Stratigraphy and formation of soils on dolerite and their implication for landscape history in southeastern Tasmania

This study presents work from Mt Nelson and Tolmans Hill in southeastern Tasmania. Detailed field observations were combined with particle size and elemental distributions, mineralogical characteristics and radiocarbon dating information in order to investigate soil stratigraphy and pedological relationships of soils developed on Jurassic dolerite, and hence to improve understanding of soil formation history on dolerite. Field studies of soil morphology and distribution indicated that the soil profiles were stratified, and their properties and distribution are closely related to their position on landscape, site drainage and the depth of dolerite weathering. This study indicates that local dolerite has been subjected to both deep weathering and severe erosional periods. Pockets of deeply weathered dolerite occur adjacent to thin topsoil on bedrock (A/C soils) or hard outcropping rock. Deeper colluvial soil materials occur on lower slopes. The presence of protruding dolerite columns now largely buried by transported clayey slope-wash materials , indicate partial landscape stripping followed by re-burial. The presence of buried stone-lines separating the upper profile from the clayey subsoils supports the idea of a second major erosional-depositional cycle. A pronounced variation between the A and B horizons particle-size distribution, mineralogy and elemental distribution supports the conclusion that the modern soils are composed of several sedimentary layers which cap a variable thickness of in situ weathered dolerite (termed \mealy material\") above fresh dolerite. Bedrock jointing veins and rock fabric extend upward from the bedrock into the mealy material but are truncated abruptly at the contact with the clayey subsoil. Soil forming processes have operated to modify soil colours and mottling soil structure and cation chemistry. Radiocarbon dating of an A2 horizon profile TH1 (grey brown podzolic) to 7500 years BP indicates that the aeolian transport process may have continued to the early Holocene during which soil disturbance and movement have occurred. A second radiocarbon date of approximately 5500 years BP on profile WW (black vertosol) at 190 - 200cm indicates recent soil creep of colluvial activity occurred during the mid Holocene. In the same soil the presence of an aboriginal artefact at 88cm with an age of 2000 - 5000 years indicates a strong link between the soil disturbance and aboriginal occupation during mid - late Holocene. The findings of this study have improved understanding of the formation and distribution of dolerite derived soils in lowland SE Tasmania and suggest that the previous theory on in situ weathering of soils formed on dolerite could not be applied to all dolerite-derived soils that cover almost one third of Tasmania."