GRAVEL ANTIDUNES IN THE TROPICAL BURDEKIN RIVER, QUEENSLAND, AUSTRALIA

The geological record is punctuated by the deposits of extreme event p henomena, the identification and interpretation of which are hindered by a lack of data on contemporary examples. It is impossible to direct ly observe sedimentary bedforms and grain fabrics forming under natura l particle-transporting, high-velocity currents, and therefore, their characteristics are poorly documented. The deposits of such flows are exposed however, in the dry bed of the Burdekin River, queensland, Aus tralia following tropical cyclone-induced floods. Long wave-length (up to 19 m) gravel antidunes develop during short (days) high-discharge flows in the upper Burdekin River (maximum recorded discharge near the study reach over 25 600 m(3) s(-1) in February 1927). Flood water lev els fall quickly (metres in a day) and flow is diverted away from rais ed areas of the river bed into subchannels, exposing many of the high- stage bedforms with little reworking by falling-stage currents. Gravel bedforms were observed on the dry river bed after the moderate flows of February 1994 (max. 7700 m(3) s(-1)) and January 1996 (max. 3200 m( 3) s(-1)). The bedforms had wave-lengths in the range 8-19 m, amplitud es of up to 1 m with steeper stoss than lee faces and crest lines gene rally transverse to local peak-discharge flow direction. The gravel fa bric and size sorting change systematically up the stoss and down the lee faces. The antidune deposits form erosive based lenses of sandy gr avel with low-angle downstream dipping lamination and generally steep upstream dipping a-b planes. The internal form and fabric of the antid une gravel lenses are distinctly different from those of dune lee grav el lenses. The erosive based lenses of low-angle cross-bedded gravel w ith steep upstream dipping a-b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes. The antidune gravel lenses are associated with thick (to 1 m) high-angle cross bed sets. Ancient antidune gravel lenses may be diagnostic of episodic hi gh-discharge conditions and particularly when they are associated with high-angle cross-bedded gravelly sand they may be useful for palaeoen vironmental interpretation.