Recognition and correlation of subdivisions within the Whakamaru Ignimbrite, central north island, New Zealand

Abstract

Throughout most of its 2500-km² area, Whakamaru Ignimbrite has simple cooling zonation with few visible partings between individual ash flows. However, the multiflow nature of the formation has been revealed by detailed examination of its mineralogy, texture, and magnetic properties. Near source in the Western Bay, Lake Taupo area, the exposed upper part of Whakamaru Ignimbrite can bc divided into three subunits (each representing one or more ash flows) of contrasting degree of crystallisation. These subunits can be identified over distances of up to 14.4 km. To the north, at Whakamaru, Maraetai, and Waipapa on the Waikato River, the ignimbrite can be divided into six subunits on the basis of its mineralogy, texture, and previously determined magnetic properties. Using mineralogy and texture al'one, the approximate location of two of the subunit boundaries can be located in sections through distal Whakamaru Ignimbrite (=“Mananui Ignimbrites”) as far north as Arapuni, a total distance of about 42 km.