https://orcid.org/0000-0002-1963-8907
![Sample our Earth Sciences journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5930/TOC-Subject-Sample-2021-Earth-Sciences.jpg"})
ABSTRACT
A unique 10–140 cm thick veneer of greensand and glauconite-rich deposits (10–80 wt% glauconite) presently sits atop central Chatham Rise (CR) in water depths of 200-500 m. The glauconite is dominated by chemically mature, polished ovoidal and lobate pellets, with a 10–12 Å phyllosilicate structure and K-Ar ages of 7–5 Ma, indicating that the glauconite pellets are predominantly allogenic (i.e. derived/reworked) grains, and not of in situ authigenic origin. Glauconite possibly evolved from seafloor alteration of detrital and/or volcanogenic smectitic clays, likely concentrated within organic-rich faecal pellets under unique paleoceanographic conditions (nutrient cycling, upwelling, carbon isotope gradients) within the latest Miocene Subtropical Front. The distribution of glauconite abundance suggests a ‘glauconite factory’ existed at this time about Reserve Bank on western central CR. The pellets were dispersed eastwards along the crest by intensified bottom currents within the dynamic Subtropical Frontal zone, perhaps during the numerous glacial periods of lowered sea level throughout the Plio-Quaternary. The bioturbated greensand veneer, and its mix of other reworked Neogene and Quaternary skeletal, phosphatic and siliciclastic components, rest unconformably on mainly Early Oligocene chalks, forming a highly condensed, relict/palimpsest deposit that alone embodies the last up to 30 My of sedimentation history atop central CR.
Acknowledgements
This paper is based on the MSc of ASL completed in 2012 at the University of Waikato, New Zealand. Discussion of technical procedures and analytical support in the course of the MSc study were generously provided by many people including, from the University of Waikato, Roger Briggs, Steve Hood, Renat Radosinsky, Ganqing Xu, Annette Rodgers and Rochelle Hansen. Kevin Mackay (NIWA, Wellington) kindly produced a GIS base map for central CR; Ritchie Simms (University of Auckland) guided our use of their electron probe micro-analyser; and Andrew Todd (CSIRO Argon facility, Perth, Australia) assisted HZ with K-Ar dating of the glauconite concentrates. We acknowledge the enthusiastic support of NIWA (Wellington) staff, including the scientific staff, Master and crew aboard RV Tangaroa during a research cruise to CR, funded by the NIWA Coasts and Oceans Centre. Special impetus for the MSc study came from the support of Chris Castle, Robin Falconer and Ray Wood at Chatham Rock Phosphate Ltd. We are grateful for the draughting assistance of figures provided by Peter Gerring and Erika Mackay (both NIWA, Wellington). Early versions of this paper benefitted from discussions with several people, including Steve Hood, Peter Kamp, Roger Briggs (all University of Waikato), Kyle Bland (GNS Science, Lower Hutt), Tim Naish (Victoria University of Wellington), and Nick Mortimer (GNS Science, Dunedin). The perceptive review comments of Andrew La Croix and an anonymous referee are gratefully acknowledged, as is the editorial assistance of Chris Moy.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Data availability statement
The data that support the findings of this study are openly available as Supplementary Files on figshare at: https://doi.org/10.6084/m9.figshare.15506037.v1. Other support information and data for the study are available online from Anna Lawless’ MSc thesis in the University of Waikato’s Open Access Research Repository at: https://researchcommons.waikato.ac.nz/handle/10289/6502.