Late Quaternary alluvial terraces and their cover bed stratigraphy, Eketahuna and Pahiatua Districts, New Zealand

Abstract

The Upper Pleistocene terraces, loess beds and tephras in the Eketahuna and Pahiatua districts, North Island, New Zealand are mapped and correlated. Sequences of seven aggradation terraces and eight loess beds are established. The aggradation terraces are related chronologically to loess beds in the area. The Upper Pleistocene history of the area is interpreted, and is compared and related with Upper Pleistocene events in Rangitikei and Wairarapa districts. Terraces were mapped from aerial photos and were correlated by directly following out closely spaced remnants in the field; by comparing features on the terrace treads in the field; by studying the terrace cover beds, especially the nature and thickness of loess mantles and by determining the plotted terrace profiles along the river valleys. Terrace heights were determined by spot heights and heights of trigonometric stations on the N.Z.M.S. 270 maps and by theodolite. Seven aggradation terraces in order of increasing age - Hukanui, Pukewhai, Eketahuna, Greenhills, Flat Top, Nireaha and Pahiatua have been mapped. They are considered to have been formed during episodes of rigorous climate which devegetated and caused overloading of rivers draining the ranges. The terrace alluvium is mostly gravel, derived from greywacke sandstone of Mesozoic age. The terrace cover beds are mostly loess with some tephra layers intercalated within the loess. Eight loesses in order of increasing age - Hukanui, Pukewhai, Eketahuna, Greenhills, Flat Top, Ridge Road 1, Ridge Road 2 and Ridge Road 3 were recognised. They were derived from the alluvial flood-plains of the rivers and were deposited during the cold period when the rivers were aggrading. Five tephras in order of increasing age - Aokautere Ash, Cliff Tephric Paleosol, Mangamutu 1, Mangamutu 2 and Pahiatua were recognised. Except Mangamutu 2 all tephras have been recognised and named in areas other than the mapped area. They originated in the Rotorua-Taupo and Tongariro National Park and Egmont volcanic regions.