Dune-building phases in the Manawatu district, New Zealand

Abstract

Dune sands with four different stages of soil development cover 330 square miles in the coastal Manawatu district. The oldest—the Koputaroa Phase—is restricted to small areas at the north and south; the other three phases—the Foxton, Motuiti, and Waitarere—form belts parallel to the coast, the oldest belt being the furthest inland. The Koputaroa Phase is considered to be Late Pleistocene or early Holocene and 20,000 to 10,000 years old. The Foxton Phase is older than the eruption of the Taupo Shower and about 4,000 to 2,000 years old. Sands of the Motuiti Phase overlie traces of Maori occupation and a stump that is about 750 years old, and are about 1,000 to 500 years old. The Waitarere Phase started about 100 years ago and is still accumulating. Sharp differences in soil development indicate that dune formation was discontinuous and much more rapid at some periods than others. The two youngest phases are considered to have been triggered off by the destruction of vegetation on the stabilised dunes near the coast and in the inland river valleys that followed the arrival of the Maori and European. This led to renewed wind erosion of the dunes and to accelerated erosion in the valleys, which caused more sand to be brought to the beaches. The causes of the two earliest phases are unknown. In an appendix, the pollen floras in a peaty layer associated with the Koputaroa Dune-sand are described and discussed. The pollen results indicate a climate considerably cooler than at present, which is attributed to a late phase of the Last Glaciation.