Aokautere ash in the Manawatu District, New Zealand and its significance to soils, and soil formation in the sand country of the Manawatu-Horowhenua district, New Zealand

Abstract

An ash bed - named Aokautere Ash - is described from the Manawatu district in the southern part of the North Island. The ash extends over terraces older than the Ohakea Terrace and also occurs in dune-sands of the Koputaroa district and in material covering the tops of the northern Ruahine Ranges. The ash is a distinctive marker bed and provides a convenient key horizon near the Pleistocene-Holocene boundary that will be useful for correlating terraces and beds with which it is associated: It is tentatively correlated with the Rotoma. Ash from the Rotorua district which is considered to be approximately 7,000 years old. Indirectly, the ash has considerable pedological significance. It is overlain and underlain by up to 10 feet of substantially uniform, fine-textured material that extends indifferently over several terraces. This material is considered to be loess derived from river sediments and from coastal sand dunes. In places the loess may contain fine volcanic ash. Differences between soils developed on this loess in the Manawatu district depend largely on the texture, rate of accumulation and composition of the loess and are independent of the age of the terraces on which they are formed. Tokomaru silt loam, a zonal yellow-grey earth, is formed on the upper few feet of thick loess deposits which are considered to be less than 7,000 years old. This gives some measure of the time necessary to reach zonality in the soils of the yellow-grey earth group.