Sedimentology of Upper Jurassic Deposits in the Tembesi River Area, Central Sumatra

In the Tembesi River area, the Upper Jurassic crops out along two parallel NW-SE oriented lines. It consists of black, massive or bedded limestones interbedded with either black, more or less laminated shales or with more or less shaley sandstones. The limestones include numerous Madreporaria which indicate an Upper Jurassic age. Examination of the limestone microfacies leads to them being interpreted as mud-mounds of microbial origin (bioclasts encrusted with micrite and various algal structures, occurrence of filaments in the sparite, rhombohedrons of dolomite, bituminous stylolites, fenestrae, etc.). The study of the clay minerals contained both in the marls, the shales, the sandstones and in the mud-mounds themselves, indicates a close shore deposit with sporadic influx of terrigenous sediment coming periodically from the continent. INTRODUCTION Up to now, the Jurassic of Sumatra has been poorly known and Jurassic corals and coral formations have not been subject to special studies. Fontaine, eta/. ( 1984) recognised for the firsttime the importance of Upper Jurassic reefformations in the country. In 1983, a field trip organized with the participation of the CCOP-ESCAP (Bangkok), the GRDC (Bandung), the French oil company CFP and the CNRS (Paris), allowed the authors to begin more detailed studies of these formations. The Upper Jurassic of Sumatra is reliably known in four areas (Figure 1): The Atcheh area (North Sumatra), the Padang area (West coast), the Tembesi River area (Central region) and the Gumai Mountains (South Sumatra). The Upper Jurassic sediments comprise black, massive or bedded limestones interbedded either with more or less laminated black shales or with more or less shaley sandstones. These limestones were previously called "reef limestones" by Tobler (1923) due to the numerous corals occurring in these formations. Coral species in the limestones indicate a Kimmeridgian age in the Padang area, and a Tithonian age in the Jambi area. The species recorded are known throughout the Tethysian Presented at CCOP-ASCOPE-GEOSEA WORKSHOP 'Carbonate Diagnesis', 4 6 April1984, Kuala Lumpur 46 BEAUVAIS, L., BERNET-ROLLANDE, M.C., BLANC, PH. AND MAURIN, A.F.