Notes on the geology and geomorphology of De Witt Island, southwest Tasmania

Abstract

De Witt Island, off the south coast of Tasmania, is composed of a folded succession of turbiditic silstone, sandstone and conglemerate, at least 450 m thick. It is probably a correlate of the Mid to Upper Cambrian Ironbound Group. Folds are upright, open to tight and at least two generations are indicated. The broad-scale geomorphology of the island is controlled by the direction of storm waves which have eroded nearly vertical cliffs up to 340 m high on the south shore. Many sea caves are found at the base of the cliffs, particularly on the west and south coasts. Cliff retreat has progressively captured the headwaters of the island's two perennial streams. Fluvial processes, controlled by both lithology and structure, but also showing possible influence from interglacial and glacial stages, have shaped the interior of the island for a long period. Aeolian processes have deposited sand sheets in the island's central basin. Some well-developed pseudokarst systems, including sinkholes, caves, underground drainage and airflows, are related to large rotational slumps on the island's south coast.