Principles of soil occurrence in the lower Coal River Valley, South-East Tasmania

Techniques and approaches from the disciplines of pedology and geomorphology have been integrated to investigate the distribution and properties of soils, the development of landscapes, and to provide insights about previous climates and pedogenic processes in two areas in SE-Tasmania. The study areas include a coalesced fan landscape on a farm near Cambridge owned by the University of Tasmania, and a landscape of predominantly valley floor alluvial deposits in the Coal River Valley north of Richmond. Stratigraphic investigations show the landscape of the University Farm to be a pediment cut in predominantly Tertiary sediments covered by a veneer of Quaternary clastic deposits, with isolated outliers of Jurassic dolerite and Triassic sandstone rising above it. Stratigraphic units have been identified in the pedisediment based on differences in composition (source and nature of the sediment) and age (relative position in the landscape, degree of weathering and dissection). Sedimentological evidence suggests that these units have been formed by processes of lateral planation and alluvial and debris flow deposition ie., processes associated with both pediment and alluvial fan formation. Soil properties are related to the characteristics of each stratigraphic unit. They are attributable to both the composition and age of the unit. Sand infills have been identified in some soils and investigated using quartz grain morphoscopy and granulometric analysis. The infill phenomenon can be used as a time stratigraphic marker and provides insights about previous climatic conditions. Ferricrete and silcrete deposits were shown to have formed over discrete intervals of time due to precipitation from iron and silicon bearing groundwater flowing from higher level Jurassic dolerite and Triassic sandstone across hydraulic discontinuities in Tertiary sediments. The Coal River Valley has been shown to have a complex distribution of Quaternary sediments. These include alluvial deposits in a sequence of alluvial terraces (five stratigraphic units identified), pedisediment deposited on a surface cut in Tertiary basalt and Tertiary sediments (two stratigraphic units identified), and aeolian deposits (two units described). Soil profile classes have been identified developed in the stratigraphic units. Chronofunctions of properties of soils on alluvial stratigraphic units have been established, the most useful of which are depth functions of %Total K (best for soils on older units) and %Total P (best for soils on younger units). Soil morphological development with age has been found to be generally consistent with findings of previous studies in eastern Australia.