paleogeography and sedimentation dynamics of...

J. SE Asian Appl. Geol., Jul–Dec 2012, Vol. 4(2), pp. 99-107

PALEOGEOGRAPHY AND SEDIMENTATIONDYNAMICS OF UJOH BILANG-BATUBELAHLIMESTONE MEMBER, UPSTREAM MAHAKAMRIVER, UJOH BILANG AREA, EASTKALIMANTAN PROVINCE

Moch. Indra Novian∗and Herning Dyah Kusuma Wijayanti

Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia

1 Introduction

The Kutai basin has become an object of interestfor research, especially since after oil and gaspotential are found in the basin. Many stud-ies, from the order of tectonics, magmatism,stratigraphy to paleontology have been donehere. Stratigraphical studies that have been per-formed provide general insights of the rock for-mation. However, a specific description at eachstratigraphic measurement point has not beenmade. The research was conducted on rock out-crops along the upstream of Mahakam Riverclose to Ujoh Bilang village up to Batubelah(Figure 1). The outcrops include the upper partof Batu Ayau Formation, Ujoh Bilang Forma-tion and Batubelah limestone member whichis located at the northwest side of Ujoh Bilangsyncline with fairly fresh outcrop conditions,continuous, and its top and bottom can be welldetermined.

The Batu Ayau Formation represents Middle-Late Eocene rift deposit which is composed bycoarse conglomeratic facies, sandstones, silt-stones, coaly facies, and mudstones which weredeposited in fluvial-deltaic to outer shelf envi-ronment (Wain et al, 1989, Abidin et al, 1993).During the sag phase in Oligocene time, deep-

∗Corresponding author: M.I. NOVIAN, Department ofGeological Engineering, Faculty of Engineering, GadjahMada University, Jl. Grafika 2 Yogyakarta, 55281, Indone-sia. E-mail: [email protected]

ening of the basin caused a marine condi-tion. The Ujoh Bilang Formation comprises ofmonotonous claystone, minor sandstone, partlycalcareous. It also consists of two members,that is the foraminiferous limestone of Batube-lah and Lenmuring sandstone. Both Batu be-lah and Lenmuring member of Ujoh Bilang For-mation was deposited during early Oligoceneand include in the Lower Ujoh Bilang Forma-tion according to Wain et al (1989); Abidin et al,1993. However, the new biostratigraphic databy Moss and Finch (1997) noted that Batu BelahMember was deposited during Late Oligocene.The Upper Ujoh Bilang Formation is recognizedthrough its Late Oligocene age and its commonassociation with olistolithic limestones and vol-caniclastic debris flows.

2 Methodology

Recording of the vertical sequence of the rockwas conducted in 1 : 100 scale along the up-stream of Mahakam River around the Ujoh Bi-lang village up to Batubelah. From these mea-surements, the resulting stratigraphic columnwas divided into 4 sections. This was carriedout to provide a clearer paleogeography pictureof that area. Section 1 recorded the rock orderchanges from the lower part of Ujoh Bilang For-mation up to the transition to Batubelah lime-stone member. The second and third sections

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NOVIAN and WIJAYANTI

Figure 1: Geological Map of Ujoh Bilang area (Abidin et al, 1993 with modification), section 1-2located at north western part of Ujoh Bilang Syncline axis, while section 3-4 located at south eastern.

100 c© 2012 Department of Geological Engineering, Gadjah Mada University

PALEOGEOGRAPHY AND SEDIMENTATION DYNAMICS OF UJOH BILANG-BATUBELAH

LIMESTONE MEMBER

were done on the Southwestern tip of Ujoh Bi-lang syncline. Section 2 and 3 are younger thansection 1 and there is a lag between measure-ments in section 2 until 3 to section 1. Sections2 and 3 have the same relative age. Section 4is younger than section 1 but younger than sec-tion 2–3. Stratigraphic measurement of section4 was conducted in Batubelah cliff. Several rocksamples were taken for analysis. Three rocksamples were taken to be analyzed with sievemethod in order to see small foraminifera con-tent, six samples were made into thin sectionsin order to view the content of minerals and fos-sils, especially foraminifera, and one rock sam-ple was analyzed by smear slide method in or-der to view nannoplankton content.

3 Facies

The stratigraphic record resulting from fieldmeasurements combined with laboratory anal-ysis can be divided into 12 facies (Figure 2). De-scription of each facies are given as follows:

Claystone facies (A). Claystone facies hasdark grey color with iron concretions whichoften form their own concretion layers (Figure3a). Thickness of this facies is around 4–15 mwith bed thicknesses vary from 30 cm up to 300cm and contain few foraminifera fossil. Thisfacies is interpreted to be deposited in lagoonalarea.

Interbedded fine sandstone and claystone fa-

cies (B). Interbedded fine sandstone and clay-stone facies has nearly similar thickness; about10 cm. Sandstone has brown to reddish browncolor. Sedimentary structures which developedare parallel lamination, ripple, load cast and oc-casionally trace fossil ophiomorpha (Figure 3b),fine grained, grains supported, composed dom-inant by quartz and feldspar. Claystone hasdark grey in color. This facies is interpreted wasdeposited in tidal area, especially in sub tidal-offshore.

Interbedded medium sandstone and clay-stone facies (C). Interbedded medium sand-stone and claystone facies has nearly similar

characteristics with the previous facies (faciesB). The differences lie in grain size which com-poses the sandstone that is medium sand. Thisfacies is interpreted was deposited in tidal area,especially in intra-tidal – sub tidal.

Sandstone facies (D). Sandstone faciesconsists of yellowish white sandstone withlaminated-bedded, ripple and occasionallyOphiomorpha trace fossil. The grain size rangesfrom fine to medium sand, grain-supported,rounded, and composed dominantly by quartzand feldspar. Thickness of this facies is about 6– 11 m with bed thickness range from 10 – 20cm. This facies is interpreted to be depositedin shallow marine environment, especially inshore face in fair weather wave zone.

Claystone with fine sandstone intercalation

facies (E). Claystone with fine sandstone in-tercalation facies is 19 m thick with bed thick-ness ranging from 30 cm to 150 cm. Clay-stone is dark grey without Foraminifera con-tent. Sandstone is brown in color with bed-ded sedimentary structure, fine sand grain size,grain-supported, rounded, and composed byquartz and feldspar. Thickness of sandstonevaries from 10 cm to 30 cm. This facies is in-terpreted to be deposited in transition zone be-tween shore face-offshore, that is, between fair-storm weather wave base zone.

Claystone with large foraminifera facies

(F). Claystone with large foraminifera fa-cies is grey in color (Figure 3c), massive inthe lower part. In the upper part it has thinbed that curves downward and composed bylarge foraminifera with good grain orienta-tion. Towards the top, foraminifera contentincreases with more random orientation. Largeforaminifera are dominated by lenticular form.This facies is interpreted to be deposited in ashallowing upward offshore area.

Muddy allochem limestone facies (G).Muddy allochem limestone facies has light greycolor, consists of large foraminifera that mixeswith algae in the lower part, and toward the topgradually changes into large foraminifera with

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Figure 2: Facies in study area. Coarsening and shallowing up ward pattern are recognize from vertical succession.

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LIMESTONE MEMBER

Figure 3: Several outcrop of facies in study area: a) Facies A: Black clay; b) Facies B: Interbedded finesandstone with claystone; c) Facies F: Claystone with large foramiinfera; d) Facies H: Foraminiferalfloat-rudstone of Batu Belah limestone member.

lenticular form. This facies is composed bylarge foraminifera such as Heterostegina bornien-sis, Nummulites, Eulepidina and Miliolina, alongwith pelecypoda, red algae and coral. This fa-cies is interpreted to be deposited in offshorearea, especially in the fore reef.

Foraminiferal float-rudstone facies (H).

Foraminiferal float-rudstone facies has yellow-ish white color, bedded, random grain orien-tation, mud-grain supported, grain size largerthan 2 mm, composed by large foraminifera, al-gae, pelecypoda, and bed thickness range from40–100 cm (Figure 4d). This facies is interpretedto be deposited in fore reef slope.

Interbedded claystone and coarse sandstone

facies (I). The facies consists of interbed-ded claystone and coarse sandstone facies.Claystone is dark grey in color, minor largeForaminifera fossil content with bed thicknessaveragely 30 to 40 cm. Sandstone is brown incolor, massive, mud-grains supported, coarsesand grain size (2–4 mm), composed by quartz

and large foraminifera fragment. This faciesis interpreted to be deposited in offshore withsediment source came from terrigenous sourceand reef.

Interbedded foraminiferal sandstone

and claystone facies (J). Interbeddedforaminiferal sandstone and claystone facieshas nearly similar bed thickness, about 10 cm.Sandstone is brown – reddish brown in color,fine grain size, grain supported with graindominantly composed by quartz and largeforaminifera. Paralel lamination and load castare the most developed sedimentary structure.Claystone is dark grey in color. This facies isinterpreted to be deposited in open platform.

Interbedded rudstone and claystone facies

(K). The facies consists of grayish white lime-stone, grain-mud supported. Grains consist oflarge foraminifera, algae and minor quartz with10 cm thickness for each bed. Claystone is darkgrey in color with thickness of each bed ranging



from 10 cm to 20 cm. This facies is interpretedto be deposited in reef flat - open platform.

Rudstone facies (L). The rudstone facies iswhite-grayish white in color, has tangentialcross-bedding structure, and grain-supported.Grains are composed by coral, red algae frag-ment, large foraminifera (Borelis, Austrotrilina,Miogypsinoides dan Amphistegina), some micritethat has been altered into calcite, thickness ofeach bed ranges between 100–150 cm. This fa-cies is interpreted to be deposited in back reefor reef flat.

4 Paleogeography and Dynamic Sedi-mentation

Based on the examination of facies developedin the study area, it can be concluded that therehave been some changes of paleogeographyduring the deposition of Ujoh Bilang Forma-tion. This is reflected in the depositional envi-ronment shift seen in the deposited facies. Atthe lower part of this formation (initial of sec-tion 1), the paleogeography still changed fromthe Batu Ayau paleogeography setting towardthe Ujoh Bilang. These changes occur gradu-ally which reflected on the gradational facieschanges. At the beginning, the facies is domi-nated by interbedded sandstone and claystone,then begin to turn into interbedded claystoneand sandstone, then claystone, and finally isclosed by limestone facies (Figure 4).

The pattern that is developed in section 1 iscoarsening and shallowing upward. The pat-tern can be seen from the change of facies Ato B then C, B–D, A–E–B, A–F, A–G, A–E–H and I–H. In facies A–B–C, B–D and A–E–B, the depositional environment changed fromdeeper environment to mainland, that is, fromoffshore in a lagoonal area to tidal area. Ini-tially, restricted or closed lagoon was formed.By time, it became more open. This is indi-cated by the presence of dominant mudstonewith iron concretion, indicating reductive con-dition. The presence of carbonate-shelled fos-sils, especially foraminifera and nannofosil wasnot found of the prepared three rock samples(OB 1, 14 and 21). Towards the top, lagoon area

was more open, shown by the open looping fa-cies A–F, A–G, A–E–H and I–H. Claystone fa-cies that exist in these facies contain many largeforaminifera fossil and start to be intercalatedby limestone. In claystone that is intercalatedwith limestone (OB 23), the amount of smallbentonic foraminifera is greater than planktonicand dominated by Quenquiloculina teenagos.The presence of Quenquiloculina teenagos in-dicates lagoonal environment, but the pres-ence of other foraminifera fossils indicate ma-rine conditions (Boudagher-Fadel, 2008). Thisstrengthens the influence of the marine envi-ronment into the lagoon where the rock wasformed. Age of rock based on the content ofplanktonic foraminifera (Globigerina ampliaper-tura Bolli, Globigerina pseudoampliapertura Blow& Banner, Globigerina tapuriensis Blow & Ban-ner, Globigerina pseudovenezuelana Blow & Ban-ner, Globigerina and Globorotalia corpulenta Sub-botina shows P18/19 or comparable with Tc–d in larger foraminifera chart (Lunt and Allan,2004).

Towards the end of the section, the environ-mental condition become increasingly headingtowards open sea, hence very supportive forreef growth (facies H). Reef initially grew asbiostrome wherein composed of shell materialboth of larger foraminifera, corals, algae andpelecypoda which are derived from surround-ing environment. Formed reef did not grow in sufficient speed hence there was no moundmorphology. Instead, it only formed a hori-zontal bedding which thinned toward the edge.The slower reef growth compared to increas-ing accomodation space is thought to be thecause of growth halts which then the remain-ings were covered by sediment materials of ter-restrial origin. In the end, reef organisms be-gan to offset the sea level rise, forming a thicklayer of limestone at the end of section 1 and in-cluded in Batu Belah Limestone Member. Basedon larger foraminifera (Heterostegina, Eulepid-ina, Nummulites, minor number of Miliolina),corals, algae and pelecypoda (OB 24 and OB25), this limestone is interpeted to be formedin reef front (Figure) and deposited in the earlyOligocene (Tc–d) (Lunt and Allan, 2004).

Like section 1, section 2 have coarsening and



LIMESTONE MEMBER

Figure 4: Paleogeography change during Early to Late Oligocene of Ujoh Bilang Formation to BatuBelah limestone member in Upstream Mahakam River. a) Intial deposition of Ujoh Bilang Forma-tion in lagoon area; b) Sea level rise cause the environment become marine; c) Reef can catch andexceed rate of sea level rise.



shallowing upward stratigraphic pattern. Thepattern can be seen from the change of faciesJ into facies K which later turns into facies L.Facies J was deposited in open platform areaat back reef. Therefore, it often had reef mate-rial debris in form of larger foraminifera shells.Facies J that is replaced by facies K indicatesthat the environment increasingly became shal-lower toward reef flat – open platform area. Af-ter that, there was an addition of accommoda-tion space, followed by the deposition of faciesJ and K. After the deposition of facies K thereis a blank zone on the stratigraphic measure-ment which is then closed by the formation offacies L which was deposited on the back reefflat. Based on the strike and dip of bedding,section 2 is younger than section 1. SampleOB 33 that was taken from facies L containslarger foraminifera such as Borelis, Austrotrilina,Amphistegina, and Miogypsinoides which showsLate Oligocene age (Te2–3) (Lunt and Allan,2004).

Section 3 and section 4 is located in the south-east of the Batu Belah syncline axis. Section 3is 5 m thick in total, individual bed thicknessranging from 30–100 cm, and included in faciesH. The larger foraminifera content in sampleOB 35, such as Heterostegina, Neorotalia mecate-pecensis, Eulepidina, Amphistegina and Austrotril-lina shows that the lithology of facies H wasdeposited in reef front at the Late Oligocene(Te1) (Lunt and Allan, 2004). Beside largerforaminifera, the lithology was also composedby other benthic organisms such as pelecypodaand also quartz which has terrestrial fragmentsorigin. This indicates the terrestrial origin ofsediment supply was still ongoing, but not tointerfere the reef growth.

Section 5 is stratigraphically above section4, but turns to be separated by the lack ofstratigraphic measurement with unconsider-able thickness. The thickness of facies H in thissection is up to 120 mete that forms a cliff withalmost horizontal bedding. The rock sampleson the lower part of this section (OB 34) showslarger foraminifera contents; the abundance ofBorelis, Austrotrillina, Eulepidina, and Amphiste-gina shows that rocks was deposited on the reef

flat or backreef at Late Oligocene (Te) (Lunt andAllan, 2004).

5 Discussion

Based on the strike and dip of the rock andlarger foraminifera content observed in eachsection, correlation can be performed on sec-tion 4. Section 1 is the oldest section whichwas deposited on this research traverse. EarlyOligocene age was determined from the rocksample in the middle of this section. Age of rockat the beginning of this section can only be con-cluded to be not younger than Early Oligocene.The absence of small foraminifera and nanno-fossil due to reductive lagoonal-tidal environ-ment causes difficulty for age determination.Furthermore, section 3 was deposited above thesection 1 with age Te1, followed by depositionof section 2 and 4.

Paleogeographic changes observed from sec-tion 4 shows that the changes were gradational.It started from lagoonal environment which isshallow into tidal areas and this occured re-peatedly. It then turned into lagoon, and thenchanged into reef front. Initially, the increasingof accommodation space cannot be counteredby the growth of reef-forming organisms so thatreef eventually died and covered with siliciclas-tic materials. However, in the end, the reef canoffset the accommodation space increase; thegrowth was even beyond the space increase.This is indicated by rock deposition which com-prises that section 2, 3 and 4 were deposited onreef flat. Youngest age that can be identified inrock samples is from the central part of section2, which is Late Oligocene (Te2–3).

6 Conclusions

1. Ujoh Bilang Formation was deposited inlagoon–tidal environment.

2. The formation changes from Ujoh Bilangto Batu Belah limestone member occuredgradually following the sea level rise. Thedepositional environment was shifted fromlagoon to marine and accompanied withgrowth of reef organisms builder which ex-ceed rate of sea level rise.



LIMESTONE MEMBER

3. Ujoh Bilang Formation was deposited notyounger than Early Oligocene, while BatuBelah limestone member was occured dur-ing Early – Late Oligocene.

Acknowledgements

Acknowledgements are addressed to LKFTUGM for their permission to present this paper.The content of this paper were part of a regionalUpper Kutei Basin study conducted by LKFTUGM-PT ATI.

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Lunt P. and Allan T. (2004) A history and applicationof larger foraminifera in Indonesian Biostratigra-phy, Calibrated to Isotopic Dating, GRDC work-shop on micropaleontology, The Museum of theGRDC, Bandung.

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