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

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A Few of Turbidite Characteristic Feature in the Vicinity of Bantarujeg (Abdurrokhim) 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 61 Bulletin of Scientific Contribution. Vol. 4, No., 2, Januari 2007 : 61-69 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 63 Bulletin of Scientific Contribution. Vol. 4, No., 2, Januari 2007 : 61-69 semirigid plug of material. The BIBLIOGRAPHY
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