Lithostratigraphic framework summary and geological context of the southern Auckland area

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Bland KJ, Lee JM, Hill MP, Jones KE. 2020. Lithostratigraphic framework summary and geological context of the southern Auckland area. Lower Hutt (NZ): GNS Science. 62 p. (GNS Science report; 2020/04). doi:10.21420/97ZH-V040.

Abstract:
The geological context and lithostratigraphic framework of southern Auckland are essential components of urban geological maps and for the general understanding of the area’s surface and subsurface earth materials. The composition and stratigraphic relationships of important geological units are described for an 829 km2 area located around the centres of Pukekohe, Papakura, Waiuku, between the southern shoreline of the Manukau Harbour, the Waikato River, Āwhitu Peninsula, and Hunua Ranges. Knowledge of the regional geology will assist with urban and suburban planning and development. Rocks in the Hunua Ranges contain the oldest outcropping geological units in the area, where Late Triassic to Jurassic Waipapa Composite Terrane (Waipapa Group) metasedimentary ‘greywacke’ and minor volcanic rocks underlie much of the hill country. These rocks are unconformably overlain by Eocene and Oligocene marginal to fully-marine carbonaceous and calcareous strata of the Te Kuiti Group. A regional unconformity separates the earliest Miocene parts of the Te Kuiti Group from the overlying early Miocene calcareous rocks of the marine Kawau Subgroup (Waitematā Group). Thick successions of alternating sandstone and mudstone of the Meremere and Warkworth subgroups in turn overlie the Kawau Subgroup. Volcanic-derived marine sedimentary rocks of the Manukau Subgroup occur in the west of the study area. No rocks of middle to late Miocene age are known to occur within the study area. Early to mid-Pliocene shallow-marine Kaawa Formation interfingers with and is overlain by Pliocene to Quaternary non- to marginal-marine sedimentary strata of the Tauranga, Āwhitu, and Karioitahi groups. The most conspicuous landscape features across much of the study area are basaltic lava flows, tuff rings, craters, scoria cones, and tephra deposits associated with the Early Quaternary South Auckland Volcanic Field. These rocks are encompassed by the Kerikeri Volcanic Group. Many of the geological units in the area are variably offset by faults, some of which are likely to be classified as active. It is anticipated that study of the wider Auckland area will result in rationalisation and changes to the lithostratigraphic framework. These changes are most likely to occur within the lithofacies-diverse Pliocene-leistocene Tauranga, Āwhitu, and Karioitahi groups. (auth)