Characteristic of the deep-water channels in the Lower Miocene Pulau Balang Formation in Samarinda

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1 Characteristic of the deep-water channels in the Lower Miocene Pulau Balang Formation in Samarinda Abdurrokhim*, Djadjang J Setiadi, Faisal Helmi, Adi Hardiyono, and Yoga A Sendjaja Faculty of Geology, Padjadjaran University *Corresponding author: abdurrokhim_syubi@yahoo.com Abstract A composite succession of total more than 2000 m thick deposits of lower middle Miocene Pulau Balang Formation is well exposed. In general, the formation represents shallowing-up succession, typified by deep-water deposits in the lower part and shallow-water paralic deposits in the upper part. Thick-bedded limestone that exposed in the Batu Putih area is occurred in between those lower and upper successions. This limestone developed during latest lower Miocene or earliest middle Miocene. The deep-water deposits of the Pulau Balang Formation typically are represented by mudstone dominated facies intercalated with thin- to medium-bedded sandstones, with local associations of channel-fill deposits, slump deposits, bioclastic grainstone, and thin- to thick-bedded gravelly sandstones. The simple parameters of channel-fill deposits in the outcrops of the Pulau Balang Formation are typified by lenticular geometry and erosional basal contacts. There are three characteristics of channel-fill deposits that observed near Samarinda City: (1) interbedded sandstones and siltstones, (2) thick-bedded sandstone, and (3) interbedded gravelly sandstones. Keywords : Deep-water channel, Pulau Balang Formation, Samarinda. 1. Introduction Land clearing activities for housing complex provides new exposures of Miocene deposits in Samarinda City. In general, the exposures can be grouped into two settings; the deep-water deposits from the lower stratigraphic succession and shelf into fluvio-deltaic facies that represent the upper stratigraphic succession. The boundary of the deepand shallow-water deposits is characterized by thick-bedded limestone as exposed in the Batuputih area. Supriatna et al. (1995) named those deposits as Pulau Balang Formation, one of stratigraphic units in the Lower Kutei Basin. Kutei Basin infilled by various lithology of sedimentary rocks from the deep-water through the fluvio-deltaic deposits during Paleogene to present (Moss and Chambers, 1999). Intense discussion of the ancient and modern delta Mahakam have been reported in many ways (e.g. Allen, G.P. and Chambers, 1998; Hook et al., 2001), but the deepwater deposits, especially from outcrop evidence are rarely reported. In this paper, we document facies and characteristic of deep-water channel from Pulau Balang Formation in Samarinda City on basis of outcrop description at 3 locations: (1) Green Palm Housing Complex, (2) Km-18, and (3) Pinang Raya Housing Complex II (Fig. 1). 2. Stratigraphic Setting Pulau Balang Formation is Miocene sediments infill of the Lower Kutei Basin that exposed in Samarinda City (Supriatna et al., 1995, Moss and Chambers, 1999). The formation is developed during latest Early Miocene earliest Middle Miocene in the deep-water setting (Mos s and Chambers, 1999) (Fig. 2), and characterized by alternating greywacke and quartz sandstone intercalations with limestone, claystone, coal and dacitic tuff (Supriatna et al., 1995). 127

corals and some scattered massive corals floating within carbonate mud.

been observed at several stratigraphic levels. The upper part of the Batuputih limestone exhibit intense burrowing, possibly of the ichnofacies Thalassinoides or Spongeliomorpha. Fig. 1.

Lithostratigraphy of the Kutei Basin (Moss & Chamber, 1999) In the study area, the Pulau Balang Formation is a siltstone-dominated succession with locally interbedded with thin- to very thin-bedded

2 corals and some scattered massive corals floating within carbonate mud. The middle and upper parts are composed mainly of wackestone and mudstone that contain branching and massive corals with fragments of molluscs, echinoids, and larger forams, while head corals have been observed at several stratigraphic levels. The upper part of the Batuputih limestone exhibit intense burrowing, possibly of the ichnofacies Thalassinoides or Spongeliomorpha. Fig. 1. Observation stop site at Samarinda City (blue circle) Fig. 3. Limestone outcrop at Batu putih Fig. 2. Lithostratigraphy of the Kutei Basin (Moss & Chamber, 1999) In the study area, the Pulau Balang Formation is a siltstone-dominated succession with locally interbedded with thin- to very thin-bedded finegrained sandstones, thick- to very thick coarsegrained sandstones, thick-bedded gravelly sandstones, conglomerate and slump deposits. Pulau Balang Formation is overlaid conformable by thick-bedded limestone. The limestone is approximately 47 m in thickness (Batuputih quarry) (Fig. 3), characterized by gradual both on top and basal contact. In lower part, it is irregularly stratified packstone, wackestone, and mudstone, composed mainly of platy and branching Thick-bedded limestone (Batuputih Limestone, Moss and Chambers, 1999) is overlain by shelf deposits, which are progressively changes into fluvio-deltaic deposits of Balikpapan Formation. The best exposures of succession from shelf into fluvio-deltaic of the ancient Mahakam Delta can be observed near SMC Hospital in Jalan Pangeran Suryanata and near Rumah PAN in Jalan MT Haryono. 3. Deep-water channel of the Pulau Balang Formation Pulau Balang Formation shows various lithofacies as presented from 4 outcrop locations. The formation is typically represented by siltstone intercalated with thin- to very thin-bedded finegrained sandstones, thick-bedded sandstone, with local association with channel-fill deposits, and slump and mass transport deposits. Simple recognizing of channel-fill deposits of the Pulau Balang Formation in the field commonly characterized by a lenticular geometry bed or bedset 128

3 with sharp erosional basal contact into the underlying deposits. In general, there are three lithofacies of deep-water channel-fill deposits in this area: (1) interbedded sandstones and siltstones, (2) thick-bedded sandstone, and (3) Interbedded gravelly sandstone. A. Interbedded sandstones and siltstones Best exposure of this facies in this area study can be found at Green Palm Housing Complex. Outcrop dimension about 10 m x 60 m2 represent complete three packages of lenticular geometry deposit of interbedded sandstone and siltstone (Fig. 4) In general, each channel-fill deposits is a finingupward sequence that consists mainly of the conglomeratic sandstone in the bottom part succeeded by the alternating of medium- and finegrained sandstone, siltstone, and shale beds in the middle and upper parts (Fig. 5). The lower conglomeratic sandstones are mostly massive, while the middle sandstones are either parallel laminated or cross-ripple laminated. Contorted and convolute lamination, small-scale load cast, and flame structures also locally found, especially in the lower part. Laminated siltstones separate the sandstones beds. The mudstones (siltstone and claystone) are interpreted to represent background sedimentation in a bathyal environment, and to have developed as hemipelagites, although some possibly formed as turbiditic mudstones (e.g., Piper and Stow, 1991). The massive conglomeratic sandstones and thinbedded sandstones with parallel and cross ripple lamination sedimentary structure are interpreted as deposits from turbidity currents. B. Thick-bedded sandstone Thick-bedded coarse-grained sandstone of the channel-fill deposits can be observed at Pinang Raya Housing Complex II. The outcrop is characterized by siltstone dominated with locally interbedded with thick-bedded sandstone and thinto very thin-bedded sandstone. There are four sandy units separated by thick laminated-siltstone and claystone with locally exhibit a highly bioturbated layer. The channel-fill deposit at Pinang Raya consists of interbedded thick-bedded sandstones with maximum thickness is 13.5 m. The sandstones characterized by well sorted coarse- to very coarsegrained sandstone, cross bedding sedimentary structure, with mud clasts common found in the lower part of sandstone beds. Fig. 4. Lenticular geometry of channel-fill deposits at Green Palm Housing Complex 129

intervals of sandstone-dominated facies and alternating laminated siltstones and sandstones. Overall, the outcrop exhibit soft sedimentary deformation (Fig. 9). Fig. 5. Fining-upward pattern.

Overall, it shows fining-upward in most upper part before overlaid by a thick-bedded limestone.

Cross bedding sedimentary structure may represent lateral accretion surfaces in sinuous channel (e.g., Hein and Walker, 1982). Fig. 6.

4 intervals of sandstone-dominated facies and alternating laminated siltstones and sandstones. Overall, the outcrop exhibit soft sedimentary deformation (Fig. 9). Fig. 5. Fining-upward pattern. This interbedded sandstones shows erosional basal contact into interbedded sandstones and siltstones (Figs. 6 & 7) and sharp top contact. Overall, it shows fining-upward in most upper part before overlaid by a thick-bedded limestone. The interbedded thick-bedded sandstones facies with erosional basal contact and fining-upward pattern in the upper part suggest a channel-fill deposit (e.g., Mutti and Normark, 1991). Cross bedding sedimentary structure may represent lateral accretion surfaces in sinuous channel (e.g., Hein and Walker, 1982). Fig. 6. Interbedded thick-bedded coarse-grained sandstones with erosional basal contact. C. Interbedded gravelly sandstone The outcrop of interbedded gravelly sandstone is located in Km. 18 of the Samarinda-Tenggarong Bypass Road (Fig. 8). It composed by two thick Fig. 7. Cross-bedding sedimentary structure. The thick interval of sandstone consists mainly of alternating gravelly medium- and coarse-grained sandstone beds with thin shale intercalations. They are grey, grain-supported, composed mainly by rock fragments and quartz with minor coal fragments. Many of the coarse-grained sandstone also contain mud clasts. Gravelly sandstones locally show sharp erosional basal contact and sharp upper contact. Graded bedding, parallel lamination, and sole marks are sedimentary structures that are common found in several sandstone beds (Fig. 10). Sandstone laminae within alternating siltstones and sandstones show parallel- and cross-rippled lamination sedimentary structures. The lenticular geometry of composite gravelly sandstone beds and erosional basal surface indicate that the deposits as channel-infill. The gravelly sandstones are product of mass transport mechanism, which then deformed in slope setting. 4. Discussion Although the channel-fill deposits of the Pulau Balang Formation have similar characteristic: fining-upward pattern, erosional basal contact, and encased in siltstone-dominated facies, however characteristic of both the channel-fill facies and encompass siltstone-dominated facies are vary. 130

Fig. 8. The outcrop of gravelly coarse-grained sandstones at KM 18 Those facies variations are most likely controlled by sediment supply and processes.

This situation possibly developed in slope setting with minor sediment discharge.

The interbedded gravelly sandstone of channel-fill together with soft sedimentary structure indicates the deposits developed in the slope setting Fig. 9. Soft sedimentary deformation Fig. 10.

5 Fig. 8. The outcrop of gravelly coarse-grained sandstones at KM 18 Those facies variations are most likely controlled by sediment supply and processes. Channel-fill deposits that characterized by interbedded sandstones and siltstones possibly developed, where turbidity current and suspension interplay each other s. This situation possibly developed in slope setting with minor sediment discharge. In contrast, thick-bedded coarse-grained sandstone with cross-bedding sedimentary structure indicates that the sediments infilled into the sinuous channel in very gently slope in the basin. The interbedded gravelly sandstone of channel-fill together with soft sedimentary structure indicates the deposits developed in the slope setting Fig. 9. Soft sedimentary deformation Fig. 10. Graded bedding, parallel lamination and flute cast sedimentary structures. 5. Conclusions 1. The Pulau Balang Formation have been developed in deep-water setting, and it is typically represented by siltstones intercalated with thin- to very thin-bedded fine-grained sandstones, thick-bedded sandstone, with local association with channel-fill deposits, and slump and mass transport deposits. 2. There are three lithofacies of deep-water channel-fill deposits in this area: (1) interbedded sandstones and siltstones, (2) thick-bedded sandstone, and (3) Interbedded gravelly sandstone 3. The channel-fill deposits of the Pulau Balang Formation have been developed in slope settings. 131

6 6. Acknowledgments This study was conducted as a part of Mahakam-Kutei Basin Reservoir Characterization Field Work Project donated by BP Berau Ltd. We would like to thank Peter Butterworth, Arief Maulana and Erick Yudhanto for intense discussions on sedimentology and Kutei Basin. 7. References Allen, G.P. and Chambers, J.L.C., 1998, Allen, G.P. and Chambers, J.L.C., Sedimentation in the modern and Miocene Mahakam Delta. IPA, p. 236.: IPA. Hein, F.J., and Walker, R.G., 1982, The Cambro- Ordovician Cap Enrage Formation, Queebec, Canada: conglomeratic deposits of a braided submarine channel with terraces: Sedimentology, v. 29, no. 3, p , doi: /j tb01798.x. Hook, J.A., Butterworth, P.J., and Ferguson, A., 2001, Contrasting Miocene Fluvio-Deltaic Channel Types from Perjuangan Quarry, East Kalimantan, Indonesia: Implications for Subsurface Reservoir Correlation, in Indonesian Petroleum Association, Proceeding Twenty Eight Annual Convention,. Moss, S.J., and Chambers, J.L.C., 1999, Tertiary facies architecture in the Kutai Basin, Kalimantan, Indonesia: Journal of Asian Earth Sciences, v. 17, no. 1-2, p , doi: /S (98)00035-X. Mutti, E., and Normark, W.R., 1991, An integrated approach to the study of turbidite systems, in Weimer, P. and Link, M. eds., Seismic facies and sedimentary processes of submarine fans and turbidite systems, Springer-Verlag New York, p Piper, D.J.W., and Stow, D.A. V, 1991, Fine-grained turbidites, in Einsele, G., Ricken, W., and Seilacher, A. eds., Cycles and Events in Stratigraphy, Berlin and Heidelberg, Springer-Verlag, p Supriatna, S., Sukardi, and Rustandi, E., 1995, Peta Geologi Lembar Samarinda, Kalimantan: Pusat Penelitian dan Pengembangan Geologi, Bandung. 132

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