Volcanogenic origin of cenotes near Mt Gambier, southeastern Australia
Introduction
Cenotes are circular, cliffed, collapse dolines containing water-table lakes. They are spectacular and very popular sites for cave diving, because of their often very clear water and typically large passages (Lewis and Stace, 1980). The best known are those on the Yucatan Peninsula in eastern Mexico (Perry et al., 1995, Smart et al., 2006, Beddows et al., 2007); the term cenote is adapted from the Mayan word ‘zenots’ for these features (Cardenas, 1996). Cenotes are most common on coastal karst plains with very low topography developed in relatively soft, porous limestone, like the Yucatan Peninsula, Florida Peninsula (Katz et al., 1995), southeastern South Australia (Marker, 1975, Marker, 1976, Lewis, 1984, Grimes, 1994), and the Bahamas Banks, where they occur onshore as well as offshore as drowned cenotes, called Blue Holes (Smart et al., 1988a, Smart et al., 1988b, Whitaker et al., 1994). Drowned cenotes have also been recorded on the Great Barrier Reef off the coast of northeastern Australia (Backshall et al., 1979). Cenotes occur in higher elevation limestones in South Africa (Gomes, 1985), northeast Mexico (Gary and Sharp, 2006), and on the Anatolian Plateau in Turkey where they are called “obruk” lakes (Jennings 1985), and they are known from evaporite karst in New Mexico (Land, 2003).
Cenotes, although they are all superficially similar, have a variety of origins, including dissolution by magmatic CO2 (northeast Mexico; Gary and Sharp, 2006), and collapse into cave systems formed by marine-freshwater mixing solution (e.g. Yucatan; Smart et al., 2006) or by dissolution of evaporite beds (e.g. New Mexico; Land, 2003).
The cenotes in South Australia are well-known as cave diving sites (Lewis & Stace, 1980, Horne, 1993, Horne, 1998), but there has been little speculation on how they formed. This paper describes the cenotes and uses their distinctive characteristics to constrain their origin.
Section snippets
Geomorphological setting
The South Australian cenotes lie on a broad, low relief, coastal limestone plain around Mt Gambier (Fig. 1, Fig. 2). The plain slopes gently southwards from 30–40 m above sea level (asl) to the sea (Grimes, 1994). To the north is a slightly higher elevation plain (60–70 m asl; Fig. 1), also composed of limestone, and partly covered by irregular ill-defined swamps such as Dismal Swamp. These swamps are absent to the south. In the northwest of the area, the inland and coastal limestone plains are
Geological setting
The coastal and inland plains are composed of the latest Eocene to early Middle Miocene Gambier Limestone (Li et al., 2000), which was deposited within the northwest section of the Otway Basin in southern Australia on a cool-water open marine shelf with clear waters (Smith et al., 1995, James et al., 1993). The Gambier Limestone is a bryozoal calcarenite, made up of sand-sized bryozoan fragments and foraminifera cemented by thin rims of calcite around the grains; it has not been deeply buried,
Climate
The climate of the Mt Gambier region is classified as Mediterranean or Csb in the modified Köppen system. It is characterised by moist cool winters and hot dry summers. Rainfall in the area is 700–800 mm, and decreases to the north. Mean daily maximum and minimum temperatures range from 22–24 °C and 12–14 °C in January to 13–14 °C and 5–7 °C in July (respectively). Potential evaporation exceeds rainfall from October to April; the excess precipitation is ∼ 300 mm for the winter months (Waterhouse, 1977
Hydrogeology
The limestone in the Mt Gambier region forms a porous, high-yielding, largely unconfined aquifer that overlies a calcareous mudstone aquitard. The groundwater is extensively used for irrigation and town water supplies because of its high quality (TDS generally 300–600 mg/l) and the lack of surface water resources.
The watertable in the limestone is well-defined (Fig. 3). On the coastal limestone plain it slopes gently towards the coast at a gradient of ∼ 1:1300, so that depth to the water table
Cenotes
The coastal limestone plain is punctured by about 20 cenotes (Fig. 1, Fig. 2), circular cliffed dolines which intersect the water table (Figs. 4A–C and 5B) and contain lakes 20 m to ∼ 125 m deep. The majority of the cenotes, including most of the deepest and volumetrically largest ones, are concentrated in two small areas 12–15 km from the coast (Fig. 1), each ∼ 4 km in diameter, located 5 km west and 10 km northwest of Mt Schank (Fig. 2). Scattered through these main areas of cenotes are several
Cenote origin
Because cenotes contain watertable lakes, they are restricted to areas with shallow watertables, mostly very flat coastal limestone plains which rise only tens of meters above sea level. The morphology of most cenotes throughout the world is very similar, i.e. circular cross-section and bell-shaped vertical profile with a central cone of rubble. However, this is just a characteristic of collapse into a large chamber within limestone with low joint density; the formation of the chambers causing
Timing of deep, volcanogenic cenote formation
The dissolution of the deep caves at Mt Gambier could have been due to a single event when a large amount of volcanogenic CO2 was released during the mid-Pleistocene(?) Mt Burr eruptions and/or the late Pleistocene Mt Gambier eruption. The strontium isotope composition of the stromatolites, which began growing ∼ 8000 years ago (Table 1), shows no volcanic influence apart from a single sample that probably records ash erupted from Mt Schank. Thus there has been no input of volcanogenic CO2 for at
Conclusions
The cenotes near Mt Gambier contain lakes up to 125 m deep floored by large rubble cones, so they formed by collapse into large, deep chambers.
A few deep phreatic caves are known in the Mt Gambier area, but they are too small to represent the caves that collapsed to form the cenotes. Furthermore, the cenotes do not connect into any deep phreatic systems.
Freshwater/seawater mixing, which formed the Bahama Banks and eastern Yucatan cenotes, was not responsible for the Mt Gambier examples, because
Acknowledgements
Thanks to Tim Collins (Department for Environment Heritage and Aboriginal Affairs, South Australia), Peter Cunningham, Peter Norman and Colin Trager for access to the cenotes under their control. Strontium isotope analyses were carried out by Roland Maas (Earth Sciences, Melbourne University). Carbon dating was carried out at the Antares Mass Spectrometer, Australian Nuclear Science and Technology Organisation, Sydney. Constructive reviews by Peter Smart and an anonymous reviewer substantially
References (71)
- et al.
Decoupled fresh-saline groundwater circulation of a coastal carbonate aquifer: spatial patterns of temperature and specific electrical conductivity
Journal of Hydrology
(2007) - et al.
Stable isotope geochemistry of cold CO2-bearing mineral spring waters, Daylesford, Victoria, Austalia: sources of gas and water and links with waning volcanism
Chemical Geology
(2002) - et al.
Episodic intraplate deformation of stable continental margins: evidence from Late Neogene and Quaternary marine terraces, Cape Liptrap, southeastern Australia
Quaternary Science Reviews
(2009) - et al.
CO2-rich gases from Lakes Nyas and Monooun, Cameroon; Laacher See, Germany; Dieng, Indonesia and Mt Gambier, Australia—variations on a common theme
Journal of Volcanology and Geothermal Research
(1991) - et al.
The stranded beach-dune sequence of southeast South Australia: a test of thermoluminescence dating
Quaternary Science Reviews
(1993) - et al.
Improved methodology and new thermoluminescence ages for the dune sequence in southeast South Australia
Quaternary Science Reviews
(2001) - et al.
The age and hydrological history of Blue Lake, South Australia
Palaeogeography, Palaeoclimatology, Palaeoecology
(1995) - et al.
Groundwater flow regime within the Gambier Embayment of the Otway Basin, South Australia: evidence from hydraulics and hydrochemistry
Journal of Hydrology
(1993) - et al.
Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record
Quaternary Science Reviews
(2006) - et al.
Strontium isotopic and trace element heterogeneity in the plains basalts of the Newer Volcanic Province, Victoria, Australia
Geochimica et Cosmochimica Acta
(1997)
Strontium isotopes as tracers to delineate aquifer interactions and groundwater salinisation in the basalt plains of southeastern Australia
Journal of Hydrology
Tertiary: Otway Basin
Estimation of groundwater accession to and evaporation from a South Australian lake using environmental tritium
Australian Journal of Soil Research
The use of environmental chloride and tritium to estimate total recharge to an unconfined aquifer
Australian Journal of Soil Research
Drowned dolines—the blue holes of the Pompey reefs, Great Barrier Reef
BMR Journal of Australian geology and Geophysics
Coastal and Marine Sequences
Field Geologists Manual
Soils
Non-atmospheric noble gases from CO2 well gases. 19th Lunar and Planetary Science Conference, Houston, 1988
Papers
Cancun and the Yucatan Peninsula
Statistical methods in geology for field and lab decisions
Liquid carbon dioxide of magmatic origin and its role in volcanic eruptions
Nature
Biodiversity plan for the South East of South Australia
Physical and chemical hydrogeology
Hydrothermal speleogenesis—its settings and peculiar features
Influence of karst hydrology on water quality management in southeast South Australia
Environmental Geology
Volcanogenic karstification of Sistema Zacaton, Mexico
Modern stromatolites in a karst structure from the Malmani Subgroup, Transvaal Sequence, South Africa
The Southeast Karst Province of South Australia
Environmental Geology
Chapter 10—Tertiary
Hydrology
Our hidden heritage South Australia's waterfilled caves
Environment South Australia
Statistics for strontium isotope stratigraphy: a robust LOWESS fit to the marine Sr-isotope curve for 0 to 206 Ma, with look-up table for derivation of numeric age
Journal of Geology
Cited by (19)
Solute transport in permeable porous media containing a preferential flow feature: Investigation of non-Darcian flow effects
2022, Journal of HydrologyCitation Excerpt :The same Km values as those adopted by Sebben and Werner (2016a) were adopted in the current study as Cases A to E, with the exception of their lowest value of Km = 10−6 m/s, for which we were unable to obtain reliable results due to numerical instabilities. Km values ranging from 10−5 m/s to 10−3 m/s are typical of unfractured, un-karstified limestone (Geiger et al., 2010; Webb et al., 2010). Preliminary simulations showed that both reducing dispersivity and considering NDEs (rather than assuming Darcian flow) lead to narrower plume widths.
Petrographic, mineralogical and geochemical constraints on the fluid origin and multistage karstification of the Middle-Lower Ordovician carbonate reservoir, NW Tarim Basin, China
2022, Journal of Petroleum Science and EngineeringCitation Excerpt :For example, the Well TS1 in the Tarim Basin contains high-quality dolomite reservoirs at a depth of more than 8 km, which appear to have been formed by acid dissolution under deep burial conditions (Zheng et al., 2007; You et al., 2018a, 2018b). Although the effect of deep burial diagenesis on reservoir porosity is controversial (Machel, 2001; Shen et al., 2015; Liu et al., 2017), it has been gradually realized that large volumes of secondary porosity can be generated by processes related to the deep-seated fluids resulting from volcanism and organic matter diagenesis with the aggressiveness being independent from the surface (Palmer, 1991; Mazzullo and Harris, 1992; Galdenzi and Menichetti, 1995; Hose et al., 2000; Palmer and Palmer, 2000; Zhu et al., 2006a; Heydari and Wade, 2002; Seewald et al., 2003; Jin et al., 2009; Webb et al., 2010; Polyak and Provencio, 2011; Audra et al., 2015; Klimchouk, 2017). This type of dissolution has been defined as hypogene karstification (Palmer and Palmer, 2000; Klimchouk, 2013), hydrothermal karst (Dublyansky, 1995) or mesogenetic dissolution (Mazzullo and Harris, 1992).
Multiphase breakdown sequence of collapse doline morphogenesis: An example from Quaternary aeolianites in Western Australia
2019, GeomorphologyCitation Excerpt :This has become the term for all permanently inundated collapse dolines (Kranjc, 2013). Flooded collapse dolines, cenotes, are found worldwide, e.g. in Australia (Grimes, 2006; Webb et al., 2010), Otaco karst in Namibia (Waltham et al., 2005), Guam (Mylroie et al., 2001) and Turkey (Jennings, 1985). Collapse dolines can occur in mixed lithological settings, for example, where non‑carbonate rocks (i.e., caprock) overlie karstified bedrock.
Origin and hydrology of a large, intact Early Cambrian paleocave system and its role in overlying fluidisation structures, Arctic Canada
2017, Sedimentary GeologyCitation Excerpt :Hypogenetic caves are the result of deep CO2- and SO2-rich fluids, which are relatively aggressive. These aggressive fluids can dissolve carbonate rocks much more rapidly (1 to 2 orders of magnitude) than meteoric fluids, producing large cave systems relatively quickly (Klimchouk, 2009; Webb et al., 2010; De Waele et al., 2016). Hypogenetic dissolution in Mexico (Gary and Sharp, 2006) and Australia (Webb et al., 2010) was attributed to volcanic-derived fluids.
On the effects of preferential or barrier flow features on solute plumes in permeable porous media
2016, Advances in Water ResourcesCitation Excerpt :A simple analytical expression for the advective displacement of a solute plume encountering a DFF is also presented. We examine the distribution of solutes for a variety of matrix-DFF permeability ratios and DFF apertures, adopting aquifer properties that are representative of sedimentary rocks (e.g. sandstone and limestone) in which PFFs, BFFs, and flow in the matrix are known to occur (e.g. Webb et al., 2010; Al Ajmi et al., 2014; Mádl-Szőnyi and Tóth, 2015). Flow line displacement is comparable to the displacement of peak solute concentration only where solute molecular diffusion and mechanical dispersion can be ignored.