Characteristics, genesis, and paleoenvironmental significances of oncolites in the lower Permian (Cisuralian) Balikelike Formation from the northwestern Tarim Basin, China

The Permian marks a pivotal transition in Earth's evolutionary history, with oncolites offering insights into paleoenvironmental and evolutionary dynamics. This study documents the first occurrence of oncolites in the lower Permian (Sakmarian) Balikelike Formation in the Keping area, northwestern Tarim Basin, China. Integrated lithofacies analysis, geochemistry, and multi-scale observations reveal that the Balikelike Formation at the Subashi section comprises a complete third-order sequence, with two layers of oncolites developed in the highstand systems tract. These oncolites, predominantly developed on marl and interbedded with mudstone, exhibit elliptical, cap-shaped, hemispherical, and thin-plated morphologies. Fluorescence microscopy exhibits yellow-green fluorescence in dark laminae, while SEM identifies fossilized filamentous cyanobacteria. Geochemical data — Sr isotopes, weathering index values, and Sr/Ba ratios — suggest that oncolites develop in a warm, humid, and high-salinity environment with limited terrestrial input. The oncolite proliferation coincides with the initial magmatic phase of the Tarim Large Igneous Province, evidenced by negative carbon isotope excursions and the lower ⁸⁷Sr/⁸⁶Sr ratios compared to global seawater, signaling an interglacial warming phase. Volcanic activity likely curtailed the Asselian–Sakmarian glaciation in the Tarim Basin earlier than elsewhere, with carbon emissions, climate warming, and eutrophication, driving massive occurrence of oncolite.