Evolution of the Illawarra escarpment terrain

Phil Flentje

Drawing on the disciplines of stratigraphy, plate tectonics and passive margin development, the evolution of Illawarra Escarpment terrain in the southeastern corner of the Sydney Basin is placed within known geological constraints. The development of the terrain begins with the initial deposition of the early Permian Sydney Basin sequences, into what was a developing (yet subsequently failed) rift fracture within the eastern side of the Gondwana supercontinent (the Sydney – Gunnedah – Bowen Basin trough), through to the eventual rifting of the existing continental margin and opening of the Tasman Sea around 70 million years ago in the vicinity of Sydney. The timing of this rifting and a regional cross-section across the coastline and out into the Tasman Basin allow the assessment of a long term rate of escarpment retreat of 0.6 m per 1000 years. Cenozoic, Quaternary and in particular Holocene climate variability and the attendant sea-level changes and the impact these have on the escarpment terrain is demonstrated. It is shown that the Illawarra Escarpment has evolved as a result of the natural processes of escarpment retreat and associated slope processes, coastal erosion and marine abrasion. The escarpment can be considered to be of Quaternary age albeit with its precursor slopes extending back into the Neogene period.

A landslide inventory managed by the University of Wollongong currently identifies 665 landslide locations and a total of 1050 landslide events in this region over the last 120 years. This inventory facilitates the assessment of contemporary escarpment retreat rates. These rates range from 0.1 m to 1.0 m per 1000 years. Whilst the process rates will vary by small amounts elsewhere along the coastline, the same logic and discussion applies for much of the south-east coast of Australia.