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

Late Ordovician shales at Kuanyinchiao, southern Sichuan Province, China, c ontain a muddy micritic limestone (the Kuanyinchiao Formation) 0.4 m thick, representing the Hirnantian glaciation, and therefore contain the two exti nction events which together comprise the Late Ordovician mass extinction. Wufeng Formation shales underlie, and Lungmachi Formation shales overlie, t he limestone, and the Ordovician-Silurian boundary is c. 0.6 m above the ba se of the Lungmachi Formation. Geochemical analysis of closely spaced sampl es aids assessment of tectonic and palaeoceanographic change influencing se dimentary facies throughout the interval, but also highlights the value and dangers of geochemical applications in palaeoenvironment study. The immobi le elements Hf, Sc, Ta and Th show a consistent asymmetric pattern of abund ance; they declined gradually in the upper Wufeng Formation to lower concen trations in calcareous sediments of the Kuanyinchiao Formation because of s uppressed erosion and lower sedimentation rate during glaciation. The decli ne may represent gradual cooling and reduction in erosion rate from source, before preserved sedimentary evidence of the glaciation itself. A sharp re turn to shale (Lungmachi Formation), and Hf, Sc, Ta and Th concentrations t o pre-glaciation levels, suggest rapid glacial collapse and renewed erosion , probably of the nearby Dianqian Land. These immobile elements co-vary wit h most REE (especially Ce and La), demonstrating that these were also sourc e-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 Format ion are contradicted by benthic faunas which prove the seafloor was oxygena ted then, and indicate complex control of those elements. Ba, As and Sb tre nds depart markedly from immobiles, and show no consistent bends; 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. (C) 2000 Elsevie r Science B.V. All rights reserved.