EXPLORING JOGJA GEOHERITAGE:

THE LIFETIME OF AN ANCIENT VOLCANIC ARC IN JAVA

C. Prasetyadi

Geology Department – UPN “Veteran” Yogyakarta

(cprasetyadi@gmail.com)

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. The widespread of Oligo-Miocene andesitic rock along the

southern part of Java indicate that at that time there was a significant volcanic arc occurring in Java

and this volcanic arc has been well known as the Old Andesite Formation (OAF).

Site 2. Thick layer of ancient volcanic ash - Candi Ijo, Prambanan (Figure-5 B)

A thick layer of ancient volcanic ash deposit forming a hilly complex is found in this location. By local

people the hills are being mined resulting in fresh exposures of the rock. The rock unit consists of

white, dull colored layers of volcanic ash containing many pumice fragments. The presence of pumice

fragments convincingly proves that this deposit was produced by an explosive eruption. This kind of

rock is widely distributed and commonly found in area extended from Parangtritis, south coast of

Jogja, to the east to Wonogiri with the thickness ranges 300-600 m. The best outcrop occurs in Semilir

Village, to the south of Jogja. The name of this village is then be used to called the rock unit as Semilir

Formation.

Semilir Formation overlies stratigraphically the Berbah Pillow Lava (Site-1). Widespread

distribution along with its great thickness indicate that Semilir Formation was the product of a serial

event of big volcanic explosion back to Oligo-Miocene time approximately 20-30 Mya (and comparable

to the big explosion of the Toba supervolcano. Smyth et al (2005) called the Semilir Formation as the

product of a super eruption of the Semilir Volcano. From Berbah Pillow Lava lying underneath going

upward to Semilir Formation, it means we see a field evidence concerning the development of a

volcanic arc in which its initial stage was marked by the occurrence of a monogenetic volcanism

(producing only one type of effusive material) in a marine environment and then developed into a

polygenetic volcanism producing strato volcanoes consisting of interbedding lava and pyroclastics,

and as the final stage is the explosive eruption, the Semilir super eruption.

Going further up stratigraphically, Semilir Formation is overlain by the younger Nglanggran

Formation that consists of aglomerate, andesitic volcanic breccia and lava. The presence of

Nglanggran Formation indicates that after the formation of extensive caldera produced by the Semilir

super eruption then it was followed by the growth of a new strato volcanoes, namely the Nglanggran

andesitic strato volcano found in Site-8. In Figure-4, Semilir and Nglanggran Formations represent the

period of OAF.

Site-3. Quartz Conglomerate – West Jiwo, Bayat, Klaten (Figure-5 C)

In this location we can find outcrops of quartz conglomerate. This sedimentary rock is considered as

the oldest sedimentary rock in Java because the components are comprised by materials or fragments

derived from the oldest metamorphic rock. The metamorphic fragments are mostly consist of quartz

material which initially was part of the quartz veins found within the metamorphic rock. The

conglomerate belongs to the Eocene Wungkal-Gamping Formation. Beside conglomerates, there

quartz sandstone, Nummulitic limestone and shale in this formation. The absence of volcanic material

in this formation indicate that during their deposition, the sedimentary environment was not related

with volcanic environment or complex. In other words, when this formation deposited, there was not

yet volcanic arc developed in Java. The deposition of this formation represent the time of Pre-OAF

(see Figure-4).

Stop Site 4a. Bayat Metomorphic rocks, East Jiwo, Klaten (Figure-5 D)

In this rare location we can find an extensive outcrop of metamorphic rocks consisting of slate, phyllite,

schist and few marble. A thick weathering condition make rather difficult to observed the fresh

outcrops of the metamorphic rock. But still we can observe foliation features as the characateristic

structure of the rocks. Metamorphic rocks similar to those we find in this location are very rare in Java.

There are only three places in Java, namely Bayat, Karangsambung and Ciletuh, where we can find

the outcrops of metamorphic rocks. The age of metamorphic rock in Bayat is about 95-100 Mya and

interpreted as part of the East Java micro continent that collided with Karangsambung-Meratus Trench

in Late Cretaceous. The collision terminated the subduction tectonic in the Karangsambung-Meratus

Trench and led to the amalgamation of the East Java micro continent to Sundaland (see Figure-3A).

The erosion product of the many quartz veins found in the phyllitic rocks can be seen as the quartz

fragments in conglomerates found in Site-3. In the Historical Geology Column presented in Figure-4,

this metamorphic rock unit represents the period of initial formation of Java Island.

Site 4b. Index Fossil of Eocene Time, Watuprau, East Jiwo, Klaten (Figure-5 E)

Not far from Site-4a with basement rock outcrops, 200 m to the south, there is a site called by local

people as “Watuprau” (Watu> “rock”, Prau > “boat”, meaning “rock looks like boat”). “Watuprau” is an

outcrop of limestone that contains mostly of fauna fossils of large foraminifera such Nummulites sp,

Discocyclina sp.. This fossiliferous limestone is found along with quartz sandstone and unconformably

overlies the metamorphic rock unit. The occurrence of Nummulites sp. and Discocyclina sp. is very

important because they are index fossils for Eocene Time (50-36 Mya). Based on the presence of the

fossils, it is very convincing to considered that the metamorphic rock underneath is much older in age

and for rock units above it are younger, and the overlying rock is the igneous and volcaniclastic of the

Oligo-Miocene OAF. Along with quartz conglomerate and quartz sandstone, this Nummulitic limestone

is part of The Wungkal-Gamping Formation. Like the conglomerate, the absence of volcanic material

in this formation indicate that during their deposition, their sedimentary environment was not

influenced by volcanic events. In other words, when this formation deposited, there was not yet

volcanic arc developed in Java. The deposition of this formation represent the time of Pre-OAF (see

Figure-4).

Site 5. Enjoying Pre-OAF Landscape – Tancep Village, Klaten (Figure-5 F)

In this location, we are in a high place topographically about 300 m above sea level in Tancep Village.

From here we can enjoy the beautiful landscape sight. The sight is looking to the north where Mt.

Merapi is located. Beside Mt. Merapi, we can see all the landscapes of the site locations we just

observed. This site is part of Baturagung escarpment that extends east-west and consists of rock unit

of OAF with the upper part consists of Semilir and Nglanggran Formations. So in this site we are

occupying the OAF rocks, and to north, there are rock units of the Pre-OAF period while to the south

we will observe the rock unit of the Post-OAF period.

Site 6. Reef Platform of Wonosari – Nglipar Village, Wonosari (Figure-5 G)

Features of karst topography of the Wonosari Formation can be enjoyed nicely in this location. From

this site, we can find the outcrop of marl and bedded limestone of Oyo Formation while to the south

away from this location we can observe the topographic expressions of Wonosari Formations

consisting mainly of reefal limestone. This reefal limestone form a very extensive platform starting

from Wonosari to Pacitan area di East Java and overlies the OAF rock unit. The massive emergence

of this extensive reef platform indicate that at the time this reef complex starting to grow, the volcanic

activity that produce the OAF had been inactive or terminated. Based on its foram content, the

Wonosari reef limestone was start to be deposited in Miosen time (15-5 Mya). The occurrence of Oyo

and Wonosari Formations marked the period of Post-OAF.

Site 7. Sambipitu Bioturbation – Ngalang River, Nglipar (Figure-5 H)

In this site we observe the outcrop as the bedrock of the Ngalang river. The bedrock consists of

interbedding calcareous sandstone and polymict breccia of Sambipitu Formation. This formation

stratigraphically overlies the volcanic Nglanggran Formation and in turn underlies the limestone of Oyo

Formation. On the surface of the sandstone bedding we can see bioturbation features indicating a

coastal to shallow marine sedimentary environment. In between sandstone, there is polymict breccia

layers. Components of the breccia consists of fragments of andesitic lava and reefal limestone. The

andesitic fragments should be derived from the older Nglanggran Formation which is identical to an

andesitic strato volcano. Based on this observations, we can infer that when Sambipitu Formation was

deposited, the Nglanggran Volcano had been inactive and then eroded. The erosion product was

transported and deposited as the fragments of polymict breccia of the Sambipitu Formation which is

marked the Post-OAF period along with Oyo and Wonosari Formations.

Site 8. Situs Gunung Api Purba – Nglanggran Village, Bantul (Figure-5 J)

This location is the most interesting site. This site offer a spectacular landscape indicating the remains

of a volcano complex. The site is dominated by topographic features of eroded volcano. The rocks

found in this area is part of the Nglanggran Formation and dominated by agglomerat and brecciated

lava. Agglomerate is made of coalition of bombs, a kind of pyroclastic materials comprised by lava

ejected not far from the vent of eruption. The Nglanggran formation as shown in this site is very

different compared to the Semilir Formation that consists of volcanic material produced by explosive

eruption, and Nglanggran Formation is younger than the Semilir Formation. The presence of

Nglanggran Formation indicates that after the formation of extensive caldera produced by the Semilir

super eruption then it was followed by the growth of a new strato volcanoes, namely the Nglanggran

andesitic strato volcano found in Site-8. In Figure-4, Semilir and Nglanggran Formations represent the

period of OAF.

4. CONCLUSION

Java island is one of the largest volcanic 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

From now on it is urgent to disseminate among ordinary people the informations related with

geological hazard and, more importantly, informations about how the Java island they occupy are

formed.

In areas surrounding Jogja, such as Berbah, Candi Ijo Prambanan, Bayat and neigboring areas,

there are 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, and it is called

as “geoheritage of Jogja”.

Jogja Geoheritage includes eigth sites or locations where the rock exposures can be observed

very closely because their high accessibilities. This series of outcrop contain complete

informations concerning the geological polyhistory of Java and can be observed within one day

fieldtrip.

REFERENCES

Asikin, S., 1974, Evolusi Tektonik Daerah Karangsambung Berdasarkan Teori Tectonik Global yg Baru,

Disertasi ITB, tidak dipublikasikan.

Hamilton, W., 1979, Tectonic of Indonesia Region, Special Paper of USGS.

Prasetyadi, C., 2007, Evolusi tektonik Paleogen Jawa bagian timur, Disertati Doktor, ITB Bandung, tidak

diterbitkan, 323 hal.

Sudarno, Ign., 1997, Kendali tektonik terhadap pembentukan struktur pada batuan Paleogen dan

Neogen di Pegunungan Selatan, DIY dan sekitarnya, Tesis Magister, ITB Bandung, tidak diterbitkan

Smyth, H; Hall, R, 2005, Basin development and Basement rocks of East Jawa, IPA Proceeding..

Wartono, R., dkk, 1995, Peta geologi Lembar Yogyakarta, skala 1:100.000, P3G Bandung.

Figure-1: Location map indicating the sites and the route to visit the sites of Jogja Geoheritage.

Figure-2: The two volcanic arcs of Java that form the “backbone of Java”..

Figure-3: Geological evolution of Java (Prasetyadi, 2007).

Figure-4: Geological History column of Java.

Figure-5: The sites of Jogja Geoheritage. (A) Oligo-Miocene Pillow Lava, Berbah Village; (B) Thick layers of

Oligo-Miocene volcanic ash of Semilir Formation, Candi Ijo, Prambanan; (C) Eocene Quartz conglomerate,

West Jiwo Village, Bayat; (D) Pre-Tertiary Phyllite, metamorphic rock, East Jiwo, Bayat; (E) Eocene

Nummulitic limestone, Watuprau Village, Bayat; (F) Landscape view in Tancep Village; (G) Landscape view

of Wonosari Karst morphology; (H) Sambipitu bioturbation, Ngalang River; (J) The remains of ancient Oligo-

Miocene Nglanggran Volcano, Nglanggran Village. (B: Foto by Zaenal Fanani; D & E: by Dwi Oblo).