Studies in the Wanganui Series: Pliocene Foraminifera from Wanganui Basin

Abstract

Foraminifera have been examined from seven Pliocene sections in South Wanganui Basin, southern North Island, New Zealand, including the Waipipian and Mangapanian Stratotypes. Lithology, faunal distribution, biostratigraphy and paleoecology are discussed for each section. The systematics and ecologic and stratigraphic ranges of 193 species are discussed. The new genus Zelamarkina is erected, with Z. excavata n.sp. as type species, and the following five species are also described as new: Bolivina hornibrooki, B. vellai, B. wanganuiensis, Rotalia fastigata and Notorotalia briggsi. Three biostratigraphic zones based on benthonic foraminifera are recognised. The Hurupiensis Zone, defined by the range of Notorotalia hurupiensis in South Wanganui Basin, is considered equivalent to the Opoitian Stage. The Molestus Zone, defined by the overlapping ranges of Cibicides molestus and Notorotalia finlayi, represents the Waipipian Stage. The Finlayi Zone equals that part of the range of N. finlayi after the extinction of Cibicides molestus, and extends through to the Recent. Temperatures dropped abruptly and markedly at the time represented by the base of the Molestus Zone, accompanied by a substantial fall in sea level, then rose at the time represented by the end of this zone to a level maintained through the period studied. No marked temperature or sea level change is recorded across the presumed Pliocene-Pleistocene boundary. The region was covered during late Miocene and early Pliocene times by shallow seas, possibly with land to the west. Rapid subsidence of the centre of the basin during the early Pliocene was followed by gradual shallowing until by the late Pliocene uniformly shallow depths again pertained. The basin was bounded to the east by the emergent Ruahine and Tararua Ranges, and probably to the south and west by shallow bars or islands formed from uplifted basement blocks. The surface microstructure of nine species of Bolivina and seven species of Notorotalia is described from scanning electron micrographs. Addition of calcite to the test of Notorotalia is progressive and is controlled largely by the positions of sutural and apertural pores.