Journal of Southeast Asian Earth Sciences, Vol. 8, Nos 1-4, pp. 173-180, 1993 0743-9547/93 $6.00 + 0.00 Printed in Great Britain © 1993 Pergamon Press Ltd The sedimentological and diagenetic evolution of the Ratburi Limestone, Peninsular Thailand ANGUS BAIRD and DAN BOSENCE Department of Geology, Royal Holloway, University of London, Egham, Surrey, U.K. Abstract--The late Middle to early Late Permian Ratburi Limestone in Peninsular Thailand is a warm-water carbonate deposit formed in an extensive platform setting, suggesting that the Shan Thai craton occupied a subtropical-tropical position at this time. The distribution of facies, from peritidal, fenestral and algal laminated muds, to subtidal skeletal and ooid grainstones, bryozoa boundstones and carbonate slope deposits, is controlled by the platform topography, which is thought to have been controlled by extensional tectonics. Early diagenesis of the platform began with seawater-influenced dolomitisation, followed by uplift, producing karstification, in response to the Late Permian Indosinian orogeny. East dipping subduction of the Indian Ocean Plate resulted in granite magmatism in the peninsula during the Cretaceous. The effect on the Ratburi Limestone included neomorphism, dissolution, calcite cementation and dolomitisation. The petroleum potential of Ratburi Limestone comes from the extensive and long karstic history (Late Permian and Tertiary-present day) and the source potential of the widespread platform carbonate mudstones (TOC <4.5%). INTRODUCTION PERMIANcarbonates are widespread in Thailand (Fig. 1) and the Ratburi Limestone is restricted to western and Peninsular Thailand. Published reports on the Permian of Thailand are rare, and for the Ratburi Limestone are confined to palaeontological descriptions only. Permian carbonates are economically important, increasingly so as an offshore and onshore oil or gas reservoir target, and onshore as a quarried aggregate. The Ratburi Limestone was examined in the area around the town of Ratburi and to the west of Hua Hin in the south (Fig. l) as part of a Ph.D. project. Field work included mapping and the logging of over 50 sedimentary sections. The Ratburi Limestone is exposed as tropical tower karst outcrops above poorly exposed siliciclastics. In the west of the field area, these siliciclastics are the presumed glaciomarine pebbly mudstones of the Late Carbonifer- ous-Early Permian Kraeng Krachan Formation (Javanaphet 1969). These sediments are on the western margin of Shan Thai, suggesting this was a rifted continental margin close to Gondwana. foraminifera, green algae, corals and bivalves have also been identified that are useful in constraining the age of the Ratburi Limestone. Particularly relevant are the fusulinids, such as Parafusulina, Chusenella, Pseudo- fusulina and Arminina. These forms are present in the Middle Permian, but do not range into the Late Permian. Other biota present in the Late Permian but, not found in strata older than the Murghabian include Baisalina, Hemigordiopsis, Globivalvulina graecea, Sphaerionia sikuodies, Neohemigordius, Frondicularia, Paraglobivalvulina and Eopolydiexodina afghanensis. The examination of the position of these opposing age indicators within logged sections, suggests the Ratburi Limestone is of late Middle Permian (Middle Murgha- bian) age, with limited extension into the Late Permian (Midian). As the thickness of the Ratburi Limestone in the field area is not greater than 200 m, this age range is reasonable, though the limited section may be the result of the extensive Cenozoic erosion, and younger ages may be present elsewhere. SEDIMENTARY FACIES ASSOCIATIONS BIOSTRATIGRAPHY There is considerable confusion in the literature as to the age of the Ratburi Limestone. Figure 2 shows the ranges in ages from early to the end of the Permian quoted by previous workers. Both brachiopods and foraminifera with algae have been used in this study to date the Ratburi (Baird 1992). Brachiopods identified from the base of the Ratburi Limestone can be corre- lated with genera reported by Waterhouse and Piyasin (1970) and Grant (1976), which are warm-water forms with an age spread throughout the Middle Permian (Fig. 2). Nineteen genera of fusulinids, other Eleven main carbonate sedimentary facies have been identified (Baird 1992) from the examination of logged sections, the localities of which are shown in Fig. 3. The relationship between these facies in logged sections allows the classification of five facies associations, which are indicative of broad environments of deposition. The high-energy open platform association includes boundstones, fossiliferous grainstones and ooid grain- stones, and the low-energy open platform association includes highly fossiliferous wackestones. The restricted platform association is limited to unfossiliferous mud- stones and micritised allochem pack/grainstones. The peritidal association includes fenestral and algal lami- nated mudstones. These facies associations are shallow 173