The mid-Castlecliffian Mangatuna Formation of the Gisborne district, North Island, New Zealand

Abstract

The type area of the mid-Castlecliffian Mangatuna Formation is described and three members are proposed. The formation unconformably overlies marine Pliocene strata in an area of Quaternary uplift at Gisborne, eastern North Island, New Zealand. The strata dip <5° towards the northwest and were at least 120 m thick (top eroded) near Calhome Station, where the Calhome (oldest), Matokitoki, and Town Hill Members are mapped. The Calhome Member is composed of nonmarine siltstone, tephras, and minor sandstone and gravel, whereas the lower part of the Matokitoki Member, formed of gravel, represents the deposits of a mid-Castlecliffian? braided Waipaoa River. The Town Hill Member contains nonmarine and estuarine molluscs and at one locality fossil tree trunks. At the commencement of deposition of the formation there was, locally, substantial mid-Castlecliffian (mid-Pleistocene) relief of at least 70 m, including the north-trending Valley Road Hill, Mangatuna Ridge, and Calhome valley. This relief was buried by the time the Town Hill Member was deposited. The presence of Matokitoki Member within the Calhome valley implies that the ancient Waipaoa River flowed along the northern margin of the Poverty Bay flats. A mid-Castlecliffian age for the formation is deducted largely from nanofossils. © 1996 Taylor & Francis Group, LLC.