Year

2000

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

Detailed knowledge of the fluvial history and stratigraphy of Quaternary deposits in semi-arid central Australia is at present poorly understood. This thesis presents a detailed study of the Quaternary fluvial and aeolian deposits in the Cooper Creek floodplain of southwest Queensland.

The present Cooper floodplain is very low gradient (~0.00015) and consists several metres of near surface muds characterised by abundant shallow surficial braid-like and reticulate channels, and less common but widespread deeper and narrower anastomosing channels, some with sandy beds. These deeper channels often interconnect much larger waterholes, some of which have scoured beneath the m u d into an extensive underlying sand sheet which was deposited primarily by laterally migrating rivers during the middle to late Quaternary. Extensive subsurface excavations revealed evidence of lateral accretionary surfaces, point bar development and upward-fining sequences. In a very few locations, the planform of these meandering channels is still visible within the muds of the present floodplain surface.

Floodplain muds were deposited35 m. The TL (thermoluminescence) evidence indicates that the fluvial sand-regime dominated oxygen isotope Stages 5-7 with a maximum recorded TL date in this study of >700 ka at a depth of 27 m and represents the oldest TL dated fluvial deposit in Australia. The peak of fluvial sand activity is -100 ka, while m u d transport dominated during Stage 1 and 3 pluvials. Good agreement exists between the fluvial TL dates and worldwide interglacials (Stages 1, 3, 5 and 7). However, there is also a strong fluvial signature evident during the glacial Stage 6 (186-127 ka).

During the transitional phases between sand and mud-load regimes, source bordering dunes were developed: the remnants of which appear at the surface of the contemporary Cooper Creek floodplain. Interestingly, the bulk of the dune TL dates range between Stages 3 and 1 with a pronounced phase of activity evident during the pluvial Stage 3, whilst very few dune dates appear in Stage 2. The role of source-bordering dunes is an important factor influencing channel morphology of the anastomosing channels. A model of channel development is presented using evidence from rare but recent evidence of channel change.

Climatic change during the mid to late-Quaternary is proposed as the major factor influencing the palaeohydrology and sediment transport of Cooper Creek, however, the role of neotectonic events cannot be discounted.

Share

COinS