S. U.Choiriah, et alii: Pliocene/Pleistocene boundary, Java, Indonesia .. , p. 15-19. Journal of Nannolankton Research, 23 , 1, 2001.

THE PLIOCENE/PLEISTOCENE BOUNDARY, BASED ON CAL­CAREOUS NANNOFOSSILS,

IMPLICATIONS, AND RELATED PALAEOCLIMATIC

SOLO RIVER SECTION, NGAWI REGION, EAST JAVA, INDONESIA

Siti Umiyatun Choiriah*, Rubiyanto Kapid** & Wartono Rahardjot *Dept. of Geology, Veteran University, Jl. SWK 104 Ringroad Utara, Condongcatur, Yogyakarta, Indonesia;

umiyatun@mai !city. corn .. Dept. of Geology, Bandung Inst. of Technology, Bandung, Indonesia

tDept. of Geology, Gajah Mada University, Yogyakarta, Indonesia

Abstract : Analysis of calcareous nannofossils has been carried out on samples from the Solo River section, Ngawi Region, East Java Basinal Area. The boundary between the Pliocene and Pleistocene was determined, based on the last occurrence of Discoaster and the simultaneous first occurrence of Gephyrocapsa, following Martini & Miiller (1983). Van Gorse! & Troelstra (1981), who studied planktonic foraminifera from the same section, detennined the same boundary on the first occurrence of Globorota/ia cf G. truncatulinoides. The present study shows that the Pliocene/Pleistocene boundary, as determined by planktonic foraminifera 'biostratigraphy, is approximately 1.8m stratigraphically higher than the boundary determined from nannofossil biostratigraphy.

Rahardjo (1999) concluded that the climate became cooler around the Pliocene/Pleistocene boundary, based on forJ!minifera and pollen from the central part of the East Java Basin: a similar conclusion is inferred from the present study.

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Introduction Nannofossil analyses have been conducted on samples taken from the Solo River section. This section, located about 2km NE of Ngawi town, East Java Province, Indonesia, was measured along the bank of the Solo River (known locally as Bengawan Solo) (Figure 1). The section contains nannofossil assemblages of Pliocene to Pleistocene age. Van Gorse! & Troelstra 's ( 1981) study of the planktonic foraminifera from the same section also identified the Pliocene/ Pleistocene boundary. However, this is stratigraphically higher than the boundary as determined by nannofossils . This situation is discussed.

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Figure 1: Location and geology of the Bengawan Solo River section, Ngawi Area, East Java.

15

Correlation between forami­niferal and nannofossil biozonations around the Pliocene/Pleistocene boundary in Indonesia has already been carried out by several authors: e.g. Hasyim (1988) in the Kendeng Zone and Kapid (1991) in the Rembang Zone, although these reached differing biostratigraphical conclusions. The aim of this paper was to detennine new infonnation to allow a direct comparison between the two biozonations through this

Journal of N annolankton Research, 23, I , 200 I. S.U.Choiriah, et alii: Pliocene/Pleistocene boundary, Java, Indonesia .. , p. 15-19.

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Figure 2 : Distribution of calcareous nannofossils, Solo River section, Ngawi Area, East Java. R = interpreted as reworked.

interval (nannofossil and plankton foraminiferal) in the same section. The Solo River section was selected because it comprises continuous marine sediments of Late Miocene to Pleistocene age, and thus contained the Pliocene/ Pleistocene boundary.

Methods and materials The lithostratigraphic nomenclature used in this study follows that of Pringgoprawiro ( 1983 ), which is in accordance with the Sandi Stratigrafl Indonesia (Stratigraphic Code oflndonesia), but differs slightly from that used by van Gorsel & Troelstra ( 1981 ). In this study, the Kalibeng Formation is equivalent to the Lower Kalibeng Formation of van Gorsel & Troelstra ( 1981 ), whilst the Klitik Member, Sonde Formation, is equivalent to their Upper Kalibeng Formation. The uppermost part of the studied section belongs to the Pucangan Formation. Several key beds, or distinct lithological boundaries, were used as reference points.

Seventy-four samples were prepared as smear­slides. The nannofossil zones of Martini ( 1971) were

16

applied. Specimens were randomly counted using a grid pattern.

Results The Kerek, Kalibeng and Klitik Formations are of Late Miocene to Pleistocene age (see range-chart, Figure 2). Fifty-seven species were recorded, and ten nannofossil biozones applied (NNll to NN20), although only the Pliocene-Pleistocene zones (NN18-NN 19) are discussed in detail. The Pliocene/Pleistocene taxa are illustrated in Plate I. Fully-authored and -referenced lists of nannofossil taxa can be found in Perch-Nielsen ( 1985) or Bown ( 1998).

Discussion- the Pliocene/Pleistocene boundary Authors such as Martini (1971), Okada & Bukry (1980) and Perch-Nielsen (1985) have placed the Pliocene/ Pleistocene boundary (approximately) between NN18 and NN19. They determined that the last occurrence (LO) of Discoaster occurred in the latest Pliocene, and could be used to approximate the P1iocene/P1eistocene boundary. Hasyim ( 1988) and Kapid ( 1991 ), however, found that

S.U.Choiriah, et alii: Pliocene/Pleistocene boundary, Java, Indonesia .. , p. 15-19. Journal of Nannolankton Research, 23, I, 2001.

Discoaster was present in Pleistocene sediments in East Java. Rio (1974 in Haq et al., 1977) also inferred that specimens of Discoaster brouweri were present in the Early Pleistocene. Previous authors have regarded such occurences as reworking, and argued that these should not be used as biostratigraphic markers. Instead, they have used the first occurrence (FO) of Gephyrocapsa to define the Early Pleistocene.

The designation of the Pliocene/Pleistocene boundary by Rio (1974 in Haq et al., 1977) at the LO of Discoaster brouweri in the presence of gephyrocapsids (Gephyrocapsa oceanica and Gephyrocapsa caribbeanica) was applied to the present study, this eo­occurrence being observed in Sample U60. Samples from U61 upwards are devoid of Discoaster. Van Gorse! & Troelstra (1981) concluded that the Pliocene/Pleistocene boundary coincided with the N21/N22 Planktonic Foraminifera Zone boundary, indicated by the FO of Globorota/ia cf. G. truncatu/inoides and the LOs of Globoquadrina a/tispira, Globigerinoides obliquus extremus and Globigerinoides fistulosus. It thus appears, from a comparison of the two studies, that van Gorse! & Troelstra 's ( 1981) boundary is approximately 1. 8m above that determined herein.

The discrepancy between the foraminifera- and nannofossil-derived Pliocene/Pleistocene boundaries may be explained by one of two factors.

The sampling .interval of the foraminifera study was of lower resolution than that of the nannofossil study, especially around the boundary between the Kalibeng and Klitik Formations. If the sampling had been more detailed, it is possible that Globorota/ia cf G. truncatu/inoides might have been found below the recorded level, and thus its FO might be closer to the level ofthe LO of Discoaster. Thus, the discrepancy may not be significant.

However, Rio's (1974 in Haq et al., 1977) study of the Catanzaro section, southern Italy, found that the FO of Globorotalia cf. G. truncatulinoides was almost coincident with the LO of Discoaster and the FOs of Gephyrocapsa caribbeanica and Gephyrocapsa oceanica, whilst in his study of the Le Castella section, he found the FO of Globorotalia cf. G. truncatu/inoides 2m above the nannofossil-defined boundary. Thus, the likely sequence ofbiostratigraphic events through the boundary interval appears to be: LO Discoaster, (nannofossil boundary), FOs G. caribbeanica and G. oceanica, (planktonic foraminifera boundary), FO G. cf. G. truncatulinoides.

Palaeoclimatic implications Herein, the Pliocene/Pleistocene boundary is quantitatively indicated by a drastic decrease in nannofossil abundance. Cool-water species, such as Cocco/ithus pelagicus, Gephyrocapsa caribbeanica and Hayaster perplexus, are relatively more abundant in this interval than warm-water taxa, such as discoasters and Ceratolithus rugosus, and the latter do not occur at all in the Pleistocene sediments. Okada & Mclntyre ( 1979) and Haq ( 1980) determined that G. caribbeanica is typical of temperate to polar regions, whilst Bollrnann ( 1997) found it to have a tolerance of 5° to

21 °C, with its maximum abundance at 13 ° to 19°C. The Solo River data may thus indicate a climate cooling scenario from the Pliocehe to Early Pleistocene.

Van Gorse! & Troelstra (1981) concluded that a cold or glacial period had occurred in the Early Pleistocene, based on forarniniferal data. More recently, Rahardjo ( 1999) studied pollen and planktonic forami'nifera of the Mojokerto area, about 1 OOkm E of the present study area. Both of the localities belong to the East Java Basinal Area. Rahardjo (1999) concluded that there was a drastic climatic cooling during the Early Pleistocene, indicated by the extinction of Stenochlaeniidites papuanus. a warm-climate pollen, and characterised by the distinct increase of gramineae. Thus, the present data may support this hypothesis.

Conclusions The Pliocene/Pleistocene boundary has been identified in the Solo River section, using the LO of Discoaster and the FO of Gephyrocapsa. This level is 1.8m below the level of the boundary determined by planktonic forarninifera (van Gorse! & Troelstra, 1981) which may be due to differences in sampling resolution, or may indicate that the sequence of events across the boundary interval are: LO Discoaster, (nannofossil boundary), FOs G. caribbeanica and G. oceanica, (planktonicforarninifera boundary), FOG. cf G. truncatulinoides.

The relatively high abundances of cool-water nannofossil taxa (Coccolithus pelagicus, Gephyrocapsa caribbeanica, Hayaster perplexus), may indicate cooling conditions across the Pliocene/Pleistocene boundary in this region.

In order to establish a reference section for the Tertiary/Quaternary biostratigraphy of Java, it is proposed that research should be continued and expanded in the region of the Solo River.

Acknowledgements The authors would like to acknowledge the AAPG Grant Foundation (1999) which financially contributed to the research. Prof. Harsono Pringgoprawiro, Dr. A. T. Rahardjo and Ir. Khoiril are acknowledged for fruitful and lively discussions on nannofossils, palynology, planktonic forarninifera and their biostratigraphical and palaeoclirnatic applications. Thanks are also due to the two reviewers for .. their useful suggestions on the manuscript.

References Bollmann, J . 1997 . http ://www.erdw.ethz.ch/-bolle/

coccospheres.tml/gephyrocapsa.html. Bown , P.R. (Ed.) 1998 . Calcareous Nannofossil

Biostratigraphy. British Micropalaeontological Society Series. Chapman & Hali/Kiuwer Academic Publishers, London: 315pp.

Gorse!, J. T. van & Troelstra, S.R. 1981 . Late Neogene planktonic foraminiferal biostratigraphy and climatostratigraphy of the Solo River section (:Java, Indonesia): Chronostratigraphic implications and paleoclimatic framework of the Mediterranean Messinian 'Salinity Crisis'. Marine Micropaleontology, 6(2): 1-66.

17

Journal of Nannolankton Research, 23, I, 200 I. S.U.Choiriah, et alii: Pliocene/Pleistocene boundary, Java, Indonesia .. , p. 15-19.

Haq, B.U. 1980. Biogeographic history ofMiocene calcareous nannoplankton and paleoceanography ofthe Atlantic Ocean. Micropaleontology, 26: 414-443.

Haq, B.U. , Berggren, W.A. & van Couvering, J.A. 1977. Corrected age of the Pliocene/Pieistocene boundary. Nature, 269: 483-488.

Hasyim, N. 1988. Le Neogene marin du Nord-Est de Java, Indonesie. Etude biostratigraphique (foraminiferes et nannoplancton). Geomedia, Memoire 1: 129pp.

Kapid, R. 1991. Le Mio-Piiocene marin du Nord-Est de Java, lndonesie. Biostratigra.fi qualitative et quantitative des foraminiferes et du nannoplancton . Thesis, Univ. de Reims, France.

Komisi Sandi Stratigrafi Indonesia, 1973. Sandi Stratigrafi Indonesia, JAG!: 1-19.

Martini, E. 1971. Standard Tertiary and Quaternary calcareous nannoplankton zonation. In: A. Farinacci (Ed.). Proc. II Plank. Conf, Roma. Edizioni Technoscienza, 2: 739-784.

Martini, E. & MUller, C. 1983. Current Tertiary and Quaternary calcareous nannoplankton stratigraphy and correlations. Newsletters on Stratigraphy. 16(2): .99-112.

18

Okada, H. & Bukry, D. 1980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973, 1975). Marine Micropaleontology, 5: 321-325.

Okada, H. & Mclntyre, A. 1979. Seasonal distribution of modern coccolithophores in the Western North Atlantic Ocean. Marine Bioi., 54: 319-328.

Perch-Nielsen, K. 1985. Cenozoic calcareous nannoplankton. In: H.M. Bolli, J.B. Saunders & K. Perch-Nielsen (Eds). Plankton Stratigraphy. Cambridge University Press, Cambridge: 427-554.

Pringgoprawiro, H. 1983. Biostratigrafi dan palaeogeografi cekungan Jawa Timur utara sualu pendekatan baru. Disertasi Doktor Teknik Geologi, ITB, Bandung, Indonesia: 239pp.

Rahardjo, A.T. 1999. Perubahan iklim dan Batas Umur Pliosen­Piistosen Berdasarkan Analisis Foraminifera dan Palinologi di Daerah Mojoroto, Mojokerto-Jawa Timur. Buletin Geologi, 31(1): 1-13.

S.U.Cholrloh, et olll: Plioccnc/Pici toccnc boundary, Java, Indonesia .. , p. IS-19. Journal of aMolanlaon Rcse. rch, 23, I, 2001.

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10

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19

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