Tectonic evolution of the Tertiary basin at Batu Arang, Selangor Darul Ehsan, Peninsular Malaysia

Abstract

In the Batu Arang area are found Tertiary sedimentary rocks that outcrop within an approximately elliptical basin surrounded by undulating to hilly terrain developed over Permian meta­ sedimentary rocks consisting of quartzites, schistose quartzites, phyllites, indurated shales and rare cherts. The Tertiary strata cover an area of some 15 km2 and rest unconformably over the meta­ sedimentary rocks that generally strike NNW-SSE with variable, steep to vertical dips. The Tertiary strata can be separated into two sequences: an upper one termed the Boulder Beds that consists of semi-consolidated, sandy to gravelly and bouldery sediments that unconformably overlie the lower sequence known as the Coal Measures or Batu Arang Beds comprising interbedded shales, structureless clays, fine grained to pebbly sandstones and some lignite seams. The Boulder Beds consist mainly of sub-angular to rounded, pebble to boulder sized clasts of quartzite, sandstone and vein quartz as well as minor chert, schist, phyllite and shale fragments, in a matrix of gravel, sand or clayey sand, though a few beds of inter-stratified sand and sandy clay are also seen. Differences in colour, as well as differences in composition and roundness of clasts, has allowed for differentiation of three separate sequences within the Boulder Beds. The Beds are up to 300 m thick at the centre of the basin where their bedding is horizontal, though along the basin edges, they show inward dips of 20° to 45°. A number of small, syn-depositional, normal faults, and post-depositional, wrench faults, cut these beds whose age is not know, though a late Middle Miocene to Pliocene age is proposed by correlation with regional tectonic events. The Boulder Beds are alluvial fan deposits that have developed through erosion of steep, fault induced slopes. The Coal Measures or Batu Arang Beds consist of several facies arranged in the following stratigraphical order: i) Sandstone with shale and clay intercalations, ii) Shale, iii) Coal beds, comprising two main coal seams, shale and sandstone, and iv) Sandstone with shale and clay and minor conglomerates including a basal pebbly sandstone. The strata show a synclinal structure plunging gently southeastwards, with the strata having an average dip of 14° towards the centre of the basin, though local variations are common especially towards the basin edges. The fine grained sediments range from stiff, structureless clays to well laminated and fissile shales containing abundant carbonaceous materiaL The shales are also often silty and even sandy and grade into clayey sandstone in places. The inter-bedded sandstones are mostly fine grained, though ranging up to coarse grained, and even pebbly. The coal occurs mostly as thin laminae or streaks in shales, but also builds thicker layers of more than 30 cm thick. In the eastern part of the basin, two thick coal seams are found as well as some thin beds of coal and coaly shale. The two seams are about 65 m stratigraphically apart; the Upper Seam attaining a thickness of up to 15 m, and the Lower Seam averaging 8 m in thickness. The coal appears to be intermediate between high-grade lignite and sub-bituminous coal and is a hard, black rock with a resinous lustre. Palynomorph assemblages indicate an Eocene to Oligocene age for the coals that are likely to have been peat-swamp deposits in an alluvial plain setting, whilst the inter-bedded shales were deposited in a freshwater lacustrine setting. Correlation with known stratigraphic successions in Sumatra shows that the Coal Measures are time-equivalent (Late Eocene-Early Oligocene) with, and have a somewhat similar depositional history as, the Parapat Formation of the North Sumatra Basin, the Permatang Group and Kelesa Formation of the Central Sumatra Basin, and the Lahat Formation of the South Sumatra Basin. Differences in lithology, however, occur and reflect differences in geographical locations; the sediments at Batu Arang having been deposited at a higher altitude, and in a more inland location, as compared with the Sumatran Basins that formed at lower altitudes, close to the then Sundaland coastline. Differences in geographical locations have also influenced the subsequent geological histories of the individual basins. No lithological equivalent of the Boulder Beds is found in the Tertiary strata of the Sumatran Basins, though its deposition is considered to be time-equivalent (Middle Miocene-Pliocene) with the Baong Formation of the North Sumatra Basin, the Minas Group and Binio Formation of the Central Sumatra Basin, and the Air Benakat Formation of the South Sumatra Basin. Correlation with regional tectonic events indicates that a pull-apart basin (of half-graben shape)