Late Paleozoic glacial marine facies in Southeast Asia and its implications

Abstract

Pebbly mudstones for some years in the Paleozoi c sequences of south Thailand (Phuket Group or Kaeng Form ation ) and th e Langkawi Island of northwest Malays ia (Singa Formati on) form pan of a belt of such rocks extending about 2000 km from Sumatra to central Burma. The roc ks form a di stinct facies: typically crude ly laminated , dark grey, poorly sorted mudstones with scattered megaclasts, few fo ssil s, an d much soft-sediment defom1ation . Traditionally interpreted as slump depos its, th ese rocks are well exposed and structurall y simple in th e Langkawi Islands (Singa Formati on), and detailed studies th ere justi fy a reinterpretation of th em as glacial marine sediments . The Singa Fm. is about 1600 m thi ck an d is Carbo-Permian in age, bracketted by fossiliferous latest Devoni an and late Earl y Permi an strata. Six lithofacies defmed from measured section s include unbedded diami ctite, vari ously bedded sand-mud combination s, an d subordinate clean sandstone. Sand-s ize grain are mainly qu artz and rock fr agments. Megaclasts (from granules to boulders) form up to 4.5 % of Singa di ami ctites, but occur also in other fa ci es through the formati on. Si ze analyses (5) of diam icti tes yie ld very poorl y sort ed po lymodal di stri ­ butions with median size in th e silt range. Pebbles and megacl as ts are mainly bl ocky, subangular, often facetted and sometimes show clear dro pstone characteristics . They are dominantl y of sandstone, w ith subordinate limestone, vein quartz , graniti c rock, and metamorphi c rocks. These characteristi cs of th e Singa Form ati on, es pecially of th e megaclas ts, strongly indicate th e sequence is glacial marine depos it s with ice-rafted stones. Thi s interpretati on is sup ported by evidence for cold conditions, evidence again st deep water, and by the presence of diamond . Implications of a gl aci al marine origin for th e 2000-km belt of rocks inclu de : ( I) th e (present) west side of the Western South east As ia tectonic block was marginal to (and attached to?) Gond wana in Carbo-Permian; (2) its position in Gondwana was probabl y somewhere between Arabi a and north west Australi a; (3) wann­ climate Permian floras of Southeast Asia may li e on oth er tectonic bl ocks with separate dri ft hi stori es. Question s and problems remainin g or ra ised by thi s work in cl ude the long time interval an d possible hidden unconformities, th e o ri g in o f th e defom1 ati on stru ctures (poss ibl y from ice push?), and the actual paleoenvironments represented by the different fac ies in the sequence.