Holocene alluviation and transgressive dune activity in the lower Manawatu Valley, New Zealand

Location of an estuarine bed near Opiki enabled the approximate mid Holocene limit of the Manawatu River estuary to be determined. Since about 6000 years B.P., flood-plain accretion in that area has averaged 1.5 mm/year, although accretion rates may have diminished with time. Two riverbank sections near Rangiotu provide evidence for the initial encroachment of Foxton Phase dunes. Thin sand horizons overlying the former flood-plain surface are identified as eolian veneer deposits, preserved at the rear of former migrating dunes, which constructed sand plains. Radiocarbon dates obtained from interbedded organic material reveal that dunes first entered the area c. 2300 years B.P. with further episodic encroachment until c. 1600 years B.P. As the Foxton Phase dunes had migrated c. 16 km inland there is no absolute date for the commencement of the phase but study of contemporary rates of dun~ advance suggests that the Foxton Phase may have been initiated at the coast about 550~6oo0 calendar years ago. Attempted correlation of the Foxton dune phase with Australian and New Zealand phases is considered premature owing to insufficient radiocarbon dates and the difficulties imposed by the timetransgressive nature of major dune everits.