Abstract

Soil profiles exposed along the margins of Hutt and Leeman Lagoons contain a succession of calcrete horizons that exhibit oolitic-pisolitic grainstone, lithoclast breccia, and pelloidal wackestone-mudstone stone lithologies. The typical coastal profile consists of an upper laminar calcrete horizon which is usually overlain by a transitional pisolitic loose soil horizon. Underlying the laminar calcrete is breccia and/or massive calcrete. Downward gradation into the host is through a mottled calcrete horizon. Structure, textures, and fabrics are utilized to classify the following different calcrete types: Pisolitic loose soil is composed of sandy soils, calcrete ooids and pisolites, eolianite clasts, and shell fragments. Laminar calcrete is composed of parallel laminated sheets of micritic carbonate that develop by segregation processes. Massive calcrete is dense, relatively structureless calcrete with a "secondary" mudstone fabric, resulting from diagenetic alteration of primary constituents; calcrete mottles form in response to localized diagenesis in the host sediment. Breccia calcrete develops as a result of prolonged weathering, involving repeated episodes of calcretization, dissolution, and brecciation. Diminution of permeability of the host sediment by development of massive calcrete is a major process in genesis of the profile, and occurs in the lowermost part of the zone of alternate wetting and drying where the effect of water input overcomes the cumulative effects of evapotranspiration. Profile development is thereafter topographically controlled and restricted to horizons overlying the massive calcrete. Redevelopment of zones of alternate wetting and drying in older deposits may produce multiple soil profiles.

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