Tuesday, 8 November 2005 - 1:45 PM
185-3

"Tiger Mottles" Subsoil Weathering Features in a Ferruginised Regolith.

Richard MacEwan, Department of Primary Industries, Box 3100, Bendigo Delivery Centre, Bendigo, VIC 3554, Australia

A prominent feature of the weathering profile of a ferruginised regolith in south west Victoria, Australia, consists of strongly contrasting colours (often as red as 10R4/6 and as grey as 2.5Y6/1) in a series of alternate bands approximately parallel to the weathering front and ground surface. Individual pale bands are 10 – 20 mm thick and have thin clay lamellae within them, red bands are generally thicker and may be fringed with yellowish brown (10YR5/8) adjacent to the pale bands. The horizon displaying this banded weathering is usually at least 0.5 m thick and below 1.0 m depth. The colour sequence, grey to yellowish brown to red, across the bands is interpreted as a redoximorphic gradient from an iron-depleted matrix to hydrated-iron (possibly goethite) to haematite. Terms such as “lamellae” (USDA Soil Taxonomy), reticulate or petroreticulate (Australian soil classification), or laminar can be applied to describe these features. Lamellae generally refer to thin horizons of illuviated clay, which while certainly present within the paler bands, are less conspicuous than the red iron rich bands. The strongly developed laterally continuous banding is not well described as reticulate, laminar being a more appropriate term. The term “tiger mottles” has been used casually to describe these features and is proposed as appropriate for general communication. Pedogenesis is a combination of redox and clay illuviation processes that contribute to the breakdown of the ferruginised parent material. Residual haematite-rich cemented material gradually becomes fragmented and isolated in the soil mass where, depending on soil wetness, it may persist as a ferric or ferro-manganiferous nodule (essentially a litho relic) or it may be hydrolised and iron depleted to leave only a soft mottle. Further weathering and deflation of the land surface has resulted in accumulation of rounded ironstone gravels in the A2 or E horizons.

Back to Soil Genesis/Div. S-5 Business Meeting
Back to S05 Pedology

Back to The ASA-CSSA-SSSA International Annual Meetings (November 6-10, 2005)