Aquaculture Groundwater Resource Atlas - Carnarvon Basin
Overview
The Carnarvon Basin is a sedimentary basin extending from south of Kalbarri northwards to the mouth of the Fortescue River. The main aquifer in the basin is the Birdrong Sandstone, which is tapped by artesian and sub artesian bores, mainly for stock watering. Underlying formations may also contribute water to the aquifer. Other aquifers are locally important; the Tumblagooda Sandstone around Kalbarri; the alluvium along the lower Gascoyne River, which supports horticulture at Carnarvon; and limestone aquifers at Cape Range. Sandstones of Devonian, Carboniferous and Permian age are minor aquifers in the east of the basin.
Unlike other sedimentary basins in Western Australia, the Carnarvon Basin contains mainly shale of low aquifer potential, and there are few sandstone aquifers. Groundwater salinity is comparatively high, both because of the lack of suitable aquifers, and also because of the low rainfall.
Diagrammatic section across the Carnarvon Basin
A general account of the hydrogeology of the basin is given by Allen (1988).
Superficial aquifer
Superficial and alluvial sediments occur on the coastal plain overlying the Tertiary and Cretaceous sedimentary rocks of the Carnarvon Basin. The most important aquifer is the Gascoyne River alluvium. Alluvial aquifers may also be present along the Wooramel, Yannarie, Lyndon and Minilya Rivers, but these have not been investigated. Alluvial aquifers associated with the Ashbuton, Cane, Robe and Fortescue rivers are described in the Pilbara Coast (LINK REQUIRED HERE) section. The superficial formations range in thickness up to about 30 metres and consist of coarse alluvium along and adjacent to the river beds, and fine flood plain silts elsewhere. Groundwater levels are generally within 5 to 10 metres of the surface.
The groundwater salinity is low close to the major rivers, owing to groundwater recharge from runoff, and increasingly higher away from the rivers. Groundwater salinity is generally adequate for stock watering in the north of the area, but is too high for stock in the Shark Bay area to the south. Groundwater usage is for pastoral purposes throughout the area, and for horticultural and town water supply from the Gascoyne River alluvium.
The Tamala Limestone along the coast is a potential source of saline groundwater, but has not been investigated because of the high salinity.
References: Forrest and Coleman (1996); Allen (1988); Allen (1972), Ventriss (1980); GSWA Hydrogeology Report 1990/21 by M. Martin.
Cape Range
The Tertiary Tulki Limestone is restricted to outcrop on the Cape Range peninsula, and the coast between Cape Cuvier and Warroora Station, where it forms an unconfined aquifer. On Cape Range, the aquifer is developed around the central anticlinal core of the range where impermeable Mandu Calcaranite comes above the water table. The limestone has karst features and is highly transmissive. The water table gradient is very flat and close to sea level on the coastal plain around Exmouth. Fresh groundwater is restricted to the inland part of the aquifer, and to a thin layer overlying saline water. The aquifer is used principally for Exmouth Town water supply. Town water supply bores are relatively low yielding to avoid drawing salt water from below.
Large bore yields of saline groundwater are likely to available around the coast, if there is karst development.
Diagrammatic west–east section near Exmouth
Reference: Allen (1988); Forth (1972). Water Corporation (1996).
Birdrong Sandstone aquifer
The Birdrong Sandstone aquifer is the main artesian aquifer in the Carnarvon Basin. In places, the aquifer also includes the underlying Silurian-Devonian Kopke and Tumblagooda Sandstones. North of the Ashburton River, the aquifer also includes the Cretaceous Yarraloola Conglomerate which occurs in palaeochannels on the eastern margin of the basin at the base of the sequence. It is overlain by a thick succession of Cretaceous siltstones and clays. The aquifer underlies most of the western part of the Carnarvon Basin, extending out to sea. The aquifer is artesian over much of the western part of the area.
Extent of the Birdrong Sandstone aquifer, showing areas of artesian flow |
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Structure contours on the Birdrong Sandstone, showing depth relative to sea level |
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Along the coast the Birdrong Sandstone may be as much as 600 metres below the surface. The sandstone ranges in thickness up to about 30 metres. In places the Birdrong Sandstone is absent, and the main aquifer is the underlying Kopke Sandstone or Tumblagooda Sandstone.
The groundwater salinity increases from inland toward the coast. Groundwater is brackish along the eastern margin, and becomes saline towards the coast.
The temperature of groundwater in the Birdrong sandstone aquifer ranges from around 30 degrees Celsius inland to 45 – 60 degrees Celsius along much of the coast, depending on the depth.
Groundwater salinity in the Birdrong Sandstone |
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Temperature of groundwater in the Birdrong Sandstone |
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Groundwater from the artesian bores is used throughout the basin for pastoral purposes, for mineral processing at Dampier Salt near Carnarvon, and for desalination for town water supply at Denham and Useless Loop. Groundwater abstraction will decrease following a program of capping free-flowing pastoral bores.
Reference: Allen (1988), WRC Hydrogeology Report HR 170 by R.Wills and S. Dogramaci.
Tumblagooda Sandstone
The Tumblagooda Sandstone crops out in the Murchison Gorges near Kalbarri. In this area it is an unconfined aquifer, extending from the coast near Kalbarri along the eastern margin of the Birdrong Sandstone. It also underlies the Birdrong Sandstone and contributes groundwatrer to some of the artesian bores. Groundwater near Kalbarri is low salinity, but farther to the north, across the Murchison River, the groundwater is brackish or saline. Groundwater is used principally for town water supply at Kalbarri.
Saline groundwater is also used at Hutt Lagoon for processing of algal material, and there is minor stock usage. Groundwater exploration has been carried out for Horrocks Beach but the groundwater is brackish in this area. Bore yields vary, depending on intersection of fractures or high permeability gravel layers.
Reference: Allen (1988).
Permian sedimentary rocks
In the eastern part of the Carnarvon Basin are sedimentary rocks of Permian ages. These include sandstones which are major aquifers. The main aquifers are in the Permian Moogooloo and Mallens Sandstones and the Cundlego Formation (Allen, 1988). Minor aquifers also occur in Carboniferous and Devonian sandstones.
Groundwater salinity is fresh to brackish, ranging up to about 4 ppt, and bore yields of as much as 2000 kL/day have been obtained from some of the Permian sandstones. The only development has been for pastoral use, and from bore fields put down for the gas pipeline. Groundwater supplied to Gascoyne Junction is from surficial sediments overlying Permian rocks.
The Permian sandstones represent a significant resource of brackish to saline groundwater. Groundwater is likely to be low salinity where the outcrop is crossed by major rivers (Thomson, 2001).
Reference: Allen (1988); WRC Hydrogeology Report HR 191 by Y.Thomson.
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