Regional geology of the Gregory Karst
The karrenfield, and sub-areas of the Gregory Karst region
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Regional geology of the Gregory Karst |
The karrenfield, and sub-areas of the Gregory Karst region |
The karrenfield and underlying cave system are intimately related and have developed in step as the Supplejack surface was exposed by slope retreat. Both the karrenfield and the caves show a lateral zonation of development grading from youth to old age. As lateral erosion exposes new surface on the Supplejack, the zones of the karrenfield and the underlying cave migrate with new small passages appearing under the recently exposed surface, and the older passages at the trailing edge being unroofed and destroyed as the, by then deeply-incised, karrenfield breaks up into isolated blocks and pinnacles and eventually a low pediment surface.
Vertical development of the caves has been restricted to the epikarst zone by a 3 m bed of impermeable and incompetent shale beneath the Supplejack which first perched the watertable, forming incipient phreatic passages above it, and later was eroded by vadose flow to form an extensive horizontal system of passages 10-20m below the karren surface. Some lower cave levels in underlying dolomite occur adjacent to recently incised surface gorges.
Speleogenesis is also influenced by the rapid diffuse vertical inflow of storm water through the karrenfield, and by ponding of the still-aggressive water within the caves during the wet season – dammed up by "levees" of sediment and rubble that accumulate beneath the degraded trailing edge of the karrenfield. The soil, and much biological activity, is not at the surface (which is bare karren) but down on the cave floors, which aids epikarstic solution at depth rather than on the surface.
While earlier hypogenic, or at least confined, speleogenic activity is possible in the region, there is no evidence of this having contributed to the known maze cave systems. The age of the maze cave system appears to be no older than Pleistocene.
The strongly developed karrenfields on the Supplejack member show a zonation which results from progressively longer periods of exposure at the surface. This starts with incipient karren development on recently exposed surfaces adjacent to the contact with the overlying Skull Creek Formation and continues through progressively deeper dissected karren to a final stage of "ruined cities" of isolated pinnacles at the outer edge (Zones 1 to 4 on cross-section below). Microkarren are abundant and extremely well developed - expecially on the thin dolomite slabs of the upper Skull Creek formation.
Diagrammatic cross section of the Gregory Karst
Extensive horizontal maze caves underlie the dissected surface, being best developed under karren zones 2 & 3. The largest, Bullita Cave, is an extensive horizontal, multi-level, joint-controlled, maze system with a total passage length of 115 km(± 5) in 2007. Its overall extent is 4-5 km. Other maze caves in the area have up to 50 km of passages. Passage junctions are typically every 15-30 m. Bullita Cave is divisible into sectors, separated by narrow connections (often a single tight passage). The individual sectors have distinctive properties.
Grimes, K.G., 2006: Gregory Karst - geology and geomorphology, Preliminary report of field observations in 2005. Unpublished report to the GKSIG. 23 pp.
Note: the report refers to photos and other files "on this CD-ROM" - these were in fact on a different CD distributed to members of the GKSIG and to the National Parks officer. To see a selection of cave and surface photos use the links below to the Poster and the Profiles.
Kershaw, R., 2005a: A Brief History of Surveying the Bullita Cave System. in Cave Mania Proceedings, Goede.A., & Bunton, S., (eds), 25th Biennial Conference of the Australian Speleological Federation, Dover, Tas. p. 53-58.
Kershaw, R., 2005b: Problems of Surveying the Bullita Cave System – The Longest Cave in Australia, in Cave Mania Proceedings, Goede.A., & Bunton, S., (eds), 25th Biennial Conference of the Australian Speleological Federation, Dover, Tas. p. 92-96.
Storm R. and Smith D., 1991: The Caves of the Gregory National Park, Northern Territory, Australia. Cave Science, 18, 91-97.