Turbidites and Mass Transport Deposits in from Active Margin Setting: The Semantan Formation (Middle-Upper Triassic), Central Peninsular Malaysia

Mazlan Madon, Hasnol Hady Ismail, and Zainol Affendi Abu Bakar
Petronas Research, Kajang, Malaysia

The Middle-Upper Triassic Semantan Formation, central Peninsular Malaysia, consists of a flysch succession deposited during the closure of the Palaeo-Tethys Ocean. In most places, the succession consists of predominantly thinly-bedded, often tuffaceous, heterolithic sandstone-mudstone facies, typical of “distal”, basin-plain turbidites, where individual turbidite beds are rarely thicker than half a meter. A gradual change to proximal facies eastwards is indicative of deposition in a west-facing, actively deforming margin. Sedimentary facies in the Semantan vary from conglomerate, sandstone, mudstone to heterolithics. Turbiditic facies associations identified may be related to various parts of a submarine fan system. Nearer to the paleo-shelf, sandier turbiditic facies occur with chaotic muddy intervals of debris flow or mass transport origin. The latter include large blocks (megaclasts) of heterolithic sandstone-mudstone in muddy matrix, apparently derived from pre-existing sediment up-dip. Widespread gravity-induced syn-sedimentary deformation features include abundant slump zones, rotational slumps, glide planes and thrust faults. The slump folds and thrust faults show vergence to the west (down the paleo-slope).

The heterolithic sandstone-mudstone megaclasts contain numerous small-scale normal faults, which are evidently produced by gravitationally-induced extensional deformation. Well-stratified turbiditic intervals above and below the chaotic intervals show packages of inclined stratal surfaces bounded erosional surfaces, and represent accretion/aggradation of several turbidity flows. The close association of incoherent mass transport units with the more well-stratified deposits reflects different gravity flow processes on the actively deforming Triassic paleo-shelf-slope system

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas