iagi sangiran mud volcano and demise of homo erectus satyana 2008

Indonesian Association of Geologists 37th Annual Conference and Exhibition, Bandung 26-30 August 2008

Sangiran Dome, Central Java : Mud Volcanoes Eruption,

Demise of Homo erectus erectus and Migration of Later Hominid

Awang Harun Satyana 1)

1) BPMIGAS (Badan Pelaksana Kegiatan Usaha Hulu Minyak dan Gas Bumi) Patra Office Tower, 22nd Floor, Jalan Jenderal Gatot Subroto, Kav. 32-34, Jakarta 12950

Telp : 021-52900245, Fax : 021-52900118, E-mail : [email protected]

ABSTRACT Sangiran Dome, located 12 kms to the north of Surakarta (Solo), Central Java is a famous site in the Quaternary geology due to the exposures of Pleistocene rocks and fossils of hominids and vertebrates. Sangiran Dome is a remnant shale diapir and mud volcanoes complex. This is based on the nature of deformation, presences of several saline water and methane gas seeps, and erupted materials including exotic blocks of metamorphic basements to Pliocene rocks. Based on the deformation and age dating, the diapiric deformation and eruption is considered took place between 0.7 and 0.5 Ma –million years ago (middle Pleistocene) and could repeat until 0.12 Ma (base late Pleistocene). Hominid (early human) called the sub-species Homo erectus erectus lived in the Sangiran Dome. Their fossils were found in the upper part of Pucangan and lower part of Kabuh Formations. They lived in the Pleistocene (ages remain in dispute as 1.7-1.0 Ma, 1.3-0.7 Ma, or 1.0-0.5 Ma). The termination of the sub-species between 0.7-0.5 Ma could be contemporaneous with the eruption of the Sangiran mud volcanoes. The eruption possibly affected the demise of Homo erectus erectus. The eruption of Sangiran mud volcanoes could also affect the migration of later hominid (sub-species Homo erectus ngandongensis / soloensis). The sub-species did not live in Sangiran area possibly the area was in-habitable due to the eruption. The sub-species migrated eastward downstream of the Solo River into the areas of Sambungmacan, Ngawi, and Ngandong where they lived until the latest Pleistocene (0.05 Ma). SARI Kubah Sangiran, terletak 12 km di sebelah utara kota Surakarta (Solo), Jawa Tengah merupakan tempat terkenal untuk geologi Kuarter karena tempat ini menyingkapkan kompleks batuan dan fosi-fosill hominid serta vertebrata berumur Plistosen. Kubah Sangiran merupakan sisa struktur diapir dan gununglumpur berdasarkan pola deformasi kubah, keberadaan rembesan air asin dan gas metana di tengah kubah, serta kehadiran fragmen dan bongkah batuan ganjil dari batuandasar malihan sampai batuan berumur Pliosen. Berdasarkan umur deformasi dan pentarikhan mutlak, deformasi diapirisme dan erupsi gununglumpur terjadi pada 0,7-0,5 Ma - juta tahun yang lalu (Plistosen tengah) dan mungkin berulang lagi sampai 0,12 Ma (bagian bawah Plistosen akhir).

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Kubah Sangiran pernah menjadi habitat hominid sub-spesies Homo erectus erectus, fosil-fosilnya ditemukan di lapisan bagian atas Formasi Pucangan dan bagian bawah Formasi Kabuh. Hominid ini hidup dalam kala Plistosen (periode tepatnya masih diperdebatkan antara 1,7-1,0 Ma, 1,3-0,7 Ma, atau 1,0-0,5 Ma). Akhir periode sub-spesies ini (0.7-0.5 Ma) kelihatannya bersamaan dengan periode erupsi kompleks gununglumpur Sangiran. Erupsi gununglumpur ini mungkin saja mempengaruhi kepunahan Homo erectus erectus. Erupsi gununglumpur Sangiran juga diperkirakan telah mempengaruhi migrasi sub-spesies hominid selanjutnya, yaitu Homo erectus ngandongensis / soloensis. Sub-spesies ini tidak pernah ditemukan di kubah Sangiran, mungkin kubah Sangiran tidak layak huni karena menjadi tempat erupsi gununglumpur sampai Plistosen akhir. Sub-spesies Homo erectus ngandongensis / soloensis bermigrasi ke arah timur sepanjang hilir Bengawan Solo menuju daerah-daerah Sambungmacan, Ngawi, dan Ngandong - tempat fosil-fosil sub-spesies ini ditemukan sampai ujung Plistosen (0,05 Ma). INTRODUCTION

Sangiran area, 12 kms to the north-northeast of Surakarta (Solo), 6 kms to the west of the Solo River, Central Java (Figure 1), is one of the most famous places of paleo-anthropological discoveries in Southeast Asia. The “Sangiran Early Man Site” is one of 550 locales worldwide that UNESCO has recognized for its special cultural and natural value (Huffman, 1998). Here, the remains of early humans (hominids) called Homo erectus (formerly was called Pithecanthropus erectus) that lived over a million years ago have been discovered since the fieldwork of Koenigswald from 1936-1938 (Koenigswald, 1940) (Figure 2). Sangiran today remains one of the most active sites for Plio-Pleistocene research and is also an excellent location to study fluvial and volcanic sedimentation (Lunt et al., 1998). Geological fieldwork in this area was firstly conducted by Es (1931) and Koenigswald (1940) who revealed that Sangiran is a four-way anticlinal structure forming a dome hence it is usually called the Sangiran Dome in the geological literatures. The origin of the Sangiran Dome has been interpreted in various ways. Intensive joint expedition of Indonesian and Japanese team from 1976-1979 (reported in Watanabe and Kadar, 1985) concluded that the Sangiran Dome is a remnant mud volcano. Regionally, Sangiran area is located at the central depression of Java involving the Kendeng and Solo Zones in Central Java. Along the zones, there are many occurrences of mud diapirs and mud volcanoes (Satyana and Asnidar, 2008). One of the mud volcanoes is called “Lusi” in Sidoarjo area, East Java which has been erupting since May 2006. The eruption of Sangiran mud volcano is considered to be catastrophic based on the presence of erupted materials found in the area including various exotic rocks. The exotic rocks were “the basement” of mud volcano before eruption. The timing of the eruption is hard to determine. However, the doming process of Sangiran was very young, to be from 07-0.5 Ma (Lunt et al., 1998). This period was contemporaneous with the upper limit of Homo erectus (sub-species Homo erectus erectus in Zaim, 2006) fossils based on fission track age dating which is 0.71 Ma (Suzuki et al., 1985). The aim of this paper is to discuss the possible relationship between the eruption of Sangiran mud volcano and the demise of Homo erectus erectus in Sangiran area. Further sub-species development of Homo erectus, namely Homo erectus ngandongensis/ soloensis did not

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develop in the Sangiran area, but migrated eastward following the downstream of the Solo River in Sambungmacan to Ngandong areas (Zaim, 2006). The migration could be related to un-habitable Sangiran area after the eruption. REGIONAL GEOLOGY In the map of physiographic zones of Java (Bemmelen, 1949), Sangiran area is located at the overlapping physiographic zones of southern limit of the Kendeng Zone and northern limit of the Solo Zone (Ngawi sub-Zone). The area is also at the northwestern foot of the Quaternary Lawu volcanic cone (Figure 1). The Kendeng and Solo Zones are the central depression of Java into which Miocene to Pleistocene volcanic-clastic sediments were rapidly deposited (Satyana and Armandita, 2004). The rapid sedimentation and high geothermal gradient due to proximity to the Miocene-Quaternary volcanic arcs has generated “elisional” condition causing many mobile overpressured clays piercing upwards as diapiric flow and mud volcano eruption (Satyana and Asnidar, 2008). SANGIRAN DOME The Sangiran area is a hilly area around the Krikilan village. Its maximum altitude is 183 m, and a structural dome trending SSW-NNE occupies its central part (Figure 4). This structure which is called the Sangiran Dome is approximately 8 kms long and 4 kms wide. It has been dissected by tributaries of the Solo River, namely the Cemoro, Brangkal, Pohjajar Rivers and many smaller streams. Consequently, the strata around the dome are well exposed. The Sangiran Dome has been geologically mapped since 1930s (Es, 1931; Koenigswald, 1940, Sartono, 1961; 1970; 1975; Itihara et al., 1985a). The origin of the Sangiran Dome has been interpreted in various ways. It is an anticline forming a dome at the southern margin of the Kendeng Zone, a compressive feature related to collapse of the old Lawu volcanic cone, an incipient volcano, or a diapiric shale flow. The stratigraphic summary of Sangiran area is as follows (based on Itihara et al., 1985a; Lunt et al., 1998) (Figure 3). The oldest horizon (fission-track dating shows 2.99 Ma –middle Pliocene) (all equivalent geologic time used in this paper are based on Geologic Time Scale 2004 – Gradstein et al., 2004) exposed in the middle of the Sangiran Dome. It is a part of the Kalibeng Formation. Most of the formation is composed by bluish-grey marine clay with six thin tuff beds. The upper part of the Kalibeng is shallow marine Turritella beds, Balanus limestone, brackish to fresh Corbulina bed, and again the uppermost bluish-grey marine clay. Overlying the Kalibeng, is there volcanic breccia at the top of Pliocene (dated as 2.1 Ma – the latest Pliocene) called the Lower Lahar composing the lowermost part of the Pucangan Formation (latest Pliocene to early Pleistocene). Lower part of the Pucangan Formation is composed of bluish grey clay with brackish water mollusks, benthic forams and ostracods (estuarine or related setting). Upper part of the Pucangan Formation is made up of lacustrine Black Clays with freshwater mollusks, ostracods, and some vertebrata fossils (including hominids). Throughout the formation, there are thin tuff layers and locally reworked marine foraminifera. Overlying the Pucangan formation is the Kabuh Formation (early-middle Pleistocene). The lower part of the formation is called the Greenzbank consisting of calcite cemented

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conglomerate rich in vertebrate fossils (mammals and hominids). The upper part of the Kabuh Formation is composed of fluvial channel conglomerates and sand clays. Middle-late Pleistocene Notopuro Formation is the youngest rock formation of the Sangiran Dome. The formation is composed of volcaniclastic conglomerates and sands called the Upper Lahar. Unconsolidated strata of the Solo River terrace and alluvial deposits are the modern sediments in Sangiran area. The stratigraphy of the Sangiran Dome shows that marine conditions (Kalibeng) persisted in the area until the end of Pliocene. There was then a rapid regression through brackish conditions (lower Pucangan) to lacustrine muds (upper Pucangan) which are the Black Clays outcropping over much of the dome. It is in the upper part of the Black Clays and the base of the overlying Grenzbank fluvio-deltaic clastics (lower Kabuh) that the hominid fossils are found. Based on the fission track ages (Suzuki et al., 1985), the extent of the hominid finds are from 1.16 to 0.71 Ma (upper early Pleistocene to lowermost middle Pleistocene). Homo erectus OF SANGIRAN DOME Sangiran is among the most important paleo-anthropological sites anywhere for addressing questions about the ecology of early humans (Huffman, 1998). The remains of dozens of Homo erectus have been discovered over the last 60 years, making Sangiran the most prolific source of early human fossils in Java. Eastern Java (Sangiran to Mojokerto areas) is the only part of Southeast Asia where such remains have been recovered. Homo erectus lived in the Solo area for several hundred thousand years in the Pleistocene (Figure 2). What part of the Pleistocene is a controversial matter. Most experts place early humans at Sangiran from 1.3 to 0.7 or even 0.125 Ma (e.g. Watanabe and Kadar, 1985) (Figure 3). This viewpoint is based primarily upon fission-track dates and paleomagnetic stratigraphy. On the other hand, the age of habitation has been placed at 1.7 to 1.0 Ma on the basis of 40Ar/39Ar radioisotopic dates (Swisher, 1997). Using the 40Ar/39Ar dating, Larick et al. (2001) dated the hominid bearing- Greenzbank of the lower Kabuh and found age of 1.51 Ma. This is much older than that reported by Watanabe and Kadar (1985) which is 0.9 Ma. Larick et al. (2001) reported that the life period of Homo erectus in Sangiran area lasted from 1.6 to 1.0 Ma. Which is right for the dates is difficult to determine since each dating method has a problem (Huffman, 1998). There are problems with the application of paleomagnetic studies in some parts of the section, some researchers suspect that key fission-track dates are unreliable, and while 40Ar/39Ar determinations are generally a dependable indication of age of the material dated. The development of Homo erectus in Sangiran area was started by sub-species Homo erectus paleojavanicus (Meganthropus paleojavanicus) (started at 1.6 Ma, lower part of early Pleistocene – Zaim, 2006) (Figure 2) where their fossils were found in Black Clays of Pucangan. This is an evidence for the first arrival of early hominid and vertebrates from mainland of Asia to Java through the Sundaland. In upper part of early Pleistocene to lower part of middle Pleistocene (1.0-0.5 Ma) (Zaim, 2006), a wide corridor across the Sunda Shelf and SE Asia brought sub-species of Homo erectus erectus. The date when the first hominids arrived in Java is widely disputed (Huffman, 2001). According to most estimates, Homo erectus was present by mid-Early Pleistocene (about 1.0 Ma) (Watanabe and Kadar, 1985; Itihara et al., 1994). Significant evidence also has been

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advanced for occupation in the latest Pliocene and earliest Pleistocene (1.7-1.8 Ma; Swisher III et al., 1994). Older than 1.8 Ma is impossible since older than 1.8 Ma marine conditions persisted in eastern Java causing inhabitable ecology for hominids. ERUPTION OF SANGIRAN MUD VOLCANO AND DEMISE OF Homo erectus erectus The Sangiran Dome as a complex of mud volcanoes has been indicated since the fieldwork of Es (1931) who explained the source of exotic Eocene blocks should be brought to surface by a reverse position of the underlying Tertiary formations caused by an overthrust, and Koenigswald (1940) as well as Bemmelen (1949) who reported the occurrence of exotic rock fragments and foraminiferal fossils in Sangiran area. Itihara et al. (1985a, b) called the Sangiran Dome is a mud volcano. It is inferred that the process of updoming resulting from a diapiric rise of deepseated muddy sediments led to radial and concentric faults, eruption of mud volcanoes and depression of central blocks (Figures 4, 5). Four mud volcanoes are present in the central part of the Sangiran Dome (Itihara et al., 1985a, b). Vents of these mud volcanoes range from 120 to 30 m in maximum diameter. Several small saline water seeps and methane gas bubbles occur at the center of the dome. Exotic rock fragments to boulders distribute in this area. They are erupted materials including plastic muds with exotic blocks from the underlying Eocene – Pliocene strata and igneous rocks comprising marls, shales, sandstones, nummulitic limestones, andesites, monzonites and phyllites. The erupted materials came from the considerable depths or “basement” of the mud volcano. The erupted materials belong to rock groups of the basement and its overlying beds in southern Central Java. The phyllites correspond to pre-Tertiary metamorphic rocks in the Jiwo Hills. The Eocene and Oligocene erupted rocks correspond to the Wungkal and Gamping Formations in the Jiwo Hills, and the Nanggulan beds in the West Progo Mountains. The lower Miocene-lower middle Miocene erupted rocks correspond to the Pelang and Kerek beds in the western Kendeng Zone, and the lower and middle parts of the Sentolo Formation in the West Progo Mountains. The Miocene-Pliocene erupted rocks correspond to the Lower and Upper Kalibeng beds of the western Kendeng Zone, and probably to the upper part of the Sentolo Formation in the West Progo Mountains. The depressions of central blocks exist around the vents of mud volcanoes and was resulted from subsidence after the eruption (Figures 4, 5). Materials erupted had decreased the volume of rock materials supporting the level of ground surface. It is a usual phenomena in other mud volcanoes in eastern Java. The depression is a crater of the mud volcano. Based on a geologic schema constructed for Sangiran area, Itihara et al. (1985b) considered that the Sangiran Dome and the mud volcano were formed after the deposition of Notopuro Formation and before the deposition of the Terrace deposits. The diapiric flow of the mud volcano they considered to be affected by collapse of the Old Lawu volcano (Bemmelen, 1949) at the end of the Notopuro period. This diapiric flow when reached surface became mud volcano eruption. The mud eruption had taken place sometime between Notopuro and Old Terrace (the uppermost part of middle Pleistocene - around 120,000 years ago).

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Geologic profile of the Sangiran Dome by Koenigswald (1940) (Figure 4) however, shows that the youngest beds deformed by the dome is the Kabuh Formation. The Notopuro beds rest on the deformed Kabuh. The central part of the Sangiran Dome also subsides. Based on Koenigswald (1940)’s profile, it indicates that the deformation (and mud eruption) was earlier than that of Itihara et al. (1985b) at the end of the Kabuh Formation time (lower middle Pleistocene – around 500,000 years ago). Lunt et al. (1998) considered that the Sangiran Dome is very young, -less than half a million years old, as is shown by the age dating of the Kabuh Formation which was accumulated in a low area at about 0.7 Ma. Taken into account the period of Sangiran Dome deformation (and eruption) from Koenigswald (1940) and Lunt et al.(1998), the eruption was contemporaneous with the end period of subspecies Homo erectus erectus which was at around 0.7-0.5 Ma. Did the eruption of Sangiran mud volcano affect the demise of the subspecies ? It could be. Evolution of hominids and vertebrates in Indonesia were might be influenced by the development of geological condition (Zaim, 2006). Tectonic activities and glacio-eustatic during Quaternary might also be related closely to the dispersal and evolution of the hominids and vertebrates, as well as the modes of their migration. The middle Pleistocene seems geologically was ended by strong tectonic and volcanic activities. All the middle Pleistocene sediments and older in Java were gentle folded. Cyclical climatic change during the Pleistocene relating to coeval tectonism might also a cause for demise of Homo erectus erectus in Sangiran area (Dr. Herman Moechtar, personal communication, 2008). The climatic change had caused extreme dry and flood seasons which might be harmful for the sub-species. MIGRATION OF LATER HOMINID In late Pleistocene time, from 0.125 to 0.05 Ma, river terraces of the old Solo River were formed (Zaim, 2006). The Old Solo River terraces were formed in Sangiran, Sambungmacan, Trinil, Ngandong and Ngawi areas (Sartono, 1976). The Old Solo River terraces at Ngandong (dated about 0.05 Ma – Zaim, 2006) contain sub-species of Homo erectus ngandongensis (Homo erectus soloensis) (Sartono, 1986). Fossils of the type are also found in Sambungmacan (Sartono, 1979) and Ngawi (Sartono, 1991) areas. The vertebrate fossils from Ngandong area were grouped into Ngandong Fauna dated younger than 0.05 Ma (late Pleistocene) (Vos and Sondaar, 1994) (Figures 1, 2). There has been no Homo erectus ngandongensis (Homo erectus soloensis) discovered in the river terraces of Sangiran. It indicates that this sub-species did not choose the Sangiran area as a habitat. They preferred the downstream of the Solo River and lived in Sambungmacan to Ngawi areas. Why the sub-species did not choose the Sangiran area is intriguing since the Sangiran area had been the good place for living since the latest Pliocene. It is considered that the Sangiran area was not a longer good place to live since the area had been a site for eruption of mud volcanoes since 0.7-0.5 Ma and might continued until 0.12 Ma (Itihara et al., 1985). Therefore, the later hominid Homo erectus ngandongensis / soloensis migrated downstream away from the Sangiran area into the Sambungmacan to Ngawi areas (Figure 1) where their fossils were found.

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CONCLUSIONS 1. Based on the nature of deformation, presences of saline water and methane gas seeps, and

erupted materials consisting of various exotic blocks from metamorphic basements to Pliocene rocks , the Sangiran Dome is a remnant diapiric structure and mud volcanoes complex. The diapiric deformation and eruption is considered took place between 0.7 and 0.5 Ma (middle Pleistocene) and could occur several times until 0.12 Ma (base late Pleistocene) based on the nature of deformation and absolute dating.

2. Sangiran Dome was a home for Homo erectus erectus who lived there in the Pleistocene

(ages remain in dispute as 1.7-1.0 Ma, 1.3-0.7 Ma, or 1.0-0.5 Ma). The fossils of the sub-species were found in the upper part of Pucangan and lower part of Kabuh Formations. The termination of the sub-species between 0.7-0.5 Ma could be contemporaneous with the eruption of the Sangiran mud volcanoes. The eruption possibly affected the demise of Homo erectus erectus.

3. Eruption of Sangiran mud volcanoes was probably also a reason why later forms of

hominid (Homo erectus ngandongensis / soloensis did not develop in the Sangiran area but migrated eastward downstream of the Solo River into the areas of Sambungmacan, Trinil, Ngawi, and Ngandong where they lived until the latest Pleistocene (0.05 Ma).

ACKNOWLEDGMENTS I would like to thank : (1) Dr. Adi Kadar (Lapindo Brantas) for a discussion on several mud volcanoes in eastern Java and providing me key references on Sangiran, (2) Prof. Dr. Yahdi Zaim (Institute of Technology Bandung) for discussions on Java’s hominids and providing me recent publications on the subject, (3) Dr. Herman Moechtar (Geological Survey of Indonesia) for a discussion on climatic changes and related sedimentology in Sangiran area. The content of this paper is however, my sole responsibility. I acknowledge my affiliation, BPMIGAS, who gives me sponsorship to present this paper. Cipi Armandita (BPMIGAS) helped me providing 3D-diagram of Sangiran Dome for presentation material. REFERENCES Es, L.J.C. van, 1931, The Age of Pithecanthropus, Martinus Nijhoff, The Hague, 142 p. Gradstein, F.M., Ogg, J.G., Smith, A.G. et al., 2004, A Geologic Time Scale 2004,

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iagi sangiran mud volcano and demise of homo erectus satyana 2008

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