University of Wollongong Research Online University of Wollongong Thesis Collection University of Wollongong Thesis Collections 1991 Sediment budgets and quaternary history of the Magela Creek catchment, tropical Northern Australia Richard Graham Roberts University of Wollongong Recommended Citation Roberts, Richard Graham, Sediment budgets and quaternary history of the Magela Creek catchment, tropical Northern Australia, Doctor of Philosophy thesis, Department of Geography, University of Wollongong, 1991. http://ro.uow.edu.au/theses/1387 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] SEDIMENT BUDGETS AND QUATERNARY HISTORY OF THE MAGELA CREEK CATCHMENT, TROPICAL NORTHERN AUSTRALIA A thesis submitted in fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY from THE UNIVERSITY OF WOLLONGONG UBfcAfc* RICHARD GRAHAM ROBERTS B.Sc. (Hons) (UCW Aberystwyth) M.Sc. (British Columbia) DEPARTMENT OF GEOGRAPHY 1991 CERTIFICATE OF ORIGINALITY The work presented herein has not been submitted to any other university or institution for a higher degree and, unless acknowledged, is my own original work. Richard Roberts 10th May 1991 iii The Magela Creek catchment is situated in tropical northern Australia. This thesis examines the contemporary sediment transport regime of the sand-bed Magela Creek, the Holocene alluviation of its palaeochannel, the Quaternary history of extensive colluvial sand aprons developed along the flanks of the Arnhem Land plateau, and the chronology of isolated alluvial and lacustrine deposits on the plateau surface. Catchment sediment budgets are constructed at contemporary (AD 1950-1989), Holocene (0-7 kyr) and Pleistocene (7-20 kyr) timescales, and a sedimentation chronology for the sand aprons is extended back to -240 kyr. Contemporary sediment and solute transport rates in the Magela Creek were measured between 1986 and 1989. The measured bedload transport rates compare favourably with those computed by the van Rijn (1984c) model and the mean computed yield for 1980-1989 (1800 m3/yr) corresponds closely to the rate of infilling of Mudginberri Billabong determined for the same period from air photo and bathymetric surveys (1400-2100 m3/yr). Annual yields of washload and solutes are estimated using rating curves. Over the 1971-1989 period, the total terrigenous load transported by the Magela Creek (-12,000 t/yr) consisted of approximately 30% bedload, 15% suspended sand, 45% washload and 10% solutes. The sand fraction is larger than that commonly reported for tropical rivers. It is derived almost entirely from the Arnhem Land plateau and corresponds to a plateau erosion rate of ~5 m3/km2/yr. Washload and solutes are derived mostly from the lowlands and their yields equate with a lowland denudation rate of -28 m3/km2/yr, which lies at the low end of the range reported for other tropical savanna regions. The difference between the two rates suggests that the relative elevation of the plateau is increasing with time. Sand transported along the Magela palaeochannel prior to -8 kyr was discharged into the deep-water estuary that existed at what is now Mudginberri Billabong. Infilling of iv the palaeochannel began at -8 kyr with the downstream progradation of a sand wedge. This sand probably was supplied by gullying of nearby sand aprons, associated with the return of enhanced monsoonal activity to the region during the post-glacial marine transgression. The palaeochannel subsequently accreted vertically, although the rate of infilling over the last 3 kyr was double that of the preceding 4 kyr. Sand aprons, dated by thermoluminescence, began to develop at the foot of the Arnhem Land escarpment at 220-230 kyr and 100-120 kyr: these ages coincide with the start of the penultimate and last interglacials respectively. Since then, the aprons have accumulated at a fairly constant rate (30-70 rnm/kyr). The basal ages of the aprons and their volumetric rates of accumulation imply an escarpment retreat rate of 20-200 rnm/kyr, in contrast to a plateau lowering rate of ~5 rnm/kyr (inferred from the rate of infilling of an enclosed ephemeral lake in the catchment headwaters). The dominance of escarpment retreat over plateau lowering by a factor of 4-40 accords with the classic theories of the parallel retreat of slopes rather than the downwasting of interfluves. v Abstract iii Table of Contents v List of Tables xiii List of Figures xvi List of Plates xix List of Symbols xx Acknowledgements xxiii Preface xxiv PART I BACKGROUND AND METHODOLOGIES 1 CHAPTER 1 INTRODUCTION 2 1.1 Historical context 2 1.2 Study aims and objectives 3 1.3 Timescales and sediment budgets 7 1.4 Thesis framework 9 CHAPTER 2 DESCRIPTION OF THE STUDY AREA 11 2.1 Location and physiography 11 2.2 Geology and soils 18 2.3 Flora and fauna 19 2.4 Climate and surface hydrology 20 2.5 Fluvial sediment and solute transport 26 CHAPTER 3 METHODOLOGY: CONTEMPORARY CONTEXT 31 3.1 Review of sediment and solute discharge methods 31 3.1.1 Terminology 31 VI 3.1.2 Measurement of sediment and solute discharge 33 3.1.3 Measures of flow intensity 3 5 3.1.4 Predictive models of sediment discharge 39 3.2 Measurement of instantaneous sediment and solute discharge 43 3.2.1 Sampling site 43 3.2.2 Hydraulic parameters 45 3.2.3 Stream bed stability 45 3.2.4 Sampling procedures 47 3.2.5 Sampling caveats 51 3.2.6 Model predictions 52 3.3 Temporal integration of sediment and solute discharge 53 3.3.1 Flow duration relations 5 3 3.3.2 Hydraulic geometry relations 54 3.3.3 Predictions of bed material yield 55 3.3.4 Predictions of washload and solute yield 57 3.4 Sand deposition and extraction at Mudginberri Billabong 59 3.4.1 Bathymetric surveys 60 3.4.2 Aerial photography 61 3.4.3 Sand mining activities 63 3.5 Catchment sources of sediment 65 3.5.1 Stream banks and floodplains 65 3.5.2 Arnhem Land plateau and sand aprons 66 3.6 Channel storage of sediment 68 CHAPTER 4 METHODOLOGY: GEOLOGICAL CONTEXT 70 4.1 Quaternary dating techniques 70 4.2 Late Quaternary sediment sources and chronologies 72 4.2.1 Pleistocene floodplains 72 4.2.2 Sand aprons 73 vu 4.2.3 Arnhem Land plateau 7 8 4.3 Sediment storage in the Holocene trench 81 4.4 Sediment export from the Holocene trench 84 4.4.1 Pleistocene sediment output 85 4.4.2 Holocene sediment output 87 PART II RESULTS: CONTEMPORARY CONTEXT 90 CHAPTER 5 SEDIMENT AND SOLUTE TRANSPORT JN THE MAGELA CREEK 92 5.1 Instantaneous discharge of bed material 92 5.1.1 Bedload transport 92 5.1.1.1 Measurement of bedload transport and bedform dimensions 92 5.1.1.2 Temporal variability in bedload transport rate 94 5.1.1.3 Spatial variability in bedload transport rate 100 5.1.1.4 Efficiency of the Helley-Smith bedload sampler 101 5.1.1.5 Channel scour and fill 102 5.1.1.6 Bedload and bed material grain-size characteristics 105 5.1.2 Suspended bedload transport 107 5.1.3 Predictions of bed material transport 118 5,1.3.1 Bedload transport predictions 119 5.1.3.2 Suspended bedload transport predictions 126 5.1.3.3 Total bed material load transport predictions 127 5.1.3.4 Summary of bed material transport predictions 131 5.2 Instantaneous discharge of washload and solutes 136 5.2.1 Washload transport 137 5.2.2 Solute load transport 139 5.3 Temporally-integrated sediment and solute discharge 141 5.3.1 Channel hydraulics 143 5.3.2 Bed material yield 149 5.3.3 Washload yield 162 5.3.4 Solute yield 165 5.4 Summary of contemporary sediment and solute discharge 167 CHAPTER 6 DRILLING OF MUDGINBERRI BILLABONG 172 6.1 Bathymetric surveys 172 6.2 Aerial photography 179 6.3 Summary of contemporary bed material output 184 CHAPTER 7 CATCHMENT SOURCES AND CHANNEL STORAGE OF SEDIMENT 188 7.1 Contemporary sources of bed material 188 7.1.1 Stream banks 188 7.1.2 Arnhem Land plateau 191 7.1.3 Magela Creek gorge 194 7.1.4 Major tributaries 195 7.1.4.1 Stream banks and floodplains 196 7.1.4.2 Arnhem Land plateau 197 7.1.4.3 Sand aprons 198 7.1.5 Summary of contemporary sediment sources 199 7.2 Contemporary storage of bed material 200 7.3 Summary of contemporary bed material inputs and storage 204 IX PART III RESULTS: GEOLOGICAL CONTEXT 206 CHAPTER 8 LATE QUATERNARY SEDIMENT SOURCES AND CHRONOLOGIES 208 8.1 Pleistocene floodplains 208 8.2 Sand aprons 209 8.2.1 Stratigraphy, chronology and sedimentology 209 8.2.2 Comparison of TL and radiocarbon age estimates 229 8.2.3 Analysis and interpretation of the TL chronology 231 8.2.3.1 Ages of the basal sediments 231 8.2.3.2 Rates of vertical aggradation 233 8.2.3.3 Rates of volumetric accumulation 239 8.3 Arnhem Land plateau 241 8.3.1 Bedrock depression 241 8.3.2 Headwater gully 242 8.3.3 Plateau floodplain 243 8.3.4 Ephemeral lake 244 8.3.5 Budgetary aspects of plateau denudation 247 8.4 Summary of geological bed material inputs 252 CHAPTER 9 INFILLING OF THE HOLOCENE TRENCH 255 9.1 Geometry of the Holocene trench 255 9.2 Radiocarbon dating of Holocene alluvium 263 9.3 Thermoluminescence dating of Holocene alluvium 268 9.4 Rate of infilling of the Holocene trench 271 9.4.1 7 kyr to the present 271 9.4.2 7 kyr to 3 kyr and 3 kyr to the present 275 9.5 Summary of geological bed material storage 280 CHAPTER 10 SEDIMENT EXPORT FROM THE HOLOCENE TRENCH 282 10.1 Pleistocene bed material output 282 10.2 Holocene bed material output 286 10.3 Summary of geological bed material output 288 PART IV INTERPRETATIONS AND CONCLUSIONS 291 CHAPTER 11 CONSTRUCTION AND IMPLICATIONS OF THE SEDIMENT BUDGETS 292 11.1