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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Sedimentology and Sr Isotopic Ages of the Bau Limestone Formation in Northwestern Borneo, Malaysia 32 Y. Kakizaki 1* ,T. Ishikawa 2 ,J. Matsuoka 3 ,T. Hasegawa 1 , A. Kano 4 1. School of Natural System, Kanazawa University, Kanazawa 920-1192, Japan (E-mail: kackey@staff.kanazawa-u.ac.jp) 2. Kochi Core Research, Japan Agency for Marine Science and Technology, Nankoku city, Kochi 783-8502, Japan 3. Kochi Marine research Support Section, Marine Works Japan, Nankoku city, Kochi 783-8502, Japan 4. Division of Evolution of Earth Environments, Kyushu University, Nishi-ward, Fukuoka 819-0395, Japan The Mid-Mesozoic era (from the Late Jurassic to the Early Cretaceous) was one of the periods in terms of the largest deposition of marine carbonate. In the eutrophied ocean, reef-builders were flourished under the circumstance of a relatively high CO2 con- centration and the warm ocean environment. Sedi- mentology and stratigraphy of the Mid-Mesozoic carbonate were well studied in Europe, the former territory of the Tethys Ocean. On the other hand, information on the Mid-Mesozoic carbonate of Asia and Pacific realm was limit. Hence, more information and more studies on them have been expected. Here, we present the sedimentology and Sr isotopic age of the Bau Limestone Formation (the Upper Jurassic to the Lower Cretaceous) in north-western Borneo of Malaysia, for the first time. We surveyed at two quarries (Marup and SSF) of the Gunung Panga Area, located at 30km southwest of Kuching City (Fig. 1). The lithofacies are mainly massive limestone yielding mega fossils. The lithology of the Marup Quarry is subdivided into Bioclastic wacke-packstone including many cidaris spines and Bafflestone containing corals. The lithology of the SSF Quarry is subdivided to 5 facies: Oncoidal Pack- stone, Peloidal Packstone, Bioclastic Packstone, Bindstone, and Frame-Bafflestone. At Gunung Panga Area, mega fossils (especially Rudists and nerineoids) are yielded at all facies, and sparitic cements occupy the intergranular spaces. We surveyed at Bunkit Akut Quarry near Kuching-Serian Road, which is located at 40km south- southeast of Kuching City (Fig.1). The limestone is clearly stratified and slumping fold is developed at the upper horizon of the surveyed section. Lithofacies are fine-grained limestone and subdivided into 3 facies: Calcarenite, Lime mudstone, and bioclastic wacke- stone. Additionally, mud beddings are intercalated with limestone at the middle to upper horizon of surveyed section. Fig.1 Geological map of the Bau Limestone Formation, modified after Wilford (1955) Because of sparitic cements and hermatypic biota, the depositional environments of Gunung Panga are considered to be photic zone influenced by wave. On the other hand, because of absence of hermatypic biota and developing of slumping fold, the depositional environment of Bunkit Akut section is considered to be deeper than storm wave base, and environment where siliciclasts were intermittently supplied. Based on our description, it is concluded that the depositional environment of the Bau Limestone Formation is
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 subdivided into two environments: shallow marine and hemipelagic at least. Outer layers of rudist shells collected from Marup and SSF Quarries were powdered, and Sr was separated from them by a use of chromatography using Sr ion exchange resin and nitric acid. The 87 Sr/ 86 Sr isotopic ratios were measured by a thermal ionizing mass spectrometer (TIMS; Thermo Finnigan TORI- TON). The Sr ratios were projected on the standard age- profile of Look-up Table (MacArthur, et al., 2001) in order to evaluate the depositional age. The result shows the depositing age of Marup Quarry is from 156.40 to 153.10 million years ago, and that the depositing age of SSF Quarry is from 157.85 to 152.65 million years ago. It is obvious that 87 Sr/ 86 Sr ratio of Marup and SSF are correlated to late Oxfordian to early Kimmeridgian of the Late Jurassic. In previous studies, the age of Bau Limestone Formation was indefinitely considered to be later than the Late Jurassic by poor-preserved ammonoid fossils. But in this study, we are able to determine the depositing age of the specific horizon of the Bau Limestone Formation for the first time. Additionally, this study suggests the age of Marup and SSF Quarries, where rudists are collected, is late Oxfordian to Kimmeridgian. These rudists are possibly one of the oldest rudist assemblages of the Far East. Key Words: Bau Limestone Formation; Sr isotopic age; Jurassic; Malaysia References: McArthur J.M., Howarth R.J., Bailey T.R. Strontium Isotope Stratigraphy: LOWESS Version 3: Best Fit to the Marine Sr-Isotope Curve for 0-509 Ma and Accompanying Look-up Table for Deriving Numerical Age, The Journal of Geology, 2001, 109: 155-170. Skeleton P.W. Rudist evolution and extinction a North African Perspective. In: Gili E., Negara H., Skeleton P.W. (ed). North African Cretaceous Carbonate Platform Systems. Kluwer Academic Publishers, Norwell, 2003, 215-227. Wilford G.E. The Geology and Mineral Resources of the Kuching−Lundu area, West Sarawak in cluding the Bau Mining district. Government Printing Office. Kuching, 1955: 254. 33
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