NICKPOINT RECESSION IN KARST TERRAINS - AN EXAMPLE FROM THE BUCHAN KARST, SOUTHEASTERN AUSTRALIA

Nickpoint recession in the Buchan karst, southeastern Australia, has r esulted in the formation of an underground meander cut-off system in t he Murrindal River valley. Three nickpoints have been stranded in the surface channel abandoned by the subterranean piracy, and these can be correlated with river terraces and epiphreatic cave passages in the n earby Buchan River valley. The presence of palaeomagnetically reversed sediments in the youngest cave passage in the Buchan valley implies t hat the topographically lowest nickpoint in the Murrindal valley is mo re than 730 ka old, and the other nickpoints are probably several mill ion years old. The nickpoints are occasionally active during floods, b ut the diversion of most surface flow underground has slowed down thei r retreat to the extent that they have been effectively stationary for several million years Underground nickpoint migration has been by bot h incision within major phreatic conduits and their abandonment for lo wer-level passages. The nickpoints are all present in the upstream par t of the cave system, but have not migrated past the sink in the river channel, despite the long period of time available for this to happen . The sink is characterized by collapsed limestone blocks; these filte r out the coarse bedload from the river channel. As a result, erosion within the cave passages is dominantly solutional and therefore slower than in the surface channel, where it is mostly mechanical. In additi on, to transmit a drop in base level the cave system requires the remo val of a larger volume of rock than for the surface migration of a nic kpoint, because any roof collapse material in the subsurface system mu st be removed. These factors have slowed the migration of the base-lev el changes through the subsurface system, and may be a general feature in caves that have diffuse sinks as their main inputs.