A Late Quaternary History of the Awatere and Wairau Valleys, Marlborough, New Zealand

Abstract

The Awatere and Wairau valleys have prominent river terraces, many of which are mantled in loess which is up to 20 m thick in the lower Awatere valley. Marlborough lies within the active tectonic belt which extends north-eastwards through central New Zealand. The Awatere and Wairau valleys strike north-east along the Awatere and Wairau Faults. High rates of deformation occur, and uplift rates in each valley increase upstream from the coast. The lower 30 km of the Awatere valley, and a central 50 km section of the Wairau valley including the major tributary Waihopai, have been studied in detail. Terrace treads have been grouped into sets, with aggradation alluvium uppermost in each set. Eight loess layers, in order of increasing age (Awatere, Starborough, Downs, Upton, Clifford, Muritai 3, Muritai 2, Muritai 1) have been recognised in the Awatere valley. Except for Awatere Loess, each layer is named after the terrace set comprising former flood plains from which the loess was generated. The base of each loess layer corresponds to the onset of river aggradation. The Marlborough loess accumulation model envisages continual loess accretion, with most loess accumulating during river aggradation. Three loess layers recognised in the Wairau valley have been correlated with Starborough, Downs and Upton Loesses using the c. 20 000 years B.P. Kawakawa Tephra, which occurs in Starborough Loess and flood plain relation diagrams. Stratigraphic variations in the abundances of major elements from three loess cores confirm boundaries drawn from morphology between the upper six layers. The greatest morphological and chemical contrasts occur where Btg horizons overlie Cx horizons. Heavy minerals indicate a loess provenance within the respective catchments. Most silt appears to have been sorted fluvially prior to deflation from flood plains to become loess. The onset of Starborough Loess accumulation is estimated from the Stratigraphic position of the Kawakawa Tephra, and the onsets of Downs and Upton Loesses are estimated using flood plain relation diagrams. Clifford, Muritai 3, Muritai 2 and Muritai 1 Loesses which largely represent cold climate accumulations are related to cold climates in the oxygen isotope and sea level records. An independent time framework is provided by the tentative chemical correlation of volcanic glass shards in Muritai 3, Muritai 2 and Muritai 1 Loesses with North Island tephras. The presence of the 20 000 years B.P. Kawakawa Tephra near the top, and the c. 240 000 years B.P. Mount Curl datum near the bottom of the stratigraphic column, allows a direct comparison with North Island sequences, where a similar number of loess layers occur. River regimens, loess morphology and accumulation rates are used to interpret past climates, and these climates are compared with the present. From 25 000 - 10 000 years B.P. it was colder, probably drier, with more droughts; from 10 000 - 1000 years B.P. temperatures were similar to today, but it was wetter in the early part. The resumption of river aggradation and loess accumulation within the last 1000 years is attributed to increased erosion resulting from Man's activities combined with stormy periods.