Karst as a factor in conservation management of a multiple value area: lower Gordon River, South West Tasmania

Jason Bradbury and Ian Houshold, Earth Science Section, Parks and Wildlife Service, Tasmania

Over perhaps the last 20 million years the Gordon River and its tributaries, especially the Olga and lower Franklin, have preferentially incised Ordovician Gordon Limestone. Elsewhere in Tasmania the same rock unit hosts the well known Mole Creek, Junee-Florentine and Ida Bay karsts.

The Gordon is the largest river in Tasmania, and has a very large discharge by Australian standards. Widespread blanket bogs and topogenous peat in the catchment mean even that in the lower reaches the water is frequently weakly acidic (King et al 1978). It is not surprising that the limestone is of relatively low relief, with notable exceptions (up to 70m). However, it remains of great significance, both as host to karst features, caves and cave contents and because the limestone valley of the lower Gordon River (ie below the Franklin confluence) also hosts a mid-late Holocene levee bank - backswamp - meromictic lake landform assemblage of international geoconservation significance (Dixon 1991).

Historical records of the presence of limestone in the lower Gordon river valley date back to those of the explorer James Kelly who named the river in 1815. When a penal station was established on Sarah Island convicts quarried and roasted lime in kilns for use in building and agriculture. The Gordon Limestone was formally named by the first full-time professional geologist in Tasmania, Charles Gould, who was commissioned by the Tasmanian government to lead three expeditions to western Tasmania in the 1860s. He correctly established the broad scale stratigraphy of Ordovician to Devonian formations in western Tasmania and produced a remarkably accurate map of the distribution of the Gordon Limestone.

The natural values of the area were first acknowledged by proclamation of a public reserve along the river banks in 1908 and tourist visitation has occurred for most of this century. Considerable publicity was given to the area by the 1982/3 dispute over construction of the now abandoned Gordon below Franklin dam. Large vessels capable of cruising at 30 knots with 200 or more passengers on board were introduced shortly thereafter. Annual visitation rapidly increased to 70,000 and a decade later is now more than 100,000.

It is ironic that advertising for the tourist cruises has always featured photographs of undisturbed rainforest reflected in perfectly still waters, since the 1m+ high wash from the new boats has caused severe erosion of the Holocene river banks. With measured rates of erosion and bank retreat reaching 2 m/year at one stage the problem was seen as so severe, and a solution so politically difficult to achieve that in 1987 Kiernan mused about finding "a tourist cave close to the mouth of the river to provide an alternative attraction that will make it easier to limit the cruises." There was also concern that base level karst features and processes might be affected.

By gradually imposing speed and access restrictions the rate of erosion has now been substantially reduced (Nanson et al 1994, Bradbury et al 1995). Because of the active degradation of significant geoheritage features by human induced modification of geomorphic processes (in particular the prevailing wave regime), management of this part of the Tasmanian Wilderness World Heritage Area requires a sound understanding of earth sciences. The Parks and Wildlife Service has conducted or commissioned geoscientific investigations in the area since 1987. This paper summarises karst related aspects of the lower Gordon River area and discusses a karst management strategy that has been developed to assist in management of this high profile part of the Tasmanian Wilderness World Heritage Area.

GEOLOGICAL CONTEXT

Local basement rocks exposed in the Elliott and King Billy Ranges are garnet grade metamorphites beneath a late Proterozoic to Cambrian angular unconformity. Between the lower Franklin River and Limekiln Reach on the Gordon erosion resistant rocks of the middle Ordovician Butler Island Formation define a broad anticline. Two relatively minor limestone units occur within this formation and host some karst development (Middleton 1979, Rao and Naqvi 1981). Parts of the Gordon and lower Franklin valleys display prominent dipslopes on the flanks of the anticline. These suggest the former position of the base of the overlying major carbonate unit, the Gordon Limestone. A 4km wide synclinal basin of Gordon Limestone west of Butler Island forms the largest (approx. 20km2) karstic terrain in the area. Limekiln Reach is a pronounced strike parallel valley where one limb of the syncline is narrowly extended northwards. The Gordon Limestone also crops out near Eagle Creek and in the upper Spence River valley.

Few fossils are known from the Gordon Limestone in the type area, although by comparison to other regions some richly fossiliferous facies within the limestone are expected. A sparse fragmentary fauna within micaceous siltstone towards the faulted top of the sequence suggests a middle - upper Ordovician age (Gee et al 1969). The apparently largely clastic Gordon Limestone is commonly recrystallised, with locally abundant calcite veins. A number of whole rock analyses indicate CaCO3 contents up to 96.4% and that the limestone is partly dolomitic towards the upper part of the sampled sequence west of Limekiln Reach (Rowe 1963). The relatively ductile limestone has probably been a locus of strain during Devonian deformation and stylolitic cleavages may be developed in places.

Southwest of Eagle Creek the Gordon Limestone is paraconformably overlain by a largely clastic shallow marine sequence correlated with the Devonian Eldon Group. Devonian carbonate facies are more common in the lower Gordon area than elsewhere in Tasmania, particularly towards the stratigraphic middle of the complexly folded succession. In the vicinity of Guy Fawkes Creek a 25m thick unit of pure crinoidal limestone occurs. This is tentatively correlated with Devonian limestone at Point Hibbs. Some karst features have been noted (Gee et al 1969), while low relief areas near Lake Morrison and the lower Spence River may also be underlain by Devonian limestone. Overall the lower Devonian sequence faces west towards the core of a middle Devonian syncline. Weakness along the hinge of this major fold was later exploited by faulting related to Gondwanan breakup. A thick, siliciclastic Tertiary graben fill sequence obscures Palaeozoic rocks west of First Gorge.

LOWER GORDON RIVER KARSTS

Investigations of limestone and karst in the area have been sporadic and limited in extent, because of the remoteness and difficulty of movement through the dense rainforest vegetation. Although early timbercutters frequently ranged far from the river banks in order to selectively fell huon pine few records of karst discoveries remain from that time.

In 1959 and 1961 the Tasmanian Caverneering Club conducted surveys of the area and documented several small caves (Goede in Middleton 1979). In 1962 the limestone in Limekiln Reach was investigated as a potential source of limestone for use as smelter flux. In the 1970s and early 1980s the Sydney Speleological Society conducted several trips to the area, documenting some caves. However, most of those investigations were concentrated in upstream areas then planned to be innundated as part of hydro-electric development. Kutikina Cave and other Pleistocene occupation sites were discovered at that time. These caves were inhabited during the last ice age by the most southerly known human population on Earth (Kiernan et al 1983).

Soutberg (1991) briefly considered karst aspects but concentrated on the Holocene fluvial geomorphology due to concern about streambank erosion. The Parks and Wildlife Service has conducted minor additional investigations in order to asses the geoconservation significance and management implications of the lower Gordon River karsts. Findings include:

• Well developed surface karst, including distinct notching at river level and other outcrop scale solution features, tufa and precipitation of carbonate on cliff faces, enclosed depressions and dolines (with or without waterbodies), dry valleys, stream sinks, resurgences, draughting holes etc.
• Several small horizontal caves at a range of altitudes, possibly reflecting fossil base levels
• A resurgence near present base level almost choked by sediment derived from the contemporary river
• A cave of approximately 40m vertical extent in an area of relatively high local relief
• A number of cave entrances potentially suitable for human occupation and which may contain Pleistocene occupation deposits
• A carbonate cemented, frost shattered clastic deposit including vertebrate bones
• A diversity of cave fauna, including a fecund beetle population

In addition, the karsts are likely to contain information relevant to reconstructing the geomorphic development of the entire lower Gordon River area. Such reconstructions could be of use in understanding pre-disturbance river dynamics in relation to the streambank erosion problem and in planning to conserve other features in the area. Potential management-directed research includes:

• dating and analysis of ancient cave sediments to indicate rates of river incision/uplift of the surrounding dissected peneplain
• reconstructing the vegetation history from pollen incorporated in sediment
• reconstructing the faunal history from vertebrate bone and other deposits
• determining the height and frequency of extreme floods from sediments within caves near base level.

KARST MANAGEMENT STRATEGY

The lower Gordon River area contains some 25 km2 underlain by limestone with high karst potential. Most of this is unexplored for karst features and all caves (known and unknown) in the area are presently classified as wild caves under the ACKMA cave classification scheme. Broad policies for the management of wild caves are indicated in the Tasmanian Wilderness World Heritage Area Management Plan (1992). To assist conservation of the karst resource these policies allow for restriction of access and will permit guided cave tours only where a management plan for the cave has been drawn up in consultation with local interests, caving groups and the Tasmanian Aboriginal community, as appropriate.

In recognition of the high karst potential of the lower Gordon River area a brief management strategy for the Gordon River karsts was included in the Draft Lower Gordon River Recreation Zone Plan (1995), which was largely devised in order to address the streambank erosion problem. The draft plan states:

• Disturbance of caves and associated features must be avoided, or at least kept to a minimum until a systematic scientific survey has been conducted.
• Cave entrance locations are not to be publicised.
• The Parks and Wildlife Service karst officer should be consulted before caves in the plan area are entered.
• Archaeological finds should be immediately reported to the Tasmanian Aboriginal Land Council and Parks and Wildlife Service (Aboriginal Heritage Unit).
• Where necessary, interpretative material should be prepared detailing the nature and vulnerability of karst in any area likely to be frequented by either private or commercial tours.
• If necessary, site plans should be prepared for karst areas at risk from recreational activities.
• Otherwise all activities in karst or related areas will be guided by the Tasmanian Wilderness World Heritage Area Management Plan(1992)

CONCLUSION

The karsts of the lower Gordon River are significant because:

• They contain pristine wild caves - undeniably a finite and diminishing resource
• They contain rare cave fauna and bone deposits
• They probably contain Pleistocene Aboriginal occupation sites
• The lower Gordon River is the stratigraphic type area of the highly karstified Ordovician Gordon Limestone
• They host a globally rare levee - backswamp - meromictic lake landform assemblage

The area is also of significance with respect to:

• Ecological values associated with a large tract of relatively undisturbed temperate rainforest.
• A physical record of pioneer society
• The history of the increasing role of nature conservation issues in both state and federal politics
• A tourism industry catering for more than 100,000 visitors per year
• Independant recreational activities - angling, kayaking, rafting etc.

The lower Gordon River must be managed to sustain all of these various values. The karsts remain in a largely natural condition because they have traditionally been subject to only very low pressure from cavers, due to difficulty of access and a perceived lack of karst development. This situation, coupled with reasonable knowledge and predictive understanding of the geology and geomorphology of the area allows the management of the lower Gordon River karsts and wild caves to be conducted in a more pro-active manner than is commonly the case.

Recent concerns over management of the lower Gordon River (eg. Bingham 1994) have focussed on obtaining a balance between conservation and presentation, as required under the 1972 UNESCO Convention concerning the Protection of the World Cultural and Natural Heritage. However, the special values associated with wild caves are often destroyed as soon as a pristine cave is entered. Presentation of such features would therefore be contrary to conservation obligations under the UNESCO convention. A strict interpretation of the precautionary principle must be the basis of all management decisions if the intrinsic value of the wild Gordon River karsts is to be maintained. Whilst it is not expected that exploration of karst features shall be actively discouraged in the near future, this remains a management option if circumstances warrant. In the meantime it is strongly recommended that the Tasmanian Parks and Wildlife Service and the Tasmanian Aboriginal community be involved in any future exploration of the wild caves of the lower Gordon River karsts.

ACKNOWLEDGEMENTS

This paper was invited to ACKMA by K. Kiernan. J. Hamill, C. Green and E. Firth assisted in the field. G. Dixon, M. Pemberton and G. Middleton commented at draft stage.

REFERENCES

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Bradbury, J., Cullen, P., Dixon, G and Pemberton, M. 1995 Streambank erosion, revegetation and management of the Lower Gordon River, Tasmanian Wilderness World Heritage Area. Environmental Management 19: 259 - 272

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UNESCO. 1972. The UNESCO Convention concerning the Protection of the World Cultural and Natural Heritage (1972). Paris.