Stratigraphy, facies, structure, and setting of the Waiheke and Manaia Hill Groups, east Auckland

Abstract

Upper Jurassic, eugeosynclinal sedimentary rocks of the Waiheke Group (belonging to the Hunua Facies) are more than 8.5 km thick at the centre of the Hunua Eugeosyncline. Ponui Subgroup, the oldest, has rare Inoceramus cf. galoi at its base, and consists of two cycles of mainly interbedded, graded sandstone and dark-coloured mudstones that become thinner and finer upwards and end in a common association of chert, red mudstone, cupriferous manganese ore, and spilite. The Subgroup is divided into the following units: Chamberlins Formation (oldest), Waikorariki Formation, Mangatangi Formation, and Rocky Bay Formation. The overlying Otau Subgroup has rare Inoceramus sp. cf. Anopaea sp. at its base and is typified by rapid lateral change in facies, from sandstone (Orere Sandstone) on the margins through zones of interbedded graded sandstone and mudstone (Duders Formation) to a massive mudstone (Mangawheau Mudstone) at the centre of the geosyncline, capped by chert-red mudstone beds (Kiripaka Formation). Kawakawa Subgroup, the youngest, appears non-fossiliferous, and consists largely of a widespread, massive mudstone (Clevedon Mudstone)—from which exfoliation produces about 50% conchoidal-shaped spalls—overlying each of the Otau facies, with minor interbedded mudstone and graded sandstone (Paparata Formation). The Waiheke Group belongs to the prehnite-pumpellyite mineral facies; pumpellyite is found only at certain horizons including parts of the uppermost Kawakawa Subgroup. Sediments of the Manaia Hill Group (belonging to the Morrinsville Facies) were deposited in the Tapu Miogeosyncline and are typified by the common presence of chipstone and cannibalistic roundstone conglomerate, and the absence of chert, red mudstone, and spilite. Te Mata Subgroup, which coarsens south-eastwards, is an evolutionary unit suggesting transgression at its base and regression at its top. It is divided into three formations: Paraunahi Sandstone (oldest), Waimana Formation, and Kirita Sandstone. The youngest mudstones of the Te Mata Subgroup exhibit mainly conchoidal spalling and are separated by an erosional break from the overlying Ahimia Conglomerate, which is largely cannibalistic. The Waiheke Group is folded into the Hunua Synclinorium with its central axis near the axis of the Hunua Eugeosyncline. Meridional folding is sinuous, with asymmetric folds on the flanks and open folds more common near the centre of the synclinorium. Folds with both limbs overturned are fan folds and pass along their striks into asymmetric folds, i.e., they are not completely overturned folds, which could be a more localised interpretation. Faulting is that expected from an east-west compression. Besides the Hunua Eugeosyncline and the Tapu Miogeosyncline, several other regional structures of Puaroan age are recognised: the Kawhia Miogeosyncline is separated from the Hunua Eugeosyncline by the Te Puroa Geanticline; the Rangitoto Geanticline separates the Hunua and Tapu geosynclines. Sedimentary and structural evidence suggests that the Hunua Eugeosyncline died southwards as it was replaced by the growing and merging Te Puroa and Rangitoto Geanticlines; northwards it may have been subdivided by another unnamed geanticline containing the Permian rocks of Northland in its core. These geanticlines are considered minor when compared with the Bay of Plenty Geanticline which was present further to the east.