Lowstand rivers need not incise the shelf: An example from the Great Barrier Reef, Australia, with implications for sequence stratigraphic models

A key tenet of many sequence stratigraphic models is that sea-level lowering causes widespread fluvial incision of the subaerially exposed continental shelf, and that river-borne terrigenoclastic sediments bypass the lowstand shoreline via canyons to the continental slope and basin floor. Consequently the occurrence of incised channels is considered a fundamental criterion for the recognition of sequence boundaries in ancient shelf successions. Contrary to this, we argue that rivers may not necessarily incise during glacio-eustatic low stands 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 last glacial maximum (LGM), but instead, subaerial accommodation was created and infilled as contemporary rivers graded to the "LGM-bayline." Incision was restricted to the lowstand shelf break, where canyons of limited extent formed by nickpoint retreat.