Evolution of a Permian carbonate platform to siliciclastic basin: Indochina Plate, Thailand

In Thailand, during the Permian, an extensive carbonate platform developed on the margin of the Indochina Plate and near to a coeval and deeper silici clastic-dominated marine basin. This basin separated the Indochina Plate, t o the east, from the Shan Thai Plate to the west. Sedimentation in the area of both platform (Pha Nok Khao Platform) and basin (Nam Duk Basin) was end ed by a late Permian/early Triassic orogenic event that caused closure of t he Palaeo-Tethys ocean. Tectonism has controlled the pattern of platform se dimentation and supplies of carbonate and elastic sediments into the basin area. The Pha Nok Khao Platform developed on an extensive distally steepene d ramp-like margin along which carbonates and siliciclastics accumulated. A djacent to this platform, to the west, the Nam Duk Basin received coeval he mipelagic siliciclastics and minor amounts of carbonates. Biostratigraphic evidence, based on foraminiferal assemblages, suggests an early to middle P ermian age equivalence between platform and basin lithofacies. The platform strata are exposed along the Loei fold and thrust belt in northeast Thaila nd and basin strata are exposed within the Phetchabun fold and thrust belt in central Thailand, respectively. The two belts extend in a north-south di rection and their rocks exhibit extensive karstification. On the basis of s eismic data, platform lithologies extend further east in the subsurface, be neath the continental sediments of the Khorat Group into Kampuchea and Laos . Facies groups of the platform environments include restricted platform, p latform interior, and outer platform. The facies groups of the basin enviro nments include basin margin and basin plain. Minor layers of volcaniclastic s and green coloured tuffites are interbedded with both the platform carbon ate layers and basinal pelagic layers, suggesting active volcanism during s edimentation. The carbonate lithofacies of the shallow platform to the east tin the Loei Fold Belt) grade westward into turbidites tin the Phetchabun fold belt), and include shales and sandstones with minor carbonates. Farthe r to the west they pass into hemipelagic lithofacies of shale and carbonate . Lithologies and fauna of the limestones suggest tropical to subtropical c limate during deposition. (C) 1998 Elsevier Science B.V. All rights reserve d.