Cenozoic landscape evolution in central Queensland

Central Queensland lies on the passive margin of eastern Australia and owes its landscape to processes that began following rifting and opening of the Tasman Sea. The modern landscape is the result of long-term processes of landform development, and the landforms themselves are the evidence of these processes. Hence, interpreting their significance provides an understanding of long-term landscape evolution. Along the eastern Australian coast, numerous rivers drain into the sea but among these, there are two that stand out: the neighbouring Fitzroy and Burdekin Rivers in central Queensland. These two streams have by far the largest catchments of any rivers along the eastern seaboard of Australia. The Burdekin and Fitzroy catchments contain widespread remnants of Cenozoic deposits, which accumulated predominantly in fluvial and lacustrine environments established during the Palaeogene. Alluvial sediments were supplied by erosion of nearby uplands, and accumulated in depressions and basins on a prior land surface. Volcanic activity also resulted in large lava flows in central western areas. Water was the main agent of sediment transport, distributing unconsolidated deposits along the drainage networks of the time, some of which were directed inland. It is inferred that during the Palaeogene, the divide between coastward and inland draining streams was further to the east than it is at present. Several basins were located west of the former coastal divide, and were characterised by continental environments of deposition in a generally westward drainage system. With continued accumulation of sediments, individual basins overflowed and merged to form a widespread flat-lying Palaeogene landscape that concealed an earlier land surface on which bedrock was more extensive. In the Early Cenozoic, there was a change from the depositional phase that resulted in the continental sequence, to an erosional phase that developed the modern landforms. The change from deposition to erosion probably started during the Palaeogene. Erosion continued through to the present, re-exposing parts of the basal Palaeogene sequence and earlier Mesozoic land surface. The erosional phase that shaped the landforms of the modern Burdekin and Fitzroy catchments can be explained by slowly evolving drainage basins in the interior being captured by small coastal streams—the predecessors of the Burdekin and Fitzroy Rivers. The coastal streams were short and steep in comparison with those in the interior, allowing a more active erosional environment along the coast. As the coastal streams expanded, the drainage divide moved rapidly westwards. Stream capture began a phase of regional erosion, which transported large quantities of sediments to the coast. The sediments contributed to coastal and nearshore features similar to the Holocene high sea-level examples at the mouth of the Burdekin River in the north, and the Fitzroy Delta and the Keppel Coast in the south. Large volumes of sediments were also transported beyond the present coast during low sea-levels of the Cenozoic, forming similar coastal features and contributing to a major eastward bulge on the central Queensland continental shelf. The emptying of continental basins has paralleled the development of the continental shelf bulge from the coast to the Marion Plateau.