Petrographic studies of Permian carbonates in southern Thailand

Abstract

Detailed petrographic studies were conducted on Permian carbonates from four areas: Ratburi, Prachuab Khirikhan, Ko Ang Thong, and Surat Thani-Phang Nga. Six carbonate facies were identified in these areas—mudstone, wackestone, packstone, grainstone, algal boundstone, and intramicrite. The allochems of these carbonates are similar and composed of bioclastic and non-bioclastic grains. The bioclastic grains are blue-green algae (stromatolite), green algae (dasycladacean), tabulate corals, solitary corals, crinoids, foraminiferas ( Shanita sp. and Hemigordius sp.), brachiopods, bryozoan, gastropods and unidentified skeletal fragments. The non-bioclastic grains are ooids, peloids, and intraclasts. Three phases of paragenesis were observed: micritization and neomorphism, dolomitization and non-ferroan/ferroan calcite fractures and veinlets. Micritization, neomorphism, and dolomitization are extensive. Rhombic and mosaic-crystalline dolomites had replaced carbonate mud, sparry calcite cement as well as the allochems. Abundant non-ferroan calcite veinlets cross cut allochems, mud matrix, and dolomite rhombs which suggests they postdate dolomitization. These carbonate facies were interpreted as interior-platform carbonate with a generally well developed shoaling-upward sequence starting from mudstone to wackestone, packstone, and grainstone. Local algal boundstone or algal mound and intramicrite could indicate low energy, regressive cycles.