Palaeochannel Systems Research Articles

Revisiting an arid LGM using fluvial archives: a luminescence chronology for palaeochannels of the Murrumbidgee River, south‐eastern Australia

ABSTRACTThe Riverine Plain in south‐eastern Australia contains numerous palaeochannels that are much larger than the present rivers and provide evidence about past hydrological conditions. Previous research suggested optima in fluvial activity both before and after the peak of the Last Glacial Maximum (LGM; 21 ± 3 ka), and, in some cases, throughout the LGM. In this study, we revisit palaeochannel remnants of the Gum Creek and Yanco palaeochannel systems along the Murrumbidgee River, which drains the high‐elevation catchments of the Australian Alps in south‐eastern Australia. We date fluvial and aeolian sediments using single‐grain optically stimulated luminescence (OSL) and apply thermoluminescence (TL) dating to a subset of samples. We compare the OSL ages to new and previously published TL ages and investigate reasons for age discrepancies between these methods, possible effects of partial bleaching and other factors that may affect luminescence ages. We propose a new OSL‐based chronology for the Gum Creek and Yanco palaeochannel systems and assign periods of enhanced fluvial activity for the Tombullen and Yanco phases to 41–29 and 29–18 ka, respectively. Importantly, we infer that conditions of increased sediment and water discharge persisted for the Murrumbidgee River at the time of the LGM.

Geophysics and Uranium in the Curnamona and Billeroo Palaeochannel Systems, Frome Embayment, South Australia

SUMMARY Tectonism is believed to have exerted a strong control on the current disposition of sedimentary hosted uranium mineral deposits in the Frome Embayment, in South Australia. However the extent of this control has not been well understood, nor documented. Here, we examine the combined use of regional and finer-scale TEMPEST AEM data sets, linked to a structural interpretation of airborne magnetics and ground gravity, to extend our understanding of the evolution, geometry and variability of sediment packages associated with sedimentary uranium mineralisation in the Curnamona and Billeroo Palaeochannel systems. Through the analysis of both smooth and blocky model LEI inversions of these AEM data, we contend that structural control was critical in determining the initial orientation of the palaeovalleys and the location of basal sequences of the Eyre Formation, the host to known uranium mineralisation. We examine this influence in the context of the Goulds Dam uranium deposit. The presence of reactivated basement faults which controlled the initial orientation of the palaeovalley systems may also have a role in providing the loci for mobile reductants from underlying basin sequences, although this requires further investigation.

Late Quaternary changes in flow-regime on the Gwydir distributive fluvial system, southeastern Australia

Ages for large palaeochannels of the Gwydir distributive fluvial system (DFS) in northern New South Wales, Australia have been determined using single grain optically stimulated luminescence. Two palaeochannel systems have been found to dominate; the here named Coocalla (43–34 ka) and Kamilaroi (19–16 ka) which have inferred palaeodischarges 25–100 times the bankfull discharges of nearby channels of the contemporary Gwydir system, which appears to have been established during the Mid-Holocene. This scale differential is very much larger than that reported for other catchments in southeastern Australia, and reflects both a decline in catchment runoff through the Last Glacial cycle and the adoption of a distributary pattern sometime after 16 ka. Actual decline in catchment runoff, determined by comparing estimated palaeodischarge with contemporary flows upstream of the DFS where flow is confined to a single channel, indicate contemporary discharge to be 0.1 times and 0.25 times that of the Coocalla and Kamilaroi, respectively.The chronology presented here shows periods of increased discharge in the Gwydir to be more or less coincident with those observed elsewhere in the Murray Darling Basin. Although no evidence of a ‘Gum Creek’ fluvial phase (from 35 to 25 ka) was found, the Coocalla and Kamilaroi palaeochannel systems broadly conform in age to ‘Kerarbury’ and ‘Yanco’ fluvial phases on the Murrumbidgee and Murray systems. This synchronicity with more southern catchments supports the hypothesis that La Nina – like conditions were semi-permanent for much of the Last Glacial cycle with moisture derived largely from the western Pacific Ocean.

Geo-electric resistivity evidence for subsurface palaeochannel systems adjacent to Harappan sites in northwest India

It has been proposed that a major palaeo-river channel course, the Ghaggar-Hakra, flowed in the interfluve between the modern Yamuna and Sutlej rivers in the western Ganges basin during the Late Quaternary. This palaeochannel course has been associated with extensive Bronze-age Harappan civilisation archaeological sites that are located with the channel. The abrupt abandonment of urban centres at ∼3500 BP has been explained as a consequence of river diversion, although alternative explanations for cultural decline have also been offered. A major problem with earlier interpretations has been that little information exists on the geology of the palaeochannel system. Electrical resistivity soundings were used to map the large-scale geometry and architecture of the palaeochannel system. A thick and extensive sand body is present in the subsurface in parts of north-western Rajasthan, Haryana and Punjab. The dimensions of the palaeochannel bodies imply that these are the deposits of a large river system, though detailed sedimentological analysis is necessary to validate this. Two of the resistivity transects are close to important Harappan sites, Kalibangan and Kunal, suggesting a possible link to archaeological site distribution. However, detailed chronological constraints are required to establish such links. Nevertheless, this study reports the first geophysical evidence for the subsurface geometry of the palaeo-Ghaggar-Hakra river system.

Australian sandstone-hosted uranium deposits

Australia’s sandstone-hosted uranium deposits occur in sedimentary basins of Carboniferous, Cretaceous and Tertiary age; these include some of Australia’s largest and highest grade uranium deposits. The conventional model for sandstone-hosted uranium deposits has proved robust and a predictive model leading to the discovery of many deposits in Australia. The location of deposits is strongly influenced by the presence of Mesoproterozoic and Archaean age leachable uranium-rich source rocks in the headwaters of channels draining into basins developed in the Northern Territory, Western Australia and South Australia. Australia’s production of uranium from sandstone-hosted deposits is limited to two in-situ leach operations and is relatively minor when compared to production from Kazakhstan and the USA. Nevertheless, Australia has a large inventory of uranium resources in 19 undeveloped sandstone-hosted deposits, amounting to 123 kt U3O8. The average grade of all 21 Australian deposits that have resources is 0·15% U3O8. Tertiary palaeochannels host the greatest number of deposits and include the largest and highest grade deposits. The Callabonna Sub-basin in South Australia is the most richly endowed basin, accounting for 62·4 kt U3O8, being 38% of Australia’s sandstone-hosted resource inventory. Australia remains highly prospective for the discovery of new palaeochannel hosted uranium deposits, with regional airborne geophysical surveys likely to be of great assistance in continuing to define palaeochannel systems that may host uranium in basins draining uranium rich source rocks.

Stratigraphy of alluvial fills and phases of the Holocene floods in the lower Wisłok River valley, SE Poland

Stratigraphy of alluvia and chronology of the Holocene flood events were studied on the basis of 50 radiocarbon dates as well as sedimentological and palynological analyses made on 20 depositional sequences of sediments in the lower section of the Wislok River valley. These sites evidence the occurrence within the 4–5 m and 6–7 m high levels of the floodplain of separate alluvial fills and palaeochannel systems, recording phases of increased Wislok activity and frequent floods dated at: the Allerod-Younger Dryas Phase, 9,900–9,600 BP, 8,500–8,000 BP, 6,600– 6,100 BP, 5,400–5,000 BP, 4,400–4,000 BP, 2,100–1,700 BP, as well as the 11–13th centuries AD and the middle of the 18th century AD. These periods are in accordance with the phases documented earlier within the Upper Vistula River drainage basin, particularly in the for-mountainous section of the Wisloka River valley, Vistula River valley downstream of Krakow , as well as in the Lower San River valley.

Episodic fluvial activity of inland rivers in southeastern Australia: Palaeochannel systems and terraces of the Lachlan River

Inland rivers in southeastern Australia preserve a long record of surface palaeochannels, accompanied by alluvial terraces that reflect changes in water and sediment discharges from the southeastern highlands through much of the last glacial cycle. A compilation of ages on rivers of the southern Murray-Darling Basin together with new dates from the Lachlan River suggest that the fluvial response to climate change has been regionally consistent and responsive to climate change on timescales <10 ka. For much of the last glacial cycle, inland rivers were mixed-load, sinuous single and anabranching channels that carried bankfull discharges of between 3 and 11 times present day rivers. Incision of the upper alluvial plains and valleys occurred in the lead up to the coldest part of the last glacial cycle. This was followed at 34 ka by the synchronous development of large meandering channels that may have been associated with lower evapotranspiration from alpine highland catchments accompanied by higher precipitation. Smaller channels and declining lateral channel sedimentation suggests arid climates during the last glacial maximum and late glacial period with seasonally large, actively migrating channels being maintained in rivers dominated by snowmelt runoff from the Australian Alps. During the Holocene, sensitivity to recorded changes in precipitation amounts and variability differed between catchments with some rivers constructing larger or more actively migrating channels in the early to mid-Holocene.

Palaeochannels of the Namoi River Floodplain, New South Wales, Australia: the use of multispectral Landsat imagery to highlight a Late Quaternary change in fluvial regime

Abandoned channels of ancient rivers are clearly visible on aerial photographs of the Namoi River floodplain in northern New South Wales, Australia. However, the spatial distribution of these palaeochannel systems over many tens of kilometres makes their broad-scale mapping with air photos difficult. Satellite imagery has previously been used to a limited degree in the identification of palaeochannels in many areas, but typically only in single image bands. This study finds that combining and enhancing multiple bands of Landsat 7 ETM+ data improves subtle differences in the spectral reflection of soil moisture and vegetation, aiding palaeochannel identification. These results are much better than using single band data alone in this important agricultural area, one suffering strong water and soil resource pressures. There are several generations of palaeochannels visible on this floodplain, and analysis shows they were very different from the modern rivers; they had greater discharges, longer meander wavelengths and flowed along different courses to the modern rivers. This study supports other recent investigations of the geomorphology and chronology of the Namoi River floodplain that showed that these changes only occurred during the Late Quaternary. These historical changes were not fully recognised until recently, and are different from what was assumed from studies of other riverine plains in south-eastern Australia.

Archaeological and Palaeo-environmental Investigations of the Upper Allen Valley, Cranborne Chase, Dorset (1998–2000): a New Model of Earlier Holocene Landscape Development

A combination of on- and off-site palaeo-environmental and archaeological investigations of the upper Allen valley of Dorset conducted in 1998–2000 has begun to reveal a different model of landscape development than those previously put forward. A combination of off-site geoarchaeological and aerial photographic survey and palynological analyses of two relict palaeochannel systems, and sample investigations of four Bronze Age round barrows and a Neolithic enclosure, have been combined with inter-regional summaries of the archaeological and molluscan records to re-examine the prehistoric landscape dynamics in the study area. Preliminary results suggest that woodland development in the earlier Holocene appears to have been more patchy than the presumed model of full climax deciduous woodland. With open areas still present in the Mesolithic, the area witnessed its first exploitation of the chalk downs, thus slowing and altering soil development of the downlands. Consequently, many areas perhaps never developed thick, well structured, clay-enriched soils (or argillic brown earths), but rather thin brown earths. By the later Neolithic these under-developed soils had become thin rendzinas, largely as a consequence of human exploitation. The presence of thinner and less well-developed soils over large areas of downland removes the necessity for envisaging extensive soil erosion and thick aggraded deposits in the valley bottom in later prehistory. The investigations have suggested that, if there were major changes in vegetation and soil complexes, these had already occurred by the Neolithic rather than in the Bronze Age as suggested by previous researchers, and the area has remained relatively stable since.

Optical Dating of Paleochannel Deposits in the Riverine Plain, Southeastern Australia: Testing the Reliability of Existing Thermoluminescence Dates

We present the first quartz optically stimulated luminescence (OSL) ages for palaeochannel sediments from the Riverine Plain in southeastern Australia. For young fluvial sediments, we agree with the notion that analysis of the leading edge of a dose distribution curve provides an objective method for determining the OSL age. For a modern flood deposit (less than 200 years old), the OSL ages estimated using the leading edge method (250 +/- 50 years) and by using the lowest 5% of the measured dose in single aliquots (230 +/- 50 years) agree within experimental errors. For older sediments, we suggest that the mean estimate of the dose distribution is likely to provide a reliable estimate of the OSL age. The luminescence ages suggest that the Coleambally and Kerarbury palaeochannel systems were active between 105 and 80 ky and 55 and 35 thousand years ago; the Yanco palaeochannel system could have been active as recently as 9000 years ago.

Hydrological Monitoring of an Alluviated Landscape in the Lower Great Ouse Valley, Cambridgeshire: Interim Results of the First Three Years

A five year research project was set up to monitor soil moisture and groundwater levels of an extensive buried, mainly alluviated landscape located at the interface between the lower Great Ouse valley and the fen-edge in western Cambridgeshire. The intention was to monitor changes in hydrology, soil structure and geochemical status before, during and after large scale gravel extraction. The results presented here concern the pre-extraction monitoring period over three years. The project has identified three major landscape zones – shallowly buried former dryland, several sets of relict palaeochannel systems which still remain waterlogged, and a marginal zone between these where the monitored parameters fluctuate on a seasonal basis. The moisture levels of the alluvial overburden, buried soils and infills of the palaeochannel systems appear to act independently of each other, controlled by a combination of rainfall, agricultural land management, height above sea level and depth of burial. It is predicted that the removal of 1–3m of overburden accompanied by water abstraction when gravel extraction commences will lead to changes in the moisture regime of the immediate area and therefore of any contained archaeological contexts and the superficial sediments in this river valley/fen-edge landscape.

Hydrological Monitoring of an Alluviated Landscape in the Lower Great Ouse Valley, Cambridgeshire: Interim Results of the First Three Years

A five year research project was set up to monitor soil moisture and groundwater levels of an extensive buried, mainly alluviated landscape located at the interface between the lower Great Ouse valley and the fen-edge in western Cambridgeshire. The intention was to monitor changes in hydrology, soil structure and geochemical status before, during and after large scale gravel extraction. The results presented here concern the pre-extraction monitoring period over three years. The project has identified three major landscape zones – shallowly buried former dryland, several sets of relict palaeochannel systems which still remain waterlogged, and a marginal zone between these where the monitored parameters fluctuate on a seasonal basis. The moisture levels of the alluvial overburden, buried soils and infills of the palaeochannel systems appear to act independently of each other, controlled by a combination of rainfall, agricultural land management, height above sea level and depth of burial. It is predicted that the removal of 1–3m of overburden accompanied by water abstraction when gravel extraction commences will lead to changes in the moisture regime of the immediate area and therefore of any contained archaeological contexts and the superficial sediments in this river valley/fen-edge landscape.

The influence of a palaeochannel on seam thickness in the coal measures of Derbyshire, England

A study of coal-bearing Westphalian B rocks in Derbyshire (East Midlands, England) has shown that a palaeochannel system influenced the thickness of both the underlying and overlying seems, and the thickness of the interseam strata containing the palaeochannel. The Top Hard/Lower Coombe Seam, which underlies the palaeochannel, contains washouts, resulting in partial or total removal of the coal and anomalous seam sections. The interval between this seam and the overlying Upper Coombe Seam shows dramatic thickness variations where the palaeochannel is present, with the interseam interval diminishing rapidly to the north away from the palaeochannel. The Upper Coombe Seam, which overlies the palaeochannel system, shows marked thickness variation where it overlies the channel, with less thickness variation outside the channel. The palaeochannel is interpreted as a distributary channel, formed on an upper delta plain to lower alluvial plain, comparable with the Atchafalaya Basin, Louisiana. There is little evidence for bedload transport within the channel, which was abandoned and infilled mainly by deposition of silt from suspension and bank collapse. These mechanisms resulted in an infill of irregular thickness, causing the thickness variation of the interseam strata and the seam overlying the channel. A thick sequence of overbank flood deposits was laid down adjacent to the channel which thins rapidly away from it. Compaction influenced the development of the channel system, and modified the post-depositional geometry. Channels similar to the one described show patterns of thickness variation, which may enable their detection in other areas. The presence of erosion surfaces, bank collapse deposits, anomalous dips, and unlaminated siltstones may also enable their delination in areas of coal mining. Isopachyte patterns of thickness of seams and interseam strata are potentially very useful in the detection of palaeochannel systems in coal-bearing sequences and a number of different patterns of thickness variation accompanying palaeochannels are briefly described.

Plio-Pleistocene raised channel systems of the western Sharqiya (Wahiba), Oman

Summary Extensive alluvial fans bounding the western edge of the Sharqiya (or Wahiba) Sands are characterized by complex palaeochannel systems now forming a series of superimposed gravel ridges. The oldest, most extensive fan system is crossed by numerous sinuous, superimposed palaeochannel courses comprising highly weathered, chert-rich gravels cemented by clear crystalline calcite. Many of these old fan sediments have been chemically altered to depths of over 200 m to form a pink dolomitic clayey deposit here termed barzamanite. The formation of this calcrete-like deposit was probably associated with rising water tables during long-term fan growth. The younger, more limited fan deposit, by contrast, comprises thin, coarse-grained, broad spreads and terrace veneers of weakly cemented ophiolitic gravels. The fans have been subject to entrenchment and deflation, resulting in extensive land-surface lowering and the exhumation of buried palaeochannels. Although no dates are yet available, the older channel systems are likely to date from more humid phases during the Pliocene-Early Pleistocene, whereas the younger, terraced deposits, and the period of subsequent channel exhumation, may date from semi-arid periods of lower sea level during the Pleistocene.