Stratigraphy and Mid-To-Late Quaternary Chronology of the Cooper Creek Floodplain, Southwest Queensland, Australia Jerry Christopher Maroulis University of Wollongong
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2000 Stratigraphy and mid-to-late quaternary chronology of the Cooper Creek floodplain, Southwest Queensland, Australia Jerry Christopher Maroulis University of Wollongong
Recommended Citation Maroulis, Jerry Christopher, Stratigraphy and mid-to-late quaternary chronology of the Cooper Creek floodplain, Southwest Queensland, Australia, Doctor of Philosophy thesis, School of Geosciences, University of Wollongong, 2000. http://ro.uow.edu.au/ theses/1982
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STRATIGRAPHY AND MID-TO-LATE QUATERNARY CHRONOLOGY OF THE COOPER CREEK FLOODPLAIN, SOUTHWEST QUEENSLAND, AUSTRALIA
A thesis submitted in fulfilment of the requirements for the award of the degree
Doctor of Philosophy from
UNIVERSITY OF WOLLONGONG by
JERRY CHRISTOPHER MAROULIS
B. Sc. (Hons.) (Syd.), Grad. Dip. Ed. (Sec.) (U.C.S.Q.), M. Sc. (Hons.) (Woll.)
School of Geosciences
June, 2000 I, Jerry Christopher Maroulis, declare that this thesis, submitted for the award of Doctor of Philosophy, in the School of Geosciences, University of Wollongong, is wholly my own work unless otherwise referenced or acknowledged. The document has not been submitted for qualifications at any other academic institution.
Jerry Christopher Maroulis
Dated: 19 June 2000
ii 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 of 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 mud 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 deposited <100 ka and provided evidence of westward migration of the floodplain, a process which continues to the present. The change in sediment transport regime from a mixed-load to a mud-dominated regime occurred between 200-80 ka which itself was proceeded by sand-load fluvial processes. The sand sheet underlies most of the muddy floodplain to depths of >35 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 mud 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).
iii 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.
iv Acknowledgements
There have been many, many generous individuals and organisations that have assisted me to complete this thesis. I would like to take a moment to thank some of them.
To my principal supervisor, Professor Gerald Nanson (School of Geosciences) University of Wollongong), thank you for your friendship and for being so patient with me over the years. It was always going to be a difficult proposition to finish the thesis especially when working in a different discipline area at another university in another state. But with your guidance and subtle prodding, I was able to see the thesis to completion. For that and a whole lot more, I will always be in your debt.
To Associate Professor Brian Jones (School of Geosciences), I appreciate all the assistance that you have given me including your constructively critical feedback on sedimentological/stratigraphical aspects of the thesis.
This study would not have been possible without the financial support of the Australian Research Council, Wollongong University and the financial and logistical support offered by SANTOS Pty Ltd. Thanks also go to John Hughes, Rick Smith, Steve Aufderheide and Lyn Pommery from Operations Geophysics at SANTOS in Adelaide. I would also like to thank Mark Thompson and Ake Thormansson and the numerous other SANTOS personnel at Jackson. Your friendship and support was very much appreciated.
My appreciation also goes to Dr. Colin North (University of Aberdeen) for his initial financial support via Shell (UK) to kick start this study. This was an important catalyst for more intensive work in the study area.
Many thanks go to all those field assistants that have toiled with me across the dusty Cooper Creek floodplain including: Professor Martin Gibling (Dalhousie University), Tom Bradbury, Richard Walsh, Dr. Richard (Bert) Roberts, Dr. Stephen Tooth, Dr. Rainer Wende, Mike Breunig, Maria Coleman, Dr. Ivers Reinfeld, Ingrid Wootton, Stephen Beaman and Dr. Alan Mclnally and especially Geoff Black who provided me with endless hours of bad jokes whilst drilling on the floodplain. Thanks also to Joe Caravallero from Delco (Adelaide) for excavation work, to David Price and Jose Ambrantes for assistance with TL dating and to Rachel Nanson for sediment analysis.
Thanks also to David Martin and Tony Skinner, for their assistance in producing some of the 'excellent' diagrams for the thesis. Also, thank you to the Department of Education at the Faculty of Education at the University of Southern Queensland for financial assistance and encouragement to complete the thesis, especially to Dr. Ron Skilton. To Dr. Tony Rickards, thanks for the words of wisdom.
v To my friends and collegues over the duration of my studies that have helped me along the way, in particular, Vickie Williams, Leanne Wirth, Carol Nanson, Jacqueline Shaw, Professor Brian Roberts, Marie Schulz, Sandy Kidd (Windorah), Dr. Roger Callen, Dr. Toni O'Neill, Dr. Rob Loch and my old mate Anthony Sherratt.
To my darling family, my wife Samantha, my 3 year old son Jeremy and my 4 month old son Daniel, thanks for putting up with me for so long. You may actually get to see me more often from now on. Also, to my mother Vickie, my sister Maria and brother Jimmy, and also to my extended family, thank you for your support and understanding. Finally, this thesis would not have eventuated without the support and wisdom of my father. By his own admission, he was not an intellectual giant but he was in my mind. Thanks Dad. This is as much your achievement as it is mine. You will never be forgotten.
VI Contents
ABSTRACT Ill
ACKNOWLEDGEMENTS V
CONTENTS VII
LIST OF FIGURES IX
LIST OF TABLES XI
CHAPTER 1 INTRODUCTION 1 1.1 CONTEXT OF STUDY 1 1.2 QUATERNARY DEVELOPMENT OF THE LAKE EYRE BASIN - A LITERATURE REVIEW 2 1.2.1 Quaternary aeolian activity and dune development 3 1.2.2 Quaternaryfluvial and lacustrine processes 6 1.3 AIMS AND OBJECTIVES 11 CHAPTER 2 PHYSICAL CHARACTERISTICS OF THE COOPER CREEK STUDY AREA .. 12 2.1 INTRODUCTION 12 2.2 CLIMATE 13 2.3 HYDROLOGY 16 2.4 GEOLOGY 20 2.5 PHYSIOGRAPHY 22 2.5.1 Dissected tablelands 24 2.5.2 Rolling downs 24 2.5.3 Dune complexes 26 2.5.4 Alluvial plains 27 2.6 SOILS 27 2.7 VEGETATION 28 CHAPTER 3 DATA COLLECTION TECHNIQUES 29 3.1 INTRODUCTION 29 3.2 TRENCHING 29 3.3 DRILLING 29 3.3.1 Uphole drilling 30 3.4 GEOPHYSICAL TECHNIQUES 31 3.4.1 Gamma-logging of auger holes 31 3.5 DATING TECHNIQUES 35 3.5.1 Thermoluminescence (TL) Dating 36 3.6 METHODS OF SEDIMENT ANALYSIS 40 CHAPTER 4 REGIONAL STRATIGRAPHIC ASSOCIATIONS 42 4.1 INTRODUCTION 42 4.2 GROUP 1: MUD-CAPPED SAND DEPOSITS (MAIN FLOODPLAIN SECTIONS) 43 4.2.1 Seismic line evidence 43 4.2.1.1 Surface Clay: 49 4.2.1.2 Clay at depth: 50 Wackett: 50 Costa: 50 Tanu: 50 4.2.1.3 Gypsum and carbonised wood: 51 Wareena: 51 Costa: 52 Tanu: 52 4.2.1.4Silcrete/Ferricrete: 52 Costa: 52 4.2.2 Durham Downs Road Transect 53 4.2.3 Shire Road Transect 56 vii 4.2.4 The Mt Howitt palaeochannel system 69 4.2.5 Goonbabinna Waterhole 75 4.2.6 Eastern and Southern Floodplain Margins 78 4.2.6.1 Baryulah 83 4.2.6.2 Bolan 3D Seismic Grid (Wilson Swamp) 84 4.2.6.3 Lignum Creek 85 4.3 GROUP 2: AEOLIAN-FLUVIAL INTERACTIONS 89 4.3.1 Chookoo Sandhill Complex 89 4.3.2 Durham Dune Site 105 4.3.3 Narberry Waterhole 112 4.4 CONCLUSION 117 CHAPTER 5 STRATIGRAPHIC AND CHRONOLOGICAL INTERPRETATIONS OF THE COOPER CREEK FLOODPLAIN 118 5.1 INTRODUCTION 118 5.2 CHARACTERISTIC FEATURES OF THE FLUVIAL DEPOSITIONAL UNITS 119 5.2.1 Near-surfacefloodplain clays 119 5.2.2 Sub-surface alluvial sand sheet 123 5.3 FORMATION OF THE FLOODPLAIN DUNES 127 5.3.1 Interactions betweenfloodplain clay anddune sand 128 5.3.2 The age of thefloodplain dunes relative to the floodplain 131 5.3.3 Was there a common period of dune development? 132 5.3.4 Are thefloodplain dunes source-bordering ? 134 5.4 DUNE AND FLOODPLAIN DEVELOPMENT OVER THE PAST 100 KA 138 5.5 A WESTWARD SHIFT OF THE COOPER CREEK FLOODPLAIN 147 CHAPTER 6. SUMMARY AND REGIONAL CONTEXT 157 6.1 INTRODUCTION 157 6.2 COMPARISON WITH OTHER QUATERNARY STUDIES IN CENTRAL, NORTHERN AND EASTERN AUSTRALIA 163 6.2.1 Lake Eyre Basin 163 6.2.2 Northern Australia 767 6.2.3 Southeastern Australia 170 6.3 A MODEL OF QUATERNARY FLOODPLAIN DEVELOPMENT OF COOPER CREEK 174 Pre-Stage 7/8 (Mid-Quaternary): 174 Stage 7/8: 174 Stage 6: 7 75 Stage 5: 7 76 Stage 4: 777 Stage 3: 7 77 Stages 2 and 1: 7 77 6.4 SOME IMPORTANT ADDITIONAL POINTS 178 6.5 CONCLUSION 179 REFERENCES 181
APPENDIX A - THERMOLUMINESCENCE DATES A.1-A.3
viii List of Figures
Figure 1.1 - Map of Australia showing the study area (refer to inset), Lake Eyre Basin, Simpson Desert, Strzelecki Desert, Lake Amadeus, Lake Lewis, Murray Basin, Channel Country, Diamantina & Georgina Rivers 4
Figure 2.1 - Isohytes of mean annual rainfall (mm) for the Lake Eyre Basin (Source: Bureau of Meteorology, 2000). Location of study area is shown in shaded box 15 Figure 2.2 - Mean annual evaporation (mm) for the Lake Eyre Basin (Source: Bureau of Meteorology, 2000). Location of study area is shown in shaded box 15 Figure 2.3 - Mean annual runoff (km3) at the Currareva (top) and Nappa Merrie (bottom) gauging stations, 1967-1988 (Knighton and Nanson, 1997) 18
Figure 2.4 - Selected relative flow magnitude (Q/Qpeak) curves from the flow records of the Currareva and Nappa Merrie gauging stations (Knighton and Nanson, 1997) 18 Figure 2.5 - Geology of the Cooper Creek study area (modified from the Durham Downs 1:250 000 Geological Series SG 54-15) 23 Figure 2.6 - Geomorphology of the Cooper Creek study area 25
Figure 3.1 - Field set-up of gamma logger with DLS-2, computer and gamma log probe at Shire Road site(SR3) 33 Figure 3.2 - Gamma-log plot of a floodplain uphole from seismic line 95-FWR Station No. 340 (plot shows a uranium anomaly at a depth of 19-20) 34
Figure 4.1 - Fieldwork locations on the Cooper Creek floodplain 44 Figure 4.2 - Schematic cross-section of the stratigraphy revealed along the 12.85 km long Tanu line. ...45 Figure 4.3 - Schematic cross-section of the stratigraphy revealed along the 35.8 km long Wareena line.46 Figure 4.4 - Schematic cross-section of the stratigraphy revealed along the 15.8 km long Costa line 47 Figure 4.5 - Sediment and gamma-log plots for the 41 upholes from the 5 Wackett seismic lines (95- FWM, -FWP, -FWQ, -FWR, -FWS) straddling the Shire Road 48 Figure 4.6 - Schematic cross-section of the Cooper floodplain along the Durham Downs Road transect.54 Figure 4.7 - Schematic cross-section of the Cooper floodplain along the 13.9 km long Shire Road transect 57 Figure 4.8 - Schematic diagram depicting the stratigraphy evident on the E face of the western Shire Road trench (SR3-T). Inset shows large-scale (-5-20 cm) 'festoon'-shaped trough-cross beds in medium sands at a depth of 5.2 m 59 Figure 4.9 - Schematic diagram depicting the stratigraphy evident on the S face of the eastern Shire trench (SR8-T) ...60 Figure 4.10 - Sediment size distribution from Shire Road auger hole SR2 showing a well-sorted possibly aeolian unit at 1.5 m overlying less well-sorted samples at depth (3.0-27.0 m) 64 Figure 4.11 - Schematic diagram describing the stratigraphy evident on the E face of the western Shire trench (SR5-T) 67 Figure 4.12 - Schematic map and aerial imagery of the Mt Howitt site highlighting the location of the trench sites and the palaeochannel 70 Figure 4.13 - Schematic cross-section of the 60 m long Mt Howitt trench (MH2) evident in Figure 4.12 showing a pronounced cut-bank to the east with lateral accretionary surfaces to the west 71 Figure 4.14 - Schematic section revealing the stratigraphy of the southern bank of the Goonbabinna Waterhole on the main western Cooper Creek channel 76 Figure 4.15 - Oblique aerial photographs of the 'islands' of liner dunes and clay pans surrounded by floodplain muds near Tooley Wooley Waterholes 80 Figure 4.16 - Sediment and gamma log plots of 4 seismic lines (95-FWX, -FWY, -FWZ, -FXA) in the vicinity of Baryulah and Tooley Wooley Waterholes 81 Figure 4.17 - Schematic diagram summarising the stratigraphic details revealed from 8 auger holes drilled within the 3D shallow seismic program in the Bolan area, Wilson's Swamp 82 Figure 4.18 - Map and schematic section showing the stratigraphy from a series of in-channel shallow trenches (Pits 1-3) in the bed of Lignum Creek (shown in inset map), on the eastern side of the Cooper floodplain (refer to Figure 4.1) 87 Figure 4.19 - Oblique aerial photograph and schematic map of the Chookoo Sandhill complex comprised of Chookoo Dunes 1, 2 and 3, the Chookoo main channel between Dunes 1 and 2 and a small channel between Dunes 2 and 3. View is to the southeast. Dune 1 in foreground is 1 km in length. 90 Figure 4.20 - Ground view photograph of the main Chookoo channel looking downstream to the southwest showing large coolibahs (E. microtheca) lining the banks of the ephemeral channel: (a) when filled with water from local runoff or large overland floods, the channel resembles a more ix perennial channel or waterhole, (b) heavily cracked, shallow, saucer-shaped depression during dry conditions 92 Figure 4.21 - Schematic cross-section revealing the stratigraphy derived from 16 auger holes and three trenches along the 1.4 km long Chookoo transect 94 Figure 4.22 - Schematic diagram describing the stratigraphy evident on the E face of the Chookoo floodplain trench (CH2-T) north of Dune 1 95 Figure 4.23 - Schematic diagram describing the stratigraphy evident on the NE face of the Chookoo channel trench A (CHW-TA) north of Dune 1. Inset shows northern cut bank of the 1.8 m wide Chookoo channel trench (CHW-TA) with a thin capping of reworked aeolian sands, overlying a dark grey clay lens and brownish-yellow alluvial sand units. Depth of trench is 6.5 m 97 Figure 4.24 - Schematic diagram displaying the stratigraphy on the southwest face of the Chookoo channel trench B (CHW-TB), south of Chookoo channel trench A (CHW-TA)- Inset of Chookoo channel trench B (CHW-Tg) looking to the southwest face. Dates from two TL samples (middle foreground (3.9 m) and lower right (6.1 m)) are given. Lateral accretionary surfaces dipping into face at 12° with a bedding angle of 120°. Palaeoflow directions of the trough cross-bedded sands are 210-220° 100 Figure 4.25 - Sediment size analysis of CH-A auger hole through Chookoo Dune 1 from surface through to a depth of 21m 102 Figure 4.26 - Schematic map and cross-section of the main Durham dune revealing the proximity of the site to the Tabbareah Waterhole and the stratigraphic units within and adjacent to the dune plus the relationship of the dune to the near-surface floodplain muds 106 Figure 4.27 - Sediment size distribution from Durham Homestead Dune showing a well-sorted aeolian sand unit to -3.4 m RL (dune depth of 9 m) and to -6 m RL in augerholes DD1 and DD3, respectively, both overlie less well-sorted fluvial units (see Figure 4.26) 108 Figure 4.28 - Four Scanning Electron Microscope (SEM) images of surface features evident on quartz grains sampled at the surface of the Durham Dune 110 Figure 4.29 - Ground view photograph of the Narberry Waterhole looking westward 113 Figure 4.30 - Oblique aerial photograph of the Pritchella and Narberry Waterholes. Also shown are the West and East Narberry dunes and a sedimentary splay at the southern end of the Pritchella Waterhole. View is to the northeast 113 Figure 4.31 - Locality map and schematic cross-section of the stratigraphy and chronology of the Narberry Transect between Narberry Waterhole and South Narberry Waterhole on the Cooper Creek floodplain 115
Figure 5.1 - Shire Road (SR) transect vertical accretion rates 122 Figure 5.2 - Oblique aerial photograph looking westward at an isolated floodplain dune surrounded by floodplain muds 129 Figure 5.3 - Ground level photograph and schematic map of small Chookoo channel 130 Figure 5.4 - Plot of relative accumulation rates of aeolian and fluvial units 133 Figure 5.5 - Model showing the evolution of the main Chookoo Sandhill and large Chookoo Channel since mid-Stage 5 136 Figure 5.6 - Schematic model highlighting source-bordering dune and channel development on the Cooper Creek floodplain 139 Figure 5.7 - Model of Cooper Creek floodplain and floodplain dune development since the mid- Quaternary 140 Figure 5.8 - TL dates with error bars classified according to the depositional units: fluvial muds, fluvial sands and aeolian sands 142 Figure 5.9 - Landsat imagery of Cooper floodplain between Durham Downs and Meringhina Waterhole. Floodplain width=16 km 149 Figure 5.10 - Contraction of fluvial activity to the western third of the Cooper floodplain especially since Stage 3 151 Figure 5.11 - Cooper Creek floodplain elevations across twofloodplain transects: Durham Road and Shire Road transects 152 Figure 5.12 - Schematic cross-sections of the Cooper valley 154
Figure 6.1 - Comparison of TL frequency histograms from this study with those from Nanson et al. (1988, 1990) and classified according to the depositional processes: fluvial muds, fluvial sands and aeolian sands 157 Figure 6.2 - Regional comparison of TL frequency histograms classified according to depositional processes: fluvial muds, fluvial sands and aeolian sands from chronological studies in central and southeastern Australia 170
X List of Tables
Table 2.1 - Selected meteorological data for Hughenden, Windorah and Moomba 14
XI Chapter 1 Introduction
"The most critical need... is to extend research away from the relatively well-watered fringes of the tropical and temperate zones into the arid and semi-arid interior of the continent... the concentration [of research] on New South Wales coastal streams means there is now a danger of developing models of Australian fluvial geomorphology that are irrelevant to inland areas." (p. 58). Tooth and Nanson (1995)
1.1 Context of study
The arid and semi-arid zones constitute 60-80% of continental Australia.
Despite an increasing body of geomorphological and sedimentological research in semi-arid regions of Australia in recent times, knowledge of the climatic and fluvial-sedimentary histories relating to the evolution of the continent's anabranching river systems lacks detail (Tooth and Nanson, 1995). As a result, our ideas relating to dryland depositional environments in central Australia are in part imported from overseas and are therefore not always relevant to the interpretation of Australia's interior rivers. Most of the research done on overseas dryland rivers has focussed on relatively small steep, sandy or gravelly desert streams. In contrast, Australia is characterised by extremely large low gradient basins, often dominated by mud loads. This is especially true of the Channel Country of western Queensland where there exists an important opportunity to document the unique depositional styles and sedimentological assembleges characterising the fluvial and aeolian systems of this part of
Australia's semi-arid interior.
1 Few studies have described the interaction between fluvial and aeolian depositional systems. A study of modern or relatively recent aeolian and fluvial interactions may provide important sedimentological information for interpreting ancient fluvial-aeolian deposits (Langford, 1989; Langford and
Chan, 1989). This thesis presents a detailed stratigraphic and chronological investigation of the Quaternary development of the Cooper Creek floodplain and associated aeolian dunes in southwest Queensland (Fig. 1.1).
Preliminary work by Nanson et al. (1986, 1988) and Rust and Nanson (1986) provides a broad overview of the geomorphology, stratigraphy and chronology of the Channel Country floodplains, including Cooper Creek. They present evidence of wetter climatic phases characterised by meandering-braided fluvial systems with more regular flow regimes than at present, however, their work made regional inferences based on a small number of widely spaced sites. This study presents a far more detailed stratigraphic and chronological investigation of a localised area of the Cooper Creek floodplain.
1.2 Quaternary development of the Lake Eyre Basin - a literature review
The history of Quaternary environmental change provides a background for understanding present-day environments and climatic evolution. Unlike much of the northern hemisphere, the interior of the Australian continent was unaffected by direct glaciation and periglaciation during the Quaternary yet has preserved a valuable archive of alluvial, lacustrine and aeolian sedimentation in response to alternating global glacial-interglacial cycles (Bowler and Wasson,
1984; Bowler, 1986; Wasson and Clark, 1988; Chappell, 1991).
2 In this section, a literature review of Quaternary research in the Lake Eyre Basin is presented. In particular, the documented effects of fluctuating global climate change on aeolian, fluvial and lacustrine sedimentary processes in central
Australia are highlighted.
1.2.1 Quaternary aeolian activity and dune development
Bowler (1983) and An et al. (1986) provide evidence from ancestral 'Lake
Bungunnia' in the Murray Basin (Fig. 1.1) indicating that humid, high rainfall conditions existed between 700 ka and the Tertiary-Quaternary transition at 2.5
Ma (Kukla, 1987; Chen and Barton, 1991) with aridity increasing after the onset of the Bruhnes Normal Chron at -700 ka (Bowler, 1983; Zheng et al. 1998).
However, Chen and Barton (1991) have interpreted, from lacustrine deposits in
Lake Amadeus, northwest Lake Eyre Basin (Fig. 1.1), that the onset of pronounced aridity in central Australia occurred at or before 0.9 to 1.6 Ma with truly arid sediments and evaporites appearing only in the last 500 ka (Gardiner et al., 1987; Wasson and Clark, 1988). Australia's climate during the mid to late
Quaternary has oscillated between drier glacial or stadial and wetter essentially interglacial or interstadial episodes broadly linked to worldwide fluctuations in climate (Nanson et al, 1992b).
With each phase of aridity, there has been pronounced phases of aeolian activity and dune building in the Lake Eyre Basin. Numerous studies of the
Australian dunefields have yielded TL dates of 300 to 200 ka (Wasson, 1986,
1989; Gardiner et al, 1987; Readhead, 1988; Nanson et al, 1988; Callen and
Nanson, 1992). However, reworking of dunes during phases of enhanced aeolian activity rejuvenates their age and as a consequence, TL dating of sand dunes rarely exceeds 200 ka except on the less-active periphery of the arid zone
3 4 (Gardiner et al, 1987; Nanson et al, 1992a, 1995). Rejuvenation of the dunes of the Simpson Desert (Fig. 1.1) results in a late Pleistocene to Holocene modal age of 15 to 10 ka for a dunefield that is clearly very much older (Callen and
Nanson, 1992; Nanson et al, 1992a, 1995).
Wasson (1989) reported six significant dune building episodes in Australia in the last 280 ka, which are closely linked with pluvial Oxygen Isotope Stages 7
(256±16 ka), pluvial Stage 5 (112+7 ka), subpluvial Stage 3 (56±6 ka) and the
Holocene 'interglacial' Stage 1 (2.5 ka). In addition, dune development during dry periods of the penultimate glacial maximum and the Last Glacial Maximum occurred during early Stage 6 (183 to 143 ka) and between mid-Stage 3 (36 to 8 ka) to early Stage 1, respectively. He also estimated that the average time interval between dune building episodes over the past 280 ka is -48 ka with a range of between 80 and 28 ka. However, the unusual occurrence of Australian dune building during pluvial and subpluvial periods required some explanation.
Wasson (1989) provided a palaeoclimatic explanation for dune building during pluvial and subpluvial phases on the basis of changes in windspeed (20% higher during dune building) and the ratio of potential evapotranspiration and precipitation (-40% reduction in precipitation during dune building). In addition, Williams (1994) contended that source-bordering dune development in western New South Wales was enhanced at times of warmer and wetter climate by replenishment of the localised fluvial-sand supply and reactivation of the linear source-bordering dunes. He maintained that a lagged sedimentary response time to climatic change may have been an important factor explaining why little evidence exists for the relatively brief Stage 4 stadial.
5 Dune building episodes of quartzose, gypseous and clay-pelleted dunes have been widely reported between Stages 5 through to the present Holocene
'interglacial' (Stage 1), with the most intense aeolian activity straddling the last glacial maximum (Stage 2) (Wasson, 1983b; Bowler and Wasson, 1984; Wasson and Donnelly, 1991; Nanson et al, 1990, 1992b; Callen and Nanson, 1992).
Gypseous source-bordering dunes were active along the shores of Lake Lewis
(Fig. 1.1) in the northern regions of the Lake Eyre Basin (Chen et al, 1990) at the onset of Stage 2 (23 to 21 ka) and at Lake Amadeus (Chen et al, 1991a, 1991b,
1995) (Fig. 1.1) during mid- to early Stage 3 (60 to 45 ka), while gypsum lunettes were formed within the Lake Frome playa (Fig. 1.1) during Stage 2 (between 23 and 15 ka) (Bowler and Wasson, 1984). Clay-rich pelleted dunes of the Simpson and Strzelecki Deserts (Bowler and Wasson, 1984) (Fig. 1.1) result from pelletising of fine-grained sediment by the crystallisation of salts from rising saline water-tables. When dry, the pellets are transported by aeolian processes onto adjacent floodplains (Butler, 1956; Bowler, 1973; Dare-Edwards, 1982;
Wasson, 1986; Wasson and Clark, 1988). On the basis of ^C dated charcoal fragments, the 13 to 25 ka ages of the Strzelecki dunes in the vicinity of Cooper
Creek correspond with the period of peak aridity in Stage 2 - the Last Glacial
Maximum (Wasson, 1983b; Bowler and Wasson, 1984; Wasson and Clark, 1988;
Wasson and Donnelly, 1991).
L2.2 Quaternary fluvial and lacustrine processes
Quaternary floodplain development of the Channel Country rivers of west and southwest Queensland has been of considerable interest to sedimentologists and geomorphologists since the early 1980's (Rust, 1981; Rust and Nanson,
1986; Nanson et al, 1986,1988; Maroulis and Nanson, 1996; Gibling et al, 1998).
Unlike the aeolian record, there is widespread stratigraphic and chronological evidence of extensive fluvial activity during the Quaternary in the Lake Eyre
6 Basin. Callen and Nanson (1992) have reported pre-Stage 7 dates from stratigraphic units that they regard as the Kutjitara Formation. These units were comprised of fluvially deposited upward-fining sandy units from mixed load, wide, shallow, low sinuosity distributary palaeochannels of the lower Cooper
Creek, which flowed westward towards the present location of Lake Eyre.
Overlying the Kutjitara Formation is the Katipiri Formation which was deposited predominantly during the penultimate interglacial Stage 7 (244 to 190 ka) and last interglacial mid-Stage 5 (-100 ka) (Nanson et al, 1992a, 1998). The
Katipiri Formation in the Cooper and Diamantina River systems is comprised of fluvial sands, with interbedded muds, overlain by floodplain muds deposited by braided and meandering precursors of the modern anastomosing streams (Nanson et al, 1988). In contrast, in downstream reaches near Lake
Eyre, the Katipiri Formation contains fining-upward sequences in medium to coarse sands deposited by laterally active, wide, probably shallow, meandering river channels (Callen and Nanson, 1992; Nanson et al, 1998).
Fossil remains of megafauna are often present in the Katipiri and Kutjitara
Formations to the east of Lake Eyre (Wells and Callen, 1986; Krieg et al, 1990;
Tedford and Wells, 1990). These animals utilised the enhanced pluvial conditions during the interglacials and interstadials to transgress into the
Australian interior. However, their remains have not yet been identified in equivalent units of the middle and upper reaches of the Channel Country streams, probably because of a lack of outcrop relative to that available closer to
Lake Eyre.
There is a growing body of literature indicating that the penultimate glacial maximum (Stage 6) was characterised by considerable fluvial activity (Nanson et al, 1992a, 1998). This is surprising given the supposed dryness of this glacial
7 phase relative to interglacial Stages 7 and 5. However, further refinements in dating are probably required to more precisely define the extent of fluvial activity in Stage 6, for it may have been restricted to interstadials, as was largely the case over the last glacial cycle.
Extensive fluvial activity was evident in the pluvial interglacial (Stage 5) throughout most of the basin (Chappell, 1991; Nanson et ah, 1991, 1992a;
Kershaw and Nanson, 1993; Magee et ah, 1995; Croke et ah, 1996, 1998, 1999;
Magee and Miller, 1998). In addition, the influx of floodwaters from most parts of the basin during Stage 5 (Nanson et ah, 1988, 1998; Croke et ah, 1996, 1998) resulted in a permanent, enlarged, -25 m deep lake which probably represents the deepest phase of Lake Eyre filling during the last glacial cycle (Magee et ah,
1995). However, the wettest reported conditions, as determined from the flow regime of the Channel Country rivers, correspond to interstadials within the latter part of Stage 5 between 110 ka and 80 ka, some 10 to 30 ka after the peak of the last (full) interglacial maximum at -126 ka (Nanson et ah, 1992b; Kershaw and Nanson, 1993).
This fluvially active period in both the Cooper and Diamantina catchments in the northeast Lake Eyre Basin (Nanson et ah, 1988) was dominated by laterally active wide channels responding to flow regimes significantly larger than those of the contemporary Channel Country rivers (Rust and Nanson, 1986; Nanson et ah, 1996, 1998). These medium to coarse sand deposits display large-scale truncated trough and tabular bedforms with occasional mud drapes, fining upward to fine sands with small cross trough sets (Rust and Nanson, 1986;
Nanson et ah, 1988; Nanson and Tooth, 1999).
The effects of progressive if somewhat erratic drying of the interior since Stage
5, especially during late Stages 5 (5c-a; -100 to 75 ka; Nanson et al. 1991,1992b),
8 has had a major influence on Lake Eyre Basin channel morphology. The rivers of the Channel Country reverted to numerous low-energy anastomosing mud- braided channels incised into the muddy floodplain with sand mostly transported as limited bedload, although with some fine sand in suspension.
Nanson et ah (1988) suggested that increased aridity from the termination of
Stage 5 to the present has been largely responsible for the continued existence of mud-dominated anastomosing planforms.
Evidence exists for fluvial activity in subpluvial Stage 3, however, it is not as pronounced as that reported for Stage 5 (Nanson et ah, 1992b; Kershaw and
Nanson, 1993). Nevertheless, the monsoon-fed streams in the northeast Lake
Eyre Basin during subpluvial Stage 3 were important hydrologically in supplying runoff into Lakes Eyre and Frome and were responsible for raising lake levels to -20 m above present (Nanson et ah, 1996,1998).
Base-level lowering and fluvial incision of up to 9 m in the lower Neales River catchment between 50 and 31 ka during Stage 3 provides evidence for episodic fluvial activity (Croke et ah, 1996, 1998). Base-level lowering of Lake Eyre, caused by a combination of reduced lake levels and groundwater controlled aeolian deflation, probably occurred at the onset of the subpluvial Stage 3 at
-60 ka (Croke et ah, 1996, 1998). Subsequent aeolian deposition of pelleted clay and gypsum from reworked playa sediments, mixed with gypsum, formed playa marginal dunes which, at Lake Eyre, dated between 60 to 50 ka (Magee et ah, 1995; Magee and Miller, 1998). TL dates of 60 to 45 ka (Stage 3) from marginal playa gypseous dunes along Lake Amedeus (Chen et ah, 1991a, 1991b,
1995), in the northwest of the Lake Eyre Basin, are in agreement.
A recent study of Lake Eyre beach ridges by Nanson et al. (1998) has provided sound evidence for high lake levels during Stages 5 and 3, and they also argue
9 controversially for a high lake level during Stage 2 near the time of the last glacial maximum. Their evidence for Stages 3 and 2 is at odds with that from
Magee et al. (1995) who report that the infilling of an enlarged Lake Eyre during the early Stage 5 pluvial, was the last major filling. While Magee et al. (1995) argued for no Stage 3 evidence for high lake levels similar to those of Stage 5,
Magee and Miller (1998) soften this position in a later interpretation, while remaining convinced that the lake would not have been high during Stage 2.
Magee et al. (1995) identified a dune building and possibly deflationary phase early in Stage 3 (60 to 50 ka). Less frequent hydrologically significant wet phases (compared to Stage 5) resulted in shallow saline lacustrine conditions during early to late Stage 3 (50 to 24 ka) culminating in groundwater-controlled playa deflation during Stage 2 (24 to 10 ka) with a thick halite salt crust being formed during peak aridity in Stage 2 (last glacial maximum) at -18 ka. In the eastern Neales River catchment, Croke et al. (1996, 1998) reported dominantly aeolian and ephemeral-fluvial activity, with fluvial intensity waning during peak aridity of the last glacial maximum between 20 and 18 ka. In the Holocene
'interglacial' phase (Stage 1) the modern Lake Eyre playa evolved after a brief perennial lacustrine episode which terminated at -4 to 3 ka (Magee et ah, 1995;
Magee and Miller, 1998).
The response of the Lake Eyre Basin river systems to any enhanced fluvial conditions during the Holocene is not readily apparent. While reworking of the sandy Finke River floodplain has been reported by Nanson et al. (1995), the lower gradient and more cohesive muddy floodplains of the Channel Country show little evidence of enhanced Holocene fluvial activity. The discrepant interpretations of lake levels of Lake Eyre, and conflicting evidence for fluvial activity during the Holocene in the Lake Eyre Basin, underscore the need for further detailed stratigraphic and chronological interpretation of the
Quaternary sediments in the basin.
10 1.3 Aims and Objectives
The vast fluvial systems in the central Australian arid zone strongly influenced by northern monsoonal rainfall represent an interesting geomorphological and sedimentological contrast to the more intensively studied rivers of the northern hemisphere, many of which have been strongly influenced by Quaternary glacial activity. This study will describe in detail the sedimentology, stratigraphy and chronology of a very large semi-arid central Australian fluvial system; Cooper Creek.
The thesis has the following specific objectives: a) to describe in detail, the fluvial and aeolian sedimentology and stratigraphy of a central portion of Cooper Creek in southwestern
Queensland b) to describe aeolian and fluvial interaction on this part of the Cooper
Creek floodplain during the mid to late Quaternary c) to propose a model of Cooper Creek floodplain development during the Quaternary d) to interpret palaeoclimatic changes in the region.
11 Chapter 2 Physical characteristics of the Cooper Creek study area
2.1 Introduction
The Channel Country is the collective term given to the complex network of ephemeral channel systems that drain western Queensland towards the Lake
Eyre playa. Cooper Creek and the Diamantina and Georgina Rivers (Fig. 1.1) comprise the three major river systems of the Channel Country, and form part of the 1.14x10° km^ Lake Eyre drainage Basin, one of the world's largest areas of internal arid-environment drainage (Fisher, 1969; Nanson and Price, 1998).
Cooper Creek by name originates at the confluence of the Barcoo and Thomson
Rivers near Windorah in the northeastern section of the Lake Eyre Basin, possibly the only place in the world where two rivers combine to form a creek!
However, its characteristics are also represented in those major headward tributaries. It flows south and southwest through a complex assemblage of waterholes, anastomosing and braided channels, backswamps and aeolian dunefields before reaching its terminus at Lake Eyre (Fig. 1.1). The length of
Cooper Creek, including the Thompson River, is 1,520 km and it has a total catchment area of 306,000 km.2 upon entering Lake Eyre (Kotwicki, 1986). The
Lake Eyre Basin is mostly flat to undulating with only 30% of the area elevated more than 250 m above mean sea level while the Lake Eyre depocentre is at
-15.5 m A.H.D. (Kotwicki and Isdale, 1991; Magee et ah, 1995). Regional down- valley gradients are low and remain in the order of 0.1-0.2 m/km through the middle and lower reaches of Cooper Creek.
12 2.2 Climate
The Lake Eyre drainage Basin lies in a region of hot, dry, steppe and desert climate with short, cool to cold winters (Koppen's Bsh and Bwh). The principal synoptic influences on rainfall in the basin are in the form of moist tropical air incursions, monsoonal depressions, which spill over the Great Dividing Range during the summer months (Winkworth and Thomas, 1974; Allan, 1985, 1988,
1990). Also, tropical cyclones can greatly augment the contribution from the monsoon. As a result, the Lake Eyre Basin, especially to the north, is dominated by summer rainfall, with approximately 60-70% of the annual rainfall recorded during the period between November and March. An important secondary rainfall maxima of 15-20% occurs in the winter months of May and June (Table
2.1) resulting from the influence of southern continental and maritime air masses (Kraus, 1955). However, rainfall varies considerably from year to year.
Mean annual rainfall decreases markedly towards the centre of the Lake Eyre
Basin with the variability of rainfall in the Lake Eyre Basin being among the highest recorded in Australia (Kotwicki, 1986; Kotwicki and Isdale, 1991). For instance, mean annual precipitation ranges from 486 mm/year at Hughenden
(Fig. 2.1; Table 2.1) in the northeast of the basin, to less than 220 mm/year at
Moomba (Fig. 2.1; Table 2.1).
The air temperature regime throughout the Lake Eyre Basin is characterised by large diurnal and seasonal fluctuations, resulting from minimal interference to incoming and outgoing solar radiation from clouds and/or water vapour
(Winkworth and Thomas, 1974; Linnacre and Hobbs, 1977). The mean monthly maximum air temperature for Hughenden is 37.0°C (December) with a mean monthly minimum of 8.8°C (July). Moomba experiences a similarly hot summer and mild winter regime, with a mean monthly maximum air
3 0009 03192801 8 13 HUGHENDEN (QLD) Station No.:30024 Period of Record: 1884-1998 Elevation: 324 mA.H.D . Latitude: 20° 51' S Longitude: 144° 12' E Month Rainfall Evaporation3 Number of Temperature (°C) (mm) (mm) Raindays Maximum Minimum JAN 116.6 238.7 8.3 35.8 22.5 FEB 97.5 198.8 7.5 34.7 22.0 MAR 58.7 198.8 4.7 33.7 20.5 APR 25.6 186.0 2.2 31.4 17.0 MAY 18.2 145.7 1.8 27.9 13.2 JUN 18.6 120.0 1.8 25.0 9.8 JUL 12.2 127.1 1.4 25.0 8.8 AUG 7.1 161.2 0.9 27.5 10.3 SEP 8.9 207.0 1.3 31.0 13.9 OCT 20.3 257.3 2.6 34.5 17.9 NOV 32.9 264.0 4.0 36.2 20.5 DEC 69.6 266.6 5.9 37.0 22.0 ANNUAL 486.1 2372.5 42.3 Mean=31.6 Mean=16.5
WINDORAH (QLD) Station No.:38024 Period of Record: 1887-1996 Elevation: 126 m A.H.D. Latitude: 25° 42' S Longitude: 142° 66' E Month Rainfall Evaporation3 Number of Temperature (°C) (mm) (mm) Raindays Maximum Minimum JAN 41.1 384.4 4.6 38.0 24.0 FEB 49.2 313.6 4.2 36.4 23.4 MAR 40.2 297.6 3.6 34.4 21.0 APR 20.3 219.0 2.2 29.9 15.8 MAY 19.5 148.8 2.4 25.2 11.2 JUN 16.0 108.0 2.5 21.5 7.5 JUL 15.2 114.7 2.3 21.2 6.4 AUG 10.2 158.1 1.8 23.8 7.9 SEP 10.6 222.0 2.0 27.9 11.7 OCT 18.6 291.4 3.0 32.2 16.2 NOV 19.8 351.0 3.4 35.4 19.7 DEC 29.6 399.9 3.8 37.7 22.3 ANNUAL 290.1 3008.5 35.9 Mean=30.3 Mean=15.6
MOOMBA (S.A.) Station No.:17096 Period of Record: 1972-1998 Elevation: 39 m A.H.D. Latitude: 28° 11' S Longitude: 140° 21' E Month Rainfall Evaporation3 Number of Temperature (°C) (mm) (mm) Raindays Maximum Minimum JAN 47.6 486.7 3.8 37.3 23.1 FEB 23.8 411.6 2.8 36.7 22.8 MAR 12.1 387.5 1.8 33.9 19.5 APR 14.3 252.0 2.3 28.6 14.8 MAY 19.5 161.2 3.0 23.6 10.8 JUN 11.7 111.0 2.3 19.8 7.4 JUL 17.3 117.8 3.1 19.4 6.3 AUG 9.2 170.5 2.3 22.0 7.6 SEP 9.5 243.0 2.3 25.9 10.8 OCT 20.9 337.9 3.6 29.9 14.8 NOV 12.8 405.0 3.4 33.5 18.6 DEC 21.3 480.5 3.0 36.5 21.3 ANNUAL 219.9 3540.5 33.5 Mean=28.9 Mean=14.8
Key:a: Evaporation data derived using a Class A pan.
Table 2.1 Selected meteorological data for Hughenden (top), Windorah (middle) and Moomba (bottom) (Source: Bureau of Meteorology, 2000). 14 J3 ta tffi' TD ai B B • A ? 1 o 800 o .*» o B "0 Ct i M o c *6 u U "^ T3 (a 8 9i.. a g 3" a o -S •fl 5) ^ -= Ul 3 ^u V X 3 P - i»Vtg,"5^g ? *~ £• •8* t#7 ^ E\ * V" - > M v I PQ o V -El aiW: --t i i Ski -£ t II a • ,V/ (If.A ; ^* ** c/> V/
r.S's ?1 -I-Q< • «
CM CO _1_
15 temperature of 37.3°C (January) and a mean monthly minimum of 6.3°C (July)
(Bureau of Meteorology, 2000) (Table 2.1).
Mean annual evaporation (Class A Pan measurements) varies from about 2400 mm/year in the northeast to more than 3600 mm/year in the lower central portion of the basin (Fig. 2.2). Evaporation rates vary seasonally, with winter evaporation constituting only 25-35% of the total mean annual evaporation rate.
Mean monthly and annual pan evaporation greatly exceeds mean monthly and annual precipitation throughout the Lake Eyre Basin (Bureau of Meteorology,
2000; Table 2.1).
2.3 Hydrology
The hydrological behaviour of streams in the semi-arid to arid Lake Eyre Basin is at best poorly understood; the irregularity of rainfall and floods, the limited database, long periods of no flow, and the skewness of flow frequencies to low values all present problems in analysing the hydrological characteristics of such river systems (French and Roberts, 1975; Kotwicki, 1986, 1987; Knighton and
Nanson, 1994a). However, a number of general hydrological investigations have previously been undertaken for the major fluvial systems within the Lake
Eyre drainage basin (Nimmo, 1947; Ogilvie, 1947; Bonython and Mason, 1953;
Bonython, 1955, 1960, 1963, 1974; Wopfner, 1970; Williams, 1971; Kotwicki,
1986,1987; Bonython and Fraser, 1989; Kotwicki and Isdale, 1991).
Continuous stream-flow records are available for three gauging stations along
Cooper Creek at Currareva, Nappa Merrie and Innamincka (Fig. 2.1), although the former two stations ceased to operate in the late 1980's. The hydrological information provided by Currareva and Nappa Merrie gauging stations (Figs
2.3 and 2.4) is presented to illustrate the general hydrological trends and characteristics of the Cooper Creek drainage system. In the absence of major 16 tributary inflows between these two stations, located approximately 420 km apart, much of the inflow at Nappa Merrie can be attributed to outflow at
Currareva (Knighton and Nanson, 1994a). Stream flow records from gauges operated by the Queensland Water Resources Commission demonstrate the extreme variation in the discharge regime at, and between, these two gauging stations. Mean annual runoff is characterised by high variability, with long periods of low or zero flow and occasional periods of extremely high discharge
(Fig. 2.3). Mean annual runoff decreases downstream from 3.05 km3 at
Currareva to 1.26 km3 at Nappa Merrie, with corresponding standard deviations of 5.02 km3 and 2.83 km3 indicating the significant year to year variability of runoff. This variability can be largely attributed to the vagaries of tropical cyclonic and summer monsoonal rainfall in the headwaters of the
Cooper catchment (Kotwicki, 1986).
The largest flood on record occurred in February 1974, with maximum daily discharges of 24,974 m3/s at Currareva and 5,812 m3/s at Nappa Merrie. These record flow conditions followed widespread heavy rainfall across northern
Australia, and were probably the largest floods for over 100 years (Kotwicki,
1986, 1987). The heavy rainfall resulted in the simultaneous flooding of all major watercourses in the Lake Eyre Basin and subsequently filled Lake Eyre to its highest recorded level (Bonython, 1974). The dominance of the 1974 flood at both Currareva and Nappa Merrie (Fig. 2.4) contrasts with the mean annual flood (Q2.33) which is relatively small at both stations (96.7 m3/s at Currareva and 40.0 m3/s at Nappa Merrie) and results in extremely steep relative flood magnitude (Q/Q2.33) curves (Knighton and Nanson, 1994a, 1997).
Clearly, a salient feature of the hydrological character of Cooper Creek is the marked reduction in mean annual flow volumes and flood peaks between
Currareva and Nappa Merrie (Figs 2.3 and 2.4) which result from significant transmission losses during the passage of floods. Transmission losses vary non-
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18 linearly with stage; they are relatively low when flow is confined to the primary anastomosing channels of the Cooper system but increase at higher stages as lesser channels and the floodplain are activated (Knighton and Nanson, 1994a,
1997). Of the input flows that do reach Nappa Merrie, about 75-80% on average are lost as transmission losses over the Currareva-Nappa Merrie length. These large transmission losses have been attributed to three main causes (Knighton and Nanson, 1994a, 1997): high rates of evaporation/evapotranspiration resulting from the large surface area and shallow depth of floodplain flows; infiltration into the channel boundary and extensively-cracked floodplain surface, particularly in the early stages of floods; and drainage diffusion into extensive flat, low-lying flood basins.
Despite the long river distance and large transmission losses between
Currareva and Nappa Merrie, relative flow magnitude is maintained over a wide range of discharges (Fig. 2.4). Transmission times for flood waves between the two stations on average is 30 days. Hence, major flooding of Cooper Creek may persist for several months at a time, with flows at or near bankfull
(Kotwicki, 1986; Kotwicki and Isdale, 1991). The protracted rising and falling limbs of flood hydrographs result from the low regional gradients and the great width (up to 60 km in flood) of the floodplains bordering the Cooper. Although
Bonython and Mason (1953), Bonython (1955, 1963, 1974) and Atterton (1974) independently reported flow velocities in the order of 1.3-1.8 m/s during major flood events on Cooper Creek, velocities on the middle reaches between
Currareva and Nappa Merrie rarely exceed 1.0 m/s. Flow measurements taken in the vicinity of Naccowlah (Fig. 2.1), immediately after the peak of the 1990 flood event, indicate velocities of 0.5-1.0 m/s for the waterholes and major channels and 0.1-0.5 m/s for the flows over the general floodplain surface (G.
Nanson, unpublished data).
19 2.4 Geology
The Lake Eyre drainage Basin is not a single geological entity; its form is a combination of geomorphological and geological features enclosing sections of several geological provinces (Veevers and Evans, 1975; Kreig et ah, 1990). The
Channel Country of southwestern Queensland overlies part of the 1x10" km^
Eromanga Basin (Dawson, 1974). The following discussion briefly summarises the regional geological history of the Eromanga Basin and examines the structural and lithological influences that have influenced the evolution of the broad, low-gradient floodplain of Cooper Creek (Wopfner and Twidale, 1967;
Senior, 1968; Senior et ah, 1968) leading to the present Quaternary landscape.
The Eromanga Basin is a broad intracratonic downwarp containing up to
3,000m of Middle Triassic to Late Cretaceous sedimentary strata (Wecker, 1989;
Kreig et ah, 1990). The accumulation of these dominantly fluviatile and lacustrine sediments has been largely continuous throughout the basin's 130 million year history, interrupted only by a short period of marine sedimentation in the Early Cretaceous (Wopfner, 1963; Dawson, 1974). Structurally, the
Eromanga Basin is a large northeast-trending folds, upturned along its eastern margin. Wecker (1989) postulated that tectonic activity within the basin was minimal during the period of Triassic-Cretaceous deposition. However, several
Tertiary structural episodes have resulted in uplift and erosion of the Basin section along its eastern margin, and the development of broad, northwesterly to northeasterly trending anticlines within the basin. This structural deformation occurred during a period of tectonic activity, with the style and magnitude of this deformation being influenced by the underlying Palaeozoic structure (Wopfner, 1963; Senior et ah, 1968; Moore and Pitt, 1984; Wecker, 1989;
Wiltshire, 1989). The geological evolution of the Basin through a regime of low- gradient sedimentation and minimal tectonic activity has provided the
20 structural base for the continued sedimentation and evolution of the contemporary land surface.
The present position and character of Cooper Creek in the vicinity of
Naccowlah has been influenced by the structural and lithological characteristics of the Eromanga Basin (Senior, 1968; Senior et ah, 1968). Structural deformation of the Basin during the Tertiary produced a land surface of broad anticlines and synclines (Fig. 2.5), establishing the spatial framework for the development of the present land surface (Dawson, 1974; Hughes, 1979). The oldest sequences in the study area are those of the extensively exposed Late Cretaceous Winton
Formation, which mainly consists of interbedded, blue-grey mudstone and lithic sandstone (Wopfner 1963; Vine and Day, 1965). The Winton Formation crops out in two basic forms: a relatively unweathered succession restricted to the eroded structural highs, and a chemically weathered and silicified derivative located around the periphery of the structural highs (Fig. 2.5). The overlying Tertiary Glendower Formation rests unconformably on the silicified portion of the Winton Formation and mainly consists of fluviatile arenite, with lesser conglomerate and argillaceous rocks (Senior, 1968; Senior et ah, 1968).
Significant sections of the Glendower Formation have also been subjected to intensive silicification during the Tertiary (Hughes, 1979).
Landscape evolution during the Quaternary has been characterised by both extensive erosion and deposition (Senior, 1968; Wecker, 1989). Quaternary deposits are primarily composed of alluvium, dune sands and silcrete gravels reworked from older strata. The distribution of the Quaternary sediments reflects the general trend of sediment accumulation around the periphery of the structural highs (Gregory et ah, 1967; Senior, 1968; Senior, 1970).
Geological and seismic investigations in the Naccowlah area indicate that the style of structural deformation throughout the region, involving low
21 amplitudinal folding, has provided a structural base for the evolution of broad, low-gradient alluvial plains (Wopfner and Twidale, 1967; Wopfner, 1970; Senior et ah, 1968). Thus the geological structure has imposed broad control upon both the direction and orientation of the contemporary drainage systems. In this study area, Cooper Creek is orientated along the extensive north-trending
Cooper Syncline before taking an abrupt westward turn along the Wilson
Depression and passing through the Innamincka Dome at Nappa Merrie and into South Australia (Fig. 2.5).
2.5 Physiography
Raised physiographic features in the middle reaches of the Cooper Creek valley include low-relief (mostly <30 m), highly-fretted escarpments around the the remnant plateaux and mesas, and their associated coarse rock-debris covered pediments that extend with a very gentle gradient down to the adjacent river valleys. Floodplains in the valleys tributary to the Cooper are generally very broad (often several kilometres in width) and typically are characterised by anabranching channel networks across muddy alluvial surfaces. The Cooper valley itself is extremely broad (up to 60 km in width north of the study area) and characterised by an extensive network of waterholes and anastomosing, braided and reticulate channels. Away from the floodplains, the geomorphology is largely determined by the underlying geology.
The physiography of the middle reaches of the Cooper Creek valley reflects the degree to which erosion and deposition has modified a Tertiary land-surface
(Senior et ah, 1968). The Tertiary structural arrangement of the land-surface has established the spatial patterns of geomorphic development, involving erosion of the structural highs and the accumulation of sediment in the adjacent downwarps (Twidale, 1972). Geomorphological investigations in the region
22 Figure 2.5 - Geology of the Cooper Creek study area (modified from the Durham Downs 1:250 000 Geological Series SG 54-15)
23 prior to those by Rust and Nanson have been largely restricted to summaries in geological and land-use survey reports (Mabbutt, 1967; Senior et ah, 1968;
Mabbutt, 1968; Mabbutt and Sullivan, 1968; Dawson, 1974) and some descriptive and speculative interpretations by Whitehouse (1948) and Rundle
(1976). The geomorphology of the middle reaches of Cooper Creek can be divided into four broad geomorphic units, as shown in Figure 2.6 below:
2.5.2 Dissected tablelands
The dissected tablelands geomorphic unit represents the remnants of a Tertiary land-surface (Senior et ah, 1968; Senior, 1970). The development of this unit has resulted from differential erosion of the Tertiary Glendower Formation and the underlying, chemically altered, Late Cretaceous Winton Formation (Fig. 2.6).
The surface of the unit is partly covered by the silicified sandstones of the
Glendower Formation and upper Winton Formation, but once erosion penetrates this hard capping, tracts of the weaker Winton Formation are exposed, and simple scarp retreat occurs, resulting in a landscape of plateaux, mesas and buttes (Dawson and Ahern, 1974). The silicified sandstones of the
Glendower Formation are largely restricted to the valley margins adjacent to the floodplain, with most of the escarpments developed in the Winton
Formation.
2.5.2 Rolling downs
The evolution of the rolling downs geomorphic unit has resulted from the complete stripping of the silicified Tertiary land surface along the periphery of the anticlinal structures (Fig. 2.6). In addition, removal of the less resistant sections of the Late Cretaceous Winton Formation has left a thin mantle of coarse rock-debris, mainly silcrete, upon an undulating surface, commonly referred to as gibber plain (Twidale, 1972). The gibber surface is characterised
24 Cooper Creek
O Wilson River 0 <0
Alluvial plains Road/track Dune complexes \\yy] Rolling downs 0 10 20 N Dissected tablelands l_ _l _lkm_
Figure 2.6 - Geomorphology of the Cooper Creek study area
25 by a yellow-brown, brown or black colouration on the stones, referred to as a desert varnish, which has been attributed to the presence of manganese and iron oxide coatings (Twidale, 1972). The thin surficial mantle of silcrete gravels that dominates this unit indicates the previous existence of silcrete formations throughout the region (Senior et ah, 1968). The rolling downs unit is characterised by undulating surfaces, with slopes commonly less than 5%
(Senior, 1970; Dawson, 1974).
2.5.3 Dune complexes
There are two distinct dune types: those associated with the alluvial floodplain and those developed on the lower slopes of the rolling downs unit (Fig. 2.6).
The dunes on the floodplain are probably mostly source-bordering, lying as they do adjacent to palaeochannels. They occur in single or small groups of rounded dunes with mobile crests, and range in height from 1 to 15 m (Senior et ah, 1968) and tend to be buff couloured (7.5YR 6/4). The dunes on the lower slopes of the rolling downs are essentially broad, convex, reticulate dunes, characteristically 3-10 m high with low, predominantly rounded and stable crests. They appear to be disrupted elements from a system of roughly northward-oriented linear dunes that characterise part of the region and tend to be reddish in colour (2.5YR 6/8). These redder dunes are believed to have been derived mainly from the aeolian reworking and accumulation of quartzose sands resulting from the erosion of the Glendower and Winton Formations
(Senior et ah, 1968). The distinction between the two types is blurred on the lower slopes of the rolling downs where the floodplain has invaded prior dune systems. On the rolling downs, claypans between the dunes are common
(Senior, 1968; Senior et ah, 1968; Dawson, 1974).
26 2.5.4 Alluvial plains
The broad, very low-gradient clay-rich alluvial floodplains of Cooper Creek and major tributaries represent the accumulation of fine-grained alluvium derived from the erosion of Mesozoic sediments in the headwaters (Nanson et ah, 1988) and also locally from the erosion of Cretaceous and Tertiary sediments (Senior,
1968; Senior et ah, 1968). Across these broad alluvial plains, developed along the major depressions of the Cooper and Wilson synclines (Figs 2.5 and 2.6), the present-day Cooper and its tributaries transport mud and minor amounts of sand through an assemblage of coexistent waterholes and anastomosing, braided and reticulate channels (Rust, 1981; Nanson et ah, 1986,1988; Rust and
Nanson, 1986; Nanson and Knighton, 1996). During major flood events, water can extend over the entire floodplain, attaining a maximum width of 60 km east of Lake Yamma Yamma.
2.6 Soils
The soils on the Cooper Creek floodplain are characterised by very deeply cracked, grey to brown clay soils, with gypsum and lime in the lower sections of their profiles (Dawson and Ahern, 1974). The floodplain surface is composed of muddy alluvium which typically consists of 10-20% sand, 25-35% silt and 55-
65% clay (Rust and Nanson, 1989; Maroulis, 1992; Maroulis and Nanson, 1996).
A prominent feature of the alluvial soils are deeply cracked and gilgai patterns typical of vertisols. Vertisols are typically composed of a high proportion of smectite (montmorillonite), with some having a mixed or kaolinitic mineralogy
(Brewer, 1964). The presence of expandable clays is responsible for the repetitive swelling and shrinking during wetting and drying cycles, resulting in the development of a complex network of cracks up to 1 m deep (Hallsworth et ah, 1955; Hallsworth, 1968; Rust and Nanson, 1989). These highly absorptive soils have a profound effect on the hydrology of the floodplain, retaining a
27 large proportion of the initial overbank flows (Knighton and Nanson, 1994a).
Hence, when saturated, the surface seals to exclude further infiltration such that floodwaters almost never penetrate more than about 1 m in depth. The floodplain soils are very low in soluble salts near the surface, but there is a marked increase in calcrete and gypcrete below 1.5-2.0 m (Nanson et ah, 1988).
2.7 Vegetation
The dominant vegetation of the Cooper floodplain is low open scrubland, which is largely composed of lignum (Muelhenbeckia cunninghamii), flowering lignum (Eremophila polyclada), bluebush (Chenopodium auricomum), swamp canegrass (Eragrostis australasica) and various types of herbfields on flat plains and inter-channel flats. The changing dominance of ground flora is largely governed by the extent of flood inundation of the floodplain. When flooding occurs the dominant ground flora species include cooper clover (Trigonella suavissama) and channel millet (Echinochloa turnerana). However, scalded, flooded areas support sparse herbfields.
Coolibah (E. microtheca) and river red gum (E. camaldulensis) form a fringing woodland that dominates the anastomosing channels and waterholes
(billabongs) (Boyland, 1974). Wooded forblands with mulga, whitewood and vinetree occupy the floodplain sand dunes and low sloping sandy surfaces adjacent to the floodplain, with occasional forblands that are devoid of shrubs or trees (Boyland, 1974; SANTOS, 1993).
28 Chapter 3 Data Collection techniques
3.1 Introduction
This study of the stratigraphy and Quaternary development of the extensive, semi-arid Cooper Creek system in southwest Queensland, incorporates a combination of the following techniques: trenching, solid augering, uphole- drilling, gamma-logging of auger holes, seismic survey, TL
(thermoluminescence) dating and texture analysis, all of which are discussed below.
3.2 Trenching
Trenches dug with a tracked excavator to depths of 2 to 8 m have provided sedimentological and indirect hydrological information on the development of the floodplain through time. The depth of trenching was limited primarily to ~2 to 3 m in unconsolidated fine sand, however, in the clayey floodplain deposits it was possible to reach depths of ~8 m without the sides collapsing, the maximum extension of the excavator. A total of 13 trenches were excavated at various sites on the floodplain, within channels and in some proximal sand dune sites, the details of which are presented in the following chapter.
3.3 Drilling
Drilling was undertaken using 2 drilling rigs; a Gemco 21 OB truck-mounted solid-augering rig operated by the School of Geosciences at the University of
Wollongong, and a Bourne 2000R truck mounted drill rig, operated by Geodrill
Pty Ltd. (Toowoomba), for drilling upholes along seismic lines for Santos Pty
Ltd. (Adelaide). Hand augering using a 10 cm diameter auger was restricted to sand dunes because it proved too difficult through the tough clayey floodplain
29 sediment. In total, 120 holes were drilled during the course of this study of which 44 were attained by solid-augering to a maximum depth of 36 m using the School of Geoscience truck mounted Gemco 210D, 73 from uphole drilling to a maximum depth of 30 m by Geodrill Pty Ltd., and 3 by hand-augering to a maximum depth of 9 m.
Drilling was primarily confined to the floodplain but with several holes on sand dunes and within ephemeral channels. Sediment collected from the solid auger holes was logged for colour and texture, some was collected for thermoluminescence analysis and size characteristics for selected samples were evaluated in the laboratory.
The major limitations of augering are firstly, each individual drill hole only provides a 1-D view of the sub-surface stratigraphy. Secondly, due to the
'churning' effects of the auger, the system is not very sensitive to subtle changes in sediment texture; this makes it difficult to identify small scale depositional units such as thin upward-fining sequences of < 0.3 m. Thirdly, sedimentary flow structures are not preserved.
3.3.1 Uphole drilling
The shallow seismic drilling operation undertaken by contractors commissioned by Santos Pty Ltd. (Adelaide) was used to obtain seismic velocity profiles from upholes. While uphole drill holes were used to obtain stratigraphic evidence, no actual seismic data were used in this thesis. These upholes, located on seismic lines (refer to Fig. 4.1), provide a surface seismic signature that could be combined with deeper seismic plots to complete the seismic profile. The major benefit to this study of these upholes was firstly, they provided reasonably detailed (±lm) sedimentary information to depths of 24 m.
Secondly, they gave this information for a large number of drill holes in a
30 relatively small area (along seismic transects of -3-4 kms at intervals of -200-
300 m across the floodplain) and thirdly that they enabled the use of a gamma logger to document the lithological signature of each hole. The limitations of this technique were that the drill cuttings are pumped to the surface using water under pressure and hence the samples are too mixed up to allow accurate textural or TL sampling. Furthermore, as not all upholes were gamma logged there was the problem of accurately determining the depth of the boundaries of sedimentary units. Finally, the time taken for drill cuttings from a particular
depth to reach the surface increases exponentially with depth. Thus, at a depth of -30 m, there may be up to a ± 1 m error range in determining the accurate
depth of a sedimentary unit. However, estimates given by experienced rig
operators were useful in helping to determine the depth of individual
sedimentary boundaries.
In total, data was obtained from 147 kms of linear transect drilling involving 65
upholes in addition to data from another 8 drill holes logged in the Bolan Area
over a -20 km^ area. Stratigraphic logs from each hole and interpretations between holes are presented in the following chapter.
3.4 Geophysical techniques
3.4.1 Gamma-logging of auger holes
An A31 high sensitive gamma log tool (Auslog Pty Ltd., Brisbane) was used to
obtain detailed information about thin beds and the existence and
characteristics of upward fining sequences from auger holes and upholes
drilled along seismic transects.
The gamma ray log is used to detect radiation emanating from naturally
occurring radio-elements in the sub-surface sedimentary units. Radiation itself
31 is not of interest in this study, however, it tends to be much higher in clays and as a consequence, the gamma log can provide a surrogate index of fine- sediment concentrations in the sedimentary profiles. Potassium 40 is by far the most common radioactive element in sediment and is normally found in greatest abundance in clays, feldspars and micas (Doveton, 1994). The instrument is an excellent indicator of lithology detecting boundaries to within
±1 cm. The probe has a diameter of 42 mm, is 1130 mm in length, weighs 4.4 kg and is used with a hand-winch to control the rate of decent of the probe into the drill hole. This rate is monitored by a DLS-2 (Downhole Logging System) which is connected to a portable laptop computer whose software allows plots to be displayed in real-time. The entire system is very portable and allows for quick acquisition of data from each drill-hole. The set-up as used in the field is shown below in Figure 3.1. In this study, a total of 70 drill holes were gamma logged:
62 of these were upholes while 8 were auger holes.
The gamma-log detects differences in sediment texture via relative differences in radiation emanating from naturally occurring radioactive-elements, with lower counts per second for coarse sediments and higher for fine sediments. A typical gamma-log trace and lithological log for a floodplain drill hole (FWR-
340) from the Wackett seismic line (Fig. 4.5) is presented in Figure 3.2. A series of sand-dominated sedimentary sequences are evident between depths of 6 to
23 m whilst a radioactive anomoly, probably attributable to the top of the permanant groundwater table is evident at a depth of 19 to 20 m (Fig. 3.2).
If an upward fining sequence is detected in a drill hole, the gamma-log trace will show a typically sharp-bed response (as noted in the medium-coarse sand unit) which will also be evident in the sedimentary log. However, it is clear in
Figure 3.2 that there is no change in sediment texture to explain these high values between depths of 19 to 20 m. Subsequent Uranium-Thorium trace
32 - " ?•*• Gamma-logger in drill hole Laptop 9 Z-f^£3itSr ••*:*1 DLS-2 Data I I Computer I Logger
*axC£ •••":•
.> .- ; «sjVil M*-*"**
'AT*.
Figure 3.1 - (a) Field setup of gamma logger with DLS-2 data logger, laptop computer and (b) gamma log probe at Shire Road site (SR3) 33 FWR-310 GAMMA LOG (counts per second) DESCRIPTION 0 25 50 75 100 PROFILE _o li|§||HI P — CLAY Grey/brown :x Q_
./Jf MEDIUM SAND Orange/buff % io- i - COARSE SAND Orange/brown poorly sorted - 15 , \ FINE-MEDIUM SAND Orange/buff grading to grey/blue sand
ZO -
Figure 3.2 - Gamma-log plot of a floodplain uphole from seismic line 95-FWR Station No. 340 (plot shows a uranium anomaly at a depth of 19-20).
34 analysis (B. G. Jones, pers. comm., 1996) confirmed the existence of the short lived isotope radon and long-lived isotopes of uranium and thorium at the top of the groundwater table, and thus has probably contaminated the gamma-log signal, a common feature in traces at depths >14 m.
However, the effects of sub-surface accumulations of radioactive nuclei with enhanced gamma-counts can bias interpretation. These anomalies probably result from accumulations of radon related to groundwater accumulations at depth under reducing conditions. Radiation anomalies at depth are recorded as symmetrical bell-shaped to sharp-response spikes in the gamma-log traces rather than the asymmetrical gamma-log plots associated with upward-fining sequences. The combined sedimentary information from uphole drilling and gamma-logging provides a very detailed and useful vertical profile of subsurface stratigraphy once these anomalies are taken into account.
3.5 Dating Techniques
As one of the major objectives in this study is to establish a Quaternary chronology of the Cooper Creek floodplain, thermoluminescence (TL) dating has been adopted as the principal dating method for the dominantly quartzose sedimentary units of the Cooper floodplain. With the exception of one young archaeological site, radiocarbon (-^C) dating could not be used due to the lack of dateable organic carbon at depth in the floodplain and the age limit for ^C of 30-40 ka (Callen et ah, 1983), a small fraction of the known age range of alluvium along Cooper Creek (Nanson et ah, 1988, 1992b). The more recent optically stimulated luminescence dating (OSL) was not available for use in this study at the time the research was being undertaken. Check OSL dates are presently being conducted in the new University of Wollongong OSL laboratory and will be used to evaluate the TL record obtained here prior to any publication of the chronology.
35 3.5.2 Thermoluminescence (TL) Dating
The TL technique is widely used in archaeology (Aitken, 1990; Roberts et ah,
1990; Roberts and Jones, 1994; Fullager et ah, 1996; Roberts, 1997) and is becoming more accepted in the earth sciences with its potential to provide dates back to -10" ka for aeolian and fluvially transported quartz and felspathic minerals.
Dating of sediments using TL is based on the acquisition of TL energy by crystalline minerals buried within a sedimentary unit. The technique assumes that during transportation and prior to deposition and subsequent burial, most of the previously accumulated TL energy is removed ('bleached') by exposure to the ultra-violet (UV) component of solar radiation. After bleaching and burial, the TL energy within the sediments' crystalline lattice once again builds up as a result of the cumulative effect of prolonged exposure to the weak flux of nuclear radiation emitted by long-lived isotopes, largely uranium, thorium and potassium found in the surrounding sediment, and from cosmic radiation. In addition, rubidium provides a relatively insignificant flux to the TL sensitive crystal.
On exposure to radiation crystalline minerals produce free or 'ionised' electrons detached from parent nuclei. These electrons may become trapped in defects or lattice-charge disequilibrium sites, called electron traps within the crystalline ionic lattice (Aitken, 1990). Once within an electron trap, an electron will remain there until released by vibrations of the crystal lattice. Over time, the number of trapped electrons increases within the finite number of electron traps. As the temperature is increased, these vibrating electrons become more energetic, and thus the probability of them escaping the traps increases so rapidly that within a relatively narrow temperature range the situation changes from one where
36 electrons are firmly held within the electron traps to that of being free to diffuse
about the crystal.
Exposure of sediment to light can also cause vibrations within the crystal lattice resulting in the eviction of electrons from electron traps. Some of the released
electrons may be conducted to luminescence centres (defects in the crystal
lattice usually due to impurities such as silver or manganese atoms) causing
photons to be emitted as light. In most TL dating procedures, the UV or blue
regions of the emitted light are selected for analysis. The time since the
sediment was last exposed to sunlight is determined by measuring the total
amount of TL energy absorbed since deposition (the palaeodose or equivalent
dose, ED), and the rate at which the energy was acquired (the annual dose,
AD):
TL age = ED/AD
The regenerative/additive quartz coarse-grain technique (Readhead, 1988),
which has been used extensively (Nanson and Young, 1987, 1988; Short et ah,
1989; Shepherd and Price, 1990; Nanson et ah, 1990, 1993a, 1993b; Page et ah,
1991,1996; Roberts, 1991; Page, 1994; Price, 1994a, 1994b). Dating of the samples
for this study was carried out by David Price in the TL Laboratory in the School
of Geosciences, University of Wollongong.
TL sediment samples were collected from a range of depositional environments, including fluvially transported sediment ranging from sandy clays through to gravelly sands, and aeolian transported fine to medium sands.
All samples were collected either from freshly exposed pit faces using metal tubing or from auger holes. Each sample was sealed as outlined below to avoid
exposure to light and to preserve the environmental moisture content of the
sample.
37 Samples obtained from trenches were collected by introducing an open-ended metal tube into a freshly cleaned trench face. These 20 cm long aluminium tubes with an internal diameter of 10 cm were carefully hammered into a sedimentary unit ensuring that the tube is tightly packed so as to gain the maximum sample amount and to minimise any unnecessary movement of sediment within the tube. The samples were taken from the centre of a homogeneous sedimentary body with no dissimilar materials within a radius of
30 cm, as could best be determined, including any pedogenic nodules, calcrete rhizomorphs or pebbles. The presence of heterogeneous materials within this distance can provide an unknown radiation dose to the sample. The tube is then extracted from the face and sealed at both ends with black plastic stoppers to exclude any light and seal in the moisture.
The collection of TL samples from the Gemco 210D drill rig augers is more problematic than those taken from trench sites. There are difficulties in establishing the homogeneous nature of the stratum being sampled and in protecting the sample from exposure to light. The auger is withdrawn from the auger hole, the outer coating of contaminated sediment has been scrapped off the perimeter of the auger, and the auger slid into a black plastic sleeve.
Sediment was then removed from the bottom of the auger by hand with minimal exposure to sunlight following which the sample was transferred into light-proof black plastic bags and sealed with packaging tape. This sampling process is acceptable so long as the sediment collected is not exposed to direct sunlight and exposed to indirect light for less than 60 seconds. It needs to be appreciated that at this stage the quartz grains still coated with fines and secondary minerals, they are mostly contained with large aggregated lumps' of sediment as they are removed from the auger, and because the less-sensitive temperature plateau (375°C) used for the TL dating requires prolonged exposure to alter its value significantly (Price, 1994a, 1994b).
38 The apparent usefulness of the method for dating quartz sands in both aeolian and fluvial environments has been demonstrated in numerous studies in
Australian environments over the past two decades (Wasson, 1983a, 1983b;
Rust and Nanson, 1986; 1989; Gardner et ah, 1987; Nanson and Young, 1987,
1988; Nanson et ah, 1988, 1991, 1992b, 1993b). Comparisons with other dating methods as well as other corroborative evidence such as stratigraphic consistency lends supports for the use of TL dating in a variety of environments including fluvial ones in northern and central Australia (Readhead, 1984,1988;
Short et ah, 1989; Bryant et ah, 1990, 1992; Roberts et ah, 1990; Shepherd and
Price, 1990; Nanson et ah, 1990, 1991, 1992a, 1992b, 1993b; Bryant, 1992; Young et ah, 1993).
However, it is of some concern whether satisfactory bleaching of sediment can occur in a fluvial depositional setting due to the limited ability of UV-radiation to fully remove accumulated TL energy from quartz grains under water
(Rendell et ah, 1994). This is of special concern where turbidity in a river can minimise the penetration of UV-radiation through the water column (Ditlefsen,
1992). However, there has been considerable stratigraphic consistency of published dates for fluvial sequences in the Channel Country and rivers near
Lake Eyre (Rust and Nanson, 1986; 1989; Nanson et ah, 1988; 1990; 1992b, 1993b,
1996; Croke et ah, 1996, 1998). There was one instance in this study where a 14C date of 1420±60 years BP (Beta-84639) of charcoal (an age of A.D. 590-775 (2 sigma range at 95% probability level (Vogel et ah, 1993)) appeared to conflict with a TL date of 5300±400 ka (W2053) in baked clay both obtained from the same buried aboriginal hearth under Goonbabinna Waterhole (Fig. 4.14). This gives an age discrepency of about 4000 years, the cause of which is unknown.
Furthermore, it can be reported that surface samples in the muds date at 200-
2000 years (David Price and Gerald Nanson, pers. comm., 1998) and the sands
39 near the present main channel of Cooper Creek date at <5000 years some several metres in depth and several hundred metres from the channel (David
Price and Maria Coleman, pers. comm. 1999). Near surface muds obtained in this study also dated at <400 ka (see Fig. 4.13). If there was a serious problem with residual TL remaining in the samples, these young samples would have yielded ages larger than they did. Even if these samples were deposited very recently, these calculated ages are within most of the error bands for the dates reported in this study. Such evidence provides confidence that most of the dates in this study contain no more residual TL than would give rise to errors of about 400-4000 years.
Truly legitimate comparisons of identical sedimentary strata using two or more fully independent dating techniques are very uncommon, particularly for the age-range of strata being investigated in this thesis that is believed to extend well beyond that suitable for radiocarbon analysis. For this reason the TL chronology presented here must be interpreted as preliminary evidence and part of an ongoing investigation within the School of Geosciences.
3.6 Methods of Sediment Analysis
Sediment samples collected from trenches and auger holes were examined for size and colour. Samples coarser than very fine sand (>4(()) were dry sieved while finer samples were wet-sieved. Cumulative size distribution curves and their associated statistics can provide insights into the depositional processes responsible for a particular sedimentary unit. For instance, aeolian sediment samples are generally fine to medium sand sized (means from 0.125-0.25 mm), moderately well-sorted and are usually unimodal, leptokurtic and fine-skewed
(Cooke et ah, 1993). In contrast, fluvially transported sediment from the same deposit commonly displays a wider range of mean sizes, are often bimodally size-distributed, coarse-skewed, and are more poorly sorted (Richards, 1982;
40 Chorley et ah, 1984). Even though cumulative size distribution plots do not conclusively identify a depositional environment, they do provide useful supporting evidence. In addition, SEM (scanning electron microscopy) was used to provide further insight info the grain morphology and possible depositional processes responsible for a range of the sampled sedimentary deposits.
41 Chapter 4 Regional Stratigraphic Associations
4.1 Introduction
On the basis of stratigraphic and chronological evidence from 120 drill holes, 13 trenches and 75 TL dates, the Cooper Creek sedimentary environment between
Mt Howitt and Nappa Merrie, can be categorised into 2 broad sedimentological groupings that accord with the general geomorphology of the floodplain surface.
• Group 1. Consists of extensive sand-dominated facies, the Katipiri
Formation, that is overlain by an upper unit of fluvial mud. This group is by far the dominant sedimentary floodplain facies in the study area. The Group
1 sediments also extend eastwards into tributary alluvial fans and thin fluvial sedimentary units over weathered bedrock and degraded sand dunes, all overlain by floodplain mud. These features are primarily located on the eastern and southern margins of the Cooper Creek valley fill in the study area.
• Group 2. Consists of aeolian sand and fluvial mud units over the sands of
Group 1, resulting from the formation of source-bordering dunes and the interaction between mud-laden overbank floodwaters and these dunes. Two
Group 2 sites were studied intensively and are reported here in detail.
42 Evidence from these two sedimentary groupings is provided from a selection of uphole drilling and drill logs, gamma logs, hollow augering and trench excavations.
4.2 Group 1: Mud-capped sand deposits (main floodplain sections)
The predominance of sand deposits underlying the Cooper Creek floodplain has been reported widely in the literature (Rust, 1981, Nanson et ah, 1986,1988;
1996; Rust and Nanson, 1986; 1989; 1991; Gibling et ah, 1998), however there is little available research detailing the distribution, stratigraphy, sedimentology, depth and total lateral extent of this sand body. Evidence presented in this chapter addresses many of these points.
Overall, the sedimentary facies of Group 1 are dominated by a mud cover at the surface, commonly 2-3 m but to a maximum depth of 6-7 m, which is underlain by extensive sand deposits to maximum depths near the centre of the valley of
>40 m (Santos Pty Ltd., unpubl. report). Evidence about the Group 1 facies is provided from a variety of sites distributed across the floodplain (Fig. 4.1).
4.2.1 Seismic line evidence
The subsurface stratigraphy of the Cooper Creek floodplain was interpreted from a total of 67 upholes drilled on 8 seismic lines at Wareena, Tanu, Costa and Wackett (Figure 4.1). These data reveal extensive upward-fining sandy
43 Currareva • KEY Windorah Site Site Name Reference 1 Lignum Creek (Fig 4.18) 2 Narberry Waterhole and South Narberry dune (Fig 4.31) . 4*1**» • * * 3 Durham dune (Fig 4.26) 4 Chookoo Sandhill complex (Fig 4.21) 5 Durham Road transect (Fig 4.06) 6 Bolan Seismic grid (93-BS01>BS26 and 93 BR01 >BR18) (Fig 4.17) 7 Costa seismic line (93-EPG) (Fig 4.04) 8 Tanu seismic line (93-EPR) (Fig 4.02) 9 Wareena seismic line (93-EPJ) (Fig 4.03) 10 River Road dune 11 Tooley Wooley Waterhole (Fig 4.16) 12 Shire Road transect (Fig 4.07) 13 Wackett seismic lines (95-FWM, -FWP, -FWQ, -FWR, -FWS) (Fig 4.05) 14 Goonbabinna Waterhole (Fig 4.14) 15 Baryulah Seismic lines (95-FWX, -FWY, -FWZ, -FXA) (Fig 4.16) 16 Mt. Howitt palaeochannels (Fig 4.12) r Lake Mt. Howitt Yamma Yamma /.
LEGEND Symbol Item
L Floodplain Sand dunes
Roads
Transects
Seismic lines
Durham Downs.. <-y-i \y A North vV© Durham Dawns - Eromanga Road 0 10 20 30 40 Meringhina Waterhole km
fJ3J I ) >f/r. Shire Road
Nappa Merrie Naccowlah Waterhole . Jackson Oil Terminal v, 'V, ' Figure 4.1 "-:** .£»«• Noccundra* FIELD WORK LOCATIONS :© !JmftLwiisonR,ver ON THE COOPER CREEK FLOODPLAIN *(r/
44 45 e -a 3 Is (0 Jr, nj OJ ra •a 2 01 U •£-C i- QJ X) Ol (X III* U («A co xi ra c o C n: oi o J 1» no 2: -•: ID «u •III 1Ol CD c oo > nj O o o o Son iTio nj oi at ** g) -*- rH ^2cf « , u JO oc N m u to 01 *oc <* ''S T3 X *•3 2 OJ bD-C > bin £ I I ii3
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47 FWM406 FWM43S FWQ 335 FWR 340 FWS 380 Figure 4.5 JUmlHD 71S-AKD 7i mac SEDIMENT AND GAMMA LOG PLOTS LEGEND
•H Mud - Approx. boundary between war- Wackett Seismic Lines: HORIZONTAL SCALE 1 1 Sand surface muds & sand units. 1:45000 95-FWM, FWP, FWQ, FWR and FWS* I I Weathered Bedrodi Boundary between near- 1 1 Dalanotavalabte surface muds & sand units. 'Location and orientation of these are shown below •I LOCALITY MAP =o
FWP 310 FWP 331 FWR 291 FWS 316 FWP 444 FWP 500
TlJmAHD 7 Mm AH) 71inAHD TVteJUC 71hn»MD ^ ESE
-> (§L ^ 4 >• " £ I \ ML Howitt
=D
FWQ 205 FWQ 289 FWP 310 FWQ 322 FWQ 335 FWQ 362 FWQ 500 FWQSS0
TltnAHD 71tmUO 'uminD TlimWtD TUmUID •i'nthC TUoAtC n.uu€ >\.., # „ South °L
0 10 20 30 40 / 7*
KEY X J f r Symbol Item Durham Downs
1 1 Rood plain 1 1 Sand dunes
Roads czJ
Seismic lines
FWR 245 FWR 280 FWR 291 FWR 315 FWR 366 FWR 415 FWR 465 FWR 527 FWR 567 see below 71.1m MO 71SmW) 71.5-AHD 7iimW> 7V7B DC 71.1B AM) 7ltaAJO TlfeUC
Nappa Merrie Jackson Oil Terminal f f-" Innamincka J* ^ VpjLif Noccundra J' [UU] t WACKETT SEISMIC ,QNi;S ' ' >i ^J I
FWR 205 FWS 305 FWS 316 FWS 380 FWS 455 FWS 505
71.1m MO TIJAIUO 717m«0 TtMWC TLtaUO floodplain sequences to depths of 24 m underlying a surficial clay unit varying in thickness from 1-7 m with one exception of 18 m on the Costa line.
Schematic diagrams illustrating each of the Wareena, Tanu, Costa and Wackett sections are presented in Figures 4.2-4.5. The Wareena line straddles the eastern margin of the floodplain and valley sides, intersecting low-gradient alluvial fans on that side of the valley, while the Tanu, Costa and Wackett lines are located on the western and central portions of the floodplain (Fig. 4.1) and therefore compare with stratigraphic information obtained from holes augered and pits excavated into the central and western side of the floodplain.
4.2.1.1 Surface Clay:
The observed thickness of the floodplain clays ranges from 3-7 m across each of the Tanu, Wareena and Costa seismic transects. In contrast, some of the multiple Wackett seismic lines have near surface clay depths of <1 m (Fig. 4.5).
This massive, homogeneous, grey-brown floodplain clay varies little from one floodplain site to the next, the only exception being the presence of abundant gypsum in uphole EPR-205 (Fig. 4.2), the westernmost uphole of the Tanu seismic line. However, given the proximity of this uphole to the iron-stained western valley side (<150 m), it is possible that the prevailing winds have transported gypsum from the weathered western valley surface onto the eastern floodplain and incorporated it into the clayey floodplain surface.
49 4.2.1.2 Clay at depth: In contrast to the surface clay layer, only small lenses of clay and clay intraclasts are evident at depth in the sand dominated sedimentary profiles.
Wackett:
There is very little evidence of clay in any of the 41 upholes except where weathered bedrock is encountered (Fig. 4.5).
Costa: In general, the floodplain stratigraphy of the Costa line is characterised by a 4-6 m thick clay layer, underlain by mostly upward-fining sand sequences with occasional clay interbeds (Fig. 4.4). Sand dominates the majority of logged holes, however an extensive 10 m thick clay deposit (74-64 m AHD) is present in uphole EPG-475 (Fig. 4.4), probably representing a palaeochannel or perhaps two superimposed palaeochannels separated by a sandy-clay unit. Only very limited clay deposits are encountered at depth elsewhere on the floodplain.
Tanu: These clays overlie extensive upward-fining sand sequences and substantial clay units at depth. In upholes EPR-205 and EPR-384 (Fig. 4.2), clay units dominate the sedimentary logs at depth, however, these multicoloured clays are almost certainly weathered bedrock. For instance, uphole EPR-205, located
<100 m from the bedrock western valley side, is predominantly clay. Typically, dry grey/brown self-mulching expandable alluvial clays dominate the floodplain surface at this site. Below 66 m AHD, soil moisture increases and colour varies markedly from mottled-orange to purple, olive, black and yellow.
XRD analysis reveals the dominance of kaolinite with minor amounts of illite/ 50 montmorillonite suggesting oxidised bedrock buried beneath 10-11 m (67 m
AHD) of vertically accreted floodplain clays and sandy clays. Over time and
under reducing conditions, the bedrock has weathered to form an assortment of
clays of widely varying colours. With the exception of infrequent thick clay
units, probably representing the preservation of palaeochannel infills or
sections of overbank sediment, the great majority of upholes reveal an extensive
sand stratigraphy. However, scattered mud lenses and abundant mud
intraclasts indicate that these channels clearly had a fine-grained suspended
and wash load that would have formed mud drapes within the channel as well
as fine overbank floodplain deposits. Widespread channel migration must have
reworked most of the overbank fines leaving mostly the coarser thalweg
sediments in the stratigraphic record.
4.2.1.3 Gypsum and carbonised wood:
Wareena: Although very little gypsum was evident, carbonised wood was noted in many
of the upholes (Fig. 4.3). In some cases it was sufficiently abundant to believe
that the drill had passed through buried tree trunks. Some of this material
recovered from uphole EPJ-270 at a depth of 18 m (61 m AHD), however, due to
saturation of the sample by calcium carbonate, it was not possible to determine
the plant species involved. The material was not radiometrically dated because
TL ages for alluvium below 10 m are shown later to be »40 ka.
51 Costa:
Gypsum was evident in the fine sand units immediately underlying the near
surface clays; charcoal was identified at a depth of 9-12 m in upholes EPG-315
and EPG-362 between 63-60 m AHD and 67-61 m AHD, respectively (Fig. 4.4).
Tanu:
Carbonised wood fragments, ranging in size up to 2-3 cm were evident in
uphole EPR-280 between 67-47 m AHD (Fig. 4.2). An interpretation of wood
anatomy recorded in the charcoal was unsuccessful due to the same calcium
carbonate saturation problems encountered for the Wareena samples. However,
the abundance of carbonised wood indicates that riparian forests probably
occurred under conditions that both the anaerobic preservation of the wood
and the abundance of coarse sand suggests were much wetter and more
fluvially active than at present. Modern conditions are too dry for the
preservation of logs in contemporary alluvium, the latter not being
permanently saturated. Gypsum and degraded wood were observed in upholes
EPR-205 and EPR-280, respectively (Fig. 4.2).
4.2.1.4 Silcrete/Ferricrete:
Costa: Silcrete and ferricrete lenses were encountered at depth in several drill holes, indicating that these pedogenic processes have been active since deposition of these essentially unconsolidated sediments.
52 4.2.2 Durham Downs Road Transect
The Durham Downs Road transect is aligned approximately parallel to the
Durham Downs Road (Figure 4.1) for a distance of -12 kms and consists of three roughly equidistant auger holes -5 kms apart on the east (F/P #1), centre
(F/P #3) and west (F/P #6) sides of the floodplain. No pits were excavated along this transect hence it is not possible to describe the sedimentary structures.
The contemporary floodplain channels vary markedly. Frequent fluvial activity and a dense vegetation cover of lignum (Muehlenbeckia cunninghammii), flowering lignum (Eremophila polyclada) and swamp canegrass (Eragrostis australasica) are prevalent on the western side of the floodplain where well- vegetated tree-lined (E. microtheca) and incised anastomosing channels dominate the floodplain. The main channel of Cooper Creek is located adjacent to the western side of the valley. In contrast, the floodplain to the east is a relatively flat, featureless, scalded alluvial surface with stunted vegetation cover and very few anastomosing channels. In general terms, the eastern side of the floodplain looks much less fluvially active than the western side.
A schematic diagram summarising the stratigraphy and chronology of the
Durham Downs Road transect is presented in Figure 4.6. The floodplain stratigraphy is characterised by a -4-6 m thick surficial clay unit which extends along the entire transect. Below this is a relatively thin sandy-clay layer that is transitional to an extensive medium-coarse sand unit below about 5-6 m in all 3 auger holes (Fig. 4.6). Upholes along the Tanu and Wareena lines (Figs 4.2 and
53 H3 rt 0 *^ Pd S cd e« ,fa .fa£ 3 H 0 04 t
4» ' - * ,'
I
54 4.3, respectively) south and east of the Durham Downs Road suggest that this sand unit extends to below 30 m in places.
Three significant changes occur along this transect: (Fig. 4.6)
1) there is an increase in pedogenic gypsum eastwards in the clay-dominated near-surface sediments,
2) there is a distinctive 'younging' of the clayey floodplain sediment from 40-
100 ka in the east to very recent clays (<3 ka) in the west, and
3) there is a change from Stage 7 (-200 ka) sand in the east to Stage 5 or 4 age sand (-75 ka) in the west.
On the eastern side of the floodplain, crystallisation of gypsum in greater abundance in the profile results probably from limited percolation and possibly the slow upward movement and evaporation of groundwater (Cooke et ah,
1993). This side of the plain does not flood as frequently as those areas adjacent to the anabranching channels in the west and the soils therefore remain drier.
Gypsum is still present at depth in the middle floodplain (F/P #3) although in lower quantities. On the western side (F/P #6), where the surface is pedogenically active and heavily cracked, the vegetation is much more dense than across the middle and eastern sections of the transect and there is no evidence of gypsum in the profile (Fig. 4.6). Wetter soil conditions prevail allowing limited opportunity for gypsum to crystallise out of solution within the clay unit. No gypsum was detected in any of the sand layers underlying the clays across the floodplain in this transect, although it is present in sand units elsewhere.
55 4.2.3 Shire Road Transect
Although similar to the Durham Road transect -35 km to the north, the floodplain morphology along the 13.9 km Shire Road (SR) transect (Fig. 4.7) differs in that active waterholes and anastomosing channels are evident on both the western (Goonbabinna Waterhole) and eastern (Naccowlah Waterhole) sides of the floodplain. However, active channels are less common on the eastern side. The Nacowlah Waterhole is not part of the main flow system of the
Cooper which here is largely confined to the western side of the floodplain. As with the Durham Road transect, the western floodplain displays more micro- relief and considerable evidence of contemporary fluvial activity in the form of incised anastomosing channels and associated waterholes.
Stratigraphic and chronological data from 8 auger holes (SRI-8) and 3 trenches
(SR3-T, 5-T, 8-T) along the Shire Road transect, summarised in Figures 4.7, 4.8 and 4.9, provide a detailed sequence across the floodplain. The trenches have permitted detailed descriptions of sedimentary and pedogenic structures in the upper 6-7 m of the floodplain. The near-surface clays vary in depth from 0.5 m to 5 m, averaging -2.5 m. The expanding nature of these clays has destroyed any flow or bedding structures that may have been present. The limited depth of floodplain clay deposition in parts of the transect resulted in only 4 clay TL samples being dated, however, minimum ages for several clay units can be inferred from dated sands beneath. These dates reveal a variable chronology for the overlying clays (Fig. 4.7) ranging between <10 and 75 ka, with younger dates (11.3±1.0 ka (W2040), 14.4±1.4 ka (W2047) and 10.5±0.9 (W2030) being
56 a .—. o a; o S-'-C -9 o «B •si * £ g> o » « S - 41 5PH3 5 •5 m S ^1 j M a co T3 2 41 O H JB AS .*, l a JJ o a; n ai «Sc5 £ 3 S S 2 -M T3 _S <*> 3 aj u 3-5.8 1 1 u- i u > S ° o 2 a e o o 1 CO '43 X> , a CJ o a is ,2 § .III E-5 S
57 located in the centre and western end of the transect, and older dates (53.9+4.0 ka (W2057) and 63.1±12.2 ka (W2049)) in the thicker and more pedogenically altered clays in the eastern end.
There are marked differences in floodplain stratigraphy east of auger hole SR5 compared to west of that (Fig. 4.7). Beneath the uppermost clay unit, the eastern auger holes (SR6-SR8) penetrated tough, almost indurated sandy profiles which restricted the depth of augering to <7 m. Apart from the overlying floodplain clay, which has an average depth of -4 to 4.5 m, most stratigraphic units in the east are thin and have mixed textures with sharp erosional reactivation surfaces displaying limited bedding structures and often abundant gypsum and calcrete.
In contrast, the western auger holes (SR1-SR4) are much easier to drill (apart from SRI where silcrete probably associated with bedrock was encountered at the base), had much less near-surface floodplain clay (-1 to 1.5 m average depth), contained less gypsum and provided evidence of extensive sand deposition with numerous bedding structures (see SR3-T; Fig. 4.8).
Excavation of a pit in the eastern floodplain (SR8-T; Fig. 4.9) revealed a very tough light brownish grey (10YR 6/2) clay with 10-20 mm diameter gypsum crystals to a depth of 3.7 m, below which is a 0.3 m thick Fe-Mn stained very pale brown (10YR 7/3) fine-medium sand layer to 4.0 m (Fig. 4.9). Between 4.0 and 5.8 m, the stratigraphy changed markedly to fine to coarse light grey (10YR
7/2) sand units, with thin calcite and Fe-Mn stained lenses probably indicative of former water-table levels within the profile. Between 5.8 and 6.2 m, there is a highly oxidised unit comprised of -5 mm diameter pea gravel with an 58 Trench: SR3-T (Western Shire Road Trench) Location: ~3.3 km east of the main Cooper Channel on west side of floodplain; trench is 50 m north of Shire Rd Site Details: Flat floodplain with minor reticulate channels and mounded gilgai (~30 cm) [E face]
Grey/ brown mud (some fine sand near base)
1.5m- Low angled, large trough-cross beds more evident from 2.2- 3.0 m; pale yellow to yellow-brown; Fe-Mn coating on bedding surfaces; ~13° dip south into trench face; possibly a reworked aeolian unit
2.0m-
2.5m-
Climbing ripple facies in 3.0-3.5 m unit
3.5m- Fe-Mn coating on horizontally laminated bedding surface; basal contact planar
~0.5-1.5 cm climbing ripple sets; unidirectional (planar 4.0m- to climbing bases and some signoidal foresets) Fe/Mn coating on horizontally laminated bedding surface at 4.4-4.5 m; also contains 4 planar layers (0.5 cm thick) which dip-12° to the S 4.5m - » JlFrom 4.7-5.0 m, 3-5 cm climbing ripple sets; tougher cemented basal 5 cm unit TL Date: 41.9+5.1 ka (5.1 m) (W2296)
Large-scale cross-beds with 'festoon' appearance ~5-20 cm sets (refer to above inset) and rare scattered pebbles (qxl cm); irregular carbonate smears left by the excavator on the sides of the trench (~80 cm long of :6.1m:::::::::::^rrrT::::::: I | diffuse carbonate) appear to represent rhyzoconcretions
M=Mud; S/M=Sandy/Mud; FS=Fine Sand; MS=Medium Sand; S/M MS VCS CS=Coarse Sand; VCS=Very Coarse Sand —L_ _L_ J Sr= Ripple cross-laminated sand; St= Trough cross-bedded M FS sand; Sh= Horizontally/Plane-bedded sand
Figure 4.8 - Schematic diagram depicting the stratigraphy evident on the E face of the Western Shire Road trench (SR3). Inset shows large-scale (-5-20 cm) 'festoon'-shaped trough cross beds in medium sands at a depth of 5.2 m.
59 Trench; SR8-T (Eastern Shire Road Trench)
Location: -0.5 km west of River Rd and Shire Rd junction on the east side of floodplain; trench is -40 m north of the Shire Rd Site Details: Flat floodplain with a reticulate channels (~5 m wide and 1 m deep) spanned by the excavated trench; very heavily gilgaied; thin surface veneer of red clay/dust (~3 mm thick) generated by passing traffic along Shire Rd and transported by local winds across the floodplain surface; trench is 6m deep and 15m long.
[S face] __ -3 mm thick veneer of red clay/ dust drape
Grey/ brown mud; friable, self-mulching layer (decreasing with depth); blocky surface; irregular clods
Grey/brown amorphous compacted mud; gypsum 1.5m- increasing with depth (some crystals ~l-2 mm thick)
KEY: 2.0m- Gypsum V Calcrete rhyzomorphs Calcrete/ calcite intraclasts
3.0m-
Mostly clay but increasing 3.5m - amounts of FS appearing at depth
Fe-Mn coating on horizontally laminated J , iijffii •«) .«, gj .1. ,ij .1, +tim, .** bedding surface from 3.7- 4.0m- 4.0m .-•, 4.0 m
4.5m - Calcrete/ calcite intraclasts evident at base of this unit (at 4.8 m) 47m?
5.0m - Fe-Mn layer horizontally laminated bedding Sh = 5.2m: surface from 5.1-5.2 m
5.5m -
5.8m ; Some pea gravel (cp~5 mm); assorted mix of "~1 weathered feldspar, quartz and red ferruginous chunks; I Tertiary 6.0m -I pale-grey, 'softish' matrix; grains very well I Glendower frosted and shiny (wind blasted?) __J Formation (?)
M=Mud; S / M=Sandy /Mud; FS=Fine Sand; MS=Medium Sand; S/M MS vcs CS=Coarse Sand; VCS=Very Coarse Sand _L _l_ Sr= Ripple cross-laminated sand; St= Trough cross-bedded sand; FS cs Sh= Horizontally/Plane-bedded sand
Figure 4.9 - Schematic diagram depicting the stratigraphy evident on the S face of the eastern Shire Road trench (SR8). Inset shows Fe-Mn coating on the horizontally laminated sand between 3.7 and 4.0 m depth. 60 assortment of quartz, feldspar and red (2.5YR 5/6) ferruginous chunks contained within a light grey (5YR 6/1) weathered clay matrix with occasional
10-20 mm diameter, rounded ventifacts displaying well-frosted, shiny surfaces that appear to have been shaped by aeolian abrasion. This heavily weathered unit may represent the top of the Tertiary Glendower Formation, which exists at shallow depths beneath the eastern floodplain and is exposed on the valley sides.
Sedimentary units east of SR6 provide evidence of much flashier fluvial processes in deposits underlying the near surface clays (Fig. 4.7), similar to that described below for the Lignum Creek site located near SR7 (see Fig. 4.18).
The TL chronologies of the sand units of the eastern floodplain also exhibit relatively large errors due to their approaching saturation. For instance, there are TL dates of 169±83 ka at 6 m (W2048), >147 ka at 7.6 m (W2050) and 164±25 ka at 6 m (W2052) in auger holes SR6, 7 and 8, respectively. Not withstanding this variability, the sand units underlying the floodplain in the east appear to consistently predate Oxygen Isotope Stage 5 (i.e., >130 ka; Fig. 4.7).
In comparison, the western floodplain stratigraphy (Fig. 4.7) is comprised of extensive non-indurated fine to coarse light grey (10YR 7/1) sand beneath a commonly thinner cover of mud. In the uppermost 10 m of the floodplain these sands date at < 80 ka, in other words to mostly post-date Oxygen Isotope (OI)
Stage 5. Upward-fining units occur at considerable depth (>18 m) with the total depth of unconsolidated sand exceeding 35 m. Here the TL chronology of the 61 deeper sands has been extended well into the middle Quaternary and possibly reaching the early Quaternary beyond the Bruhnes-Matayama magnetic reversal at -780 ka (Bassinot et ah, 1994).
A trench excavated in the western floodplain (SR3-T; Fig. 4.8) revealed a sandy stratigraphy dominated by the prevalence of trough cross-beds and numerous climbing ripple sets largely free of gypsum. After 1.4 m of near-surface greyish brown (10YR 5/2) clay, the sedimentary profile grades into a very pale brown
(10YR 7/4) very fine sand unit to a depth of 5.0 m which coarsens to a fine- medium sand from 5.0 m until the base of the trench at 6.1 m. A few well- rounded pebbles (diameter of 10 mm) were identified below 5.0 m and some diffuse carbonate was detected below 5.4 m.
As with the eastern side of the floodplain, bedding structures in the clay units on the western side are poorly preserved or destroyed due to the shrink- swelling effects of the floodplain soils. However, beneath the clays, low-angled, trough-cross beds are evident with bedding planes dipping -13° into the eastern face of the trench. Cross-laminated, unidirectional ripple sets of ~0.5-1.5 cm amplitude exhibiting planar to climbing bases and sinusoidal foresets dominate the sand unit to a depth of 5.0 m. The average palaeoflow orientations are to the SW (230°). Medium sands from 5.0 m to 6.1 m revealed large-scale
(~5-20cm) 'fesfoon'-shaped trough-cross beds (Fig. 4.8) with average palaeoflow directions to the W (275°). In addition, several pronounced Fe-Mn-stained horizontally-bedded laminations are evident at three locations in the profile between 3.5 m and 4.5 m (Fig. 4.8).
62 The sedimentary profile of SR3-T trench (Fig. 4.8) appears to represent an upward-fining sequence that is probably attributable to a laterally migrating fluvial system. In addition, the bedding structures illustrate marked differences in rates of sediment accretion in the profile. Fine-medium sand below 5.0 m was transported and deposited by channelised flows oriented to the W (275°); this differs from the present flow direction that ranges between SSE-SSW (150°-
210°) and is possibly evidence for a strongly sinuous laterally-active channel.
Rapid vertical aggradation in the cross-laminated very fine-fine sand ripple sets with planar to climbing bases is evident between 3.0-5.0 m where climbing ripples and horizontal laminations were deposited by lower and upper regime flows, respectively. The unidirectional SW-oriented (230°) ripple sets may indicate the prominance of overbank deposition which, with changes in flow energy and/or sand supply, probably resulted in the deposition of the lowermost muddy floodplain alluvium (1.0-0.5 m) while the uppermost muds
(<0.5 m) were probably a product of the present regime of laterally stable channels and overbank mud deposition. From the nature of the fine and well sorted size distribution of the uppermost sands (-1.5-1.2 m) it appears that they may have been reworked by aeolian activity and deposited on top of an otherwise fluvial sand body.
Buried aeolian features also appear to exist in SR2 between depths of 0.5 and
~3m, immediately underlying the floodplain mud (Fig. 4.7). Sediment size analysis reveals positively skewed, moderately-sorted to well-sorted fine- medium (1.5-2.2(1); Fig. 4.10) sands in the upper parts of SR2, SR3 and SR3-T
63 % coarser:
99
95
84
75
50
25
16
Shire Road Drill Hole (SR2): Sediment Analysis Mean Sorting Skewness Kurtosis (40 (40 (40 (40 1.5m 1.7869 0.7320 0.3722 2.0321 3m 1.9130 0.8423 0.0318 1.4329 6m 1.6759 0.6670 -0.0011 1.1565 21m 1.8586 0.7393 -0.1271 1.4740 27m 1.7857 0.7061 -0.0272 1.0790
•4> 0 1 Figure 4.10 - Sediment size analysis from Shire Road (SR2) auger hole from 1.5-27 m. 64 which are comparable to floodplain dune samples (see Figs 4.25 and 4.27). Also,
TL dates of 34.5±3.6 ka (W2032) and 29.9±2.6 ka (W2037) from SR2 and SR3, respectively, obtained at depths of 1.5 m, compare closely with aeolian dates
(reported later in this chapter) ranging between 20-45 ka.
Three TL dates from extensive sand deposits in auger hole SR2 of 405+43 ka
(W2034), 512±83 ka (W2035) and 740±55 ka (W2036) at depths of 15 m, 21 m and 27 m, respectively, represent the oldest dates ever obtained from the
Cooper Creek floodplain, and the oldest non-saturated TL dates reported for fluvial deposits anywhere in Australia. An additional date of 454±174 ka
(W2046) at a depth of 15 m was obtained in SR5. While the relatively large errors of these dates reduces their precision, in combination they indicate that floodplain alluviation was occurring somewhere between 360 ka and 800 ka years ago.
TL dates from sand ranging between 160 ka and 200 ka at depths of <7 m in the east half of the transect compared to dates of <80 ka from similar depths in the western half show very clearly that the western part of the floodplain is substantially younger (Fig. 4.7). While there is evidence of upward-fining sequences in auger holes SR2, SR3 and SR4, auger hole SR5 shows surprisingly uniform texture with depth. Particularly noteworthy here is that there is very little clay at depth in all the sedimentary profiles along the Shire Road transect.
Apart from very occasional -1 m units and assorted smaller clay lenses (see
SR2; Fig. 4.7), the profiles are almost entirely dominated by sand. The presence of mud balls and muds intraclasts in the sands at depth indicate that mud drapes and muddy banks and overbank deposits must have been present along 65 these channels. As was interpreted from the uphole data (Figs 4.2-4.5), it appears that the fines deposited on earlier floodplains and in earlier palaeochannels have been reworked by subsequent phases of fluvial activity associated with gradual vertical accretion and relatively rapid lateral migration, leaving little or no evidence of clays at the top of each truncated upward-fining sedimentary sequence.
Shire Road trench site SR5-T, located in the centre of the floodplain (Fig. 4.7 and
4.11), displays certain stratigraphic features characteristic of both eastern and western sides of the floodplain. The upper part consists of a 1.2 m capping of greyish brown (10YR 5/2) floodplain clay. To a depth of 2.4 m, there are assorted sedimentary units (<30 cm thick) of varying texture (Fig. 4.11), ranging from thin clay beds through to coarse sands with small (-5 cm thick) trough- cross beds in coarse sands which have average palaeoflow orientations to the southeast (145°). In addition, some Fe-Mn staining delineates the bedding planes at 2.2 m while a 5 cm calcite layer exists at 2.2 m. Between 2.4 and 3.5 m, a reddish yellow (7.5YR 6/6) coarse to very coarse sand unit with a series of upward-fining trough cross-sets are intermixed with 1 cm thick clay beds.
Palaeoflow directions for this unit varies between south and southwesterly
(175°-230°). Also, some sub-angular gravel (2-3 cm diameter) appears between
3.2-3.5 m. Below 3.5 m, the stratigraphy is primarily composed of yellow (10YR
7/6) medium-coarse sand that TL dates at 197±24 ka (W2044). There are -13 trough-cross bed sets angled at 45°, with an average thickness of -8-10 cm, with the largest being 23 cm. Palaeoflow orientation of this unit is -south (185°).
66 Trench: SR5-T (Middle Shire Road Trench)
Location: Middle floodplain trench 6.9 km from either side of the floodplain edge; ~25 m north of the Shire Rd along a seismic track Site Details: Flat floodplain with mounded gilgai (~50 cm deep cracks) exhibiting 20-30 cm of relief which characterise the vertisols of the floodplain; trench located on a small mud braid-like bar ~30 cm above adjacent flood channels.
[E face] urn —.
0.5m - Grey-brown mud (some FS appearing from >1.3 m)
1.0m-
im v 1.4m V. 1.5m- st Small ~5 cm thick trough-cross beds; Mn outlining bedding units o 1.7m--.V/. FS to CS (plus some thin laminar clay from 2.1-2.2 m); Mn-Fe outlining 2.0m - 1 2.1m.V-; bedding planes; calcrete (very mixed thin units)
ii 2.2m Orange-yellow PC 2.5m- zSltr • in Stg a Various CS-VCS beds with sandy trough cross-beds (~5 cm thick) and clay laminae (~1 cm thick) at 3.0 m; some sub- 3.0m- o angular gravel ( St ~14 sets of trough cross-beds with 4.0m - average thickness -8-10 cm; maximum thickness ~23 cm m TL Date: 4.5m - 197+24 ka (4.5 m) a (W2044) o 5.0nx: 5.0m 5.0m - 1 a. m greater induration 5.5m - and orange colouring St Calcrete rhyzomorphs 6.0m - T V 6.0m M=Mud; S/M=Sandy/Mud; FS=Fine Sand; MS=Medium Sand; MS VCS S/M CS=Coarse Sand; VCS=Very Coarse Sand _L I Sr= Ripple cross-laminated sand; St= Trough cross-bedded M FS cs sand; Sh= Horizontally/Plane-bedded sand Figure 4.11 - Schematic diagram depicting the stratigraphy evident on the E face of the middle Shire Road trench (SR5-T). 67 Modern roots extend to a depth of 5.5 m while -7-10 cm diameter calcrete rhyzomorphs were noted below 2.1 m (Fig. 4.11). The sedimentary profile of SR5-T trench can be segregated into three parts: (i) unconsolidated medium-coarse sand below 3.5 m that dates at -200 ka, more typical of the western floodplain where extensive, non-indurated sands dominate; (ii) mixed assortment of thin sedimentary units between 1.2 to 3.5 m which highlights a more flashy sedimentary history of non-continuous erosional and depositional episodes, with similar stratigraphies also evident at sites to the east of SR5 including auger holes SR6, SR7 and SR8 (Fig. 4.7), SR8-T trench (Fig. 4.9) and Lignum Creek (Fig. 4.18); (iii) grey/brown clay (10YR 5/2) to a depth of 1.2 m deposited by vertically accreting floodplain muds. On the basis of augering adjacent to SR5-T, the sand deposits evident below 3.5 m extend to depths of >35 m, with unconsolidated Quaternary and possibly Tertiary sediments reportedly reaching depths of -50 m along the Shire Road (Santos Pty Ltd., unpubl. report). A single, relatively imprecise TL date of 454+174 ka (W2046) provides a probable age for the sand body at 15 m of 280- 630 ka (taking the possible errors into account). Therefore, these deeper underlying sand units, like those noted earlier in auger hole SR2 (Fig. 4.7) represent middle or possibly, at greater depth, early Quaternary development of the floodplain. 68 4.2.4 The Mt Howitt palaeochannel system Rust and Nanson (1986) and Nanson et al. (1988) reported regular, sinuous, meandering-channel planforms from aerial photographs on the eastern margins of the Cooper floodplain, northwest of the Mt Howitt homestead (Fig. 4.1 and 4.12). The meander forms, display cut-offs, scroll bars and meander loops that are visible on airphotos but are totally invisible to an observer on the ground (Fig. 4.12). These features are atypical of the generally smaller, mud-dominated, irregular anastomosing-channel systems of the contemporary low-gradient Cooper floodplain. They used these observations, along with their sedimentary investigation and TL chronology obtained from near Naccowlah Waterhole, to assert that a change from a meandering sand-dominated to anabranching mud- dominated fluvial regime on Cooper Creek took place between 50 ka and 200 ka and probably reflected increasing aridity. However, they undertook no detailed research at the Mt Howitt palaeomeander site itself. The excavation of three 6-7 m deep trenches at the Mt Howitt site in 1996 confirmed the existence of palaeochannels beneath the contemporary floodplain (Fig. 4.12 and 4.13). In particular, a 60 m long trench, oriented roughly perpendicular to the apparent channel orientation and through the apex of a meander loop, revealed stratigraphic evidence of lateral accretionary surfaces, upward-fining sand sequences, a well-defined cut-bank (Fig. 4.13) and sedimentary structures indicating palaeocurrent directions. The importance of the site is that it provides a TL chronology and the 3-dimensional geometry of a substantial palaeochannel and provides valuable insights into the meandering 69 serein in IUCIGQ LU t I 71 channel conditions that prevailed at times of higher and more regular discharges than at present. The 60 m long and 6-7 m deep excavation (MH2) revealed a steep-sided erosional palaeo cut-bank to the east of the transect while lateral accretionary point-bar surfaces, clay drapes and overbank muds were evident to the west (Fig 4.13). The cut bank displays upward-fining sedimentary units varying between lenses of fine gravel (at depths of 3.5 and 4.3 m) and coarse sand (below -4.5 m) with abundant flow structures, continuing to fine upwards to medium and then fine-medium sand with negligible primary sedimentary structures. Overlying the cut bank surface between 0 m and 15 m from the eastern side of the transect is 2-2.5 m of the floodplain clays with the steeply dipping erosional boundary (31° slope) differentiating the cut bank from the clay infill of the channel to a depth of 5.5 m (Fig. 4.13). At 5-7 m depth between 25-35 m along the transect, an -8 m wide longitudinal in-channel bar is present near the bed of the infilled channel, and is composed of mottled trough-cross- bedded (-20 cm) very coarse sand at the base of the bar, through to smaller trough-cross beds (-5-10 cm) in medium sand at the top (Fig. 4.13). From 35-60 m, numerous accretionary surfaces are evident. For instance, 5-10 cm thick light grey clay lenses dipping upwards at -12° to the west are apparent at 40-45 m, becoming thinner and more numerous towards the end of the excavation. At their thinnest these lenses taper to clay drapes over sandy sedimentary units beneath. 72 At depths >5 m, there are large scale (-10-20 cm) 'festoon' trough cross-bedding dipping westward. Palaeoflow orientations determined from 96 trough-cross bed measurements reveal a southwest (mean=211°) orientation. Similar sedimentary features were observed at the Chookoo channel excavation (CHW-TB; Fig. 4.24) some 85 km to the southwest (Fig. 4.1) where lateral accretionary surfaces and trough-cross-bedded channel sands dominated the stratigraphy beneath a large scour channel (Fig. 4.21). Overall, the excavation and surface planforms with scrolls at Mt Howitt (MH2) provide strong evidence for meandering channels exhibiting lateral migration. Below a depth of -5 m, the sedimentary units are those deposited by in-channel sandy, bedload-transporting meandering processes, while at depths of 3-5 m the stratigraphy represents overbank mud infilling of a palaeochannel. An estimated bankfull width of 60-70 m for the former meandering channel corresponds closely to the width observed from aerial photos and aerial observations of about 60 m (Fig. 4.12). Hence, there is good correspondence between the planform appearance of the channels and the sub-surface stratigraphy. It is readily apparent from the Mt Howitt site that there has been a dramatic change in water and sediment flux during the late Quaternary resulting in the abandonment of large, laterally active meandering sand-load channels during Stage 3. These channels were migrating into an extensive sand body formed substantially earlier during late Stage 5. Following abandonment of the Stage 3 73 phase of large-channel migration at about 63 ka, it appears that a channel at this site continued to exist, probably as a muddy channel until about 15 ka, following which it completely infilled with mud. These buried channel features are undoubtably more widespread than just at the Mt Howitt site, however, they have been obscured by an abundance of anastomosing channels and vegetation which characterise the surface of much of the remainder of the floodplain. Possible contraction of fluvial activity on the eastern side of the floodplain (discussed in detail in Chapter 5) has probably allowed the preservation of very subtle surface expressions of relatively deep fluvial structures near Mt Howitt on the eastern side of the floodplain. Using an estimated channel width of 60 m, a bankfull average depth of 5-6 m and a Manning's n of 0.025, the estimated palaeodischarge for the Mt Howitt channel is about 80 m3/s. This is much less than the estimated bankfull discharge of the present flow of Cooper Creek through Meringhina Waterhole (Knighton and Nanson, in press, and see Chapter 5). This suggests that either this Stage 4/3 palaeochannel was just one of a series of anabranches that made up the total flow of the Cooper at that fluvially more active time, or as is more likely, that it represents a tributary to the Cooper feeding water from the valley side north and east of Mt Howitt. Reasons for the faint surface expression of these buried channels, invisible on the ground, are not clear. Wootton (1996) examined remotely sensed data to further establish the link between the sub-surface and the meander planform images evident at the surface. It is hypothesised here that the expandable clays 74 that dominate the surficial floodplain clay mineralogy may differ from the initial clay infill of the meandering channels. The shrink-swell properties of the self-mulching clays near the surface may help to churn the floodplain clays at depth. This mixing could enable clays with different moisture retentive characteristics to be brought up to the surface resulting in the distinctive channel outline of the sub-surface channel evident in plan view. However, this has yet to be proven. 4.2.5 Goonbabinna Waterhole Located <1 km northwest of the main dune at Chookoo (Figs 4.1 and 4.14), the Goonbabinna Waterhole forms part of the interconnected network of waterholes and anastomosing channels in the western channel belt of Cooper Creek. Goonbabinna Waterhole has a maximum bankfull depth of 5 m and a width of 34 m with only slight levee development (Fig. 4.14). Lignum (M. cunninghamii) and coolibahs (E. microtheca) line the banks. Trenching and augering of the convex bank of a bend in the Goonbabinna Waterhole revealed four sedimentary units (Fig. 4.14). The cracking clay-rich grey soil (Unit 1) is underlain at about 6-7 m by a sheet of structureless clay (Unit 2), with two sandy layers that dip at 12° towards the active channel and delineate former accretion surfaces. Excavation was extremely difficult due to the very tough nature of the clay which was sufficiently impermeable that no water seeped into Trench 1 which extended -3 m below the water level at the edge of the waterhole, just a metre away (Fig. 4.14). The effectiveness of this 75 1 N E.microtheca Drill Hole (Coolibah) DH2 Drill Hole DH3 X2 Not to scale ® <-N X1 X2 Self-Mulching Layer Main Cooper Channel slope 30° TT7 Accretion •^kj/^'^T: ';•..;{ ;.\J. UNITJ_ Surface (12°)' JillV Trench 2 '.-'•.'•'.•v'/vVv'/v'.-v MUD /[.;. Trench 1 'vv.'/.V.- / / UNIT 'i 14C: 1420+60BP ' \\\\- (Beta - 84639) TL:5.3+0.4ka (W2053) Schematic Cross-section (X1-X2) through Goonbabinna Waterhole Figure 4.14-Goonbabinna Waterhole: Location of the Goonbabinna Waterhole on the western side of the Cooper Creek floodplain downstream of the Shire Road is shown in (a). The schematic diagram (b) shows the location of the two trenches and two drill holes while (c) presents a schematic cross-section through the trenches and drill holes. 76 narrow clay 'wall' at excluding water from the pit underlines just how effective the alluvial muds are in lining the floodplain and channel surfaces and preventing water from entering the dry sand aquifer beneath. On the muddy floodplain surface which dries and cracks to depths of up to 1 m, water can percolate down the cracks. However, below 1 m, the compacted clays are highly impermeable. In the waterholes, which remain wet all year, water is unable to enter more than a few centimetres into the boundary clay that forms a highly effective seal. Unit 2 clays were traced for over 100 m perpendicular to the modern channel, where they thin to <5 m. Adjacent to the active channel, this unit is underlain by pink (5YR 7/3) fine sand (Unit 3) that contained an aboriginal hearthsite with charcoal, partially baked clay fragments and a stone implement. Unit 4 is a coarse pebbly very pale brown (10YR 7/4) sand, that was encountered in drill holes away from the active channel (Fig. 4.14). The lateral extent of Unit 3 sands is uncertain. They may, however, be present in the auger holes (DH2 and DH3; Fig. 4.14) but, if thin, would be difficult to distinguish from Unit 4 sands. Charcoal from the hearthsite in Trench 1 (Sample Beta-84639) yielded an AMS radiocarbon date of 1420±60 BP (conventional 14C age) (or A.D. 590-775 (2 sigma range at 95% probability level) using Vogel et ah, (1993) calibration method) while a TL age for the baked clay unit of the same hearthsite dated at 5.3±0.4 ka (W2053). 77 Units 1-3 are interpreted as deposits laid down by the slow migration northwards of the anastomosing channel. By analogy with modern channel elements, Unit 3 is interpreted as a sandy accretionary bench or bar deposit exposed near the riverbed and available for use by aboriginal people. Unit 2 is interpreted as muddy accretionary bank deposits laid down by the northward migration of the convex bank which generated accretionary layers that covered the sandy bench sediments of Unit 3. These accretionary deposits may extend to the limits of investigation (DH3) or may rest on older muddy deposits with an unexposed contact. Unit 1 is the modern palaeosol developed on the floodplain surface. Unit 4 is interpreted as deposits of underlying sand sheet based on its coarse, pebbly nature and lithological similarity with sub-surface sediments exposed in nearby pits along the Chookoo transect and elsewhere on the floodplain. 4.2.6 Eastern and Southern Floodplain Margins Towards the eastern and southern margins of the Cooper floodplain in the study area the floodplain stratigraphy laps against low-gradient fans in the northeast and over aeolian dunes and shallow bedrock at the confluence of the Wilson River and west of the confluence near the Tooley Wooley Waterholes (Fig. 4.15). A selection of uphole sites at Wareena (Fig. 4.3), Baryulah (Fig. 4.16) and Bolan (Fig. 4.17) enable a broad assessment of the stratigraphies of these floodplain marginal sites, while excavations at Lignum Creek near Naccowlah Waterhole, and those already described at the eastern ends of the Durham 78 Downs Road transect and the Shire Road transect provide detailed examples of certain aspects of the stratigraphy. Both the Bolan upholes and Baryulah upholes are located in the southern-most extremity of the study area (Fig. 4.1) and for three reasons represent an interesting departure from sites elsewhere on the floodplain: (i) This location overlies the junction of two significant structural controls which have a major influence on the orientation of the regional fluvial landscape; the north-south oriented Cooper Syncline and the east-west trending Wilson Depression (Fig. 2.5) where the floodplain is in the order of 40 kms wide. From this junction flow direction is directed westward and follows the Wilson Depression towards Nappa Merrie (Fig. 2.5). The Wilson River is the largest tributary (catchment area of 25,000 km.2) entering Cooper Creek between Windorah (Fig. 4.1) and South Australia, a distance of some -400 km, and thus at times the Wilson River represents a significant contributor of flow and sediment to the southern extremity of the study area. (ii) From maps and aerial photographs, the combined Cooper and Wilson floodplains in the Baryulah area appear to be invading the southern-most regional dunefield over a shallow bedrock surface along this structurally defined margin of the Cooper valley. 79 Anastomosing channels >odout channels Clay pans Tooley Wooley Waterhole Figure 4.15a - Oblique photograph showing an 'island' of truncated linear dunes and clay pans east of the Tooley Wooley Waterhole. The 'island' is slowly being / inundated by vertically accreting, mud-laden overbank Flow direction floodwaters from Cooper Creek. Flow is from the top of page. View is to the north. Figure 4.15b - Oblique photograph showing both the Tooley Wooley and Little Tooley Waterholes at the confluence of Cooper Creek and the Wilson River. The 'islands' of sand dunes on either side of the waterholes are more susceptible to erosion than the muddy floodplain resulting in narrow, straight wide channels. View is to the southeast. 80 LOCALITY MAP INSET Figure 4.16 Durham Downs A SEDIMENT AND GAMMA LOG PLOTS North o to Baryulah Seismic Lines: km 0° Bolan 95-FWX, FWY, FWZ, and FXA* 0 10 20 30 40 'Location and orientation of these are shown below V FWX FWY BaryulahBaryulah "j. Tooley Wooley/ Waterhole ^h ^^^ g w jP^f' *ti. Utile Tooley T Wooley Waterhole Nappa Merrie Jackson Oil Terminal KEY Symbol item see insat *•' Noccundra 1 1 Floodplain 1 1 Sand dunes Wilson River a m Roads -n\H5 Seismic lines V/ FWX4K) FWX 540 FWX 660 FWX 722 FWX 1266 nii-C ujAfC UiUC ttoWD Ullrt (UL » i •-L-J yy =i SEND 0 100 Bour Gamma Log Scale i^B MudlMl cps (counts per second] HORIZONTAL SCALE StlfB 1 1 Sand IS) Bour 1 units M S WB Sedimentary Scale 1:30000 1 I Weathered Bedrock (WB) andi FWY 590 MJA* M S WB • 81 tjj Oj OJ £3* £ •8 8. I 8 13 gf c S Ja 2 3 i a. a. 3 D El ii? / 3£ Vfi 00 ° 2 II u > 2 c ^ o1 ) Z -«— 82 (iii) Finally, unlike the relatively isolated floodplain dune sites at Chookoo and Durham Downs, there is a prevalence of sand dunes, clay pans and floodplain clays at this location, and a dearth of fluvial sands (Fig. 4.15). 4.2.6.1 Baryulah This site is farthest from the Wilson River and is located located some 3-10 km from the present southern margin of the Cooper valley. Nineteen upholes were gamma-logged along 4 seismic lines in the area north and west of Little Tooley Wooley and Tooley Wooley Waterholes (Figs 4.1 and 4.15). These data (Fig. 4.16) reveal 8-24 m of fluvial and aeolian sediments composed of ~4 m of uppermost floodplain clay and -9-12 m of fine-medium sand overlying a heavily oxidised weathered bedrock, probably the Tertiary-aged Glendower Formation (Fig. 2.5). The Glendower Formation is a fluviatile sequence of gravels and quartzose arenaceous sediments with some minor argillaceous interbeds. Large areas of the Glendower Formation are mantled with Quaternary sediments in alluviated areas such as the Cooper Syncline (Senior et ah, 1968). The stratigraphy of the Baryulah and the Tooley Wooley dune complex indicates that aeolian deposition by regional dunes occurred prior to fluvial deposition (Fig. 4.16). Aerial inspection of the area illustrates the invasion of the floodplain onto the valley sides and into the associated regional dunes (Fig 4.15). The dunes found adjacent to the Tooley Wooley Waterhole and Little 83 Tooley Wooley Waterholes are relatively old. While they show evidence of recent reworking towards the surface, a TL date of 81.7+5.5 ka (W1888) obtained at the indurated base of a dune about 1 m above the present floodplain and adjacent to the Tooley Wooley Waterhole makes this point. The dune is presumably older than this at its base. 4.2.6.2 Bolan 3D Seismic Grid (Wilson Swamp) This site is located in Wilson Swamp, an area near the present interface between the Wilson River and Cooper floodplains (Fig. 4.1). The surface morphology of Wilson Swamp is markedly different from the general Cooper floodplain in that there is far more aeolian activity evident in the form of truncated linear dunes with clay pans and pebbly surfaces of lateritic gravels between dune swales. In addition, the Baryulah and Wilson Swamp sand dunes are much redder in colour than the pale orange-yellow dunes of the Cooper floodplain and probably represent part of the regional dunefield which is much the same colour. Sediment logs from 8 upholes scattered over an area of 27 km^ throughout the 3-D shallow seismic grid area are presented in Fig. 4.17. There are marked variations here in the sub-surface stratigraphy compared to the general Cooper floodplain (Fig. 4.17). For instance, at Bolan large amounts of gypsum dominate the profile of each uphole. The total depths of alluvial and aeolian deposits vary 84 between only 3-12 m over heavily oxidised weathered bedrock, probably weathered Tertiary Glendower Formation. However, some unusual stratigraphic features were also encountered. For instance, two upholes (R08S09, R12S20) (Fig. 4.17) contained evidence of 9 m thick clay units which were interpreted as infills of former channels or waterholes, possibly representing an earlier course of the Wilson River cut into the weathered basement. From stratigraphic evidence, the area was probably subjected to both aeolian and fluvial processes with the floodplain invading the valley side and associated regional dunefield, thereby displaying similar stratigraphic features to the Baryulah site (Fig. 4.16). The significance of both these sites in relation to the alluvial and aeolian accretionary record of the Cooper floodplain since the middle Quaternary is discussed in detail in Chapter 5. 4.2.6.3 Lignum Creek The stratigraphy of alluvial sequences distal to the main Cooper Creek flow system is not interpretable in detail by means of uphole description. Several excavations in the Lignum Creek portion of the Cooper floodplain, however, do provide such detail for a relatively old portion of floodplain subjected to slow accretion and relatively thin accretionary beds and reactivation surfaces that are the product of ephemeral floods from the valley side and from the main Cooper system. 85 The floodplain stratigraphy in the bed of Lignum Creek, the dominant anastomosing channel feeding into the Naccowlah Waterhole from the north, and excavated adjacent to the Shire Road transect (Fig. 4.7) near auger site SR7, (Fig. 4.1) displays a complex assemblage of sedimentary units. Evidence from three 3.0-3.5 m deep excavations spaced -100 m apart in the dry bed of the inset anastomosing channel, -2.5 m below the floodplain surface, revealed an assortment of sedimentary features (Fig. 4.18). Some of these include thin (-2-5 cm thick) calcitic layers with Fe-Mn nodules, probably indicative of former water-table levels (Cooke et ah, 1993), and horizontally- bedded strata with erosional contacts between pebbly coarse sand and near- surface clays. Despite the relatively close spacing between the three trenches, they display rather different stratigraphic features (Fig. 4.18). For instance, in Pit No. 1 an erosional contact at a depth of 1.6 m separates the overlying grey clay from a pebbly coarse sand lens which is Fe-stained and contains clay infilled root casts and occasional Fe-Mn nodules. Also evident at 1.6 m is a flow disturbance around a log at the interface between clays and sand. The log has triggered the deposition of a sediment lobe (-20 cm high and 40 cm long) with parallel bedding planes composed of interbedded mud pellets and fine-medium sand oriented in a southerly direction, similar to Lignum Creek. Beneath the erosional contact, there is an assortment of fine sand units of varying thickness with trough cross-sets and thin calcitic units. 86 •a OJ O) 3 CO -t-» O 01 73 01 3 C a> § r/1 OJ C 3 to 01 M T3 LH OL, -a OJ cj c (/) ra tA S OJ 01 o OJ Ct, OaJ T3 ra (/l O T3 3 u C c s fc . C CO U I" ra /—"s (3 0) ra U ra 2 rH 1 to C -3 < s u O >-, U 60 Tf OJ •J3 o E U > CS OJ s rC 53 CO D E 01 • Sr-cH CO OJ UH 0 o «d •M 0) rG <+* a CD rH •rCH v / <+H o •rH co r—H 0) •CcD o CO cd <+H roH s <1> o O ifcr O 9/ OJ •fl bO <+H •J3 cd o fc -8 '<-i ro CO OJ rH •abO C £10 CO '5 OJ OJ roG rG CO +-> C C o o OJ ToO •43 +-a> OJ 6ca CO OJ '*4 1=1 -CsO T 3 T3 C •H CS 8 PH o CC rv r* s OJ r* OJ OJ U (0H) u S S b=3C 3 3 & CO a• rH. 00 CO 1-1 •M t •rPHH 4> au 87 The profile of Pit No. 2 is dominated by a sandy clay unit for -1 m below the surface which is underlain by numerous thin beds composed of fine-very coarse sand, pebbly sands, calcite and calcrete nodules (Fig. 4.18). Below -2.5 m, the excavation is dominated by fine sand which also displays evidence of large trough sets. Pit No. 3 is quite distinct from the other two pits in that apart from a -20 cm clay interbed at a depth of 0.5 m, there is only a thin 0.2 m veneer of cracking clays at the top of the profile. Below this clay veneer is an extensive and less variable stratigraphy where medium sands dominate. Some coarse sand interbeds and 0.2 m of calcite are also evident. Calcrete nodules are more prolific in this profile (Fig. 4.18). Trenching by Rust and Nanson (1986) to depths of -7 m about 3 km to the south near the Naccowlah Waterhole, presented evidence of upward-fining sequences and laterally accreting surfaces. The shallower excavations at Lignum Creek were not so clear. Most of the interfaces at Lignum Creek appear to represent reactivation surfaces with erosional contacts implying considerable erosion, reworking and subsequent deposition (Fig. 4.18), perhaps in keeping with the site having been reworked by precursors to the channel that exists there today. In addition, the SR5-T trench (Fig. 4.11) displays a highly variable stratigraphy between depths of 1.2-3.5 m which is similar in depth to that exposed in the three pits in Lignum Creek. 88 The depositional features identified in Lignum Creek and from the auger holes east of SR5 on the Shire Road suggest that this stratigraphy is a product of both flash floods that bring coarse sand and pebbles from the valley side slopes and relatively steep tributaries, and the extensive floods from the Cooper that cover the floodplain with up to 2 m of water and lay down muddy fines. The reactivation surfaces are the result of the marginal flash floods, for such features are not evident nearer the main channels of Cooper Creek. 4.3 Group 2: Aeolian-fluvial interactions The Cooper floodplain with its sporadic distribution of sand dunes in a mud- dominated fluvial environment provides an opportunity to evaluate the separate processes and chronologies as well as the possible interactions between aeolian and fluvial depositional environments. Two dune sites were investigated in detail, these being the Chookoo Sandhill complex and the Durham homestead dune (Fig. 4.1), with the River Road dunes and the dunes near South Narberry and the Tooley Wooley Waterholes providing some additional data. 4.3.1 Chookoo Sandhill Complex The Chookoo Sandhill complex is located on the western side of the floodplain (141°54/30,,E, 27°34'45"S), 6 kms south of the Shire Road and 1.5 kms east of the western edge of the floodplain (Fig. 4.1). Encompassing an area of -4 km.2, it is composed of three sand dunes (Fig. 4.19). The 11 m high main Chookoo dune 89 Small Chookoo Chookoo dune 3 channel Chookoo dune 2 Large Chookoo channel 1 km W to main channel (Goonbabinna Waterhole) (b) s Chookoo Transect * ^ Dune crest Drill hole locations 9 Trench locations Figure 4.19 - Oblique aerial photograph (a) and schematic map (b) of the Chookoo Sandhill complex comprised of Chookoo dunes 1, 2 and 3, the main Chookoo Channel between dunes 1 and 2 and a small channel between dunes 2 and 3. View is to the southeast. 90 (Dune 1) with its steep lee face is a prominent barchanoid sand feature with its dune front (-1 km in length) aligned in a northwest-southeast orientation. The mobile dune crest is sparsely vegetated with Acacia spp., becoming increasingly more densely vegetated towards the dune margins. Two smaller Chookoo dunes (Dunes 2 and 3) are located -1 km and -1.5 km to the southwest of the main Chookoo dune (Dune 1; Fig. 4.19). They are lower in elevation (maximum of -6 m) with no clearly defined crest orientation and have gentle well-vegetated slopes and margins supporting A. murrayana and A. cambagei as well as Crotalaria eremaea and Muelhenbeckia cunninghamii. When completely or partially-full of water, the large Chookoo channel between Dunes 1 and 2, with well-established coolibah trees (E. microtheca) lining its banks, bears a striking similarity to waterholes elsewhere on the floodplain (Fig. 4.20a). However, when dry (Fig. 4.20b), only a shallow depression with a large width-to-depth ratio is revealed. On the basis of planform and the presence of coolibahs on the banks of the channel, it was believed that the channel represented a formerly active waterhole which has since become inactive and infilled (see its appearance from the air in Figure 4.19). It was, therefore, seen as a potentially valuable site for providing chronological evidence of channel change in this semi-arid floodplain setting. To investigate this possibility and the relationship between the channel and its flanking dunes, intensive subsurface investigations were conducted from 16 auger holes and three trenches along a 1.4 km transect (the Chookoo transect) across Dunes 1 and 2, the shallow large Chookoo channel and the adjacent muddy floodplain (Figs 91 Figure 4.20 - Ground level photographs of the main Chookoo channel looking downstream (southwest). Large E. microtheca line the banks of the ephemeral channel. (a) when infilled with water from local runoff or large overland flood, the channel resembles a more perennial channel or waterhole. However, maximum depth of floodwaters in this photo is 0.25m. (Photo taken: March 17, 1993) (b) shows a heavily cracked, shallow, saucer-shaped depression evident during dry conditions. (Photo taken: Jun 18, 1994). (b) was photographed 50m further downstream (southwest) of (a). 92 4.19 and 4.21). This transect provided detailed stratigraphic and chronological data which are summarised in Fig. 4.21. The stratigraphy evident in trench CH2-T (Fig 4.22), ~200m N of Dune 1 (Figs 4.19 and 4.21) reveals 1.2 m of near surface greyish-brown floodplain clays (10YR 5/2) progressively grading into a well-compacted grey-green clay with no obvious bedding structures to depths of 2.3 m then into a unit of yellow (10YR 7/6) very fine to fine sand with small climbing ripple sets (1-2 cm amplitude) with -10 cm thick Fe-Mn-stained, horizontally-bedded units up to a depth of 3.3 m. Some root traces are common above 3.3 m. Below this, the stratigraphy is characterised by light yellowish brown (10 YR 6/4) very fine to fine sand with numerous small climbing ripple sets (1-2 cm amplitude) with no evidence of roots. In addition, there are several Fe-Mn-coated horizontal laminations (-10 cm thick) separating the ripple sets at 5.4, 6.0 and 6.4 m. The variable angle of climb of the ripples indicates rapidly aggrading fluvial units between these Fe-Mn laminations. Average palaeoflow orientations below -3.3 m were 220° (SSW-SW), 40° W of the mean downvalley direction, with one 20 cm thick trough set at 5.1 m having a palaeoflow direction of 350° (NNW-N) -10° W of the mean up-valley direction. Sediment exposed in the CH2-T trench (Fig. 4.22) is believed to have been deposited entirely by fluvial processes and the fine to very fine texture of the sand suggests that flows may have been ponded against the dune forming downstream to the south. The variability in palaeoflow orientations and evidence of steeply climbing ripples below 3.3 m would suggest that sediment 93 X, o a o a LO c u bJO 73 TJ1 +-J O s s o -M co M co fl 0 -4=1 £* 0 a X £ ^ s rj cu =3 ^ rH CO CO CD rH CM .. CU -^ o cu H3 tUO IN t W m WJ.g OJ •5 r^ fj roH u 1/3 rH ir> 0) ra aj M SJj s 60 CO cu RJ £^CN S <*> ra c x .r7 h Xoj g o < ufcH =3 _N OJ CJ 'u > cu c o rH X cu H u 4H H cu ai C o c "c3 0) "H. O !H OJ o 13 o • « M-i O tn QJ o 4/ O o E o o U pa o r*i uo s* o T3 X, £ u 03 0) o tn s Ol QJ 03 Ol 01 en <=.> '— UL. r > =) 03 PH ^ LT, rr ^O> u- Si, "J ro E 01 U -a bO-'H QJ 44 rcj IH 03 > n3 C a j Xm en ,3 H-* 03 C/j co A HH C 03 OJ n3 ro "> T3r3 oX c 3 PH r-H E S c LO H OJ Cfi 03 >: Tsk.( —2' CU bo 01 03 ',ni (H • i—i San t Ae o C/j < WH I w 94 Trench: CH2-T (Chookoo Floodplain Trench) Location: ~200 m north of main Chookoo Sandhill; ~1 km southeast of Goonbabinna Waterhole; adjacent to drill hole CH2; forms part of the Chookoo transect. Inset CH2-T Site Details: Flat planar floodplain surface with minor gilgai (-20 cm N relief); only sparsely vegetated. / RL [~E face] (80O) Om" J_L-I L Flow Direction 62 m wide 260O "J -0.5m - Trench! Grey/brown mud ^v = Chookoo Transect Large Chookoo ChanneTl (CHW) -1.0m- Not to scale -1.5m - Grey/brown mud as above with grey/green clay (very tough) -2.0m- -2.5m- FS with dark Fe-Mn staining and small ripple sets -1-2 cm amplitude; Roots common in this unit (palaeosol??) -3.0m- •3.3m-V -3.5m — -4.0m- FS with Fe-Mn coating on bedding surface of basal contact planar units between 5.4 and 6.4 m (7 cm thick at ~5.4 m and -5 cm thick at -5.9 m (see below)) -4.5m - Palaeoflow direction in one 20 cm thick trough set unit at 5.1 m of 350° (~N) £ Sr iSSii / -5.0m' / 5.4m.-". -5.5m — -6.0m - -6.4m.'* -6.5m — Maximum Depth=6.4 m S/M MS vcs M=Mud;S/M=Sandy/Mud;FS=Fine Sand; MS=Medium Sand; . , , CS=Coarse Sand; VCS=Very Coarse Sand Sr= Ripple cross-laminated sand; St= Trough cross-bedded M FS CS sand; Sh= Horizontally/Plane-bedded sand Figure 4.22 - Schematic diagram depicting the stratigraphy evident on the E face of the Chookoo floodplain trench (CH2-T) north of Chookoo dime 1 (refer to inset). 95 deposition occurred rapidly from in-channel processes such as bar development. The Fe-Mn-stained horizontal laminations have probably resulted from shallow flow conditions. Above -3.3 m, deposits have resulted primarily by upward fining overbank processes which grade to dense clay at -1.2 m. Iron nodules, calcretes, thin clay lenses and a mottled sandy-clay unit and clay lenses were noted in auger hole CH2 (adjacent to trench CH2-T) at the interface between fine sand and the coarser sand unit at -9-11 m below the floodplain (Fig. 4.21). This sedimentary unit probably represents a -1-2 m thick palaeosol separating the finer sands above from the coarser sands beneath (Fig. 4.21). Detailed augering through Dune 1 (Auger hole CH-A; Fig. 4.21) shows that red (7.5 YR 6/6) fine sands grading to white (10YR 8/2) and buff (10YR 7/3) sands at depth dominate the profile, with mottled sandy-clay units appearing only at depths of 16-19 m (i.e., -9 m below the present floodplain surface) and medium-coarse sand evident only below 19 m (Fig. 4.21). The lack of floodplain or channel-infill clay underlying Dune 1 has important implications for sequencing channel, dune and floodplain development at this site during the Quaternary. It is apparent that the aeolian dune has formed directly on channel sands, probably a within-channel bar, with no overbank mud between these sands and the dune above. In order to examine the stratigraphy beneath the large Chookoo channel, a 6.2 m deep trench CHW-TA was excavated starting at the northern bank of the channel (Figs 4.21 and 4.23). The stratigraphy of the eastern and western faces 96 Trench: Chookoo Channel Trench (CHW-T) (Trench A) Location: ~300 m south of the main Chookoo Sandhill within the Large Chookoo Channel; trench forms part of Chookoo transect Site Details: Smaller of the two Chookoo Channel trenches (~20 m long) on north side of the Large Chookoo Channel 62 m wide [NW face] (350°) Reworked aeolian FS unit [SW face] (260°) Grey/brown mud FS with traces in -2.5m- massive sediments -3.0m- Photo of NW face of the Large Chookoo Channel (Trench A) showing surficial aeolian FS unit overlying fluvial muds and sands. SW face (described here) is to the left. MS unit dominated by trough cross-beds; 10-25 cm sets with curved bases and downvalley cross-stratified dips M=Mud; S/M=Sandy/Mud; FS=Fine Sand; MS=Medium Sand; CS=Coarse Sand; VCS=Very Coarse Sand Sr= Ripple cross-laminated sand; St= Trough cross bedded sand; Sh= Horizontally/Plane-bedded sand Maximum Depth=6.2 m S/M MS VCS Figure 4.23 - Schematic diagram showing the I— _L_ r stratigraphy on the NW face (see inset) and SW face FS cs M of the Large Chookoo channel trench (CHW-TA) north of Chookoo dune 1. 97 of this trench revealed only 0.8 m of floodplain mud overlying 5.4 m of alluvial sand. Typically, the near surface mud unit is soft and puffy for the upper 0.3 m, becoming compacted and tough with depth. Weak planar very fine sandy stratification and large modern roots are evident at the basal contact. At depths between 0.8-3.8 m, the structures are cryptic, however, some traces of ripples and horizontal laminations in the fine to very-fine sand unit are evident. Strata are gradational upwards from trough cross-beds through to ripple sets. Carbonaceous vertical tubes (some >1 m long) were scattered below -2.8 m with very little trace of Fe-Mn staining except around quasi-modern roots. Between 3.8-6.2 m, sand texture varies between fine-medium to medium sand with coarse and fine sand layers. Trough cross-bed sets of 10-25 cm with curved bases (Fig. 4.23) and down valley cross-stratified dips dominate the remainder of the profile. The northern bank of the large Chookoo channel (in CHW-TA) has a near surface stratigraphy different to that of the east and west faces. At the surface, a 0.5 m light red (2.5YR 6/6), well-sorted, medium sand wedge rests above a 0.7 m weak red (2.5YR 4/2) friable fine sand, both of which dip down-face (Fig. 4.23). Underlying these sand units is 1.3 m of solid clay which corresponds to the 0.8 m mud layer that forms the base of the present channel and is visible on the western and southern faces of the trench. Below 2.5 m, the stratigraphy reverts to the same sandy textures and bedding structures described in trench CHW-TA • The light red (2.5YR 6/6) sands on the bank of the large Chookoo channel probably represent a wedge of fluvially reworked aeolian dune sand associated with Dune 1. In general, both Chookoo channel trenches revealed 98 similar stratigraphic evidence although CHW-TB provided clear evidence of lateral accretionary surfaces below about 3.0 m depth (Fig. 4.24). In trench CHW-TB, on the southern side of the large Chookoo Channel, very heavily cracked light greyish brown (10YR 6/2) clays exist from the surface to a depth of -0.2 m progressively grading into a thick homogeneous greyish-brown (10YR 5/2) clay layer to -1.2 m and then into a light yellowish brown (10YR 6/4) very fine to fine sand unit containing some small-scaled (2-5 cm) diffuse, fine-sandy trough cross-beds to a depth of 3.0 m. Below 3.0 m, medium to coarse brown (10YR 5/3) sand with 10 cm trough cross-beds dominated the remainder of the profile. Several light grey (7.5YR 7/0) clay bands of varying thickness (-15 cm) at a depth of 3.5 and 5.0 m possessing abrupt contact surfaces separate packages of sandy trough cross-bed sets (Fig 4.24). These clay bands in CHW-TB have bedding surfaces of 120° that dip 12° into the surface of the SW face of the trench (Fig. 4.24). Sporadic pebbly units (clasts of 1 cm diameter) occur at the base of the trough cross-beds at trench depths of -6.0 m. The orientation of the palaeocurrent indicators along the strike of the dipping sand body below 3.0 m provides evidence for its formation by point bar accretion within a laterally active fluvial system, whereas the muddy unit above 3.0 m is interpreted as the infill of an abandoned meander (Fig. 4.24). Similar evidence of former lateral accretionary surfaces associated with meandering channel processes was presented by Rust and Nanson (1986) at a trench site on the Cooper floodplain near the Naccowlah Waterhole (Fig 4.1). Examination of the stratigraphy along the Chookoo transect reveals that there are in fact two 99 Trench: Chookoo Channel Trench (CHW-TB) (Trench B) Location: ~400 m south of the main Chookoo Sandhill within Large Inset| N Chookoo Channel; trench forms part of the Chookoo transect 200 m from dune fringe Site Details: Large trench (~30 m long) on south side of Chookoo Channel / Trench A . J_J L-Q-1 uJ L [~SW face] (280°) Flow Direction XI ^ 62 m ' Trench B wide '-—i rr-r l >v = Chookoo Transect LargTe Chooko o Channel (CHW) Grey/brown sandy-mud Not to scale Gradational change -1.5m - FS; tends to be a mixture of grey clay, yellowish VFS/silt with thin VFS-FS lenses especially in bottom 1 / 3 of unit- faint bedding structures in massive -2.0m - sediments Change to diffuse 2-5 cm trough cross-beds; broadly fining- upward with lots of Fe-Mn staining; Fe-Mn stained vertical roots evident from 2.0-3.0 m (no roots evident below 3.0 m) -3.0nvV-.-V-A' MS-»FS; trough cross-beds -10 cm . Grey-silty clay; abrupt base and top (dip 12°, strike 120°) MS-»FS; trough cross-beds -10 cm Grey-silty clay; abrupt base and top (dip 12°, strike 120°) CS; trough cross-beds -10 cm tf . Grey-silty clay; abrupt base and top (dip 12°, strike 120°) MS-CS; pebbles ( M=Mud; S/M=Sandy/Mud; FS=Fine Sand; MS=Medium Sand; S/M VCS MS CS=Coarse Sand; VCS=Very Coarse Sand _| I Sr= Ripple cross-laminated sand; St= Trough cross-bedded M FS CS sand; Sh= Horizontally/Plane-bedded sand Figure 4.24 - Schematic diagram showing the stratigraphy on the southwest face (see photograph above), south of Large Chookoo channel trench A (CHW-TA) (refer to inset). 100 ~6 m deep palaeochannels infilled with massive clays and located some 300 m apart north and south of the large Chookoo channel (Fig. 4.21). Interestingly, the present position of the channel does not entirely overlie either of these, but is positioned between them. The stratigraphy, sedimentology and chronology of the extensive sand sheet into which the two palaeochannel fills are inset, and that underlies Dune 1, is shown clearly in Figure 4.21 to be a Stage 5 unit with 6 TL ages ranging from 103±10 ka to 81.7±7.4 ka. On the basis of colour and grain-size analyses (Fig. 4.25), red to buff coloured dune sand is recognised as sitting directly upon these Stage 5 channel sands. The basal dune age of 83.9+6.7 ka (W1709) corresponds closely to the youngest date for the channel sands (81.7+7.4 ka), suggesting that this source bordering dune started to form from sand blown out of the adjacent sand-load channel during late Stage 5 (Fig. 4.21). The Stage 5 alluvium sits unconformably above coarser sands that on the basis of two TL dates of 162±14 ka (W1718) and 250±23 ka (W1711), are clearly older than Stage 5 and probably represent Stages 6 or 7 (Fig. 4.21). The northern clay palaeochannel infill appears to have been abandoned first, as evidenced by its mud infill TL dates of 65.9±5.8 ka (W1716) and 53.9±5.2 ka (W1715) (late Stage 4 and early Stage 3), whereas a date of 33.3±2.6 ka (W2056; late Stage 3) was obtained from the basal mud in the southern palaeochannel infill (CHW-TB; Figs 4.21 and 4.24). There are no channel sands in the northern 101 • ... % coarser 99 95 84 75 / / III Surface 50 / 12mi// IIII 25 16 / / \.5m / 9m / y7 / Chookoo Drill Hole (CH-Ah Sediment Analysis /l8m III Mean Sorting Skevvness Kurtosis (•) ( I I l 1 1 1 1 * -1 0 1 Fig 4.25 - Particle size analysis of CH-A auger hole through Chookoo dune l from surface to a depth of 18 m. 102 Chookoo palaeochannel windward of Dune 1 (Fig. 4.21) that date younger than -80 ka. With the infilling of the northern palaeochannel with mud from about 70 ka, the aeolian sand supply would have terminated. This shows that the upper aeolian sands in Dune 1 have been reworked from earlier dune sand, for they date at Stages 2 and 1. Dune 2 was also reworked during the same period (Fig. 4.21). In contrast to the large waterholes on the Cooper system described in detail by Nanson et ah, (1988) and Knighton and Nanson (1994b), the large Chookoo channel bares little detailed morphological and stratigraphic similarity. Although the large Chookoo channel has the appearance of a typical waterhole in planform, (Fig. 4.19) the geometry of a true waterhole extends some 4-10 m below the floodplain surface and into the sand body beneath. These deep waterholes are formed either by scour penetrating both the surficial floodplain muds and the underlying sand sheet, or alternatively by the stabilisation of channels once formed within the underlying sand sheet (Knighton and Nanson, 1994b). Palaeoflow orientations of the coarse channel sand units between 3.5 and 6.2 m depth (CHW-TB (Fig. 4.24)) were to the SSW-SW (210-220°) as compared to the orientation of the present large Chookoo channel to the W-SW (-260°) Dune 2 also sits directly upon sands but in this case the channel sands date at 48.6±3.8 ka or Stage 3 (Fig. 4.21). It appears that following Stage 5 and the abandonment of the channel system that fed aeolian sand to Dune 1, there was 103 a period of fluvial activity to the south that cut into the Stage 5 alluvium beneath the present large Chookoo channel. Clearly identified aeolian transported sands in Dune 2 dating at 44.1±3.4 ka (W1938) and 47.5±2.9 ka (W1940) suggest that this dune was fed from an adjacent channel probably immediately to the south of this dune. In fact, Dune 2 appears to be built partly on the northern extent of this southern palaeochannel. Like Dune 1, these aeolian deposits were reworked in Stage 2 and probably Stage 1 as well. In summary, the research findings from the Chookoo site reveal the following: 1) Extensive sands underlie the site and indicate laterally active channels in the area during Stage 5 sitting unconformably over coarse sands from Stages 6 or 7. 2) Dune 1 appears to have been formed about 85 ka from a palaeochannel upwind and immediately south, with the channel infilling with mud at 70-55 ka. 3) Dune 2 appears to have been formed at about 50-40 ka from sands deposited about the same time up-wind and immediately south (Stage 3). Termination of this latter period of activity is evidenced by the muddy palaeochannel infill dating at 33.3±2.6 ka (W2056) north of Dune 2. 4) The crests of Dunes 1 and 2 have been reworked at about 25 ka and then again during the Holocene. 5) The confinement of flow between Dunes 1 and 2 has produced a scour feature (large Chookoo Channel) with the superficial impression of being an infilled waterhole, but the stratigraphy reveals is only a shallow scour feature penetrating the alluvial muds by only about 2 m. 104 6) A similar but much smaller scour channel (small Chookoo Channel) exists between Dunes 2 and 3 (Figs 4.20 and 5.3). 7) There is no evidence of significant sand transport in the vicinity of the Chookoo dunes after about 48 ka and no substantial channels present after about 33 ka. 4.3.2 Durham Dune Site The Durham Dune site consists of two adjoining sand dunes (141°54'42"E, 27°03'43"S) and covers an area of 1 km^, -1.5 km east of the Durham Downs homestead and Tabbareah Waterhole, ~2 km north of the Durham Downs to Bundeena Road and ~5 km west of the Narberry site (Fig. 4.1). A vehicular track bisects the two low amplitude dunes with the larger dune being located on the northern side of the track (Fig. 4.26). Both dunes are well-colonised by assorted shrubby plants such as sandhill canegrass (Z. paradoxa), blue bush pea (C. eremaea) and along the margins by the occasional Murray's wattle (A. murrayana) and gidgee (A. cambagei). Research focussed on the northern Durham dune which at 6.8 m elevation is the highest of the two dunes. Three auger holes were drilled at the site: DD1 on the dune at an elevation of 5.6 m, -25 m southeast of the dune crest; DD2 on the floodplain between DD1 and DD3, i.e., 110 m north of the base of the Durham dune or 140 m from DD1; while DD3 was located on the floodplain 240 m northwest of DD2 (Fig. 4.26). The sub-surface stratigraphy of these three auger holes is presented in Fig. 4.26. 105 ' "3 m a in C oi £j bo oi in HH en ,< 1 j 106 Despite the prevalence of floodplain clays surrounding the dunes at the surface, there is no clay beneath the dune. The maximum depth of floodplain clay at the site was -2 m RL in DD3, the most distant auger hole from the Durham Dune. The only other clay was in auger hole DD1 at -14 m RL where a thin indurated grey clay layer was encountered that is interpreted as weathered bedrock (Fig. 4.26). The dune stratigraphy in both DD1 and DD3 is dominated by reddish yellow fine-medium sand (7.5 YR 7/6) grading into yellow and light yellowish brown fine-medium sands (10YR 7/6; 10YR 6/4) with depth (Fig. 4.26). Mottled brownish yellow to white poorly sorted coarse sands (10YR 6/6; 10YR 8/1) occur at depths > -8 to -10 m RL. Detailed size analyses for sediments in holes DD1 and DD3 confirm the very considerable extent of dune sand beneath the floodplain mud. Sediment size analysis of sediment samples from auger hole DD1 (Fig. 4.27) shows size distributions for the moderately well sorted aeolian sands at depths of 0, 1.5, 6 and 9 m while samples from depths of 15, 16-16.5 and 17 m are relatively coarse and poorly sorted sand (Fig. 4.27). In addition, there appears to be good correspondence between near-surface sediment size distributions from dune sand at DD1 and sediment samples from DD3 at -3 and -6 m RL (Fig. 4.27). On the basis of this evidence, source-bordering dune sands exist above a depth of -8 to -10 m RL, and these overlie significantly coarser sands. The Durham Dune represents a northward oriented source-bordering dune with its flanks now extensively buried by floodplain accretion. 107 % coarser 99 95 84 75 16m / / / J Surface / Om 50 / 15m / / ¥ 25 16 17m / / / V Durham Drill Hole (DD1): Sediment Analysis Mean Sorting Skevvness Kurtosis (40 (4-) (+) (4>) 0m 2.1167 0.7783 0.0960 0.8680 1.5m 1.9864 0.5442 0.1859 1.2676 1.9535 0.8282 0.1512 0.8560 6m 1.8815 0.7104 0.04% 0.7496 9m 1.9190 0.4641 0.0859 1.1057 15m 1.1817 0.7655 -0.1923 1.3022 16m 0.2669 1.0002 0.1104 1.1214 9m / //1.5m 17m 0.4463 1.1447 -0.0525 0.9274 6m I 1 1 l III -2 •1 0 1 d> Figure 4.27 - Sediment size analysis of auger hole DDi through Durham dune from surface through to a depth of 17 m. 108 The SEM analysis of quartz sand grains at the surface of the Durham Dune reveal very few impact features to indicate prolonged long distance aeolian transport. Quartz sand grains at the surface of the dune are mostly sub-angular and exhibit limited evidence of mechanical impact (Fig. 4.28a and b). Frequent impacts during aeolian transport should result in substantial rounding of the grains (Krinsley and Doornkamp, 1973; Krinsley and McCoy, 1978; Trewin, 1988; Cooke et ah, 1993; Zhou et. ah, 1994; Newsome and Ladd, 1999) and the formation of impact features such as mechanical V-forms, however, very few were evident on quartz grains of the Durham Dune (Fig. 4.28c). There may have once been more mechanical impact features on the grain surfaces, however, chemical alteration of the grain surfaces in this hot climate with some 200-250 mm of annual rainfall may have dulled mechanically induced features over time (Fig. 4.28d). The lack of appreciable mechanical impact features, even on quartz grains at the present surface of the dune, provides evidence that aeolian transport has been very limited. Local reworking by aeolian transport of adjacent sands to form source-bordering dunes has probably been very localised. SEM and sediment size analysis in the studies of Pell and Chivas (1995), Newsome and Ladd (1999) and Pell et al. (2000) have shown that the notion of long distance aeolian transport of quartz grains is not plausible. However, Folk (1971), Pell and Chivas (1995) and Pell et ah (2000) endorse an in situ origin or short-distance transportation of fluviually transported quartz grains that are subsequently reworked into source bordering dunes by aeolian processes. Also, the relative dearth of mechanical impact features attributable to aeolian 109 110 transportation provides strong evidence for a fluvially-transported origin (Newsome and Ladd, 1999). Textural sorting has proven to be a more reliable guide than grain impact features as to the form of transport, for sorting will occur over relatively short distances. Figure 4.27 shows how clearly sand samples separate into two distinct groups on the basis of textural sorting brought about by selective transport. Thermoluminescence evidence indicates that the coarse sand unit at > -8 m RL was deposited >100 ka, after which the yellowish fine-medium sand was laid down between about 85 ka and 50 ka by aeolian selection of channel sand and aeolian reworking of the Durham dune to about 8 ka. The crests of dunes in the area remain partially active today, however, this could be due in part to the introduction of rabbits and other grazing animals. Research results from the Durham dune site suggest that: 1) Well-sorted fine to medium sands with no coarse fragments provide sound evidence of abundant aeolian sand at this site to depths of 6 m below the present floodplain. 2) Extensive fine to medium aeolian sand deposits underlie both the dune and the floodplain to the north, indicating a much more extensive dune now substantially buried by floodplain accretion. Ill 3) The dune was initially deposited at between -120 and 85 ka prior to the deposition of the present clayey floodplain in this area. The floodplain is gradually burying the dune which is now starved of a supply of sand. 4) SEM provides evidence for the short-distance aeolian transportation of largely unaltered fine-medium quartz sand. These source-bordering dunes were formed adjacent to an available source of fluvially-derived sand. 5) An extensive coarse alluvial sand unit dating at > 100 ka appears ~9 m below the floodplain surface, with aeolian deposition post-dating this period (Fig. 4.26). 4.3.3 Narberry Waterhole The Narberry site is located ~3 km N of the Durham Road in the centre of the Cooper floodplain (Fig. 4.1) and is characterised by the ephemeral Narberry Waterhole with mature coolibah trees (£. microtheca) lining its banks. The waterhole, which displays considerable similarity to the large Chookoo channel described earlier, is 410 m long, features a shallow, wide cross-section with a maximum depth of 1.8 m and a width-to-depth ratio of ~45, and has a bed surface of heavily cracked clays (Fig. 4.29). In addition, there are two small, very low relief (<2 m) aeolian dunes, one to the west-northwest, the other to the east-southeast of the waterhole (Fig. 4.30). In appearance, the feature was thought to represent an abandoned and infilled channel or waterhole, the legacy of an avulsion from the main anastomosing channel to the northwest. However, as at Chookoo, augering and TL dating at 112 Figure 4.29 - Ground view photograph of the Narberry Waterhole looking westward across the shallow, saucer-shaped depression (max. depth=i.8m) displaying a heavily cracked clay surface. The site of drill hole NHi is clearly shown (see Fig. 4.31 for sedimentary description). Large coolibahs (E. microtheca) line the banks of the ephemeral channel. Photograph was taken on the eastern bank of the channel between drillholes NHi and NH2 (refer to Fig. 4.31). Figure 4.30 - Oblique aerial photograph of the Pritchella and Narberry Waterholes. Also shown are the West and East Narberry dunes and a sedimentary splay at the southern end of the Pritchella Waterhole. Photograph was taken after a moderate flood event which filled the ephemeral Narberry Waterhole and overtopped the anastomosing channel to the extent delineated by the green floodplain vegetation on the drier floodplain surface. Large coolibahs (E. microtheca) line the banks of the anastomosing and ephemeral channel. View is to the northeast. Distance between Narberry and Pritchella Waterholes is 2 km. 113 the site revealed no evidence for a former deep waterhole infilled with overbank mud (Figs 4.29 and 4.30). Two auger holes NHI and NH2, -100 m apart, were drilled in the dry waterhole and adjacent floodplain respectively (Fig. 4.31). In floodplain auger hole NH2, 3.1 m of very dark greyish brown floodplain clay (10YR 3/2) grades into a 1.5 m thick brown sandy-clay (10YR 5/3) unit. Between 4.6-12.0 m, a 3 m fine yellowish brown sand (10YR 5/6) unit grades into a light yellowish brown medium sand (10YR 6/4) and then into a white coarse sand (10YR 8/2). A TL age of 92.1+10.6 ka (W2297) was obtained from the middle of the fine sand unit in auger hole NH2, at a depth of 6 m. Several thin (-10 cm) clay lenses are evident in the sandy units of the profile between 5.4 and 10m. In comparison, the 7 m deep waterhole (NHI) auger hole displays only 1.0 m of very dark greyish brown floodplain clay (10YR 3/2) overlying 1.0 m of brown sandy-clay (10YR 5/3). The remaining 5 m of the profile is comprised of 3.0 m of yellowish brown fine sand (10YR 5/6) grading to a 1.2 m pale brown medium sand (10YR 6/3) and finally a 0.8 m thick white coarse sand (10YR 8/2). No clay lenses were evident in the sandy portions of this profile. A TL age of 146+11 ka (W2029) was obtained from the base of the fine sand unit in auger hole NHI, at a depth of 4.5 m (-7 m RL). The stratigraphies of each auger hole are almost identical apart from the thickness of the surface clay which in the waterhole is only 0.5 m thick (NHI) compared with 3.0-3.5 m thick on the floodplain (NH2); there is no evidence for a previous deep waterhole or channel infill (Fig. 4.31). The Narberry Waterhole represents a shallow scour channel (similar to the Chookoo channels) resulting 114 from only about 2-4 m of incision into the floodplain clays. The depth of scour was not sufficient to allow the storage of floodwaters for months or years after flood events as occurs in the major waterholes along the Cooper, and for that reason it is not a true waterhole. The reason for its existence is probably the two adjacent low-elevation aeolian dunes (Fig. 4.30) that have slightly confined the flow between them, inducing local scour into the floodplain surface (Knighton and Nanson, 1994b). The sand body beneath the Narberry Waterhole dates at 146±11 ka (W2029) or Stage 6 whereas that to the east dates at 92.1+10.6 ka (W2297) or late Stage 5. It is interesting that far from the Narberry Waterhole representing an infilled palaeochannel, it is actually located over alluvium that is older than that flanking it to the east. The West Narberry and East Narberry dunes (Fig. 4.31) are very low amplitude features each with a maximum relief of <2 m. Preliminary research by Nanson and Knighton (pers. comm.) on the South Narberry dunes (-2.5 km south of the Narberry Waterhole) revealed somewhat older TL dune chronologies compared to equivalent depths below the dune crest of the Chookoo and other dune sites reported here. However, much of the mobile near-surface dune sand has probably been removed from both Narberry dunes, that now barely protrude above the floodplain surface, thereby giving them greater near-surface TL ages (Fig. 4.31). For instance, a TL date of 14.5±1.7 ka (W1041) at a depth of 1 m into the dune surface is equivalent to dune dates obtained elsewhere in the study area at dune depths of -4 m. Furthermore, the TL dates of 35-45 ka at 4.5-6.0 m into the South Narberry Dune are equivalent to those obtained from about 7-9 m in Chookoo Dune 2. Thus the Narberry dunes may have undergone about 3 m of deflation of their dune crests, culminating in their present low-relief only 116 slightly higher than the accreting floodplain surface. This relief is still sufficient to concentrate the 1-2 m of flodwaters across the floodplain and to maintain the scour channel that is termed Narberry Waterhole. Presumably, these dunes will be buried by floodplain accretion in the future. 4.4 Conclusion The floodplain stratigraphy of Cooper Creek in this study is the product of a mix of fluvial and aeolian processes. However, it is clear that the processes responsible for the stratigraphy at the surface and at depth have varied markedly during the Quaternary. There have been prolonged periods of sand deposition by probably laterally migrating channels in the mid and late Quaternary at times of more reliable and higher energy runoff. Flow regimes appear to have progressively waned due to increasing aridity during the mid and late Quaternary resulting in dune building episodes and the onset of low energy fluvial processes associated with the contemporary mud-domianted anastomosing systems. From this evidence, models are developed (Chapters 5 and 6) to explain the mid to late Quaternary floodplain development of the Cooper Creek floodplain. 117 Chapter 5 Stratigraphic and Chronological Interpretations of the Cooper Creek floodplain 5.1 Introduction The results in Chapter 4 have identified the dominance of three depositional units characterising the Cooper Creek floodplain of southwest Queensland since the mid-Quaternary. These include: • alluvial clay soils which dominate the morphology of the present floodplain surface mostly <70 ka in age; • an extensive alluvial sand sheet mostly >70 ka in age which underlies the alluvial clay; • sporadically distributed floodplain dunes largely <60 ka in age, probably source-bordering in origin, and with buried portions between 80-110 ka. This chapter examines the characteristics of each sedimentary assemblage highlighting the processes responsible for their development. From this evidence the Quaternary evolution of the Cooper Creek floodplain is interpreted, showing a strong causative link between the river channels and adjacent aeolian dunes. The TL chronologies obtained for the aeolian and fluvial units in this study are then compared to similar previous TL based chronology studies within the Lake Eyre Basin (Nanson et ah, 1988,1991,1992a; Magee et al. 1995, 1998; Croke et ah, 1996, 1998) and to possible global trends identified from changes in marine oxygen isotopes during the past 300 ka (Shacklefon and Opdyke, 1973; Martinson et ah, 1987). Finally, consideration is given to the apparent migration of the active Cooper floodplain westward since Stage 3. 118 Nanson et al. (1988) provided a preliminary but useful synthesis of sedimentary features, stratigraphy and early TL chronology characterising the anastomosing Channel Country streams in western Queensland, including Cooper Creek, and Gibling et al. (1998) have summarised the prominent surficial sedimentological characteristics of these anastomosing channels. However, the Channel Country is a highly complex sedimentary system formed over several hundreds of thousands of years by the interaction of fluvial and aeolian processes under conditions of changing climate and flow regime and, as such, much remains to be described and explained of its detailed stratigraphy, chronology and Quaternary evolution. This chapter builds considerably upon the work of Rust (1981), Nanson et al. (1986, 1988, 1992b) and Rust and Nanson (1986) by presenting a detailed interpretation of the fluvial and aeolian depositional processes and Quaternary chronology that characterise the floodplain of Cooper Creek in southwest Queensland. 5.2 Characteristic features of the fluvial depositional units 5.2.1 Near-surface floodplain clays The depth profile of floodplain clay soils can be sub-divided into two units: (a) an upper -0.5 m of grey-light brown (10YR 5/2) fluffy, friable, heavily cracked alluvial clay soil assemblage exhibiting both vertical and horizontal jointing, below which is, (b) a compacted, massive, featureless, homogeneous light brownish grey (10YR 6/2) clay unit. By far the most prominent surface feature of the floodplain is the heavily cracked, aggregated alluvial soils (vertisols). These cracking clay soils cover the entire floodplain surface within the study area except where interrupted by incised channels and aeolian sand dunes. These vertisols, which typically comprise >50% clay (Northcote, 1965; Stace et ah, 1968; Loch and Silburn, 1993; 119 Maroulis and Nanson, 1996), exhibit significant shrink-swell characteristics which result in low-relief (<20 cm) heave-type structures. Soil heaving has produced pedogenic gilgai patterning at the surface (Dawson and Ahern, 1974). After rainfall and/or inundation by overland flow, swelling of the floodplain clays causes the cracks to seal such that, following initial inundation of the surface, floodplain infiltration ceases with the soil forming a very effective moisture seal (Nanson et ah, 1988; Maroulis and Nanson, 1996). In the top metre of the floodplain, clay aggregates are readily apparent, however, at depth compaction results in little evidence of an aggregated ped structure (Rust and Nanson, 1989; Maroulis and Nanson, 1996). Immersion wetting during overland flow, causes the irregular-shaped blocks of surface soil to disperse into water-stable, low-density, fine-sand sized mud aggregates whose hydraulic transport properties have been demonstrated in a laboratory flume (Maroulis, 1992; Maroulis and Nanson, 1996). The highly mobile aggregated sediment is readily transported as bedload (Nanson et ah, 1986, 1988; Rust and Nanson, 1986; Maroulis and Nanson, 1996) and, as such, appears to play a role in the development of braid-like features on the floodplain surface. Primary sedimentary structures formed by the deposition of mud aggregates on the floodplain surface are largely destroyed soon after formation by the churning action of self-mulching expandable clays. Nevertheless, sedimentary structures of aggregated mud have been observed immediately after flooding by Nanson et al. (1988) and Rust and Nanson (1989; 1991) in both the Thomson and Barcoo Rivers (Figs 2.1 and 4.1) while small cross-laminated ripple structures of mud (amplitude < 1 cm) were observed by the author in a cut bank -400 m downstream from the Cooper Creek bridge crossing near Windorah (Figs 2.1 and 4.1). 120 The depth of alluvial clay is quite variable across the floodplain ranging between 0.5 m and 10 m with an average of -3 m. The thickest units were observed in some of the drill holes at Tanu (Fig. 4.2) and near the Wilson River at Bolan (Fig. 4.17) and are interpreted as 10 m deep clay palaeochannel infills. Vertical accretion rates of the floodplain muds were found to vary considerably across the floodplain. The western Durham Road and Shire Road transects closest to the presently active channels, (Figs 4.6 and 4.7) are accreting -4 times faster than the eastern floodplain, with rates of 0.019-0.023 m/100 yr and 0.004- 0.006 m/100 yr, respectively (Fig. 5.1). TL dates for the floodplain muds vary from 118 to 2.2 ka, with the bulk of ages lying between 70 and 30 ka. The most significant aspect of the TL chronology from these uppermost clays is the apparent decrease in sedimentary ages from east to west across the floodplain, a point considered in detail below. The distribution of pedogenic secondary minerals such as gypsum provides additional insights into wet and dry climate phases (Nanson et. ah, 1992b). Gypsum is relatively soluble and is thus precipitated only under relatively arid conditions (Cooke et ah, 1993). It was evident in numerous auger holes and excavations, however, its distribution was more prevalent in the eastern rather than western floodplain muds. Unlike gypsum, Fe-Mn oxides/hydroxides were confined to thin, horizontally- bedded fine sand laminations at the base of the clay units and at depth within the underlying sand body. These thin, Fe-Mn-rich layers were readily evident in numerous floodplain excavations and may have been more widely reported from drilling across the floodplain, however, the churning effects of augering and uphole drilling on these thin beds (<10 cm thick) made detection of these thin layers almost impossible. Furthermore, modern plant roots penetrate 121 o 03 DC CO cc 00 • o^•rl CO cc o3 CO CN o CC cc o C/) CO < 0) T3 ai w 5) C • «0 i_ c 4-o« 1— o 0) t/> (D re > <: UJ i —r~ —r o CM S3J13UJ ui mdaa 122 through the clay deposits and into the underlying sands and disturb the Fe-Mn- rich layers. 5.2.2 Sub-surface alluvial sand sheet Stratigraphic evidence presented in this study and similar to that reported elsewhere (Nanson et ah, 1986, 1988; Rust and Nanson, 1986, 1989) illustrates that almost ubiquitous beneath the muddy near-surface floodplain is an alluvial, and sometimes an alluvial and aeolian, sand body. Augering along the 13.9 km Shire Road transect intersected the alluvial mud-sand contact at depths ranging from 0.5 to 4 m (Fig. 4.7). The sand body is very extensive, and reaches a thickness in excess of the maximum drilling depth of 36 m in auger hole SR5 (Fig. 4.7). The dominance of subsurface sand in the drilling was also encountered along the 1.4 km Chookoo Sandhill transect (Fig. 4.21), the 12 km Durham Road transect (Fig. 4.6), Durham dune (Fig. 4.26), the seismic lines at Costa (Fig. 4.4), parts of Tanu (Fig. 4.2), Wareena (Fig. 4.3) and Wackett (Fig. 4.5), and at Narberry Waterhole (Fig. 4.30) and Lignum Creek (Fig. 4.18) sites. Rust and Nanson (1986) provided stratigraphic evidence from several excavations near the Naccowlah Waterhole indicating that the mud-sand interface is often sharply defined. In this study, there are instances where the boundary is very abrupt, however, gradational sequences are also plentiful. For example, the western floodplain auger hole (F/P #6) along the Durham Road transect (Fig. 4.6), revealed an abrupt change in stratigraphy at -6 m below the floodplain surface, from a well-compacted grey clay unit to a medium-coarse sand with some occasional gravel and sub-angular silcrete fragments. In contrast, the stratigraphy of both the central (F/P #3) and eastern auger holes (F/P #1) is more gradational with the grey clay units being separated from coarse sand by 1-2 m of sandy-clay (Fig. 4.6). Many of the floodplain trenches 123 revealed a gradational mud-sand contact, as did y-logs from the seismic drill holes (Figs 4.5 and 4.16). The underlying stratigraphy in many respects does not conform with floodplain drill log data presented by Veevers and Rundle (1979) and Rust (1981). These authors reported (from drill logs obtained by others) alternating cycles of mud and sand deposition within the top 100 m of the floodplain alluvium. However, in the present study, it has been verified that beneath the surface muds there is an almost unbroken sequence of fine to coarse sand with only minor amounts of clay at depth in the profile. Augering along the Shire Road transect (Fig. 4.7) revealed that apart from near the surface, there was little clay present. Below 2 m no clay was evident in the 36 m deep SR5 auger hole while only thin clay lenses were found separating upward-fining sequences in auger holes SR2 and SR3 (Fig. 4.7). Despite the apparent lack of buried mud units, there is ample evidence of buried upward-fining sequences suggesting repeated cycles of sedimentation as the basin filled. For example, the Wackett area (Fig. 4.5), Shire Road transect (Fig. 4.7), Narberry Waterhole (Fig. 4.30), Durham Road transect (Fig. 4.6) and the Costa (Fig. 4.4), Tanu (Fig. 4.2) and Wareena (Fig. 4.3) seismic lines. Sedimentary data that may agree with Veevers and Rundle (1979) and Rust (1981) comes from near Baryulah (Fig. 4.16) where a more cyclical stratigraphy exists to depths of 24 m. Here, the near surface is characterised by floodplain clays and sometimes aeolian sands while at depth alternating bands of alluvial sand and some mud were deposited above a weathered argillaceous sandstone unit that is probably Tertiary in age (Fig. 4.16). Similar but less extensive units were encountered at the nearby Bolan (Fig. 4.17) site on the Wilson River floodplain, however, such alternating mud and sand units were not common elsewhere on the Cooper floodplain. 124 The TL dates obtained in this study provide the oldest absolute ages ever recorded from the Cooper floodplain or from any other fluvial deposits in Australia. Three of the oldest TL dates were obtained on the Shire Road transect from auger hole SR2 which gives stratigraphically consistent ages of 405+43 ka (W2034) at 15 m depth, 512±83 ka (W2035) at 21 m depth and 740+55 ka (W2036) at 27 m (Fig. 4.7). A date of 454+174 ka (W2046) obtained at a depth of 15 m in auger hole SR5 is comparable to those from SR2 but has a sizeable error band due to its approaching TL saturation (Figs 4.7 and 4.14). At depths of -5 to 12 m, alluvial sand TL dates ranged between 35 ka and 200 ka with most dates being >80 ka. Holocene (Stage 1) dates were obtained in alluvial sands near the western boundary of the floodplain. Excavations of the unique sinuous palaeochannels beneath the contemporary floodplain surface at Mt Howitt (Figs 4.12 and 4.13) revealed a strong association between the planform expression of the meander channels and the sub-surface stratigraphy. In particular the planform revealed meander characteristics such as scroll bars, meander cutoffs and meander loops with sinuosities of -1.9 (Rust and Nanson, 1986), channel widths of -50-60 m and sedimentological features all indicators of a former meandering channel habit including upward-fining sequences, a well-defined erosional cut-bank and lateral accretionary surfaces (Fig. 4.13). The dimensions of the palaeochannel revealed a bankfull channel width of -60 m, a maximum depth of -5-6 m (W/D ratio=~10) with an estimated palaeochannel bankfull channel area of 135 m2, from which the average bankfull palaeochannel velocity was calculated at 0.6 m/s with a bankfull discharge of -80 m3/s (assuming s=0.0008 m/m; Manning's n=0.025). A much larger palaeochannel was identified near Naccowlah Waterhole by Rust and Nanson (1986; Fig. 5) and revealed the following palaeochannel 125 dimensions: width -500 m; depth -4 m; wavelength -4 km; radius of curvature -2 km; slope -0.0001-0.00008 m/m; Manning's n=0.025. From this data the estimated palaeochannel bankfull cross-sectional area was 1750 m2 with a velocity of 0.8-1.0 m/s and bankfull discharge of 1440 m3/s. The palaeochannel bankfull discharge is estimated to be over three times the bankfull discharge of Meringhina Waterhole, the only waterhole in the study area known to take the entire flow of the present Cooper up to bankfull (330 m3/s; Knighton and Nanson, in press). In comparison to the Mt Howitt palaeochannel, the Naccowlah palaeochannel has a bankfull discharge 18 times greater and 13 times the bankfull cross-sectional area. In addition the TL dates of the Mt Howitt clay fill (Fig. 4.13) are quite young in comparison to the dates from the Naccowlah palaeochannel. An age of 16.7+1.6 ka (W2294) at the deepest point, 4.5 m below the floodplain surface within the Mt Howitt clay infill, is appreciably younger than the 51±4.5 ka age obtained at the Naccowlah palaeochannel (Rust and Nanson, 1986). Furthermore, the Mt Howitt palaeochannel ages for both the lateral accretionary surfaces at -60 ka and the older cut bank at 95 ka are all younger than the >200 ka ages obtained by Nanson and Rust (1986). From the above, it appears that the Mt Howitt palaeochannel represents a small tributary which meandered across the sand-dominated floodplain from its eastern valley source towards a larger probably meandering sand-load palaeochannel farther to the west. The Mt Howitt palaeochannel was probably active during late Stage 5 but with the abandonment of the large, laterally active meandering channels in Stage 4/3 (-63 ka) the Mt Howitt palaeochannel probably continued to exist as a mud channel until -15 ka before infilling with mud. 126 5.3 Formation of the floodplain dunes While the coexistence of anastomosing and braided planforms on the mud- dominated, low-gradient Cooper Creek floodplain has been widely reported (Nanson et ah, 1986, 1988; Rust and Nanson, 1986, 1989; Maroulis and Nanson, 1996), the relationship between the aeolian dunes and the floodplain has received little attention. Indeed the interaction between aeolian and fluvial deposits and the palaeoclimatic significance of such processes are only rarely reported in the literature (Langford, 1989; Langford and Chan, 1989). Yet, stratigraphic evidence shows that aeolian processes and dune development during the late Quaternary have had a significant impact upon the floodplain development of Cooper Creek. The presence of floodplain aeolian sand dunes along Cooper Creek provides an opportunity to address a number of research questions, including: • What effect does the present muddy sediment regime have on the contemporary floodplain dunes? • Do existing floodplain dunes influence the character of the modern anabranching channel network and floodplain? • Do the floodplain dunes pre- or post-date the clayey floodplain? • Were there common periods of aeolian dune development indicative of distinct episodes of climate and/or flow regime in the area? • Were the floodplain dunes formed as source-bordering dunes adjacent to active channels or where they formed essentially independently of the channels? 127 5.3.1 Interactions between floodplain clay and dune sand Most floodplain dunes display an irregular but rounded planform, some being near circular. A zone of slightly indurated or compacted sand between the main body of the dune and the floodplain forms a 'halo' or 'areola' around the margin of each dune. This is commonly characterised by a gently sloping, hard, clayey sand surface that appears to form during flooding when fluvial clays in suspension mix with dune sands to form a resistant surface. Rainfall landing on this surface, and that collecting within the aquifer of the dune and draining subsurface to merge at the edges, runs across it eroding shallow rills in the form of dendritic drainage nets that fan out to the margin. The relative absence of vegetation on this hard, scalded surface produces a zone that is distinctive when viewed both on the ground and from the air (Fig. 5.2). These indurated areola reveal almost no sedimentary structures although in the bank of an incised gully leading into the small Chookoo channel (Fig. 5.3) some thin (-10-20 mm) horizontally-bedded strata comprise alternating units of grey- brown clay aggregates and iron-stained sands. This stratigraphy reveals that dune sands are water eroded, transported and deposited into -15-20 mm thick strata within the areola by localised rainfall-runoff events. Intervening aggregated mud layers about 10 mm thick result from overbank flooding from channels often distant from the site as well as from dust blown onto the dune from the floodplain. Each depositional layer displays well-defined depositional planes with no evidence of intermixing (Fig. 5.3). The mud layers act to cement the intervening sand units producing a resistant unit with surface scalding inhospitable to vegetation. Because mud now covers almost the entire floodplain, contemporary floodplain dune development is starved of a sand source; TL dating reveals recent 128 Figure 5.2 - "Poached egg dune". Oblique aerial photograph looking eastward at an isolated floodplain dune surrounded by floodplain muds with a zone of intermixed sand and mud in the form of an indurated areola (~ioom wide) around the margin of the dune. Acacia spp. line the contact between unconsolidated dune sand and the indurated areola interface. Vegetated dune circumference = 470 m. 129 3 to w O O —i MX (3 130 reworking of only the upper few metres of the dunes. At present, surface sands are entirely confined to existing aeolian dunes or, in places, on the beds of anastomosing channels. Wind is unable to entrain the channel sands due to their very limited extent, the narrow, inset nature of the channels, and the wind protection provided by the tree-lined banks. With this lack of available sand, it is anticipated that deflation will result in the dune sands being gradually distributed onto the floodplain surface and encorporated in the self-mulching soils. Furthermore, the floodplain muds vertically-accreting at rates of 0.004 to 0.020 mm/yr are continuing to slowly bury the dunes. For example, a mound of only -2 m relief is all that remains of the heavily deflated South Narberry Dune (Chapter 4) surrounded by an accreting floodplain (Fig. 4.30). 5.3.2 The age of the floodplain dunes relative to the floodplain On the basis of stratigraphic and chronological evidence, it is clear that the Chookoo and Durham dunes were initiated as dunes on the margins of wide sandy channels prior to the formation of the now ubiquitous muddy floodplain. It is clear that the floodplain dunes are not still forming, for there is at present no source of sand other than the dunes themselves. Furthermore the dunes appear to be relatively stable. However, Nanson (pers. comm., 1998) identified one small dune on the eastern fringe of the floodplain near the Wilson River confluence which has been blown across the muddy floodplain surface but this was a small feature about one hectare in area and <2 m in height. Compared to the much larger dunes reported in the same area, it represents an interesting anomaly, but a negative TL age (W1050) (Appendix A) for the sand immediately overlying the mud surface indicates that the dune had probably been recently reworked. If the large dunes were still active features migrating over the floodplain muds, then excavation or drilling of them would reveal extensive floodplain clays 131 beneath. However, the Chookoo and Durham dunes (Figs 4.21 and 4.26) are connected to the underlying alluvial sand deposits and do not overlie floodplain clay. The floodplain muds have clearly been lapped onto the outside of the aeolian sand units with the floodplain clays thickening away from the dune margins, with TL ages provided further evidence of the dunes predating the adjacent floodplain muds (Figs 4.21 and 4.26). 5.3.3 Was there a common period of dune development? There is a degree of consistency in the TL ages obtained from dunes in this study. Dune dates to depths of -9 m ranged between 4 and -60 ka (Figs 4.21 and 4.26). However, three TL dates >80 ka were obtained from three different aeolian dunes. These include a TL date of 81.7+5.5 ka (W1888) from an exposed indurated sand dune unit ~lm above the present floodplain surface near the Tooley Wooley Waterhole, while the main Chookoo Dune and the Durham Dune provided similar TL dates of 83.9+6.7 ka (W1709) and 86.7±8.2 ka (W1712) at depths of -1 m and -4 m, respectively, below the present adjacent muddy floodplain (Figs 4.21 and 4.26). A plot of dune accumulation rates reveals rapid phases of source-bordering dune building during oxygen isotope Stage 3 (Fig. 5.4) for the Durham and Chookoo Dune 2, while Chookoo Dune 1 revealed more rapid accumulation or reworking during Stages 2 and 1. There is also evidence of brief but rapid sand and mud deposition during Stages 3 and 1, however, apart from indicating rapid pulses of fluvial and aeolian sand deposition, there are insufficient data to make further inferences. There are two other minor groupings of dune dates: <25 ka and between 40 and 50 ka which may represent phases of dune building however, these 'peaks' are only characterised by an extra two dates each and cannot therefore be seen as particularly significant at this stage. The regional climate and flow-regime implications of these and other TL dates are discussed 132 OH o .22 0) 6 CD S3 CU -a 0 s: u 1 Q 9 q3 II >- « g « a ul o CD cd *+- cu s| 2| o g '•g"g p 3 co O u rn oi o*>' 133 further in Chapter 6, as are comparsons between the results obtained here and those from other researchers. 5.3.4 Are the floodplain dunes source-bordering? Studies describing the nature of aeolian transport in central Australia provide useful observations for interpreting aeolian processes operating on the Cooper floodplain. Wasson (1983b) contended that dune development in the Simpson and Strzelecki dunefields was largely a function of localised reworking of sand deposits and aggregated muds. In addition, no evidence was found for long distance transport of aeolian sand during the Cainozoic, an observation supported by more recent work (Pells et ah, 2000). Source-bordering dunes of quartz sand derived from rivers and lunettes composed of clay-pelleted mud, gypseous and quartz sands derived from nearby lacustrine sources have been reported in inland and southeastern Australia (Bowler, 1973, 1976,1983; Butler, 1974; Dare-Edwards, 1982; Wasson, 1983b; Bowler and Wasson, 1984; Chen et ah, 1990, 1991a, 1991b, 1993, 1995; Williams et ah, 1991; Williams, 1994; Nanson et ah, 1995; Page and Nanson, 1996; Page et ah, 1996). Butler (1956) and Bowler (1973) observed the development of clay aggregates (pellets) in playa lakes of central Australia. They suggested that the highly saline lake waters helped to aggregate fine clays washed into these lakes during floods. Aeolian processes transport the aggregates from the dry playa lake-bed onto lunettes on the lee side and possibly farther afield, such as onto adjacent floodplains. However, Nanson et al. (1986), Rust and Nanson (1986, 1989) and Maroulis and Nanson (1996) have demonstrated convincingly that salinity is not required for the formation of fluvial mud aggregates. Furthermore, despite the abundance of mud aggregates on the Cooper floodplain there is almost no evidence of the development of dunes by aeolian transport of these aggregates. The only exception are some low (<1.5 m) flat- 134 topped aeolian deposits formed almost entirely of mud aggregates north of Moomba on the Cooper floodplain (G. Nanson & M. Coleman, pers. comm., 1998). Source-bordering dunes of largely quartz sand derived from rivers on the Riverine Plain of southeast Australia have been described by Butler (1974), Pels (1969), Bowler (1978,1983), Page and Nanson, 1996 and Page et al. (1996). Work by Nanson et al. (1995) on the Finke River provides evidence for some of the oldest (-100 ka) TL dated fluvial source-bordering quartz dunes in central Australia. Williams (1994) identified 3 requirements for the development of source- bordering dunes, each of which is addressed in the accompanying discussion. These include: (i) a regular source of sand from a seasonally-flowing sand-bed channel, (ii) predominant uni-directional winds for the majority of the year (although strong uni-directional winds during the dry part of the year would probably suffice), and finally, (iii) limited vegetation adjacent to the sand source which would otherwise impede aeolian sand movement. The Chookoo Sandhill complex represents a series of three dunes, of which one is clearly source-bordering for it is derived from the reworking of adjacent alluvial sand deposits, the channels of which have since been infilled (Fig. 5.5). Two buried sand palaeochannels were revealed along the Chookoo transect (Fig. 4.21), providing evidence of lateral accretionary surfaces, palaeocurrent flow directions and upward-fining sequences, all indicative of former laterally migrating channel processes. The northern palaeochannel represents an older channel infilled prior to the southern channel and is believed for the following reasons to be the sand source for Dune 1. Firstly, Dune 1 is located on the 135 Evolution of Chookoo Dunes l and 2 and Large Chookoo Channel Variable wind Chookoo direction N Dune 1 Large Chookoo -< Chookoo Dune 2 Channel Stage 1 (10-0 ka) Variable wind direction N Stage 3-2 (35-10 ka) stable suspended- load Prevailing Wind * N Stage 3 (-50-35 ka) almost stable sand bed Prevailing Wind * q Late Stage 5 (-90-80 ka) imited channel N "" '""i- Li '"' Mid-Stage 5 ^ channel ^ miaration Sandy bedload ~2ml_ Key ~100m Alluvial Mud approx scale Aeolian Sand ] Fluvial Sand Figure 5.5 - Model showing the evolution of the Chookoo Sandhills and large Chookoo Channel since mid-Stage 5. 136 leeward side of the channel in relation to the former dominant south- southwesterly sand-transporting winds (Wasson, 1989; Williams, 1994). Secondly, the channel is close to the present dune crest (<250 m) an ideal circumstance for the formation of a source-bordering dune. Finally, the TL dates from the channel infill and adjacent dune indicate the contemporary nature of the wind and water-lain sediments at similar elevations. The similar configuration of all 3 Chookoo dunes suggests that they are all source- bordering in origin. Figure 5.5 summarises the probable evolution of the main Chookoo Sandhill and the large Chookoo scour channel during the last -100 ka. The model initially highlights the prevalence of wide, sandy laterally migrating channels. With increasing seasonality, possibly associated with increasing aridity, the channel sand deposits were readily exploited by the prevailing wind enabling the lee-ward formation of source-bordering dunes. In the latter stages, the floodplain dunes were surrounded and the sandy floodplain partly buried by vertically accreting overbank muds, with a modern scour channel formed between the dunes and essentially unrelated to the position of either of the buried palaeochannels north and south of it (Figs 4.21 and 5.5). The evidence presented in Chapter 4 illustrates that the majority of dunes on the Cooper floodplain were probably formed as source-bordering dunes developed before the hydrology of the river changed from that of a possibly perennial sandy system to a more seasonal sand and mud-load river. As specified by Williams (1994), the development of source-bordering dunes requires there to be both a regular supply of sand and a period of low or non existent flow to allow deflation of the river bed or adjacent bars. Local aridity in the study area would be necessary to suppress vegetation, as would, for part of the year at least, strong local winds to build the dunes. Of particular significance is the observation that source-bordering dunes on the Cooper 137 floodplain are not widespread. They tend to be isolated, probably associated with individual large channels or point bars. This suggests that by the time the source-bordering dunes were forming after about 60 ka, flows from northern Australia were probably reduced relative to those that provided the abundant subsurface sand bodies formed prior to about 90 ka. The channels probably became less laterally active and therefore not so actively reworking any source- bordering dunes that did form. While source-bordering floodplain dunes were deposited prior to 90 ka as aeolian sand dates older than this indicate, rapid lateral activity of the channels probably reworked most of them. The sporadic distribution of floodplain dunes and the paucity of older aeolian units >90 ka in age are probably the result of such active channel migration. Mud was certainly present on the floodplain during the period of alluvial and aeolian activity as evident by the presence of mud intraclasts in the sand units, and by buried muds in several of the stratigraphic profiles through the floodplain. However, the absence of mud pellets in the dunes indicates that the pelleted floodplain muds were never a significant source of aeolian sediment for the dunes. It is quite possible that the muddy floodplain has remained sufficiently well vegetated to prevent their mobilisation by wind. A schematic generalised model summarising the late Quaternary development of source-bordering dunes and channel development on the Cooper floodplain is presented in Figure 5.6. 5.4 Dune and floodplain development over the past 100 ka Detailed dune and floodplain stratigraphies and chronologies obtained for this study permit the development of this model of sequential environmental change for the Cooper system in the study area for the past 100 ka (Fig. 5.7). In 138 Source-bordering dune and channel development N S .<> -— Variable Qtano 1 Jf ^ .. -yr Variable _ <-<* wind S •^ direction Stage 3-2 i % = (35-10 ka) termination of ,.,.,, ,,,. .. T fluvial sand source sandy bed|oad overbank mud deposition N ^ Prevailing S Wind * sta9e 3 % $ y^lNl^V (65-35 ka) ,. ^^=s=5v,y almost stable sandbed aeolian reworking channe| of channel sands N S „ ... Late Stage 5 Pr ng — fA^ (90-80 ka) Wind $> limited lateral channel aeolian reworking minrn*;nn • migration of channel sands N S * ^ Stage 6-7 ( .; c kM f$ % (150-250 ka) _K extensive W lateral channel migration Sandy bedload Key -2mq 1 gg Alluvial Mud ~100m M Aeolian Sand approx scale • Fluvial Sand Figure 5.6 - Schematic model highlighting source-bordering dune and channel development on the Cooper Creek floodplain. 139 <15ka Further mud deposition and infilling of remaining e.g.: Mt Howitt palaeochannel; A contraction of Longreach Waterhole Nanson et ai. (1988) Increase in mud deposition and <40ka filling of some Stage 3 channels e.g.: southern Chookoo palaeochannel Minor reactivation of the Cooper palaeochannels with seasonal flow (e.g.: Mt Howitt STAGE 3 55-30 ka palaeochannel) and the dominant period of Late SUBPLUVIAL Quaternary source-bordering dune construction (e.v.: Durham Homestead Dune and Chookoo Dune) Increasing mud deposition and 65-55 ka burial of some Stage 5 channels e.g.: northern Chookoo palaeochannel Mostly drier (possibly more seasonal flooding) as evidenced by the lack of fluvial and source- ~ STAGE 4 80-65 ka bordering dune activity and the onset of substantial floodDlain mud deDosition e.g.: Durham Homestead Dune and Chookoo Dune 1 increasing aridity Initial development of 90-80 ka Stage 5 source-bordering dunes increasing aridity and flow seasonality STAGE 5 PLUVIAL 120-90ka Laterally-active, possibly- perennial sand- dominated meandering MID-QUATERNARY Braided to meandering TO PRE-STAGE 5 sand-dominated channels Figure 5.7 - Model of Cooper Creek floodplain and floodplain dune development since the mid-Quaternary 140 addition, Figure 5.8 summarises by mode of deposition the TL dates and error bands and compares these depositional episodes to established world-wide climatic phases associated with d^O isotope stages (Bassinot et ah, 1994). Previous research has shown conclusively that >90 ka the Cooper floodplain displayed the characteristics of a sandy bedload transporting, laterally migrating, upward-fining meandering channel system with muddy upper banks (Nanson et ah, 1986, 1988; Rust and Nanson, 1986). In addition, flow regimes would have been more regular and predictable than at present, possibly even perennial. Rainfall must have been much heavier and more widespread with vegetation more prevalent. Interestingly, overbank-fines were not very well preserved on these sandy-floodplains although mud intraclasts are present in the sands. There could be a number of explanations for this lack of mud facies preservation in these older floodplains: 1) A more energetic fluvial system may have flushed the fines through as washload, resulting in the selective deposition of mostly coarse sediment. 2) Fines may have been deposited on the floodplains but reworked by vigorous laterally-active. Gradually aggrading rivers would have reworked the uppermost floodplain sediment leaving a stratigraphic record of mostly deeper within-channel coarser-grained sediments. This would explain the presence of mud intraclasts. The muddy infilled northern channel at Chookoo (Fig. 4.21) and the mud filled channel at Mount Howitt (Fig. 4.13), and observations by Rust and Nanson (1986), suggest that substantial mud deposition must have been present at least during the latter part of this sand-dominated phase in Stage 5. Mud intraclasts, 141 a o spues uBijoav s:pnp\[ pjiAnij spues IBiAnu 142 mud balls and mud lenses, particularly in the uppermost sandy alluvium of Stage 5 supports this proposition. However, the presence of aeolian dunes directly connected to underlying sands with almost no intervening mud units suggests that at the height of sand dominant transport, the active channels were probably relatively wide and shallow, depositing almost no mud within channel. The dunes appear to have formed directly on extensive alluvial sand bars within or adjacent to the active channels. It is likely that there were mud- capped floodplains distal to these extended channels, as evidenced by the mud fragments within the alluvial sands, probably derived from upstrem bank erosion. Aeolian sand deposition spans from -80 ka to the present with no muddy aeolian units of any significance present within the dunes. This shows that at no time were the mud aggregates on the floodplain reworked by aeolian processes. This is significantly different from lunettes in the Strzelecki and lower Cooper to the southwest which are often comprised almost entirely of mud pellets (Wasson, 1983a). Only quartz sand has been worked by aeolian processes on this section of Cooper Creek upstream of the Innamincka Dome, however work by Coleman (in prep.) confirms the existence of mud pellets in the dunes downstream of Innamincka. It is possible that the Strzelecki mud-pelleted lunettes reported by Wasson (1983a) represent either the reworking of more saline-enriched muddy alluvium, or that the more arid setting closer to Lake Eyre, enhanced the aggregation process making clay pellets more readily available for aeolian working. Regardless, the role of salinity in the aggregation processes of the muds on the Cooper floodplain upstream of the Innamincka Dome is not important (Nanson et ah, 1986,1988; Maroulis and Nanson, 1996). Sediment size analysis was used to determine the sand size and degree of sorting from which the provenance of the sands contained at depth in the source-bordering dune could be ascertained. At the surface, dune sands display 143 a unimodal, well-sorted sediment size distribution while at depth, the samples were more bimodal and less well-sorted (Figs 4.25 and 4.27). At Chookoo Dune 1, on the basis of sediment size distribution, the boundary between fluvial and aeolian deposits occurs at a depth of -12 m below the crest (—3 m RL; Figs 4.21 and 4.25) while in the Durham Dune this occurs at -11 m below the crest (—6 m RL; Figs 4.26 and 4.27). At -90 ka, there was an apparent change to drier or at least more seasonal conditions when the flow regime and sandy bedload transport probably became more irregular. The resultant effect would have been an accumulation of sand within the channels and much greater aerial exposure of channel sands to aeolian processes during seasons of low flow. Dune building phases are commonly associated with periods of low temperatures, high winds and prolonged drought (Wasson and Clark, 1988). Wasson (1989), using a dune mobility index that is based on windiness, a surrogate for vegetation cover, estimated that at times of aeolian activity in Australia, sand shifting winds were 20% higher than present. Also, the estimated maximum reduction in precipitation in the dunefields of central Australia in the Last Glacial Maximum was 40%. The dominance of south-southwest prevailing winds in the study area during the formation and orientation of the region's linear dunes resulted in sands being blown out of the bed of ephemeral sand-abundant channels after about 85 ka. This enabled the formation of the source-bordering dunes on the northern sides of these channels (Figs 5.5 and 5.6). Over time these dunes became larger but did not migrate far from their immediate source of channel sand. The onset of substantial alluvial mud deposition at about 80 ka (Rust and Nanson, 1986; Nanson et ah, 1988) resulted in a change in both channel form and sediment supply for dune building. The channels would have become 144 narrower and steeper sided, thereby limiting the sandy surfaces available and hence the supply of sand to the dunes. In the case of Chookoo Dune 1, the northern channel supplying sand to the dune ceased to transport sand and had infilled with mud by about 50 ka (Figs 4.21 & 5.5). With further overbank mud deposition and vertical aggradation, the sand dunes progressively became confined and sometimes even partially buried by floodplain mud (Fig. 5.6). Dune migration was always negligible but deflation of sand from the margins of the dunes probably became more prevalent. In addition, the channels probably became narrower and trees, requiring a regular water source due to increasing aridity, probably contracted to the stream banks from about 80 to 65 ka. Where dunes were initially small, or where they have experienced extensive deflation such as the South Narberry dune (Fig. 4.30), then such dunes will eventually become buried by aggrading alluvial muds. Following Stage 3, there is evidence of aeolian reworking but no evidence of any significant fluvial activity adjacent to these dunes. Since then the dunes have remained essentially stationary with their surfaces partially stabilised by vegetation. What is surprising is that despite clear evidence of considerable aeolian reworking of the upper parts of these dunes since the end of the Stage 3 subpluvial (<30 ka) (Fig. 5.4), the dunes remain. TL dates indicate that aeolian reworking, without replenishment from channel sands, has occurred through to the present day. While South Narberry dune has all but disappeared, Chookoo and Durham dunes remain dominant features on the landscape despite the gradual vertical accretion of floodplain mud. While they may have been once larger features, they are clearly long-lived and stable. While their sand has been mobile for tens of thousands of years, the dune feature itself has managed to resist complete deflation over this period. However, there there may have been more dunes in the past than are visible today, for it is difficult to determine the proportion that may have survived such continued aeolian activity. 145 Clearly, the dunes have been active during the very late Pleistocene and Holocene but because of the lack of a sand source, it appears that this was a period of reworking of the existing dunes. Floodplain dunes may have an important role in channel development during the Holocene. For instance, scour channels were initiated when floodwaters were funnelled between the intervening area between dunes (Fig. 4.15) (Knighton and Nanson, 1994b). The additional flow competence between the dunes was sufficient to scour the muddy alluvium and generate shallow depressions which enlarged until such time as the constricting effects of the dunes on overland floods waned. An example of a recent or incipient scour channel is the small Chookoo channel between Chookoo Dunes 2 and 3 (Fig. 5.3) which was probably formed within the last century. The young coolibah (E. microtheca) trees colonising its banks are good indicators that this is probably a relatively young scour channel. In comparison, the large Chookoo channel between Dunes 1 and 2 is an older, larger-scaled scour channel with flow energy being less confined (Fig. 4.19). If scour channels were able to incise through to the underlying sand units, it is possible that the scour channels may become a more permanent channel feature, such as a perennial waterhole. The Narberry Waterhole is another example of a scour channel formed as a result of flow concentration between dunes (Fig. 4.30). This shallow waterhole also displays similar attributes to the large Chookoo channel in terms of its W/D ratio and the maturity of the coolibahs (E. microtheca) lining its banks (Figs 4.20 and 4.29). Finally, the 'islands' of truncated linear dunes with clay pans (shown in Fig. 4.15) on the southern margins of the Cooper Creek floodplain do not appear to be source-bordering for they are positioned within shallow alluvium over 146 bedrock. This appears to be the case at both the Baryulah (Figs 4.15 and 4.16) and Bolan (Fig. 4.17) sites. 5.5 A westward shift of the Cooper Creek floodplain Knighton and Nanson (1994b) hypothesised that the mud-laden Cooper floodplain is migrating westward and invading numerous dune fields in the process. However, they acknowledge that at their time of writing "... given the piecemeal nature of the evidence, the migration hypothesis cannot be regarded as definitive" (p. 323). If the floodplain migration theory is to be substantiated, some thought must be given to its cause. Evidence from various sources is presented in relation to whether both the channel and floodplain have shifted westwards and, if so, what might be the cause. Evidence for a progressive westward shift of the Cooper is apparent in several forms. Along the Durham Road transect (Fig. 4.6) the 'yotmgrng' of TL ages in the upper floodplain muds from east to west at equivalent depths of 1.5-3.0 m provides support for the proposal that the active part of the floodplain has moved or contracted westward over the past -100 ka. Ages range from 100+-18 ka (W1701) in the basal muds and 38.9±3.1 ka (W1700) near the surface in the east, to 32.7+2.4 ka (W1704) near the base and 7.8±0.8 ka (W1703) near the surface in the centre of the floodplain transect and 2.5±0.3 ka (W1706) near the surface of the floodplain in the west (Fig. 4.6). The TL date of 100±18 ka (W1701) at the base of the clay unit in the eastern floodplain represents the oldest floodplain clay date obtained thus far from the Cooper Creek floodplain and contrasts dramatically with the youngest date obtained at 2.5+-0.3 ka in the west (Fig. 4.6). TL dates of 10.5+0.9 ka (W2030) (Fig. 4.7), and 5.3±0.4 ka (W2053) and an AMS 14C date of 1.42±0.6 ka BP (Sample Beta-84639; Fig. 4.14) from the western margin of the Cooper floodplain further illustrate its relatively young age. 147 The trend across the Shire Road transect is not so clear cut with regards to the ages obtained from the floodplain clays (Fig. 4.7). However, the position of an approximate 80 ka isochron shows that the active floodplain appears to have shifted or contracted westwards in a similar fashion to 35 km farther north along the Durham Road. Aerial reconnaissance, aerial photographs and satellite imagery (Fig. 5.9) of the floodplain between Durham Downs and the Shire Road provides further evidence of a westward trend to floodplain sedimentation. The muds from the Cooper floodwaters are invading the mouths of and blocking tributaries on the western side of the valley, whereas those tributaries on the eastern side are prograding low-gradient alluvial fans onto the Cooper floodplain. A sediment lobe actively invading the mouth of one unnamed western tributary south of the main bridge crossing of the Cooper channel along the Shire Road in July 1996 measured 0.5-0.8 m high, 1.5 m wide and 20 m long and consisted of inter- fingered mud and sandy alluvium oriented info the tributary. This suggests that, despite the infrequent nature of floods, the Cooper plays a role in controlling sedimentation in the mouths of these western tributaries, effectively blocking their lower reaches. Undoubtedly the most dramatic evidence for this western invasion is the encroachment of the Cooper flood system into Lake Yamma Yamma (Fig. 4.1) and, as reported by Knighton and Nanson (1994b), into the adjacent longitudinal dunefield. The origin of Lake Yamma Yamma is as yet unknown, however, it is clear that it owes much of its contemporary lacustrine-playa character to not infrequent incursions of floodwater from the western side of the Cooper system. 148 a a a c >> < CO " ^•1C.-.":V--- M«s .-.PIS "'* *Sfc.tfW ^ 1 a c •>'^^«^- ,.f;! s o T3 • a" v C 2 a B tji w> a cu 5 o rS a P o ° rt|rl|.S , o« o ° 03 '43 ft O » £ oflC >> h C O^rS 3 O t3 ° _, 3 -H OJ rr-J 3r^U ft O O -C rH O 0 M +- 0)0(3 S wi ft^ OJ ft co U 4-1 ^ "S 2 fi °rQ O c "5 C T3 53 R O °rg 1 2 gj S > S -5 .> ,* a T3 "3 i QJ ft w "-1 S o o ^^ « -^ ° g « s s QJ^3£ 8 OJ R cj ,2 S O r-H3 ^ a 2^ 3 QJ - ,-J T3 .r-> -H CO ftd^S ra o ^+i to 3 fc 2 g, s IS "3 fc a. 60 QJ QJ QJ 3 « The relatively small, single-channelled catchments of the western tributaries have average basin lengths of -5-10 kms, with floodplain widths generally <1 km. These dimensions are in contrast to the multi-channelled eastern tributaries with average basin lengths of 30-40 km and floodplain widths of 3-5 km. Both Wareena (Fig. 5.9) and Okena Creeks have well-developed multi channel distributary systems which merge with and deposit sediment on the eastern Cooper floodplain, forming extensive depositional fans 3-5 km in length and 5-7 km in width. These eastern tributary catchments are very much larger and tend to exert a significant influence on sediment loads, flow hydrology and surface morphology of the eastern Cooper floodplain. In contrast, there are no catchments of any great size nor splays of any significance on the western side of the Cooper valley within the study area (Fig. 5.9). Aerial observations of a moderate Cooper Creek flood sourced by monsoon rains in the headwaters of the catchment in January-February, 1997 revealed floodwaters being forced upstream for several kilometres in several of the western tributaries while the eastern tributaries remained relatively unaffected. Evidence for greater frequency of flows and sediment transport on the western side of the valley suggests that vertical accretion of the Cooper floodplain would be more rapid here than to the east. The western Durham and Shire Road transects (Figs 4.6 and 4.7) are accreting four times faster than the eastern floodplain, rates of 0.019-0.023 m/100 yr, and 0.004-0.006 m/100 yr respectively (Figs 5.1 and 5.4). Furthermore, fluvial aggradation has been concentrated in the western third of the floodplain since Stage 3 (Fig. 5.10). However, detailed surveys reveal no slope across the floodplain (Fig. 5.11). If the present surface is horizontal but vertical accretion rates for the past in the western portion of the floodplain were four times faster than in the east then there can be only two possibilities: 150 Cooper Creek Floodplain near Naccowlah IJMeringhina W.H. 27°20'- ) 'ogaller W.H. Karmona" ) :/»:30-::>.«•.•; .170- • •'-.'( 2 0 k a &l?S?:&'^'- ?. . . . HShire Road 27° 30' - •.::-•/. :•:.•/ .""*•'.• *• V\Naccowlah Goonbabina W.H. / Naccowlah W.H. <= i/ 0 10 27°40' km 141° 50' 142° 00" 142°10' I i Figure 5.10 - Contraction of fluvial activity to the western third of the Cooper floodplain especially since Stage 3. 151 o rH fe o CO x CO •i*n 13 0 X rt cd ri c 0 o a £ +H o +H a ra o cd o rH rraH OJ o rfi rH bX3 bJO o bJj rC ra 53 ra OJ Q X CO X 0) rH w "rWa u ra o u o o +H +H QJ cu T3 o rH a QJ > o O o O u -S i cu CO +J CO CO C CO O r- O u o rH o o r£ >> CO 3 cu C rH 3 3 Q r £ o 00 IN \0 LO < 6 IN IN S N S 152 • the alluvial surface in the west was lower in the past, has accreted more rapidly than the east and by coincidence is now exactly the same elevation right across the flooplain (an extremely unlikely conicidence). • the western side of the valley is subsiding more rapidly than the east and that sedimentation is maintaining an equilibrium rate of accretion across an unevenly subsiding basin. The stratigraphy on the western compared to the eastern sides of the Cooper floodplain in the study area reflects their different depositional histories. The eastern Shire Road auger holes (SR6-SR8; Fig. 4.7) and Lignum Creek (Fig. 4.18) exhibit tough, almost indurated profiles where the stratigraphic units are mostly thin, poorly-sorted and with sharp erosional/reactivation surfaces displaying small amounts of trough-cross and tabular bedding structures and often containing abundant gypsum and calcrete. Many of the interfaces represent reactivation surfaces with sharp contacts implying erosion, reworking and deposition by shallow, high energy localised fluvial processes. This contrasts with the western Shire Road (SR1-SR4; Fig. 4.7) where there are extensive sand deposits with numerous large bedding structures (see SR3-T; Fig. 4.8) including epsilon cross-beds and upward-fining sequences indicative of much larger more laterally-active, palaeo-meandering mixed and sand-load fluvial systems. Limited calcrete and gypsum induration of these deposits suggests they have remained moist for longer. Hence, the distinctive stratigraphic units in the eastern portion of the floodplain display a fluvial signature of a much flashier tributary flow regime compared to those in the west that reflect large prolonged flows in the main part of the Cooper channel system (Fig. 4.7). 153 CZ) b IN c CN w uo o ra o 0 1— CO CO II cc UJ QJ (0 > 3 o r2 Ml 3 o "ra W b CM o CM rH 3 ra W b o QJ 3 +H • rH W) 3 O l—I 3 cu QJ rO QJ -H ra 3 _o CJ QJ C/3 I cfl C/3 o rH O C o CJ 1/5 (fi I/) 0 rH u r>> OJ r>N cd > CJ CJ u O s- o 0 uCJ CJSH fas, 154 The reason for the apparent shift in the active Cooper floodplain westward is unclear, however, possible explanations are proposed. Firstly, subtle tectonism beneath the Cooper floodplain during the late Quaternary (possibly due to earth movements along a fault on the eastern side of the Cooper valley; Figs 2.5 and 5.9) may be uplifting the eastern side of the valley causing the active Cooper anabranches to shift towards the west. Figure 5.12 presents three cross- sections of the Cooper valley which illustrates differences in elevation between the eastern and western tributary valley systems. Uplift would explain the larger catchment sizes to the east of the Cooper. However, no terraces have been observed in the catchments of these eastern tributaries. Also, detailed surveying across the floodplain revealed no significant change in elevation across either the Durham Road or Shire Road transects (Fig. 5.11). Furthermore, no differences in the elevation of Tertiary geological units (Glendower and Winton Formations; Fig. 2.5) could be identified on either side of the Cooper valley syncline to invoke a neotectonic cause for the westward shift of the floodplain. The second suggestion is that subsidence is occurring in the western portion of the Cooper floodplain. In addition the larger elevated tributary catchments on the eastern side of the valley have undoubtedly produced more sediment for deposition on the floodplain. With subsidence in the western floodplain, the deposition of alluvial fans along the eastern margin could have deflected the active channels westward and in the process contributed to the drowning of the western tributaries. While the reason for larger tributary valleys on the eastern side of the Cooper valley is not clear, the subsidence explanation appears to be the most plausible for the westward shift of floodplain sediments during the last 100 ka. 155 Chapter 6. Summary and Regional Context 6.1 Introduction This study provides 13 fully exposed and logged excavated sections, 120 auger and up-hole descriptions and 75 TL dates, and as such provides the most detailed description of an alluvial stratigraphy for any Quaternary sedimentary basin in Australia. It provides an excellent opportunity to detail the Quaternary development of a monsoon fed, semi-arid, alluvial-aeolian depositional environment over more than two glacial cycles. The dominant sediment types on the Cooper floodplain are broadly divisible into fluvial sands, fluvial muds and aeolian sands. The dominance of alluvial mud deposition is climatically related to periods of relative aridity while extensive reworking and deposition of fluvial sands relate to periods of enhanced flow (Nanson et ah, 1988). Of the 37 fluvial-sand TL dates presented in this study, 15 dated between the Stage 5/4 boundary and the Bruhnes Normal Chron at -780 ka (Bassinot et ah, 1994), and 7 are from the penultimate glacial Stage 6 (Fig 6.1). The Stage 6 glacial period is generally interpreted as relatively dry with only limited fluvial activity (Nanson et ah, 1992b) however, no Stage 6 aeolian deposits have been identified in the Cooper sequences. It is possible that reworking of the relatively isolated and exposed dune sands effectively rejuvenated them or that lateral river activity reworked them since Stage 6. Furthermore, evidence presented here shows Stage 6 to have had some fluvial activity associated with it, possibly during interstadial periods. It is here assumed that the frequency of TL ages presented for a particular sediment type is indicative of the level of depositional activity. 156 £ CS u © «5 •r* s s Of .** <4-l rJ H o c o •r* S- o (qs66T 'O66T'986T) Aprils siq i •p j?a UOSUPM inspg ajAg a^fBq jjSBaqjjJOjy 157 The majority of TL dates from fluvial deposits on Cooper Creek occur in Stage 5 with 10 of these dates ranging from only 105 to 90 ka in mid Stage 5. The pooled mean age of the pluvial Stage 5 dates is 92.3 ka, -30 ka after the peak of the last interglacial maximum at 126 ka. This is some 17 ka after the pooled mean age obtained for Channel Country rivers, including the Diamantina River and Warburton Creek, by Nanson et ah, (1992 a, 1992b). However, most of the Stage 5 dates in the present study come from the most recently reworked western portion of the floodplain at Chookoo and near the western end of the Shire Road transect. In combination, those obtained by Nanson et ah, (1986, 1988, 1992a, 1992b) and those in this study support the contention by Nanson et ah, (1992b) and Kershaw and Nanson (1993) that the peak of fluvial activity postdated the temperature peak (-126 ka) of the last interglacial. The stratigraphic and chronological data suggests that wide, shallow, laterally active, palaeochannels operated during mid to late Stage 5 (Figs 5.5, 5.6 and 5.7). It is difficult to establish the onset of recent aridity on the Cooper Creek floodplain. The earliest recorded aeolian-sand TL date from this study is 83.9±6.7 ka (W1709) from the main Chookoo Dune (Fig. 4.21) in late Stage 5 (Fig. 5.8). However, as stated above, aeolian dunes are vulnerable to reworking by wind and destruction by channel migration and therefore the lack of evidence for aeolian activity here prior to late Stage 5 may distort the true picture. The abundance of fluvial muds in preference to fluvial sands is also believed to be an indication of increasing aridity; the presently semi-arid environment floodplain is entirely dominated by mud deposition. However, the upper-floodplain muds are also vulnerable to reworking by subsequent channel migration. The earliest fluvial mud TL date obtained in this study was a date of 100±18 ka (W1701) from mud immediately overlying fluvial sands on the eastern margin of the floodplain (Durham Road transect; Figs 4.6 and 5.8). Two TL dates from mud probably deposited during the brief and fluvially relatively 158 inactive Stage 4 stadial between 74 to 60 ka, are the next oldest. Out of a total of 19 fluvial mud and 19 aeolian-sand TL dates in the present study, only one mud and three aeolian-sand dates were obtained for the period prior to Stage 4. The possibility of reworking not withstanding, this does lend support to an argument for increasing aridity and a far less fluvially active channel system on Cooper Creek since the end of Stage 5. Regardless of whether aridity was increasing following Stage 5, the abundance of mud and aeolian sand indicates that conditions were relatively arid. However, this period was punctuated by what appear to be phases of sand- channel activity from early-Stage 3 or late Stage 4 (-65 to 58 ka; at Mt Howitt and on the western Shire Road transect) to later Stage 3 (50 to 35 ka; e.g., southern Chookoo palaeochannel; Figs 4.21, 5.8 and 5.10). Evidence of both aeolian and fluvial activity is readily apparent during Stage 3 (Figs 5.8 and 6.1). While the oldest source-bordering dunes on Cooper Creek date at between 90 and 80 ka, and although they probably existed before then, they appeared to have reached the peak of their development between 50 and 35 ka if the number of dates for this period is any indication. In terms of the frequency of dates, there is a correspondence between peak fluvial mud and fluvial sand deposition between 40 and 30 ka, as reworking and deposition of both sand and overbank muds probably occurred during this period, especially in the western margins of the Cooper floodplain (Shire Road transect; Fig. 4.7). The Stage 3 subpluvial was probably not as humid or pronounced a pluvial phase as was Stage 5. Alluvial sand dates for Stage 3 are less widely distributed across the floodplain than are those from Stage 5, being restricted to the more active western side of the Shire Road transect (SRI-3; Figs 4.7 and 5.10), with a late Stage 4-early Stage 3 channel at the Mt Howitt site (Fig. 4.13). 159 The former sinuous, laterally active, sand-load channels at the Mt Howitt site (Figs 4.12 and 4.13) had bankfull channel widths of -60 m and depths of -4-5 m. They were supplied by more regular flows than that supplying the contemporary Channel Country rivers. It is not clear whether the Mt Howitt channel is part of an anabranching network of channels or whether it is transporting flow from the eastern tributaries towards the main channel of the Cooper that probably lay towards the western side of the floodplain, as shown on the Durham Road and Shire Road transects (Figs 4.6 and 4.7) farther south. A Stage 5 TL date of 95.4±8.9 ka (W2289) was obtained from the outer cut bank of a buried channel (Figs 4.12 and 4.13) containing point bar deposits, dated at 63.1±8.9 ka (W2290) and 60.1±6.4 ka (W2291), indicating that the Stage 3 channel was migrating into an older Stage 5 alluvial unit. TL dates from the clayey channel infill at Mt Howitt indicate a surprisingly recent period of infilling. Mud samples taken at depths of 0.5 m, 2.5 m and 4.5 m gave TL dates of 0.33±0.04 ka (W2292), 2.6+0.2 ka (W2293) and 16.7±1.5 ka (W2294) respectively, (Fig. 4.13) much younger than the -60 ka dates for the point bar sediments beneath. It appears that this early Stage 3 channel continued to operate as a mud lined, non-migrating channel until late Stage 2, infilling in the Holocene. A similar stratigraphy and chronology for the mud infilling of a channel was noted by Nanson et al. (1988) at the Longreach Waterhole on the Thomson River some 400 km to the north. Mud and aeolian sand dates are prominent during Stages 2 and 1 (Fig. 5.8) with only two fluvial-sand TL dates being recorded in this period. The latter, 10.5±0.9 ka (W2030) and 5.3±0.4 ka (W2053), come from only a few hundred metres from the present position of the main Cooper channel on the western margin which has been fluvially active much later than other parts of the floodplain (Figs 4.7 and 4.14). The next oldest period of fluvial sand activity 160 recorded in this study was at 29.9±2.6 ka (W2037) on the western Shire Road, some -19 ka earlier in late Stage 3 (Fig. 5.8). In conclusion to this section, the TL chronology and stratigraphy presents: • an extensive fluvial sand TL record dating back to the Bruhnes Norm Chron at -780 ka (Bassinot et ah, 1994) and thus represents the oldest and one of the most detailed sequences of Quaternary fluvial activity in Australia. • a revealing but low resolution TL-based interpretation of fluvial activity during interglacial Stage 7 and glacial Stage 6. • evidence that the interglacial maximum during Stage 5e (-126 ka) was probably not especially humid or fluvially active compared to that later in Stage 5. • evidence from the last full glacial cycle of two dominant periods of fluvial sand activity, one in mid to late-Stage 5 and a lesser one in Stage 3, with fluvially relatively quiescent periods in late Stage 5-early Stage 4 and again during most of Stages 2 and 1. • evidence of aeolian sands and fluvial muds first appearing in the record in mid to late Stage 5, in the form of source-bordering dunes associated with fluvial activity at that time. • evidence that the present interglacial probable maximum (the Holocene) has been fluvially very inactive with no development of source-bordering dunes. 161 • evidence that from prior to Stage 5 that the Cooper channel/floodplain system appears to have been much more fluvially active than during Stage 5, and that it has been contracting westward due to increasing aridity. 6.2 Comparison with other Quaternary studies in central, northern and eastern Australia Until recently, Quaternary chronologies have been limited to -30 ka because of a reliance on ^C dating. However, recent developments in sedimentary luminescence dating has enabled researchers to push back the chronological boundary to the last full glacial cycle (130 ka to present) and, with less precision, to the mid-Quaternary. In this section, comparisons are made between the research results of this study and published findings for the Channel Country and elsewhere in the Lake Eyre Basin, and for northern Australia and southeastern Australia. 6.2.1 Lake Eyre Basin In the last few decades there has been a steady increase in TL studies of Quaternary depositional environments within the Lake Eyre Basin. These include studies of aeolian, lacustrine and fluvial sedimentary environments watered by both monsoonal and westerly-fed catchments of the Lake Eyre Basin (Rust and Nanson, 1986; Gardner et ah, 1987; Nanson et ah, 1988, 1992a, 1992b, 1998; Wasson and Donnelly, 1988; Wasson, 1989; Chen et ah, 1990; Magee et ah, 1995; Croke et ah, 1996,1998,1999; Magee and Miller, 1998). In this section, a regional comparison between the TL data from this study and those of Nanson et ah (1988, 1992b) on the Channel Country rivers of the northeast Lake Eyre Basin, Gardner et al. (1987) and Wasson (1989) on the arid dunes of central Australia, and Croke et al. (1996, 1998, 1999) on the Lower 162 Neales River of western Lake Eyre will be presented. Additional TL dates from Williams Point at Madigan Gulf, Lake Eyre (Magee et ah, 1995) and the beach ridges of the playa lakes in the Lake Eyre Basin (Magee and Miller, 1998; Nanson et ah, 1998), are also discussed. Initially, TL data from the Channel Country rivers are considered by comparing the TL frequency histograms of Kershaw and Nanson (1993) and Nanson et al. (1990,1992b) with the TL dates presented in this study (Fig. 6.1). In general, the TL dates reported here correspond to pluvial episodes defined by Nanson et al. (1988,1990,1992b) with enhanced periods of fluvial activity dominating the last two interglacials (Oxygen Isotope Stages 5 and 7) however, results from this study suggest fluvial activity continuing to some extent between the pronounced pluvials. In combination, these flows resulted in an extensive subsurface alluvial sand unit described in detail in Chapter 4 and reported by Nanson et al. (1992b) as the Katipiri Formation (Nanson and Tooth, 1999). Although there is broad agreement between the two TL data sets, there are however some notable differences (Fig. 6.1): • both studies show significant fluvial activity beyond 200 ka in Stages 7 and 8 although this study suggests that rather than peaking in the penultimate interglacial as suggested by Nanson et ah, (1990, 1992b) there was more continuous activity that continued through Stage 6. • both studies show a peak of substantial fluvial activity in Stage 5 but the results here suggest that the peak occurred at about 92 ka, 18 ka after that reported by Nanson et al. (1988,1990,1992b) at 109 ka. • the peaks of aeolian and fluvial activity rarely coincide with low and high sea level stands associated with glacial and interglacial maximums, 163 respectively, often lagging behind these lowstands and highstands by several thousands of years. Chappell (1991) reported that coastal dune and near-shore marine systems also took several thousand years to equilibrate after sea level stopped rising. • both studies suggest a subpluvial Stage 3 which, based on the evidence from this study, peaked at -40-30 ka, • whereas Nanson et ah (1988, 1990, 1992b) found no evidence for significant Holocene fluvial activity, this study shows some Holocene (Stage 1) fluvial activity evident very close to the present position of the main channel of Cooper Creek. It should be remembered that the majority of the TL evidence of Nanson et al. (1988, 1990, 1992b) was from the upper reaches of Cooper Creek and the Diamantina River and also from Warburton Creek on the lower Diamantina River, with only 4 TL dates located within the present study area of the middle Cooper and each of these from locations distal to the main channel. These 4 dates were 193±73 ka (W551), 239+96 ka (W552), 232±29 ka (W566) and 252±74 ka (W567) from the sandy Katipiri Formation underlying the mud in the eastern floodplain. They were some of the oldest fluvial TL dates presented by Nanson et al. (1988,1990,1992b) and agree with evidence from this study that shows the eastern side of the floodplain to be formed of basal sands older than Stage 5. They also support the contention that the active Cooper floodplain has contracted westwards since Stage 6 and 7. Nanson et al. (1988) obtained a TL age of 274±22 ka (W627) from a dune near Diamantina Lakes on the upper Diamantina River. The lack of dune ages in this study greater than 90 ka suggests that the preservation potential of original 164 aeolian dunes decreases towards the arid centre of the Lake Eyre Basin due to aeolian reworking (Nanson et ah, 1995). Major fluvial activity during Stage 5 appears to exist in most TL data sets throughout the basin. Apart from the Stage 5 alluvial TL chronology of the Cooper and Diamantina systems, this phase is tentatively correlated with lacustrine facies at Williams Point (Magee et ah, 1995) and possibly indicates a western expansion of Lake Eyre North during this lacustrine and fluvial episode (Croke et ah, 1996, 1998, 1999). Additional but spatially limited fluvial activity was noted in early Stage 3 and from late Stage 3 to present, however further research is required to determine the distribution of the subpluvial Stage 3 throughout the Lake Eyre Basin (Croke et ah, 1996,1998,1999). In this study, the majority of TL dates from aeolian facies were found to date between -30 ka to present with the oldest age for aeolian activity being -60 ka (Stage 3). This compares well with Wasson's (1986) study which showed that dune-building occurred during the period 9-34 ka peaking at 20 ka, while Callen and Nanson (1992) and Magee et al. (1995) identified aeolian facies throughout the Lake Eyre Basin from about 60-50 ka. Although the early to mid-Stage 3 (60 to 32 ka) was identified as a fluvially enhanced period across Australia by Nanson et al. (1992b) and Kershaw and Nanson (1993), they found no actual stratigraphic or chronological evidence for Stage 3 fluvial activity on the Cooper floodplain and only limited evidence within the headwaters of the Diamantina. This led Nanson et al. (1988, 1990, 1992b) to suggest that the Stage 3 pluvial episode diminished in its effectiveness northwards. Nanson and Tooth's (1999) recent assertion, that the Stage 3 fluvial episode in the Channel Country was most active in the headwaters and less so farther downstream is not supported by the present results where a small but significant phase of Stage 3 fluvial activity was detected in the middle reaches 165 of the Cooper (Fig. 6.1). However, compared to the Stage 5 pluvial, the influence of the Stage 3 subpluvial was more localised than even Stage 5 and confined to the western portions of the floodplain (Shire Road transect; Fig. 4.7) of the middle Cooper, possibly associated with the continued westward migration of fluvial activity and contraction of the fluvial system as aridity has progressively increased following Stage 7 and 6. The latter has resulted in the development of the mud-dominated fluvial regime which persists to the present day (Fig. 5.10). It appears that the fluvially active Stage 3 restricted mostly to the western side of the floodplain was an important source of sand for the development of source-bordering dunes such as the Chookoo and Durham dunes between 50 and 35 ka. These aeolian landforms may also suggest that the Stage 3 fluvial episode was more arid than previous ones, although the reworking of older dunes, has to be taken into account. Finally, the two most recent fluvial sand TL dates of 5.3±0.4 ka (W2053) and 10.5+0.9 ka (W2030) near the present main channel of Cooper Creek on the western side of the floodplain emphasises that Cooper Creek has undergone little activity during the Holocene (Figs 4.7,4.14,5.8 and 5.10). 6.2.2 Northern Australia Climatic changes that characterised monsoonal northern Australia during the late Quaternary are poorly understood. In comparison to Quaternary climatic studies in the Lake Eyre Basin and those from southeastern Australia, there are few detailed studies of Quaternary climatic history of the monsoonal areas of northern Australia. Research in the tropical-monsoon valley of Magela Creek, a major tributary of the east Alligator River in the Northern Territory, shows that from -300 kyr, cyclical episodes of channel incision and sediment transport have become less 166 effective due to increasing aridity in the late Quaternary (Roberts, 1991; Nanson et ah, 1993a). Subsequent episodes of channel incision and channel infilling in Magela Creek resulted from a combination of sea-level fluctuations and variable monsoonal activity. The most recent underlying palaeochannel began filling at -8 ka (Roberts, 1991; Nanson et ah, 1993a) when sea level was -12 m below present (Woodroffe et ah, 1987) and continued to do so rapidly as stream competence warned with drier conditions prevailing in the late Holocene, and backwater effects resulting from rising sea-level. The reduced flow regime and rising sea level in the late Holocene resulted in the latest phase of alluvially- dammed tributary lakes and deferred junction tributary streams in the mouth of Magela Creek emphasising the reduced erosional effectiveness of coastal streams in monsoonal northern Australia (Woodroffe et ah, 1985,1987; Roberts, 1991). Wende et al. (1997) report that the 'White Sands' dune near Cabbage Tree Creek in the eastern Kimberley region of Western Australia provides evidence of pronounced fluvial activity during Stage 3 (-37 ka). This was followed by a phase of aeolian activity at the last glacial maximum (-22 ka), a return to a fluvial phase in the early to mid Holocene (12-5 ka) and finally aeolian activity again in the mid to late Holocene (-6 ka to present). Kershaw (1978) identified the Last Glacial Maximum as a period of pronounced aridity in northeastern tropical Australia with the early Holocene being wetter than at present. More recent work by Nanson et al. (1991,1993a), Nott and Price (1994) and Nott et ah (1996,1999) identify enhanced fluvial activity in the early to mid Holocene, but that by Nott and co-workers also found enhanced fluvial activity occurring around the time of the Last Glacial Maximum . Studies reported here (Kershaw, 1978; Lees et ah, 1990; Nanson et ah, 1991, 1993a, 1995; Roberts, 1991; Schulmeister, 1992, 1999; Wasson, 1992; Nott and 167 Price, 1994,1999; Schulmeister and Lees, 1995; Nott et ah, 1996,1999; Nott and Roberts, 1996; Wende et ah, 1997) highlight several important aspects of climate change from the late Pleistocene to present in monsoonal northern Australia: • Stage 5 fluvial activity appears to have been pronounced in the region (Roberts, 1991; Nanson et ah, 1993a). • The last 30 kyr were dominated by alternating periods of extreme and relatively low magnitude flood events which are linked to changes in glacial-interglacial cycles and sea level and also variations in the intensification of the northern Australian monsoon (Nott et ah, 1996; Nott and Price, 1999). • Although it was not a major fluvial episode, there is evidence of enhanced fluvial activity during Stage 3 probably associated with intensification of monsoonal activity relative to the present (Nanson et ah, 1991). • There was an enhanced late Pleistocene episode of coastal dune development in northern Australia between 24 and 17 ka which is coincident with the last glacial maximum and associated low sea level (Lees et ah, 1990). • In Stage 2, there is evidence for post-last glacial maximum fluvial activity (-22-18 ka) although this tended to decrease until -17 ka (Nott and Price, 1994,1999) with Lees et ah, (1990) reporting conflicting evidence of enhanced coastal dune development between 24-17 ka. • Holocene (Stage 1) provides conflicting evidence for enhanced fluvial and aeolian activity. However, there is mounting evidence that the late to mid Holocene was significantly drier than the early to mid Holocene (Nanson et 168 ah, 1991,1993a; Schulmeister, 1992, 1999; Nott and Price, 1994; Schulmeister and Lees, 1995; Nott et ah, 1996; Wende et ah, 1997). Possible anthropogenic impacts may be a factor in accounting for the variability of the Holocene climatic record (Wende et ah, 1997). 6.2.3 Southeastern Australia A detailed study of the Riverine Plain palaeochannels of the Murray and Murrumbidgee Rivers by Page et al. (1991,1996) and Page (1994) have identified 4 phases of fluvial activity since the penultimate glacial. These phases (shown in Fig. 6.2) are: • mid to late Stage 5 (105-80 ka) but mostly in late Stage 5 (Coleambally), • Stage 3 (55-35 ka) (Kerarbury), • late Stage 3 to early Stage 2 (35-25 ka) (Gum Creek) and, • late Stage 2 (20-13 ka) (Yanco). Aeolian activity associated with the uppermost sequences of source-bordering dunes and lunette on the Riverine Plain (the cores of the dunes were not dated) is largely confined to the period 40-11 ka (Page et ah, 1991, 1996; Page, 1994) with a concentration of these dates between 20 and 12 ka (Fig. 6.2). This suggests that enhanced aeolian activity during the Last Glacial Maximum persisted until close to the boundary of the Oxygen Isotope Stages 1 and 2 (Wasson, 1989; Page et ah, 1996). These original dunes and lunettes on the Riverine Plain are probably older, contemporaneous with the age of the channels that supplied their sand, but have been by aeolian activity between 20- 12 ka. Nanson and Young (1987) found evidence for fluvial activity in Stages 5 and 3 within the Cranebrook Terrace of the Nepean River near Penrith, west of 169 Regional comparison of TL frequency histograms Oxygen Isotope Stages | (Bassinot et al. 1994) NEALES n=i5 RIVER, WEST LAKE EYRE •CHANNEL COUNTRY' NORTHEAST LAKE EYRE BASIN RIVERINE PLAIN, SOUTHERN N.S.W. HUON SEA LEVEL CURVE (ChappeU and Shackle ton, 1986) T—1—1—1—1—r~i—n—rn—1—1—1—rn—1—1—1 120 140 160 180 200 220 240 260 280 300k a Total No. of TL dates=is8 Figure 6.2 - Regional TL frequency histograms from this study combined with those from Nanson et al. (1988,1990,1992b)from eastern Lake Eyre basin and compared with thosefrom the west of Lake Eyre (Croke et al. 1996,1999) and elsewhere in southern Australia and classified according to the depositional processes: fluvial and aeolian sands from chronological studies in central and southeastern Australia, and compared to the Huon sea level curve (Chappell and Shackleton, 1986). 170 Sydney. Since original publication of this work, the TL chronology has been substantially revised (Nanson et ah, 1990). Extensive exposures revealed a laterally active Stage 5 braided channel system with flow regimes estimated to be several times larger than present with flow competences to transport coarse gravels. In Stage 3, a less dynamic fluvial phase was able to rework the western half of the Cranebrook Terrace including the basal gravels. Enhanced fluvial activity in Stage 2 between 21 and 12 ka was able to rework the overbank fines but not the basal gravels in this western portion. These periods of Stage 5, 3 and 2 activity are similar to the Riverine Plain findings of Page et al. (1991,1996) and Page (1994). Since Stage 2, fluvial conditions on the Nepean River have been comparable to the present (Nanson and Young, 1987). While Stages 5 and 3 show periods of fluvial activity in southeastern Australia similar to those in northern and central Australia, the two regions show little synchoneity in Stages 2 and 1. In northern Australia there was a marked decline in fluvial activity after the Last Glacial Maximum with intensification again at -16 ka and in the early Holocene (Nanson et ah, 1991, 1993a; Nott and Price, 1994; Nott et ah, 1996,1999). The TL frequency histogram (Fig. 6.2) allows comparisons between major episodes of climate change in central and southeastern Australia to the sea level curve (Chappell and Shackleton, 1986) and oxygen isotope stages (Martinson et ah, 1987). In total there are 157 TL dates presented in Figure 6.2 derived from a variety of sources including 75 from the current study of Cooper Creek combined with those from elsewhere in the Lake Eyre Basin (Nanson et ah, 1988, 1992b, 1996; Croke et ah, 1996, 1998), and those from the Riverine Plain and the Nepean River in southeastern Australia (Page et ah, 1991, 1996; Page, 1994; Page and Nanson, 1996). The TL evidence presented in Figure 6.2 indicates that: 171 Fluvial activity has been ongoing in the Channel Country with extensive sand bodies indicative of fluvial conditions much more active than the present for most of the middle and late Quaternary. There is clear evidence for a pronounced period of fluvial activity in Stage 5 throughout all the study areas of the Lake Eyre Basin including the western side (Neales River) (Croke et ah, 1996, 1998) and in Lake Eyre itself where a major lacustrine phase has been reported by Magee et al. (1995) and Nanson et al. (1998). Evidence of pronounced aeolian activity starts at about 90 ka and reaches a peak at around the Last Glacial Maximum. It is probable, given the very considerable age of dunes on the periphery of Lake Eyre Basin, that aeolian activity started before 90 ka. It does appear that there has been a progressive increase in aridity in central Australia which has spread coastward since at least Stage 5 (Nanson et ah, 1992b; Kershaw and Nanson, 1993). There is little correspondence between the almost coincident peaks of aeolian and fluvial activity and higher and lower sea levels linked with glacial-interglacial extremes. The stadial Stage 4 period appears to have been relatively inactive in terms of aeolian processes, however, glacial Stage 2 which is of similar duration to Stage 4, provides sedimentological evidence for having been much more active. Interstadial Stage 3 provides sedimentological evidence for a combination of fluvial and aeolian activity which is not surprising given that the dunes have derived their sand from active channels. 172 6.3 A model of Quaternary floodplain development of Cooper Creek The development of the Cooper Creek floodplain in southwest Queensland provides an extensive stratigraphic and chronological record of fluvial and aeolian activity from the mid-Quaternary to the present (Figs 5.6 and 5.7). Pre-Stage 7/8 (Mid-Quaternary): Extensive alluvial deposits of medium to very coarse sands and grits dating between -800-400 ka at depths of 30-15 m are indicative of active, sandy bed- load braided or meandering river systems. Lateral migration accompanying slow vertical accretion would have resulted in these rivers depositing their bed- material load while progressively reworking their overbank muds during hydrological regimes much more active than at present. An abundance of carbonised wood in certain drill holes suggests a forested floodplain environment. Evidence from the Shire Road transect (Fig. 4.7) indicates that in the mid-Quaternary, the active floodplain was some 15 to 30+ m below the present surface in a more confined bedrock valley with a floodplain width of probably about 10-11 km compared to the present 14 km width. The resolution available for TL dating from the mid-Quaternary, and the slow vertical accretion relative to lateral reworking of any overbank muds, make it impossible to say whether there were markedly wet and dry phases or a relatively continuously active fluvial episode. Stage 7/8: Nanson et al. (1992b) have interpreted Stage 7/8 to have been a distinctively fluvial period with enhanced flows in the Channel Country rivers. While this 173 study generally supports this contention, very few deposits from Stage 7 were located and dated. Furthermore, the large TL errors associated with such old TL samples makes it difficult to confine ages to particular isotope stages beyond Stage 5 or possibly Stage 6. As a consequence, it is not possible to say if the dates from this time are from Stage 7 or Stage 8. Nevertheless, the results obtained from Nanson et al. (1992b) and Callen and Nanson (1992) suggest Stage 7/8 was at least in part a discrete episode of fluvial activity characterised by extensive sandy braided and meandering channels providing abundant evidence of lateral activity. As with the pre-Stage 7/8 deposits, an absence of mud units does not mean that overbank deposits were not present but that lateral migration could have reworked them, leaving only the basal sands (Fig. 5.6). Mud intraclasts and mud balls within the sand body suggest this to have been the case. Deposits that date prior to Stage 6 extend across the floodplain at the Shire Road transect (Fig. 4.7) and are much more widespread than later fluvial deposits, suggesting that this level of fluvial activity was not repeated during the fluvial episodes of Stages 5 and 3. The laterally-active meander forms, probably remnant of declining conditions at the end of Stage 7, were very much larger than any meander forms present today (Rust and Nanson, 1986). Calculations of palaeodischarges based on stratigraphic evidence of the likely channel dimensions give values three times bankfull discharges of Meringhina Waterhole, the only location today where the entire channel discharge of the Cooper in the study area occurs within a single channel. Stage 6: This was a period of some 60 ka (Martinson et ah, 1987; Bassinot et ah, 1994) and chronostratigraphic evidence indicates that it too contained episodes of considerable fluvial activity similar to those in the earlier Stage 7 interglacial. Deposition resulting from laterally-active sandy rivers of upward-fining facies are readily apparent in the sedimentary profiles visible along the Shire Road 174 transect and at Chookoo dune. Given the length of Stage 6 and the errors associated with TL dates from Stages 6 and 7, it is difficult to differentiate between units within these stages. Just as the last glacial cycle has had an interstadial and fluvial episode in Stage 3, so Stage 6, nominally a 'glacial', was characterised by one significant interstadial of higher sea level at about 175-160 ka (Petit et ah, 1990). Of the six fluvial dates from within Stage 6, three of them lie within this 15,000 year period which was a period of higher sea levels globally (Chappell, 1991). Stage 5: As identified in earlier work by Nanson et al. (1988; 1992b), Figure 5.8 reveals very clearly the existence of a pronounced fluvial phase during Stage 5. Of the 15 new fluvial TL dates from the 56 ka time span of Stage 5, ten of them lie in the 15,000 year period between 105 and 90 ka suggesting that this was a period of marked fluvial activity. Stage 5 deposits were not encountered by Rust and Nanson (1986) in their investigation of the study area, for their excavations were restricted to the older eastern side of the floodplain. What is noteworthy is that Stage 5 fluvial deposits are in relative abundance only on the western side of the floodplain at less than 10 m depth (Figs 4.7 and 5.10). The Stage 6 and 7 alluvium is more widespread, occurring at relatively shallow depths on the eastern side of the floodplain and on the western side at depth. Associated with late Stage 5 (-85 ka) was the onset of source-bordering dune development, at the Chookoo Sandhill, Durham dunes and at the Tooley Wooley dunes (-85 ka). Earlier aeolian dunes may have been present on the floodplain but if that was so, they were probably reworked by subsequent fluvial and aeolian activity. Also of note is that overbank mud deposition has been preserved from as early as Stage 5, as evidenced by a date of -100 ka in muds in the eastern side of the floodplain (Durham Road transect, Fig. 4.6) a finding which supports similar old overbank deposits reported by Rust and Nanson (1986) from the eastern 175 floodplain. In the wetter periods of Stage 5 it is likely that the Cooper was perennial, therefore, the western floodplain would be the product of continuous flows and substantial reworking, the eastern floodplain receiving only overbank muds at that time. Stage 4: This stadial appears to have been relatively dry and fluvially inactive on Cooper Creek, with only one alluvial TL date reported from this period (Fig. 5.8). Since the dunes on the Cooper are source-bordering and supplied from the channels, it seems from the lack of aeolian TL dates representing this period that dune activity along the rivers may also have been low. Alternatively, dunes were active then but reworked in Stage 3. Stage 3: With the onset of greater runoff from -60 ka to -26 ka (the last interstadial), it appears that renewed fluvial activity commenced on the western side of the floodplain forming channels of fine-grained sand and floodplain mud, an episode associated with the formation of source-bordering aeolian dunes. Floodplain muds become sequentially younger westwards (Fig. 5.12), commensurate with the westward shift of active channels (Durham Road and Shire Road transects; Figs 4.6 and 4.7, respectively). The renewal of an alluvial sand supply may also have been the reason, perhaps along with seasonal aridity, for the abundance of source-bordering dune activity during Stage 3. Stages 2 and 1: After late Stage 3, conditions became particularly arid and fluvially relatively inactive, judging by the lack of alluvium dating from this time. Contemporary 176 anastomosing channels and waterholes with high mud banks and sandy but not highly mobile beds, were restricted mostly to the western side of the valley, although a complex system of braid-like flood-channels reworked the pedogenic clay aggregates across much of the remaining floodplain surface. Almost no evidence exists for alluvial sand activity during Stage 1 (there is one date of 10.5±0.9 ka only a few hundred metres from the main western channel and another of 5.3±0.4 ka at Goonbabinna Waterhole which is part of the main western channel). There is abundant evidence for the reworking of source- bordering dunes and the deposition of floodplain muds during Stages 1 and 2 (Fig. 5.8). Thus, with increasing aridity and declining flows at the end of Stage 3 and in Stage 2, Cooper Creek converted from a mixed-load river with abundant sand transport on the western side of the floodplain, flowing probably single- channelled but a relatively large and laterally-active fluvial system with erodible banks and source-bordering dunes, to the present low-energy, mud- dominated, laterally-inactive anastomosing system. 6.4 Some important additional points 1) The early fluvial episodes in Stage 7 and even within the Stage 6 interstadial appear to have been much more effective at reworking alluvium than were similar but less energetic events in Stage 5 and 3. 2) Evidence from Nanson et al. (1992b) suggests that maximum fluvial activity during Stage 5 occurred at about 110 ka while evidence presented here (Figs 5.8 and 6.1), suggest the maximum occurred between 105 and 90 ka. Both results are substantially later than the Stage 5 temperature optimum at -126 ka. Of course, rivers will continue to rework their alluvium even following a period of maximum activity, therefore the results could be recording the latter stages of fluvial activity rather than the peak period. However, it is noteworthy that of 177 the 16 TL dates from Stage 5 fluvial sediments obtained in this study only one is older than 110 ka. 3) Although the Holocene 'interglacial' commenced about 12 ka and sea levels reached their maximum at about 7 ka (Chappell, 1991), there is no sign yet of the Cooper reverting to a laterally active sand-transporting phase characteristic of previous interglacials and interstadials. As noted by Nanson et al. (1992b), either: • Australia is drying out and this interglacial will be drier than the previous one, or • if the suggestion from the prior record is correct, pluvial events occur later in the interglacials than do temperature maxima, and we may therefore not expect a dominant pluvial episode for another 5-10 ka. Indeed, both propositions could be correct. From evidence of the active Cooper channels contracting westwards, it does appear that the last three interglacials have been characterised by progressively decreasing episodes of fluvial activity, and that Australia has been drying out in an attenuating cyclical fashion. 6.5 Conclusion The Cooper floodplain provides a detailed record of fluvial and aeolian activity that suggests that Australia's Quaternary climate was very much wetter than at present during the mid Quaternary. From at least Stage 7 climate has oscillated between dry and wet (pluvial) episodes probably broadly linked to the worldwide glacials and interglacials, respectively. Fluvially active conditions with large sandy bedload discharges appear to have dominated parts of the last two interglacials (Stages 5 and 7) and probably an interstadial within the penultimate glacial (Stage 6). During Stage 5, fluvial activity appears to have peaked in the mid to latter part, considerably after the temperature maximum 178 at -126 ka. Stage 5 was not as energetic as previous fluvial episodes, for the Cooper only reworked about half of its floodplain within the vicinity of the Shire Road during this period. Following a brief arid phase in Stage 4, a period of moderate fluvial activity and sand transport during Stage 3 was even more restricted in lateral reworking of the floodplain than Stage 5, confined mostly to the far western side of the floodplain (Fig. 5.10) and a tributary system near Mt Howitt in the east. Following this, there was an intensification of aridity associated with the end of Stage 3 and with the last glacial in Stage 2. Sand-bed channels reverted to numerous low-energy inset anastomosing channels while surficial braided flood channels transported mud over extensive floodplains (Figs 5.6 and 5.7), a condition that has prevailed throughout the Holocene. Cooper Creek has provided the detailed record of tropical and sub-tropical climatic and associated flow regime changes in central Australia. It compares well with the Riverine Plain where a similar detailed record has been presented for southeastern Australia. Both regions present convincing evidence that Australian flow regime changes in the late Quaternary have been cyclical, probably associated with the broad astronomical cycles that have driven the world's major glacial events. In addition to other regions extending from the far north of Australia to the southeast, Cooper Creek suggests strongly that Australia has been drying out and flow regime declining in an alternating fashion over the past several glacial cycles. 179 References Aitken, M. J. (1985) Thermoluminescence Dating, Academic Press, London. Aitken, M. J. (1990) Science-based Dating in Archaeology, Longman, London and New York. Allan, R. J. 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