INORGANIC AND STABLE ISOTOPE GEOCHEMISTRY OF ORDOVICIAN STRATA IN THE CANNING BASIN: THE MANIFESTATION OF GLOBAL BIOEVENTS IN NORTHERN GONDWANA

New inorganic geochemistry (major and trace elements) and stable isotope (δ¹³C_carb, δ¹⁸O_carb) data are integrated herein to resolve the age and depositional setting of the Willara, Goldwyer (I, II, and III) Nita, and Bongabinni formations in the Canning Basin of Western Australia.

Carbon isotopes display low values through the Goldwyer III Formation, above this, δ¹³C_carb values rise over 3-4 ‰ to a local peak in the Goldwyer II Formation. Both the negative and positive excursion are consistent with the patterns expected of the globally recognized “Mid-Darriwilian Carbon Isotope Excursion” (MDICE). Values descend into the Goldwyer I Formation before rising to another positive excursion in the overlying Nita. This is interpreted as the late-Sandbian to early Katian “Guttenberg Carbon Isotope Excursion” (GICE). Values decline again through the Nita and overlying Bongabinni Formations.

High concentrations of CaO in the Willara and Nita formations confirm these units as carbonates. Enrichment of MnO and MgO in the upper Nita may suggest that these calcareous strata are slightly dolomitic. The Goldwyer and Bongabinni formations are enriched in Al₂O₃, Fe₂O₃, K₂O, Ga, Th, and Rb, which are commonly associated with fine grained clastic lithologies. Distinct subdivisions within the Goldwyer can be founded on geochemical patterns: higher abundances of Cu and V are found in the Goldwyer III, elevated Y and Na₂O characterize the Goldwyer I.

Enrichments of redox sensitive trace elements in the Goldwyer III suggest that local environments (associated here with back-reef facies) were subjected to periods of anoxia during the time leading up to the MDICE. Paired stable isotope and elemental geochemical analysis elucidate the local manifestation of this global paleoceanographic event on the northern end of Gondwana, permitting a more complete understanding of the Middle Ordovician world.