Orientation and Spacing of Columnar Peds in a Sodic, Texture Contrast Soil in Australia

The orientation and size of very coarse columnar peds in a sodic, texture contrast soil in Australia is largely influenced by the inherited fracture system from the underlying bedrock. The study is based on field measurement of fracture metrics including aperture (width), depth of penetration and orientation revealed along 36m of trenches and from of over 100 columnar peds displayed in a 50 m$

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$ area where the topsoil had been removed. Four classes of fractures (mega, major, minor and incipient) were used to explore these associations more fully. The first three classes are continuous and define the columnar peds, whilst the incipient fractures represent a partial fracture across a dome. An orientation analysis indicates that the fracturing of the saprolite into large polygons is not random, but follows preferred orientations that coincide with jointing in the underlying bedrock. These joints define mega blocky peds (>1 m$

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$) of which the pentagonal to hexagonal columnar peds are a subset. These features indicate that brittle fracturing is the main mechanism involved in the initiation of columnar peds at the study site. A tentative model for the development of polygonal fractures in the saprolite is proposed in which relict joints in the bedrock provide the zones of weakness along which initial fracturing of the saprolite occurs. These fractures relieve tension produced by seasonal desiccation. Superimposed on these widely-spaced deep fractures is a smaller, shallower more random system of fractures that relieve surficial tension developed by rapid desiccation after rain. However, the presence of flow structures on the surface of domes and between domes indicates that ductile deformation occurs too, presumably when the soil expands on wetting and the apparent explosive nature of some of these may signify the release of confining pressure even though the capacity for expansion is not particularly high (COLE 7.7). It is possible that this behaviour may assist in the development of domes (the convex-up top of columnar peds) though a full explanation of this feature has yet to be developed. It is suggested that brittle fracturing is the prime factor involved in the development of several ped types including columnar, prismatic, blocky and parallelepiped and that this mechanism needs to be included with aggregation into a general theory of peds. In contrast gilgai, that also shares a similar polygonal structure to columnar peds, is normally viewed as responding to ductile deformation and associated shearing.