Guide Book of Geological Excursion - Bayat, Central Java - October 2014

GUIDE BOOK Geologic Excursion to Bayat, Central Java SANTOS (Sampang) Pty Ltd

Trip leader:

Salahuddin Husein and Moch. Indra Novian

Dept. of Geological Engineering UGM

October 10, 2014

Geologic Excursion to Bayat, Central Java

Why Bayat? Bayat, Klaten, Central Java, is one of three location1 in Java that has a complete types of rocks exposed at the earth surface. In geoscience, rocks generally divided into three major groups based on their origin, i.e. igneous (crystallized from molten magmas), sedimentary (lithified from eroded surface materials), and metamorphic (compacted from other existing rocks). All of those rocks are scattered in a short distance in Bayat, revealing past processes of rock deformation (folding, fracturing and faulting) and present-day processes of rock weathering and erosion. And more important, all principal geologic outcrops in Bayat have easy access and located in easy terrain. Klaten

Prambanan Temples

Jiwo Hills

Bayat

Sheraton Yogyakarta

Southern Mountains

Figure 1. Location of Bayat.

Geology of Bayat Geomorphology2 of Bayat composed of two separated low-lying hills, its peaks only reach 280 m above sea level, collectively also known as Jiwo Hills (see Figure 1). The eastern Jiwo Hills elongates in NNE-SSW trend, with the core built by G 3. Pendul and G. Konang. The western Jiwo Hills elongates in NE-SW trend, and G. Jabalkat, G. Cakaran, and G. Sari form the main peaks. Both are separated by the valley of Dengkeng River which flows to NE. There are some isolated hills dotted the peripheral of Jiwo Hills, such as G. Wungkal, G. Kampak, G. Jeto, and G. Lanang. Geologically, those isolated hills are still part of Jiwo Hills, but they are now separated by the alluvium4 deposited by ancient rivers and lake.

1

The other two are Karangsambung, Kebumen, also situated in Central Java, and Ciletuh, Sukabumi, West Java. Geomorphology pictures topographic shapes of a terrain and how rocks respond to surficial processes. 3 Abbreviation of G stands for Gunung, or hill. 4 Loose, unconsolidated soil or sediments, which has been eroded and redeposited in a non-marine setting. 2

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4

3

2

1

Figure 2. Geological map of Bayat (after Rahardjo, 1984). Yellow circles with number indicate location of visited outcrops.

Geologically, Jiwo Hills built by elevated basement5 rocks covered by a relatively thin marine sedimentary rock layers and intruded by numerous igneous rock bodies (Figure 2). The basement complex mostly composed of metamorphic rocks, such as phyllites, schists, and marbles (Figures 3 and 4). Among them, phyllites are the most widespread, generally in weathered condition with some quartz veins6 and calcite veins aligned with their foliation. The age of this basement rocks were first reported by Bothé (1929) by a finding of Orbitolina, a large foraminifera fossil indicates an age of Cretaceous (older than 66 million years ago7). A more accurate method, by applying K-Ar absolute dating in two samples of quartz-mica schists, was done by Prasetyadi (2007) and indicate an age of 98 mya, which belong to Late Cretaceous epoch. This metamorphic rocks is interpreted to be formed during a regional metamorphism that involved along an ancient subducting lithospheric plate and the plates collision afterward (Setiawan et al., 2014). Subsequent erosion took place for million of years (some researchers suggest a number of 30 million years) had uplifted and peneplained8 those metamorphic rocks, and placed them as basement rocks for the next oncoming sedimentary sequences.

5

Basement refers to any older crystalline rocks (such as igneous or metamorphic) that overlaid by thick sedimentary sequences. 6 Veins are mineral-filled fractures, could be cut through rock bedding or aligned them. Commonly the minerals crystallized long after the fracture formation. 7 Later will be shortened as 'mya' (million years ago). 8 Peneplain means terrestrial erosional work that makes the landscape nearly flat or with subdued topographic relief. Page 2 of 12

Figure 3. Outcrop of calc-schists at G. Jokotuo (left), and how its minerals look like under microscope9 (right) (Prasetyadi, 2007).

Figure 4. Outcrop of marbles at G. Jabalkat (left), and how its minerals composition in thin section (right) (Prasetyadi, 2007).

Figure 5. A close-up photo of Nummulitic limestones at Sekarbolo, Western Jiwo Hills (left), folded Nummulitic limestones at Sekarbolo as a product of submarine slump (right).

Unconformably overlying the metamorphic basement rocks is Wungkal-Gamping Formation10. It has two distinct member, Wungkal Member at its lower part and Gamping Member occupies the upper part. 9

Rocks can be studied under microscope by cutting them into thin (~ 30 µm) sections on glass plates, mineral identification could be done much easier this way, rather than using naked eyes in the field. Page 3 of 12

Wungkal Member deposited during Early to Middle Eocene (55-40 mya) composed of quartz conglomerates, polymict breccias, quartz sandstones, calcareous sandstones, calcareous siltstones, and some intercalation of Nummulitic limestones. Gamping Member, younger than the Wungkal Member (deposited during Middle to Late Eocene, approximately 48-34 mya) but interfingered each other, composed of Nummulitic limestones (Figure 5), micritic sandstones, and quartz-arenite sandstones. The Wungkal-Gamping Formation gradually become Kebo-Butak Formation, which at the lower part composed of sandstones, gravelly sandstones, siltstones, claystones, tuff, and shales. The middle part composed of pebbly sandstones, whilst the upper part composed of polymict breccias, sandstones, pebbly sandstones, claystones, siltstones, and shales. Age of this formation is Late Oligocene to Early Miocene (2820 mya). During deposition of Kebo-Butak Formation, magmatic activites took place, possibly in submarine volcanic setting, as suggested by some fragment of basaltic pillow lavas (Husein & Sari, 2011) at the lower part of formation. Numerous igneous intrusion that dotted the Bayat landscapes also formed during Late Oligocene, producing diorite, diabase, andesite, and basalt (Surono et al., 2006). The next stratigraphic sequences were product of violent and massive volcanic processes, conformably and gradually replaced the Kebo-Butak Formation were Semilir and Nglanggran formations. These two formations were not deposited in Bayat, but they formed impressive mountain ridges to the south of Jiwo Hills, known as the Baturagung Ridge. It suggests that during Semilir and Nglanggran deposition, the volcanic sources located further south in the Southern Mountains, and the Jiwo Hills remained as topographic highs where subaerial erosion work kept it clean from any looming volcanic materials (Husein, 2013). The Semilir Formation are composed of tuff, lapilli, tuffaceous sandstones, autoclastic breccias, polimict breccias, calcareous tuffaceous sandstones. Some andesitic lavas were observed at the lower part. In the upper part a distinct Buyutan Member developed as sequences of tuffaceous sandstones, carbonaceous siltstones, and coals (Novian et al., 2012). The age of Semilir Formation ranges from Late Oligocene to Early Miocene (28-16 mya). Radiometric dating using U-Pb method by Smyth (2005) indicates an absolute age of 20 mya (Early Miocene). The Nglanggran Formation was interfingered11 with the Semilir Formation, composed of polimict conglomerates, pebbly sandstones, tuffaceous sandstones, andesitic breccias, with some tuff and basaltic-andesitic lavas. Subsequently, volcanic activites had gradually deceased, forming Sambipitu Formation that composed calcareous sandstones and tuffaceous siltstones. The rising sea-level at the end of Early Miocene gave a chance for deeper marine carbonate sequences to be developed, forming Oyo Formation that composed foraminiferal limestones. In Bayat, the Jiwo Hills that remained as topographic highs were covered by shallow marine carbonates of Wonosari Formation. It was composed of well-bedded limestones that directly rested on much older metamorphic basement rocks. The deposition of Wonosari Formation was occurred for quite a long time, approximately since 16 to 5 mya. At some places in the Southern Mountains, the deeper marine carbonates of Oyo Formation were continuing its deposition, although is given a new formation name of Kepek. Figure 6 depicts all those stratigraphic formations described above in a chart. Early Pliocene remarks a regional tectonic uplift of the Southern Mountain and Jiwo Hills, mostly due to isostatic rebound in response to the deepening of Kendeng back-arc basin which located to the north of them. 10

Formation is a term to name a certain number of rock strata that have comparable lithology, facies, and other similar properties. A formation can be made up by few members, a smaller unit of rock group. 11 Interfingering is a lateral stratigraphic relationship between two rock units being deposited at the same time. Page 4 of 12

Limestones and marls

Bedded limestones, reefal limestones, marly limestones, tuffaceous sandstones, siltstones

Tuffaceous limestones, tuff, tuffaceous marls

4 Calcareous sandstones and shales

Volcanic breccias, tuff, agglomerates, pillow lavas, autoclastic breccias, epiclastic breccias

2

Pumiceous breccias, tuff, tuffaceous sandstones, shales

1

Bedded sandstones, siltstones, shales, claystones, agglomerates, tuff, andesitic breccias, andesitic lavas

Limestones, sandstones, sandy marls, claystones

3 Metamorphic rocks

Metamorphic rocks

3

Schists, phyllites, meta-volcaniclastics, marble, slates

Figure 6. Stratigraphic chart of Jiwo Hills and Southern Mountains (after Sudarno, 1997). Yellow circles with number refer to stopsites order in this excursion.

What to observe? We will stop at 5 places, each has distinct geologic features that are interesting to observe and to discuss. 1. 2. 3. 4.

Siliciclastic sequences at Trembono Diabasic igneous rocks and their exotic fragments at Pendul Metamorphic rocks and Nummulitic limestones at Watuprahu Carbonate sequences at Kampak

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1. Trembono This stopsite exposes a delicate sequences of Kebo-Butak Formation, and suggests the importance of volcanic activities at that time for the sedimentary basin (Figure 7). The outcrop is showing two rock sequences, both are bounded by a large normal fault. The lower part is composed of alternating carbonaceous siltstones and sandstones, with frequent occurrence of tuff layers. They were in lamination12 and arranged in low angle channel structures. Progradation of those layers to the south suggests a lateral migration of a east-west trending sub-marine channels. Trace fossils13 of Thalassinoides14 ichnogenera presents in the bedding surface of siltstones. Most of carbon flakes accummulated in thin sheets between tuff layers, indicates alternating deposition of high and low energy currents, a remark for submarine turbidity current. The upper part is composed of massive15 quartz-rich volcanic sandstones. This type of rock are related with rhyolitic (felsic) magma at an environment of high temperatures and low pressures (close to the surface in a volcanic vent) and they were ejected forcefully in a Plinian-type eruption. (a)

(b)

(c) Figure 7. (a) Outcrop of Kebo-Butak sequences at Trembono, photo to the north. The red dashed line is a normal fault, with the western block as the downthrown block. This fault also a boundary for carbonaceous tuffaceous sandstones at the lower part and the quartz-rich volcanic sandstones at the upper part. (b) Close-up of the quartz-rich volcanic sandstones. (c) A minor conjugate block faulting exposed in the carbonaceous tuffaceous sandstones.

Those two rock units were bounded by a N-S trending normal fault, with the western part that occupied by the quartz-rich volcanic sandstones as downthrown block (Figure 7). Although the fault might be occurred long after the deposition of those rock sequences, observation to similar bounding fault in another location suggests that the fault was a syn-sedimentary16 fault and they shifted the underlying rocks with the massive weight of quartz-rich volcanic sandstones.

12

Lamination is a thin bed of sedimentary layers, usually less than 1 cm thick. Trace fossils, also called ichnofossils, are geological records of substratum (near-surface) biological activity. 14 Thalassinoides are cylindrical and branching burrows that occur parallel to the bedding plane, commonly made by crustaceans, such as crabs, lobsters, and shrimp. 15 Massive refers to beddingless sedimentary rocks, where no apparent layers observed. 16 Syn-sedimentary means an occurrence of a geologic structures during ongoing sedimentation processes. 13

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A closer look to the fault plane reveals an intricate fault system working in an extension tectonic regime, triggered by the weight of quartz-rich volvanic sandstones (Figure 8). They worked in conjugate pattern, some have jogged fault plane that suggests a lateral direction of extension, made an acute angle to the downward fault slip direction. Later those jog voids were filled by altered volcanic clays.

(a)

direction of extension

(b) Figure 8. (a) Block faulting on the quartz-rich volcanic sandstones. Photo taken to the southeast. (b) Jogged fault plane that filled by weathered clay. The jog pattern suggests a lateral direction of extension, which responsible for the fault plane opening and widening.

2. Pendul This stopsite is located in the eastern slope of Pendul Hill, one of the highest peak in eastern Jiwo Hills, that mainly composed of diabase. This kind of igneous rock is characterized by ophitic texture17, where plagioclase crystals randomly oriented around pyroxene minerals. Since plagioclase turned to white color when weathered, whilst pyroxene retained dark color, it is much easier to observe this texture when the diabase is weathered (Figure 9). When the diabase was subjected to tectonic stress, a set of systematic joints were developed. Commonly those joints or fractures were perpendicular each other, dividing the rocks into smaller cubes. Later, those joints infiltrated by groundwater that vigorously attacked the rock cubes from all sides. This process leading to a series of weathering 'union-skins' layers, enveloping the remaining unweathered, rounded-spheroidal rock in the cube core (Figure 10). Thus, this unique weathering process is known as spheroidal weathering.

17

Texture refers to arrangement of the grains or crystals within a rock, also considering are their size, shape, and color. Page 7 of 12

(b)

(a)

Figure 9. (a) Outcrop of diabase at Pendul. The round boulders of diabase are in situ, they are resulted from spheroidal weathering processes in a jointed rocks. (b) Close-up of diabase, showing ophitic texture, where randomly oriented plagioclase laths (white color) enclosed by pyroxene grains (dark color).

core of spheroidal weathering

weathered union skins

(a)

(b)

Figure 10. (a) Spheoridal weathering intensively occurred in diabase. It occurs in a jointed rock rich in plagioclase minerals that easily reacted with infiltrating ground water and turn them into clay mineral. The clay minerals constructed weathering layers, like union skins, around the corestone. (b) Schematic diagram on spheoridal weathering process.

Next to the diabase outcrop, a significant and important exposure of Kebo-Butak Formation is present (Figure 11). It mainly composed of alternating volcanic siltstones and volcanic claystones, with some volcanic sandstones that exhibit slump structures. The geometry of slump structures suggests a southward transport direction during their sliding downward along a submarine slope. A remarkable outcrop of Nummulitic limestones and calc-schists, both were found as fragments with different size in the volcanic siltstones. It suggests a subaerial environment existed at that time of Late Oligocene in the surrounding area of Jiwo Hills, where all of those fragments were eroded and swept to be incorporated in submarine environment. It also further deduces that the subaerial environment were tectonically created by a horst (basement high) and graben (basement trough) system, in which the eroding material came from horst and filled the adjacent graben. In this location, from those scattered fragments of polymict breccias, Prasetyadi (2007) took some samples of calc-schists for K-Ar radiometric dating, revealing an age of 98 mya. Page 8 of 12

claystones diabase siltstones claystones - siltstones

(a) (b) (c) (d) siltstones

siltstones Nummulitic limestones

polymict breccias (schists fragments)

Figure 11. (a) Intrusion contact between diabase and sedimentary rocks of Kebo-Butak Formation. (b) Well-laminated siltstones overlaid by claystones. (c) Nummulitic limestones (originated from Wungkal-Gamping Formation) as fragments in the siltstones. (d) Polymict breccias dominated by schists fragments (also originated from Wungkal-Gamping Formation) as fragments in the siltstones.

3. Watuprahu This stopsite is located in a saddle18 morphology between G. Pendul and G. Semangu to the north. There are two kind of rocks to be observed. The first one is phyllites and marbles, exposed at the southern slope of G. Semangu (Figure 12). General foliation19 of phyllites steeply dip to southeast. Phyllites are in weathered condition, as their minerals easily decomposed at the earth's surface. A 20 cm thick lense of marble was found in between the foliation, its orientation follows foliation attitude. Marbles have no foliation, they are massive or structureless. However this marble at Watuprahu fractured sub-parallel to the phyllitic foliation, suggesting those fractures formed due to unloading20 process during long period of exhumation of Jiwo Hills. A high-angle reverse fault cut the marble and disturbed the foliation, suggesting they once experienced a compressive tectonic regime. Apparently as marble and phyllite have different strength to the metamorphism, their boundary became a narrow corridor where the applying stress worked heterogeneously, resulting an irregular, contourted pattern of foliation. 18

Saddle morphology refers to a small valley connecting to adjacent hills or peaks, looks alike a shape of bicycle saddle. 19 Foliation is lamination-like structure in metamorphic rocks, they are made by orientation of platty minerals, such as micas. 20 It is common for deep-seated crystalline rocks, such as granites and marbles, to be fractured when they were brought to near-surface due to loss of overburden weight. Page 9 of 12

(a)

(b)

(c) Figure 12. (a) Outcrop of phyllites, a low grade (low pressure - low temperature) metamorphic rock. Foliation is generally steeply dipping to southeast direction. A faulted layer of marble (shown by yellow-dashed lines) is observed as a lense in phyllites. (b) Marble lense, sub-parallel fractures (shown by red-dashed lines) were developed in general trend of foliation of phyllite, most likely related to the unloading process during long period of exhumation. (c) Changing in foliation pattern as the phyllites getting close to the marble, from a sub-parallel pattern (shown by orange-dotted lines) to an irregularly-contourted pattern (shown by yellow-dotted lines). The first pattern sugests a regular homogenous 21 stress apply to the protolith , whilst the later pattern indicates an heterogeneous stress concentrated along a narrow corridor, mainly along fault zones. The white-dotted line suggests a boundary between those two patterns.

(a) (b)

Figure 13. (a) Nummulitic limestones of Watuprahu. The shape of this outcrop comparable to an overturned ship's hull, where the name of Watuprahu (ship's rock) was originated. (b) Close-up photo of Nummulites sp. fossil, a spesies of large foraminifera which often used as an Eocene age indicator. 21

Protolith refers to original, existing rock under metamorphism. Some protolith might partially survived from metamorphism and reveals information on the original rock. Page 10 of 12

To the east of metamorphic rocks, a different type of rock is exposed. It is a Nummulitic limestones. Its color is grey or bluish grey. Fossils of Nummulites were scattered randomly in a lime mud, suggesting they were deposited in place of origin (autochthonous). Some of limestones body was slightly metamorphosed due to incresing temperature and probably magmatic fluids, delivered by the adjacent diabasic intrusion at G. Pendul.

4. Gunung Kampak This location is fully occupied by limestones of Wonosari Formation, the youngest Tertiary formation in the Southern Mountains in Jiwo Hills. In G. Kampak, the limestones unconformably rest directly on metamorphic rocks. In the northern part of G. Kampak, a relatively horizontal well-bedded limestones overlying by reef talus deposits (Figure 14). Its contact was erosional, as indicated by scouring of the talus to the underlying bedded limestones. The bedded limestones mostly composed of intercalation of wackstones and packstones. The reef talus contains numerous broken coral fragments. (a) reef talus

bedded limestones

(c)

(b)

coral fragment

Figure 14. (a) Limestone sequences at G. Kampak, bedded limestones overlying by reef talus that composed of coral fragments. The contact between those rock units was erosional, indicated by scouring of reef talus to the bedded limestones. (b) Close-up photo of coral fragments in the reef talus. (c) Two types of terminations of strata were observed in the limestone sequences: downlap between the middle unit toward the underlying lower unit, and onlap between the upper unit toward the underlying middle unit. White-dotted lines are bedding planes, while the yellow-dotted lines are sequence boundary between each unit.

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In the southern part of G. Kampak, the exposure of bedded limestones suggests well-stacked rock units with distinct sequence boundary as marked by downlap and onlap strata termination. It suggests that sealevel fluctuation had played important role during deposition of Wonosari limestones, which constantly shifted the whole reef system in lateral sense during their growth.

References Bothé, A.Ch.D (1929) Jiwo Hills and Southern Range Excursion Guide. IVth Pacific Science Congress, Java, Bandung, pp. 1-14. Husein, S. (2013) Perkembangan Tektonik Pegunungan Selatan Yogyakarta: dari busur volkanik hingga patahan bongkah, sebuah kontribusi pemikiran. Presentasi pada Seminar Nasional memperingati 30 tahun Stasiun Lapangan Geologi 'Prof. R. Soeroso Notohadiprawiro' Bayat, Jurusan Teknik Geologi FT UGM. Husein, S. dan R. Sari (2011) Sedimentasi Terpicu Gaya Berat di Bagian Bawah Formasi Kebo, Mojosari, Bayat, Jawa Tengah. Prosiding Seminar Nasional Ilmu Kebumian ke-4 dan Pelepasan Purna Tugas Dosen Teknik Geologi FT UGM, pp. 119-140. ISBN 978-979-17549-9-6. Prasetyadi, C. (2007) Evolusi Tektonik Paleogen Jawa Bagian Timur. Disertasi, Program Doktor Teknik Geologi, Institut Teknologi Bandung. Novian, M.I., P.K.D. Setiawan, S. Husein, dan W. Rahardjo (2012) Stratigrafi Formasi Semilir bagian atas di Dusun Boyo, Desa Ngalang, Kecamatan Gedang Sari, Kabupaten Gunung Kidul, DIY: Pertimbangan untuk penamaan Anggota Buyutan. Geologi Pegunungan Selatan Bagian Timur, Publikasi Khusus Pusat Survei Geologi, pp. 27-37. Setiawan, N.I., S. Husein, and M.F. Alfyan (2014) Speculative models of exhumation on high-pressure low-temperature metamorphic rocks from central part of Indonesia: an implementation of concepts and processes. Prosiding Seminar Nasional Kebumian ke-7, Yogyakarta. Smyth, H. (2005) Eocene to Miocene Basin History and Volcanic Activity in East Java, Indonesia. PhD Thesis, University of London, 470 p. Surono, U. Hartono, dan S. Permanadewi (2006) Posisi stratigrafi dan petrogenesis intrusi Pendul, Perbukitan Jiwo, Bayat, Kabupaten Klaten, Jawa Tengah. Jurnal Sumber Daya Geologi, XVI/5, pp. 302-311.

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