Alluvial fans and fan deltas in the Paparoa Formation, Greymouth Basin: a new rift model

ABSTRACT

Sedimentary facies analysis of conglomerate deposits in the Greymouth Rift Basin has identified the latest Cretaceous to Paleocene alluvial fan and fan delta environments on the northwestern side. The Gravelly Braided River Facies Association is interpreted as high energy, braided river streamflow in a streamflow-dominated alluvial fan environment. The Gravelly Delta Front Facies Association was deposited by high bedload mouth bars and channel avulsion. The Gravelly Prodelta Facies Association is interpreted as high-density turbidity currents, and subaqueous debris flows in a fan delta slope environment. Bedding geometries suggest the fan deltas were Hjulström-types formed on lower angle slopes. The gradual decrease in conglomerate thickness from northwest to southeast indicates that the primary basin bounding fault was located offshore to the northwest, most likely the Cape Foulwind-Canoe Fault Zone. Overall facies distribution and paleoflow directions indicate the Greymouth Basin formed as a half-graben in a purely extensional setting with no strike-slip movement. The presence of contemporaneous sub-basins in the West Coast region suggests they likely experienced similar depositional history to the Greymouth Basin. Our findings agree with previous models from the Taranaki Basin that indicate that rifting was purely extensional, suggesting that the West Coast-Taranaki Rift System recorded primarily orthogonal extension.

KEYWORDS:

• Greymouth Basin
• West Coast-Taranaki Rift
• Paparoa Formation
• half-graben rift
• fan delta
• Cretaceous faults

Acknowledgements

The authors would like to acknowledge the University of Canterbury Mason Trust Fund for financial supports to field trips to Greymouth and the National Core Store in Featherston. The authors are thankful to Brad Field (Senior Scientist at GNS Science, New Zealand) and Dr. Guy Plint (Professor at the Department of Earth Sciences in the Western University, Canada) for their valuable comments on this research.

Disclosure statement

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

Data availability statement

The supplementary data that support the finding of this study and figures are openly available in Zenodo at https://doi.org/10.5281/zenodo.4706562 and in Figshare at https://figshare.com/s/b50cdd99dfe47a15bfd9.

Additional information

Funding

This research was funded by the Ministry of Business, Innovation, and Employment of New Zealand (MBIE) through GNS Science-led research program on New Zealand petroleum source rocks, fluids, and plumbing systems (contract C05X1507), Research Aim 1.4: Discovering our lacustrine source rock potential.