f Submarine Mass-Transport Deposits in The Semantan Formation (Triassic), Central Peninsular Malaysia

Triassic sedimentary rocks in the Central Belt of Peninsular Malaysia represent syn-orogenic sedimentation associated with the eastward-subduction and closure of the Palaeo-Tethys Ocean. This deepmarine “flysch” succession, of middle-upper Triassic age, is mapped as the Semantan Formation over much of central Pahang. The Semantan Formation consists of predominantly thin-bedded sandstone-mudstone facies, deposited in mud-dominated submarine fan systems. Individual turbidite beds are rarely thicker than 1 metre thick. Relatively fresh exposures of the Semantan Formation along the Karak-Kuantan Highway have given new insights into the sedimentary processes in the Triassic flysch basin. A change from distal to proximal facies eastwards between Karak and Maran indicate west-facing, active continental shelf-slope sedimentation. Hence, outcrops between Karak and Temerloh, east of the Bentong-Raub suture is characterized by “classical” flysch-like, thinly-bedded sandstone-mudstone facies, but west of Temerloh, and nearer to the paleo-shelf and slope, more sandy facies and thick-bedded turbidites occur. A fine example of proximal deep-marine facies association in the Semantan Formation is exposed at the Chenor Junction (Exit 821), kilometer 139 along the highway. South- and north-facing cuts on either side of the highway reveal interesting sedimentary features, which include large slide blocks (megaclasts), slumps, debris flow deposits, and associated syn-sedimentary thrust faults and glide surfaces. These features are strongly indicative of large-scale submarine mass-transport processes on the paleo-slope of the Triassic active margin. The Chenor mass-transport complex is made up of zones of incoherent slump deposits intercalated with well-bedded turbidite/debrite facies. In the lower part of the succession, there are at least two large blocks or megaclasts of sandstone-mudstone facies, measuring several metres in size, are encased in silty matrix. Along with other smaller sandstone blocks, these megaclasts are interpreted as slide blocks derived from slope failure up-dip of the basin plain. The internally stratified sandstone-mudstone blocks are strongly deformed internally by numerous meso-scale normal faults, which are evidently formed by ravitationally induced extensional deformation. Plastic deformation is also evident from the slump-related soft-sediment folds within the muddy matrix. Besides the chaotic “broken beds” and slump blocks, there are gravity-induced structural features such as rotational slumps, glide surfaces, thrust faults and associated soft-sediment folds. The slump folds and thrusts show vergence to the west, in the opposite sense to the tectonic vergence observed at other outcrops. A few of the well-stratified units show strongly inclined stratal surfaces, which may be attributed to lateral accretion of turbidite sand-lobes. Several sets of these inclined surfaces are bounded by erosional surfaces, which could have resulted from different episodes of turbidity flow. The association of incoherent mass-flow units with the better-stratified deposits reflects the close spatial and temporal relationship between submarine mass-transport processes and turbidity flows on the active Triassic paleo-slope and basin plain.