Variations in character and preservation potential of vegetation-induced obstacle marks in the variable discharge Burdekin River of north Queensland, Australia

Vegetation-induced obstacle marks are described from Dalrymple Bend in the Burdekin River of Queensland, Australia, and their preservation potential is discussed. The bend is divided into a sand- and gravel-covered lower bar, and a vegetated upper bar. Obstacle marks, comprising an erosional scour and a depositional sediment tail, are recognised on the lower bar and the lower margin of the upper bar. Two types of obstacle marks are dominant in the lower part of the lower bar; sediment tail-dominated obstacle marks associated with inclined trees of Melaleuca argentea (Silver-leaved Paperbark), and comparatively small obstacle marks around stranded driftwood. Obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel. At the lower margin of the upper bar, scours and gravelly sediment tails are recognised around mature tall trees. In these cases, the scour is >4 m in width, and covered with alternating beds of fine sand and mud. Among the obstacle marks described, sediment tail-dominated obstacle marks around groves of M. argentea have the highest preservation potential. Obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential. Obstacle marks around stranded driftwood have the lowest preservation potential. Obstacle marks around grass clumps survive more than 1 year, but their long-term preservation potential is nonetheless low. Implications for the stratigraphic record are also discussed. (C) 2002 Published by Elsevier Science B.V.