Lower Miocene geology of North Kaipara, northern New Zealand, with emphasis on the paleontology

Abstract

Onerahi Supergroup (new) is erected to incorporate most of the Cretaceous - Paleogene sediments in west Northland (ie Patutahi and Callaghan Formations, Mangakahia and Opahi Groups, and revised Waiomio and Motatau Groups, all of Hay 1952, 1960). The Lower Miocene Akarana Supergroup (Ballance et al 1977) is extensively revised. The new Kapowairua Group (mid Waitakian to early Otaian) is erected for volcanic-rich biomicrites and associated volcaniclastic sediments that unconformably overlie Onerahi Supergroup and Whangakea Volcanics (Bell and Clarke 1910). It incorporates Wainui Siltstone (Waterhouse 1966), Tom Bowling Formation (Leitch 1970), and Puriri Formation (Ballance 1976a). The latter includes the new Te Kopua Member. The Waitemata and Waitakere Groups and their respective subgroups (Ballance 1976a, Hayward 1976a, Ballance et al 1977) are revised, with the Kaipara Subgroup (Ballance 1976a) becoming redundant. Waitemata Group in north Kaipara (Warkworth Subgroup: revised after Ballance 1976a) is of early to late Otaian age and includes three formations. Timber Bay Formation is revised after Carter (1971) and Ballance and McCarthy (1975), and the new Waikanai Member is erected. Matapoura Conglomerate (Carter 1967, Ballance and McCarthy 1975) is elevated to formation status, and includes the new Hargreaves Member and informal Ngamotu Slump. Waihangaru Member (Ballance 1976a) is also elevated to formation status and incorporates the new Waitieke Member. Okahukura Formation (Ballance and McCarthy 1975) is made redundant. The Waitakere Group in north Kaipara (Hukatere Subgroup: revised after Hayward 1976a) includes six formations and is of latest Otaian to probable mid Altonian age. Pakaurangi Formation (Jones 1969) is revised to include the Waiteroa, Hollands, Waipukua, Funnel, and Arapaoa Miogypsina Sandstone Members (all revised after Jones 1969), and the Waipahihi Member (revised after Carter 1971). Oruawharo Hyaloclastite Member (Ballance and McCarthy 1975) is elevated to formation status, and Pakaurangi and Tapu Members are made redundant. Puketi Formation (Jones 1969) is revised to include the Yellow Point Sandstone and Waititi Tuff Breccia Members of Jones (1969), and the new Tinopai and Ngawhero Members. Motuouhi Formation (Carter 1971) is retained. Pupuia Formation (new) is erected for intrusive and extrusive lavas within Hukatere Subgroup, and incorporates the Morgans Point Basalt Member of Jones (1969). Okaroro Formation (new) is erected for volcaniclastic sandstones and breccia on northwest Hukatere Peninsula. Paleontologic studies have led to recognition of additional biostratigraphic criteria for determining the Otaian/Altonian boundary (species replacement in Vaginella) and lower/mid Altonian boundary (first appearance of Globigerinoides tri lobus Form B populations). New species are described, and some overseas taxa are recorded for the first time from the New Zealand Tertiary within phyla Foramini feri da, Mollusca and Coelenterata. Foramini feral associations, scleractinian coral associations, and paleocommunities are described, and their paleoecologic significance is discussed. Most observed spatial and temporal faunal changes in north Kai para can be related to varying water depth, oxygenation of bottom waters, and substrate type. Rapid temporal changes in habitat produced abrupt turnover of associations/paleocommunities, while gradually changing physical parameters produced heterogeneity within an association/paleocommunity, or its transition to another. Most shelf and uppermost slope paleocommunities were both biologically and physically controlled. Bathyal paleocommunities and those in "high-stress" shelf environments were predominantly physically controlled. Paleogeographic/paleoenvironmental studies indicate that a bathyal flysch basin existed in the north Kaipara area from early to late Otaian. Bottom waters within the basin were poorly oxygenated. In the early Otaian thinly-interbedded mudstone-sandstone of Timber Bay Formation accumulated unconformably on a highly irregular surface of Onerahi Supergroup. Sandstones have Onerahi Supergroup and volcanic provenance, and were deposited by southwards-travelling mass flows. In the mid Otaian uplifted fault-blocks northeast of Hukatere, Puketotara and Okahukura Peninsulas isolated the basin from the northern source area. Sediment derived from a western source area (Onerahi Supergroup and igneous provenance) was channelled eastwards into the basin, where it accumulated as an elongate submarine fan extending south eastwards over Puketotara and Okahukura Peninsulas. Proximal conglomerate and gravelly sand (Matapoura Conglomerate Formation) were deposited in a broad inner-fan channel by traction and mass-flow processes, and mud and fine sand (Waihangaru Formation) accumulated in adjacent areas as a result of overbank spill and hemipelagic sedimentation. In the latest Otaian (Waitemata/Waitakere Group boundary) the basin floor was tectonically elevated to shelf depths, and volcanism began on Hukatere and Okahukura Peninsulas (Pupuia and Oruawharo Hyaloclastite Formations). Muddy, fine sand (Pakaurangi Formation) was deposited over much of the Kaipara shelf in the latest Otaian - earliest Altonian, with tuff, lapilli-tuff, and volcani clastic gravelly sand (Oruawharo Hyaloclastite, Okaroro and Motuouhi Formations) accumulating adjacent to submarine eruptive centres. During the early Altonian the northern part of the Kaipara shelf shallowed, and the Hukatere area became emergent. Pyroclastic eruptions from breccia pipes on Hukatere Peninsula and northeast of Puketotara Peninsula (Ngawhero Member) mantled the low-lying, swampy land surface with tuff-breccia lahar deposits, ignimbrites, and airfall tuff and lapilli-tuff (Puketi Formation). Tentative models of the Cretaceous-Paleogene geologic history of northern New Zealand, and the early to mid Miocene evolution of the Waitemata Basin are presented.