Saturday, 15 July 2006

Landscape and Soil Profile Development in a Dissected Coastal Plain.

Richard MacEwan, Dept of Primary Industries, PO Box 3100 Delivery Centre, Bendigo, VIC3554, Australia

In a recent Tertiary (Neogene) coastal plain in south-west Victoria (Australia), landscape evolution and pedogenesis can be explained in relation to processes dominated by ferruginisation, subsequent redox weathering, gleying, clay illuviation and podsolisation. Soil distribution is related to landscape position and proximity to the coast. Soil distribution on the relict plain is easier to predict than in the valleys where dissection of the landscape has been dominated by landslides, which have served to redistribute materials on slopes and valley floors in a complex manner. This paper describes the landscape and proposes processes that have produced the contemporary soil profiles. A conceptual model for understanding the distribution of different soils in this landscape and their implications for management is presented. The landscape originates from a large graben that was progressively filled, over the last 90 million years, first with terrestrial sediments and subsequently marine sediments (clays and marls) and ultimately coastal outwash and alluvial material (sands and gravels). Today, a remnant plain of coastal material (Hanson Plain Sand and Moorabool Viaduct) overlies the marine marl (Gellibrand Marl), which has been exposed on valley sides by dissection of the plateau. The coastal plain material was strongly ferruginised two to six million years ago and subsequent weathering of the ferricrete has resulted in accumulation of large quantities of ironstone gravel in the E or A2 horizons of soils in much of the landscape. A conspicuous feature in the ferricrete weathering profile is strongly contrasting alternate bands approximately parallel to the weathering front and to the ground surface with colour often as red as 10R4/6 and as grey as 2.5Y6/1. Individual pale bands are 10 to 20 mm thick and have thin bands of clay 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. This feature has been referred to as ‘tiger mottling'. The upper two to three metres of the regolith profile can be viewed essentially as a weathering chronosequence with residual partially weathered ferricrete at depth, overlain by mottled material (often tiger mottles) at about 1.5 metres depth. The depth of maximum weathering occurs at around a metre depth and exhibits features dominated by redox processes and clay illuviation. Depending on the redox regime hard ferro-manganiferous gravels may be present or absent. The upper part of the soil profiles are dominated by clay loss and waterlogging and characteristically have bleached E or A2 horizons (with or without Fe-Mn gravels). Gravels have been mobilised in the E horizons in wet conditions and are sometimes found as re-cemented pisolitic masses. Near the coast, siliceous sands have been reworked on the residual plain covering the land surface with acidic, nutrient poor, material and Spodosols have developed with strongly cemented thick spodic horizons, described locally as ‘coffee rock'. Spodosol profiles are sometimes superimposed on the older Alfisol profiles, and, in such cases, the E or A2 horizons show a dull mottling that is the result of gradual loss of iron and disintegration of the Fe-Mn gravels. Perched watertables are common in winter in the Alfisols and the Spodosols. The valley sides are characterised by old and recent landslides and the soils are dominantly heavy clays. Convex slopes with better drainage have soils with calcareous B2 horizons, whereas others are strongly gleyed and have no remaining calcium carbonate within the soil profile. Landslides have been responsible for the redistribution of the coastal sand material as colluvium and alluvium in lower slope and valley floor sites. Older, fragmented buried surfaces occur in colluvial positions as a result of sequential land slips. Alluvial soils range in texture from sandy loams derived from the coastal plain to heavy clays derived from the marine marl. The sandier alluvial soils have developed profiles similar to the plateau Spodosols with Ah, E, occasional thin Bhs, and well developed Bg horizons. Management of this landscape for agriculture requires an understanding of the soil distribution, their hydrological characteristics, susceptibility to waterlogging, vulnerability to damage when wet and their suitability for subsurface drainage.

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