Mesozoic regolith and palaeolandscape features in southeastern Australia: Significance for interpretations of denudation and highland evolution

Abstract

The long‐term survival of regolith and landscape features of Mesozoic origins in southeastern Australia is supported by the sedimentary record, pre‐volcanic remnants, oxygen‐isotope signatures of secondary minerals, and the associated geomorphological context. The recognition of these ancient landscape remnants reflects the establishment of a major paradigm in Australian landscape studies after early interpretations emphasised a predominantly Late Cenozoic landscape history. The Mesozoic regolith and landscape remnants constrain interpretations of the evolution of the highlands in this region, indicating that Late Mesozoic and Cenozoic denudation did not extend beyond the complete removal of Mesozoic landscape features. Apatite fission track thermochronology interpretations invoke kilometre‐scale denudation levels across the southeastern highlands during the mid‐Cretaceous, which at first may appear contradictory to the geomorphological evidence of restricted denudation. Rather than necessarily being mutually exclusive interpretations, possible scenarios allowing for the preservation of Mesozoic palaeosurfaces along with kilometre‐scale denudation in the mid‐Cretaceous include: (i) exhumation of palaeosurfaces from beneath a thick sedimentary cover; (ii) truncation of formerly more extensive weathering profiles; and (iii) local‐scale variations in denudation. Local‐scale variations in denudation may not have been detected in previous studies due to dangers associated with overextending regional extrapolations and interpretations, as well as a tendency for the field basis of the different denudation models to emphasise different parts of the landscape. Field studies from areas where there has been localised deep incision into Mesozoic landscape remnants highlight the problem. Geomorphological studies have tended to emphasise areas of relative stability, featuring the preservation of regolith materials and the associated long‐term landscape record. In contrast, apatite fission track thermochronology interpretations may tend to emphasise a regional thermal history related to maximum denudation. Local variations in denudation leading to the preservation of Mesozoic palaeosurfaces have mainly been facilitated by localised lithological and structural controls on stream base‐levels and knickpoints, tectonic setting, and sedimentary and volcanic burial. When palaeolandscape interpretations are considered at the local scale, arguments proposing long‐term stability based on palaeolandscape remnants and apatite fission track thermochronology interpretations of large‐scale denudation may therefore not necessarily be contradictory.