Palaeozoic redbeds and radiolarian chert: reinterpretation of their relationship in the Bentong and Raub areas, West Pahang, Peninsular Malaysia

Abstract

The Foothills Range in the Bentong-Raub area east of the Main Range in Pahang is composed of three distinct rock units: schist, chert/argillite, and redbed conglom­ erate/sandstone, separated by at least one and probably two major unconformities. Ultra­ basic rocks, mainly serpentinite, occur in parts of the range. The chert/argillite unit contains· fossil radiolarians, and shows evidence of having been deposited in a marine, reducing environment; whereas the conglomerate/sandstone unit is almost certainly continental and was probably deposited by fluviatile processes, in a piedmont environment. The structure in the Raub area is homoclinal, eastward dipping, whereas in the Bentong area, a major over­ turned syncline and a complementary anticline, have caused a repetition of the outcrop of the units. Hitherto these units have been regarded as facies of a single stratigraphic unit, the "Bentong Group". The schist, chert/argillite and conglomerate/sandstone continue along strike from Ben­ tong southeast to the Karak and Jelebu areas, where the argillite contains Lower Devonian graptolites, and have been mapped together as facies of the "Karak Formation" West of Bentong, in the Genting Sempah area a similar sequence of schist, chert/argillite and redbeds has been mapped as facies of the "B1:IIttmjGroup". Evidence presented here shows that the "Bentong Group" and "Karak Formation" are not valid lithostratigraphic units, but comprise three separate formations of contrasting lithology. The suggested sequence in the Raub-Bentong area is: (Raub Group: Early Carboniferous and Younger) Conglomeratic redbeds: Earliest Carboniferous, or Devonian Unconformity Chert/argillite: Early Devonian Probable unconformity Schist: Earliest Devonian or older In the Raub and Bentong Districts of Pahang (Fig. 1), between the granite of the Main Range and low-lying areas further east is a-chain of hills, the Foothills Range, mostly 600m high or less, extending from Karak north-northwest through Bentong-and passing immediately west of Raub. This range is composed of various sedimentary and metasedimentary rocks (intruded by basic dykes and some ultrabasic bodies) trending in this same direction, with general steep easterly dips. Because the Foothills Range includes ultrabasic intrusions and radiolarian cherts and seems to mark a major facies boundary between West Malaya on the one hand and Central and East Malaya on the other, it has 45