Integrated Graptolite Biostratigraphy and Geochemistry of the Ordovician-Silurian Boundary in Southern Thailand: Implications for Sibumasu and Adjacent Terranes

This study presents new integrated graptolite biostratigraphic and geochemical studies of the uppermost Ordovician–Lower Silurian Wang Tong Formation of southern Thailand, and compares the results with coeval successions of the Jenhochiao Formation at the Laojianshan section, Baoshan (western Yunnan, China), and the Tanjung Dendang Formation of Langkawi, northwestern Peninsular Malaysia. Three graptolite biozones are recognized from the black shale successions of the Wang Tong Formation at Khao Noi, Wang Hon, and Wang Sai Thong, in ascending order: the Metabolograptus extraordinarius, the Parakidograptus acuminatus, and the Coronograptus cyphus biozones, respectively. Biostratigraphically, the basal Wang Tong Formation (i.e. the graptolitic shales underlying the shelly Hirnantia fauna bed) is revised to correspond to the M. extraordinarius Biozone, rather than the previously assigned M. persculptus Biozone, and thus aligns the Thai succession with contemporaneous records from Langkawi. Strong faunal correspondence between southern Thailand, Langkawi of Malaysia, and Baoshan of China supports close palaeobiogeographic affinity along the Sibumasu margin, despite minor lithological and preservational differences. Integration of total organic carbon (TOC) and organic carbon isotope (δ¹³Corg) data reveals consistent geochemical trends across the studied regions, characterized by elevated TOC values in graptolitic black shales, a marked depletion within Hirnantia fauna beds, and a shift toward more negative δ¹³Corg values across the Ordovician–Silurian boundary. These patterns indicate regionally coherent changes in redox conditions and carbon cycling linked to glacial–postglacial environmental transitions during the Hirnantian–Rhuddanian interval. Within the Sibumasu Terrane, comparison between biostratigraphic zonation and geochemical signatures supports the development of a laterally continuous depositional system. In contrast, comparison with adjacent terranes including Indochina, South China, and Australia highlight their shared biostratigraphic frameworks and local stratigraphic complexities. These results refine the biostratigraphic framework of southern Thailand and contribute to understanding the stratigraphy and palaeogeographic relationships of the Sibumasu margin during the Late Ordovician to early Silurian.