Parent material stratigraphy of an egmont loam profile, Taranaki, New Zealand

Abstract

The Egmont loam of Taranaki, New Zealand, is regarded as a classic andosol developed in andesitic tephra (a yellow-brown loam in the N.Z. genetic soil classification or an entic dystrandept in the US. Soil Taxonomy). Variations in grain size distribution and mineralogy within a representative profile show it to consist of two distinct units, an upper unit of andesitic tephra and a lower unit, containing up to 30% quartz, which is interpreted as a tephric loess. Correlation of peaks in andesitic glass distribution within the profile with eruptions from Mt Egmont suggest an accumulation period of circa 10000 years for the tephra unit, while the presence, in places conducive to its preservation, of the Aokautere Ash, a rhyolitic ash of widespread distribution in the Central North Island, dates (NZ1056A) the base of the profile at less than 19 850 ± 310 years B.P. Peaks in distribution of the minor rhyolitic glass component in the tephra unit are correlated with three major post-glacial rhyolitic eruptions from the Central North Island; the Taupo eruption of 1840 ± 50 years B.P. (NZ1548A), the Waimihia eruption of 3440 ± 70 years B.P. (NZZA), and the Rotoma eruption of 7330 ± 235 years B.P. (NZ1199A). Variations in the rate of quartz accumulation in the silt fraction of the Egmont profile are correlated with climatic changes, a higher rate of quartz accumulation occurring during the colder climate of the last stadial, in contrast with a lower rate of quartz accumulation occurring during the warmer climate of post-glacial time.