Milankovitch-driven terrigenous deposit influx in Middle Ordovician marine successions of Western Australia: Insights for paleoclimate and geochronology

Abstract

This study investigates the terrigenous sediment influx in the Canning Basin, Western Australia, during the Darriwilian Stage of the Middle Ordovician, specifically correlating it with Milankovitch obliquity cycles. The basic stratigraphic data came from the Goldwyer Formation within Theia 1 well which was dated using the U-Pb zircon geochronology. Four ash beds, located in the lower part of the formation, yielded distinct radiometric ages at depths of 1550.81 m (466.07 ± 0.12 Ma), 1559.81 m (466.31 ± 0.11 Ma), 1572.55 m (466.84 ± 0.11 Ma), and 1587.10 m (467.12 ± 0.14 Ma) which following the age of conodont biostratigraphy. Based on the astrochronological analysis, duration of the Darriwilian in the Canning Basin is estimated at 9.03 ± 0.3 Myr, providing a precise chronological framework for the studied sedimentary successions. Core sample examinations indicated notable lithological discrepancies with a pronounced increase in terrigenous sedimentation recorded during intervals of elevated obliquity. Quantitative analysis revealed that sedimentation rates reached their zenith in correlation with ∼1.2 Myr obliquity cycles, resulting in substantial accumulations of fine-grained sediments, notably shale and mudstone. Petrographic examination identified a predominance of silt and clay-sized particles during these high influx periods, confirming a direct relationship between obliquity and sediment composition and led to enhanced fossil preservation. ICP-MS analysis of 39 samples indicated variations in elemental abundance that corresponded with the identified obliquity cycles, further supporting the correlation between climate changes and sediment influx. Scatter diagrams illustrated the relationships between various geochemical proxies, revealing trends that align with the sedimentary data. The findings confirm that the Canning Basin experienced significant shifts in sedimentation patterns driven by obliquity-induced climate changes during the Middle Ordovician. This research demonstrates how obliquity affects sedimentation rates and fossil preservation in the Canning Basin, enhancing our understanding of its geological history.