Late Paleozoic glacial marine facies in Southeast Asia and its implications

Abstract

Pebbly mudstones for some years in the Paleozoic sequences of south Thailand (Phuket Group or Kaeng Formation) and the Langkawi Island of northwest Malays ia (Singa Formation) form pan of a belt of such rocks extending about 2000 km from Sumatra to central Burma. The rocks form a distinct facies: typically crudely laminated , dark grey, poorly sorted mudstones with scattered megaclasts, few fossils, and much soft-sediment defom1ation. Traditionally interpreted as slump depos its, these rocks are well exposed and structurally simple in the Langkawi Islands (Singa Formation), and detailed studies there justi fy a reinterpretation of them as glacial marine sediments. The Singa Fm. is about 1600 m thick and is Carbo-Pe rmian in age, bracketted by fossiliferous latest Devoni an and late Early Permi an strata. Six lithofacies defmed from measured sections include unbedded diamictite, variously bedded sand-mud combinations, and subordinate clean sandstone. Sands ize grain are mainly qu artz and rock fragments. Megaclasts (from granules to boulders) form up to 4.5 % of Singa diamictites, but occur also in other facies through the formati on. Si ze analyses (5) of diam ictites yield very poorly sorted po lymodal distributions with median size in the silt range. Pebbles and megaclasts are mainly blocky, subangular, often facetted and sometimes show clear dro pstone characteristics. They are dominantly of sandstone, with subordinate limestone, vein quartz, granitic rock, and metamorphic rocks.