Phases Of Aggradation Research Articles

Widespread aggradation in the mountainous catchment of the Alaknanda–Ganga River System: timescales and implications to Hinterland–foreland relationships

Cut-and-fill type fluvial terraces are ubiquitous in the Lesser Himalayan zone of the Alaknanda–Ganga (Ganges) rivers system, which flows perpendicular to Himalayan lithotectonic units and traverses a steep climatic gradient. Lithofacies analysis of the sedimentary sequences of cut-and-fill terraces indicated that the valley aggradation took place via (1) channel bar development and excess sediment supply, (2) debris flows composed of mixed rounded to sub-rounded lithoclasts, resulting from episodic high intensity rainfalls in the upper catchment or (3) debris flows or rockfalls generated by local landslides. The luminescence chronology indicates that valley aggradation took place in two phases: ∼49 –25 ka and 18 –11 ka. The incision of the fill started soon after 11 ka. Paleoclimatic records from marine sediments indicate that the aggradation and incision in the Alaknanda–Ganga River has oscillated in-phase with global climatic variations. Glaciation –deglaciation processes in the upper catchment produced huge amounts of sediment between 63 and 11 ka, which was fluvially transferred to the lower valley via several cycles of erosion and deposition, leading to extensive aggradation. The climatic amelioration at ∼11 ka and the completion of deglaciation processes led to increased fluvial discharge and decreased sediment supply, conditions conducive for incision of the alluvial fills. Records from the Indo-Gangetic plain and the Ganga Delta demonstrate that the phase of aggradation was regional but that incision in the foreland started at least 2 –3 ka later, after 7 ka. Bedrock incision rates, as calculated from dated alluvial covers of terraces that are separated by bedrock steps, are spatially variable and fall within the range of rates reported from across the Himalaya. These estimated rates, however, are higher than the basin average erosion rates calculated using isotopic mass balance in riverbed sediments. This study suggests that during the last 50 ka river dynamics in the Himalayas were dominated by monsoon variability and the role of tectonic activity was limited to bedrock incision in few reaches only.

The position and context of Middle Palaeolithic industries from the Ebbsfleet Valley, Kent, UK

The late Middle Pleistocene sites in the Ebbsfleet Valley, Kent, UK, have yielded archaeological assemblages critical to understanding the early Middle Palaeolithic of northwestern Europe. Despite a long history of research, the nature and context of these assemblages are still poorly understood. This paper clarifies the stratigraphic, environmental and archaeological records at Ebbsfleet. These reflect a cold–warm–cold sequence of climatic events, preserved within part of the Taplow/Mucking Formation of the Thames (Marine Isotope Stage (MIS) 8/7/6). Levallois artefacts are shown to be restricted to the lower part of the Ebbsfleet Channel sequence (Phases I and II) and are assigned to late MIS 8/early MIS 7. This material is associated with fauna indicative of open environments during both cool and temperate conditions. Handaxe assemblages are recorded from higher up the sequence (Phases III–V), but have been redeposited from higher terrace units nearby. No primary context archaeology is apparent during these later phases of aggradation. This may indicate that humans abandoned the site once available raw material became inaccessible, and may also reflect a decline in human presence in Britain during the latter part of MIS 7. Copyright © 2010 John Wiley & Sons, Ltd.

Quaternary evolution of the Marrakech High Atlas and morphotectonic evidence of activity along the Tizi N'Test Fault, Morocco

Here, we consider the Ourika and Rheraia drainage basins on the Tizi N'Test Fault zone in the mountains of the Marrakech High Atlas (western Morocco) with regard to Late Pleistocene tectonic activity. New insights into geomorphological changes in drainage patterns and related landforms are based on geological fieldwork in conjunction with DEM analysis. Lithological and structural data combined with certain geomorphometric indices provide clues to the ongoing uplift of Quarternary surfaces in this region. Five geomorphological indices are utilized: 1) drainage network, 2) shape of stream long profiles, 3) hypsometric integral and curves, 4) valley-floor width valley-height ratio (Vf index), and 5) stream gradient index (SL index). We also considered the temporal evolution of alluvial-deposit complexes with diverse lithofacies, such as debris flows, channel gravels, rockslide-debris avalanche, stratified slope deposits, terrace gravels, and fan deposits in the Ourika and Rheraia valleys. Pleistocene talus deposits and fluvial sediments are offset by the Tizi N'Test Fault in the Upper Ourika and Upper Rheraia valleys. Such deformation of thick, continental deposits strongly points to thrust reactivation along the Tizi N'Test Fault. We define the chronology and overall aggradation phases, or lateral incision phases, showing how they are the consequences of variations in tectonic uplift and climate. As a result, we are better able to access recent morphotectonic evolution in part of the Marrakech High Atlas.

Fluvial aggradation phases in the Upper Rhine Graben—new insights by quartz OSL dating

The Upper Rhine Graben (URG) is characterized by a thickness of up to 500 m of unconsolidated Quaternary sediments, providing excellent records of the Rhine river system and its responses to tectonic and climatic changes. The most complete Quaternary sequence of fluvial and limnic-fluvial deposits is found in the Heidelberg Basin, due to its long-term subsidence since the mid-Eocene. The aim of this study is to provide a chronological framework using optically stimulated luminescence (OSL) dating of aeolian and fluvial sands derived from the upper 33 m of a sediment core, which was drilled into the Heidelberg Basin infill close to the village of Viernheim, Germany. The OSL ages demonstrate that the dated fluvial sediments were deposited during the last glacial period (Weichselian) and that there were at least three aggradation periods during this episode. The coversands that cap the sequence were emplaced during the early Holocene.

Holocene flooding and river development in a Mediterranean steepland catchment: The Anapodaris Gorge, south central Crete, Greece

Many Mediterranean steepland rivers are flanked by extensive alluvial and colluvial deposits, but Late Quaternary histories of channel and hillslope behaviour remain poorly constrained, primarily because of the limited availability of material suitable for dating. Study of a 4.8 km long reach of the Anapodaris Gorge, located in the lower part of an ∼ 500 km 2 catchment in south central Crete, reveals a succession of well preserved, coarse-grained (predominantly cobble to boulder) and fine-grained (predominantly silt to sand) alluvial deposits that locally interfinger with, or are overlain by, coarse colluvial and tributary stream deposits. Detailed ground surveys, geomorphological mapping, sedimentological investigations, and geochronology (optically stimulated luminescence, radiocarbon, and lichenometry) have allowed detailed reconstruction of the timing and pattern of sedimentation and erosion over the mid to late Holocene. Widespread, coarse-grained aggradational episodes at c. 4.86–4.20 and c. 3.40–3.00 ka have been punctuated by incisional episodes and coarse sediment export, resulting in a suite of alluvial terraces. Comparison with other proxy Mediterranean environmental change records, particularly high-resolution marine and lake records, suggests that these aggradational/incisional episodes were primarily climatically driven, reflecting changes in the balance between hillslope/tributary sediment supply and high-energy flood events. By contrast, phases of widespread fine-grained aggradation at c. 1.90 ka, 1.13 to 1.10 ka, 0.85 to 0.70 ka and 0.21 ka provide evidence for decreases in flood competence, possibly coupled with up-catchment historical land use changes. Since the middle of the nineteenth century, several large floods have formed localised boulder berms and splays and have contributed to stripping of the fine-grained deposits from many parts of the gorge. The findings from the Anapodaris Gorge demonstrate the sensitivity of Mediterranean steepland catchments to rapid and/or short-lived Holocene climate change but also highlight the need for higher resolution data on historical and prehistoric land use changes.

Depositional facies framework, evolution, and reservoir architecture of the Upper Devonian Jean Marie Member (Redknife Formation) in the July Lake area of northeastern British Columbia

Abstract The Jean Marie Member of the Upper Devonian Redknife Formation is a gas-saturated, dominantly limestone unit with minor dolostone that occurs throughout a widespread area in the subsurface of northwestern Alberta, northeastern British Columbia, and the southern Northwest Territories. In British Columbia this unit is estimated to most likely contain approximately 10 TCF of gas in place and 6.5 TCF of marketable gas. This paper discusses the depositional facies framework, evolution, and reservoir architecture of the Jean Marie Member in the July Lake area of extreme northeastern British Columbia, based on the description and investigation of 18 cores from vertical wells drilled in the early 1990s. The Jean Marie in these wells varies from 11.7 to 21.2 m thick and consists of the upper portion of one Transgressive-Regressive (T-R) cycle and two other complete, overlying T-R cycles. The basal 3 to 4 m of the Jean Marie consists of brachiopod-bearing lime wackestones that were deposited on an areally extensive carbonate ramp. These carbonates were deposited during the regional progradation of limestones out over deeper-water shales of the underlying Fort Simpson Formation. The base of the second T-R cycle marks the change from regional progradation to the start of a more aggradational phase of sedimentation. The second T-R cycle attains a maximum cumulative thickness of 16.5 m and consists of coalescing patch reef deposits. The patch reefs are speculatively interpreted to be only 50 to 100 m across to begin with, but expanded in certain directions by lateral growth during progradation. The uppermost T-R cycle consists of either a continuation of these patch reef facies, the nucleation of Amphipora shoals on underlying T-R cycle 2 patch reefs or intervening, detrital stromatoporoid and coral-bearing foreslope deposits and deeper-water branching coral-bearing limestones. Unlike the second T-R cycle, foreslope facies of T-R cycle 3 consist of more fragmental carbonates with grain-support (pellet packstone) matrices. Progradation during this interval caused further coalescence of patch reefs and Amphipora shoals, and the appreciable infilling of low-lying areas between these reefs and shoals. This resulted in lower depositional relief at the top rather than at the base of this T-R cycle. Preferential partial dolomitization and dissolution of these deposits produced permeable strata between the patch reefs and/or Amphipora shoals. Reservoir facies consist of two main types. Firstly, and most importantly, are lime framestones that make up the patch reefs of the second and third T-R cycles, consisting of in situ platy stromatoporoids and pendent Renalcis that grew downward into shelter cavities beneath the platy stromatoporoids. Porosity occurs mainly in molds or partial molds of Renalcis that are connected by cm-scale, non-planar fractures. Secondly are somewhat less fractured limestones and partially dolomitized limestones of the detrital stromatoporoid-coral foreslope facies that occur as prograding aprons around the patch reefs in the upper T-R cycle. Pores in this facies are predominantly micropores or coalesced micropores, of dissolution origin. The degree of calcite dissolution is generally greater in samples with a higher matrix replacement dolomite content.

Influence of steady base‐level rise on channel mobility, shoreline migration, and scaling properties of a cohesive experimental delta

Here we describe results from an alluvial delta physical experiment with and without steady base‐level rise. Using a unique cohesive sediment mixture that promotes the development of persistent channels and a rugose shoreline, we quantitatively characterize channel network properties as a function of base‐level rise in a distributary system that reproduces many aspects of the geometry of natural deltas. Analysis of the experimental data shows clear dynamical differences between the predominantly progadational and aggradational phases of the experiment. The experiment was conducted in two phases: a first in which the delta prograded into standing water of constant depth in the absence of base‐level rise and a second during which steady base‐level rise was imposed on the system, forcing a twofold increase in topset aggradation due to greater sediment retention in the fluvial reach. The shift in sediment mass balance to enhanced fluvial deposition in the second phase caused channel network mobility to increase, reducing the autogenic channel time scale from 23.5 to 12.5 h and supporting a positive correlation between deposition and channel avulsion frequency. An independent shoreline time scale that characterizes the dominant time over which shoreline regression is persistent closely correlates with measurements of the channel network (28.3 h during fan progradation and 9.5 h during fan aggradation). These metrics suggest a strong coupling between channel network and shoreline kinematics and a more active fluvial surface during fan aggradation by a factor of 2 to 3, similar to the increase in aggradation rate. Spatial scaling of shoreline roughness reveals that maximum shoreline variability is set by the scale of distributary lobes. Strong coupling exists between delta growth and shoreline geometry during progradation. During aggradation, however, shoreline variability is not solely due to distributary lobe growth but is also set by shoreline transgression over inactive portions of the delta, illustrating a decoupling between fan kinematics and fan geometry. This decoupling, together with the matched increase in channel network mobility and fluvial aggradation, suggest the stratigraphic architecture may not be a strong geometric signal of different fluvial surface conditions either.

Fluvial response to climate change during and after the Last Glacial Maximum in Central Anatolia, Turkey

Fluvial response to climate change during and after the Last Glacial Maximum (LGM) included incision–aggradation phases, recorded in sediment cores from the valley floor of the Kızılırmak in Central Anatolia. A numerical chronology was established by AMS dating of fluvial sediments and Ar–Ar dating of a basalt sample capping the youngest terrace (Late Pleistocene terrace), indicating that the main incision phase was completed at the end of the LGM. The succeeding aggradation phase in the valley took place from the end of the LGM onwards. It appears that the major climatic transition from the LGM to the Late Glacial gave rise to aggradation in the fluvial system, and this event seems to be consistent with the timing of the regression phase of pluvial lakes and the termination of paleoglacier advances in the high mountains of Anatolia. This study of fluvial systems in Anatolia indicates that the coldest periods are not always characterized by fluvial aggradation.

Changes in soil N mineralization and nitrification pathways along a mixed forest chronosequence

Changes in soil N mineralization pathways occurring along a full rotation cycle have received little attention to date, while tree uptake for N may change during forest ageing. The aims of this study were (i) to characterize changes in potential net N mineralization and potential net nitrification within organic layers and the topsoil (organo-mineral horizon) along a 100-year chronosequence for a temperate oak–hornbeam forest and (ii) to reveal covariances between potential net N mineralization pathways and the properties of the humic epipedon (defined as the sum of organic layers and topsoil). For that purpose, a space-for-time substitution procedure and aerobic laboratory incubation method for 28 days at 28 °C in the dark were used. In addition, acetylene and captan were used to discriminate between autotrophic and heterotrophic (bacterial and/or fungal) nitrification. Several humic epipedon properties were determined, e.g. pH, exchangeable cation concentrations, effective cation exchange capacity, total C and N, dissolved organic C and N, fungal and microbial biomass N. Potential net N mineralization and nitrification pathways changed greatly along the mixed forest chronosequence. Potential net N mineralization in the organic layers increased with stand maturation whereas potential net nitrification in the topsoil decreased significantly. Selective inhibitors revealed changes in nitrification pathways along the chronosequence, i.e. potential net nitrification was autotrophic in the topsoil while it was mainly heterotrophic within the organic layers. In the organic layer, potential net nitrification was autotrophic at the onset of the chronosequence while it appeared heterotrophic during the aggradation phase and finally fungal in mature stands. A Co-Inertia Analysis was used to reveal covariances between N mineralization pathways and humic epipedon properties. The analysis showed two functional temporal shifts within N cycling along the chronosequence, one probably controlled by organic matter quality and high competition for available N resulting in the autotrophic versus heterotrophic nitrification shift in the organic layers and one mainly controlled by (i) fine organic matter abundance, allowing high N mineralization in the organic layers and (ii) acidity inhibited autotrophic nitrification in the topsoil.

Do indicators of nitrogen retention and leaching differ between coniferous and broadleaved forests in Denmark?

The area of broadleaved forests is projected to increase in Denmark as well as in the rest of Europe. However, studies of the N leaching response to elevated N deposition have focused on coniferous stands and considerable uncertainty still remains on whether broadleaved and coniferous forests respond differently to elevated N. We studied N input–output relations for eight stands intensively monitored during 2002–2005 and literature data for 37 additional stands which together formed a comprehensive dataset on Danish forests including 26 broadleaved stands and 19 coniferous stands. Nitrate leaching was significantly higher in first generation stands on former arable land with mineral soil C/N ratios 10–15, but both low and high rates were observed independent of the N input. A net N loss was observed in some of these stands even though they are in the aggrading phase and accumulate N in the biomass. Broadleaved stands had significantly lower throughfall N deposition than coniferous stands and this seems to be the main process where forest type exerts an influence on the N cycle. Lower soil C/N ratios offset the effect of throughfall N deposition and thus N leaching did not differ between the two forest types. The best regression models for prediction of nitrate leaching included throughfall N deposition and C/N ratio, but only a minor part of the variability was explained. The C/N ratio of the upper mineral soil was more generally applicable than that of the organic layer. The N retention of the soil was reasonably well predicted above a C/N ratio of 25, but below this threshold the importance is not known. We suggest focusing future efforts on quantifying the relative retention functions (sink strength) of the vegetation and the soil organic matter to improve the predictions of N retention and N leaching.

Drainage basin response to climate change in the Pisco valley, Peru

Research Article| June 01, 2009 Drainage basin response to climate change in the Pisco valley, Peru Damian Steffen; Damian Steffen 1Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Fritz Schlunegger; Fritz Schlunegger 1Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Frank Preusser Frank Preusser 1Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Geology (2009) 37 (6): 491–494. https://doi.org/10.1130/G25475A.1 Article history received: 26 Aug 2008 rev-recd: 30 Dec 2008 accepted: 15 Jan 2009 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Damian Steffen, Fritz Schlunegger, Frank Preusser; Drainage basin response to climate change in the Pisco valley, Peru. Geology 2009;; 37 (6): 491–494. doi: https://doi.org/10.1130/G25475A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Quaternary development of the Pisco valley in central Peru has been characterized by multiple phases of sediment accumulation and erosion that formed distinct levels of cut-and-fill terraces and alluvial fans. Luminescence dating shows that they formed in response to at least two different stages of sediment accumulation and erosion during the past 60 ka, the main phase of sediment aggradation occurring between ca. 54 and 38 ka ago. The ages show that sediment accumulation was contemporaneous with the time intervals of the Minchin (47.8–36 ka ago, with enhanced precipitation beginning ca. 54.8 ka ago) and Tauca (26–14.9 ka ago) paleolakes on the Altiplano, where the headwaters of the Pisco River are located. We conclude that sediment accumulation was triggered by shifts toward a more humid climate, whereas erosion is the response of the fluvial system to the depletion of the hillslope sediment reservoirs. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Quaternary Paleohydrology of the Madre de Dios River, southwestern Amazon Basin, Peru

Late Quaternary climatic and hydrologic variability triggered changes in fluvial deposition and erosion along the course of the Madre de Dios River, Peru, the largest tributary basin of the Madeira basin, itself the largest tributary basin of the Amazon. Three laterally extensive, Quaternary-age, terrace tracts are present within the Madre de Dios basin. Analysis of sedimentary facies, present in the modern cut banks and terraced sequences, along with radiocarbon dates on fossil wood and leaf material preserved in the terraced strata, allow reconstruction of the Late Quaternary depositional history of the sedimentary sequences, including determination of the approximate timing of aggradation and downcutting episodes and its relationship to the timing of past climate change in this portion of the Amazon basin and beyond. The Quaternary sediments underlying the terraces most often recorded deposition in a coarse-grained meandering fluvial system. The T3 terrace, the highest terrace, is underlain by the Miocene (?) Ipururi Formation, which is unconformably overlain by the late Miocene–Pleistocene (?) (> 48,000 cal yrs BP) Madre de Dios Formation, a multistory coarse-sandy to gravelly channel and point bar complex. The latter was downcut before 29,850 ± 100 cal yrs BP. This downcut landscape was infilled by meandering fluvial strata characterized by gravelly channel deposits in a sequence dominated by floodplain and lateral accretion deposits. These strata were in turn downcut to form the T2 terrace before 11,970 ± 100 cal yrs BP. A third episode of aggradation resulted in the deposition of a sand-dominated meandering channel complex that infilled the T2 valley and was subsequently downcut after 3780 ± 50 cal yrs BP. This most recent terrace is infilled by the modern fluvial sediment, which has been actively aggrading since at least 870 ± 50 cal yrs BP. Importantly, the Madre de Dios fluvial system actively aggraded between 30,000 and 25,000 cal yrs BP, (and likely much younger, as dated samples were, thus far, only found near the base of the T2 sequence). This observation implies that some combination of (1) increased precipitation and decreased temperature, (2) decreased evapotranspiration and increased runoff, (3) increased Andean glacial erosion and increased sediment supply, and (4) decreased atmospheric CO 2 (hence decreased rain-forest primary productivity and altered rain-forest physiology/ecology), entering the last glacial maximum period brought about increased floodplain deposition in the southwestern Amazon. Elsewhere in the Amazon basin few, if any, fluvial sediments of this age range have been observed. The start of the next major phase of aggradation coincided with the Younger Dryas and suggested that floodplain sedimentation in the lowlands was again related to cold and wet conditions in the adjacent highlands (and perhaps in the lowlands as well) and that Madre de Dios history was also tied to large-scale global climate. This aggradation may have continued throughout the early and mid-Holocene, until at least 3,780 cal yr BP. If so (and this is uncertain), this episode of sedimentation took place during a dry period.

Monsoon triggered formation of Quaternary alluvial megafans in the interior of Oman

A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.

Conceptual framework for assessing the response of delta channel networks to Holocene sea level rise

Recent research has identified two fundamental unit processes that build delta distributary channels. The first is mouth-bar deposition at the shoreline and subsequent channel bifurcation, which is driven by progradation of the shoreline; the second is avulsion to a new channel, a result of aggradation of the delta topset. The former creates relatively small, branching networks such as Wax Lake Delta; the latter generates relatively few, long distributaries such as the Mississippi and Atchafalaya channels on the Mississippi Delta. The relative rate of progradation to aggradation, and hence the creation of accommodation space, emerges as a controlling parameter on channel network form. Field and experimental research has identified sea level as the dominant control on Holocene delta growth worldwide, and has empirically linked channel network changes to changes in the rate of sea level rise. Here I outline a simple modeling framework for distributary network evolution, and use this to explore large-scale changes in Holocene channel pattern that have been observed in deltas such as the Rhine-Meuse and Mississippi. Rapid early- to mid-Holocene sea level rise forced many deltas into an aggradational mode, where I hypothesize that avulsion and the generation of large-scale branches should dominate. Slowing of sea level rise in the last ∼6000 yr allowed partitioning of sediment into progradation, facilitating the growth of smaller-scale distributary trees at the shorelines of some deltas, and a reduction in the number of large-scale branches. Significant antecedent topography modulates delta response; the filling of large incised valleys, for example, caused many deltas to bypass the aggradational phase. Human effects on deltas can be cast in terms of geologic controls affecting accommodation: constriction of channels forces rapid local progradation and mouth-bar bifurcation, while accelerated sea level rise increases aggradation and induces more frequent channel avulsion.

Fluvial response to Holocene glacier fluctuations in the Nostetuko River valley, southern Coast Mountains, British Columbia

Abstract Mountain rivers, like alpine glaciers, are sensitive indicators of climate change. Some rivers may provide a more complete record of Holocene climate change than the glaciers in their headwaters. We illustrate these points by examining the record preserved in the upper part of the alluvial fill in the Nostetuko River valley in the southern Coast Mountains, British Columbia (Canada). Glacier advances in the upper part of the watershed triggered valley-wide aggradation and complex changes in river planform. Periods when glaciers were restricted in extent coincide with periods of incision of the valley fill and floodplain stability. As many as 10 overbank aggradation units are separated by peat layers containing tree roots and stems in growth position. Twenty-five radiocarbon ages on roots, tree stems and woody plant detritus in several of the peat layers closely delimit periods of aggradation. The oldest phase of aggradation occurred about 6500 years bp and coincides with the Garibaldi Advance documented elsewhere in the southern Coast Mountains. A second phase of aggradation, recorded by several units of clastic sediment, dates to about 2500 years bp , near the peak of the middle Neoglacial Tiedemann Advance. The third phase occurred shortly after 1400 years bp during or shortly after the First Millennium Advance, which has been recently documented in coastal British Columbia and Alaska. The most recent phase of aggradation began about 800 years bp and continued until recently. It coincides with the Little Ice Age, when glaciers in the Nostetuko River basin and elsewhere in the southern Coast Mountains attained their greatest Holocene size. Several periods of peat deposition during the Little Ice Age indicate periods of floodplain stability separated by brief intervals of floodplain aggradation that coincide with Little Ice Age glacier advances in western Canada. The results imply that the west fork of Nostetuko River is sensitive to upvalley glacier fluctuations and, indirectly, to relatively minor changes in climate.

Climatic signature of cyclic fluvial architecture from the Quaternary of the central Po Plain, Italy

Detailed investigation of middle-upper Quaternary deposits from central Po Plain was carried out on the basis of integrated sedimentological and pollen analyses of a 114 m-long core, and stratigraphic correlations of well data. Stratigraphic architecture of alluvial strata beneath modern Po River reveals distinctive cyclic changes in lithofacies and channel stacking patterns. Each cycle includes basal, silt–clay overbank deposits with thin and lenticular fluvial-channel sands, with upward transition to increasingly amalgamated and more laterally extensive fluvial-channel sand bodies. Lower cycle boundaries, corresponding to the top of laterally extensive fluvial complexes, are locally associated with organic-rich, paludal clays. Pollen records show distinctive cyclic changes that parallel facies architecture. Overbank deposits in the lower parts of cycles are invariably associated with forest expansions. These indicate that major phases of channel abandonment and widespread floodplain aggradation took place at the onset of warm-temperate (interglacial) climatic conditions. The middle portions of cycles record transition to pollen associations diagnostic of glacial periods. Lack of pollen data from sandy facies prevents climatic characterization of laterally extensive fluvial bodies in the upper parts of cycles. On the basis of the overall stratigraphic framework, internal facies architecture and distinctive pollen attributes, the cycles identified in the subsurface of the study area are interpreted to fall in the Milankovitch (100 ka) band, and inferred to be correlative with the transgressive–regressive sequences recently recognized in the coeval successions of the Po coastal plain.

Fashion and phases of late Pleistocene aggradation and incision in the Alaknanda River Valley, western Himalaya, India

We study the aggradation and incision of the Alaknanda River Valley during the late Pleistocene and Holocene. The morphostratigraphy in the river valley at Deoprayag shows the active riverbed, a cut terrace, and a fill terrace. The sedimentary fabric of the fill terrace comprises four lithofacies representing 1) riverbed accretion, 2) locally derived debris fan, 3) the deposits of waning floods and 4) palaeoflood records. The sedimentation style, coupled with geochemical analysis and Optically Stimulated Luminescence (OSL) dating, indicate that this terrace formed in a drier climate and the river valley aggraded in two phases during 21–18 ka and 13–9 ka. During these periods, sediment supply was relatively higher. Incision began after 10 ka in response to a strengthened monsoon and aided by increase of the tectonic gradient. The cut terrace formed at ~ 5 ka during a phase of stable climate and tectonic quiescence. The palaeoflood records suggest wetter climate 200–300 yr ago when the floods originated in the upper catchment of the Higher Himalaya and in the relatively drier climate ~ 1.2 ka when locally derived sediments from the Lesser Himalaya dominated flood deposits. Maximum and minimum limits of bedrock incision rate at Deoprayag are 2.3 mm/a and 1.4 mm/a.

Geomorphology, Stratigraphy and 14C-Chronology of Ancient Tufas at Louie Creek, Northwest Queensland, Australia

Louie Creek is a karst spring-fed stream situated in the seasonally humid tropics of northwest Queensland, Australia. It rises as a series of small exsurgences along the eastern edge of the Barkly Tableland. As it enters the lowlands of the Carpentaria plain, the creek deposits tufa which produces a series of cascades. This modern tufa extends discontinuously for about 1.5 km. A series of ancient tufas, in places lying adjacent to sites of modern deposition, extends discontinously for about 8 km downstream. At least two ancient tufa units are preserved at one location, Little lndarri site. The older unit comprises a sequence of well-preserved barrages with an orientation transverse to present-day stream flow. In places, erosion has reduced these barrages to their calcrete substrate. The older tufa is overlain in places by sediment which has become cemented to form a second calcrete unit. This sediment is in turn succeeded by the younger ancient tufa. Subsequent river incision has removed part of the sediment from the older unit and exposed several contact points between the ancient tufa and calcrete units. Radiocarbon dating of the Little lndarri site tufas, as well as other ancient Louie Creek units, yielded apparent ages ranging from ~ 30 to ~ 14 ka BP, suggesting that conditions were sufficiently wet during the period immediately preceding and throughout the Last Glacial Maximum for tufa depostion to occur. However, ancient tufa formation occurred during a phase of net river aggradation. There is geomorphic evidence that such aggradation was a result of an increased sediment supply to the fluvial system, most likely in response to conditions drier than present. Results from studies elsewhere in the region support such a Late Pleistocene trend. Incision of Louie Creek, which postdates the youngest of the dated ancient tufas, is most likely to have resulted from a shift to wetter conditions during the early Holocene,

Late Holocene debris cone development and vegetation and land-use history in the Pasture Beck valley, Lake District, NW England

SUMMARY Investigations of a sedimentary sequence exposed in the distal margin of a small debris cone in the valley of Pasture Beck, eastern Lake District, have yielded detailed information concerning the time and nature of cone aggradation phases and the local vegetation and land-use history. Radiocarbon dates from two buried peat beds indicate that the cone surface was generally stable from the late 8 th to the early 13 th centuries AD (1165–736 cal. BP). An interlude of colluvial gravel deposition interrupted peat accumulation and was probably the result of a single short-lived event. Gravel-dominated beds represent deposition of reworked rockfall debris and glacial drift by cohesionless debris flows, whereas finer-grain beds are probably the result of sheet-flows derived by washout of fines from debris-flow deposits. The stratigraphic evidence indicates that the minerogenic sediments accumulated in quick succession. Pollen and charcoal data from peat samples reveal intensification of land-use before burial of the peats by minerogenic sediments. It is proposed that landscape instability was associated first with Norse-Irish settlement and woodland clearance, and later, probably, to the expansion of upland pastures and more intensive grazing in the 13 th century AD. When the Pasture Beck data is considered along with information from elsewhere in the British Isles, it is apparent that not all patterns of Late Holocene landscape change in upland areas can be ascribed to a single cause.

Alluvial megafans in the Venetian–Friulian Plain (north-eastern Italy): Evidence of sedimentary and erosive phases during Late Pleistocene and Holocene

The Venetian–Friulian Plain is the eastern part of the foreland basin of the Southern Alps and is characterized by the presence of alluvial megafans. Existence of these large landforms results from the tectonic setting, but their Late Pleistocene and Holocene evolution has been mainly controlled by climatic change and eustasy. Geomorphological, geological information and radiocarbon dating allow the recognition of phases of sedimentation and incision in the megafans of the Brenta, Piave and Tagliamento rivers. The main phase of aggradation took place during the Last Glacial Maximum (LGM) (24–15 ka BP), when the glaciers emanating from the Alpine valleys reached the plain and supplied large amounts of sediments. During the Late LGM the rate of aggradation lowered in the Brenta megafan and a wide incision of the fanhead developed in the Tagliamento megafan. During Late Glacial and Early Holocene an important phase of incision took place, and smaller telescopic lobes formed in the distal portion of Brenta and Tagliamento megafans. Sedimentation was absent or very low between 14 and 8 ka BP and only since the Middle Holocene a new phase of deposition affected the coastal areas, probably related to a marine highstand. Widespread aggradation started once more around 4–3 ka BP, with the formation of fluvial ridges along the terminal tract of Alpine rivers. Since the Roman period, human influence affected the alluvial sedimentation, especially in the last centuries, due to land reclamation and construction of river embankment.