Deep cementation in late quaternary sands near Westport, New Zealand

Abstract

Deeply cemented quartzofeldspathic and ilmenitic sands occur on and beneath uplifted late Quaternary marine terrace surfaces over wide areas south of Westport on the west coast of the South Island. These terraces are weakly dissected, but are mainly flat to rolling. Rainfall averages about 3000 mm per year. Present vegetation consists of pasture, scrub and rush/fern associations which have replaced podocarp forest. Cementation of sands has resulted in impeded drainage, limited rooting volumes for trees and crops, and other limitations to land development. Two soil series, Charleston and Marris, were studied, as they were distinguished by having different forms of cementation in the field. Techniques used to investigate cementation included the determination of Fe, Al and Si contents by extraction with standard chemical reagents, X-ray diffractometry to identify minerals of short-range order, SEM to examine microstructures, EDAX to determine elemental composition, and heating in an induction furnace to evaluate C content. Two apparent causes of cementation were identified, namely ferrihydrite in the Charleston soil, and allophane with humus-aluminium complexes in the Marris soil. Microstructures of sand cemented by these materials were clearly different. In the Charleston soil, iron-rich material coating sand grains and bridging between them is smooth and gel-like and has a fine globular appearance under high magnification. In the Marris soil, the cementing material has a rough-textured fibrous appearance. The contrasts in types of cementing agents are thought to be related to firstly, differences in physiography at time of formation and secondly, different drainage conditions related to topographic position.