Geological Foundations of Shark Bay
Geological cross section of Shark Bay
The Shark Bay area is part of the Carnarvon Basin, a geological depression stretching along thecoast of Western Australia. It comprises two main layers of marine sediments: a 4 km-thick Silurian sequence, dating from about 434–410 million years ago, and a 2 km-thick Devonian-Lower Carboniferous sequence, dating from about 410–320 million years ago. On top of this is another layer of chalky limestone called Toolonga Calcilutite, formed during the Cretaceous period some 144–65 million years ago.
By contrast, Shark Bay’s surface geology is relatively young. It is mostly sandstone and limestone, composed of sediments deposited by the wind and sea during the last two million years (Quaternary period). This map shows how Peron Sandstone and Tamala Limestone dominate the Shark Bay region.
Peron Sandstone underlies most of the region, but is exposed only on Peron Peninsula and Fauré Island, off Monkey Mia. Its rich red colour comes from the thin layer of iron oxide coating each grain of quartz. The sandstone originated in ancient seabeds hundreds of kilometres east of Shark Bay. About 250,000 years ago sediments from this seabed were blasted to the coast by easterly winds, or shorn off by torrential rains and carried down rivers to the sea.
Tamala Limestone was formed from marine deposits – shells, skeletons and corals – eroded by the severe Shark Bay winds.
The white Tamala limestone, which overlays most of Shark Bay, was formed from shells and other marine skeletons. During the last glacial period (or ‘ice age’) some 125,000 years ago, falling sea levels exposed the shells to the winds, which eroded them into sand particles and blew them into enormous dunes. These dunes accumulated against an anticline that underlies present-day Edel Land, Dirk Hartog Island, and Bernier and Dorre Islands. Over time, calcium carbonate (lime) from percolating rainwater cemented the sand into stone. The Zuytdorp Cliffs are made of Tamala Limestone.
Later, a rise in sea levels during the Holocene epoch (the most recent 10,000 years) flooded the region and created Shark Bay’s unique double-basin form. It also left more marine deposits, which were exposed during a later drop in sea-level and slowly eroded into sand to form giant white dunes overlaying the Tamala Limestone.