Index macrofossils and New Zealand pliocene and lower pleistocene time-stratigraphy

Abstract

The present and suggested stage and substage divisions of the Wanganui Series are discussed and stratigraphic sections in Wanganui, Hawke' Bay, southern Waira-rapa, and North Canterbury are reviewed to determine the ranges of molluscs formerly used to define stages and substages. It is concluded that the Opoitian, Waipipian, Mangapanian, Nukumaruan, and Castlecliffian are recognisable stages, and the Okehuan and Putikian are recognisable substages of the Castlecliffian Stage. The faunal change at the Mangapanian-Nukumaruan boundary (currently accepted as the Pliocene-Pleistocene boundary) is gradational, and there is no molluscan criterion by which the boundary can be recognised in all areas; in North Canterbury it cannot as yet be recognised. The most useful molluscan criterion for the recognition of the Opoitian, Waipipian, and Mangapanian Stages is the lineage of Phialopecten, in which distinct forms are restricted to each stage. The Waipipian is regarded as a full stage because its fauna is more like that of the Opoitian Stage than that of the Mangapanian (with which it was formerly grouped as a substage of the Waitotaran Stage), and because many generic groups such as Austrotoma, Eucrassatella, Manaia, Marshallena, Miltha, and Perirhoe, and several species such as Mesopeplum crawfordi (Hutton) and Austroiosus pagoda Finlay, last appear during the unit. The name Mangapanian is used instead of Waitotaran (restricted) of recent authors to avoid conflict with old usage of the name Waitotaran, and because a Waitotaran “superstage” could still be usefully used where sections cannot be subdivided by means of key molluscs such as phialopecten. The zone of Chlamys delicatula (Hutton), previously known as “the Hautawan Stage”, has diachronous boundaries. Its time range narrows to the north, being from Waipipian to late Nukumaruan in North Canterbury, but restricted to a 12 ft basal zone of the Nukumaruan in southern Wanganui District The "stage" is not recognisable where Chlamys delicatula is absent, and is thus a biostratigraphic unit rather than a time-stratigraphic one, and is designated the Delicatula Ecozone.