Exploring Jogja Geoheritage: the Lifetime of an Ancient Volcanic Arc in Java

EXPLORING JOGJA GEOHERITAGE: THE LIFETIME OF AN ANCIENT VOLCANIC ARC IN JAVA C. Prasetyadi Geology Department – UPN “Veteran” Yogyakarta ([email protected]) Abstract In addition to their art and cultural heritages, Jogja and surrounding areas have also an important and interesting geoheritage. In areas not far from Jogja, within radius around 40 km and only one day visiting, there are characteristic and important rock exposures that can be reached easily because their locations are nearby the asphalted roads. These exposures are very special due to geological informations they contain. The informations include a whole story of the lifetime of an ancient volcanic arc of Java known as the Oligo-Miocene Arc or called the Old Andesite Formation (AOF), “the backbone of Java Island”, which forms the southern part of Java Island. The story is a polyhistory consisting of periods with different geological events. These events consist of pre-OAF, syn-OAF and post-OAF. The pre-OAF period is a non-volcanic period as shown by the outcrops of Pre-Tertiary basement and Paleogene carbonate and silisiclastic rocks in Bayat areas. The syn-OAF period comprises three phases, namely early phase (or monogenetic volcanism, indicated by the pillow lava of Berbah Village), constructing phase (or polygenetic volcanism, marked by the sequence of volcanic breccia and aglomerate of Nglanggran Formation), and destructing phase (or explosive volcanism, as evidenced by thick layer of volcanic ash of the Semilir Formation). The post-OAF period is indicated by the widespread exposures of Miocene-Pliocene carbonate rock which is well-known as the reefal limestone of Wonosari Platform to the south of Jogja. This paper is aimed to introduce some easily reachable outcrops along with the route or traverse where we can explore them in order to find out the whole geological history of Java as the main volcanic arc in western Indonesia region. There is no other route or traverse of this kind in Java so it is appropriate enough to considered it as Jogja geoheritage. 1. INTRODUCTION Java island is one of the largest volcanic island, beside Sumatera Island, in western Indonesia region. As the main island with its highly dense population, Java island has a high potential of geological hazard due to the presence of active volcanoes and high seismic activity. The years of 2006 and 2010 are years that have been noted and memorized by most people of Jogja and surrounding areas as years of disaster because of the occurrences of Jogja earthquake and Merapi eruption. Both geological hazard events are traumatic for most people, especially for ordinary people. Facing and coping all the consequences of the disasters, most people were shocked and unprepared mentally. This is due to the lack of basic knowledge and awareness that they live in a region with highly geological hazard prone. Therefore in the future it is urgent to disseminate among ordinary people the informations associated with geological hazard and, more basic and important, informations about how the Java island they occupy are formed. To achieve this purpose, namely to inform and help ordinary people in Java to get the very basic knowledge of the geological evolution of their island, we need to find a simple way or method. Just by exploring the areas surrounding Jogja (Figure-1) we have found the method, and we called it as “introduction to Jogja geoheritage” and described as follows: In areas surrounding Jogja, such as Berbah, Candi Ijo Prambanan, Bayat and neigboring areas, we can find field evidences in the form of rock exposures that contain informations on geological history back to a hundred million years ago. The geological informations contained by the series of outcrop construct a complete history of the lifetime of Java as the main volcanic island arc in western Indonesia. The informations include the lifetime of an ancient volcanic arc of Java known as the Oligo-Miocene Arc or called the Old Andesite Formation (AOF), “the backbone of Java Island”, which forms the southern part of Java Island (Figure-2). The story is a polyhistory that consists of periods with different geological events. These events consist of pre-OAF, syn-OAF and post-OAF. The pre-OAF period is a non-volcanic period as shown by the outcrops of Pre-Tertiary basement and Paleogene carbonate and silisiclastic rocks. The syn-OAF period comprises three phases, namely early phase (or monogenetic volcanism), constructing phase (or polygenetic volcanism), and destructing phase (or explosive volcanism). The post-OAF period is indicated by the widespread exposures of Miocene-Pliocene carbonate rock which is well-known as the reefal limestone of Wonosari Platform to the south of Jogja. This paper is aimed to introduce some easily accessible outcrops along with the route or traverse where we can explore them in order to find out the whole geological history of Java as the main volcanic arc in western Indonesia region. There is no other route or traverse of this kind in Java therefore it is appropriate enough to considered it as Jogja geoheritage. With the dissemination and socialization of this geoheritage through some ways and method, espcially through a one-day fieldtrip, it is hope that more people in Jogja, and generally in Java, will improve their knowledge and awareness to the fact that Java island is a highly geohazard prone region. 2. BRIEF GEOLOGICAL HISTORY OF JAVA The following is a brief geological history of the eastern part of Java since Late Cretaceous to Miocene summarized from Prasetyadi (2007) (Figure-3): Late Cretaceous – Paleocene (Figure-3 A) The occurrence of Cretaceous Luk-Ulo Melange Complex in Karangsambung area, north of Kebumen, Central Java indicate that during Late Cretaceous the area was part of subduction zone of Indo- Australian Plate northwestward beneath the Sundaland continental plate. This subduction zone extends to the west to Ciletuh area in West Java and to the NE to Meratus Mountain area in the SE tip of Kalimantan. In Late Cretaceous in its movement to the west East Java microcontinent collided with the subduction zone of Karangsambung-Meratus resulted in subduction termination and forming the suture zone of Meratus Mountain. Middle Eocene (Figure-3 B) During 54 to 45 million years ago plate interactions in Indian Ocean region were marked by a significant decrease of northward movement of India Continent. This sudden drop of India movement to the north resulted in the slow down of convergent rate of Indo-Australian plate and causing an extension phase to take place in eastern region of Indian Ocean. This extension regime was a period of massive basin formations in some areas such as Karangsambung, Nanggulan, and Bayat in Central Java, and in East Java. In Nanggulan and Bayat areas the sediments were deposited ini a shallow marine environment and interpreted to be located in the eastern shelf area of East Java microcontinent and adjacent to the west with deep basin of Karangsambung. Most of Eocene rocks in Java is free from volcanic material. Late Eocene – Early Oligocene (Figure-3 C) Most of upper Eocene sediments have an unconformable contacts with the younger Oligocene rock units above it. The unconformity was probably associated with a deformation that took place at that time. The deformation was also related with the northward movement of Australia continent. The increasing movement of Australia continent caused a higher rate of convergence of Indo-Australian Plate in Java trench and pushing East Java microcontinent to the northwest. This microcontinent convergence resulted in the deformation of Paleogen formations ini Karangsambung, Nanggulan, and Bayat. Oligocene – Miocene (Figure-3 D) The increase of movement rate of Australia continent northward might continue until Middle Oligocene time. This event triggered a volcanic activity associated with the emerging of volcanic zone in the southern part of Java known as the Old Andesite Formation (OAF) and its area is called as the Southern Mountain zone. This volcanic activity did not reach regions to the north of Java where carbonate and silisiclastic sedimentation took place continuously in this area. When Australia continent was approaching the subduction zone of Java-Banda in Late Miocene, there was a massive tectonic compression occurring in SE Asia region. This compression regime inversed most of older sediments. In southern Java because of the arrival of Australia continent in Banda Arc, the volcanic activity became inactive and the area underwent uplift. The uplift of the area was marked by the widespread sedimentation of reefal limestone in southern part of Java. The reefal complexes are well- known as Punung formation in East Java, and as Wonosari, Sentolo, and Kalipucang Formations in Central Java. The above geological history in general can be divided into periods as follows (Figure-4): 1. Period of modern volcano 2. Period of post-OAF (Period of inactive Oligo-Miocene Volcanic Arc) 3. Period of OAF (Period of active Oligo-Miocene Volcanic Arc) 4. Period of pre-OAF (Period of non-volcanic arc) 5. Period of Pre-Tertiary Geology or Initial formation of Java Island (Period of microcontinent amalgamation) 3. THE SITES OF JOGJA GEOHERITAGE EXPOSURES There are eigth sites or locations where the rock exposures can be observed very closely because their high accessibilities (Figure-5). This series of outcrop contain complete informations concerning the geological polyhistory of Java. Site 1. Pillow Lava – Berbah Village (Figure-5 A) Based on stratigraphic correlation, the age of Berbah pillow lava is about Oligosen (approximately 30 Million years ago). Its age and basaltic composition indicate that this pillow lava was a product of monogenetic volcanism, an initial stage of volcanism in a volcanic arc. In the next stage this monogenetic volcanism will develop into a polygenetic volcanism forming strato volcanoes in which their composition turn into andesitic.

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