Forum Geografi, 31(1), 2017; DOI: 10.23917/forgeo.v31i1.2821
Caldera of Godean,
Sleman, Yogyakarta: A Volcanic Geomorphology Review
Sekolah Tinggi Teknologi
Nasional Yogyakarta
*) Corresponding author (email: hilghartono@sttnas.ac.id)
Received: 07 December 2016 / Accepted: 19 May 2017 / Published: 01 July
2017
Abstract
Godean Hills is located approximately 10 km westward from the Yogyakarta City. The landscape of Godean hills and plains is affected by various factors, such as lithology, geological structure, and sub-aerial process. The purpose of this study was to reveal the landscape of Godean. The method consisted of field study, morphological variables assessment, rock sampling, and laboratory analysis. The results of field mapping indicated that the landscape of Godean is an isolated hill with a steep slope of >40° and an elevation of +231 m a.s.l, passed by the rivers flows from northeast to south-west that disembogue into the west part of Kulon Progo. The morphology of Godean hills varies including G. (Gunung/Mountain) So (+173 m a.s.l), G. Gede (+218 m a.s.l), G. Wungkal (+187 m a.s.l), G. Butak (+154 m a.s.l), and G. Berjo ( + 175 m a.s.l), dominated by the lithology of igneous rock, which is composed of porphyry andesite-micro-diorite, pumice lapilli, and quartz-rich lapilli-tuff. In addition, most of the igneous rocks have weathered and have been altered to clays, while the deposition from Merapi volcano formed a landscape with an altitude between +100–+150 m a.s.l surrounding Godean Hills. Sentolo Formation was found in Kembang, Bantul, which is located approximately ±5km in the south of the study area N93ºE/12º, while the distribution in the south-west and northeast relatively covers the Godean hills in curve shape. The results of the analysis provide information related to Godean landscape that it is the remains of the volcanic caldera, with various igneous rock types and volcaniclastics deposits, as well as endured the occurrences of hydrothermal alteration and mineralisation. Further geophysical research is required to determine the configuration of igneous rocks under the earth's surface.
Keywords: Genesis, geophysical, micro-diorite, landscape,
caldera.
Abstrak
Daerah Godean terletak lebih kurang 10 km di sebelah barat kota Yogyakarta. Genesis bentang alam tinggian dan dataran daerah Godean dipengaruhi oleh berbagai faktor utama yang berbeda, seperti litologi, struktur geologi, dan proses geologi di permukaan bumi. Tujuan penulisan makalah ini adalah mengungkap keberadaan bentang alam daerah Godean. Metode pendekatan yang dilakukan adalah kunjungan lapangan, pengukuran variabel morfologi, pengambilan contoh batuan, dan analisis batuan di laboratorium. Hasil pemetaan lapangan menunjukkan bahwa daerah Godean berupa tinggian terisolir yang terdiri atas beberapa bukit dengan kelerengan terjal (>40o), ketinggian +231 m dpl., dilewati sungai-sungai yang relatif berarah ke timur laut–barat daya dan bermuara di K. Progo di sebelah barat. Morfologi tinggian Godean seperti G. So (+173 m), G. Gede (+218 m), G. Wungkal (+187 m), G. Butak (+154 m) dan G. Berjo (+ 175 m) disusun oleh litologi yang sangat keras berupa batuan beku berkomposisi andesit porfiri–diorit mikro, batu lapili pumis dan batu lapili tuf yang kaya kuarsa, dan sebagian besar batuan beku telah mengalami pelapukan dan teralterasi menjadi tanah liat, sedangkan material endapan G. Merapi membentuk bentang alam dataran dengan ketinggian antara +100-+150 mdpl. mengelilingi tinggian Godean. Formasi Sentolo yang tersingkap di desa Kembang, Bantul (±5 km) sebelah selatan daerah kajian berkedudukan U93oT/12o, sedangkan sebaran di bagian barat daya dan timur laut relatif melengkung melingkupi tinggian Godean. Hasil analisis memberikan pemahaman bahwa tinggian Godean merupakan bentang alam sisa tubuh gunung api berupa kaldera atau bregada, dengan variasi litologi batuan beku, batuan vulkaniklastik dan kehadiran alterasi hidrotermal dan mineralisasi. Penelitian geofisika lanjut perlu dilakukan untuk mengetahui konfigurasi batuan beku bawah permukaan bumi.
Kata kunci: Genesis,
geofisika, diorit mikro, bentang alam, bregada.
Introduction
Godean is part of the Godean
District, Sleman Regency, Yogyakarta (Figure 1), which
is situated precisely in the southwestern of Sleman Regency and approximately
10 km westward from Yogyakarta City. In general, Godean is well-known for the tiles and red brick manufacture by utilising local raw material while the area is surrounded by very fertile land in
the form of rice fields. Geologically, Godean
Hills are an isolated area that consists
of G (Gunung or Mountain). So (+173 m
a.s.l), G. Siwareng (+194 m a.s.l), G. Gede
(+218 m a.s.l), G. Wungkal (+187 m a.s.l), G. Ngampon (+222 m a.s.l), G. Gedang (+193 m a.s.l), G. Patuk (+231 m a.s.l), G. Butak (+154 m a.s.l) and G. Berjo (+175 m a.s.l) surrounded by plains.
Those isolated hills are constituted of intrusion
igneous rocks that consist of andesite and diorite in the Old Andesite
Formation, while the plains are constituted of younger volcaniclastic rocks
from G. Merapi (van Bemmelen, 1949; Rahardjo et al.,
1977). This formation has been studied several times for the fulfilment of Sandi Stratigrafi Indonesia and yet the
understanding to reveal the genesis of the landscape of hills has not been investigated comprehensively and
continuously. The genesis of a landscape involves some important aspects such
as the source of material, the mechanical
process of the formation, the age and environmental factors of the formation.
The material sources describe the origin
of intrusion rocks and volcanic rocks that made up the hills and plains of Godean including the formation process and the
formation period, while the environmental aspect refers to the sub-surface or
sub-aerial formation. The genesis of the formation of hills landscape and the
plains surrounding the hills became the main focus of this study.
Figure 1. Red
Rectangular Showing the Study Area Location.
Rahardjo et al. (1977)
mentioned that geologically, Godean hills
are a part of Kulonprogo Mountains. Kulonprogo Mountains are built up from the
ancient volcanoes of G. Gadjah, G. Idjo,
and G. Menoreh, well-known as “Oblong
Dome” due to its elongated dome from northeastern
to southwestern with a length of 32 km
and a width of 15-20 km (Figure 2). Godean hills, in
general, are also an igneous body building that has been deformed from weathering, alteration,
intrusion, and fault. It is a high possibility that those processes formed the modern landscape.
The stratigraphy of
Kulonprogo Mountains is initiated by siliciclastic sediments classified as Nanggulan Formation (Tabel
1), which member of Seputih consists of marks in upper part, then conformable
deposit of intercalation volcanic and
coherent lava, in addition to shallow
intrusion of old Andesite Formation. Jonggrangan Formation and Sentolo Formation were overlapped and
distributed upon Old Andesite Formation.
Lelono (2000) mentioned that Nanggulan Formation was formed in the Eocene Epoch, which can be
concluded based on the findings of Palynomorphs
that is the characteristic of lowland/rainforest
and the climate indicator. In addition,
Budiadi (2008) concluded
that the landscape of Kulonprogo Mountain was
generated from the geological structure. Hartono and Pambudi (2015) mentioned that the rocks building up Nanggulan
Formation are the deposits of small basin
between two hills. Meanwhile, Sipatriot (2016)
concluded that Godean was a relic of ancient volcano and Bronto (1999) mentioned
about industrial mineral deposit in
Godean that consists of clays originated from the weathering of igneous rocks.
Research Method
This research is a
surface geological study that included geological mapping, measurement of
geomorphological aspect, geological structure, and rock sampling. Geological mapping was done based on previous studies
and literatures that was specifically performed on Nanggulan hills and in
addition, the measurement of geological structure elements to determine their
role in the formation of the landscape of the study area, as well as the
measurement of geomorphological aspect
to determine the slope, height difference, pattern of flow, stadia, and
the type of streams in the context of the landscape characteristic, and the
process that accompanied it were arrived out. Furthermore, petrographic analysis was carried out to
identify the basic mineral composition,
texture, and structure of deposition, the process of formation, and
depositional environment.
Results and
Discussions
The field study
showed that the rocks making up the Godean
hills are composed of volcanic rocks and sub-volcanic rocks. Volcanic rocks generally contain lapilli tuff and lapilli pumice,
while the sub-volcanic rocks are intrusion rock composed of basalt, andesite,
dacite, and micro-diorite (Figure 3). Lapilli tuff, lapilli pumice, tuff, and others
volcanic rocks are originated from eruption activity, where pumice indicates
magma series composed of acid and the massive eruption of volcanoes.
Furthermore, the intrusion rocks are generally
the igneous rock composed of basalt, andesite and rare rocks including xenolith
in igneous rocks composed of porphyritic andesite-dacite. It shows the volcanic
activities that occur repeatedly and some
rocks that are intruded enter into the
intrusion rocks, as in tuff within the
dacite rocks. It is important in the context of this study that this process
took place in the body of magma or the magma chamber of a volcano body.
Based on the
variation of the rock composition revealed in the study area, an explanation of
the process of magma differentiation that leads to the behaviours of Godean volcanic eruptions is provided. Period of destructive eruption
probably contributed to the formation of the volcanic landscape in Godean. The
volcanic eruptions formed craters that vary in diameter (>2 km) and are characterized by perfect circular and semi-circular
shape of crescent (half moon, horseshoe shape). On the contrary, the inside
of the circular structure composed of intrusion rock, alteration rock, volcanic
neck, and lava dome. The remains of the outer landscape generated from volcanic
eruption activity frequently have symmetrical
shape, steep rock ramps of volcanic beds, and constituted of intercalation of
pyroclastic rocks and coherent lava.
The petrographic
analysis showed the igneous rocks that made up the Godean hills are generally
intermediate igneous rocks consisting of hornblende andesite, pyroclastic
rocks, and acid rocks of dacite. It indicated the amount of quartz mineral that reaches
5%-30% and the pyroclastic rocks with glass texture in tuffs, pumice, while the intrusion rocks is porphytic aphanitic, with
plagioclase mineral type of andesine (An44Ab56) (Figure 4). Volcanic rocks
consisting of coherent lava and pyroclastic made up the Godean landscape that lies
in the form of circular to semicircular landscape (Figure 5),
where rocks that had undergone alteration was found, while the plains that
surround it constituted of volcaniclastics material produced by the volcanic
activity of Mount Merapi located ± 25km to the north (Rahardjo
et al., 1977).
Geomorphology can be used as basic
framework to land resources evaluation
with landform unit as land mapping unit
or evaluation unit (Sutikno, 1990). Geomorphological
analysis indicated that the landscape was built
from active volcano (intrusion, melt, and
eruption) where material and composition is
associated with magma and lava, as well as its geological structure. Godean hills reflect the phenomenon, as demonstrated
in Figure 5A where the Hill is situated inside a
circular structure of 2 km in diameter and the distribution was covered by
carbonate rock of Sentolo formation that shows the rocks layering tilt to south
with the position N92°E / 12° (Figure 5B), while the
distribution of the south western and northwestern north parts follows a
circular structure.
The circular structure
on the west side, followed by a curved shape of the Progo river is a reflection
of the body architecture of ancient Godean
volcano. The curving river stream might also be affected by the resistance of
volcanic rocks and the structure of the fire mountain. In contrast to the
eastern part, there is neither river nor curved structures such as in the west.
This is because the eastern part covered by clastic
material from Mount Merapi and the river flow patterns evolve over the sediment
of Merapi. In the geological map (Figure 5A and 5B), the
hills built up Godean separately and
apparently unrelated. The explanation to this is the possibility of the faulted
igneous body hence the faults become
resistant and eroded, or the landscape is a dome in the surface (cryptodome),
the body of an intrusion, or volcanic neck in the volcanic crater.
The oldest
stratigraphy of Godean is constituted of Nanggulan Formation
consisting of Eocene mudstone and sandstone covered by the deposit of Old
Andesite Formation composed of igneous rock, andesitic breccia and tuff.
Igneous rock is in the form of lava and intrusive rock (Rahardjo
et al., 1977). It confirms that Godean was built by a group of volcanic rock
from eruption process activity and shallow intrusion. Nevertheless, Sutanto et al. (1994)
asserted the results of absolute age dating from igneous rock shows the Old
Andesite Formation aged 25.35-29.63 million years ago. It means that the
volcanic activity in Godean occurred
simultaneously with those in the area of Kulon Progo, or is possibly older when
it is assessed from the morphology and
weathering type.
In Godean, hydrothermal alteration and mineralisation occurred particularly in G.
Wungkal that indicates the ancient volcanic activity, which can be assumed as the geothermal system in ancient
time. The occurrences of hydrothermal alteration (Verdiansyah,
2016) can verify the presumption of Godean
as the product of volcanic process.
Figure
5C shows Godean landscape as volcanic
geomorphology formed from self-destruct process of volcanic eruption. This
eruption is associated with caldera
eruption type, hence, it is the remains
of the caldera. Genesis of the landscape of Godean
is relatively similar with G. Gajahmungkur in Wonogiri, Central Java, which is also characterized by caldera morphology (Hartono,
2010). In fact, the identification of hypothesis is also verified by the findings of pumice and dacitic tuff as the
product of big explosive activity
associated with acidic magma. In addition,
opened circular and a half circular landform have
more than 20 km in diameter, meanwhile
there is a crater of 6km in diameter. The opened structure might have endured weathered
process and erosion causing only the west side can be clearly observed and measured.
The plains
constituted of the clastic material from
G. Merapi is predicted to be the early
body of G. Godean, which occupies the
proximal facies, medial facies, and
distal facies. Further study is required
to prove that Godean is an isolated
volcanic cone between Kulonprogro Mountains and Southern Mountains.
Reconstruction of
the lower plain of Godean has been
interpreted by Pertamina (2008 in Winardi et al., 2013).
The reconstruction explained that the geophysical gravity measurements
illustrate that under the Godean hills,
there was the rock mass of 2.49 g/cc to a depth of 2500m classified as
volcanic rock aged approximately 28.5 million years ago (Figure 6).
Reconstruction model of the earth's surface and subsurface implied on the basin
model in volcanic arc and the discovery
of new natural resource sites.
The volcano genetic
perspective to understand the characteristics of basin has been put forward by Bronto et al., (2006) and the model can be
applied in relation with the
assessment of the relation and distribution of rocks formations in basin. It can used
to estimate the sedimentation between two hills or more, with the basin as the result of eruption that is associated with caldera in the study area.
Conclusions
The volcanic
landscape or geomorphology of Godean hills is
associated with the features of a caldera. Volcanic rocks covering the
land surface of Godean are constituted of
andesite and dacite. In addition, the
central part of Godean caldera is represented by G. Wungkal that is a circular
crater approximately 6000m in diameter.
Acknowledgment
The author would
like thank to STTNAS management for research funds and other committees who
have helped the research and the publication of this paper.
References
Bronto, S. (1999) Petrologi dan Bahan Galian Industri di Daerah Godean Yogyakarta. Departemen Pendidikan dan Kebudayaan, Kantor Koordinasi Perguruan Tinggi Swasta Wilayah V Yogyakarta.
Bronto, S., Budiadi, Ev., and Hartono, G.H. (2006) A New Perspective of Java Cenozoic Volcanic Arcs, Proceedings International Geosciences Conference and Exhibition, Jakarta.
Budiadi, Ev. (2008) Peranan Tektonik dalam Mengontrol Geomorfologi Daerah Pegunungan Kulon Progo, Yogyakarta, Disertasi Doktor, UNPAD, Bandung, 204 hal. (Unpublished).
Hartono, G. (2010) Peran Paleovolkanisme dalam Tataan Produk
Batuan Gunung Api Tersier di Gunung Gajahmungkur, Wonogiri, Jawa Tengah,
Disertasi, UNPAD, Bandung, 335 h. (Unpublished).
Hartono, H.G. and Pambudi, S. (2015) Gunung Api Purba Mujil,
Kulonprogo, Yogyakarta: Suatu Bukti dan Pemikiran, Prosiding ReTII ke 10, STTNAS
Yogyakarta.
Lelono, E.B. (2000) Palynological Study of the Eocene Nanggulan Formation, Central Java, Indonesia. Unpublished PhD Thesis. Dept. of Geology, Royal Holloway Univ. of London
Marks, P. (1957) Stratigraphic lexicon of Indonesia. Republik
Indonesia, Kementerian Perekonomian, Pusat Djawatan Geologi.
Pandita, H.,
Sukartono, S., & Isjudarto, A. (2016) Geological Identification of Seismic
Source at Opak Fault Based on
Stratigraphic Sections of the Southern Mountains. Forum Geografi, Vol. 30, No. 1, pp. 77-85.
Rahardjo, W., Sukandarrumidi, and Rosidi, H.M.S. (1977) Peta Geologi Lembar Yogyakarta skala 1:100.000. Direktorat Geologi, Bandung.
Rahardjo, W. (1995) Geological map of the Jogjakarta Sheet, Jawa. 2nd edition. Geological Research and Development Centre, Bandung.
Sipatriot, R. F. (2016) Geomorfologi Gunung Api Purba Godean, Kecamatan Godean, Kabupaten Sleman, Provinsi DIY, Seminar Geologi, J. Teknik Geologi, STTNAS, Yogyakarta. (Unpublished).
Sujanto F.X. and Roskamil (1975) The Geology and hydrocarbon Aspects of the South Central Java, Prosiding Pertemuan IAGI, Bandung, Desember 1975.
Sutanto, Soeria-Atmadja, R.C. Maury,
and H. Bellon (1994) Geochronology of Tertiary Volcanisme in Java, Kumpulan
Makalah Seminar Geologi dan Geotektonik Pulau Jawa, Sejak Akhir Mesosoik Hingga
Kuarter, Jurusan Teknik Geologi, F.
Teknik UGM, Yogyakarta, p. 53-56.
Sutikno, S. (1990)
The Role of Geomorphology in Physical Geography and its Application in
Research. Forum Geografi, Vol. 4,
No. 2.
Van Bemmelen, R. W. (1949) The Geology of Indonesia, Vol. 1A, The Hague, Martinus Nijhoff, p. 732.
Verdiansyah, O. (2016) Perubahan Unsur Geokimia Batuan Hasil Alterasi Hidrotermal di Gunung Wungkal, Godean, Yogyakarta. Kurvatek, 1(1), 9.
Winardi, S., B. Toha, M. Imron, and D. H. Amijaya (2013) The Potential of Eocene Shale of Nanggulan Formation as a Hydrocarbon Source Rock. Indonesian Journal of Geology, Vol. 8 No. 13-23.
© 2017 by the authors. Submitted for possible open access publication under the terms and
conditions of the Creative Commons Attribution (CC-BY-NC-ND) license
(http://creativecommons.org/licenses/by/4.0/).
Article Metrics
Abstract view(s): 2035 time(s)Refbacks
- There are currently no refbacks.