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THE OCCURRENCE OF A NEWLY FOUND EOCENE TECTONIC MELANGE

IN KARANGSAMBUNG AREA, CENTRAL JAVA

Prasetyadi, C.(1); Suparka, E.R.(2); Harsolumakso, A.H.(2); Sapiie, B.(2) (1)Jurusan Teknik Geologi, FTM-UPNV, Jalan Lingkar Utara, Condongcatur, Jogjakarta.

(2)Department of Geology, FIKTM- ITB, Jalan Ganesha No.10, Bandung.

ABSTRACT

Based on the 2004-2005 field work in the northern part of Karangsambung area this study presents new evidences for the occurrence of Eocene tectonic melange in the area. Previous studies have mapped the northern area of Karangsambung as part of the Cretaceous Luk Ulo Melange Complex. In this area, especially in the Larangan area, the present study discovered for the first time Middle Eocene Asterocyclina-bearing limestone blocks within the tectonic melange. The occurrence of Middle Eocene limestone blocks indicates the age of tectonic melange rock in this area is no longer of Cretaceous but at least of Late Eocene. If Luk Ulo melange complex indicates the presence of Cretaceous-Paleocene northwestward-plunging subduction zone then it is expected to the north we should find the older part of the melange. The presence of Eocene tectonic melange to the north of the Cretaceous-Paleocene subduction melange does not fit with this model and indicates that deformation phases responsible for both melanges are different. Based on our field work we propose a new model for the tectonic evolution of the area that the shifting of NE-SW trend of Cretaceous subduction to E-W trending in the Oligocene time has occurred due to collision of a micro-continent. It is concluded that the Late Cretaceous to Paleocene subduction occurred in the subduction zone where the Luk Ulo Mélange Complex was formed. The sub-sequence overlying Paleogene (Eocene) series were deposited, following by collision in the Late Eocene – Early Oligocene as indicated by the occurrence of Eocene tectonised rocks prior to the onset of the OAF (Old Andesite Formation) new subduction-related volkanism.

INTRODUCTION In addition to the main period of formation of major Tertiary basins, Paleogene was also marked by a significant tectonic evolution, namely the shifting of subduction zone in the southeastern margin of the Sundaland from a NE-SW trend in the Cretaceous to Paleocene into an E-W trend in the Tertiary (FIGURE 1). Until recently this important tectonic evolution has not been studied appropriately and its interpretation was generally made as part of regional studies (Hamilton, 1979; Daly et al, 1991; Bransden and Matthew, 1992; Soeria-Atmadja et al, 1994; Parkinson et al, 1998; Hall, 2002). Eastern part of Java offers a good opportunity to study this important tectonic phenomenon in more detail. The possibility is supported mainly by the facts that in the onshore area of Eastern Java there are two locations, Karangsambung and Bayat, where pre-Tertiary basement rocks along with Paleogene sediments exposed. Karangsambung in Central Java has been well known as the best place where the Cretaceous subduction-related melange rocks called the Luk

Ulo Melange Complex exposes. The occurrence of this Pre-Tertiary rocks along with the Paleogene (Eocene) provide a rare opportunity in revealing geological problems associated with the K-T boundary history along the southeastern margin of Sundaland. To address this problem a field work has been carried out recently in the northern part of Karangsambung area which is poorly studied compared to the southern part (FIGURE 2).

GENERAL GEOLOGY In Karangsambung area the basement rock has been identified as the Luk Ulo Melange Complex (Asikin, 1974) which is unconformably overlain by Eocene to Oligocene Karangsambung - Totogan Formations (FIGURE 3). Many informations have been produced by previous studies regarding structures, petrology, paleontology, age of metamorphism, and rock distributions (Asikin, 1974; Ketner et al, 1976; Hamilton, 1979; Suparka, 1988; Wakita et al, 1994; Harsolumakso et al,1995). This melange complex consists of tectonic blocks of various

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rocks embedded in a sheared shale matrix. The blocks comprise sandstone, greywacke, chert, limestone, pillow basalt, ultramafics, rhyolite, marble, phyllite and schist. Some of the sandstone blocks are interbedded with shale, while cherts are in close association with limestones (calcilutites) and pillow basalts. Long axis of the blocks have general trend of ENE-WSW (Wakita et al, 1994; Harsolumakso et al, 1996). The large tectonic slabs consisting of dismembered ophiolite (Suparka, 1988) are dominant in the central part of the complex. Concerning the age of the melange complex, Ketner et al. (1976) reported the late Early Cretaceous foraminifer Orbitolina from limestone in the Luk Ulo Melange Complex. Furthermore Wakita et al. (1994) showed that the duration of subduction and accretion in the complex can be estimated based on the age of the terrigenous and hemipelagic (siliceous) shales along with the associated pillow lava. The age of pillow lava in Kali Muncar is of Early Cretaceous (120-130 Myr), while the oldest terrigenous rock is the shale of middle Cretaceous age (100-110 Myr). The youngest rock is the siliceous shale of latest Cretaceous age in the Kali Medana. As the terrigenous rocks in the accreted sequences are usually as young as or slightly younger than hemipelagic siliceous shale, the youngest terrigenous rocks may be of latest Cretaceous or earliest Paleocene age. These age data suggest that the accretion of pillow lava and pelagic to hemipelagic sediments in the Luk Ulo Melange Complex occurred during middle Cretaceous to latest Cretaceous or earliest Paleocene time (Wakita et al. 1994). Data from metamorphic blocks dating indicate ages, ranging from 110-117 Myr (Miyazaki et al. 1998) to 119-124 Myr (Parkinson et al.1998), which are older than the age of the shale matrix. Based on its rock assemblage, the Luk Ulo Melange Complex has been considered to be caused by tectonic mixing during subduction of the Indian oceanic plate under the SE Asia continental plate (Asikin, 1974; Hamilton, 1979; Parkinson et al. 1998) and appears to be resemble many other late Mesozoic melange belts around the Pacific rim (Nilsen, 2002). Unconformably, marked by a tectonic contact, this melange complex is overlain by Tertiary sediments, in an ascending order, the Karangsambung, Totogan, Waturanda, and Penosogan Formations. Karangsambung and Totogan Formations are olistostromal deposits

consisting of sedimentary mixture of rocks fragments and blocks (olistoliths) such as sandstone, conglomerate, fossiliferous (Nummulites) limestone in clay matrix. These two formations have lateral gradual changes (Harsolumakso et al., 1995). Overlying unconformably on these olistostromal deposits is the Waturanda Formation that consits of volcanic breccias and sandstones. The Waturanda Formation is, in turn, overlain conformably by Penosogan Formation consisting of alternation of calcareous sandstone and marl. There are two major structural trends in Karangsambung. The first trends NEE-SWW developed in the pre-Tertiary rocks, and the second has E-W orientation developed in the younger Tertiary rocks.

NEW EVIDENCES FOR EOCENE TECTONIC MELANGE

The evidences for the occurrence of Eocene tectonic mélange were found in Bulukuning and Larangan areas, Banjarnegara. In Bulukuning area, in the NW part of the Luk Ulo Melange complex, a previously mapped as a Pre-Eocene sedimentary sequence is well exposed in an area 3 by 7 km along south tributaries of Kali Sapi. This sequence is in a fault contact with the surrounding Luk Ulo Melange unit consisting of a chaotic, sheared agglomeration of sedimentary, metamorphic, and igneous rocks. Ketner et al. (1976) mapped the sedimentary sequence as of Pre-Eocene sediments, and Condon et al (1996) assigned a Cretaceous age for this sequence. Pre-Eocene or Cretaceous ages suggested by previous workers are not based on fossils. The ages have only been interpreted based on its relationship with the melange rocks. In the Kali Poh traverse, evidence for an Eocene age was indicated by the finding for the first time fossils of foraminifer Nummulites sp in interbedded limestone (FIGURE 4). Unlike in other Eocene areas (Bayat, Nanggulan, and Karangsambung) where Nummulites fossils are abundantly found in white limestone and have larger size (0.5-1.5 cm), the Nummulites sp from Kali Poh occur in a 2.5 m interbed of black, hard, limestone and smaller in size (0.3-0.5 cm). The species type of this newly found Nummulites, until the writing of this paper, has not been determined. Compared to Nummulites

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from other areas (Bayat, Nanggulan, and Karangsambung), the Nummulites from Kali Poh generally show less rooms (5-6 rooms) in vertical sections suggesting the possiblity they have an older age range of Early to Middle Eocene (Lunt, 2005; pers. comm). North-dipping thrust mark the contact between the lower part of the sequence with the Luk Ulo Melange. The lower part of the sedimentary sequence consists of bedded (5-20 cm), grey in color, fine to medium-grained, sandstones, with interbedded shales, few calcareous sandstones and limestones. In thin sections the sandstone is mostly composed of quartz, chert, basalt and schist, and slightly metamorphosed indicated by the growth of micas in between the grains. The upper part of the sequence consists mostly of black, non-calcareous shales containing non-stratified polimict conglomerate lenses 0.5-5 m in diameter. Most of the conglomerate components, with 2-12 cm in size, consist of milky quartz, slate, basalt, sandstone, few schist and chert. Some of the conglomerates are of the pebbly mudstone type in which pebbles are widely dispersed in a very large proportion of fine-grained matrix material. Others are more ordinary conglomerates in which the pebbles are densely packed. In thin sections the shale also appears to be metamorphosed shown by strong fissility of sericitic materials. Like the lower part, the upper part of this sequence is also a fault contact with the Luk Ulo Melange unit. Structurally this sedimentary sequence dips generally to the south with thickness of 350 m. The lower and upper parts of this sequence are in a fault contact with the melange unit. Structurally this sedimentary sequence dips generally to the south with thickness of 350 m. We call this sequence as “Bulukuning metasediment” (Prasetyadi et al., 2005, 2006 a & b). In the NE part of the Luk Ulo Melange Complex (Larangan area), another tectonised Eocene rock has been discovered in the Kali Sabeng and Kali Mondo traverses (FIGURE 5 and 6). Rock unit exposed in this area has been mapped as Cretaceous melange rock (Ketner et al. 1976, Condon et al. 1996). The Eocene rocks in this area were found as limestone blocks, containing abundant Discocyclina sp and Asterocyclina sp. (FIGURE 7). These fossiliferous limestones along with bedded sandstones, breccia,

greywacke, red shale, basalt, and dacite are found as blocks embedded in black, sheared shale matrix. Blocks of sandstone occur as the predominant blocks. We name this Eocene melange rock as “Larangan melange complex”. Foraminifer Orbitolina sp has usually been mentioned by previous studies (Asikin, 1974; Ketner et al. 1976; Hamilton, 1979) as the Cretaceous guide fossil for the melange complex but the location of the fossil was not clearly reported. The present study has been able to find the location of the Orbitolina fossils in Kali Mondo, about 2 km to the NE of the Discocyclina limestone location (FIGURE 8). These Orbitolina fossils were abundantly found in wackstone-type limestone blocks, 1-5 m in sizes, embedded in sheared shale matrix along with others various blocks such as bedded sandstones, mudstones, breccia, greywackes, and black phyllite (FIGURE 9).The size of the fossils ranges from 0.3 – 1.0 cm, with characteristic helmet-like form, and randomly oriented. Based on traverses along the creek that dissect the melange complex, such as Kali Mondo, Kali Sabeng, Kali Muncar, and Kali Loning, an interesting structural feature has been documented during the present study. Measuring structural dip of the components of the melange along with the shale matrix, the results indicate a general structural dip of the melange components to the south and southeast. Similar results have been recognized by Kamtono et al.(1996) whose interpretation based on a gravity measurement.

DISCUSSION The new discoveries of Eocene tectonised sediments in Bulukuning and Larangan areas, respectively in the NW and NE parts of the Luk Ulo Melange Complex change significantly the geological feature of Karangsambung and the surrounding areas as shown in the revised geological map generated by the present study (FIGURE 10). These new findings also indicate the Eocene strata in Karangsambung-Banjarnegara area is more various and complex than those in Bayat and Nanggulan areas. Eocene sediments in Bayat and Nanggulan areas share similar stratigraphic features, such as they form upward-deepening, transgressive sequence

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and almost undeformed (Smyth et al. 2003). With the new findings, the Eocene sediments in Karangsambung are considered tectonised or deformed sedimentary sequence as also repoterd by Harsolumakso et al. (1996) which, based on several detail river sections mostly in the southern part of Karangsambung area, suggested that besides occurring as olistostromal deposit, the Eocene formation has also been undergone severe deformation after deposition. The newly found Eocene metasediment of Bulukuning section shows a sedimentary sequence of internally coherent, slightly metamorphosed, consisting mostly of meta-shale with conglomerate lenses in the upper part and bedded meta-sandstone and interbedded limestones in the lower part. Here the conglomerates may not be of basal-type conglomerate because they are always found as lenses within the shale. The continuous succession indicates the Eocene Bulukuning section may not be a olistostromal deposit like those found in southern Karangsambung section but deeper and older Eocene formation in Karangsambung area. Another newly found Eocene sediment is the Eocene Larangan section. Previous workers have been considered the rocks as Cretaceous melange. The finding of Middle to Upper Eocene Discocyclina-Asterocyclina limestone blocks within the melange indicates that the age of the melange is not Cretaceous anymore but younger, at least as young as or younger than the Middle to Upper Eocene limestone block. The highly deformed (tectonic melange) Eocene rocks in the Larangan area at least indicates that there is a major deformational event in this area during the late Eocene. Measuring structural dip of the components of the melange along with the shale matrix, the results indicate a general structural dip of the melange components to the south and southeast. Similar results have been recognized by Kamtono et al.(1996) whose interpretation based on a gravity measurement. The structural features indicated by a general south-dipping of the components of the mélange may be associated with a later deformation and could not be related with the original structural dip of the accretion wedge of the subduction melange. If it is still in original orientation then the structural dip should be to the north or northwest following the direction of the subducting plate.

The occurrence of Middle Eocene limestone blocks indicates the age of tectonic melange rock in this area is no longer of Cretaceous but at least of Late Eocene. If Luk Ulo melange complex indicates the presence of Cretaceous-Paleocene northwestward-plunging subduction zone then it is expected to the north we should find the older part of the melange. The presence of Eocene tectonic melange to the north of the Cretaceous-Paleocene subduction melange does not fit with this model and indicates that deformation phases responsible for both melanges are different. Based on our field work we propose a new model for the tectonic evolution of the area that the shifting of NE-SW trend of Cretaceous subduction to E-W trending in the Oligocene time has occurred due to collision of a micro-continent which its occurrence has been interpreted by previous studies (Smyth et al., 2005). It is concluded that the Late Cretaceous to Paleocene subduction occurred in the subduction zone where the Luk Ulo Mélange Complex was formed. The sub-sequence overlying Paleogene series were deposited, following by collision in the Late Eocene – Early Oligocene as indicated by the occurrence of Eocene tectonised rocks prior to the onset of the OAF (Old Andesite Formation) new subduction-related volkanism. The proposed tectonic evolution is presented in FIGURE 11.

CONCLUSION

• Two discoveries of new type of Eocene rocks in the Bulukuning and Larangan areas show that the Eocene strata overlying the Luk Ulo Melange Complex is more complex than those exposed in any other areas in the Sundaland.

• The age of melange rock in the NE part of Karangsambung (Larangan area) is no longer Cretaceous. The finding of Discocyclina limestone block within the melange indicates that the melange rock is of, at least, Late Eocene.

• The structural features indicated by a general south-dipping of the components of the mélange may be associated with a later deformation and could not be related with the original structural dip of the accretion wedge of the subduction melange. If it is still in original orientation then the structural dip should be to the

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north or northwest following the direction of the subducting plate.

• It is concluded that the Late Cretaceous to Paleocene subduction occurred in the subduction zone where the Luk Ulo Mélange Complex was formed. The sub-sequence overlying Paleogene (Eocene) series were deposited, following by collision in the Late Eocene–Early Oligocene as indicated by the occurrence of Eocene tectonised rocks prior to the onset of the OAF (Old Andesite Formation) new subduction-related volkanism.

ACKNOWLEDGMENT

We thank Dirjen MIGAS, BP Migas and Kodeco, Departemen Teknik Geologi ITB and Jurusan Teknik Geologi UPN Veteran Yogyakarta for their invaluable supports to this study.

REFERENCES Asikin, S., 1974, Evolusi geologi Jawa Tengah dan sekitarnya ditinjau dari segi tektonik dunia yang baru. Lap. Tidak dipublikasikan, disertasi, Dept. Teknik Geologi ITB, 103 hal. Condon, W.H., Pardyanto, L., Ketner, K.B., Amin, T.C, Gafoer, S., and Samodra, H., 1996, Geological map of the Banjarnegara and Pekalongan sheet, Java, P3G, Bandung. Hamilton, W., 1979, Tectonics of the Indonesian Region, Geol. Survey. Prof. Paper No. 178., USA. Harsolumakso, A.H., M.E.Suparka, D.Noeradi, R.Kapid, Y.Zaim, N.A.Magetsari, C.I.Abdullah, 1995, Karateristik Struktur Melange di Daerah Luk Ulo, Kebumen, Jawa Tengah, Prosiding Seminar Sehari Geoteknologi dalam Industrialisasi, PPPG-LIPI, Bandung. Harsolumakso, A.H., M.E.Suparka, D.Noeradi, R.Kapid, Y.Zaim, N.A.Magetsari, C.I.Abdullah, 1996, Status olistostrom di daerah Luk Ulo, Jawa Tengah: suatu tinjauan stratigrafi, umur dan deformasi. Kumpulan makalah seminar Nasional.

Kamtono, Lumban Gaol, K., and Praptisih, 1996, Konfigurasi batuan-dasar daerah Karangsambung dengan pendekatan studi penampang gayaberat, Proceeding of the 25th annual convention of IAGI. Ketner, K.B., M.Kastowo, Subroto et al., 1976, Pre-Eocene rocks of Java, Indonesia, US Geological Survey Journal of Research 4, 605-614. Parkinson, C.D., K.Miyazaki, K.Wakita, A.J.Barber, and D.A.Carswell, 1988, An overview and tectonic synthesis of the pre-Tertiary very-high-pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia, The Island Arc (1988) 7, 000-000. Prasetyadi, C., Harsolumakso, A.H., Sapiie, B., and Setiawan, J., 2002, Tectonic significance of pre-Tertiary rocks of Jiwo Hill, Bayat and Luk Ulo, Karangsambung areas in Central Java: A comparative review, Proceeding: 31st annual convention of IAGI, p. 680-700. Prasetyadi, C., Suparka, E.R., Harsolumakso, A.H., and Sapiie, B., 2005, Eastern Java basement rock study: Prelimenary results of recent field study in Karangsambung and Bayat areas, Proceedings JCS 2005-HAGI-IAGI-PERHAPI, Surabaya. Prasetyadi, C., Suparka, E.R., Harsolumakso, A.H., and Sapiie, B., 2006a, An overview of Paleogene stratigraphy of the Karangsambung area, Central Java: Discovery of a new type of Eocene rock, Proceedings Jakarta 2006 Internat. Geosci. Conference and Exhibition, Jakarta. Prasetyadi, C., Suparka, E.R., Harsolumakso, A.H., and Sapiie, B., 2006b, The Larangan Complex: A newly found Eocene tectonic melange rock in Karangsambung area, Central Java, Indonesia, Extended abstract Internat. Sedimentological Congress, Fukuoka Pulunggono, A., dan S. Martodjojo, 1994, Perubahan tektonik Paleogen - Neogen merupakan peristiwa tektonik terpenting di Jawa, Proceed. Geologi dan Geotektonik Pulau Jawa sejak akhir Mesozoik hingga Kuarter, Seminar Jurusan T. Geologi Fak. Teknik UGM, p.253-274.

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Setiawan, J., 2000, Kompleks batuan pra-Tersier, mula-jadi dan implikasi tektonik daerah Perbukitan Jiwo, Bayat, Jawa Tengah, tesis magister ITB, unpublished. Smyth, H., Hall, R., Hamilton, J., and Kinny, P., 2003, Volcanic origin of quartz-rich sediments in East Java, Proceeding IPA, 29th Annual Convention. Smyth, H., Hall, R., Hamilton, J., and Kinny, P., 2005, East Java: Cenozoic basins, volcanoes, and ancient basement, Proceedings 30th IPA convention and exhibition, Jakarta. Soesilo, J., and Sutanto, 2000, Preliminary study on garnet bearing quartz-muscovite schist blocks

of the Luk Ulo Melange Complex, Kebumen, Central Java, Proceeding IAGI 2000 annual meeting, Bandung. Suparka, M.E.R., 1988, Studi petrologi dan pola kimia kompleks ofiolit Karangsambung utara Luh Ulo, Jawa Tengah, Evolusi geologi Jawa Tengah , Lap. Tidak dipublikasikan, disertasi, Dept. Teknik Geologi ITB, 181 hal. Wakita, K., Munasri, and B.Widoyoko, 1994, Cretaceous radiolarians from the Luk-Ulo Melange Complex in the Karangsambung area, Central Java, Indonesia, J. of SE Asian Earth Sci., Vol.9, No.1/2, pp. 29-43.

FIGURE 1: Subduction zone development along the SE margin of Sundaland

FIGURE 2: Location map of the 2004-2005 field study

FIGURE 3: Published geological map of Karangsambung area (compiled from Asikin et al, 1992, and Condon et al 1996)

FIGURE 4: Early Eocene Bulukuning metasediment

FIGURE 5: Tectonised Late Eocene rock exposed in the Larangan area

FIGURE 6: Various sandstone blocks in the Larangan Eocene tectonised unit.

FIGURE 7: Asterocyclina-bearing limestone blocks as the evidence for Middle Eocene age found in the Larangan melange

FIGURE 8 : Orbitolina-bearing limestone block indicating Middle Cretaceous age found in the Larangan melange

FIGURE 9: The other various blocks found in the Larangan melange

FIGURE 10: Geological map and stratigraphy generated by the present study

FIGURE 11: Proposed schematic tectonic evolution of the Karangsambung area.