anatomy of the buried burdekin river channel across the great barrier reef shelf how does a major river operate on a tropical mixed siliciclastic carbonate margin during sea level lowstand abstract a large palaeochannel on the northeastern australian continental shelf has been imaged by a series of shallow seismic reflection profiles the buried channel forms an important pleistocene route of the burdekin river and extends almost continuously for 160 km from the present coast to the outermost reef the channel floor profile steps across the shelf with alternating segments of gentle gradient flats and steeper gradient ramps channel sinuosity as interpreted from seismic records varies among segments between 1 and 1 72 with no consistent relationship between sinuosity and gradient the lower and upper parts of the channel fill have different geometry and reflection character suggesting channel excavation and initial filling occurred during a different regime than final filling in one section of the shelf about the 50 m isobath the channel is difficult to define and appears to have wandered significantly either because it has been modified by shoreface erosion ca 10 5 ka or because the river encountered a change in topography in front of karstified reefs as the channel passes between the numerous outer shelf reefs in water depths of 70 80 m it becomes progressively smaller conspicuously underfilled and absent entirely over the outermost 10 km of the shelf no discrete lowstand river mouth could be recognised on the present shelf edge the elevations of flat segments on the channel floor profile show considerable similarity to published elevations of stillstands or brief rises in sea level attained during the long term drawdown associated with the last glacial cycle 125 20 ka and are interpreted to have formed during this stepwise drop in sea level channels were cut and partially filled during the fall and lowstand and then backfilled during the holocene transgression the ancestral channel of the burdekin river therefore preserves a rare insight into the stratigraphic record of falling sea level during the last glacial

particle size related mineral magnetic source sediment linkages in the rhode river catchment maryland usa characterization of both the source materials and the bottom sediments of an estuary by magnetic characters on a particle size basis allows the source of the bottom sediments to be determined and indicates a shift in sediment sources corresponding to changing land use patterns the major source of the sediments in the lower part of cores from the middle of the estuary is eroding shorelines soil derived particulates from the uplands dominate the upper part of the cores this change in source occurred in the early part of the 19th century the magnetic character of the most recent sediments suggests a progressive depletion through time of the upland soils in the finest fractions

a pseudo thellier relative palaeointensity record and rock magnetic and geochemical parameters in relation to climate during the last 276 kyr in the azores region summary in the pseudo thellier method for relative palaeointensity determinations tauxe et al 1995 the slope of the nrm intensity left after af demagnetization versus arm intensity gained at the same peak field is used as a palaeointensity measure we tested this method on a marine core from the azores spanning the last 276 kyr we compared the pseudo thellier palaeointensity record with the conventional record obtained earlier by lehman et al 1996 who normalized nrm by sirm the two records show similar features intensity lows with deviating palaeomagnetic directions at 40 45 ka and at 180 190 ka the first interval is associated with the laschamps excursion while the 180 190 ka low represents the iceland basin excursion channell et al 1997 the pseudo thellier method in combination with a jackknife resampling scheme provides error estimates on the palaeointensity spectral analysis of the rock magnetic parameters and the palaeointensity estimates shows orbitally forced periods particularly 23 kyr for climatic precession this suggests that palaeointensity is still slightly contaminated by climate fuzzy c means cluster analysis of rock magnetic and geochemical parameters yields a seven cluster model of predominantly calcareous clusters and detrital clusters the clusters show a strong correlation with climate for example samples from detrital clusters predominantly appear during rapid warming although both the pseudo thellier palaeointensity m a and fuzzy clusters show climatic influences we have not been able to find an unambiguous connection between the clusters and m a

holocene depositional history of the burdekin river delta of northeastern australia a model for a low accommodation highstand delta abstract the burdekin river of northeastern australia has constructed a substantial delta during the holocene delta plain area 1260 km2 the vertical succession through this delta comprises 1 a basal coarse grained transgressive lag overlying a continental omission surface overlain by 2 a mud interval deposited as the coastal region was inundated by the postglacially rising sea in turn overlain by 3 a generally sharp based sand unit deposited principally in channel and mouth bar environments with lesser volumes of floodplain and coastal facies the holocene burdekin delta was constructed as a series of at least thirteen discrete delta lobes formed as the river avulsed each lobe consists of a composite sand body typically 5 8 m thick the oldest lobes formed during the latter stages of the postglacial sea level rise 10 5 5 kyr bp are larger than those formed during the highstand 5 5 3 kyr bp which are in turn larger than those formed during the most recent slight sea level lowering and stillstand 3 0 kyr bp radiocarbon ages and other stratigraphic data indicate that inter avulsion period has decreased through time coincident with the decrease in delta lobe area the primary control on holocene delta architecture appears to have been a change from a pluvial climate known to characterize the region 12 4 kyr bp to the present drier enso dominated climate in addition to decreasing the sediment supply via lower rates of chemical weathering this change may have contributed to the shorter avulsion period by facilitating extreme variability of discharge more frequent avulsion may also have been facilitated by the lengthening of the delta plain channels as the system prograded seaward

magnetic mineral tracing of sediment provenance in the central bengal fan abstract we present a comprehensive magnetic mineral analysis of surface sediments from 110 sites in the central bengal fan to trace sediment provenance multiple magnetic parameters including magnetic mineralogy concentration and grain size were generated rock magnetic measurements indicate that the dominant magnetic minerals within the studied sediments are detrital magnetite and titanomagnetite which is further confirmed directly by scanning electron microscope sem and transmission electron microscope tem observations on magnetic extracts fuzzy c means clustering analysis of magnetic parameters indicates that the study area can be divided into three clusters considering the position of active valley we further divide cluster 1 into clusters 1a and 1b cluster 1a primarily covers the west side of the active valley with relatively high magnetic concentrations and dominantly coarse grained magnetic minerals which likely correspond to basaltic materials derived from the deccan plateau from the godavari river mouth to the inner bay of bengal magnetic concentration gradually decreases with a fining trend of magnetic grain size clusters 1b 2 and 3 are on the east side of the active valley where magnetic concentration and magnetic grain size decrease with increasing distance to the active valley combined with published data in the indian region and the upper bengal fan we suggest that the main provenances are the himalayan and the deccan plateau whose contributions to sediment distribution are quantitatively assessed our analysis suggests that the position of active valley plays a key role in sediment distribution in the central bengal fan while the effect of ocean currents is negligible

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

sediment provenance in a tropical fluvial and marine context by magnetic fingerprinting of transportable sand fractions the sources and fluxes of sediment to the great barrier reef lagoon from northeastern australian rivers have been the subject of much concern and study with the large catchments of the burdekin and fitzroy rivers thought to be the key sources at present here the utility of newly developed magnetic fingerprinting methods for identifying sediment provenance both onshore and offshore and in association with individual large flood events is investigated within the burdekin catchment sediments are mobilized from different subcatchments by runoff generated by intense localized rainfall events magnetic measurements were made on untreated and acid treated samples of river channel sediments within the burdekin river subcatchments and from the estuarine and inner shelf depocentres of burdekin river sediments the acid treatment removes all discrete magnetic particles and coatings and leaves magnetic inclusions protected within host silicate grains as the basis of the measured magnetic signature of a sample the magnetic properties of the acid treated samples display statistically distinct sediment provenance groupings sand samples from the upper burdekin river appear magnetically distinct from samples from tributaries of the burdekin e g hann creek fanning river and also from nearby coastal rivers including the haughton suspended sand samples from a burdekin flood event in 2000 appear to have a different source compared with those from floods in 1998 and 1999 comparisons of the terrestrial acid treated sand fractions with the same acid treated sand size fractions from transects taken offshore suggest that the surface sediments in upstart bay and bowling green bay have different sources some of these sources are as yet unidentified but may represent the unsampled lower discharge south western burdekin subcatchments and or along shore drift of sand from the south perhaps even from the fitzroy river over millennial timescales of cyclone pumping the magnetic inclusion method precludes any obfuscation or confounding of sediment source which might arise from hydraulic sorting and or post depositional magnetic diagenesis or authigenesis

an extension of the insar based probability integral method and its application for predicting 3 d mining induced displacements under different extraction conditions underground extraction can be roughly classified into three types i e subcritical critical and supercritical extraction in accordance with the geological conditions in the overburden and the geometry of mined out areas in 2016 we proposed an approach based on the interferometric synthetic aperture radar insar technique and the probability integral method pim for the cost effective prediction of 3 d mining induced displacements abbreviated as insar pim due to the inherent assumption of critical extraction in the pim the insar pim method performs well in predicting the 3 d displacements caused by critical and or supercritical extraction but poorly for subcritical extraction in this paper we first propose a generalized pim gpim by modifying the traditional pim with a simplified boltzmann function we then replace the pim of the insar pim with the proposed gpim to develop an extension of insar pim referred as to insar gpim the insar gpim was tested in the qianyingzi coal mining area china the results show that the insar gpim predicted horizontal and vertical displacements caused by subcritical critical and supercritical extraction agree well with the in situ observations with average root mean square errors of about 0 032 and 0 050 m respectively these accuracies represent improvements of 60 9 and 59 when compared with the accuracies predicted by the insar pim in the horizontal and vertical directions the results indicate that the insar gpim is capable of accurately predicting 3 d mining induced displacements under different extraction conditions i e subcritical critical and supercritical extraction and it performs much better than the insar pim in the case of subcritical extraction it is therefore believed that insar gpim will have a wider scope of applications than the previous insar pim

fuzzy c means cluster analysis of early diagenetic effects on natural remanent magnetisation acquisition in a 1 1 myr piston core from the central mediterranean abstract the influence of early diagenesis on the natural remanent magnetisation nrm in sediments from the calabrian ridge central mediterranean is analysed with the help of fuzzy c means fcm cluster analysis and non linear mapping nlm the sediments are variably coloured white beige purplish greenish and grey layers occur with occasionally intercalated sapropels the nrm acquired depends on both depositional conditions and diagenetic processes to describe these fcm was performed with in arm caco3 ba mn and s as variables an eight cluster model was derived with the clusters belonging to two categories one expressing mainly diagenetic processes i e dissolution and precipitation and the other expressing mainly depositional conditions the impact of diagenesis on nrm acquisition is profound and not restricted to the close vicinity of the anoxic sapropelitic layers as a consequence the influence of diagenetic processes on the nrm should be thoroughly assessed when selecting samples e g for the determination of the relative palaeointensity of the geomagnetic field application of multivariate classification techniques appears to be useful because it links rock magnetic parameters to the geochemical environment in the present piston core three short reversed geomagnetic events in the brunhes chron are preserved and indeed occur in clusters expressing no or minor diagenesis the recording of the blake event however has been prevented by later precipitation of magnetite in the corresponding interval

variations in character and preservation potential of vegetation induced obstacle marks in the variable discharge burdekin river of north queensland australia abstract vegetation induced obstacle marks are described from dalrymple bend in the burdekin river of queensland australia and their preservation potential is discussed the bend is divided into a sand and gravel covered lower bar and a vegetated upper bar obstacle marks comprising an erosional scour and a depositional sediment tail are recognised on the lower bar and the lower margin of the upper bar two types of obstacle marks are dominant in the lower part of the lower bar sediment tail dominated obstacle marks associated with inclined trees of melaleuca argentea silver leaved paperbark and comparatively small obstacle marks around stranded driftwood obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel at the lower margin of the upper bar scours and gravelly sediment tails are recognised around mature tall trees in these cases the scour is 4 m in width and covered with alternating beds of fine sand and mud among the obstacle marks described sediment tail dominated obstacle marks around groves of m argentea have the highest preservation potential obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential obstacle marks around stranded driftwood have the lowest preservation potential obstacle marks around grass clumps survive more than 1 year but their long term preservation potential is nonetheless low implications for the stratigraphic record are also discussed

cyclone pumping sediment partitioning and the development of the great barrier reef shelf system a review abstract the modern great barrier reef gbr is part of the world s largest and best known mixed terrigenous carbonate continental margin the gbr shelf contains three shore parallel sedimentary belts an inner shelf zone of terrigenous sedimentation at depths of 0 22 m a middle shelf zone of sediment starvation at depths of 22 40 m and an outer shelf reef tract with its inner edge at ca 40 m depth these zones are controlled by the dynamics of northward fair weather along shelf drift driven by southeasterly trade winds and by the regular passage of tropical cyclones cyclones cause wind driven north directed middle shelf flows in excess of 130 cm s which erode the seabed concentrate the sparse mobile sediment into sand ribbons and advect suspended load onto the outer part of the nearshore terrigenous sediment prism and into inter reef depocentres within the outer shelf reef tract cyclones largely control the input of new sediment into the gbr system via river flooding seabed erosion or reef breakage they also help to control the partitioning and dispersion of the three main shore parallel belts of sediment and hence stratigraphic accumulation acting as a sediment pump especially during interglacial highstands cyclones have exerted great control on the development of the modern gbr province and its sediments by maintaining a broad shelf parallel zone of episodically mobilised sediment and scoured seabed upon which coral reefs have been unable to form cyclones may also have partly controlled the timing of initiation of the first gbr at 0 6 mybp contrary to current models gbr storm beds are most likely to be preserved intact close to the shoreline and become coarser grained away from the shoreline for the central gbr highstand shedding only applies to carbonate sediment at the scale of local reefs system wide oceanographic controls cause high rates of carbonate sedimentation on the slope during both sea level rise and highstand concomitantly terrigenous sediment accumulates fastest on the slope during sea level rise and slowest during sea level lowstand and highstand

evaluation of sediment provenance using magnetic mineral inclusions in clastic silicates comparison with heavy mineral analysis magnetic fe oxide inclusions within framework clastic grains in sediments provide an indication of the provenance of the enclosing host silicate particles magnetic mineral inclusion characterisation is performed using a variety of magnetic properties that are related to magnetic mineral abundance magnetic grain size domain state oxidation state and magnetic grain interaction the magnetic methodology extends that based on a conventional set of environmental magnetic measurements using a variety of recent sediments transported from known rock sources within the uk and eire it is demonstrated using discriminant function analysis that the magnetic properties of the fe oxide inclusions provide a clear distinction of primary provenance this is both at the large scale in terms of various gneiss schist and granite sources and also at a smaller scale when considering subdivisions of these sources using case studies from the marine upper jurassic piper formation and non marine triassic sediments otter bank sandstone and foula sandstone formations it is shown that provenance differences clearly expressed by the results of heavy mineral analysis are also sensitively displayed by the magnetic mineral inclusion data the provenance classification is based around use of hierarchical cluster analysis and multidimensional scaling simple statistical tools are developed to determine a least noisy subset of magnetic parameters which are most suitable for stratigraphic provenance discrimination the triassic case study has indicated the magnetic mineral inclusion technique is most sensitive when using the sediment fractions larger than 150 m

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

the significance of various sediment magnetic mineral fractions for tracing sediment sources in killimicat creek abstract the mineral magnetic properties of sediment are increasingly being used to determine the sources of sediment and associated nutrients and contaminants in drainage basins this study was undertaken to measure the relative contributions of the magnetic mineral components in sediment i e components associated with surface bound fe the heavy mineral fraction and as inclusions in particles to determine the extent to which they represent the bulk of the sediment deposited channel sediment samples from the tributaries and downstream reach at the first major confluence in the headwaters of killimicat creek new south wales were sieved to separate 6 particle size fractions and the mineral magnetic properties measured to determine the relative contribution of sediment from the smaller tributary basin the finest sand component 63 125 m was then separated into light and heavy mineral fractions and magnetic measurements made on each fraction magnetic parameter data from the 63 125 m fraction measured before density separation show a dominance of sediment 70 12 derived from the smaller tributary basin measurements of the light sediment fraction give a similar result 73 13 the proportion of the heavy mineral fraction delivered from the smaller tributary is 78 38 the light mineral fraction of all of the samples were treated with hcl to remove surface bound fe and subsequent magnetic measurements show that 41 94 of the mass specific magnetic susceptibility is attributable to magnetic minerals associated with surface bound fe the heavy mineral component while 60 is associated with sediment particles rather than the discrete heavy mineral component

supply limited sediment transport in a high discharge event of the tropical burdekin river north queensland australia interactions between catchment variables and sediment transport processes in rivers are complex and sediment transport behaviour during high flow events is not well documented this paper presents an investigation into sediment transport processes in a short duration high discharge event in the burdekin river a large sand and gravel bed river in the monsoon and cyclone influenced semi arid tropics of north queensland the burdekin s discharge is highly variable and strongly seasonal with a recorded maximum of 40 400 m s sediment was sampled systematically across an 800 m wide 12 m deep and straight reach using helley smith bedload and us p 61 suspended sediment samplers over 16 days of a 29 day discharge event in february and march 2000 peak 11 155 m s about 3 7 10 tonnes of suspended sediment and 3 10 tonnes of bedload are estimated to have been transported past the sample site during the flow event the sediment load was predominantly supply limited wash load included clay silt and very fine sand the concentration of suspended bed material including very coarse sand varied with bedload transport rate discharge and height above the bed bedload transport rate and changes in channel shape were greatest several days after peak discharge comparison between these data and sparse published data from other events on this river shows that the control on sediment load varies between supply limited and hydraulically limited transport and that antecedent weather is an important control on suspended sediment concentration neither the empirical relationships widely used to estimate suspended sediment concentrations and bedload e g ackers white 1973 nor observations of sediment transport characteristics in ephemeral streams e g reid frostick 1987 are directly applicable to this river

flood behaviour of the burdekin river tropical north queensland australia strongly seasonal rainfall in north queensland australia causes extremely variable fluvial discharge and tropical cyclones commonly produce high magnitude short duration floods in the burdekin river catchment 129 500 km2 of eastern queensland rapid runoff and channel floodplain configuration lead to very fast flood wave speeds up to 4 34 m s 1 calculated the large catchment and varying tropical cyclone paths result in extreme variations in flood history at different sites changes in channel conditions bed elevation vegetation bedforms sediment surface character in successive events combined with complex rainfall patterns and highly unsteady flow make flood estimates extremely difficult because of the short duration of flow events and very rapid rate of change in discharge during individual events sediment transport rate bedforms and bed elevation change continuously for most of the year the channel is occupied by an underfit stream but this small perennial stream does not significantly rework the deposits or change channel form before the next major discharge event consequently there is little channel form recovery between major discharge events successive events have very variable size and consequently the bedforms and channel morphology are normally out of equilibrium with the flow and record the result of the most recent major events high magnitude floods transport coarse sediment onto the floodplain and the short duration of flooding and rapid runoff results in relatively low preservation of mud in the channel or overbank environment the holocene flood deposits include thick coarse sand and medium sand units and generally thin fine grained units

magnetic characterization of holocene sedimentation in the south atlantic surface sediment samples representative for the tropical and subtropical south atlantic 15 n to 40 s were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant holocene climatic and oceanographic controls individual parameters reveal distinct yet frequently overlapping regional sedimentation characteristics a probabilistic fuzzy c means cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine particle magnetite grain size and coercivity spectra the resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes authigenic biogenic magnetite accumulation in high productivity areas transport by ocean current systems and effects of bottom water velocity on depositional regimes distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the south atlantic reflect prominent contrasts of both oceanic and continental influences

magnetic characterisation of present day deep sea sediments and sources in the north atlantic the north atlantic plays a key role in global climate through formation of north atlantic deep water which drives the thermohaline circulation to understand past and future climate change it is essential to understand the processes occurring within the present day north atlantic but ocean wide studies of present day sediment distribution and sources are relatively sparse here we use magnetic measurements to characterise the surface sediments of the north atlantic and identify the major climatic and oceanographic controls on their magnetic signatures the magnetic data and subsequent cluster analysis identify distinct spatial patterns of sediment sources and transport pathways much of the sedimentary magnetic signal appears to be controlled by detrital inputs especially of windblown dust and ice rafted debris ird with a range of different sources these sediment transport pathways can be validated by making direct comparison of the sediment magnetic properties with source rocks and soils and with iceberg trajectory and observation data the spatial distributions of the ird dominated sediments substantiate those mapped previously using lithological tracers provide additional spatial information on sediment sources and pathways and suggest that deep water currents are less significant than proposed in controlling present day sediment mineralogy and distribution the data form a present day basis for comparison with glacial stage ird and dust distributions and modes of ocean circulation

gravel antidunes in the tropical burdekin river queensland australia the geological record is punctuated by the deposits of extreme event phenomena the identification and interpretation of which are hindered by a lack of data on contemporary examples it is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle transporting high velocity currents and therefore their characteristics are poorly documented the deposits of such flows are exposed however in the dry bed of the burdekin river queensland australia following tropical cyclone induced floods long wave length up to 19 m gravel antidunes develop during short days high discharge flows in the upper burdekin river maximum recorded discharge near the study reach over 25 600 m s in february 1927 flood water levels fall quickly metres in a day and flow is diverted away from raised areas of the river bed into subchannels exposing many of the high stage bedforms with little reworking by falling stage currents gravel bedforms were observed on the dry river bed after the moderate flows of february 1994 max 7700 m s and january 1996 max 3200 m s the bedforms had wave lengths in the range 8 19 m amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak discharge flow direction the gravel fabric and size sorting change systematically up the stoss and down the lee faces the antidune deposits form erosive based lenses of sandy gravel with low angle downstream dipping lamination and generally steep upstream dipping a b planes the internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses the erosive based lenses of low angle cross bedded gravel with steep upstream dipping a b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes the antidune gravel lenses are associated with thick to 1 m high angle cross bed sets ancient antidune gravel lenses may be diagnostic of episodic high discharge conditions and particularly when they are associated with high angle cross bedded gravelly sand they may be useful for palaeoenvironmental interpretation

coarse grained floodplain deposits in the seasonal tropics towards a better facies model abstract floodplain deposits in semiarid and subhumid settings with erratic rainfall may contain relatively high proportions of sand because of climatically controlled patterns of discharge and vegetation cover ancient formations with abundant floodplain sands may record fluvial systems with extreme discharge variability of this type but are difficult to interpret because of the paucity of published modern analogues the lower burdekin and haughton river alluvial and delta plains of north queensland australia are used here to illustrate this type of fluvial architecture the modern burdekin and haughton channels have gravelly sand beds and transport a mixed load in the wet season the channels have low relief levees that decrease in amplitude downstream across the low gradient floodplains tropical cyclone activity results in high magnitude short duration sediment charged floods that inundate large areas of the coastal lowland bankfull discharge is infrequently exceeded return interval 25 years because the channels are enlarged by major cyclone induced runoff events and between these events flows are too small to modify the channel form significantly suspended sediment loads and the proportions of these that are sand are particularly high when intense rainfall occurs after prolonged drought when vegetation cover is reduced as a consequence of these factors most of the deposition on the levees and floodbasins occurs in very large discharge events with significant suspended sand load these floods carry a lot of sand out of the channel and efficiently in suspension over the floodplains because of deep inundation boreholes drilled in these floodplains encountered sharply bounded and gradationally bounded sand beds that fine upward or show no vertical grain size trend these are interbedded with thin silt clay and minor gravel beds holocene paleochannels make up about 30 of the floodplain area and consist predominantly of coarse sand that is similar to modern channel bed deposits the overbank sand in contrast is predominantly fine medium grained and beds have distinctly different granulometry from each other and from channel floor deposits efficient sediment partitioning in the runoff and extreme variation in flood magnitude explain variations in sand bed granulometry in the alluvial valley and upper delta plain individual sand beds are difficult to correlate between adjacent boreholes even over very short distances tens of meters in contrast on the lower delta plain overbank sands have a more sheet like geometry this is explained by the lower lying landscape negligible levee development greater inundation depths and reduced surface roughness in addition the channel bodies are thinner and wider and may be connected to overbank sand wings

sedimentology of the upper burdekin river of north queensland australia an example of a tropical variable discharge river the burdekin river is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge in response to seasonal but erratic monsoonal rainfall floods of the burdekin rise abruptly reaching peak discharges of up to 40 000 m s in less than 24 h maintain peak flow for up to a few days and recede exponentially the geomorphology and deposits of these rivers reflect the extreme discharge fluctuations and have not previously been described a stretch of the upper burdekin river comprising four bends and one straight reach was examined near the town of charters towers the river bed is largely exposed for most of any year with a small misfit perennial channel carrying low stage flow major geomorphic elements of bends include point bars with ridge and swale topography three distinct types of chute channels avalanche slipfaces up to 5 m or more high around the downstream edges of bars and on the outer part of one point bar an elevated vegetated ridge straight reaches are flat or gently inclined sand and gravel covered surfaces much of the river bed is covered by well sorted in places gravelly coarse to very coarse grained sand with local accumulations of pebble to boulder gravel lower parts of the river bed are periodically draped by mud which is desiccated on exposure dunes and plane beds are the most commonly occurring bedforms with local development of gravelly antidunes most bank tops and upper vegetated bars are covered by silt and fine grained sand the river bed also hosts a low diversity but locally high abundance flood tolerant flora dominated by the paperbark tree melaleuca argentea which plays an important role in controlling the distribution of sediment the gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major bankfull or near bankfull flows which have a recurrence of about 18 years based on 65 years hydrographic data the initial rapid drop in discharge following flood peaks appears to preserve flood peak features on upper bars more or less intact whereas lower areas are subjected to variable degrees of modification during falling stage and by more frequent non bankfull discharge events

late oligocene early miocene major tectonic crisis and related basins in bolivia recent advances in biochronology geochronology magnetostratigraphy structural geology and basin analysis make untenable the traditional correlation of the first major andeanage deformational episode in bolivia with the middle eocene incaic tectonic phase defined in central peru and instead demonstrate that the episode took place in late oligocene and early miocene time this major tectonic crisis resulted in contemporary development of the subandean external foreland basin and altiplano intermontane basin which were separated by the initiation of thrusting in the present cordillera oriental area the deformation suggests that the bolivian orocline began to develop at that time it is likely that this tectonic upheaval is genetically linked to the marked increase in rate of plate convergence produced by the contemporaneous breakup of the farallon plate