Geochemical and facies evidence for palaeoenvironmental change during the Late Ordovician Hirnantian glaciation in South Sichuan Province, China

Abstract

Late Ordovician shales at Kuanyinchiao, southern Sichuan Province, China, contain a muddy micritic limestone (the Kuanyinchiao Formation) 0.4 m thick, representing the Hirnantian glaciation, and therefore contain the two extinction events which together comprise the Late Ordovician mass extinction. Wufeng Formation shales underlie, and Lungmachi Formation shales overlie, the limestone, and the Ordovician–Silurian boundary is c. 0.6 m above the base of the Lungmachi Formation. Geochemical analysis of closely spaced samples aids assessment of tectonic and palaeoceanographic change influencing sedimentary facies throughout the interval, but also highlights the value and dangers of geochemical applications in palaeoenvironment study. The immobile elements Hf, Sc, Ta and Th show a consistent asymmetric pattern of abundance; they declined gradually in the upper Wufeng Formation to lower concentrations in calcareous sediments of the Kuanyinchiao Formation because of suppressed erosion and lower sedimentation rate during glaciation. The decline may represent gradual cooling and reduction in erosion rate from source, before preserved sedimentary evidence of the glaciation itself. A sharp return to shale (Lungmachi Formation), and Hf, Sc, Ta and Th concentrations to pre-glaciation levels, suggest rapid glacial collapse and renewed erosion, probably of the nearby Dianqian Land. These immobile elements co-vary with most REE (especially Ce and La), demonstrating that these were also source-controlled, and unaffected by contemporary environmental, or diagenetic, processes. However, partial co-variance of Eu and U with immobiles suggests a combined control of Eu and U by both source variations and redox changes. Anomalies in Ce, Eu and U are often used to identify shifts in oxidation state; at Kuanyinchiao, anomalies which suggest anoxia in the Wufeng Formation are contradicted by benthic faunas which prove the seafloor was oxygenated then, and indicate complex control of those elements. Ba, As and Sb trends depart markedly from immobiles, and show no consistent trends; they are interpreted as modified by diagenesis. Overall, the immobile element suite suggests that the site received clastic supply from local uplift throughout the interval, which was interrupted by the glacial phase.