Although very small in area compared to mainland Australia, Tasmania has a remarkably varied geology, a wide range of landforms, and the most mountainous terrain on the continent. Geology Tasmania has rocks representative of every geological period (see geological time scale). The rugged terrain and long coastline mean the complex geology is exceptionally well exposed. Deformed sedimentary and metamorphic rocks-some as old as 1,000 million years-make up the Precambrian 'basement' of Tasmania. These are the oldest exposed rocks in eastern Australia. During the Cambrian period, extensive volcanism occurred and one suite of volcanic rocks in western Tasmania, the Mt. Read volcanics, contains the State's main metalliferous ore deposits. There are thick and extensive lower Palaeozoic sedimentary sequences, including coarse conglomerates, shallow marine sandstones, deep marine shales and extensive limestones. Folding, faulting and widespread granitic intrusion occurred during the Devonian. The folded rocks are overlain by extensive, relatively flat-lying sediments from the Carboniferous, Permian and Triassic periods (Parmeener Supergroup). Some of these show glacial influence while others contain the coalified remnants of ancient forests. Much of the land surface in central and eastern Tasmania consists of Jurassic dolerite, which was intruded during the initial phase of the break-up of the supercontinent Gondwana. The eroded remnants of these intrusions are a striking feature of the Tasmanian landscape, capping most of the State's tallest mountains. Australia began drifting north from Antarctica around 95 million years ago. As the continents separated, block faulting produced prominent horst and graben structures in Tasmania. Geomorphology Many of Tasmania's present-day landforms are a result of modifications of older Mesozoic topography by Tertiary block faulting. Fluvial processes shaped the surface during the Tertiary period, etching out geological structures, including faults and dolerite sills in eastern and central Tasmania, while fold structures and granite bodies in western Tasmania were exposed after flat-lying sediments and dolerite sills were removed by erosion. Some overlying structures, now long gone, are still reflected in western rivers that flow east-west across the grain of the now exposed folded basement rocks. Basaltic lava flows modified Tertiary drainage patterns, giving rise to the precursors of the modern drainage system. The Tertiary landforms were rapidly modified by energetic glacial, periglacial and glacio-fluvial processes during the last few million years, but are today being only slowly modified by fluvial processes. Glacial and periglacial landforms in Tasmania are fossil features produced by processes that are no longer acting, so they cannot be regenerated if damaged.
Complex surface and subsurface drainage and landform systems, including caves, formed in water-soluble rock such as limestone are known as 'karst systems'. They are an important part of Tasmania's environment. Karst systems are particularly sensitive to disturbance, since they depend on delicately balanced chemical and physical systems that can be easily upset, resulting in destructive effects. Proportionately, Tasmania contains more karst areas than any other state in Australia: 4.4% of the State is underlain by potentially karstic limestone or dolomite (see Karst area map). Fluvial landforms Fluvial landforms are those landforms produced by the action of running water in the terrestrial environment, and fluvial geomorphic processes are those natural processes that produce, maintain and change fluvial landforms. Fluvial landform (or geomorphic) systems include river and stream channels and their catchments (including slopes). Fluvial landform development is dominated by the action of running water on catchment slopes and in stream and river channels, as well as by groundwater infiltration and flows within the catchment. Fluvial processes are by far the dominant land-forming process operating in the Tasmanian environment today (in contrast to glacial climatic phases when glacial, periglacial and aeolian processes have played a more active role in Tasmanian landscape development). Karst landforms Karst landforms are those landforms produced where the ('karst') processes of rock solution in natural waters and removal of rock material in solution exert a major influence on landform development compared to mechanical weathering and erosion processes. Karst landforms are most commonly developed in carbonate rock types such as limestone, dolomite and magnesite, although the can occur in other rock types as well. Karst landform (or geomorphic) systems include underground drainage systems (stream-sinks, caves, underground streams and springs) as well as a variety of characteristic surface landforms including sinkholes, pinnacles and surface weathering forms known as karren. In common with fluvial processes, karst geomorphic processes are dominated by the action of running water and groundwater flows, and the condition of a karst catchment area is crucial in determining the condition of the karst itself. Although karst processes are the dominant geomorphic process over a much smaller percentage of Tasmania's land area than fluvial processes, they are nevertheless significant land-forming processes in Tasmania. Coastal landforms Coastal landforms are those landforms produced in the zone where terrestrial and marine processes interact, and so coastal geomorphic processes involve a variety of controls including wave, current and wind (aeolian) action, as well as the effects of fluvial and mass movement processes on river estuaries and coastal slopes. Coastal land-forming processes are active (and commonly highly energetic) in a zone around the entire Tasmanian coast, and coastal landform change is often rapid on certain types of coastline such as sandy coasts. Soils Despite the small geographic area of Tasmania its soil pattern is extremely diverse. This is due to the variety of climatic conditions, parent materials, landforms and ages of the developed soils. The soil types include highly leached sandy soils (siliceous sands and podzols) in coastal areas, organosols (peats) in western and highland Tasmania, and well structured deep clay loam soils (ferrosols, previously known as krasnozems) in the northwest. In the drier regions, such as the east coast, Midlands, Coal River and the Derwent Valley, sedimentary rocks of the Parmeener Supergroup, Tertiary lake sediments and recent aeolian, fluvial and slope deposits, have produced a range of duplex, gradational and uniform soils. These duplex soils have sandy to sandy-loam surface horizons over variously coloured and mottled clayey subsoils, while the uniform soils have deep clayey or sandy profiles. In eastern, southern and central Tasmania, there are large areas dominated by soils on dolerite. In alpine areas, yellowish brown gradational soils have developed on dolerite detritus, and are characteristically very rocky. At lower altitudes, dolerite soils vary from duplex profiles to black cracking clays and brown gradational soils. In the north-east, granitic rocks are common and support a considerable variety of soil types which is a reflection of topography, climatic variations and changes in the minerology of the granites. Various acidic, duplex and gradational soils occur above the extensive Mathina beds.
Geoheritage features The following is a sample of features of high geoconservation significance whose values were, for the most part, intact at the time of the first SoE Report. It was not possible to update this assessment for this second report. It is intended that an update will be included as part of the ongoing work program. The features illustrate the quality and diversity of the geoheritage of Tasmania:
|
