By Perinne Tyler

Approximately 1/3 of Tasmania’s surface geology consists of dolerite; an area of approximately 30 000 km2. Exposures of this rock form many of the hills and mountains of Tasmania, including some of Tasmania’s most well-known landmarks. During the field trip, we unsurprisingly observed a large amount of dolerite, most notably at Mount Wellington, Cradle Mountain, Cataract Gorge and the Tasman Peninsula.

The Jurassic dolerites of Tasmania form just a small part of the Ferrar Large Igneous Province (FLIP); a 3 500 km long linear belt of tholeiitic rocks which extends along the Transantarctic Mountains of Antarctica and into South-East Australia (Elliot and Fleming, 2004). The FLIP was emplaced during the early stages of Gondwana break-up from a source located in the Weddell Sea region (a part of the Southern Ocean, located above what is now the North-West Coast of Antarctica), with magma transport controlled by an early Jurassic zone of extension (Figure 1) (Elliot and Fleming, 2000, 2004).

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Figure 1. Gondwana reconstruction showing distribution of the Ferrar Large Igneous Province and the inferred source of the Ferrar tholeiitites in the proto-Weddell Sea region (Weddell Triple Junction). From Elliot and Fleming, 2000. CTM – central Transantarctic Mountains, EWM – Ellsworth-Whitmore Mountains Block, ENZ – east New Zealand, KD – Kirwans dolerite, MEB – Maurice Ewing Bank, NVL – north Victoria Land, SVG – south Victoria Land, TI – Thurston Island, WNZ – west New Zealand.

Within Tasmania, the preserved tholeiites are almost entirely represented by intrusive rocks which form large sheets, sills (~100-400 m thick) and dyke-like bodies (approximately 1 km or greater in width) (Hergt et al., 1989). These intrusions consist predominantly of dolerite, with granophyre and quartz diorite differentiates (McDougal, 1962). The tholeiites outcrop over an area of approximately 30 000 km2 and are generally confined to the Permo-Triassic sediments of the Parmeener Supergroup, with very little occurring in the basement rocks (Figure 2) (Hergt et al., 1989). The total volume of magma emplaced in Tasmania has been estimated at a minimum of 15 000 km3, based on the extent of outcrop and thickness of sheets (Hergt et al., 1989). Re-Os dating has been used to constrain the age of the Tasmanian tholeiites to 177.3 ± 3.5 Ma (Hergt and Brauns, 2001).
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Figure 2. Map of the surface geology of Tasmania, showing location of dolerite and Parmeener Supergroup sediments. From Wikipedia Commons.

The dominance of dolerite in the Tasmanian landscape owes not only to the sheer volume of dolerite intruded but also to uplift caused by extensive block faulting that was both contemporaneous with and post-dating emplacement (Carey, 1961). As dolerite is significantly more resistant to erosion than the sediments it intruded, subsequent erosion has resulted in numerous exposures of vertical columns of columnarly-jointed dolerite throughout central, northern, eastern and south-eastern Tasmania. An excellent example of such an exposure is at Mount Wellington, located in the Wellington Ranges, near the city of Hobart. Mount Wellington was formed during extensive block faulting that occurred throughout central and eastern Tasmania approximately 150 Ma. Erosion has since exposed a 350 m tall section of columnar dolerite on the side of Mount Wellington, known as the Organ Pipes (Figure 3).
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Figure 3. Photo of the summit of Mount Wellington, showing the exposure of columnar dolerite known as the ‘Organ Pipes’. From Wikipedia Commons.

Similarly impressive exposures of columnar dolerite can be observed in the coastline of the Tasman Peninsula, located approximately 75 km south-east of the city of Hobart. Here, dolerite columns rise as far as 300 m above sea level (Figure 4).

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Figure 4. Columnar dolerite columns observed on the coastline of the Tasman Peninsula.

Dolerite was also observed at Cradle Mountain, a 1545 m tall peak located in the central highlands of Tasmania (Figure 5). Here, dolerite columns form the summit of the mountain, which has four distinct and named peaks: Cradle Mountain (1545 m), Smithies Peak (1527 m), Weindorfers Tower (1459 m) and Little Horn (1355 m).
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Figure 5. Dolerite columns form the summit of Cradle Mountain and many of the other peaks found in Tasmania’s central highlands.

Within Cataract Gorge, dolerite forms the walls of the gorge (Figure 6). Although dolerite is resistant to erosion, earlier faulting in the region had formed two faults within the rock, allowing the Meander River to cut out a deep gorge along these lines of pre-existing weakness.

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Figure 6. Dolerite exposed in the walls of Cataract Gorge, Launceston. From Wikipedia Commons.

REFERENCES

Carey, S. W. 1961. The isostrat, a new technique for the analysis of the structure of the Tasmanian dolerite. In: Dolerite: A symposium. University of Tasmania, 130-164.

Elliot, D. H. and Fleming, T. H. 2000. Weddell triple junction: The principal focus of Ferrar and Karoo magmatism during the initial break up of Gondwana. Geology, 28(6): 539-542.

Elliot, D. H. and Fleming, T. H., 2004. Occurrence and dispersal of magmas in the Jurassic Ferrar Large Igneous Province, Antarctica. Gondwana Research, 7(1): 223-237.

Hergt, J. M. and Brauns, C. M., 2001. On the origin of Tasmanian dolerites. Australian Journal of Earth Sciences, 48: 543-549.

Hergt, J. M., Chappell, B. W., McCulloch, M. T., McDougal, I. and Chivas, A. R. 1989. Geochemical and isotopic constraints on the origin of the Jurassic dolerites of Tasmania. Journal of Petrology, 30(4): 841-883.

McDougall, I., 1962. Differentiation of the Tasmanian dolerites: Red Hill dolerite – granophyre association. Geological Society of America Bulletin, 73: 279 – 315.