Chert geochemistry, depositional setting, stratigraphic and structural significance for the Permian Nong Pong Formation, Khao Khwang Fold and Thrust Belt, Saraburi, Thailand

Abstract

The deepwater chert-shale-limestone-tuff Permian Nong Pong Formation (NPF) of the Saraburi Group is a major unit within the Triassic (Indosinian Orogeny) Khao Khwang Fold and Thrust Belt (KKFTB). Whether the NPF was deposited in a continental margin or ocean basin remains uncertain and impacts structural, sedimentary and stratigraphic interpretations of the KKFTB. This study investigates chert geochemistry from a 60 m section of the formation, near Muak Lek town, to determine its depositional environment. Radiolarian fauna from the chert beds includes Follicucullus porrectus Rudenko, (probable late Guadalupian, late middle Permian), indicating the NPF is time-equivalent to adjacent carbonate platform deposition (Khao Khwang and Khao Khad formations). High ratios of SiO2/(SiO2 + Al2O3 + Fe2O3) and Fe-Al-Mn plots indicate a biogenic origin (radiolarians, sponge spicules) for the cherts, far from a hydrothermal source. Trace element ratios for U/Th, Sc/Th and Sr/Ba and Ce anomalies in the NASC-normalized REE distribution are ambiguous, and range between shallow to deep ocean basin, and continental margin settings. Major elements analysis of cherts show: (1) high ratios of SiO2/(SiO2 + Al2O3 + Fe2O3), range: 0.60–0.82 (0.76 average), (2) Al/(Al + Fe + Mn), range: 0.67–0.82 and (3) average MnO/TiO2 = 0.14. The MnO/TiO2 and Al2O3/(Al2O3 + Fe2O3) ratio, 100 × Fe2O3/SiO2 vs. 100 × Al2O3/SiO2 and Fe2O3/(100-SiO2) vs. Al2O3/(100-SiO2) diagrams indicate deposition on a continental margin. The Lan/Cen versus Al2O3/(Al2O3 + Fe2O3) discrimination diagram indicates a pelagic, continental margin setting. Combined geochemical, structural and sedimentary facies analyses suggests the NPF was deposited in a deep marine, normal fault-bounded basin on a rifted, ribbon continent margin where carbonate platforms grew on structural highs.