Chronology and paleoenvironmental interpretation of talus flatiron sequences in a sub‐humid mountainous area: Tremp Depression, Spanish Pyrenees
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ABSTRACT Talus flatirons are debris‐covered relict slopes, disconnected from the source area, which are relatively common in arid and semi‐arid areas. Talus flatiron sequences record the alternation of accumulation and incision phases. These chronosequences may be used for infer temporal changes in the morphogenetic processes acting on the slopes as well as information on the local paleoclimatic history. Talus flatiron sequences developed in the Tremp Depression, eastern Spanish Pyrenees, are analysed from the geomorphological, chronological and paleoenvironmental perspective. The two groups of relict slopes differentiated by means of detailed geomorphological mapping have been dated by optically stimulated luminescence (OSL) and radiocarbon dating at 25–20 kyr (S3) and 5·4–1·7 cal kyr (S2). The talus flatiron group S3 is correlated with a fluvial terrace of the Noguera Pallaresa River ( c . 23 kyr bp ). The comparison of the ages obtained in the Tremp Depression with chronologies published for talus flatiron sequences in semi‐arid areas and other paleoclimatic proxies suggests that the aggradation phases in the slopes occurred during periods with higher humidity and vegetation cover. The chronological differences observed between semi‐arid Spain and the Tremp Depression may be partly related to the more humid climate of the latter mountain area. Copyright © 2013 John Wiley & Sons, Ltd.Keywords:
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Tufas are constructional landforms composed of calcium carbonate. They are common in karst terrains, and are known worldwide as useful archives of palaeoenvironmental information. In the case of fluvial tufas, these carbonate deposits experience net aggradation when environmental conditions are stable, whilst incision through the deposits occurs when conditions become wetter. In this study, the first of its type in temperate Australia, we used geomorphological and sedimentological evidence preserved in fossil tufas and alluvial terraces to investigate the fluvial history of Davys Creek (NSW, Australia). The nature and timing of fluvial changes along two quasi-independent reaches of the creek were synchronous, suggesting the operation of externally driven processes rather than local internal fluvial adjustments. Following a period of sustained aggradation, rapid incision occurred along the creek between 1500 and 1600 years before present ( bp ) in response to a shift to a moister climate. This was succeeded by a second major aggradation event (1500 years bp to 150 years bp ), then subsequent incision following the arrival of Europeans at 150 years bp.
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Abstract Accurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Amblève catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Amblève catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking.
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ABSTRACT The influence of permafrost growth and thaw on the evolution of ice‐rich lowland terrain in the Koyukuk‐Innoko region of interior Alaska is fundamental but poorly understood. To elucidate this influence, the cryostratigraphy and properties of perennially frozen sediments from three areas in this region are described and interpreted in terms of permafrost history. The upper part of the late Quaternary sediments at the Koyukuk and Innoko Flats comprise frozen organic soils up to 4.5 m thick underlain by ice‐rich silt characterised by layered and reticulate cryostructures. The volume of visible segregated ice in silt locally reaches 50 per cent, with ice lenses up to 10 cm thick. A conceptual model of terrain evolution from the Late Pleistocene to the present day identifies four stages of yedoma degradation and five stages of subsequent permafrost aggradation‐degradation: (1) partial thawing of the upper ice wedges and the formation of small shallow ponds in the troughs above the wedges; (2) formation of shallow thermokarst lakes above the polygons; (3) deepening of thermokarst lakes and yedoma degradation beneath the lakes; (4) complete thawing of yedoma beneath the lakes; (5) lake drainage; (6) peat accumulation; (7) permafrost aggradation in drained lake basins; (8) formation of permafrost plateaus; and (9) formation and expansion of a new generation of thermokarst features. These stages can occur in differing places and times, creating a highly complex mosaic of terrain conditions, complicating predictions of landscape response to future climatic changes or human impact. Copyright © 2014 John Wiley & Sons, Ltd.
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Foreland river channels and river systems are characterised by extreme variability caused by changes in external factors driving fluvial morphology. Two foreland areas were considered: the southern part of the Minusinskaya Basin in the Sayan Foreland and the Priazovskaya Plain of the Western Caucasian Foreland. In the former area there is an example of a huge transformation of the River Yenisey channel and in the latter area there is the gradual formation of flatland in the River Kuban delta. In both cases the transformation of the fluvial morphology is driven by the changing volume of water in the rivers during the Quaternary glacial and interstades.
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Detailed characterization of fluvial strata or fluvial reservoirs can be achieved by the integration of all the facies within such succession. In this study we investigate the stratigraphic organization within a previously defined ~ 100,000 year non-marine sequence. Detailed description and correlation of channel facies, floodplain facies, paleosols, and coals reveal a hierarchical organization of strata, which include fluvial aggradation cycles, fluvial aggradation cycle sets, and 3 higher frequency sequences. Fluvial aggradation cycles in the succession are represented by simple floodplain bedsets or single channel storeys that are capped by immature paleosols or erosionally truncated. Fluvial aggradation cycle sets consist of multiple fluvial aggradation sets that are capped by relatively mature paleosols as well as coals and carbonaceous strata, or may be erosionally top-truncated. Fluvial sequences consists of multiple fluvial aggradation cycle sets that are bounded by extensive (or regional) erosional surfaces or surfaces of non-deposition that are represented by the most mature paleosols in the entire succession. Fluvial aggradation sets are interpreted as representing floodplain or channel depositional events that are followed by short-lived abandonment and immature paleosol development. They span not more than 2000 years. Fluvial aggradation cycle sets represent multiple episodes of floodplain and channel deposition followed by a relatively long period of abandonment and paleosol development, which is attributed to long-term river avulsions. Marine trace fossils and dinocysts associated with coals beds that cap fluvial aggradation cycle sets suggest marine transgression due to compactional subsidence or sea-level rise. Fluvial aggradation cycle sets span not more than 14, 000 years. Fluvial sequences are attributed to higher frequency Milankovitch-scale base-level rises and falls in sea-level.
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The fluvial process of establishing new equilibrium in river section upstream of Aojiang River estuary due to constructed reservoir is studied by means of 1-D tidal flow-sediment mathematical model coupling with 2-D model.The characteristics of hydrology,sediment transport and fluvial process are analyzed.The calculation condition, the techniques for solving some key problems and verification of computation results are presented.The flow and fluvial process are predicted.It is found that the aggradation develops rapidly in the early stage after construction of reservoirs and it tends to degrade afterward until a new equilibrium is established.The aggradation process develops from upstream to downstream and the thickness of deposition at the river bend is smaller than the transition section between two bends.The water depth of navigation at the port will be affected by the sediment deposition.
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The stream power law model has been widely used to represent erosion by rivers but does not take into account the role played by sediment in modulating erosion and deposition rates. Davy and Lague (2009, https://doi.org/10.1029/2008JF001146 ) provide an approach to address this issue, but it is computationally demanding because the local balance between erosion and deposition depends on sediment flux resulting from net upstream erosion. Here, we propose an efficient (i.e., O ( N ) and implicit) method to solve their equation. This means that, unlike other methods used to study the complete dynamics of fluvial systems (e.g., including the transition from detachment‐limited to transport‐limited behavior), our method is unconditionally stable even when large time steps are used. We demonstrate its applicability by performing a range of simulations based on a simple setup composed of an uplifting region adjacent to a stable foreland basin. As uplift and erosion progress, the mean elevations of the uplifting relief and the foreland increase, together with the average slope in the foreland. Sediments aggrade in the foreland and prograde to reach the base level where sediments are allowed to leave the system. We show how the topography of the uplifting relief and the stratigraphy of the foreland basin are controlled by the efficiency of river erosion and the efficiency of sediment transport by rivers. We observe the formation of a steady‐state geometry in the uplifting region, and a dynamic steady state (i.e., autocyclic aggradation and incision) in the foreland, with aggradation and incision thicknesses up to tens of meters.
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ABSTRACT The nature and timing of fluvial response and coincident changes in climate as inferred from multiple, independent proxies are examined in the wet tropics of north‐eastern Australia. Chronostratigraphic data provide evidence for regionally synchronous fluvial aggradation at 30–13 ka, 8–5 ka and again from 1 ka. Terrace incision and removal occurred at 13–8 ka and 5–1 ka. A new synthesis of regional palaeoenvironmental proxy records spanning the last 30 ka highlights key periods of environmental change. Comparison of the records of fluvial and environmental change reveals the relationship is complex with no clear relationship between river aggradation/incision and wetter/drier conditions. The nature and timing of fluvial response in the wet tropics also shows broad similarities with coastal valleys in south‐eastern New South Wales, suggesting continental‐scale controls on fluvial response to changing climate during the Late Quaternary in Australia. Copyright © 2017 John Wiley & Sons, Ltd.
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