How the Great Australian Bight Was Formed

The Great Australian Bight is the largest indentation on the Australian coast, has the longest line of sea cliffs in the world, and even holds the title for being the longest south-facing coastline in the world. Simply stated, a Bight is a bend in the coast that forms an open bay, and this particular one starts in the west at West Cape Howe and stretches all the way to the South West Cape in Tasmania. While Europeans didn’t explore the shores of the Bight until the 19th century, the area has been known to and occupied by Aboriginal people since time immemorial.

It’s easy to see the Bight, or at least part of it. Anyone who has been on a sightseeing tour in Australia, such as a Great Ocean Road tour, who has driven across the Nullarbor Plain, or who has spent much time on the West Coast of Tasmania has been able to catch a glimpse of it.

A bit about the Bight

The continental shelf that makes up the Bight is very shallow and wide, and in some spots the shelf break is close to 190 kilometres away from the shore. Beyond the shelf break, the depth quickly descends from an average of 200 metres deep to four kilometres deep by the time the slope reaches the abyssal plain.

The white-coloured rock that sits near the base of the cliffs is known as Wilson Bluff Limestone, and was formed on the seabed between 38 and 42 million years ago. Like the cliffs of the Bight, the seabed is also made up of the same rock, with sediments such as seashells layering overtop. The ocean currents that come through the Bight make it a biologically productive area, which attracts plenty of ocean life.

Geological formation of the Bight

The fragments of land that are now called Australia have gone through plenty of periods of amalgamation and fragmentation throughout the last few billion years due to constant tectonic action. But it wasn’t until the breakup of Gondwana that Australia as we know it began to take form. Between the Early Permian and the Late Jurassic (280-160 million years ago), a rift valley began to form between what is now Australia and Antarctica, with the Bight representing the edge of that division.

Australia and Antarctica had been together for about a billion years, but only took about 100 million years to completely separate from each other. The break-up commenced as a result of sea-floor spreading of the Bight Basin, proceeding from west to east. The break-up isn’t thought to have been accompanied by much magmatism, and thus the margin has been classified as a “magma poor” (or non-volcanic) rifted margin.

The sea-floor spreading was slow in the beginning, but by the mid-Eocene (45 million years ago) a widening seaway appeared between the two continents. About 10 million years later, the Bight Basin had become a well-developed distal ocean-continent transition zone, which only widened as Australia continued on its northward path.

After Australia and Antarctica severed their final connection along the Tasman Fracture Zone, and the current that circumnavigates Antarctica was established, the sea in the Bight Basin began evolving into the form it’s in today.

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