Mineralogy, stratigraphy, and provenance of soil coverbeds in the Kumara district, Westland

The sand and silt mineralogy of four soils preserved on fluvioglacial outwa sh terraces at Kumara indicates that the main soil body is developed in up to I in of loessial coverbeds that accumulated subsequent to cessation of g ravel terrace formation. Oxygen isotope abundances of the dominant mineral quartz in these soils show a higher temperature origin for the > 250 mum fr action, consistent with derivation from local granodiorites. Finer quartz i s of a mixed provenance origin consistent with the variety of rock types in the region. The finest quartz, <5 mum, of aerosolic origin, can be used as a proxy time record for the coverbed accumulations. The widespread 22 590 yr BP Aokautere Ash chronohorizon is preserved in thr ee of the profiles, whilst wind redeposition of the glass shards is respons ible for traces of rhyolitic glass in all subsequent horizons. Widespread landscape erosion is postulated either immediately before or dur ing deposition of the K-1 terrace, so that similar subsequent coverbed sequ ences are preserved on the K-1 and all older terraces in this district. Mag netic mineral assemblages in the profiles indicate a slightly different loe ssial provenance for the Cockeye, K-1, and K-2.1 profiles, but the K-2.2 is significantly different from the rest. This is probably a function of aeol ian winnowing and variability of source materials. In the past, different m ountain catchments have connected with different lower river reaches, due t o transcurrent movement on the Alpine Fault. This is interpreted as having led to changing provenance of river sediment supplying loess to local soils .