Biostratigraphy, organic petrography and carbon isotope chemostratigraphy of the Ordovician-Silurian black shales from the Northwestern Domain of Peninsular Malaysia

Abstract

Integrated biostratigraphy, petrography, organic carbon and carbon isotope chemostratigraphy were used in tandem to assess the paleoenvironment and hydrocarbon potential of the latest Ordovician to Early Silurian basin in the Northwestern Domain of Peninsular Malaysia. Graptolite biostratigraphy indicated a Hirnantian to Aeronian age for the Tanjung Dendang Formation in Pulau Langgun, Langkawi. The graptolite fauna from the Langkawi Islands demonstrates a close association with the South Chinese fauna. The graptolite fragments show a non-granular surface morphology and are mostly present in a long, stipe-like, lath shape when observed under reflected white light microscopy. The pyrites present are relatively small and sparse during the latest Ordovician, suggesting a euxinic condition, but during the Silurian, the pyrites are larger and more common, suggesting anoxic and later dysoxic bottom water. The mean random reflectance of non-granular graptolite in sections perpendicular to the bedding range from 1.8% to 3.07%, and the equivalent vitrinite reflectance ranges from 1.86% Ro to 3.12% Ro, suggesting an overmature stage in term of hydrocarbon generation. The TOC of the Langkawi black shale averages 2.1 wt%, while the rock from the Mahang and Sungai Petani areas measures 1.37 wt% and 0.47% wt%, respectively. δ13Corg chemostratigraphy for the Tanjung Dendang Formation resulted in δ13Corg level between −31.4 ‰ to −28.7 ‰. The Hirnantian Isotope Carbon Excursion (HICE), signalled by a high δ13Corg value, is documented near the base of the formation, followed by a sudden negative shift towards the Ordovician-Silurian boundary. δ13Corg more or less maintained during the Silurian except for slight carbon excursion events during the middle Rhuddanian and early Aeronian. The δ13Corg excursion events from the Tanjung Dendang Formation in Langkawi can be accurately correlated with successions from China, the United Kingdom and Artic Canada.