Lithostratigraphy of upper palaeozoic carbonate rocks in the southeastern part of Changwat Loei

Abstract

The objective of this research is to establish the stratotype of carbonate succession representing the Nam Maholan Formation in the southeastern part of Changwat Loei. Additional attempts have been made to conduct the lithofacies analysis and to reconstruct the depositional environment. The succession of carbonate rocks of the Nam Maholan formation in the study area is dated palaeontologically from late carboniferous to late early permian. This formation conformably overlies the Wang Saphung formation, but unconformably underlies the Huai Hin Lat formation. The about 500 metre-thick sequence is subdivided into three members in an ascending order: 1) the Tham Suae Mop member (about 50 metre-thick), characterized by limestone and shale, 2) the Ban Nong Hin member (about 200-250 metre-thick) comprising limestone-chert and dolomite, 3) the Phu Khao member (about 250-300 metre-thick) consisting of limestone and dolomite. Petrographic study reveals seven microfacies types of packed biomicrite, sparse biomicrite, biosparite, crystalline, algal lamination, oosparudite, and pelmicrite, respectively in decreasing order. These microfacies types together with the diversity of fossils suggest the intertidal and subtial regimes under the influence of low-to high-energy shallow shelf sea with partly restricted water condition. Additionally, the values of triangle18 O vary from -7.51 to -6.20 per mil (PDB) and triangle13 C values between +2.54 to 3.67 per mil (PDB) advocate shallow marine origin. Salinity values between 18.44 and 30.08 per mil suggest fresh water contamination and indicate partly restricted water circulation of the depositional environment. lithology and fossils as well as isotopic results suggest that these upper palaeozoic strata occurred in tropical climate condition, implying that the carbonate depositional basin was situated close to palaeoequator in palaeotethys. The appearance of middle to late palaeozoic basaltic ocean floor associated deep-water radiolarian chert stratigraphically beneath limestone terrane suggest the development of carbonate onto the ocean basin prior to the advent of subduction-related permo-triassic arc-type magmatism. This may also advocate that the late palaeozoic carbonate terrane has become part of the indochina terrane during triassic period.