Origin of cenotes near Mt Gambier, South Australia

John A. Webb1. Ken G. Grimes2, Roland Maas1 and Russell Drysdale3
l Department of Earth Sciences, La Trobe University, Bundoora, Vic 3083
2 P.O. Box 362, Hamilton, Vie 3300
3 Department of Geography, University of Newcastle, Callaghan, N.S.W. 2308
Email: john.webb@latrobe.edu.au

Abstract in Thurgate, M., (editor) Program of papers, Abstracts and Field Trip program. Fifth Karst Studies Seminar, Wellington, February 2000, pp 33-34.

The Southeast Karst Province of South Australia is notable for its cenotes, collapse dolines containing water-table lakes (Grimes 1994), up to 50m wide at the surface and extending down to 95m below the land surface. They are developed in the flat-lying Oligocene - Early Miocene Gambier Limestone, which is exposed at the surface over much of the province. The cenotes are all characterized by collapse; they have vertical and overhanging sides and are floored by large cones of rubble, which may be overlain by finer grained sediment. Exploration and diving have revealed no major phreatic passages extending off any of the cenotes, despite the fact that typical shallow phreatic joint maze caves, both dry and water-filled, are common in the Gambier Limestone throughout the Southeast Karst Province. Collapse dolines are associated with some of these phreatic caves, but are shallower than the cenotes in that they do not have deep lakes.

The distribution of the cenotes is uneven; they are concentrated in two small areas, each ~3km in diameter, located 5km W and 10km NW of Mt Schank (a few other cenotes are scattered through the province). Within these areas they are distributed along two joint sets, a dominant set trending 320° and a subsidiary one at right angles. These are the dominant regional joint directions (Marker 1975).

Lewis (1984) noted that the depth of the cenotes indicates that they represent collapse into large caverns dissolved at or close to the base of the Gambier Limestone; the boundary between the Gambier Limestone and the underlying Tertiary siliciclastics lies at about 100m below ground surface around the cenotes west of Mt Schank. He suggested that the caverns had been dissolved by acidified groundwater containing large amounts of volcanogenic CO2, which had ascended up fractures from deep-seated reservoirs related to the magma chambers that fed the Quaternary volcanoes Mt Schank and Mt Gambier. Approximately 10km east of Mt Schank is a CO2-producing well (Caroline); the isotopic composition of the CO2 identifies it as magmatic in origin, and it is probably related to the Quaternary volcanics in the area (Chivas et al. 1990).

Direct evidence for the influence of volcanogenic fluids on the cenotes comes from strontium isotope analyses of a stromatolite collected at ~8m depth from Black Hole, one of the larger cenotes. Stromatolites grow on the walls of many of the cenote lakes, and are large structures up to 4m long formed of calcite precipitated by the microbial communities growing on their surfaces (Thurgate 1996). In cross-section the calcite of the Black Hole stromatolite shows submillimeter-scale laminations, which may be annual. Detailed sampling of one section of this stromatolite showed that overall it has a 87Sr/86Sr ratio of around 0.7088, slightly greater than the isotopic composition of the host Gambier Limestone (0.7083, corresponding to the Early Miocene age of the limestone at this location). However, one sample has a lower ratio (0.7079), probably due to an input at this time of volcanic fluids, which have a much lower Sr isotopic ratio (0.7037-0.7058 for the Quaternary volcanics of the region; Price et al. 1997). The layer with the anomalous 87Sr/86Sr ratio has an age of -6000 BP, from C14 dates on the stromatolite either side of the layer (Kelly 1998), and assuming a uniform rate of growth. This age corresponds closely to that of the eruption at Mt Schank (6000 BP, based on recent thermoluminescent dating).

Thus the Sr isotope data show the apparent influence of volcanogenic fluids within the cenote lakes during a time of eruption (although it must be noted that additional work is still needed to confirm the Sr isotope data and the dating). It is possible that larger amounts of fluid, including volcanogenic CO2, could have been injected during previous eruptions, dissolving the caves that collapsed to form the cenotes. The oldest date obtained from the base of the stromatolites within the lakes is 23000 BP (Kelly 1998), so it is possible that the cenotes formed during the eruption of Mt Gambier, which has been dated as ~28000 BP (Leaney et al. 1995), although this age is controversial. In addition, the old stromatolite C14 date (if correct; it needs confirmation) indicates that the watertable did not drop greatly during sealevel lowstands. At 23000 BP, sea level would have been >50m below its present height, yet the stromatolite was growing only ~8m below the current water table.

References

Chivas, A.R., Barnes, L., Evans, W.C., Lupton, J.E. and Stone, J.O., 1990. Liquid carbon dioxide of magmatic origin and its role in volcanic eruptions. Nature, 326, 587-589.
Grimes, K.G., 1994. The Southeast Karst Province of South Australia. Environmental Geology, 23, 134-148.
Kelly, M.D., 1998. Freshwater stromatolites as potential palaeoenvironmental indicators: a case study from lower southeast South Australia. B.Sc. (Rons) thesis (unpubl.), Dept of Geography, University of Newcastle, 107 pp.
Leaney,  F.W.J., Allison, G.B., Dighton, J.C., and Trumbore, S., 1995. The age and hydrological history of Blue Lake, South Australia. Palaeogeography, Palaeoclimatology, Palaeoecology, 118, 111-130.
Lewis, I.D., 1984. Cave and sinkhole morphology in the lower southeast karst region. B.A. (Hons) thesis (unpubl.), Department of Geography, Flinders University, 119 pp.
Marker, M.E., 1975. The lower southeast of South Australia: a karst province. Department of Geography and Environmental Science, University of Witwatersrand, Occasional paper, 13, 68pp.
Price, R.C., Gray, C.M. and Frey, F.A., 1997. Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia. Geochimica et Cosmochimica Acta, 61, 171 -192.
Thurgate, M.E., 1996. The stromatolites of the cenote lakes of the lower southeast of South Australia. Helictite, 34, 17-25.