Ponding and major drainage diversions by late Palaeogene basalts, Shoalhaven River catchment, New South Wales, Australia

摘要

There is a continuing debate about the long term stability or otherwise of drainage lines and divides, and the importance or otherwise of Cenozoic volcanism and tectonics in the landform evolution of eastern Australia. This paper reports new observations and interpretations in the Shoalhaven River catchment of southeast New South Wales, which has featured prominently in this debate. The Shoalhaven River flows north northeast in a broad valley in the highland plateau at about 600 m before bending abruptly east, to flow to the coast. Upstream and downstream of the bend the river is incised into the plateau in a 500 m gorge. Not far downstream of the bend four outcrop areas of late Eocene to Oligocene basalt and sediments occur either side of the gorge. The basalts, comprising multiple flows with sediment interbeds, filled palaeovalleys incised around 100 m into the plateau. They are truncated by the gorge, which entirely post dates the basalt. The basalts dammed a late Eocene cast-flowing former Shoalhaven channel, forming a long lake upstream about 100 m deep. The lake overflowed north through a narrow gap in the low watershed into another lake in the adjacent Wollondilly River catchment; and it slowly filled with sediment over several million years. The river, flowing north over the sediment fill, was then re-diverted to the cast, initiating the present lower Shoalhaven course. Much of the basalt dam is still preserved as the fill of the former late Palaeogene east-flowing tract of the Shoalhaven, now 3-4 km north of the present gorge. Regional gentle southerly tilt of the plateau caused late Palaeogene alluviation in the gap in the Shoalhaven/Wollondilly divide; and also reduced to near zero the gradient of the sediment-filled late Palaeogene former north-flowing tract of the Shoalhaven. Results of this study support the conclusion of TAYLOR (1911) that drainage lines in eastern Australia were substantially modified by Cenozoic basalts. Conclusions of recent authors that the course of the Shoalhaven has not changed during the Cenozoic are based largely on an erroneous interpretation that the basalts flowed from shallow palaeovalleys over the sides of the gorge and have remained there ever since. This error led also to serious underestimates of Cenozoic erosion rates.