Proposed new reference section for the base of the Bortonian stage (middle Eocene), New Zealand

The boundary between the New Zealand Porangan and Bortonian stages (Middle Eocene) is no longer exposed in the original Porangan type section, and was never documented from the original Bortonian type section. Although the stage boundary is inferred to exist at Hampden Beach, North Otago, a site that has been the Bortonian reference section for many years, it is poorly exposed. Consequently, this paper proposes a new reference section for the base of the Bortonian Stage within an approximately 30m-thick outcrop of Wanstead Formation exposed in Tahuokaretu Stream, approximate to 3km east of Weber, Tararua District (southern Hawke's Bay), New Zealand. Twenty-four samples from four collections have been analysed in support of this proposal. Fossil foraminifera and dinoflagellate assemblages are described. Following current practice, the base of the Bortonian Stage is placed at the stratigraphically lowest sample having the planktic foraminifera, Globigerinatheka index. The underlying Porangan is identified by the benthic foraminifera, Elphidium saginatum. There is no significant change in the paleoenvironment at the Porangan-Bortonian boundary, indicating the boundary is not affected by facies change.