A Few of Turbidite Characteristic Feature In the Vicinity of Bantarujeg (Abdurrokhim)

A FEW OF TURBIDITE CHARACTERISTIC FEATURE IN THE VICINITY OF BANTARUJEG

Abdurrokhim Laboratorium Sedimentologi dan Geologi Kuarter, Jurusan Geologi FMIPA – UNPAD

ABSTRACT

The traversing of Cilutung River exposes the sedimentary rock which is corresponding to turbidity system sedimentation. In around Bantarujeg area, the composite lithology is characterized by monotonous of poorly sorted tuffaceous sandstone interbedded with mudstone of classical turbidite; coarse grain of pebbly sandstone; interbedded tuffaceous sandstone and mudstone of slump deposits; disorganized pebble-coble-sized components of mud supported; massive sandstone and polymix breccia. A genetic unit based on sedimentary processes is more reliable in order to grouping the lithostratigraphic units.

Keyword: system sedimentation, turbidity

ABSTRAK

Lintasi S. Cilutung menyingkapkan batuan sedimen yang sesuai dengan sistem sedimentasi turbidit. Di sekitar area Bantarujeg, batuan ditandai oleh batupasir tufaaan porositas buruk selang-seling dengan batulempung turbidite klasik; batupasir kerikilan berbutir kasar; selang-seling batupasir tufaan dan endapan slum batulempung; batupasir masive dan breksi polimiks. Satuan genetika berdasarkan proses sedimentasi lebih dapat dipercaya dalam rangka menggolongkan satuan litostratigrafi.

Kata kunci : sistem sedimentasi, turbidit

INTRODUCTION of the observation product from the field will be documented and reported Bantarujeg is the town, lies about periodically. Since the lithology 15 km south of Majalengka. characteristic, disperse and variation Department of Geology Universitas of composition vertically or laterally Padjadjaran have been utilizing this are rapid changing, the sedimentary area for student mapping exercise rock characteristic is the focused since three years ago in order to observation in this time. accelerate the time of student To simplify the grain size-based of graduation without reducing the lithology name, sand/sandstone is tutoring stuff. The students of terms applied to average grain sizes Geology Unpad are very familiar with coarser than 4φ (1/16 mm). If the Bantarujeg since they were doing the bulk of sediment is finer than 4φ, the first field work of Physical Geology term mud/mudstone used with an course. additional word to emphasize the This passage is a little part of the major component (Bouma and major scenario and emphasized on Rozman, 2000). the outcrops feature observation of sedimentary rock characteristic as the METHOD AND DATA basic scope for recognizing sedimentary rock in the field. The Geological field work is deducted major line of the work is establishing especially to observe outcrops of the geological setting in that area and sedimentary rock which are exposed will be reported after the whole of around Bantarujeg by traversing geological aspects observation Cilutung River and streams. completely done. So, every little thing

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The characteristic of sedimentary The Feature of Turbidite rocks, particularly the texture and Characteristics structure led by genetic processes is Turbidites are defined as the the main subject in this time. deposit of turbidity current and Absolutely, the deposit characteristics generally regarded as deep water are not presenting all the variety of deposits. The turbidity current process turbidites due to the limitation of can operate in any depth, but to observed sedimentary outcrops. preserve the deposits as turbidites,

they must not be reworked into Regional Stratigraphy Setting different-looking deposits by other The geology of West Java has currents. This effectively places been studied by many research turbidites below storm wave base; workers since the first decades of the depending on the basin, this probably 20th century. The results of their implies minimum depths of 250-300 studies were summarized and well m. (Walker, 1992). documented by Van Bemmelen Present day Cilutung exposure in (1949) and Martodjojo (1984). The Majalengka area, displays the West Java, especially Bogor Trough segmented of Neogene turbidite revealed that the basin is mostly filled succession more than 3600 m in with marine deposits during Neogene thickness (Cimanuk Group, Djuhaeni time. These deposits are consisting of and Martodjojo, 1989). The group is intercalation tuffaceous sandstone; consisting of Cinambo Formation, vary of polymix breccia and shally Cantayan Formation and Bantarujeg facies which corresponding to Formation (Halang Formation, Djuri turbidity sedimentary processes. The 1973). The sedimentary rocks are young E-W trending anticline is outcropped around Bantarujeg and formed during a recent episode of reported in this paper is only included north-directed compressive of Bantarujeg Formation. structuring and authentic event The outcrops of sedimentary rocks related to subduction of Indian obtain in the walls and base of stream Oceanic Plate below Sundaland discontinuously but relatively easy to Continent touch and observe. The beddings surface are striking ESE-WNW and In Majalengka area, the dipping to the south. It is successions are commonly characterized by intercalation of characterized by fine to coarse of poorly sorted tuffaceous sandstone, poorly sorted tuffaceous sandstone, polymix breccia and mudstone. The polymix breccia intercalation with thickness and characteristics are siltstone and claystone of Cimanuk various in vertical successions along Group N14-N18 (Djuhaeni and perennial or intermittent streams. Martodjojo, 1989), while in the Regarding to field-based major Rajamandala and Cianjur area, the characteristic, the type of turbidity similar type is obtainable Citarum deposits in the Bantarujeg area can Formation which older in age (Lower be described as follow: Miocene) and overlaying the limestone of Rajamandala Formation.  The Interbedded Tuffaceous Sandstone and Mudstone of In the Bumiayu and Kuningan area Classical Turbidite the similar characteristic have the The characteristic of classical same age with Cimanuk Group and turbidite in Bantarujeg area is named from the older to younger as monotonous alternation of sharp- Halang Formation, Kumbang based poorly sorted tuffaceous Formation and Tapak Formation. sandstone and interbedded

62 A Few of Turbidite Characteristic Feature In the Vicinity of Bantarujeg (Abdurrokhim)

mudstone. The sandstone is (Dzulynski and Walton, 1965 in commonly medium and coarse Reading, 1982). grain, calcareous, grey, hard, and Slump deposits consist of about 10 - 40 cm in thickness and interbedded poorly sorted defined as thin-bedded turbidite tuffaceous sandstone and (figure 1). These individual beds mudstone. The thickness of are excellent lateral continuity. deposits is more than 2 meters. The sharp bases of the beds have The fix thickness is ignored due to abundant markings, such tool recent eroded and weathered at marks and scour mark as indicator the top (figure 6). The original of for local flow direction (figure 2). typical and facies is interbedded Within the sandstone beds, the tuffaceous sandstone and graded bedding (decrease mudstone of classical turbidite. upward) and parallel lamination of Bouma sequence is commonly  Pebbly Mudstone present. The climbing ripple is Pebbly mudstones or pebbly rarely founded (figure 3). The mudstone-sandstone breccia in sandstone-mudstone beds ratio is Cilutung River and Cijurey River vary from low value to high value consist of pebbles-boulders of (figure 4). intermediate igneous rock and

distorted clasts of sandstone,  Pebbly Sandstone mudstone and rare limestone, The pebbly sandstone is founded dispersed in a silty mudstone- rarely in around Bantarujeg and sandstone matrix. Basically the common associates with classical matrix can be divided into silty turbidite facies. Pebbly sandstone mudstone matrix and coarse consists of coarse grain, well grain sandstone matrix (figure 7 graded (normal graded bedding of & 8). The composition ranges pebble-size grain), poorly sorted from mudstone containing only of sandstone-size matrix and no a few clasts to large number of internal imbrications. The top of clasts (clasts supported). The bedding surfaces are sharp, and thickness of individual bed is erosional as scour and channeling commonly less than 10 meters. type at the bottom (figure 5). The The deposits are internally usual thickness of single bed is structureless, poorly sorted of about 30-40 cm and laterally mixing intra and inhomogeneous. The pebble extraformational clasts, random components are sub-rounded and fabric and matrix supported mostly the diameter is less than 1 framework. The clast cm. components of sedimentary rock are varying in type, size and  Slump Deposits fabric. Soft sedimentary deformation is common as the Slump deposits can be block of sedimentary rock clast recognized easily in the field and erosional base contact while the deposit is sited in (figure 9 and 10). between the undisturbed turbidite. The main body of the Pebbly mudstone breccia is de- slump mass is not totally posit of mud flows/grain flow. disturbed which indicates that These are mudrich flows in which the slump has moved largely as the mud-water mixture has an undeformed block along a matrix strength. This strength basal plane of decollement supports the coarser clasts during their transport in a

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semirigid plug of material. The BIBLIOGRAPHY resulting deposit commonly has Bolliger, W and de Ruiter, P.A.C., large blocks projecting up above 1975, Geology of Central Java the top of the bed, or even Offshore Area, Proceedings resting almost entirely on top of Indonesian Petroleum Association. the bed. The deposits show no Bouma, A.H. and Stone, C.G (eds) internal evidence of slumping 2000, Fine Grained Turbidite (Walker, 1992). Systems, AAPG Memoir 72 and SEPM Special Publication No. 68. Tulsa Oklahoma, USA. 342p RESUME AND DISCUSSION Bouma, A.H, 2000, Fine-Grained, Sedimentary rock characteristics Mud-Rich Turbidite Systems: of Bantarujeg Formation in around Model and Comparison with Bantarujeg and which have been Coarse-Grained, Sand-Rich observed is consisting of classical Systems (AAPG Mem72 and SEPM turbidite, pebbly sandstone, pebbly Spec Publ 68) mudstone and slump deposit. The Cook, H.E, Field, M.E., and Gardner, other types of turbidity deposit are J.V., 1982, Characteristics of still opened and possible to be set up. Sediments on Modern and Ancient Those facies variation are interpreted Continental Slopes, (Sandstone as the product of slump, mass flow Depositional Environments. AAPG (grainflow/mudflow) and low turbidity Memoir 31. editors: Peter A. current on the base of slope – Scholle and Darwin Spearing) submarine fan depositional setting. Darman, H., and Sidi, F.H., 2000, An To delineate the lateral distribution Outline of The Geology of of each characteristic genetically Indonesia, Ikatan Ahli Geologi needs the parallel sections and Indonesia. relatively perpendicular to the Djuri, H. 1973. Geologic Map of bedding surface. The bedding surfaces Arjawinangun Quadrangle, Scale 1 lead the time of correlation as long as : 100.000. Bandung: Geological the detail of time line of stratigraphy Survey of Indonesia. succession is not available. Howell, D.G. and Normark, W.R., The major criteria which is 1982, Sedimentology of controlled the variety of turbidite Submarine Fans (Sandstone characteristic in Bantarujeg could be Depositional Environments. AAPG determined as a function of the Memoir 31. editors: Peter A. composition of source material and Scholle and Darwin Spearing) the physical and chemical forces Katili, J.A., 1972, Plate Tectonic of during sedimentation processes Indonesia with Special Reference without refusing to consider the to the Sundaland Area, tectonic uptilting during that Proceedings Indonesian Petroleum deposition period. Association. Koesoemadinata, R. P., and Siregar, ACKNOWLEDGMENTS S., 1984, Reef Facies Model of the Rajamandala Formation, West I thank to Geology Department, Java, Proceedings of IPA 13th Universitas Padjadajaran and students Annual Convention, Jakarta, p.1- for giving the opportunity to see the 18. best outcrops of turbidity system Marks, P. 1957. Stratigraphic Lexicon deposits in West Java. Pak Boy and of Indonesia. Publikasi Keilmuan Pak Adam did all the best of 31, Ser. Geologi. Bandung: discussion along time in the field. Any Djawatan Geologi. 233 p. errors remaining are mine.

64 A Few of Turbidite Characteristic Feature In the Vicinity of Bantarujeg (Abdurrokhim)

Martodjojo, S. 1984. Evolusi Schiller, D.M., Garrad, R.A., and Cekungan Bogor, Jawa Barat, Prasetyo, L, 1991, Eocene Bandung Institute of Technology. Submarine Fan Sedimentation in 396 p. (Unpublished Ph.D Southwest Java, Proceedings dissertation; in Indonesian with Indonesian Petroleum Association. English abstract). Sujanto, F.X, and Sumantri, Y.R., Martodjojo, S., dan Djuhaeni. 1989. 1977, Preliminary Study on the Stratigrafi daerah Majalengka dan Tertiary Depositional Pattern of hubungannya dengan tatanama Java, Proceedings Indonesian satuan litostratigrafi di Cekungan Petroleum Association. Bogor. Geol. Indon. v. 12, no. 1, Van Bemmelen, R. W. 1949. The p. 227-252. (in Indonesian with Geology of Indonesia. Vol. IA. The English abstract) Hague: Government Printing Office. 732 p.

Figure 1. Thin bedded classical turbidite

Figure 2. Sole marking, common found at the bottom of sandstone bed as indicator of local paleo current

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Figure 3. Classical turbidite deposits, commonly characterized by intercalation of tuffaceous sandstone and mudstone with erosional base contact. Bouma sequence is common present in sandstone layers

Figure 4. Slump deposit of interbedded tuffaceous sandstone and mudstone overlaid the normal strata of classical turbidite deposits.

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Figure 5. Claystone clast in the med-coarse grain sandstone, normal grading in the bottom become reverse grading in the middle overlaying classical turbidite with erosional contact. The pen is 20 cm long.

Figure 6. Tabc of Bouma’s sequence within medium sandstone

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Figure 7. The strata of classical turbidite within mudflow deposit which is faulted

Figure 8. Polymix component supported breccia, igneous is the major type of component with matrix is poorly sorted coarse grain sand material.

68 A Few of Turbidite Characteristic Feature In the Vicinity of Bantarujeg (Abdurrokhim)

Figure 9. Limestone block, gravels of sandstone, claystone, and igneous rock igneous rock as disorganized component in mud supported breccia.

Figure 10. The soft sedimentary deformation (slump deposit) shows likely the “micro folds” as a “ block” within mud/ grain flow.

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