Pre-miocene lithostratigraphy of taranaki basin, New Zealand

Abstract

The stratigraphy and sedimentology of the Paleocene-Oligocene sedimentary sequence of Taranaki Basin is reviewed using data obtained from exploration wells. Sixteen of these wells are sited in the Taranaki Graben, an infilled, down-faulted block extending westwards from the Taranaki Fault to the Cape Egmont Fault Zone. A further seven wells are sited on the Western Platform, which extends westwards from the Cape Egmont Fault Zone seaward to beyond the edge of the continental shelf. In the past, the lithostratigraphic nomenclature applied to the sequences encountered in the wells was adopted from adjoining onshore areas. Where existing names were inadequate, new names were created, but never formally defined. The term “Kapuni Formation” is an informal name for the Paleocene-Oligocene sandstone-coal measure sequence which was first encountered in the Kapuni-1 well. It is proposed that the thick sequence currently known as the Kapuni Formation be formally named the Kapuni Group. Kapuni Group is subdivided into four formations. From oldest to youngest they are Kaimiro Formation, Omata Formation, Mangahewa Formation, and McKee Formation. A lateral, fully marine equivalent of the Kapuni Group is described. A further three formations are defined. The Whaingaroan marine mudstone (Kaiata Formation of Shell BP and Todd Oil Services Ltd) is here named the Turi Formation; the Whaingaroan-Wai-takian calcareous siltstone sequence informally known as the lower Mahoenui Group is named the Otaraoa Formation; and the Waitakian limestone immediately underlying the Mahoenui Formation proper is here named the Tikorangi Formation The stratigraphy of Taranaki Basin is compared with that of adjoining onshore areas to the north and south. Similarities are apparent and posszible northern and southern correlatives are proposed. The environments of deposition of the various units range from bathyal to outer shelf through inner shelf to lagoonal and terrestrial. Based on these interpretations, a paleogeographic reconstruction of Taranaki Basin shows a major pre-Miocene transgression. It progressed across the basin in a generally southeasterly direction. The proximity of sediment source combined with a transgressive shoreline accounts for the diachron-ous nature of the lithostratigraphic boundaries. © Crown copyright 1985.