Phases Of Aggradation Research Articles

Two Late Pleistocene climate-driven incision/aggradation rhythms in the middle Dnieper River basin, west-central Russian Plain

In valleys of the River Seim and its tributaries in the middle Dnieper basin (west-central Russian Plain), two low terraces (T1, 10–16 m, and T0, 5–7 m above the river) and a floodplain (2–4 m) with characteristic large and small palaeochannels exist. A range of field and laboratory techniques was applied and ∼30 new numerical ages (OSL and 14C dates) were obtained to establish a chronology of incision and aggradation events that resulted in the current valley morphology. Two full incision/aggradation rhythms and one additional aggradation phase from the previous rhythm were recognized in the Late Pleistocene – Holocene climate cycle. The following events were detected. (1) Late MIS 5 – early MIS 4: aggradation of Terrace T1 following the deep incision at the end of MIS 6. (2) Late MIS 4 (40–30 ka): incision into Terrace T1 below the present-day river, formation of the main scarp in the bottom of the valley between Terrace T1 and Terrace T0/Floodplain levels. (3) MIS 2: aggradation of Terrace T0, lateral migrations of a shallow braided channel located few meters above the present-day river since ∼25 ka through the LGM. (4) 18–13 ka: incision into Terrace T0 below the modern river. Multiple-thread channels concentrated in a single flow that at some places formed large meanders. In the period 15–13 ka, high floods that rose above the present-day floods left large levees and overbank loams on Terrace T0. (5) Younger Dryas – Holocene transition: aggradation up to the modern channel level, transformation of large Late Glacial to small Holocene meanders. The established incision/aggradation rhythms are believed to be manifested over the Central Russian Plain outside the influence of ice sheets in the north and base level changes in the south.The two-phase deepening of the valley occurred in the last quarter of the last glacial epoch but can not be attributed directly to the glacial-interglacial transition. Both the detected incision events correspond to relatively warm climate phases – late MIS 3, post-LGM warming including the Bølling-Allerød interstadial. Anomalously large size of the preserved river palaeochannels prove that the post-LGM incision phase was induced by a climatically forced large increase of water runoff. Considerable increase of water discharges is considered the most probably cause for the late MIS 4 incision phase also. Therefore river incision seems to have been governed rather by changing water runoff that oscillated in phase shift with the thermal regime.

Depositional evolution of a progradational to aggradational, mixed-influenced deltaic succession: Jurassic Tofte and Ile formations, southern Halten Terrace, offshore Norway

Predicting the hydrodynamics, morphology and evolution of ancient deltaic successions requires the evaluation of the three-dimensional depositional process regime based on sedimentary facies analysis. This has been applied to a core-based subsurface facies analysis of a mixed-energy, clastic coastal-deltaic succession in the Lower-to-Middle Jurassic of the Halten Terrace, offshore mid-Norway. Three genetically related successions with a total thickness of 100–300 m and a total duration of 12.5 Myr comprising eight facies associations record two initial progradational phases and a final aggradational phase. The progradational phases (I and II) consist of coarsening upward successions that pass from prodelta and offshore mudstones (FA1), through delta front and mouth bar sandstones (FA2) and into erosionally based fluvial- (FA3) and marine-influenced (FA4) channel fills. The two progradational phases are interpreted as fluvial- and wave-dominated, tide-influenced deltas. The aggradational phase (III) consists of distributary channel fills (FA3 and FA4), tide-dominated channels (FA5), intertidal to subtidal heterolithic fine-grained sandstones (FA6) and coals (FA7). The aggradational phase displays more complex facies relationships and a wider range of environments, including (1) mixed tide- and fluvial-dominated, wave-influenced deltas, (2) non-deltaic shorelines (tidal channels, tidal flats and vegetated swamps), and (3) lower shoreface deposits (FA8). The progradational to aggradational evolution of this coastal succession is represented by an overall upward decrease in grain size, decrease in fluvial influence and increase in tidal influence. This evolution is attributed to an allogenic increase in the rate of accommodation space generation relative to sediment supply due to tectonic activity of the rift basin. In addition, during progradation, there was also an autogenic increase in sediment storage on the coastal plain, resulting in a gradual autoretreat of the depositional system. This is manifested in the subsequent aggradation of the system, when coarse-grained sandstones were trapped in proximal locations, while only finer grained sediment reached the coastline, where it was readily reworked by tidal and wave processes.

The Holocene evolution and geomorphology of a chain of ponds, southeast Australia: Establishing a physical template for river management

Chains of ponds are a discontinuous river type found in Australia. Their unusual morphology, important ecological functions and increasing rarity make them a priority for conservation, and yet very little research has investigated their physical structure, behaviour and evolution. This paper reconstructs the Holocene evolution and environmental history of Crisp's Creek, a headwater chain of ponds in the Southern Tablelands of NSW Australia. This history establishes baseline information on the physical template that can be used to assess a range of other biophysical processes and design appropriate river rehabilitation and management strategies for these rivers. Sedimentary aggradation began in the Late Holocene, at least 3.7–1.3ka, broadly synchronous with aggradation phases at other sites in southeast Australia. Since European arrival in the Nineteenth Century, parts of the river incised, destroying intact ponds and smothering formerly swampy floodplains with post-incision alluvium. However, sections unaffected by incision provide a rare opportunity to examine the evolutionary trajectory of an intact variant of these rivers. This research deepens our understanding of the evolutionary context for contemporary river behaviour as relevant for designing appropriate and effective conservation and rehabilitation strategies for chains of ponds.

The role of pre-existing topography in the evolution of post-glacial fluvial landforms in the middle Satluj valley, north-western Himalaya, India

Abstract In tectonically active orogen like the Himalaya while computing the bedrock incision rate it is assumed that the underlying incised bedrock is genetically related to the overlying alluvium. This assumption may not be applicable universally, considering that the trans-Himalayan rivers were in existence before Miocene or earlier. Therefore, the sediments preserved at various locations represent the youngest phase of aggradation on the pre-existing valley topography – a cumulative expression of multiple cycles of erosion and aggradation. Considering that the fluvial terraces are extensively used to estimate the incision/uplift rates, it is therefore pertinent to ensure that the underlying bedrock topography is indeed genetically related to the overlying fluvial sediments and is not a relic of the pre-existing topography. The present study is aimed at understanding the relationship between the valley and channel-fill sediments and the underlying bedrock topography in the evolution of fluvial landforms. Based on the detailed field mapping of the relict channel/fossil valley-fill sequences, geomorphometry and limited optical ages the study suggests that the bedrock topography is cumulative expression of multiple cycles of erosion while the post glacial valley-fill sediments (dated to 14 ka and 8 ka) represent the most recent aggradation phase. The discreet linear strath terraces and epigenetic gorges represent a combination of climate and tectonically induced mid-Holocene river adjustment particularly in the areas proximal to local thrusts. Further, the study suggests that the securely dated strath terraces may provide the average tectonically induced incision/uplift rates whereas the epigenetic gorges largely represent climatically controlled focused and local uplift/incision in the study area. A preliminary estimate based on the strath terraces indicate that the terrain is being incised/uplifted at ∼0.6–1.3 mm/yr whereas as expected the epigenetic gorges provide a relatively higher incision rate (3.8 mm/yr to 12.5 mm/yr). Finally, the study cautions against using the conventional age-depth relationship of valley-fills and the underlying incised bedrock to ascertain the incision/uplift rates. Instead it suggests that the morphology of the underlying bedrock must be taken in to account in order to avoid erroneous estimate of incision/uplift rates (crustal deformation) in a tectonically active orogen.

Initial micromorphological results from Liang Bua, Flores (Indonesia): Site formation processes and hominin activities at the type locality of Homo floresiensis

Liang Bua, a karstic cave located on the island of Flores in eastern Indonesia, is best known for yielding the holotype of the diminutive hominin Homo floresiensis from Late Pleistocene sediments. Modern human remains have also been recovered from the Holocene deposits, and abundant archaeological and faunal remains occur throughout the sequence. The cave, the catchment in which it is located and the gross aggradational phases of the sediment sequence have all been subject to a great deal of scientific scrutiny since the discovery of the holotype of H. floresiensis in 2003. A recent program of geoarchaeological research has extended analyses of the site’s deposits to the microstratigraphic (micromorphological) level. The stratigraphic sequence in the cave is well defined but complex, comprising interstratified sediments of diverse lithologies and polygenetic origins, including volcanic tephras, fine-grained colluvium, coarse autogenic limestone gravels, speleothems and anthropogenic sediments, such as combustion features. The sedimentological and chemical heterogeneity suggest that processes of site formation and diagenesis varied markedly through time, both laterally and vertically. We present initial results from samples collected in 2014 from an excavation area near the rear of the cave, which yielded radiocarbon ages from charcoal that fill an important temporal gap in the chrono-stratigraphic sequence of previously excavated areas of the site. The results indicate marked changes in site environment and hominin activity during the Late Pleistocene, relating primarily to the degree to which the cave was connected to the hydrogeological system and to the varying intensities of use of the cave by hominins. Importantly, we identify anthropogenic signs of fire-use at the site between 41 and 24 thousand years ago, most likely related to the presence of modern humans.

The MIS 5.5 terraced deposit of Fosso del Cupo (Montecelio, Central Italy) and its Mousterian lithic assemblage: Re-evaluation of a nineteenth-century discovery

Abstract By means of a geomorphological study and the correlation with the geochronologically constrained terraced deposits of the greater area of Rome, we attribute to the aggradational phase during marine isotopic stage (MIS) 5.5 a terraced deposit firstly described in the earliest local geological reports of the 19th century, in which a small set of lithic artifacts and vertebrate fossil remains was recovered. After this correlation, the sedimentary deposit of Fosso del Cupo represents the only inland occurrence of an aggradational deposit of MIS 5.5 so far recognized in the area of Rome, where a rich record of lithic industries and faunal assemblages has been yielded by the sedimentary successions deposited in response to deglaciation during sea-level rises of MIS 15 through MIS 7. We have implemented the lithic and faunal assemblages through a collection during the geological survey performed in the area where the terraced deposit crops out. With an age tightly constrained around 125 ka, the lithic industry and the faunal assemblage from Fosso del Cupo, although limited in number, represent an important witness of the early development of the local Pontinian culture and should be considered a regional marker, which may concur to improve the knowledge on the Mousterian of Central Italy.

Reconstruction of Late Quaternary climate and seismicity using fluvial landforms in Pindar River valley, Central Himalaya, Uttarakhand, India

Fluvial landforms in the Pindar River valley are investigated to understand the role of temporal variability in the Indian Summer Monsoon (ISM) and the spatial changes in crustal deformation. Employing the conventional geomorphological and sedimentological concepts, supported by optical dating, three major phases of valley-fill aggradation separated by phases on non-deposition are discerned. An oldest aggradation phase is dated to 33.5 ka during relatively strengthened ISM corresponding to the later part of pluvial Marine Isotopic Stage-3 (MIS-3). Following this, alluvial fan sedimentation impersistently continued during the declining phase of the ISM (until around 21 ka). A second major aggradation phase (17.5 ka–13 ka) occurred during the post-LGM/lateglacial strengthening of the ISM in which contribution from tributary valleys overwhelmed the valley-fill sedimentation. A youngest aggradation phase dated to the middle to late-Holocene (8 ka–3.6 ka), represents the transitional climate during which sporadic high sediment fluxes both from the upper catchment and tributary streams led to the development of fossil valleys (abandonment of old channels).The spatial variability in the incision/uplift rate, based on river strath terraces reveals that the terrain is undergoing differential crustal deformation. Two zones of relatively high crustal deformation are identified. These are located in the vicinity of the Main Central Thrust (MCT) and is attributed to the activity along the transverse fault and the in the inner Lesser Himalaya for which the thrust propagated fold associated with regional north-south compression is implicated. The study suggests that fluvial landforms (valley-fill and strath terraces) in the Pinder river valley are genetically related to the multi-millennial scale changes in the ISM and spatial and temporal changes in the crustal deformation associated with regional compression.

Late Quaternary tectonic landforms and fluvial aggradation in the Saryu River valley: Central Kumaun Himalaya

The present study has been carried out with special emphasis on the aggradational landforms to explain the spatial and temporal variability in phases of aggradation/incision in response to tectonic activity during the late Quaternary in the Saryu River valley in central Kumaun Himalaya. The valley has preserved cut-and-fill terraces with thick alluvial cover, debris flow terraces, and bedrock strath terraces that provide signatures of tectonic activity and climate. Morphostratigraphy of the terraces reveals that the oldest landforms preserved south of the Main Central Thrust, the fluvial modified debris flow terraces, were developed between 30 and 45ka. The major phase of valley fill is dated between 14 and 22ka. The youngest phase of aggradation is dated at early and mid-Holocene (9–3ka). Following this, several phases of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces. Seven major phases of bedrock incision/uplift have been estimated during 44ka (3.34mm/year), 35ka (1.84mm/year), 15ka (0.91mm/year), 14ka (0.83mm/year), 9ka (1.75mm/year), 7ka (5.38mm/year), and around 3ka (4.4mm/year) from the strath terraces near major thrusts. We postulate that between 9 and 3ka the terrain witnessed relatively enhanced surface uplift (2–5mm/year).

Late Pleistocene river migrations in response to thrust belt advance and sediment-flux steering — The Kura River (southern Caucasus)

One reaction of rivers toward allogenic triggers is the large-scale river channel migration in the form of avulsions or progressive lateral migrations (combing) that are widespread phenomena around the world during the late Quaternary. Because they potentially cause significant human and economic losses and significantly change geomorphic processes in the affected regions, a deeper knowledge about causes and rates is essential and furthermore helps to identify the dominant drivers of regional landscape evolution during different periods. One possible cause for river channel migrations is sediment-flux steering, i.e. the shift of rivers in sedimentary basins against a tectonically driven trend caused by transverse sediment discharge. During the last 30years, sediment-flux steering has been investigated by field and experimental studies in extensional half-grabens with generally small-sized transverse catchments and/or volcaniclastic sedimentation.This study presents geomorphologic, geochronologic, and heavy mineral analyses together with complementary tectonomorphometric and earthquake data to investigate late Quaternary channel migrations of the Kura River in the southern foreland basin of the Greater Caucasus, a region where the late Quaternary landscape evolution is rather fragmentarily understood so far. Special emphasis of this study is given to the interplay between axial river flow and transverse sediment supply leading to sediment-flux steering. Large-scale migrations of the course of the Kura River during the late Quaternary reflect the interplay between tectonic processes leading to the southwestward advance of the Kura Fold-and-Thrust-Belt and climatically-triggered sediment-flux steering caused by aggradation phases of transverse rivers with comparatively large catchment areas in the Lesser Caucasus. During generally warmer periods such as the Holocene with fluvial incision and low sediment supply from the transverse rivers, the main Kura River could follow its tectonically driven trend toward the southwest. In contrast, during generally colder periods such as the upper late Pleistocene, sediment-flux steering caused by aggradation of the transverse rivers forced the main Kura River to migrate >10km against that tectonically induced trend toward the northeast. Apart from giving information about the main drivers of the late Quaternary landscape evolution in this part of the southern Caucasus region, this study also helps to understand the cause of a permanent threat of settlements and the loss of fertile agricultural land in the intensively labored Marneuli Depression of southern Georgia.

Geomorphic evolution of Dehra Dun, NW Himalaya: Tectonics and climatic coupling

The Dehra Dun is a good example of a piggyback basin formed from the growth of the Siwalik hills. Two large rivers, the Ganga and the Yamuna, and their tributaries deposit a significant part of their sediment load in the Dun before they enter the Gangetic plains. This work documents the geomorphic complexities and landform evolution of the Dehra Dun through geomorphic mapping and chronostratigraphic investigation of the incised fan sections. Lesser Himalayan hills, inner and outer dissected hills, isolated hills, proximal fan, distal fan, dip slope unit, floodplains, and terraces are the major geomorphic units identified in the area. Isolated hills of fan material (IHF), proximal fan (PF), and distal fan (DF) are identified as fan surfaces from north to south of the valley. The OSL based chronology of the fan sediments suggests that the IHF is the oldest fan consisting of debris flow deposits with a maximum age of ~43ka coinciding with the precipitation minima. The proximal fan consisting of sheet flow deposits represents the second phase of aggradation between 34 and 21ka caused by shifting of deposition locus downstream triggered by high sediment supply that exceeded the transport capacity. The distal fan was formed by braided river deposits during 20–11ka coinciding with the deglacial period. The IHF, PF and DF surfaces were abandoned by distinct incision phases during ~40–35, ~20–17, and ~11–4ka respectively. A minor phase of terrace deposition in Dehra Dun was documented during 3–2ka. Our results thus show that the evolutionary history of the alluvial fans in Dehra Dun was primarily controlled by climatic forcing with tectonics playing a minimum role in terms of providing accommodation space and sediment production.

Late Pleistocene–Holocene uplift driven terrace formation and climate-tectonic interplay from a seismically active intraplate setting: An example from Kachchh, Western India

Fluvial terrace formation is often regulated by external forcings like climate, tectonic and eustatic changes. These terraces, particularly in a dryland environment, preserves the discrete signatures of these external forcings, thus enabling us to reconstruct the fluvial response to the late Quaternary palaeoenvironmental changes and factors governing them. The present study focuses on reconstructing the aggradation/incision phases in the Lotia River which is located in the eastern segment of the Northern Hill Range (NHR) of the Kachchh Peninsula. The Lotia river drains through Mesozoic rocks before cutting across the Kachchh Mainland Fault (KMF) and finally debouch in the Banni Plains. Reconstruction based on tectonic geomorphology, sedimentology, sediment geochemistry, mineral magnetic, and OSL chronology suggests the fluvial response to monsoon variability archived during the last 15ka. The time frame was also marked by incision enhanced by uplift along the KMF, which led to strath terrace formation. The accommodation space thus created was filled by an aggradational event between 14.8ka and 10.6ka. Sedimentological and geochemical parameters have also suggested that the time period between 12.5ka and 11.5ka showed a decline in the monsoon strength, which coincides with ‘Younger Dryas’. It has been observed that the sediments spanning between 10.6ka and 7.8ka are absent from the archive, which is most likely the manifestation of the early Holocene optimum that led to severe erosional processes. The period between 7.8ka and 3.3ka is marked as another aggradational phase with fluctuating climatic conditions. At 3.3ka, the region has experienced an incision of 4m, which led to the formation of Holocene terrace T1, most likely due to tectonic uplift. During the last 3.3ka, another pulsative uplift has occurred, which led to the formation of unpaired Holocene terrace T2, along with tilting of the Lotia basin. Based on the OSL chronology of bedrock strath terrace, we reported a minimum uplift rate of 1.04mm/a for the eastern KMF during the Late Pleistocene–Holocene period, hinting seismically active nature of the KMF during this period. The terrace formation in the eastern Northern Hill Range is mostly regulated by tectonic uplifts along the KMF.

Spatial characteristics of the Pliocene to modern alluvial fan successions in the uplifted sedimentary basins of Almería, SE Spain: review and regional synthesis

Abstract Conventional sedimentological models of alluvial fan sequences tend to be based on fans in subsiding sedimentary basins. Fans in uplifting terrains are rather different. We examined the sequence of fan delta and alluvial fan deposition from the Pliocene to the Late Quaternary in the uplifting terrain of the Neogene sedimentary basins of Almería, Betic Cordillera, SE Spain. There were three phases of fan deposition, each yielding particular morpho-sedimentological characteristics and each having distinctive spatial distributions. The first phase was the Pliocene fan deltas, deposited where the larger river systems entered the receding early Pliocene seas. This phase was followed during the late Pliocene–Early Pleistocene by a phase of extensive alluvial fan deposition extending from the mountain fronts to the basin centres. Continued uplift occurred during the Pleistocene, resulting in incision of the drainage network and dissection of the Plio-Pleistocene fan systems. The third phase, during the Quaternary, involved much smaller mountain front alluvial fans set well below the Plio-Pleistocene fans, except in local areas of tectonic subsidence where extensive fans were formed. The older fans exhibit stacked sedimentary successions with very little evidence of sustained vertical incision. However, the Quaternary fans show much evidence of sustained incision between major aggradational phases, often resulting in inset rather than stacked stratigraphic relationships. We interpret this to imply that, although the tectonic context was the primary temporal control on variations within the older fan successions, within the Quaternary fans the overall tectonic control has been modified by responses to the Quaternary climatic sequence.

OSL‐based chronostratigraphy of river terraces in mountainous areas,Dunajec basin,WestCarpathians: a revision of the climatostratigraphical approach

The technique of optically stimulated luminescence (OSL) dating applied to fluvial sediments provided a geochronological framework of river terrace formation in the middle part of the Dunajec River basin – a reference area for studies of evolution of river valleys in the northern part of the Carpathians (West Carpathians). Fluvial sediments at 18–90 m above valley bottoms were dated in the valleys of the Dunajec River and one of its tributaries. The resulting ages range from 158.9±8.3 to 12.2±1.3 ka. This indicates that some of the terrace sediments were deposited much later than previously assumed on the grounds of a combined morphostratigraphical and climatostratigraphical approach. TheOSL‐based chronostratigraphy of terrace formation consists of seven separate phases of fluvial aggradation, separated by periods of incision and lateral erosion. Some of the ages determined correspond to warm stages of the Pleistocene – Marine Isotope Stage 3 (MIS3) andMIS5 – demonstrating that some terraces were formed during interstadial or interglacial periods. The results provide a key for evaluating rates of neotectonic uplift, allowing us to decipher the response of a fluvial system to climate change within the context of the glacial–interglacial scheme.

Palaeogeographical reconstruction of the Sierra de Atapuerca Pleistocene sites (Burgos, Spain)

Abstract The Sierra de Atapuerca is an anticlinal ridge of Mesozoic carbonate rocks on the NW edge of the Iberian Chain (Northern Spain, Burgos), surrounded by subhorizontal continental sediments of the NE Duero Cenozoic Basin under endorheic conditions. The shift to exorheic conditions in the Duero Basin lead to the onset of an episodic downcutting phase and the development of the Atapuerca multilevel cave system, containing several sites from the Early and Middle Pleistocene. In this work, we have reconstructed the Pleistocene palaeogeographical evolution of the SW flank of the Sierra de Atapuerca, where these archaeological sites are located. The study is based on a detailed geomorphological and geological analysis, combined with Global Navigation Satellite System (GNSS) and 3D LiDAR data, and GIS modeling. These techniques have been applied to analyse the small valleys and the interfluve on the SW flank of the SW Sierra de Atapuerca. The results were combined with the regional base levels recorded by fluvial terraces and the chronostratigraphic sequences of the Sierra de Atapuerca sites. These reconstructions have allowed us to model the palaeogeographical evolution in the nearby area of the cave sites during the Early–Middle Pleistocene, coupling the main formation phases of the sites with the local physical landscape changes that occurred outside the caves. Surface processes are defined by incision phases entailing mitigate knickpoint recession and slope retreatment, and local aggradational phases associated with caves opened and captured by fluvial incision. This reconstruction provides the local physical palaeogeographical habitats developed during the Pleistocene hominid occupation of the Sierra de Atapuerca.

Luminescence chronology of alluvial fan in North Bengal, India: Implications to tectonics and climate

Alluvial fans are important geomorphic archives because of its strategic location at mountain front which can provide clues to the past climate and tectonics. In general, they provide information about optimum climate condition suitable for fan formation. We provide a detailed luminescence chronology of alluvial fan in north West Bengal, India. The regional fan surface (T<sub>4</sub>4) has been dissected by E-W trending Himalayan thrusts known as Matiali and Chalsa thrusts and have various terraces named as T<sub>3</sub>, T<sub>2</sub> and youngest one as T<sub>1</sub>. Luminescence ages suggest that the formation of the alluvial fan (regional surface) started before 171 ka and continued till 72 ka covering a time span of nearly 100 ky; suggesting of weaker monsoon at 72 ka. Matiali fault activated after 171 ky. Chalsa fault is suggested to be active during 48–41 ka. The last aggradational phase was around 6 ka which led to the formation of T<sub>1a</sub> terrace. The study suggests that climatic fluctuation during the period were shaping the morphology of the alluvial fan, along with tectonic activities on the two faults.

Sediment mixing in aeolian sandsheets identified and quantified using single-grain optically stimulated luminescence

Post-depositional mixing processes are extremely common and often obscure a record of deposition in dune and sand sheet deposits. We show that the upper half metre of a dune in southeastern Australia is currently being turned over through bioturbation, but that single-grain OSL dating and contextual knowledge can be used to identify and model these modern mixing processes. In the sandy deposits investigated, mixing processes were observed to be acting to a predicable depth of ∼50–60 cm. This observation was used to develop a conceptual framework that can be applied to buried deposits and used to temporally constrain the evolution of the landform and quantify rates of mixing. When our mixing zone conceptual framework was combined with the MAM we show that phases of significant dune aggradation occurred at ∼29.9, ∼18.3, ∼10.3 ka, and continued through the Holocene. We also present an approach using single-grain OSL data to estimate downward mixing rates, which show a strong depth dependency and are coherent with previously reported mixing rates. Modern downward mixing rates indicate that the upper ∼50 cm (Zone 1) will be completely turned over on millennial time scales. While caution needs to be used when interpreting archaeological and OSL data from bioturbated sandy environments, our results demonstrate that contextual knowledge and single-grain OSL data can resolve mixing processes and contribute to an understanding of landscape evolution.

A Holocene fluvial archive of the Mulgrave River, Notheastern Australia: Influence of tropical cyclones and sediment delivery to the Great Barrier Reef

The sedimentology, geomorphology and chronology of late Quaternary fluvial landforms and sedimentary sequences within the Mulgrave River catchment in northeast Queensland suggest that episodic stripping or wholesale erosion of Holocene floodplains is a major mode of sediment delivery to the Great Barrier Reef lagoon. The last major phase of Holocene valley sediment removal likely occurred sometime between approximately 1200 and 250years ago and was possibly associated with a phase of heightened tropical cyclone activity and consequent riverine flooding that occurred between AD1400 and 1800. Since then relative tropical cyclone quiescence may be the cause of a phase of valley aggradation that has been occurring over the past two centuries. The results of this investigation suggest that in this catchment there have been alternating phases of Holocene valley floodplain stripping and subsequent aggradation, with the latter being the current dominant mode. This suggests that at least here, in this relatively confined valley, sediment delivery to the Great Barrier Reef may be relatively low compared to other periods over the past millennium and this may be due to low levels of tropical cyclone activity over the past 200years.

The influence of Late Pleistocene geomorphological inheritance and Holocene hydromorphic regimes on floodwater farming in the Talgar catchment, southeast Kazakhstan, Central Asia

In comparison to Southwest Asia and the Indian subcontinent, the relationship between Holocene river dynamics, climate change and floodwater farming in Central Asia is significantly under researched. To address this, a multi-disciplinary research project was begun in 2011 centred on the Talgar catchment, a south-bank tributary of the Ili River, southeast Kazakhstan. Building on archaeological excavations and surveys conducted over the past 20 years, we have undertaken investigations of Holocene human adaptations to changing hydromorphic regimes in the Tien Shan piedmont region, Central Asia. Fluvial geochronologies have been reconstructed over the last 20,000 years using Optically Stimulated Luminescence and 14C dating, and are compared with human settlement histories from the Eneolithic to the medieval period. Phases of Late Pleistocene and Holocene river aggradation at c. 17,400–6420, 4130–2880 and 910–500 cal. BC and between the mid-18th and early 20th centuries were coeval with cooler and wetter neoglacial episodes. Entrenchment and floodplain soil development (c. 2880–2490 cal. BC and cal. AD 1300–1640) coincided with warmer and drier conditions. Prior to the modern period, floodwater farming in the Talgar River reached its height in the late Iron Age (400 cal. BC – cal. AD 1) with more than 70 settlement sites and 700 burial mounds. This period of agricultural expansion corresponds to a phase of reduced flooding, river stability and glacier retreat in the Tien Shan Mountains. Late Iron age agriculturists appear to have been opportunistic by exploiting a phase of moderate flows within an alluvial fan environment, which contained a series of partially entrenched distributary channels that could be easily ‘engineered’ to facilitate floodwater farming. Holocene climate change was therefore not a proximate cause for the development and demise of this relatively short-lived (c. 200 years) period of Iron Age farming. River dynamics in the Tien Shan piedmont are, however, strongly coupled with regional hydroclimatic fluctuations, and they have likely acted locally as both ‘push’ and ‘pull’ factors for riparian agriculturists.

Facies, geometry and growth phases of the Valdorria carbonate platform (Pennsylvanian, northern Spain)

AbstractThe 14 km wide Valdorria outcrop (Pennsylvanian, northern Spain) is one of the few examples of entirely exposed flat‐topped and high‐relief carbonate platforms reported in the fossil rock record. Laterally and vertically traceable stratal patterns expose three phases of growth. Phase I is a 430 m thick platform to slope succession that prograded over 6 km, and is dated as Early Bashkirian (Akavasian–Askynbashian). Phase II aggraded and prograded, exhibiting 180 m thickness of cyclical platform top deposits, dated as Late Bashkirian (Asatauian). Phase III is a mound‐shape structure that developed over the platform top of Phase II as a new phase of platform nucleation. It is 535 m thick and 2 km wide, and dated as Late Bashkirian (Asatauian–Transition interval). The observed changes of growth styles during platform evolution, from a prograding to an aggrading–prograding system, and a rapid aggradational phase, are inferred to be controlled by flexural subsidence in the active Cantabrian foreland basin, at the Variscan orogenic front. The metre‐scale shallowing‐upward cycles of the platform top are most probably due to glacioeustasy, as evidenced by well‐recorded subaerial exposure surfaces superimposed on subtidal deposits, and by a stratal pattern recurrent in a short interval of about 160 kyr. Observations of outcropping Bashkirian cyclothems in an isolated carbonate system, devoid of siliciclastic input, are relevant for a better understanding of the impact of high‐frequency sea‐level fluctuations on the carbonate factory. Moreover, progradation of the platform margin during Phase I reaches a rate of 2500 m/Myr, and 1810 m/Myr during Phase II; rates that are high when compared to other Pennsylvanian examples. The aggradation rate of 447 m/Myr calculated for the Late Bashkirian–Transition interval (Phases II and III; uncorrected for compaction, missing beats and erosion) is uncommonly high in comparison to coeval Pennsylvanian examples. The platform exhibits a self‐nourishing prograding microbial boundstone‐dominated slope. Thus, the slope‐shedding model applies well to Valdorria. However, Phase II recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea‐level stands; this is marked by common slope red‐stained breccias synchronous to platform top subaerial exposure phases. Contrarily, periods of relative high sea‐level and rapid subsidence in Phase III registered a greater development of cemented microbial boundstone. These observed, partly opposing relationships of sea‐level stands, shedding modes and slope architecture provide an improvement of the currently used slope‐shedding model. The overall architecture of the Valdorria outcrop compares well with that of other contemporaneous platforms, such as Sierra del Cuera and Bolshoi Karatau. Valdorria shares the high‐relief and flat‐topped, steep slopes, cyclothemic patterns and occurrence of karst features with the Pricaspian Basin platforms (Tengiz, Karachaganak and Kashagan), with minor variations in facies distribution of the internal platform. Furthermore, the continuous seismic‐scale outcrop of Valdorria, together with its isolated setting and asymmetrical growth, makes it a very good candidate for potential subsurface analogues of hydrocarbon‐bearing systems.

Excavations at Site C North, Kalambo Falls, Zambia: New Insights into the Mode 2/3 Transition in South-Central Africa

We report on the results of small-scale excavations at the archaeological site of Kalambo Falls, northern Zambia. The site has long been known for its stratified succession of Stone Age horizons, in particular those representing the late Acheulean (Mode 2) and early Middle Stone Age (Mode 3). Previous efforts to date these horizons have provided, at best, minimum radiometric ages. The absence of a firm chronology for the site has limited its potential contribution to our understanding of the process of technological change in the Middle Pleistocene of south-central Africa. The aim of the excavations was to collect samples for luminescence dating that bracketed archaeological horizons, and to establish the sedimentary and palaeoenvironmental contexts of the deposits. Four sedimentary packages were identified with the oldest containing Mode 2 and Mode 3 horizons. In this paper we consider the implications of the luminescence ages for the archaeological record at Kalambo Falls, and place them in a regional context. The reworking and preservation of the archaeological horizons is interpreted as the result of successive phases of meander migration and aggradation. Limited pollen evidence suggests a persistent floodplain palaeoenvironment with intermittent swamp forest and adjacent valley woodland, while mineral magnetic susceptibility data support an interpretation of river flow variability without any significant change in sediment provenance. The dynamics of the fluvial system cannot as yet be linked directly with regional climate change. The age range of ~500–300 ka for the oldest sedimentary package places the Mode 2/3 succession firmly in the Middle Pleistocene, and contributes to an expanding African record of technological innovation before the evolution of Homo sapiens.