Chemical, physical, and mineralogical properties of the cover bed sequence on the Pahiatua terrace north-western Wairarapa

Abstract

The Pahiatua terrace in northern Wairarapa was first investigated by Kaewyana (1980) during mapping of alluvial terraces in the Eketahuna-Pahiatua district. The terrace is the oldest of a series of river terraces and Vella et al. (1988) found the cover bed sequence to include nine loess beds representing cold climate phases of past glaciations and paleosols representing interglacial and interstadials stages. The sequence contains two rhyolitic tephras, the Aokautere Ash (22,560 ± 230 yrs old), and the Mt Curl Tephra (240,000 ± 50,000 yrs old). Andesitic tephric material is present throughout the cover bed sequence. In this study the cover bed sequence has been investigated in detail by examination of the physical, geochemical, and mineralogical properties of 15 cm diameter continuous cores taken from the terrace to a depth of some 18m. Magnetic susceptibility measurements show a systematic fluctuation down the sequence, which is attributed to the presence of titanomagnetite from andesitic tephras that are concentrated in zones throughout the sequence. The zones occur because andesitic tephra continued to accumulate when loess accumulation slowed or ceased during interglacials and interstadials, at which times soil forming processes left their strongest imprint on the loess. The high susceptibility zones therefore record the presence of paleosols. These same zones contain high concentrations of carbon, nitrogen, phosphorus, and manganese, and low concentrations of potassium. This is further evidence for the zones being paleosols. Bulk density and water content curves parallel the fore mentioned changes. Allophane from the weathering of the tephra is also high in the paleosols. A stratigraphy for the Pahiatua terrace bed sequence is presented which contains 14 loess units representing about the last 350,000 years. The chronology of this stratigraphy is correlated with the oxygen isotope record from deep sea cores, and with the loess stratigraphy proposed for the southern Wairarapa, Rangatikei, Wanganui, Taranaki,and the Awatere Vallev in the northern North Island.