Nullarbor Karst (Western Australia and South Australia)

Sorry, This Page is still "Under Construction". But it's nearly done

Introduction

The Nullarbor Plain is a large area of arid to semi-arid karst developed on Tertiary "softrock" limestones. The plain extends ~300 km inland and ~800km along the coast where 40-90m high cliffs drop directly to the sea or back a pair of narrow coastal plains. Rainfall varies from 400mm at the coast to 200mm at the railway and < 150 mm at the inland edge. [add climate charts??] The plain is treeless saltbush/bluebush & tussock grass except at the coastal margin where small mallee eucalypts and myall acacias form a patchy low woodland. [photo below]

The host rocks

The limestones are marine calcarenites of Eocene and Miocene age. They are generally soft, but the upper, Miocene, beds tend to be better cemented. They still have a strong primary matrix porosity, but the permeability varies. Dips are shallow and there is only weak jointing.

The Surface

The surface is monotonously flat, but has low relief (3-5m) of stony ridges and corridors aligned to the joint directions. Small pans are known locally as "dongas" (not to be confused with the southern African use of that term). There is a well developed calcrete caprock, but little karren. [Landsat image]

There are numerous blowholes: small shafts a few metres deep that connect to either shallow caves or to a zone of spongework. These breath in or out according to the barometric pressure. Winds up to 70 km/h have been measured. [photo]

Collapse Dolines are found only in the southern part of the plain, mostly within 60 km of coast. These are large, deep, and steep sided, and connect to caves at various depths, including the deep systems. [PHOTO Koonalda]

Geological & karst history

In spite of the arid climate (at present) there are large cave systems, including extensive flooded passages. Speleogenesis may be partly inherited from earlier (late Tertiary) climates that were more humid.

The arid climate, coupled with the breathing which desiccates the cave air, produces a variety of minerals: gypsum, halite, etc. There are also ancient black calcite speleothems, dating from the wetter Tertiary period, which are being broken up by crystal growth of the younger gypsum and halite minerals.

This near-surface spongework was thought to be phreatic and dating from past higher watertables, but James et al (2005) and Webb & James (2006) suggest that it could be a recent vadose effect resulting from the rare rain storms. Within the spongework zone there are many shallow caves. At greater depth, but still above the present watertable are larger systems such as Old Homestead Cave (with ~30 km of passage) and Thampanna Cave - these show extensive phreatic sculpturing.

In the southern part of the plain there a many deep, steep-walled collapse dolines, which connect to large deeper cave systems, that have large watertable lakes and extensive sections of flooded passage. [see below and http://www.trimixdivers.com/index.html for additional diver's maps etc]. These caves have been extensivly modified by collapse and crystal weathering.

Further reading

Gillieson, D., 2004: Nullarbor Plain, Australia. in Gunn,J [ed] Encyclopedia of Caves and Karst Science. Fitzroy Dearbon, NY. pp. 544-546.

James, J.M., Contos, AK., & Barnes, CM., 2005: Nullarbor Caves, Australia. in Culver, DC. & White, WB., [eds] Encyclopedia of Caves. Elsevier, Amsterdam, pp. 418-426.

Lowry, DC. and Jennings, JN., 1974: The Nullarbor Karst, Australia. Zeitschrift fur Geomorphologie 18(1): 35-81.

Webb, J.A., Grimes, K.G., & Osborne, R.A.L., 2003: Black Holes: Caves in the Australian Landscape. in Finlayson, B. & Hamilton-Smith, E. (eds.) Beneath the Surface: A Natural History of Australian Caves. Sydney: UNSW Press. pp. 1-52.

Webb, JA, & James, JM., 2006: Karst evolution on the Nullarbor Plain, Australia, in Harman, RS & Wicks, C [eds] Perspectives on karst geomorphology, hydrology and geochemistry. Geological Society of America, Special Paper 404: 65-78.

Selected photographs and diagrams

To view full size images, click on the displayed image.
At the ocean edge of the plain, the Bunda Cliffs run as an unbroken vertical scarp for 200 km with a height between 40 and 100m.
S9211-02.jpg
Landsat image of the central Nullarbor Plain in the vicinity of Old Homestead Cave (6N-83).
Even grey is saltbush on broad rises, pale & darker patches are shallow "dambo" hollows.
Click the image to see a broader view
6N83.at.jpg
Typical saltbush vegetation on the plain. The bare patch is exposed calcrete in a small "donga" developed around a blowhole.
S921507.jpg
Blowholes are a common features of the surface of the plain. They are vertical shafts which may connect to small shallow caves, though most just end in a fine impenetrable spongework.
They "blow" or "suck" according to the barometric pressure.
Note the saltbush vegetation in the background.
S921512.jpg
Map of Old Homestead Cave (6N-83). The cave is an extensive horizontal phreatic system developed on two levels, lying about 63 metres and 70 metres below the plain. The passages often show strong phreatic sculpturing (see photo below).
6N-83.Map.png
Phreatic sculpture in a passage in Old Homestead Cave
Photo by Ken Boland
KB-3.c.jpg
Map of Koonalda Cave, showing watertable lakes.
Collapse doline of Koonalda Cave
Shallow-level spongework exposed in the wall of Koonalda Doline (5N-4D).
Stereo-pair - view cross-eyed.
S921113.jpg & S921112.jpg
Aboriginal finger markings in area of chert mining, NW Passage of Koonalda Cave.
S921117.jpg
Anastomosing half-tubes exposed along a bedding plane that has fallen from the ceiling, Webbs Cave.
Stereo-pair - view cross-eyed.
S9212-14.jpg & S9212-13.jpg
Old black calcite pool speleothems, in Webbs Cave (6N-132)
S921215.jpg
Calcite speleothem cracked open by crystal wedging. Webbs cave.
S9212-09.jpg
Go to Main Index menu