Geomorphology: Geology, Karst & the Gnargoo Range

Gnaraloo features a magnificent geological and dynamic landscape, including its very own Gnargoo range.

The South Girilia Plateau, which stretches from Exmouth south to Gnaraloo, is considered an area of Environmental Management Priority, as it is a region of high conservation value (Department of the Environment, Water, Heritage and the Arts, Commonwealth, 2008).

The landscape on the Gnaraloo area is beautiful in its stark contrasts. It is characterised by the striking blue and greens of the Indian Ocean and the Ningaloo Reef, bordered by an area of relic linear dune plateau, with good biodiversity levels. Heading inland are vast red aeolian sand fields sparsely covered with spinifex (Spinifex squarrosus) grassland, before encountering the Gnargoo range. Underlaying all of this is an extensive karst system characterised by limestone bedrock. A unique feature of this area of coastline is the presence of tertiary karst systems as well as incised river channels that are directly linked to the coast. The climate along the coast varies between arid and semi-desert to subtropical conditions, with frequent annual cyclonic activity.

Karst

The Ningaloo coastline has principle characteristics of a tertiary karst environment, which is unparalleled in Australia, making the area of great importance to natural heritage (Department of the Environment, Water, Heritage and the Arts, Commonwealth, 2008). Tertiary karst environments form when the tides of the ocean interact with the hydrological network in the karst system, mixing the ground water and seawater. This increased salinity further dissolves the limestone bedrock, and combined with the flow created by the tidal pumping, creates interconnecting tubes between pre-existing cavities within the system. The presence of a tertiary karst system in Australia is very rare and there are no comparable systems within Australia (Department of the Environment, Water, Heritage and the Arts, Commonwealth, 2008).

In the Gnaraloo area, the predominant limestone bedrock results in the presence of aquifers, groundwater resources and incised river channels, which form a direct link between the Indian Ocean and the significant inland Lake MacLeod wetland system via subterranean channels (Department of the Environment, Water, Heritage and the Arts, Commonwealth, 2008).

Karst is a three-dimensional geological feature which is formed over time by the action of water dissolving soluble layers of bedrock. The bedrock usually consists of carbonate rocks such as dolomite, limestone, marble or less commonly gypsum (Waele et al. 2008). These formations occur when rain gains acidity in the atmosphere through the collection of carbon dioxide. Once it reaches the ground surface, the mildly acidic rain begins to drain through the surface and the acidity can be amplified by collecting more carbon dioxide, creating carbonic acid (H2C03). As the acidic water drains through the bedrock, it begins to eat away at the fractures, causing them to enlarge. These fractures then begin to form an underground drainage system, which in turn increases the flow of water through the bedrock and begins to create the unique underground karst system. The results of karstification can vary greatly, producing small and large features both underground and on the surface.

Above ground, the features of a karst system can vary anywhere from small flutes, karren, runnels, grikes and clints, all the way through to sinkholes (dolines), vertical shafts, disappearing streams and reappearing springs (Waele et al. 2008). Below the surface extends a large underground drainage system, including features such as aquifers as well as an expanse of caves and cavern systems.

In areas where a karst system is present, there is a lack of surface water as it quickly drains through the crevices. In addition to this, the high permeability of karst systems means that underground water supplies are likely to be contaminated, as water travelling through the bedrock travels at such a fast rate that it does not undergo the same filtering processes as ground water would in other areas. These features make living on karst systems particularly difficult. Often the ground water, which in other areas would be heavily relied upon, cannot be used and there is a risk of sinkholes forming and underground caverns collapsing. This means that human activities in karst areas need to be monitored, especially waste disposal, freshwater supply, construction and mining (Waele et al. 2008).

Gnargoo Range

The Giralia Range district includes the anticline structures of the Giralia Range, Rough Range, Gnargoo Range and numerous small folds adjacent to Lake MacLeod (Western Australian Planning Commission, 2004). The geological history of the district follows similar patterns as the Cape Range district in Exmouth with marine sedimentation, tectonic stress and the uplift and exposure of tertiary sediment.

Reseach needed

Very little is currently known about the Gnargoo Range, which is located in the southern Carnarvon basin, extending approximately 500m below Cretaceous strata. Gnargoo exists on the Gascoyne Platform, which extends from Kalbarri to Coral Bay, while the range itself stretches from north of the Gnaraloo Homestead towards Warroora Station (Western Australian Planning Commission, 2004).

The information was compiled by Karen Hattingh, Simone Bosshard, Tessa Concannon, Tess DeSerisy, Heather Shipp and Megan Soulsby, 2017/18

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