Jurnal Geologi , Vol. 5 No. 4 Desember 2010: 263-268

The Evolution of Gajahmungkur Paleovolcano, Wonogiri, , as A Reference to Revize the Terminology of “Old Andesite Formation”

I. SYAFRI1, A. SUDRADJAT1, N. SULAKSANA1, and G. HARTONO2

1Faculty of Technical Geology, Padjadjaran University, Jatinangor 45363 2Geology Department, University College of National Technology Yogyakarta

ABSTRACT Gajahmungkur is a Tertiary paleovolcano located in Wonogiri , Central Java. The volcanic product of this volcano are widely distributed and composed of important elements of the stratigraphic sequence in the Southern Mountain area. The volcanic products so far have been simply classi›ed as “Old Andesite Formation” which apparently is not in line with the stratigraphic code and the Indone- sian Stratigraphic Code. The description of paleovolcano therefore might contribute to the revision of the “Old Andesite Formation”. The evolution of Gajahmungkur paleovolcano commenced with the formation of a submarine volcano, and then at the second phase a composite volcano emerged above sea level forming a volcano island. The third phase was the self destruction resulting in a formation of a caldera. Pumiceous components dominated the products. At the fourth phase, the activities began to decline producing more basaltic rocks. The statistical analysis of the interrelation between various physical properties of the clastic rocks leads to the identi›cation of volcanic facies and the location of the paleovolcano vent. Keywords: Gajahmungkur paleovolcano, statistical interrelation, volcanic facies, Old Andesite

SARI Gunung Gajahmungkur yang terletak di Kabupaten Wonogiri, Jawa Tengah merupakan gunungapi purba yang berumur Tersier. Produk gunungapi purba ini tersebar luas dan membentuk elemen penting dalam urutan stratigra› di Pegunungan Kidul. Sejauh ini, batuan gunungapi Tersier digolongkan sebagai —Formasi Andesit Tua“. Penamaan ini tidak sesuai dengan aturan tatanama stratigra› maupun Sandi Stratigra› Indonesia. Karena itu pemerian terhadap gunungapi purba dapat membuka jalan untuk menyempurnakan tatanama “Formasi Andesit Tua”. Evolusi Gunungapi Purba Gajahmungkur dimulai dengan pembentukan gunungapi bawah laut dan pada fase kedua terjadi pembentukan sebuah gunungapi komposit yang membangun sebuah pulau. Pada fase ketiga terjadi penghancuran diri yang membentuk sebuah kaldéra. Produk letusan didominasi oleh batuan piroklastika pumis. Pada fase keempat, aktivitas menurun dan menghasilkan batuan yang lebih basaltis. Analisis secara statistik terhadap hubungan antar karakteristik ›sik batuan klastik menjadi dasar bagi pengenalan fasies gunungapi dan lokasi pusat kegiatan. Kata kunci: Gunungapi purba Gajahmungkur, interrelasi secara statistik, facies gunungapi, Andesit Tua

INTRODUCTION South (Figure 1). The Tertiary volcano of Gajah- mungkur is part of the Southern Mountain. The range The investigated area is located in Wonogiri consists of various types of volcanic rocks sub aerially Regency, Central Java, at the longitudes 110047’43”- and subaqeously deposited. The age ranges between 110057’17” East and latitudes 7052’28” – 7052’7” Eocene to Miocene (Bothe, 1929; van Bemmelen,

Naskah diterima 17 Juni 2010, revisi kesatu: 20 Agustus 2010, revisi kedua: 15 September 2010, revisi terakhir: 26 November 2010

263 264 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 263-268

1949). However the recent investigations showed The subaqeously deposition of the volcanic rocks syn- younger ages up to Pliocene which moved the upper genetically deposited with shallow and littoral marine unconformity boundary of the rock sequence from deposits adds the complication of the stratigraphy of Mid-Upper Miocene to Upper Pliocene (Surono et al., the area. Therefore, the volcanic sequence of Southern 1992) or even to Pleistocene (Samodra et al., 1992). Mountain is described as a formation. However the On the other hand, some other investigators favored terminology has so far been controversial as among to assign Cenozoic Era to the volcanic rock sequence others pointed out by Marks (1957). He further sug- of the Southern Mountain (Smyth et al., 2008). gested re-describing the terminology. Moreover the The Tertiary volcanic activity of Southern Moun- Stratigraphic Code of Indonesia published by the As- tain so far has been simply described as a single for- sociation of Indonesian Geologists is not in favor to mation called “Old Andesite Formation” with the age such a description (Martojoyo et al., 1973; Martojoyo supposedly ranges between Oligocene to Miocene. and Djuhaeni, 1996).

° 111 Legend: Mount Provincial city 7° S Regencial city Sub-regencial township Mount Airport Unggaran 342m Road River 2050m Provincial boundary CENTRAL JAVA Investigated area

Mount Merbabu S.Solo S. Se r ang 3142m Mount Merapi Mount 2911m Lawu 3265m

Mount

Kukusan YOGYAKARTA 2298m Bayat Wonogiri

Semin YOGYAKARTA Wuryantoro Gajahmungkur 1033m SPECIAL REGION Reservoir 1248m S. Opak 8° 8°

IN IAND O EC AN EAST JAVA N

0 20 Km 111° E

Figure. 1 Locality map of the investigated area, where the Gunung (Mount) Gajahmungkur is situated. The Evolution of Gajahmungkur Paleovolcano, Wonogiri, Central Java as A Reference to 265 Revize the Terminology of “Old Andesite Formation” (I. Syafri et al.)

The present investigation attempted to fully ap- shape caldera with the undulated topography of the ply the lithologic unit description in line with the volcanic products. These phenomena clearly depict code. Gajahmungkur Volcano has been selected as the self destruction of Gajahmungkur Volcano at the the focus of the investigation taking into account latest stage of the activity combined with the sector the complexity of the depositional environment of failure (Figure 2). The undulated topography typi- the volcanic rocks produced by the volcano. The cally exhibited the deposit produced by sector failure description and delineation of the volcanic rock units as recently demonstrated by Mount Saint Helens give the idea of the development of Gajahmungkur eruption in 1980 (Lipman and Mullineaux, 1981). Volcano. Hence the status and position of the rock The stratigraphic sequence of Gajahmungkur sequence in the formation might be constructed. The Volcano complex is commenced with the oldest same method would be able to be applied in other rocks consisting of micro diorite. The rock is over- area in the so called “Old Andesite Formation”. lain by a tuff unit with the lithology of alternations between ›ne- and coarse- tuffs, ›ne- grained brec- cia, and calcareous sandstone. The rocks contain METHODOLOGY benthonic foraminifera fossils indicating a shallow marine environment. Overlying the rocks, predomi- The investigation mostly relied on the physical nantly andesitic lava was fiowed down. The fiow rock determination in the ›eld. Petrographic analysis was interrupted with the intercalation of ›ne- and has also been carried out to support the result of the coarse-grained sandstones. Auto breccias were also ›eld evidence. The remote sensing method was in- observed. Part of the lava fiows exhibits pillow tensively used in the initial phase of the investigation. structures with the aphanitic texture containing The digital analysis of the Alos images produced in ma›c minerals. 2007 with 10 m resolution was integrated with radar Based on the characteristic mentioned above, image prepared in 1997 with a resolution of 90 m. presumably the volcano was formed in the subma- The Alos image was fused with the topographic map rine environment. The rock sequence was intruded resulted in SRTM. by the pyroxene andesite. At the same time the The image analysis was aided by the software of amphibole andesite intrusions took place. The ER Mapper 6.4, MapInfo 8.0 SCP, and Global Map- radiometric age dating con›rmed these intrusions per 8. The topographic expression obtained from the of having the same age of the Middle Miocene age images has clearly depicted the development of the (Surono et al., 1992). volcanic activity. A very impressive horse shape cal- All those volcanic products were ›nally overlain dera was readily exhibited and guided further detailed by an intensive sub-aerial deposition of pumiceous investigation in the ›eld. breccia. The deposit indicates typically cataclysmic In accordance with normal mapping procedure, eruption of the Katmaian type. The sandstone, ›ne the random sampling with GPS aided coordinate breccia, and clay intercalations were present exhib- location was carried out. It was conducted particu- iting the reworked products of the pumice deposit. larly for the statistical analysis of the distribution of Among the pumice components, angular fragments volcanoclastics. The process involved the dimension of metamorphic rocks consisting predominantly and orientation of the fragments in the ›eld and under of schist were found. The fragments supposedly the microscope to observe the same phenomena in abraded from the inner wall of the diatrema during minerals. The sampling procedure was guided by the the powerful outburst of Plinian type. general distribution of volcanic rock based on volcano Pumiceous breccias might have been deposited facies method developed by Vessel and Davies (1981). under the pyroclastic fiow mechanism of Krakatau type. Base surge deposits were clearly identi›ed showing cross bedding strati›cations. The rock RESULTS OF INVESTIGATION partly consists of massive tuff deposit presumably the result of a high temperature cooling of a great The very impressive geomorphologic expression pile of tuff. The radiometric age determination us- of Gajahmungkur Volcano complex is the horse shoe ing hornblende indicated 9.6 ± 0.3 my (Setijadji 266 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 263-268

Gajahmungkur Caldera 0 > 2 km ?

Gajahmungkur Mount Gandul Mount ?

Ngroto Mount Tumbu Mount Tenong Mount

Figure 2. The reconstruction of Gunung Gajahmungkur proper shows the ancient Gajahmungkur Volcano. Three remnants of calderas are observable, namely Gajahmungkur-Gandul Caldera (the largest), unnamed caldera, and Ngroto Caldera. In the central part the basaltic andesite domes namely Tenong Hill and Tumbu Hill are also observable. These domes developed in the Gajahmungkur Caldera (somma) indicating the terminating stage of the ancient Gajahmungkur Volcano. The crater is about two kilometers across (Hartono, 2010). The photograph was taken from NW of Gajahmungkur by H. G. Hartono. and Watanabe, 2009), whilst the other one using Beside the topographical expression, the statist- zircon was resulted in 16.0 ± 0.7 millions years to ical analysis of the grain size and the phenocryst of 17.0 ± 1.1 million years (Surono, 2008). Based on the rock forming minerals (RFM) concluded that the general correlation it was concluded that the age the volcano facies could be recognized based on the was presumably Early Miocene to Middle Miocene. following veri›ed criteria: The deposition environment was mainly terrestrial; - The interrelation between the grain sizes. The however the littoral and shallow marine deposits grain size distribution, the ratio of sizes, the ratio were also identi›ed. of the fall materials, and the steepness of the ratios The volcanic product of ›ner materials con- represent the speci›c type of volcano facies. taining calcareous- and pumiceous- tuffs might - The physical distribution of igneous rock pheno- have been deposited in a marine environment. cryst. The diameters of the RFM phenocryst iden- The sub-aqueous calcareous tuff represents such a tify the central, proximal, and medial facies. The deposit containing fossils. The fragments of large amount of glass proportion of the RFM identi›es foraminifera in places were abundantly recognized. the central, proximal, and medial facies. Fragments of plankton fossil were also present. The The statistical analysis of various types of chemi- low energy depositional environment was probably cal elements however proved to be insigni›cant predominant a showing shallow marine condition. in delineating the volcano facies. The proportion The pumiceous tuff was named as Semilir Forma- between effusive materials might in cases con›rm tion. The latest phase was the development of the the interrelation of various physical dimensions of basaltic rocks in the caldera represented by the the clastic materials. Tenong and Tumbu lava domes. The development of Gajahmungkur paleovol- cano is illustrated as follows: ›rst, a submarine DISCUSSION volcano was born, then it developed to become a volcano island, and ›nally it destructed itself The physical properties of the rock sequence in (Figure 3). volcanic products are readily identi›able including The Evolution of Gajahmungkur Paleovolcano, Wonogiri, Central Java as A Reference to 267 Revize the Terminology of “Old Andesite Formation” (I. Syafri et al.)

Sea level

Pillow lava

West East

Magma

1. Andesite-basaltic pillow-lava, early volcanism

Alternating lava Alternating lava and pyroclastics and pyroclastics

Limestone Limestone Sea level

West East

Magma

2. Andesitic volcanic rocks, composite volcano

ø > 2km Co-ignimbrite Co-ignimbrite breccia breccia Pumice Pumice West East

Magma

3. Dacitic-rhyolitic pyroclastic rocks, caldera volcano

Figure. 3. The illustration shows the development of Gajahmungkur Volcano starting with the formation of lava fiows with pillow structure indicating subaqueous deposition (above), followed by the formation of a composite strato volcano island (middle), and ›nally the self destruction resulting in the formation of caldera (below). the environmental condition of the deposition. determination of the vent or the central of volcanic Hence, the lithologic rock unit in line with the activity of a particular rock unit. The volcanic rock Stratigraphic Code of Indonesia can be constructed. sequence therefore, can be identi›ed based on the The volcanic rock facies analysis might lead to the lithologic unit produced by a particular observation 268 Jurnal Geologi Indonesia, Vol. 5 No. 4 Desember 2010: 263-268

with the vent as the center. In such a case the REFERENCES individual volcanic products can be delineated. The Tertiary volcanic products so far were Bothe, A. Ch. D., 1929. Djiwo Hills and Southern Range, Fourth Paci›c Science Congress Excursion Guide, 14 pp. grouped into a unit without distinguishing between Hartono, G., 2010. Peran Paleovolkanisme Dalam Tataan the individual volcanic rock units. The strongly Produk Batuan Gunungapi Tersier Di Gunung eroded topography of the volcanic body hampered Gajahmungkur, Wonogiri, Jawa Tengah. Disertasi Doktor the recognition. It was therefore quite reasonable to di Fakultas Teknik Geologi, Universitas Padjadjaran, group the andesite rocks of the Tertiary age as older Bandung, p.74-176. andesite in view of the readily recognize Quaternary Lipman P.W. and Mullineaux, D. R., 1981. The 1980 Eruption of Mount St. Helens. US Geological Survey Professional. volcanoes. However with the analysis of various Paper 1250, Washington. interrelations between the physical properties of Marks, P., 1957. Stratigraphic Lexicon of Indonesia, Scienti›c the clastic rocks, the locality of the vent can be Publication No. 31, Geological Series, Geological interpreted. The “Old Andesite Formation” might Survey of Indonesia, Bandung, 145 pp. be divided into the formations based on group of Martojoyo, S., Koesmono, M., and Sudradjat, A., 1973. The volcanic bodies identi›able in the formation. Indonesian Stratigraphic Code. Indonesian Association of Geologists. The identi›cation of individual Tertiary volcano Martojoyo, S. and Djuhaeni, 1996. The Indonesian in particular in the area occupied by Old Andesite Stratigraphic Code. Indonesian Association of Geologists, Formation might lead to the renaming of the formation Jakarta, 25 pp. in line with the Stratigraphic Code of Indonesia. Samodra, H., Gafoer, S., and Tjokroseputro, S., 1992. The Geological Map of Pacitan Quadrangle, scale 1:100.000. Geological Research and Development Center, Bandung. Setijadji, L. D. and Watanabe, K., 2009. Updated Age Data CONCLUSIONS AND RECOMMENDATION of Volcanic Centers in Southern Mountains of Central- East Java Island, Indonesia. Proceedings of International From the above discussion, the following Conference of Earth Science and Technology, Gajah conclusions might be drawn: Mada University, Yogyakarta, p. 1-7. 1. The history of the volcanic activities of Smyth, H.R., Hall, R., and Nichols, G.J., 2008. Cenozoic Volcanic Arc History of East Java, Indonesia: The the Gajahmungkur paleovolcano can be stratigraphic record of eruptions on active continental constructed based on the interrelation of the margin. The Geological Society of America, Special physical properties of the deposit; Paper 436, p. 199-222. 2. The application of volcanic facies might Surono, Sudarno, I., and Toha, B., 1992. The Geologic Map contribute to the solution of the terminol- of Surakarta Quadrangle, scale 1:100.000. Geological Research and Development Center, Bandung. ogy of “Old Andesite Formation” which Surono, 2008. The Sedimentation of Semilir Formation in was not in line with the Stratigraphic Code Sendang Village, Wuryantoro, Wonogiri, Central Java. of Indonesia. Journal of Geological Resources, 18(1), p.29-41. It is suggested therefore to apply the concept Van Bemmelen, R. W., 1949. The Geology of Indonesia, Vol. IA, The Hague, Martinus Nijhoff, 732 pp. and delineate the individual volcanic bodies of Vessel, R. K and Davies, D. K., 1981. Non Marine Sedi- Tertiary age already ruined by the intensive ero- mentation in Active Fore Arc Basin, SEPM Special. sion. The geologic mapping in old volcanic terrain Publication., No. 31, p. 31- 48. is the key to solve such a problem.

Acknowledgement---The authors wish to express their gratitude to Prof. Dr. H. R. Febri Hirnawan who has kindly supervised the last author in the preparation of his PhD dis- sertation of which this paper is based. The acknowledgement is also due to the Head of Geological Survey Institute (PSG) for the ›eld support and laboratory services.