Geomorphic and tectonic significance of early Cretaceous lavas on the coastal plain, southern New South Wales

Summary

1. The stratigraphy of the Sydney Basin strata and the presence of palaeolandforms on the New South Wales south coast dispute the notion that the coastal lowland in these specific locations resulted from tectonic lowering of the upland plateau. These regions therefore cannot be used as evidence for a tectonic origin for parts of the New South Wales far south coast cut into fold‐belt strata.

2. The attitude of flow structures within intrusive rocks of the Mt Dromedary Igneous Complex dispute the notion that these structures were emplaced vertically and that they can be used to determine the extent and direction of subsequent tectonic movements.

3. There is no published evidence for kilometres of Mesozoic volcanic and volcaniclastic rocks having covered all or much of southeastern Australia and it is unlikely given the recognised evidence that either of the cooling events was due to burial and later removal of this volcanic cover.

4. The presence of lavas older than 100 Ma on the New South Wales south coast shows that this erosional land surface has remained largely unchanged apart from possible exhumation since this time irrespective of the altitude of this surface at any time during its existence.

5. The fact that relief and landscape morphology play a major role in controlling both the rates and extent of denudation in any given landscape and that much of the southeastern highlands are drained by streams adjusted to base‐levels within the highlands poses difficulties for understanding why and how thick and extensive covers of volcanic and volcaniclastic rocks, and for that matter any other thick sequences of strata, could have been removed between the mid‐Cretaceous (ca 100–90 Ma) and earliest Tertiary. The potential energy gained by streams draining the highlands as a consequence of uplift is converted into energy for erosive work largely at the heads of gorges and at major nickpoints, not in the plateau landscapes upstream.