Coarse-grained floodplain deposits in the seasonal tropics; towards a better facies model

Floodplain deposits in semiarid and subhumid settings with erratic rainfall may contain relatively high proportions of sand because of climatically controlled patterns of discharge and vegetation cover. Ancient formations with abundant floodplain sands may record fluvial systems with extreme discharge variability of this type but are difficult to interpret because of the paucity of published modern analogues. The Lower Burdekin and Haughton river alluvial and delta plains of north Queensland, Australia, are used here to illustrate this type of fluvial architecture. The modern Burdekin and Haughton channels have gravelly sand beds and transport a mixed load in the wet season. The channels have low-relief levees that decrease in amplitude downstream across the low-gradient floodplains. Tropical-cyclone activity results in high-magnitude, short-duration, sediment-charged floods that inundate large areas of the coastal lowland. Bankfull discharge is infrequently exceeded (return interval >25 years) because the channels are enlarged by major cyclone-induced runoff events and between these events flows are too small to modify the channel form significantly. Suspended-sediment loads and the proportions of these that are sand are particularly high when intense rainfall occurs after prolonged drought, when vegetation cover is reduced. As a consequence of these factors, most of the deposition on the levees and floodbasins occurs in very large discharge events with significant suspended sand load. These floods carry a lot of sand out of the channel and efficiently in suspension over the floodplains because of deep inundation. Boreholes drilled in these floodplains encountered sharply bounded and gradationally bounded sand beds that fine upward or show no vertical grain-size trend. These are interbedded with thin silt, clay, and minor gravel beds. Holocene paleochannels make up about 30% of the floodplain area and consist predominantly of coarse sand that is similar to modern channel-bed deposits. The overbank sand, in contrast, is predominantly fine-medium-grained, and beds have distinctly different granulometry from each other and from channel-floor deposits. Efficient sediment partitioning in the runoff and extreme variation in flood magnitude explain variations in sand-bed granulometry. In the alluvial valley and upper delta plain individual sand beds are difficult to correlate between adjacent boreholes even over very short distances (tens of meters). In contrast, on the lower delta plain overbank sands have a more sheet-like geometry. This is explained by the lower-lying landscape, negligible levee development, greater inundation depths, and reduced surface roughness. In addition the channel bodies are thinner and wider and may be connected to overbank sand "wings."