Chemostratigraphic resolution of widespread reducing conditions in the southwest Pacific Ocean during the Late Paleocene

Abstract

Paleogene strata throughout much of the southwest Pacific is comprised of thick successions of fine-grained siliceous to calcareous sediments. However, during the Late Paleocene the south Pacific experienced widespread deposition of organic-rich and δ13Corg-enriched sediments that are recorded in the Waipawa Formation and the correlative Waipawa Organofacies. The mechanisms responsible for the elevated input and preservation of organic matter are still poorly understood. Using a multi-proxy chemostratigraphic approach we assess paleoceanographic conditions, including paleo-redox states and paleo-productivity of the Waipawa Formation. Eight key sections from the East Coast Basin, New Zealand were sampled and analysed for multi-element geochemistry and bulk pyrolysis. Sedimentological, stratigraphic and chemostratigraphic evidence indicates the south Pacific underwent an abrupt regression event during the Late Paleocene resulting in on-shelf erosion and/or bypassing of continental sourced sediments from the shelf into deeper waters. The change in sediment delivery further increased nutrients and terrestrial organic matter loading to the site of deposition. Coeval excursions in Ti/Zr and Rb/Sr indicate a shift to an aluminosilicate-dominated sediment supply, resulting in a modest increase in marine productivity that cannot account for the anomalous organic matter enrichment. Changes in the hydrogen index show a transition from terrestrial to mixed terrestrial–marine organic matter occurred during the Late Paleocene, in which high concentration of Type II-III kerogens were buried and preserved in association with enhanced water column reducing conditions in deep marine settings. Elevated concentrations of redox-sensitive trace elements (e.g. U, Mo, Fe, S) are consistent with alternating phases of dysoxic to oxic conditions suggesting an expansion of an oxygen minimum zone, enhancing preservation and delivery of organic matter to the seafloor, leading to the subsequent burial and preservation of organic matter within anoxic sediments. These new data provide insight into the depositional conditions and the petroleum potential of prospective source rocks in this basin using an integrated organic and inorganic geochemical approach to refine our knowledge of the depositional environment and paleoceanographic changes in shelfal to bathyal marine sediments of the Late Paleocene.