Plant-material deposition in the tropical Burdekin River, Australia: implications for ancient fluvial sediments

Abstract

In the deposits of the Burdekin River (north Queensland, Australia), plant material (character and directional fabric) together with the sedimentary facies character are generally diagnostic of depositional environment. Plant material is selectively entrained, transported, and deposited in all flow conditions ranging from dry season minimal flow (<10 m 3 s −1) to tropical cyclone-induced catastrophic flood (up to 40,393 m 3 s −1). Deposition of plant material occurs in five main ways: (1) burial in situ, (2) fall directly onto sediments of the emergent bars and burial, (3) wind transport before burial, (4) entrainment and deposition by flowing water, and (5) pre-existing accumulations of plant material within sediment may be eroded and re-deposited. Each of these depositional styles produces plant-material assemblages with distinct characteristics: (1) in-situ plant remnants (roots and stems) overturned in the flow direction, (2) mostly leaf material within falling stage mud near-parallel to bedding surfaces and showing weak–strong alignment often with more than one mode, (3) dense accumulations of complete leaves and small twigs in dune troughs and scour hollows, often with locally strong clast alignment but wide directional scatter through the deposit, (4) (i) isolated, palaeoflow-parallel logs within bar deposits, dense chaotic accumulations of logs and other debris, either (ii) associated with in-situ trees, or (iii) with bank irregularities, and (iv) strandline deposits (dominantly wood fragments, seeds, and minor proportions of other plant parts with a wide range of orientations), (5) finely comminuted plant material in cross-bedded and ripple cross-laminated sand, often composed dominantly of leaf fragments with long-axis alignment related to sedimentary structure. The predominance of different assemblage types and the nature of those assemblages vary with discharge and with location within the channel belt. Identification of characteristic plant-material assemblages, combined with sedimentary facies should allow improved interpretation of the depositional environment of ancient deposits and help assess discharge variability. In the Burdekin River and similar settings, the plant-material assemblages within the channel are dominated by plant parts from plants growing within the channel, and the floodplain community is relatively unrepresented.