A Contribution to the Geology of Part of the Indonesian Tinbelt: the Sea Areas Between Singkep and Bangka Islands and Around the Karimata Islands

An acoustic continuous pro filer survey over extensive sea areas between Singkep and Bangka and around the Karimata Islands, supplemented by sea drillings, indicates the following sedimentary succession: below the sea, the basement, which on the islands consist of folded sediments and granitic rocks, is covered by unconsolidated subhorizontal, mostly sandy sediments with intercalated peat layers near the top, probably of Tertiary age. Next a sequence of sed iment-filled gullies incised into the older sediments and sometimes also into the basement is present. Again, peat layers are of common occurrence, indicating a terrestrial origin for most of these sediments ; at least part of these deposits seems to be of young Tertiary age. An extensive, nearly horizontal planation surface is found at depths of 20--30 m below sea level. This surface is at remarkably constant depth and very flat. indicating a marine origin. Spotty red clay is present on part of this surface, indicating a temporary emergence of the abraded surface. A young sedimentary deposit, mainly of marine origin, represents the latest postglacial transgression. Part of its material is derived from the surrounding islands. INTRODUCTION Whereas in Burma, Thailand and Malaysia the tin belt is composed of chains of hills and mountains, in Indonesia it consists of a few islands with extensive, seacovered stretches in between. Consequently, it has been difficult to obtain insight into the regional geological problems of the tin belt in Indonesia. During submarine prospecting for tin ores carried out by the Billiton Company in the sea areas between Singkep and Bangka and in the Karimata Archipelago (see fig. 1), much data became available which may throw some light on these problems. In this paper an outline is given of the information now available and an attempt is made to assess its significance. The Indonesian tin islands Bangka, Belitung and Singkep, together with the Riau Archipelago, the small islands of the Tudjuh group and the Karimata Islands were generally thought to be remnants of the divide areas of the submerged continuation of the Malayan tin belt. (Van Bemmelen, 1949, p. 319). In 1967 the Indonesian Government offered some of the sea areas in this region for offshore tin prospection. From the brochure distributed by the Ministry of Mines at that time (see reference) we quote the following: "It is a generally accepted conception that the shallow sea surrounding the tin islands represents a peneplain, submerged by the last post-glacial eustatic rise of the sea level. An aureole of submerged river valleys, containing the valuable tin ores, is surrounding the islands of Singkep, Bangka and Belitung at present" . 1. Billiton International Metals B.V. , The Hague, Netherlands. 2. International Institute for Aerial Survey and Earth Sciences (LT.G), Enschede, Netherlands. 3. Formerly Billiton N.V., The Hague, Netherlands.