Chemostratigraphic Framework for Changing Depositional Conditions of Prospective Late Cretaceous-Paleogene Marine Source Rocks of the East Coast Basin, New Zealand

Abstract

Late Cretaceous – Eocene marine successions in the East Coast Basin, New Zealand, are dominated by thick, lithologically homogenous siliceous to moderately calcareous mudstones of the Whangai and Wanstead formations. In many localities, this sequence is interrupted by the Late Paleocene highly prospective organic-rich source rocks of the Waipawa organofacies. The importance of these source rocks to the understanding of New Zealand's petroleum systems is well recognized; however, the depositional controls on their formation are not yet well constrained. We have studied the geochemistry of several key outcrop sections in the East Coast Basin in moderate to high resolution using portable X-ray fluorescence. Elemental concentrations were validated using conventional, laboratory-based analytical procedures (e.g. inductively coupled plasma mass spectrometry and wavelength dispersive X-ray fluorescence). Samples were further analysed by X-ray diffraction (XRD) to determine modal mineralogy. Bulk pyrolysis was used to determine the organic component of samples. Data analysis using principal components analysis combined with model-based cluster recognition identifies systematic vertical and lateral geochemical compositional variation, enabling correlation between stratigraphic sections. These high-spatial resolution datasets provide robust chemostratigraphic divisions, both between and within units. Within this framework, XRD analyses indicate no significant change in detrital source occurred across the Whangai–Waipawa–Wanstead transition. This confirms that a change in the paleoceanographic setting, rather than a change in provenance, was responsible for the widespread deposition of the lithologically and geochemically distinct Waipawa organofacies. Trace metal indices and total organic carbon (TOC)–sulphur (S)–iron (Fe) relationships reveal evidence of shifting paleo-redox conditions, from oxic to dysoxic conditions during the Late Paleocene, associated with enhanced preservation of organic matter. Arsenic and uranium are enriched relative to average shale values, and molybdenum is depleted, consistent with an unrestricted marine paleogeographic setting. Integrating chemostratigraphic approaches into the existing stratigraphic framework provides new constraints on Late Cretaceous – Eocene paleoenvironmental conditions and provides a new tool for correlating lithologically monotonous, prospective mudstone formations in the eastern sedimentary basins of New Zealand.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015