North Island (Houtman Abrolhos) - Geology and Physiography

Geology and Physiography

The basement of North Island is the Wallabi Limestone, a dense calcretised, coral limestone platform that underlies the entire Wallabi Group. Arising abruptly from a flat shelf, it is about 40 m (131 ft) thick, and of Quaternary origin. Areas of reef that formed during the Eemian interglacial (about 125,000 years ago), when sea levels were higher than at present, are now emergent in places, and these form the basement of the group's central platform islands, namely West Wallabi Island, East Wallabi Island and North Island.

North Island's basement for the most part does not exceed 1.5 m (4 ft 11 in) in elevation. Much of it is capped by aeolianite, and nearly all of it is covered with sand, but there are some exposed outcrops. The southern margin of the island, for example, takes the form of a low cliff, which is severely undercut by the sea in many places.

There are extensive dunes of unconsolidated Holocene sand along both the western and eastern sides of the island. The topography of these dunes varies with time: in 1913, Dakin recorded the dunes as being a good deal higher in the east than in the west, but in 1960 Storr found the eastern dunes to be severely eroded, apparently because of a fire that burnt much of the island's vegetation in 1935. In the centre of the island is a low plain with a sinkhole in its southwest corner and a small salt lake near its northern edge. The soil in the centre of the plain is shallow loam, whereas the rim is deeper and composed largely of shell fragments.