“Ombilin Basin”

Tugas mata kuliah Geologi Tektonik

Disusun Oleh :

Hanagia Saputra

270110130106

GEOLOGI B

PROGRAM STUDI S1 FAKULTAS TEKNIK GEOLOGI

UNIVERSITAS PADJADJARAN

BANDUNG

2015/2016

Cekungan Ombilin

Ombilin basin is a pull apart basin. The wide of the basin is

approximately 25 X 10 km, elongate parallel direction with the main structure of

sumatera. It has 4600 m of thickness, it is too thick for the basin that length 60 km

and wide is 25 km.It shows that the rapid degradation of the basin floor is closely

related with the main dextral Sumatra fault . Stratigraphy of ombilin basin is

composed of the succession of tertiary sedimentation that uncomformity deposited

on the pratertiary rocks.

(picture 1) Structural of Ombilin Basin (Situmorang dkk., 1981)

Ombilin basin located in the magmatic arc. This magmatic arc didominsi

also by cesarean horizontal trans Sumatra / Semangko (Katili ,, 1975). Ombilin

basin is a low area that is filled by sediment layers of early Tertiary age, result

destruction of pre-Tertiary age rocks from the ranks of the front row and row

ranks Schiefer rock in the west. In the east, the basin is bounded by faults Takung

where the Pre-Tertiary rocks on top of a layer of Tertiary (Cameron et al, 1981).

To the west, the basin Ombilin in rapidly growing, where layers of

Tertiary tersesarkan and move down through the fault-fault ride northwest-

southeast direction (Gb.1), which may be associated with cesarean mandatar (T.

Koning, 1986).

Geological structure of the basin Ombilin :

a. Fault

Ombilin basin geological structure has been studied in detail by

Koesoemadinata and Matasak (1981) the eastern edge of the basin bounded by

faults takung where pre-Tertiary age rocks cover the old rock tertiary (Cameroon

et al ,, 1981). The southern part of sekungan not limited by cesarean. orogen cause

a rise in the southeastern part of sekungan and erosional truncation led to the

formation of the tertiary age limit memperlihatakan ekungan the south and

southwest in its present form.

Fault headland divides the basin Ombilin Ampolo into two large groups,

where the northern part of the fault forming protruding piece that separates the

deeper part of the basin Ombilin against sigalut altitude in the north.

Reverse fault in the basin are known to all parallel and interconnected by cesarean

takung. Southwestern part of the reverse fault that is found in the north basin

separating the pre-Tertiary age rocks tarhadap Tertiary age rocks. Relations with

this reverse fault is a fault horizontal directional heading to the US to UTL-SBD

and go to the left horizontal fault with NE-SW direction.

Fault promontory ampalo a normal fault that beraarah US and TTG-BBL

TB up to, as well as in the southern basin fault that caused the western block

down. All the structures including the folds that occur during the Plio-Pleistocene

associated with cesarean takung.

b. Fold

Ombilin basin especially in the eastern bloc form a large syncline

structure, in the northern part of the syncline structure is not symmetric about the

axis of the basin trending NW-TG, asymmetric folds is associated with takung

reverse fault that forms the northeastern edge of the basin.

Besides syncline structure in the basin is also dominated by Palangki

anticline. Where this Palangki shaped anticline formed by the movement of the

fault on the block of bedrock. Both edges of the basin may be controlled by

normal faults on the part of the parent. Incongruent contact between sediments

Oligocene and early Miocene age in Palangki thought to be caused by anticline

Palangki experiencing uplift between these times.

In the western part of fault cape ampalo strktur suimbu trending folds with

EW, fold axis is often found in shale formations Sangkawerang and formation of

Sawahlunto, this illustrates that this structure is a big fold.

(Picture 2) Longitudinal Section from North West-South East

(picture 3) Longitudinal Section Ombilin Basin from west-east

Stratigrafi cekungan ombilin

As explained above, the basin is a basin Ombilin pull apart basin (pull

split) that have traits - traits, among others:

a. High deposition process.

b. Atimetri pattern of sequence and sedimentary facies.

c. Form of precipitation that shows the boundary fault on the edge of the

basin sediment alluvial fans 'konglomerates', affluent flood, lacustrine

etc.).

The properties of the deposition as above, found in the basin Ombilin. Of

seismic data and drilling (T. Koning, 1986) as well as the calculation of the

erosion that has taken place during the Tertiary. it is predicted that this basin has

received no less than 9100 cubic meters of sediment during its deposition.

Ombilin basin stratigraphy is made up of runs sedimentation Tertiary lithologies

precipitated tidakselaras above lithologies Pratersier. Pratersier lithologies consist

of Kuantan Formation, Formation and Formation Silungkang Tuhur that

terinstrusi granite. The third formation menjari-finger touch. Tertiary lithologies

consist of Brani Formation, Formation Sangkarewang, Sawahlunto Formation,

Formation Sawahtambang and Ombilin Formation.

(pictur 4) Stratigraphy of Ombilin Basin

1. Brani Formation

Brani formations are generally found on the slopes of the hills of

bedrock as fan alluvium sediment consisting of breccias and

conglomerates. Rock composer composed of cutouts - cutouts rock

Pratersier which include granite, limestone, quartzite, filit, quartz and

marble. The spread of these formations originated from the northwest to

the east and southeast. Based keratannya position, lithologies at Brani

Formation can be divided into two parts. In the western part, keratannya

derived from Silungkang Formation, Formation Tuhur, covering quartzite

and granite, quartz, basalt, chert, and granite. The grain size is composed

of gravel - gravel with a grain shape angled - angled responsibility. In the

eastern part keratannya derived from Kuantan and granite formations,

consisting of marble, limestone, filit, and volcanic. Rounded granular

shape - rounded responsibilities, sized gravel - gravel. Towards the center

of the basin, increasingly fining grain size and shape of the rounded grains

- rounded responsibilities. Brani Formation depositional environment is a

fan alluvium. On Formation Brani not found fossils that can be used for

age determination.

2. Formation Sangkarewang

Sangkarewang Formation consists of sandstone, siltstone and

claystone with inserts conglomerate in some parts. Sandstones show

decker bedding and in some places, conglomerates occupy the bottom.

Medium grain size - coarse fining upwards. Formation deployment there

Sangkarewang northwest and northern part of the basin. The abundance of

rocks consisting of shale and carbonaceous mudstone indicate depositional

environment close to the source material - organic materials. The

discovery of fossils of freshwater fish in the area Tigotanjung, is indicative

of the environment of the lake. Inserts sandstone with bedding granulated

sediment structure change gradually, indicating that the rock deposition

process does not take place in a stable condition but interspersed basin

subduction - subduction sediments towards the center of the lake.

Based on the analysis of sample Palynology claystone in the area btukuali,

age Sangkarewang Formation is Middle Eocene - Late Eocene (Hima &

Thunder, 1992).

3. Formation Sawahlunto

Sawahlunto formation characterized by shale, silty shale, siltstone,

quartz sandstones and coal seams are fairly thick. Gray flakes in general -

brownish gray, carbonaceous, and contains a drain - the drain of coal.

Brown sandstone, compact, shows the structure of refining grains towards

the top, sometimes - kang layered thin, planar manifold cross maze, and

surging waters. Coal on umumnyaberselingan with siltstone and mudstone.

Formation Sawahlunto limited deployment in the northwest and

north basin.

Formation depositional environment Sawahlunto is a meandering

river that marked encountered carbonaceous shale, coal and charred river

sediment. This formation contains no fossils of foraminifera that can be

used as a pointer age, but based on the analysis Palynology Sawahlunto

known that formation is not older than Eocene precipitated out of tune

over Sangkarewang formations and formations Brani.

4. Formation Sawahtambang

Sawahtambang formation composed of sandstone unit volkanikan,

tuffaceous sandstones, and sandstones kuarsaan. The lower part of this

formation consists of alternating units of quartz sandstones with mudstone

unit. At the top, the unit kuarsaan colored sandstone - gray, medium-sized

- gravel, with a thin layered sedimentary structures cross- confusing. The

unit was deposited on colored claystone unit - gray-green which is above

the formation Sawahlunto. Sawahtambang the spread formations located

in the southern, central and eastern basins.

Sawahtambang Formation depositional environment is

characterized by rivers woven encountered sedimentary structures cross-

maze, grain refinement upward, pasirkasar-sized gravel, and not found a

thick layer of mudstone. This formation is aligned over Sawahlunto and

Oligocene formations.

5. Formation Ombilin

Ombilin formation mainly composed by gray shale -abu generally

calcareous and sandstones containing organic matter, nodule calcareous

sandstones and lenses - thin lenses of limestone at the bottom. At the top

of the formation, tuffaceous sandstones encountered with inserts

containing glauconite marl and molluscs. Deployment of this formation is

in the east and southeast basins.

The thickness of the formation Ombilin never know for sure,

because some have been eroded. Maximum thickness measured is + 1600

meters are found in Sub-basin Sinamar. Based on the presence of

glauconite mineral and fossil collection of mollusks, depositional

environment is neritik in. Paleontological analyzes carried out on

limestone, give the age of the early Miocene Middle Miocene top to

bottom. This formation is not aligned at the top of the formation

Sawahtambang (LEMIGAS Tertiary Basin Study Team, 1994).

6. Formation Ranau

Ranau formation mainly composed by sediment tuff, lapilli, and

lava breccia laharan andesitan. The precipitate tuffs generally maroon, not

experienced Compaction and fine grain size. This precipitate Pleistocene

age, derived from volcanic dust Singgalang, Malintang, and Merapi which

is located in the north basin Ombilin.Dengan met Lava andesitan, lapilli

and tuff and marine influence is not met, this formation was deposited in

the terrestrial environment.

7. Alluvium

The youngest sediment basin Ombilin are aged Holocene alluvium

sediment. These deposits consist of a mixture of loose rock lempung- sized

lumps that come from old rock result destruction older. Contact with the

rock below it in the form of field erosion.

The geological history of the basin Ombilin

Tertiary beginning probably Paleocene, block-faulting occurred as

a result of tensional stress as a result of old orogen chalk on. As a result,

the basin-type intramountaine graben formed by steep cliffs on its side

alluvial fan formed on all sides of the basin produces Brani conglomerate

formation. A lake formed in the central basin of the results of turbidite

deposited in the northwestern part represented by sangkawerang

formation.

At the time of Eocene precipitation occurs due to the continuing

decline in the basin, except in the center and northwest of the former lake

basin where the marshy areas with vegetation of the lake formed by the

type of river flood plains winding, this deposition resulted in the formation

of coal from Sawahlunto. Precipitation continues to make swamp areas dry

out and removal of the surrounding hills followed by sediments klastika

rough shape Rasau members members paddy mines, leading to the end of

the deposition of a river meanders winding along the flood plains of Poro

member, a member of the formation paddy mine where baubara thin layer

is formed and bedding flakes.

Late Oligocene occur pengankatan locally produce surface erosion

in palangkipada earliest Miocene decline in large areas and trangresi of the

sea resulting in changes in depositional environment which is sealed from

the environment neritik be abysal This can be proved from the lenses of

limestone danserpih the Miocene beginning of formation Ombilin ,

Beberqapa volcanic activity occurred in tuff intercalation at the top of the

formation Ombilin.

The historical record precipitation in the Miocene and Pliocene is

unknown but it appears folds and faults that occur in the Plio-Pleistocene

when the ranks of experienced mountain chain appointment followed by

erosion and deposition of Pleistocene age tuff formations ranau.

Potential basin Ombilin

Basin Ombilin turns mepunyai petroleum potential as evidenced

from shale results lacustrine berurumur Eocene and flakes precipitated

marine old Oligocene estimated sourcerock for hydrocarbons and is where

oil seeps located along the edge of the basin such statements are based on

research conducted in the well Sinamar 1 by T.Koning 1981.

At Singkarak block there are two oil seeps located along the edge

of the southern and western basin, in the area found the flow of oil is very

small to flow into a small river close to the southern side of the basin are

both found in the area kolok where the area is known to have oil seeps

from results coal drilling before the second world war 2. The oil seeps

have a similar type to the type of crude oil that is contained in a field of

thorns, central Sumatra.

Both of these oil seeps prove that there is a source rock in the basin

Ombilin and with a high enough temperature will produce oil.

Daftar pustaka

Hastuti, sulistya,dkk, 1996. Analisis Tegasan Purba dan Implikasinya Pada

Evolusi Tektonik Cekungan Ombilin Sumatera Barat. Jogjakarta : UGM.

Hardjanto, sigit, 1992. Laporan ekskursi heologi sumatra tengah. Jatinangor :

Unpad.