Litho- and biostratigraphy of a late Oligocene–Early Miocene succession in the Weber area, southern Hawke’s Bay, and implications for early Hikurangi subduction-margin evolution

ABSTRACT

A deep-marine sedimentary succession of Whaingaroan to Altonian age (Early Oligocene–Early Miocene), cropping out near Weber, southern Hawke’s Bay, records abrupt changes in depositional paleoenvironments and sedimentary lithofacies. Highly calcareous early Waitakian (latest Oligocene) Weber Formation (Mangatu Group/Waka Supergroup) is unconformably overlain by terrigenous-dominated late Waitakian–Otaian (Early Miocene) Coast Road or Whakataki formations (Tolaga Group/Māui Supergroup). Most notably, in some localities, lowermost parts of the Coast Road Formation (Mangapuku Mudstone Member) contain prominent matrix-supported extra-formational olistoliths of centimetre to decametre scale derived from the underlying Weber Formation. Deposition of these blocks is attributed to deep-water collapse of parts of rapidly growing, over-steepened, reverse-fault-controlled thrust ridge(s). The onset of Coast Road and Whakataki formation deposition and associated abrupt changes in sedimentary lithofacies correlate to prominent Early Miocene lithological, mineralogical, and paleoenvironmental changes elsewhere along the Hikurangi Margin. We attribute these bio- and lithostratigraphic changes as evidence for significant shortening and upper-plate reverse faulting marking the onset of subduction beneath eastern North Island during the mid-Waitakian (c. 23 Ma). Proposed lithostratigraphic revisions presented herein help clarify depositional events and correlative sedimentary packages within the Miocene Hikurangi margin.

KEYWORDS:

• Olistostrome
• Akitio River
• Tahuokaretu
• Mangatu Group
• Tolaga Group
• Weber Formation
• Coast Road Formation
• Ihungia Formation
• foraminifera
• lithostratigraphic revision

Acknowledgements

We warmly thank landowners for generously allowing access to geological sections, especially the Bell family at Waipatiki, Margaret, Peter, and Simon Hales at Kereru Farm, and Steve Lewis of Mangatoitoi Farm. Sue and Shaun Morgans are thanked for providing field accommodation at Spring Creek Farm. We thank Randall McDonnell and Henry Gard for processing biostratigraphic samples. Chris Clowes is thanked for extracting digital data from the New Zealand Fossil Record File, and for enthusiastically providing a thorough review of a very early manuscript draft, resulting in significant changes in scope and focus of this paper. Julie Palmer and Jonathan Proctor (Massey University, Palmerston North) are thanked for their support and supervision of Hannah Harvey’s MSc project. We thank Malcolm Arnot and Angela Griffin for their reviews and comments on the manuscript prior to submission. We gratefully acknowledge Associate Editor Julien Bailleul and journal reviewers Geoffroy Mahieux and Andrea Festa for their helpful comments and suggestions, which have improved clarity within the paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

A supplemental file that gives details of new biostratigraphic analyses carried out as part of this study is available via Figshare (https://doi.org/10.6084/m9.figshare.19720219.v1). All biostratigraphic data are lodged in the New Zealand Fossil Record File (FRF; Clowes et al. 2021), accessible at https://fred.org.nz/.

Additional information

Funding

This project gratefully acknowledges Strategic Science Investment Funding from the New Zealand Government (Ministry of Business, Innovation and Employment) to GNS Science as part of the Petroleum Basins, Sedimentary Basins, and Te Riu-a-Māui/Understanding Zealandia research programmes.