The Development of the Miocene Wonosari Formation South Central Java (Lokier)

© IPA, 2006 - 27th Annual Convention Proceedings, 2000 IPA99-G-027

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty Seventh Annual Convention & Exhibition, October 1999 THE DEVELOPMENT OF THE MIOCENE WONOSARI FORMATION, SOUTH CENTRAL JAVA Stephen W. Lokier*

ABSTRACT

Both ancient and recent tropical active volcanic settings have acted as sites for carbonate development. development. Although this association has long been recognized, very few studies into the interactions  between volcaniclastic volcaniclastic and carbonate lithologies lithologies have  been undertaken. An ideal site for f or such a study of an ancient example is the Miocene Wonosari/Punung Formation of south central Java, J ava, Indonesia. During the Miocene, andesitic volcanism in Java occurred along an extensive island-arc associated with the subduction of the Indo-Australian Oceanic  plate. To the south of this island-arc a shallow-water carbonate platform developed under moderate to

Periodic inputs of volcaniclastic material, either as sub-aerial ash-falls or high-density flows, into the marine environment had dramatic effects on the development of shallow carbonate platforms. Where inputs were small, an increase in nutrients resulted in an increase in the numbers of species and the numbers of individuals within the palaeoenvironment. Larger, or more sustained periods of volcaniclastic sedimentation resulted in a decrease in the numbers of species but increased numbers of individuals. This response can be attributed to an increase in nutrients, a lack of competitors and changes in substrate. INTRODUCTION

The aim of this study is to establish the effects of

The area lies between latitudes 07°45’S and 08°15’S and longitudes 110°15’E and 111°15’E, an area of 2 ~3,200 km . The area is bordered by the Indian Ocean to the south, and towns of Nglipar, Parangtritis and Pacitan lie to the north, west and east respectively (Figure 1). The middle Miocene to lower Pliocene age Wonosari Formation has previously been studied by a number of authors, most notably Sartono (1964), Surono et al (1992), Sampurno and Samodra, (1991) and Rahardjo et al. (1995). While these previous studies have identified the occurrence of volcaniclastic horizons interbedded with carbonate lithologies, they have not considered the direct effects of siliclastic sediments on the marine biota or development of the platform as a whole. METHODS

During fieldwork, lithologies were systematically logged and sampled to establish stratigraphic sequences and facies variations across the area. Samples were returned to the UK where detailed  petrographic, geochemical and palaeontological analyses were undertaken. These analyses have been used to constrain the age, microfacies, diagenetic history and porosity development of the observed

emplaced by debris flows, turbidity currents and as subaerial ash falls. Later there was a reduction in volcaniclastic activity and deposition of redeposited carbonates derived from the shallow marine platform south of the basin occurred. These carbonates were emplaced as a series of turbidites. Where the volcaniclastic and carbonate lithologies interacted an inter-fingering relationship developed. However, in some localities extensive bioturbation has resulted in the mixing of sediments. South and east of the study area a shallow water carbonate platform developed on a topographic high (Figure 3). Calcareous algae and larger benthic foraminifera packstones, rudstones and grainstones dominated the carbonate lithologies on this platform. Although the mid-Miocene was a period of abundant global coral development (Wilson and Rosen, 1998), corals and other marine biota occurred only as tertiary elements within the lithologies. This carbonate platform was isolated from most of the volcaniclastic sediments by the deep-water basin to the north acting as a sediment trap. However,  periodic episodes of explosive volcanic activity resulted in volcaniclastic sediments being deposited from subaerial ash-falls. In platform top locations the influence of air-fall deposits on shallow water carbonates, while at platform margins the

b. Moderate volcaniclastic input

REFERENCES

During short periods of elevated volcanic sediment input there was a rapid increase in the number of individuals and number of species of molluscs, foraminifera, arthropods and solitary corals present within the palaeoenvironment. Sustained volcanic input resulted in a decrease in the number of species  present, but with elevated numbers of individuals.

Rahardjo, W., Sukandarrumidi and Rosidi, H.M.D. 1995. Geological Map of the Yogyakarta Sheet. Jawa. 1:100,000. Geological Research and Development Center.

c. High volcaniclastic input

During periods of higher sedimentation, either rhodoliths or larger benthic foraminifera and  burrowing fauna dominated assemblages. Energy levels and volcaniclastic sediment grain size appear to have been the main factors controlling the  prevalent biota. High energy levels and coarse volcaniclastic material resulted in a dominance by laminar concentric rhodoliths whereas lower energy conditions, along with finer volcaniclastics, are associated with abundant larger benthic foraminifera. CONCLUSIONS

While sites of contemporaneous volcaniclastic and carbonate sedimentation are common, research in these important environments has been neglected.

Sartono, S. 1964. Stratigraphy and Sedimentation of the Easternmost part of Gunung Sewu (East Java). Publikasi Teknik Seri Geologi Umum No. 1. Republik Indonesia Departemen Perindustrian Dasar/Pertambangan Direktorat Geologi - Bandung Sampurno and Samodra. 1991. Geology of the Ponorogo Quadrangle, Jawa. 1:100,000. Geological Research and Development Center. Sumarso, & Ismoyowati, T. 1975. Contributions to the Stratigraphy of the Jiwo Hills and their Southern Surroundings (Central Java). Proceedings Indonesian Petroleum Association,4 th Annual Convention Surono, 1992. The Stratigraphic Relationship Between the Punung and the Wonosari Formations, Central Java. Geological Research and Development Center Bulletin. 15. 31-37. Surono, Toha, B., Sudarno, I., and Wiryosujono, S. 1992. Geology of the Surakarta - Giritontro