during the Paleogene along initially pre-Tertiary weak

tectonic trend. The NE SW structure is interpreted as the normal graben fault in

the rifting stage which was inverted in the Neogene time.

Fold and thrust structures dominated the Kutai Basin area. The fold structure

E-W trend on the Mangkalihat Peninsula and an N-S to NNW-SSE trend in the

southern part of the area. The N-S trending fold system is typical of the Kutai

basin, broad gentle synclines and sharp thrusted anticlines.

The NNE-SSW trending fold and thrust (reverse) faults are bending in Upper

Kutai Basin area toward NE-SW and farther east towards E-W. These thrust folds

are typically sharp-crested with broad intervening syncline, typical for the Kutai

Basin. These fold and thrust structures involve mainly Neogene strata and are

related to the inversion of the graben related normal faults at the base of the

Paleogene section (Fig. III.3.1).

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Figure III.3.1 Structural pattern of the Kutai Basin (Biantoro et al, 1992)

Several theories or structural models have been put forward for the origin of the

deformation styles:

Gravity gliding (Ott, 1987)

Contractional tectonics

Wrench inversion, Strike-slip faulting (Biantoro et al, 1992)

Basement controlled deformation -extensional block/inversion (van de

Weerd and Amin, 1992)

Differential loading and inversional underlying regional overpressure

(Chambers and Daly, 1995) small 20 km Eocene fault block can be

inverted.

Delta progradation, (McClay, 1997).

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III.4 Regional Stratigraphy of Kutai Basin

Discussion on the stratigraphy of Sangatta will be following a tectono-

stratigraphic framework, which is according to Feriansyah et al 2000; Guritno et

al, 2000. This tectonic framework consists of the following cycles or stages from

sedimentation point of view:

1. The Paleogene Syn-rift Depositional Stage

2. The Oligocene Post-rift Sag

3. Unconformity- due renewed extensional tectonics along E-W faults

4. Early Miocene Subsidence Depositional Stage

5. Unconformity due to basin inversion

6. The Mid-Miocene-Present Syn-inversion Depositional Stage (Neogene

Delta Progradation System)

7. Regional Unconformity

8. Pliocene-Recent Depositional Stage

The Paleogene rift system is capped by a post rift carbonate deposition

represented by the Kedango Fm. The post rift depositional stage is characterized

by cessation of rifting as tectonic activity, followed by continuing subsidence

during Early Miocene with volcanism and initial delta formation.

The Mid-Miocene to present day Syn-inversion depositional stage is characterized

by inversion of the grabens/half-grabens and the uplift of the Kuching Zone,

accompanied by an eastward prograding deltaic deposition stage Renewed

extension at the end of Oligocene was not followed by a distinct type of (syn-rift)

deposition, but further subsidence during Early Miocene time, while deltaic

deposition took place during continuing basin inversion. The Neogene system is

dominated by the Sangatta delta system (Sadirsan et al, 1994).

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III.4.1 The Paleogene Syn-rift Stage

The Paleogene in this area is represented by the Eocene Mangkupa Formation and

the Beriun Sand formation, as syn-rift deposits in a graben and horst system

varied environments ranging from non-marine lacustrine, fluviatile to deep a

marine environment. Some of the Beriun sand is a fan delta deposit, an alluvial

fan which enters directly a marine environment (Sunaryo et al, 1987) while other

facies of the Beriun sand is interpreted as a submarine fan (Idris et al 1994).

The Paleogene was represented by a patch-work of graben/half-graben and horst

system, generally submerged below sea level as syn-rift deposits in a graben a

varied environments ranging from non-marine lacustrine, fluviatile to deep a

marine environment, called facies tracts (Tardjamah et al, 1997).

III.4.2 The Oligocene Sag Phase Deposition

The Sag phase actually began at Late Eocene time with the Paleogene rift system

is engulf by a by a post rift marine transgression capped by carbonate deposition

represented by the Kedango Fm (Koesoemadinata, et.al, 2007). In the Sangatta-

Bungalun area Oligocene deposition is represented by the Kedango, Pamaluan,

Kariorang formation, which over the deeper previous graben area developed into a

more marly basinal shales of the Atan formation.

III.4.3 The Neogene Deltaic Progradation Phase

During the Neogene the Sangatta Bungalun area consisted of at least 2

stratigraphic provinces; a shallow marine clastic carbonate platform in Bungalun

area, separated by a horse-shoe shaped reefal shelf edge toward the southeast, and

a delta depositional system in the Sangatta area. The shallow marine carbonate

platform also extended toward the Mangkalihat Peninsula (Koesoemadinata, et.al,

2007, 2007).

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The Neogene deltaic progradation is started as early as Early Miocene, when the

uplift in the Kuching Zone took place as the results of the collision between the

Luconia micro-continent and the Sunda subcontinent. This uplift is associated

with inversion of the Paleogene grabens. The delta sequence consists from top to

bottom: The Kampungbaru Formation, the Balikpapan Formation and the

Pulubalang Formation.

III.4.4 Recent

Recent sedimentation is represented by alluvial deposition and deltaic deposition

of the Mahakam rivers in the southern and Sangatta -Bungalun Rivers and in the

Sangkulirang estuary (northern area).

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Figure III. 4.1 Lithostratigraphy summary of Kutai Basin and its regional tectonic summary (Satyana, et.al, 1999)

2012-02-03T13:08:23+0700Digital Content