QUATERNARY STRATIGRAPHY, GEOCHRONOLOGY AND EVOLUTION OF THE MAGELA CREEK CATCHMENT IN THE MONSOON TROPICS OF NORTHERN AUSTRALIA

Magela Creek, a major tributary of the East Alligator River in northern Australia, has left a detailed sedimentary record of a fluvial landscape dominated by climatic and eustatic changes associated with Quaternary glacial-interglacial cycles. Uranium-series dates from young pisoliths in floodplain deposits indicate that ferruginisation is probably ongoing under present conditions while ferricretes in degraded terraces that flank the lower valley reveal a fluvial history extending back to early Pleistocene or Tertiary time. Inset within this older alluvium is a valley fill which, from thermoluminescence dates, was initiated about 300 kyr ago. With each glacial climate change and associated fall in sea level, distinct palaeochannels have been eroded into these floodplains, infilling later with alluvium when climate and base-level conditions were conducive to fluvial deposition. Radiocarbon dates show that the most recent palaeochannel beneath the modern Magela Creek last started to fill by downstream progradation and vertical accretion of bedload sand about 8 kyr. The palaeochannel filled at an accelerating rate, probably as a result of declining stream competence associated with drier conditions in the late Holocene augmented by the backwater effects of sea-level rise. Continued aggradation blocked the mouths of tributary valleys along Magela Creek, forming alluvial-dammed tributary lakes and deferred-junction tributary streams. From about 300 kyr, cyclic episodes of channel incision and sediment evacuation in this tropical-monsoon river valley have become less effective, possibly because increasing aridity in the late Quatemary has reduced the erosional effectiveness of Australia's northern rivets. Reduced flow regime and rising sea level in the late Holocene has resulted in the latest phase of alluvial accretion.