Lithofacies and diagenesis of the Khao Khad formation in the vicinity of changwat Saraburi, Central Thailand

Abstract

The Khao Khad Formation of Saraburi Group in the vicinity of Changwat Saraburi area is a repetitive sequence of limestone, dolomitic limestone and silt-shale with nodular and banded chert. The age of the Khao Khad Formation lies between Asselian of Lower Permian to Capitanian of Middle Permian. Twenty nine measured sections, totally 28,603 meters long, were conducted to determine the lithological characteristics of sedimentary sequences. Altogether 536 rock samples of stratified sampling type were obtained from all the measured sections for petrographic determination of textures, composition and rock classification. The Khao Khad Formation consists of dismicrite, biomicrite, biomicrudite, packed biomicrite, biopelsparite, biosparite, intrasparite, intraparudite, biothite and calclithite. Fifteen lithofacies were distinguished from the sequence indicating the depositional environment varying from shallow restricted marine to barrier bar and foreslope. The rocks of Khao Khad Formation have suffered a complex diagenetic alteration. The processed occurred during early diagenesis were micritization, microcrystalling calcite cement, meniscus cement, acicular fibrous cementation, burrowing, microcrystalline dolomitization, radiaxial fibrous calcite cement, dog-tooth calcite cement, early compaction, equant calcite cement, blocky calcite cement, syntaxial overgrowth cementation, dissolution microcrystalline dolomitization, mesocrystalline dolomitization and silicification. The processes occurred during late diagenesis were macrocrystalline dolomitization, calcitization or dedolomitization, ferroan calcitization, dissolution compaction, and neomorphism. The individual carbonate fabrics and the whole-rock samples have the isotopic composition varying from -5.96 to 5.49% PDB for the δ13C values and from -16.75 to -2.31% PDB for the δ18O values. Even though such the isotopic signature still reflect marine source, both δ18O and δ13C values are somewhat lighter than the normal Permian seawater carbonates probably due to the isotopic exchange with lighter isotopic sources during diagenesis. The low contents of most trace elements in the rocks of Khao Khad Formation are similar to those found in the Ratburi limestone and Nam Maholan limestone from Loei.