Anatomy of the buried Burdekin River channel across the Great Barrier Reef shelf: how does a major river operate on a tropical mixed siliciclastic/carbonate margin during sea level lowstand?

A large palaeochannel on the northeastern Australian continental shelf has been imaged by a series of shallow seismic reflection profiles. The buried channel forms an important Pleistocene route of the Burdekin River and extends almost continuously for similar to 160 km from the present coast to the outermost reef The channel floor profile steps across the shelf with alternating segments of gentle gradient (flats) and steeper gradient (ramps). Channel sinuosity as interpreted from seismic records varies among segments between I and 1.72, with no consistent relationship between sinuosity and gradient. The lower and upper parts of the channel fill have different geometry and reflection character, suggesting channel excavation and initial filling occurred during a different regime than final filling. In one section of the shelf, about the - 50 in isobath, the channel is difficult to define and appears to have wandered significantly, either because it has been modified by shoreface erosion ca. 10.5 ka or because the river encountered a change in topography in front of karstified reefs. As the channel passes between the numerous outer shelf reefs, in water depths of 70-80 m, it becomes progressively smaller, conspicuously underfilled, and absent entirely over the outermost 10 kin of the shelf No discrete lowstand river mouth could be recognised on the present shelf edge. The elevations of flat segments on the channel floor profile show considerable similarity to published elevations of stillstands or brief rises in sea level attained during the long-term drawdown associated with the last glacial cycle (125-20 ka) and are interpreted to have formed during this stepwise drop in sea level. Channels were cut and partially filled during the fall and lowstand and then backfilled during the Holocene transgression. The ancestral channel of the Burdekin River therefore preserves a rare insight into the stratigraphic record of falling sea level during the last glacial. (C) 2002 Elsevier Science B.V. All rights reserved.