title: Revised stratigraphic framework for the Pliocene to Holocene sedimentary deposits of the Auckland area
Creators: name: Barrell, David J. A.
name: Bland, Kyle J.
name: Hill, Matthew P.
name: Lee, Julie M.
Publisher: name: GNS Science
Year: 2021

description: Existing terminology for the relatively young and weak sedimentary geological formations of the Auckland area has limited utility for geological and geotechnical classification, because there is considerable variety in their material content. A revised stratigraphic naming framework has been developed for the Pliocene to Holocene sedimentary strata of the Auckland area, in consultation with Auckland geological and geotechnical practitioners. Sedimentary strata previously identified as Tauranga Group and/or its constituent Puketoka Formation in the Auckland area are now defined as Takanini Formation (new name). The formation contains eight members, based on their dominant composition and without explicit age stratigraphic context. The members (all new names) are Puketoka Gravel Member (distant source fluvial deposits), Pahurehure Member (local-source fluvial deposits), Runciman Member (carbonaceous deposits), Ardmore Member (highly organic deposits), Curlew Member (mainly fine-grained lacustrine, estuarine, or shallow-marine deposits), Otahuhu Member (conspicuously fossiliferous (shelly) deposits), Hobsonville Member (otherwise unnamed silicic volcaniclastic or pumiceous deposits) and Kepa Member (landslide debris deposits). Reference can also be made to undifferentiated Takanini Formation. Where a named silicic volcaniclastic deposit is present, e.g. Potaka Ignimbrite, we recommend that the existing name is used. We restrict the Kaawa Formation to fossiliferous strata of Early Pliocene age in the Waikato area. Unless an Early Pliocene age can be clearly demonstrated for conspicuously fossiliferous strata in the Auckland area, we recommend that those beds are referred to as Otahuhu Member (Takanini Formation). Existing terms Puketoka Formation, and the Piako and Walton sub-groups of Tauranga Group and their other constituent formations, are now restricted in their use to the Waikato area. Takanini Formation is tentatively placed in Tauranga Group, but it is recommended that Tauranga Group no longer be used for site-specific stratigraphic unit classification in the Auckland area. We propose no high-level changes for the Pliocene–Holocene dune sand(stone) deposits of the western coastal area, although we recommend the use of the informal name ‘undifferentiated Kaihu group’ where deposits cannot be distinguished between Pliocene–mid Pleistocene Āwhitu Group and late Pleistocene–Holocene Karioitahi Group. No formation names have been proposed for the Āwhitu Group. The lowermost mafic-rich sand body in the Port Waikato area is named Hood Sands. Mitiwai Sands is expanded to encapsulate loose or poorly consolidated sand in mobile or recently stabilised dune sand and beach sand along exposed coasts and associated sedimentary deposits. It is suggested that because the previous mapping framework using the term Tauranga Group was extended throughout the Auckland and Northland regions in the 1:250 000-scale (QMAP) geological map series, the revised stratigraphic terminology can also be applied through those regions. (auth)