Petrology and mineralogy of ‘laterites’ in southern and eastern Australia and southern Africa

Abstract

Field observations of ‘laterites’ in southern and eastern Australia and in southern Africa reveal a variety of ferruginous horizons and crusts referred to herein as ferricretes. Their geomorphic and stratigraphic relationships with bedrock, sediments and soils indicate formation throughout long intervals of geological time in landscapes which are also characterised by zones of bleached and iron-mottled materials. There does not appear to be a genetic relationship between the ferricretes and the weathered zones in the sense of the so-called ‘laterite profile’. Many of the ferricretes form part of existing soil profiles. Petrographic studies of a variety of ferricretes have identified three broad categories: (a) ferruginised bedrock; (b) Fe-impregnated and -indurated sediments, including sands, clays and organic sediments; and (c) ferricretes of complex sedimentary and pedogenic origin. Type-(a) and -(b) ferricretes characteristically have simple fabrics, often with single-generation, secondary Fe-oxides. Type-(c) ferricretes have complex fabrics, with many generations of hematite, goethite and in some variants, gibbsite, in the matrix and in ferruginous clasts and pisoliths. Maghemite is a common constituent of the pisoliths. The characteristics of ironstone gravelly duplex soils, which are common in the contemporary landscapes, provide the framework for a model involving multiple stages in the development of these ferricretes. The origins of the various secondary oxide minerals in ferricretes are assessed on the basis of knowledge about the formation of these minerals in pedogenic environments. Examples are given of the intricate patterns of distribution of the minerals in thin section from which definitive data may be obtained on environmental conditions for integration with field-based geomorphic studies.