LOWSTAND RIVERS NEED NOT INCISE THE SHELF - AN EXAMPLE FROM THE GREAT-BARRIER-REEF, AUSTRALIA, WITH IMPLICATIONS FOR SEQUENCE STRATIGRAPHICMODELS

A key tenet of many sequence stratigraphic models is that sea-level lo wering causes widespread fluvial incision of the subaerially exposed c ontinental shelf, and that river-borne terrigenoclastic sediments bypa ss the lowstand shoreline via canyons to the continental slope and bas in floor. Consequently the occurrence of incised channels is considere d a fundamental criterion for the recognition of sequence boundaries i n ancient shelf successions, Contrary to this, we argue that rivers ma y not necessarily incise during glacio-eustatic lowstands if they flow out onto a coastal plain flanked by a broad, low-angle shelf. On the Great Barrier Reef shelf, fluvial incision did not occur during the la st glacial maximum (LGM), but instead, subaerial accommodation was cre ated and infilled as contemporary rivers graded to the ''LGM-bayline.' ' Incision was restricted to the lowstand shelf break, where canyons o f limited extent formed by nickpoint retreat.