Kawakawa Tephra in Wairarapa, New Zealand, and its use for correlating Ohakea loess

Abstract

In the eastern part of Wairarapa, the 20 000-year-old Kawakawa Tephra occurs as a 0.060–10 m band of coarse silt to fine sand with medium and fine ash size glass and pumice fragments within the uppermost extensive loess unit. Loess enclosing Kawakawa Tephra occurs mainly on alluvial fan and terrace deposits' older than the Waiohine Surface. The stratigraphic position of Kawakawa Tephra within the loess enables confident correlation with the Ohakea loess of Rangitikei and Manawatu. Kawakawa Tephra and enclosing loess have contrasting particle shape, particle packing, grainsize distribution, and mineralogy. Consequently, the dry bulk density and natural water content of the tephra, where thick and coarse, differ markedly from that of the loess. The dry bulk density/natural water content method can usually be used to locate the position of the tephra within the loess column at sites where the tephra is not macroscopically visible. Identification of Kawakawa Tephra is confirmed by ferromagnesian mineralogy.