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Browsing by Author "Nanson, GC"

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Continental aridification and the vanishing of Australia's megalakes
(Geological Society of America, 2011-02) Cohen, TJ; Nanson, GC; Jansen, JD; Jones, BG; Jacobs, Z; Treble, PC; Price, DM; May, JH; Smith, AM; Ayliffe, LK; Hellstrom, JC
The nature of the Australian climate at about the time of rapid megafaunal extinctions and humans arriving in Australia is poorly understood and is an important element in the contentious debate as to whether humans or climate caused the extinctions. Here we present a new paleoshoreline chronology that extends over the past 100 k.y. for Lake Mega-Frome, the coalescence of Lakes Frome, Blanche, Callabonna and Gregory, in the southern latitudes of central Australia. We show that Lake Mega-Frome was connected for the last time to adjacent Lake Eyre at 50–47 ka, forming the largest remaining interconnected system of paleolakes on the Australian continent. The final disconnection and a progressive drop in the level of Lake Mega-Frome represents a major climate shift to aridification that coincided with the arrival of humans and the demise of the megafauna. The supply of moisture to the Australian continent at various times in the Quaternary has commonly been ascribed to an enhanced monsoon. This study, in combination with other paleoclimate data, provides reliable evidence for periods of enhanced tropical and enhanced Southern Ocean sources of water filling these lakes at different times during the last full glacial cycle. © 2011, Geological Society of America
Discordant 26Al/10Be ratios as an indicator of bedrock plucking: case studies from northern Australia
(Australian Nuclear Science and Technology Organisation, 2021-11-17) Fujioka, T; May, JH; Fink, D; Nanson, GC
With the number of multi cosmogenic nuclide data from detrital samples increased, it becomes apparent that discordant ²⁶ Al/¹⁰ Be ratios are not exception but common. Traditionally, depressed ²⁶ Al/¹⁰ Be ratios, below the nominal production ratio of 6.8, have been interpreted as prior burial. However, in northern Australia, such scenario is highly unlikely as the region is subject to intensive annual floods and therefore river channels normally lack long-term sediment storage. Another possible interpretation for lower ²⁶ Al/¹⁰ Be ratios is non-steady state erosion, such as stochastic bedrock plucking. Such interpretation has an important implication to the sediment source and production mechanism, and therefore to the basin-wide erosion rate approach that assumes a steady-state erosion at sediment source. In this study, we measured ¹⁰ Be and ²⁶ Al concentrations from three different waterfall sites in northern Australia with contrasting lithological and physical characteristics, in an attempt to capture depleted ²⁶ Al/¹⁰ Be ratios. Our results indicate that ²⁶ Al/¹⁰ Be ratios from two sandstone-dominated sites show consistently lower values (4.3-6.1), consistent with non-steady erosion interpretation, whereas data from a quartzite-dominated site (5.9-6.9) are indistinguishable from steady-state interpretation. Detrital samples collected downstream at each site indicate the similar trend as respective bedrock sites, implying that sediments are largely derived from the waterfall bedrock surfaces. In this paper, we discuss the results in the context of lithological difference and physical erosion mechanism between the sites. © The Authors
Erosion and the sediment conveyor in central Australia
(Geological Society of Australia, 2016-02-29) Jansen, JD; Chappell, J; Struck, M; Eccleshall, SV; Fujioka, T; Codilean, AT; Fülöp, RH; Fink, D; Cohen, TJ; Nanson, GC
Why are the Neogene sedimentary fills across central Australia generally thin and discontinuous? One long-standing explanation is that sluggish tectonism and intensified aridity have combined to suppress rates of erosion and sediment production yielding a landscape crowded with inherited, preMiocene forms. Quantifying rates of sediment production, residence time and transport is possible with numerous methods, but the recent growth of cosmogenic nuclide (CN) analysis has provided unprecedented quantitative insights to rates of landscape evolution. Measurements of in situ produced cosmogenic 10Be and 26Al integrate rates of surface processes over million-year timescales—the last part of the Neogene in which aridity has strengthened across the continental interior. We present a compilation of ~600 published and unpublished 10Be and 26Al measurements from central Australia with a focus on the Neogene Eyre Basin and its periphery. Outlying and inlying bedrock uplands serve as engines of sediment production via erosion of bedrock. Surrounding the bedrock outcrops are vast sediment conveyors of varying efficiency and tempo: hillslopes, pediments, and alluvial fans are interim storage/burial zones for sediment in transit to the network of low-gradient rivers, dunes, and playas towards base level. Interactions between fluvial and aeolian processes are especially pertinent to sediment flux in the Eyre Basin. Major rivers such as the Cooper and Finke traverse dunefields in their lower reaches where quantities of alluvia are recirculated into dunes and vice versa. Tracking the trajectories of sediment from source-to-sink (including aeolian recirculation) remains a major challenge, but is central to unravelling the sedimentary dynamics of central Australia's Neogene basins. Based on the CN compilation we estimate 1) spatially averaged erosion rates at the scale of a hillslope or river catchment; 2) pointbased erosion rates on bedrock surfaces; 3) residence time of sediment in hillslope regolith and alluvial fans; and 4) cumulative burial history of sediments in transit. Catchment-scale erosion rates (n~100) are consistently low (<10 m/Myr) and include some of the lowest rates ever measured (~0.3 m/Myr); however, a small group of catchments in the Flinders Ras yield higher erosion rates (~30–60 m/Myr). Bedrock hillslopes (n~200) tend to erode even slower (<5 m/Myr), with a subset of Flinders Ras sites again being the exception (~10–30 m/Myr) and suggesting the influence of recent tectonism. Several CN depth-profiles measured on hillslopes and alluvial fans indicate sediment residence times >0.5 Myr, and high-resolution sampling along three hillslopes with differing morphology (linear, convex, and concave) reveals major variations in sediment production and transport rates that hint at the long-term evolution. In the rivers, fluvial sediments show a weak tendency to increase cumulative burial history downstream (1–2 Myr), consistent with the expanding accommodation space for storage and burial. Dune sediments sampled in the Simpson and Tirari dunefields (n~16) contain cumulative burial histories (up to 1.5 Myr) similar to that of the intersecting rivers. This points to an intimate mix of fluvial and aeolian processes in areas approaching base level. Curiously, these sediments occur in the lowest part of the continent and contain the longest histories of cumulative burial, yet do not form part of the thickest sedimentary fills in the Eyre Basin.
Flood-flipped boulders: in-situ cosmogenic nuclide modeling of flood deposits in the monsoon tropics of Australia
(Geological Society of America, 2014-12-04) Fujioka, T; Fink, D; Nanson, GC; Mifsud, C; Wende, R
During the Quaternary, extreme floods along the Durack River, in the Kimberley, northern Australia, dislodged, transported, and stacked massive meter-sized boulders from the underlying bedrock channel floor. Field evidence identified a population of the boulders to have been overturned after detachment. We measured in situ cosmogenic 10Be and 26Al concentrations in six imbricated boulders to constrain the timing of flood events. We present a simple numerical model that simultaneously solves the expressions for the predicted nuclide concentrations from the exposed and hidden surfaces of a flipped boulder to calculate the time since it was overturned. The ability of the model to unequivocally discern whether a boulder was overturned depends on boulder thickness and the site-specific steady-state erosion rate. Of the six boulders sampled, our model successfully determined four finite flip ages, whereas the other two boulders indicated steady state and were either not flipped or flipped sufficiently long ago for the nuclide profile to have returned to steady state. While the two older model ages (ca. 150 ka and ca. 260 ka) are strongly sensitive to assumptions made for the local erosion rate correction, the two younger flip ages, 5.6 ± 1.0 ka and 10.3 ± 1.9 ka, are robust against such corrections. Early to mid-Holocene major floods have been recorded in other parts of northern Australia. We suggest that similar Holocene floods occurred in the Kimberley and that such extreme events may have been widespread in northern Australia in the late Quaternary. Our boulder-flip model can be applicable to similar deposits associated with other extreme events such as paleo-tsunamis. © 2014, Geological Society of America.
The floodplain sedimentology of Cooper Creek. Why billabongs (waterholes) in the Channel Country start and stop
(Universidad Nacional de Tucumán, Fundación Miguel Lillo, 2009-08-24) Nanson, GC; Larsen, JR; Cendón, DI; Fagan, SD; Jones, BR
The Channel Country of western Queensland consists of a low gradient anastomosing channels inset in muddy floodplains in a desert climate. Annual transmission losses are large, averaging 75-80% of total volume along a ~400 km reach of Cooper Creek. Billabongs (waterholes) are common and consist of local channel expansions in the mud-lined anastomosing network, as well as isolated scour-channels on a floodplain that is formed of 2-3m of clay-rich mud. Both the primary anabranching and overbank flood channels carry water during large floods but only the billabongs store water for long periods of drought, making them vital to the ecology and agriculture of the region. Despite penetrating a very extensive underlying Pleistocene-age sand body, they are impermeable much of the time. However, transects away from several billabongs reveal marked increases in salinity and provide compelling evidence that they act as fresh-water ‘entry valves’ through an otherwise impermeable muddy floodplain. The base of each billabong is scoured during floods, enabling large volumes of surface water to be dumped into the 10 m deep saline aquifer. As flows decline the billabongs self-seal with mud such that fresh water can be stored for a year or more. Transmission losses along Cooper Creek were initially believed to be due to evaporation, however, the lack of any solute concentration between gauging stations along the ~400 study reach, suggests that of the ~1.8 km3 of surface flow lost from the ~3.0 km3 entering the upper end of the study reach, most leaks through the base of the billabongs. Such massive losses also account for why the billabongs in the form of large and efficient channels, terminate after just a few kilometres. They reform repeatedly at locations where conditions are suitable, but their numbers decline downstream in accordance with the loss of total flow-volume.
Freshwater recharge into a shallow saline groundwater system, Cooper Creek floodplain, Queensland, Australia
(Elsevier, 2010-10-15) Cendón, DI; Larsen, JR; Jones, BG; Nanson, GC; Rickleman, D; Hankin, SI; Pueyo, JJ; Maroulis, J
Freshwater lenses have been identified as having penetrated the shallow regional saline groundwater beneath the Cooper Creek floodplain near Ballera (south-west Queensland). Piezometers were installed to evaluate the major-element chemistry along a floodplain transect from a major waterhole (Goonbabinna) to a smaller waterhole (Chookoo) associated with a sand dune complex. The floodplain consists of 2–7 m of impermeable mud underlain by unconsolidated fluvial sands with a saline watertable. Waterholes have in places scoured into the floodplain. The transect reveals that groundwater recharge takes place through the base of the waterholes at times of flood scour, but not through the floodplain mud. Total dissolved solids rise with distance from the waterhole and independently of the presence of sand dunes. Stable water isotopes (δ2H and δ18O) confirm that recharge is consistent with, and dependant on, monsoonal flooding events. Following floods, the waterholes self-seal and retain water for extended periods, with sulfate-δ34S and δ18O isotopes suggesting bacterial reduction processes within the hyporheic zone, and limited interaction between the surface water and groundwater during no-flow conditions. The area occupied by the freshwater lenses (TDS < 5000 mg/L) is locally asymmetrical with respect to the channel flow direction, extending down gradient along distances of 300 m. © 2010, Elsevier Ltd.
Geomorphic controls on groundwater evolution in the arid Cooper Creek system, SW Queensland, Australia: inferences from element and stable isotope hydrogeochemistry
(International Association of Hydrogeologists, 2007-09-17) Cendón, DI; Larsen, JR; Jones, BG; Rickleman, D; Nanson, GC
Quaternary climatic changes have had a remarkable impact on the biological and geomorphological evolution of the Australian continent, and in turn can exhibit considerable control on the current hydrological cycle. In the absence of glaciation, changes in precipitation and wind strength have resulted in alternating fluvial, aeolian and lacustrine deposits over much of inland Australia. In the currently arid anabranching floodplain-channel system of Cooper Creek (SW-Queensland), this is manifest as extensive late Pleistocene fluvial and aeolian sand bodies overlain by floodplain and channel mud deposits. The alluvial muds are the result of the much reduced Holocene transport capacity of the Cooper Creek system, and are punctuated at the surface by remnant aeolian sand dunes which are stratigraphically connected to the underlying sand bodies. These Quaternary sand bodies (Chookoo dune-floodplain complex) have in turn become the main aquifers for the region, where the water table is ~10-12m below the floodplain surface. The presence of shallow groundwater is especially crucial for ecosystems in arid environments because evaporation quickly removes any available surface waters. Considering the importance of this resource, and the fragility of the hydrological cycle in arid zones, the shallow groundwaters in this region have received surprisingly little attention. This study aims to determine the basic recharge/evaporation processes of the Chookoo dune-floodplain-channel system using major and minor element chemistry together with water stable isotopes (δ18O, δ2H) and dissolved sulfate isotopes (sulfate-δ34S and δ18O). We hypothesise that groundwater recharge predominately occurs as diffuse rainfall infiltration via the dunes while chemical variations also occur through the dunes, modifying the original chemistry of the recharged water. Presented chemical data suggests that the main channel of the creek has little or no hydraulic connectivity with the shallow aquifers except during large flood events when the mud seal over their base is scoured and fresh water temporarily recharged. Major-element chemistry: All waters are Na-Cl-rich with appreciable amounts of Ca and SO4. All major elements increase along a transect from the sand dunes to the floodplain within the same aquifer. In general, major element ratios show a marine derived signal for groundwater, while a few surface water samples deviate from marine ratios (Fig. 1A). This difference is interpreted as an event based signature, with most solutes incorporated from dissolution of surfaces salts. Evaporation models also provide evidence of major element evolution (Fig. 1B). Simple evaporation of surface waters cannot reproduce the concentrations found in the groundwater. Only when a mixture of surface and groundwater and/or dissolution of previously precipitated salts along the recharge path are considered, do the evaporation models match with the observed concentrations.
Integration of ice-core, marine and terrestrial records for the Australian Last Glacial Maximum and Termination: a contribution from the OZ INTIMATE group
(Wiley, 2006-10) Turney, CSM; Haberle, SG; Fink, D; Kershaw, AP; Barbetti, M; Barrows, TT; Black, M; Cohen, TJ; Corrège, T; Hesse, PP; Hua, Q; Johnston, R; Morgan, VI; Moss, PT; Nanson, GC; van Ommen, TD; Rule, S; Williams, NJ; Zhao, JX; D'Costa, D; Feng, YX; Gagan, MK; Mooney, SD; Xia, Q
The degree to which Southern Hemisphere climatic changes during the end of the last glacial period and early Holocene (30-8 ka) were influenced or initiated by events occurring in the high latitudes of the Northern Hemisphere is a complex issue. There is conflicting evidence for the degree of hemispheric ‘teleconnection’ and an unresolved debate as to the principle forcing mechanism(s). The available hypotheses are difficult to test robustly, however, because the few detailed palaeoclimatic records in the Southern Hemisphere are widely dispersed and lack duplication. Here we present climatic and environmental reconstructions from across Australia, a key region of the Southern Hemisphere because of the range of environments it covers and the potentially important role regional atmospheric and oceanic controls play in global climate change. We identify a general scheme of events for the end of the last glacial period and early Holocene but a detailed reconstruction proved problematic. Significant progress in climate quantification and geochronological control is now urgently required to robustly investigate change through this period. © 2006 John Wiley & Sons, Ltd.
Lake Quaternary mega-lakes fed by the northern and southern river systems of central Australia: varying moisture sources and increased continental aridity
(Elsevier Science BV, 2012-10-15) Cohen, TJ; Nanson, GC; Jansen, JD; Jones, BG; Jacobs, Z; Larsen, JR; May, JH; Treble, PC; Price, DM; Smith, AM
Optically stimulated and thermoluminescence ages from relict shorelines, along with accelerator mass spectrometer C-14 ages from freshwater molluscs reveal a record of variable moisture sources supplied by northern and southern river systems to Lake Mega-frome in southern central Australia during the late Quaternary. Additional lacustrine, palynological and terrestrial proxies are used to reconstruct a record that extends back to 105 ka, confirming that Lakes Mega-frome and Mega-Eyre were joined to create the largest system of palaeolakes on the Australian continent as recently as 50-47 ka. The palaeohydrological record indicates a progressive shift to more arid conditions, with marked drying after 45 ka. Subsequently, lake Mega-Frome has filled independently at 33-31 ka and at the termination of the Last Glacial Maximum to volumes some 40 times those of today. Further sequentially declining filling episodes (to volumes 25-10 those of today) occurred immediately prior to the Younger Dryas stadial, in the mid Holocene and during the medieval climatic anomaly. Southern hemisphere summer insolation maxima are a poor predictor of palaeolake-filling episodes. An examination of multiple active moisture sources suggests that palaeolake phases were driven independently of insolation and at times by some combination of enhanced Southern Ocean circulation and strengthened tropical moisture sources. © 2012, Elsevier Ltd.
Land-atmosphere coupling during the last glacial maximum: an Australian perspective
(International Union For Quaternary Research (INQUA), 2011-07-21) Larsen, JR; Nanson, GC; Cendón, DI
The last glacial climate experienced extreme variability, however during the peak of the glacial conditions, the last glacial maximum (LGM), many areas of the Southern Hemisphere do not appear to experience corresponding minima in temperature and precipitation. This presents a considerable conundrum for our understanding of glacial climates, and more specifically for the land-atmosphere energy balance. In an effort to gain further insight, we examine potential scenarios for the water budget on the Australian continent during the LGM, and compare these with a variety of direct and proxy evidence for temperature, evaporation, and precipitation. Current research suggests some areas of Australia were wetter than expected during the LGM, however many researchers have also suggested precipitation in most areas was lower than the present. Thus in order to rectify the water balance, lower global surface air temperatures have been used to infer correspondingly lower evaporation rates. We examine both the theoretical and the palaeoclimatic evidence for lower evaporation, and find that in many cases, the relationship between lower temperature and evaporation is not straightforward. If the global hydrological cycle was slowed by lower global temperatures, then reduced overall actual evaporation could potentially increase atmospheric capacity depending on the vapour pressure deficit. Furthermore, atmospheric demand strongly depends on whether the environment is water limited or energy limited, and since much of the Australian continent is water limited, we expect actual evaporation to be much more sensitive to other external forcings such as wind speed and solar irradiance than temperature. These results have significant implications for the continental water balance, and alterative scenarios for precipitation, runoff, evaporation, vegetation and atmospheric feedback are proposed.
Late quaternary aeolian and fluvial interactions on the Cooper Creek Fan and the association between linear and source-bordering dunes, Strzelecki Desert, Australia
(Elsevier, 2010-02) Cohen, TJ; Nanson, GC; Larsen, JR; Jones, BG; Price, DM; Coleman, M; Pietsch, TJ
The Innamincka Dome and associated low-gradient fan in the Strzelecki Desert is the product of Cenozoic crustal warping that has aided formation of an extensive array of palaeochannels, source-bordering transverse dunes and superimposed linear dunes. These dunes have impeded the course of Cooper Creek and provided a repository of evidence for Quaternary climate change as well as the interactive processes between transverse and linear dune formation. At Turra, Gidgealpa and sites nearby are extensive fluvial and aeolian sand bodies that date from marine isotope stages (MIS) 8–3 and the Last Glacial Maximum (LGM) and are now surrounded or buried by overbank mud. The sandy alluvium was deposited on the downstream slope of the dome by large channels transporting abundant bedload, subsequently blown northward to form transverse dunes from what were probably seasonally-exposed bars in a palaeo-Cooper system. Thermoluminescence (TL) and optically stimulated luminescence (OSL) ages demonstrate that the base of the dune complex is at least MIS 7 in age (~250 ka) but that it has been subsequently reworked by wind with additional sand blown from the river. Source-bordering dunes formed during a period of enhanced river flow and sand supply from ~120 to 100 ka, with another short episode of the same at ~85–80 ka and from ~68 to 53. The LGM was associated with enhanced flows and the supply of dune sediment, from 28 to 18 ka. Pronounced river flow and dune activity occurred in the early to mid Holocene, but there is no evidence of dunes being supplied from Cooper Creek since the LGM. The dunes forming the oldest basal sand units appear to be largely transverse in form and are aligned roughly parallel to adjacent east–west trending palaeochannels. Linear dunes have formed from and over these, and yield basal ages ranging from MIS 5 or MIS 4 but continuing to accrete and rework through to the Holocene. The study results in one of the few detailed chronological investigations of the interaction between transverse and linear dunes. It is apparent that long-distance sand transport has played no significant role in dune formation here for the linear dunes show no significant downwind decline in ages. Linear dunes appear to have accreted vertically from underlying transverse dunes. A wind-rift vertical accretion model with only minor lengthwise extension is the dominant mode of linear dune formation in this section of the Strzelecki Desert, the bulk of dune sediment being sourced from adjacent swales since the LGM. © 2010, Elsevier Ltd.
Late Quaternary landscape evolution in the Keep River region, northwestern Australia
(Elsevier, 2005-09) Ward, IAK; Nanson, GC; Head, LM; Fullagar, R; Price, DM; Fink, D
This paper evaluates the Late Quaternary chronostratigraphic context of archaeological sites in the Keep River region, Northern Territory, Australia. Cosmogenic dating, luminescence dating and sediment characterisation reveal sedimentary processes commencing from erosion of the escarpment and plateaux source through temporary storage in sand sheets, to final deposition in alluvial floodplains. Erosion of the sandstone plateaux (∼5 mm ka−1) and escarpment faces (probably ∼50–100 mm ka−1) provide the main sediment source for the adjacent sand sheets which have evolved over the past 100,000 years as the product of ongoing cycles of accumulation and denudation. The rate of sediment accumulation is lowest near the escarpments on the low-energy sediment-limited sand sheets (<100 mm ka−1) and greatest near the main streams (>400 mm ka−1) that have more numerous sediment sources. Collectively, luminescence ages indicate an apparent increase in sediment accumulation rate in the sand sheets from ∼100 mm ka−1 in the late Pleistocene to over 200 mm ka−1 in the Holocene. This most likely reflects enhanced monsoonal activity following postglacial marine transgression. Palaeosol horizons in the creek profile distinguished by sediment mottling mark potentially significant palaeoenvironmental and palaeoclimatic changes during the Quaternary. © 2004 Elsevier Ltd.
Late quaternary palaeoenvironmental change in the Australian drylands
(Elsevier, 2013-08-15) Fitzsimmons, KE; Cohen, TJ; Hesse, PP; Jansen, JD; Nanson, GC; May, JH; Barrows, TT; Haberlah, D; Hilgers, A; Kelly, T; Larsen, JR; Lomax, J; Treble, PC
In this paper we synthesise existing palaeoenvironmental data from the arid and semi-arid interior of the Australian continent for the period 40–0 ka. Moisture is the predominant variable controlling environmental change in the arid zone. Landscapes in this region respond more noticeably to changes in precipitation than to temperature. Depending on their location, arid zone records broadly respond to tropical monsoon-influenced climate regimes, the temperate latitude westerly systems, or a combination of both. The timing and extent of relatively arid and humid phases vary across the continent, in particular between the westerly wind-controlled temperate latitudes, and the interior and north which are influenced by tropically sourced precipitation. Relatively humid phases in the Murray-Darling Basin on the semi-arid margins, which were characterised by large rivers most likely fed by snow melt, prevailed from 40 ka to the Last Glacial Maximum (LGM), and from the deglacial to the mid Holocene. By contrast, the Lake Eyre basin in central Australia remained relatively dry throughout the last 40 ka, with lake high stands at Lake Frome around 35–30 ka, and parts of the deglacial period and the mid-Holocene. The LGM was characterised by widespread relative aridity and colder conditions, as evidenced by extensive desert dune activity and dust transport, lake level fall, and reduced but episodic fluvial activity. The climate of the deglacial period was spatially divergent. The southern part of the continent experienced a brief humid phase around ∼17–15 ka, followed by increased dune activity around ∼14–10 ka. This contrasts with the post-LGM persistence of arid conditions in the north, associated with a lapsed monsoon and reflected in lake level lows and reduced fluvial activity, followed by intensification of the monsoon and increasingly effective precipitation from ∼14 ka. Palaeoenvironmental change during the Holocene was also spatially variable. The early to mid-Holocene was, however, generally characterised by moderately humid conditions, demonstrated by lake level rise, source-bordering dune activity, and speleothem growth, persisting at different times across the continent. Increasingly arid conditions developed into the late Holocene, particularly in the central arid zone. © 2012 Elsevier Ltd.
Long-term waterfall dynamics in monsoonal Australia based on cosmogenic Be-10
(AMS-13 The Thirteenth International Conference on Accelerator Mass Spectrometry, 2014-08-24) Fujioka, T; May, JH; Fink, D; Nanson, GC; Jansen, JD; Codilean, AT
Extensive plateaus, arrays of escarpments and a variety of waterfalls are iconic to northern Australia. How old and stable are these features ? Tectonically, northern Australia has been quiescent during the Quaternary. Rainfall is highly seasonal and dominated by the summer monsoon. In this setting, regional landscape dynamics should be strongly afected by uctuations in monsoon and the associated uvial processes. Here, we examine timescales and processes of waterfall evolution in northern Australia. Situated in the Kimberley sandstone plateau, Durack Falls comprise a series of 1-3 m falls, while Bindoola Fall is a large 15 m fall. Surprise Creek, 100 km south of Darwin, has three 3-5 m waterfalls with deep plunge pools developed at the edge of a quartzite plateau. Over 30 samples were collected from bedrock straths up- and downstream of the waterfalls and on their headwall. Their 10Be exposure ages (assuming zero erosion) reveal contrasting results. While two waterfalls in the Kimberley show relatively young, variable ages (15-110 ka for Durack and 11-57 ka for Bindoola), Surprise Creek indicates old, but uniform ages (94-160 ka). Out-of-channel,undisturbed bedrock exhibits consistently high 10Be equivalent to steady-state erosion rates of 2-5 mm/ka, in agreement with typical bedrock erosion rates observed across Australia. Based on these data, we here present a model to evaluate process and rates of waterfalls formation, and discuss the controlling factors.
Lowland river responses to intraplate tectonism and climate forcing quantified with luminescence and cosmogenic 10Be
(Elesevier Science BV, 2013-03-15) Jansen, JD; Nanson, GC; Cohen, TJ; Fujioka, T; Fabel, D; Larsen, JR; Codilean, AT; Price, DM; Bowman, HH; May, JH; Gliganic, LA
Intraplate tectonism has produced large-scale folding that steers regional drainage systems, such as the 1600 km-long Cooper Ck, en route to Australia's continental depocentre at Lake Eyre. We apply cosmogenic Be-10 exposure dating in bedrock, and luminescence dating in sediment, to quantify the erosional and depositional response of Cooper Ck where it incises the rising Innamincka Dome. The detachment of bedrock joint-blocks during extreme floods governs the minimum rate of incision (17.4 +/- 6.5 mm/ky) estimated using a numerical model of episodic erosion calibrated with our 10Be measurements. The last big-flood phase occurred no earlier than similar to 112-121 ka. Upstream of the Innamincka Dome long-term rates of alluvial deposition, partly reflecting synclinal-basin subsidence, are estimated from 47 luminescence dates in sediments accumulated since similar to 270 ka. Sequestration of sediment in subsiding basins such as these may account for the lack of Quaternary accumulation in Lake Eyre, and moreover suggests that notions of a single primary depocentre at base-level may poorly represent lowland, arid-zone rivers. Over the period similar to 75-55 ka Cooper Ck changed from a bedload-dominant, laterally-active meandering river to a muddy anabranching channel network up to 60 km wide. We propose that this shift in river pattern was a product of base-level rise linked with the slowly deforming syncline-anticline structure, coupled with a climate-forced reduction in discharge. The uniform valley slope along this subsiding alluvial and rising bedrock system represents an adjustment between the relative rates of deformation and the ability of greatly enhanced flows at times during the Quaternary to incise the rising anticline. Hence, tectonic and climate controls are balanced in the long term. © 2013, Elsevier Ltd.
Modern depositional processes in a confined bedrock setting: benches of the Shoalhaven river
(International Association of Geomorphologists, 2013-08-27) Kermode, SJ; Cohen, TC; Reinfelds, IV; Nanson, GC; Jones, BJ
The lower Shoalhaven River provides an opportunity to examine bench processes in a confined setting. Stratigraphic analysis of trenches and augur holes, ground penetrating radar, Hec-RAS modelling and geochronological techniques combine to identify that benches of multiple levels along Bull Reach are composed of coarse material and have been extensively eroded and reworked by modern events. Kermode et al. (2012) established the long-term polycyclical nature of the higher alluvial surfaces (up to 193 ka in age), and this is contrasted with the youth of the lower inset alluvial surfaces, which are shown to be less than 270 years in age. This study evaluates the relative significance of both flood regime and effects of European settlement on the geomorphic effectiveness of high magnitude events and investigates the characteristics of bench formation in this confined setting. It characterises the nature of depositional events and the relationship between facies at an event scale. Using Hec-RAS modelling, events of different recurrence intervals are compared to explore the relative impact of varying flood magnitudes. The results bring into question the theory that inundation frequencies of these surfaces are constant, or associated with formative processes. © Authors
Overturned mega boulders on coastal cliff-tops and in bedrock river channels : can cosmogenic nuclides constrain tsunami and palaeo-flood
(12th International Conference on Accelerator Mass Spectrometry (AMS-12), 2011-03-23) Fink, D; Fujioka, T; Mifsud, C; Nanson, GC; Felton, A
Jacks Waterhole at the Durack River in the Kimberley region of north west Australia is an exposed bedrock paleo-channel, excavated by hydraulic plucking of well-jointed bedrock. This section of the channel consist of arc-shaped disconnected stacks of imbricated meter-sized slabs dislodged from bedrock steps immediately upstream of the boulder-filled channel. The semi-arid climate is punctuated by summer tropical storms/cyclones causing occasional violent floods. High magnitude floods with high flow velocities are required to erode and transport such large rock slabs. Along the south-eastern Australian coastline, at Little Beecroft Head, large detached sandstone boulders are found unconformably on horizontal cliff top escarpments and benches of identical lithology some 20-35 meters above present day sea-level. For some boulders, local stratigraphy indicates transport from the nearby cliff face, implicating tsunami or exceptional storm events. In other cases, an interpretation of differential erodibility along bedding strata, slow emergence and preservation from the contemporary platform is applicable. For both locations, detailed geomorphic mapping, cross-bedding orientation and tracing from the identified detachment site clearly indicates that boulders have experienced at least one flipping event. Consequently, previously buried surfaces are instantaneously exposed to an enhanced production rate of cosmogenic nuclides. The possibility of dating the ‘flipping’ event depends largely on a comparison of measured cosmogenic concentrations from 4 surfaces (upper and lower boulder, shielded and exposed bedrock) to that predicted on the boulder as a function of boulder thickness. In this paper, we describe our model and its sensitivity to boulder thickness, inheritance and postflipping time. Preliminary results of 10Be and 26Al analysis from flipped and non-flipped boulders at Jack’s Waterhole and Little Beecroft Head are given.Copyright (c) 2011 AMS12.
Overturned mega boulders on coastal cliff-tops and in bedrock river channels : can cosmogenic nuclides constrain tsunami and palaeo-flood events in Australia?
(18th INQUA Congress, 2011-07-21) Fink, D; Fujioka, T; Mifsud, C; Nanson, GC; Felton, A; Crook, KAW; Switzer, A
Jacks Waterhole at the Durack River in the Kimberley region of north west Australia is an exposed bedrock paleo-channel, excavated by hydraulic plucking of well-jointed bedrock. This section of the channel consist of arc-shaped disconnected stacks of imbricated meter-sized slabs dislodged from bedrock steps immediately upstream of the boulder-filled channel. The semi-arid climate is punctuated by summer tropical storms/cyclones causing occasional violent floods. High magnitude floods with high flow velocities are required to erode and transport such large rock slabs. Along the south-eastern Australian coastline, at Little Beecroft Head, large detached sandstone boulders are found unconformably on horizontal cliff top escarpments and benches of identical lithology some 20-35 meters above present day sea-level. For some boulders, local stratigraphy indicates transport from the nearby cliff face, implicating tsunami or exceptional storm events. In other cases, an interpretation of differential erodibility along bedding strata, slow emergence and preservation from the contemporary platform is applicable. For both locations, detailed geomorphic mapping, cross-bedding orientation and tracing from the identified detachment site clearly indicates that boulders have experienced at least one flipping event. Consequently, previously buried surfaces are instantaneously exposed to an enhanced production rate of cosmogenic nuclides. The possibility of dating the ‘flipping’ event depends largely on a comparison of measured cosmogenic concentrations from 4 surfaces (upper and lower boulder, shielded and exposed bedrock) to that predicted on the boulder as a function of boulder thickness. In this paper, we describe our model and its sensitivity to boulder thickness, inheritance and post-flipping time. Preliminary results of 10Be and 26Al analysis from flipped and non-flipped boulders at Jack’s Waterhole and Little Beecroft Head are given. Copyright (c) 2011 INQUA 18
Radiocarbon and geochemical constraints on shallow groundwater recharge in a large arid zone river, Cooper Creek, SW Queensland, Australia
(Copernicus Publications, 2010-05-02) Larsen, JR; Cendón, DI; Nanson, GC; Jones, B
In the arid and semi-arid internally drained Lake Eyre Basin of central Australia, large mud dominated anabranching river systems transport monsoon derived floodwaters into the centre of the continent during the summer months, and subsequently spend much of the year under low to no flow conditions. Cooper Creek has the largest catchment in this basin, and in south west Queensland has a wide (20-60km) floodplain and multiple channel system. Enlarged channel segments, known as waterholes or billabongs, can retain water throughout much of the dry season, and their mud base can often be scoured during floods into the underlying sandy alluvium where the shallow groundwater table exists 3-5m below the base of the waterholes. Little is known of the groundwater recharge mechanisms in this ecologically important and hydrologically unregulated river system, thus a number of piezometer transects were construct across the floodplain between two waterholes to investigate groundwater recharge processes in further detail. Samples recovered from all piezometers were analysed for major-trace element, water stable isotopes (δ2H and δ180), 3H and 14C. Water stable isotopes reveal shallow groundwater is recharged by high magnitude, low frequency monsoonal flood events, with minor evaporative enrichment probably linked to recent smaller flooding events. 14C dating of dissolved inorganic carbon reveals recharge is most effective beneath the deepest channel segments of the waterholes, and that residence time of the shallow groundwater increases with distance from major waterholes, with the post 1950’s 14C bomb pulse signature present only in close proximity to the channels. 3H allows further refinement of the shallow groundwater residence times, with no 3H detected in groundwater over ~500m from the waterholes, indicating groundwater recharge is slow and restricted to major flooding events. The increase in groundwater residence time with distance from waterholes, is also accompanied by an abrupt increase in salinity, and suggests recent recharge has formed local freshwater lenses above the regional, more saline groundwater. This increase in salinity with increasing distance from the waterholes is not accompanied by an increase on the evaporative signal of water stable isotopes, suggesting evapotranspiration is the dominant mechanism of salinisation within the shallow groundwater beneath the floodplains and minor channels. This study demonstrates that detailed chemical analysis of groundwaters from arid and semi arid areas can provide a useful estimate of recharge where the remote location makes traditional detailed borehole monitoring difficult or impossible to achieve.
Surface and groundwater hydrology of arid-zone billabongs (waterholes) in Queensland, Australia
(International Association of Geomorphologists, 2009-07-06) Larsen, JR; Cendón, DI; Nanson, GC; Jones, BG
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