SEQUENCE- AND CHEMO-STRATIGRAPHIC FRAMEWORK FOR THE WAIPAWA FORMATION, EASTERN NEW ZEALAND: CONTROLS ON LATE PALEOCENE BLACK SHALE DEPOSITION IN THE SOUTHWEST PACIFIC

Late Cretaceous–Eocene marine successions in eastern New Zealand are dominated by thick, lithologically monotonous siliceous to moderately-calcareous mudstones of the Whangai and Wanstead formations. In many localities, this sequence is interrupted by organic-rich highly-prospective source rocks of the Late Paleocene Waipawa Formation. Elsewhere, the Waipawa Formation occurs within fine-grained Paleocene limestones. Understanding the depositional controls on the Waipawa Formation is important for characterising prospective source rocks and paleo-oceanographic settings in eastern New Zealand.

Eleven stratigraphic sections have been sampled in moderate to high-resolution throughout the region. Over 700 samples from latest Cretaceous to Early Eocene strata were analysed by portable X-ray fluorescence (pXRF) for multi-element geochemistry. Elemental concentrations were validated using inductively coupled plasma mass spectrometry and wavelength dispersive X-ray fluorescence. A subset of samples were also analysed by X-ray diffraction to determine modal mineralogy, and bulk pyrolysis was used to determine the organic content and source rock potential.

This geochemical and mineralogical dataset provides a robust chemostratigraphic framework that indicates no significant change in detrital source across the Whangai–Waipawa–Wanstead transition. These data suggest that a change in Late Paleocene paleoceanography, rather than a change in provenance, was responsible for the widespread deposition of the organic-rich Waipawa Formation in the southwest Pacific. Trace metal indices, elemental enrichment factors and total organic carbon–sulphur–iron relationships reveal shifting paleo-redox conditions, from oxic to dysoxic conditions during the Late Paleocene, associated with enhanced preservation of organic matter. Integrating high spatial resolution pXRF data into the existing stratigraphic framework provides new constraints on Late Cretaceous–Eocene paleoenvironmental conditions and provides a robust correlation between prospective mudstone formations in the eastern sedimentary basins of New Zealand.