West Australian Basin

a depression in the eastern part of the Indian Ocean. It is enclosed in the south by the West Australian, or Broken, Ridge, in the west by the East Indian Ridge, in the north by a chain of underwater mountains between the Cocos and Christmas islands, and in the east by the continental slope of Australia. A rugged hilly relief predominates. There are large mountain massifs near Australia, which evidently are the remains of old continental structures. Depths range from 5,000 to 6,500 m; over several ridges they subside to 1,555 m. The seabed consists of red clays, as well as radiolarian and foraminiferal oozes. Ferromanganesian concretions are prevalent, and outcrops of bedrock are found on the summits of the mountains and hills.