Tertiary basins of inland Peninsular Malaysia: review and tectonic evolution

Abstract

Sedimentary rocks of Tertiary age have been reported from several areas of inland (or onshore) Peninsular Malaysia, though a review of published data indicates that only some of them are known, or likely, to be so (Table A). In the known and likely localities, which occur as a series of small basins along the West Coast, are found partly consolidated gravels and sands inter-bedded with soft, often carbonaceous, shales, clays and lignite seams, as well as rare limestone, calcareous shale and volcanic ash. The sediments are entirely continental deposits and mostly gently dipping, though dips of 30° to 45° occur with a synclinal or basinal structure shown by some of them. The sediments mostly unconformably overlie, much older, Palaeozoic to early Mesozoic, generally folded and partly metamorphosed, rocks. The sediments have been considered to have been of a more extensive distribution than they presently are, for in places dipping beds are truncated, at the surface, or along an unconformity, though it is doubtful ifthey ever formed a connected sheet. At Bukit Arang in Perlis and Kedah, the Tertiary strata form two broad, gently northward plunging, synclinal basins with moderate bedding dips « 35°). The strata consist of an upper sequence of poorly sorted gravels and boulders in a sandy to clayey matrix (Boulder Beds) that unconformably overlies a lower sequence of sand and clay layers with a few thin coal seams. The lower sequence has a minimum thickness of 130 m and was deposited in a lacustrine environment, whilst the Boulder Beds with a total thickness of some 90 m were deposited under fluvial conditions. At Enggor in Perak, the Tertiary strata form a small, circular, basin with a thin layer of surface wash sediments overlying a sequence of shales, sandstones and clays with two coal seams. The coal-bearing strata, which dip 10° towards the NW, have a minimum thickness of 65 m and unconformably overlie folded, Upper Palaeozoic sedimentary rocks. At Batu Arang in Selangor, the Tertiary strata form a broad, gently southwestward plunging, synclinal basin with moderate dips « 45°). The strata consist of an upper sequence of pebble to boulder sized clasts in a sandy to gravelly matrix (Boulder Beds) that unconformably overlies a lower sequence of shales, structureless clay layers and sandstones with some thin to thick coal seams. The lower sequence, which is of an Eocene to Oligocene age, has a maximum known thickness of 265 m and was deposited in a lacustrine environment, whilst the Boulder Beds with a minimum thickness of 300 m were mainly developed as alluvial fan deposits. At Kampung Durian Chondong in Johore, Tertiary strata lie under a cover of soil and Older Alluvium that is at least 5 m thick. The Older Alluvium, of a probable Pliocene to Early Pleistocene age, consists mainly of fluvially deposited sands and unconformably overlies a sequence of shales, clays and volcanic ash with some thin to thick coal seams. The coal-bearing strata, which dip some 20° westwards, have a maximum known thickness of 195 m and were deposited in a lacustrine environment. At Kluang-Niyor in Johore, the Tertiary strata consist of a sequence of shales and clays with some lignite seams that unconformably underlie Older Alluvium of some 5 m thick. The Older Alluvium consists mainly of fluvially deposited sands, whilst the Tertiary strata, which show gentle « 20°) dips, have a maximum known thickness of 67 m and were deposited in a lacustrine environment. At Layang-Layang in Johore, the Tertiary strata (Layang-Layang Formation) consist of an upper sequence (3.5 m thick) of sands, clayey sands and clays unconformably overlying a gently dipping sequence of clay-shales and mudstones with a minimum thickness of 95 m. The upper sequence (Pengeli Sand Member) is equivalent to the Older Alluvium and was deposited in a fluvial to deltaic environment, whilst the lower sequence (Badak Shale member) is of a probable Miocene age and was deposited in a lacustrine environment. At Lawin in Perak, the Tertiary sediments only consist of inter-stratified sand, grit and pebble beds that gently dip inwards to form a basin. The sediments are likely to be more than 300 m thick and were most likely deposited as alluvial fans. The development of the Tertiary basins has not been discussed by most workers, except for Stauffer (1973) who postulated that they were produced by Late Tertiary structural adjustments mainly involving Paper presented at GSM Seminar on Tertiary basins of Peninsular Malaysia and its adjacent offshore areas, 21 February 1998