THE GAMBIER KARST REGION

Prepared by Ken Grimes

The Host Rocks

 

Gambier Karst Region: Host Rocks for Cave & Karst features 

The Tertiary limestones were deposited in a shallow sea that flooded the region about 15-30 million years ago. This limestone is relatively soft in the subsurface but develops case hardening and calcrete cappings on exposure. It is locally well jointed with a dominant north-west trend. The Tertiary limestones are similar to those in the Nullarbor and there are similarities in the cave styles also, though in the Gambier Region we have none of the huge passage systems that occur in the Nullarbor.

 The Quaternary dune limestones are a series of calcareous sand ridges which formed as coastal dunes along old shorelines during an overall retreat of the sea during the Quaternary (the last 2 million years). In South Australia they form discrete, linear, north-west trending ranges, which extend northwards beyond the limit of the Gambier Basin to overlie the (non-calcareous) sediments of the Murray Basin. In Victoria the distribution of the dune limestones is less regular, and east of Portland they are mainly restricted to a belt along the modern coast, though some older ridges (without karst) occur further inland. These Quaternary dune limestones are similar to those on Kangaroo Island (e.g. the Kelly Hill caves), the Eyre Peninsula, and the coastal areas of West Australia (e.g. at Augusta, Margaret River etc.).

The dune ridges are now partly consolidated calcarenites (sandy limestones) and contain syngenetic karst features in which caves and solution pipes developed at the same time as the sands were being cemented into a limestone. The dune limestone has well developed bedding which shows up well in some cave walls. In contrast to the Tertiary Limestone it shows little or no jointing. Just to confuse things, some caves are developed mainly in the Tertiary limestones but have their entrances in the overlying dune limestones.

Hydrology

The aquifer is karstic, but as the limestone is also quite porous a well developed water table exists. During glacial periods the lowering of sea levels would have caused a significant drop in the groundwater levels in the coastal parts of the region - as shown by submerged speleothems and mudcracks in some South Australian caves.

 The interaction of uprising hot volcanic magmas and the groundwater in the limestones were responsible for the steam-driven explosions that formed the large crater lakes of the region such as Blue Lake at Mount Gambier, and Tower Hill crater near Warrnambool.

Dolines and cenotes

	The most spectacular surface karst features are the collapse dolines (or sinkholes) especially those in the Mount Gambier area that extend below the water table to form cenotes. These have formed by the collapse of the roof of large phreatic caverns. The Figures show several typical cenotes and related features: in the Gambier area the watertable is lower and the collapse dolines have relatively shallow lakes (e.g. Hells Hole & Umpherstons), however, nearer to the coast the water levels are higher and we find deep lakes (e.g. Little Blue Lake).   An interesting feature of some of the cenotes is the existence of stromatolites: columnar or platy underwater calcareous growths formed by algae. These have been found as deep as 25m, and also extend 2m above the present water level - implying a higher watertable at some time in the past.  Fields of shallow dolines and uvalas (larger, more complex hollows) are extensive in South Australia and extend into the westernmost part of Victoria. Isolated doline fields also occur further east, e.g. beside the Great Ocean Road northwest of Peterborough. These shallow hollows generally have sandy or muddy floors and rarely have cave entrances. Instead we find cave entrances typically in collapse dolines, via solution pipes (which may have a small conical subsidence doline above them), in small outcrops of calcreted caprock, or in cliffs along streams or on the coast.
A Cenote (Little Blue Lake, see cross-section above). Click the photo for a larger view (74kb .JPG)
Stromatolites exposed at Goulden Hole. Click it for a 61kb.JPG

Soft-rock caves

Phreatic sculpturing in a dune limestone cave. Click it for a bigger view (58kb.JPG)	The caves in the Gambier Karst Region are dominantly phreatic in origin, i.e. formed by slow moving groundwater below the water table. The limited local relief means that vadose stream flow features are extremely rare in South Australia, but some streams occur in the caves of the Glenelg River gorge in Victoria and also further east in the Warrnambool and Timboon areas.  Many of the original phreatic caverns and passages have been modified by breakdown to form collapse domes and rubble filled passages.   Cave diving has demonstrated the existence of extensive underwater cave systems in South Australia, and it appears that south of Mount Gambier the bulk of the cave development may be below the present water table.   Typical syngenetic caves in the dune limestones are shallow horizontal systems developed beneath the caprock or at the level of an adjoining swamp. They have multiple entrances (often via solution pipes or the collapse of the surface crust) and an irregular outline of chambers, pillars and short connecting passages, generally with a roof height less than one metre throughout. The walls are often difficult to see (and map) as they are out of reach where the roof slowly drops to floor level. Breakdown tends to modify this original structure so that many of the caves are now mainly collapse domes with only small remains of the original solutional system visible at the base of the walls.   The caves in the Tertiary limestone are similar but generally have larger chambers and passages, and also show better joint control, with many fissure style passages. Many are partly or wholly submerged.   Speleothems are generally not abundant - a consequence of the frequent destructive collapse. However, there are some spectacular exceptions to that rule - and these include extensive and very delicate forms - especially clusters of long straws and soft deposits of moonmilk. Cave "coral" is well developed. Still pools may be partly covered by thin floating calcite rafts.   Although these caves lack the extensive vertical pitches found in eastern "hard-rock" caves, they are not without their challenges: tight, vertical, solution pipe entrances; unstable sand and rubble cones; confusing mazes; and extensive crawlways - not to forget the ultimate challenge of long water-filled sections reaching to depths in excess of 80m !
Stream cave near the Glenelg River. Click it for a 76kb.JPG
Submerged mudcracks indicate a lower watertable in the past. The laminated mud suggests an even earlier lake deposit. Click it for a 78kb.JPG
Straws in a dune limestone cave. Click it for a 91kb.JPG

Cave Biology and Archaeology

The solution pipes form excellent pitfall traps and thus bone deposits of Quaternary age have been found in a number of caves. The most important, and world famous, bone deposit is in the Victoria Fossil Cave at Naracoorte, but other significant sites have been found in both states, including some underwater bone deposits in the cenotes.

The Tertiary limestones in the Mount Gambier area have bands of flint nodules which make excellent stone tools. Evidence of aboriginal mining of flint is seen in several caves, as is also aboriginal art in the form of scratch marks and finger marks.

Volcanic Caves

Copyright (c) Ken Grimes, 1998: ken-grimes@h140.aone.net.au
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