Models of soil development and distribution on alluvial, composite and debris-flow fans : case studies, Central Canterbury intermontane region

Abstract

Fans are a major landform of piedmont plains, intermontane basins and wide-floored valleys. This study presents landform-regolith-soil pattern models for small fans, 40 to 500 ha, based on case studies in Cass basin, eastern South Island mountainlands.

Field and scale model studies indicate that alluvial and debris-flow transport give rise to distinguishable surface styles (micro topographic patterns). Three fans representing distinctly different fan building processes were surveyed. Field observations were made using free survey techniques and maps detailing surface style, sedimentary deposits, geomorphic surfaces and soil profile forms were compiled.

Soil stratigraphy established the chronological sequence of geomorphic surfaces and identified buried geomorphic surfaces. This allowed temporal and spatial soil patterns to be recognised. Within the identified geomorphic surfaces the distribution of aggrading, composite and compound classes of simple soil profile forms were related to surface style and sedimentary deposits.

The results revealed a dominant radial organisation to surface style, sedimentary deposits and soil pattern for all the fans in this study.

The fans in this study displayed distinct and different patterns of surface style, ranging from braid bar and channel surface styles on the alluvial fan to debris-flow lobes, levees and associated channels on the debris-flow fan.

Plots of mean grain size and sorting of samples from the fans indicated that a clear distinction could be made between debris-flow and alluvial sediments. Alluvial transport tended to produce better sorted, open packed, clast supported fabrics while the more poorly sorted, matrix supported debris-flow sediments had high matrix densities, low permeabilities and a concrete like appearance. Some alluvial and debris-flow deposits were not easily distinguished in the field. A wide range of geomorphic surfaces and buried geomorphic surfaces indicated by soil profile development was a feature of all the fans in this study.

The Alluvial fan was characterised by an unweathered debris mantle predominantly with stage 1 soil development and a smaller proportion of stage 2 soil development. The soils were sands to sandy loams (fine earth) and gravels (whole soil) overall.

The Composite fan was characterised by a well expressed debris-mantle regolith, predominantly stage 3 soil development plus areas of stage 1 and 2 soil development. The soils were silt loams (fine earth) and muddy gravels (whole soil) overall.

The Debris-flow fan was characterised by a very well expressed debris-mantle regolith with predominantly stage 4 soil development with small areas of stage 5 soil development. The soils were clay loam (fine earth) and muddy gravels to muddy sandy gravels (whole soil) overall.

A complex history of erosion and deposition is envisaged for each fan in this study; the pattern of geomorphic surfaces and buried geomorphic surfaces within each fan reflects previous cycles of instability originating in the catchments associated with each fan and the pattern of soil development is a record of periods of stability between such events.

No major consistent differences, with respect to soil texture, were recognised between hypothetical fanhead, midfan and fantoe zones.

Surface styles, often used as a basis for delineating soil patterns, were only partly related to the distribution of geomorphic surfaces and soil mapping units.