Bulletin of the Geological Society of Malaysia, Volume 20, August 1986, pp.363 – 397
1Geology Dept., Palo Alto, California, USA
2Universiti Malaya, Kuala Lumpur
Abstract: Pebbly mudstones for some years in the Paleozoic sequences of south Thailand (Phuket Group or Kaeng Formation) and the Langkawi Island of northwest Malaysia (Singa Formation) form part 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 deformation.
Traditionally interpreted as slump deposits, these rocks are well exposed and structurally simple in the Langkawi Islands (Singa Formation), and detailed studies there justify a reinterpretation of them as glacial marine sediments. The Singa Fm. is about 1600 m thick and is Carbo-Permian in age, bracketted by fossiliferous latest Devonian and late Early Permian strata. Six lithofacies defined from measured sections include unbedded diamictite, variously bedded sand-mud combinations, and subordinate clean sandstone. Sand-size grains are mainly quartz and rock fragments.
Megaclasts (from granules to boulders) form up to 4.5% of Singa diamictites, but occur also in other facies through the formation. Size analyses (5) of diamictites yield very poorly sorted polymodal distributions with median size in the silt range. Pebbles and megaclasts are mainly blocky, subangular, often facetted and sometimes show clear dropstone characteristics. They are dominantly of sandstone, with subordinate limestone, vein quartz, granitic rock, and metamorphic rocks.
These characteristics of the Singa Formation, especially of the megaclasts, strongly indicate the sequence is glacial marine deposits with ice-rafted stones. This interpretation is supported by evidence for cold conditions, evidence against deep water, and by the presence of diamond. Implications of a glacial marine origin for the 2000-km belt of rocks include: (1) the (present) west side of the Western Southeast Asia tectonic block was marginal to (and attached to?) Gondwana in Carbo-Permian; (2) its position in Gondwana was probably somewhere between Arabia and northwest Australia; (3) warm-climate Permian floras of Southeast Asia may lie on other tectonic blocks with separate drift histories.
Questions and problems remaining or raised by this work include the long time interval and possible hidden unconformities, the origin of the deformation structures (possibly from ice push?), and the actual paleoenvironments represented by the different facies in the sequence.