Soil development on dolerite and its implications for landscape history in southeastern Tasmania

Soil genesis has been examined using field description, particle-size distributions, chemical properties, mineralogy and elemental distributions of five soil profiles developed on dolerite on Mt. Nelson and Tolmans Hill near Hobart in Tasmania. The soils form a sequence ranging from a Black Vertosol (P8) to four texture contrast soils, namely, a Eutrophic Brown Chromosol (P5), two Mottled-Subnatric Grey Sodosols (MN8 and P4), and a Mottled-Mesonatric Grey Sodosol (P7). The soil stratigraphic and pedological relationships of these soils have been investigated to help understand their distribution and improve understanding of soil formation history. The knowledge of the soil stratigraphy and weathering features aided in the determination of the broader landscape history. The field observations show the local dolerite has been subjected to both deep weathering and severe erosional periods. Pockets of deeply weathered dolerite occur adjacent to thin A/C soils or hard outcropping rock. Deeper colluvial soil materials occur on lower slopes. The presence of protruding dolerite columns now buried by transported clayey slope-wash materials indicates partial landscape stripping followed by reburial. 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. These findings have important implications for landscape history, slope processes and the improved understanding of the distribution of dolerite-derived soils in Tasmania.