Further Hydrogeological Investigations of the Mill Creek - Kansas Creek Area, Northern Tasmania K. Kiernan!, R. Eberhard! and C.H.C. Shannon2 ! Forestry Commission, Tasmania 2Northern Caverneers Inc., Launceston, Tasmania Abstract Valley and has been explored for more than 0.5 km. It appears to be the former outlet for Surface exploration has revealed three additional a spring that now discharges directly into the karstic streamsinks on the lower slopes of Western bed of the Mersey River outside the cave, but Bluff All appear genetically related to Croesus the cave itself now serves as an overflow Cave, but today discharge their base flow to the conduit. Just under 1 km down the Mersey more recently evolved Tailender Cave Spring. Valley lies Croesus Cave (305 m altitude) in This is despite the fact that one streamsink lies which 2 km of passages have been mapped, only 100 m from Croesus Cave. Drainage from many of which are of large size and Blue Lake, a sinkhole pond, has been traced into spectacularly decorated with speleothems, Rat Hole Cave, and the connection from that cave notably flowstones (Photo 1). This cave to Tailender has been confirmed. Aquaduct discharges about 10 m above river level and is Swallet probably drains directly to Tailender, the largest of the three outflow caves, despite 345 m lower and 3.6 km distant. A major the fact that under normal flow conditions it extension has also been discovered in Rubbish contains by far the smallest stream. Lynds Heap Cave, source of the stream that discharges Cave (290 m altitude) lies a further 650 m from Lynds Cave, but from which drainage down the Mersey Valley, and presently connections to Croesus under high discharge contains the largest underground stream. It conditions have previously been hypothesised. is also a spacious cave and has been explored These data compound the evidence for the for more than 1 km. existence of a major integrated karst drainage system and a high probability of interflow between This paper reports new investigations of karst some of the components under high discharge in the area: water-tracing experiments in the conditions. The highest level of protective Tailender Cave - Croesus Cave area; management is warranted for this very important investigation of the water chemistry and complex. related aspects of Croesus Cave; and the exploration and mapping of a major new extension in Rubbish Heap Cave, source of Introduction the stream that flows through Lynds Cave. Ordovician limestone that crops out between The complex of caves in this locality is of the Mersey River and the lower slopes of major significance for a variety of scientific, Western Bluff (about 1400 m altitude) at the recreational and aesthetic reasons. It is an western end of the Mole Creek karst area has excellent example of a major integrated karst long been known to contain three major hydrogeological system comprising various outflow stream caves (Figure 1). Tailender components at different stages of evolution. Cave (305 m altitude) lies at the upstream This complex of caves is important for the boundary of the limestone in the Mersey major insight it offers into the evolution of the surrounding landscape and patterns of past more recent cave sediments is providing climatic change. Some of the caves contain information relevant to a better evidence of multiple phases of sedimentation understanding of natural hazards in karst Trimmer C. and re-elaboration, probably over hundreds areas, such as the sudden formation of cover of thousands of years, that are related to collapse sinkholes (Kiernan 1989b, 1990b; phases of relative instability and stability in Kiernan et al. 1993). Rimstone pools in Execution Poi~~ the surrounding landscape during late Croesus Cave have been single'd out as 41 °35' Cainozoic glacial and interglacial climatic outstanding examples of this type of stages (Jennings 1967; Kiernan 1984, 1990a; speleothem (Jennings 1971, 1985). More Spate and Holland 1990). Radiometric dating recently discovered clastic speleothems in and stable isotope analysis of stalagmite Rat Hole Cave are of national significance samples from Lynds Cave have also (Kiernan 1989a) while the whole area has produced a sensitive record of temperature recently figured prominently in an change over the period 12 600-2 800 years BP international text on karst hydrology (Goede and Hitchman 1983). More recently (Kiernan 1992). The caves are also an Goede and Vogel (1991) have analysed important recreational venue, their possible variations in trace element composition in potential for development for tourism having another dated stalagmite from this cave and long been recognised. have shown that trace elements can also provide important information about Protection of these various values demands o environmental change. These records are of an adequate understanding of the cave intrinsic interest and of relevance to current catchments since caves are continuously concerns regarding possible warming of the interactive with water and many of the Figure 1. Relationships between karst phenomena and baseflow underground drainage routes planet by the Greenhouse Effect. Study of natural processes within them need the demonstrated by water-tracing experiments, The present land tenure in the area is superimposed: CR = Croeslis Cave State Reserve; SF = State forest; WHA = World Heritage Area; MR = Marakoopa Cave State Reserve; PL = private land. maintenance of natural stream and seepage covered barely one-third of either cave, the flow regimes and water chemistry. remainder of the area overlying those caves, Determining drainage routes in karst areas the other caves in the complex, and all the is not straightforward since underground cave-stream catchments being State forest streams commonly ignore the dictates of the that had been subject to logging in the 19405 surface topography, sometimes sinking to the 1960s (Figure 1). The entire catchment underground into inaccessible cave systems was included on the Register of the National to rise again in another valley altogether. Estate in September 1984. A study of karst Hence, valleys conspicuous on air in the Mole Creek area undertaken by the photographs and maps often give a totally Forestry Commission, Tasmania, in 1983-84 misleading impression of the drainage. recommended a major extension to the Catchment disturbance, vandalism in the reserve, the establishment of Forest Reserves caves, the impact of careless visitors around the streamsink caves, and riparian underground, the theft of stalactites and other protection measures, but to date none of the speleothems, and public safety, are among reserve proposals has been implemented. the issues that demand management Part of the upper Kansas Creek catchment responses (Kiernan 1984, 1989c; Spate and was included within the Tasmanian World Holland 1990). Heritage Area (WHA) in 1988, above a bifurcation of the stream high on Western Land tenure and conservation status Bluff from which the northern anabranch descends into the tourist cave in the In 1972, the Croesus Cave State Reserve was Marakoopa Cave State Reserve. The WHA proclaimed over the outflow entrances of boundary extends southwards along the crest Photo 1. Speleothems in Croesus Cave. (Photo: Andrew Briggs) Croesus Cave and Lynds Cave. However, it of Western Buff. The catchment of Lynds, T.s AS Q Croesus and Tailender Caves was Underground drainage contained a large and vigorous stream, and tortuous upper level passages that were the recommended for inclusion in the WHA in a with the very large dimensions of the main only part of the cave known for several report by the Department of Parks, Wildlife This study focusses on an area bounded in stream passage in that cave. However, the decades. The discovery that the Rat Hole and Heritage, Tasmania (Dixon 1990). The the north by the catchment of Kansas Creek, present day drainage patterns in the area are stream drained to Tailender was unexpected, total area was recommended for protection in to the south by the catchment of Mill Creek, far more complicated than the surface given that Rat Hole lies in the uvala only a the report of the Balanced Panel of Experts to the east by Western Bluff and to the west topography suggests. few hundred metres distant from April Fools compiled during the Forests and Forest by the Mersey River. A further pronounced and Top Hole. The main pa'ssage of Tailender Industry Strategy process (Podger et al. 1990) valley, formed by Vanishing Creek, descends Previous studies (Kiernan 1984, 1989c, 1992) Cave carries water only under high flow and has the status of Deferred Forest under from Western Bluff between Kansas Creek have shown that Vanishing Creek, which conditions. An older, upper level passage the Public Lands (Administration and Forests) and Mill Creek. All the water that flows into sinks at about 530 m altitude, travels via Rat contains large dolerite boulders in a matrix of Act 1991. this area sinks into the limestone, with the Hole Cave to the Tailender Spring. The silt and clay. The active stream flows through exception of Mill Creek which, at about 380 m stream that discharges from Lynds Cave has a conduit a few metres below the level of the Despite the presence of a gate on the outflow altitude, flows onto older non-carbonate rocks been shown to originate from Kansas Creek, main passage and has been entered but not entrance to Croesus Cave where the that underlie the limestone. This geological which sinks into Rubbish Heap Cave (400 m fully explored by cave diving (P. Cover, pers. Department of Parks, Wildlife and Heritage boundary cuts off any underground drainage altitude) at high stage, or into the creek bed comm.). Both Croesus and Lynds Caves are had for many years sought to administer an down the lower Mill Creek Valley. It several hundred metres upstream, and up to characterised by large suites of fossil clastic access permit system, two other unmanaged probably deflects towards Tailender Cave any 20 m higher than the cave when the stage is sediments, often embedded in speleothem entrances on State forest continued to allow seepage that has occurred into the limestone lower.