Parent materials of Tirau silt loam

Abstract

The parent materials of Tirau silt loam (a moderately leached yellow-brown loam/Humic Andosol) at the Reference Site near Tirau have been established with the aid of a correlation line carried westwards over the Mamaku Plateau from Rotorua where the individual tephras can be identified. However, supplementary laboratory tests were needed to establish the identity of some tephras that have been removed by erosion over an intermediate part of the correlation line. In addition to air-fall tephras, several beds of loessial material were recognised. These have originated from rhyolitic tephras that were erupted towards the end of the last glaciation, and lie immediately above the tephras from which they have been derived. Although the tephras recognised are all rhyolitic, frequent additions of andesitic ash from sources to the south have altered the physical and chemical properties of the soil so that it resembles a yellow-brown loam rather than the expected yellow-brown pumice soil. No evidence has been found