Sedimentary Facies from Ground‐Penetrating Radar Surveys of the Modern, Upper Burdekin River Of North Queensland, Australia: Consequences of Extreme Discharge Fluctuations

Abstract

Ground-penetrating radar (GPR), mapping and sediment-sampling techniques were used to investigate a reach of the upper Burdekin River, Australia after successive discharge events (1994-1997). The river is moderately sinuous, partly bedrock-controlled and has incised a channel 5-25 m deep and 300-400 m wide. It drains a large, mainly subhumid to semi-arid catchment that receives erratic summer rainfall, related to the passage of tropical cyclones across north-east Australia. Larger, short-duration discharge events cause major changes to the channel, bars, banks and bed but, because of their short duration (days to weeks), equilibrium channel form does not develop, channel-forming discharge is difficult to define and the deposit characteristics depend on the sequence of individual events. Discharge events that cause large-scale change to the deposits have a recurrence interval of about 8.5 yr. The extreme discharge variability of the Burdekin is reflected in its facies assemblage. A variety of barforms, including braid, lateral, transverse and point bars, are exposed on the river bed for much of any year and these are covered by dunes of varying geometry, plane beds, gravelly antidunes and gravel patches, with significant, stable arborescent vegetation (Melaleuca argentea), mainly in flow-parallel, linear groves. Successive GPR surveys, before and after the March 1997 (17 808 m s) channel-modifying event, demonstrate up to 6 m of bed erosion and deposition. The GPR data show distinct facies zonation relative to position on or in the major complex bars. Lower bar deposits are well-sorted, coarse- to very coarse-grained, gravelly sands and upper bar and bank-top deposits are principally silts to very fine-grained sands, with some interbedded coarser grained beds close to the active channel. Above the basal erosion surface, large-scale (to about 3 m) cross-bedded facies pass up-dip of major bedding surfaces and in some places vertically up into smaller scale cross-bedded facies. In places and at times the very large-scale cross-strata are truncated, recording reworking during the falling stage of major discharge events or during smaller events. Planar-stratified and cross-bedded facies are preserved above truncation surfaces. Lesser discharge events cause minor modification of topography, and deposit sediments on the bed and lower bar surfaces. The upper bar deposits are formed in major discharge events and deposits of individual events drape topography and thicken into swales. The facies assemblage and architecture is distinctive of this style of river and may be useful in identifying the deposits of variable-discharge rivers in the rock record.