Phases Of Incision Research Articles

Fluvial aggradation and incision in the Brazilian tropical semi-arid: Climate-controlled landscape evolution of the São Francisco River

Large rivers are dynamic systems whose evolution depends on both internal and external forcing, particularly tectonics, sea level, and climate. Associating fluvial responses to a specific driver is a complex task that has been debated for a long time. Thus, rivers that flow exclusively under tectonically stable areas and without direct influence of relative sea level changes are suitable targets to understand how large fluvial systems responded to past climate changes. The São Francisco River is one of the largest cratonic rivers across South America, and its late Quaternary sedimentary deposits record the fluvial landscape evolution in a thousand-year timescale. The São Francisco River flows northward over different climate zones, with its upper course in a semi-humid setting, but with most of its watershed under semi-arid conditions. To understand the controls on sediment erosion, transport, and storage from uplands to lowlands, we investigated a 200-km section of the medium course of the São Francisco River in northeast Brazil. Several geomorphological zones were characterized, mapped, and dated by optically stimulated luminescence (OSL). Two zones are represented by degraded terraces with lakes, but no preserved alluvial features: (zone 1) high-level terrace (87.7 ± 12.7 ka) and (zone 2) low-level terrace (65.5 ± 5.3 to 39.3 ± 4.3 ka). Three zones comprise the active confined aggradational plain, with features such as scroll bars and abandoned channels: (zone 3) older meander belt (18.1 ± 1.6 ka); (zone 4) young meander belt (15.5 ± 1.5 to 9.5 ± 1.0 ka), and (zone 5) modern channel belt (0.4 ± 0.1 to 0.3 ± 0.1 ka). Zone 6 comprises an eolian dune field composed of parabolic dunes with two phases of active sedimentation (45.1 ± 5.2 to 25.5 ± 4.4 ka and 14.3 ± 2.6 to 5.2 ± 1.4 ka). Sediment deposition ages allowed the recognition of at least four phases of fluvial aggradation (⁓90 ka; ⁓66 to 39 ka; ⁓18 to 9 ka and ⁓0.3 ka to recent), three phases of incision (⁓85 to 66 ka; ⁓39 to 18 ka and ⁓9 to 1 ka), and two phases of dune field stabilization (⁓25 to 15 ka and ⁓5 ka to recent). Development of the eolian dune fields occurred during drier conditions, when the inland activity of trade winds reworked sediments deposited on the fluvial plain. We interpret the incision events as having been set in motion by an increase of fluvial discharge in the upper catchment area, produced by rainfall intensification due to activity of the South Atlantic Convergence Zone (SACZ). The aggradation and incision phases on the São Francisco River during the last 100 ka are therefore likely controlled by multi-millennial precipitation changes, possibly related to precession cycles. The events of high sedimentation rate in the São Francisco river mouth are partially correlated with incision phases in its middle course. This suggests that sedimentation in plains of large plateau rivers can be decoupled from the coastal area.

Sinkhole development in the Sivas gypsum karst, Turkey

The extensive gypsum karst of Sivas, Turkey is one of the most outstanding examples of bare gypsum karst in the world. It displays a number of remarkable geomorphic features, including: (1) two stepped planation surfaces cut-across folded gypsum developed during an initial phase of slow base level deepening punctuated by periods of stability; (2) unusual deeply entrenched gypsum canyons related to a subsequent phase of rapid fluvial incision and water table lowering; (3) a polygonal karst of superlative quality mainly developed in the upper surface; (4) relict valleys disrupted by sinkholes in the lower erosional surface; (5) a large number of bedrock collapse sinkholes mostly associated with the lower surface; and (6) numerous cover subsidence sinkholes developed in the valley floors. This work analyses the spatial distribution, characteristics and evolution of the sinkholes within the broad Plio-Quaternary geomorphological and paleohydrological evolution of the epigene karst system dominated by autogenic recharge. A cartographic sinkhole inventory has been produced in an area covering 2820 km2 with morphometric data and including 295 bedrock collapse sinkholes and 302 cover subsidence sinkholes. The different sinkhole types show a general spatial zonation controlled by the hydrogeological functioning of the different sectors: (1) solution sinkholes (polygonal karst) in the upper recharge area; (2) bedrock collapse sinkholes in the lower denudation surface and close to the base level, where well developed caves are inferred; and (3) cover subsidence sinkholes, with high densities probably associated with areas of preferred groundwater discharge. The morphology of the bedrock collapse sinkholes, varying from small cylindrical holes to large and deep tronco-conical depressions with gentle slopes reflect to geomorphic evolution of these sinkholes that reach exceptionally large hectometre-scale diameters. Their evolution, involving substantial enlargement and deepening, is attributed to the solutional removal as solute load of large volumes of gypsum by downward vadose flow. This type of morphological evolution with significant post-collapse solutional denudation differs from that observed in carbonate rocks characterised by lower solubility and erodibility. The analysis of historical imagery reveals that bedrock collapse sinkholes currently have a very low probability of occurrence and that buried cover subsidence sinkholes are used for urban development creating risk situations.

Active tectonics, landscape evolution and sediment dynamics in Dehra Dun, Northwest Himalaya inferred from geomorphic indices and GIS tools

Geomorphic evolution of the frontal parts of active mountain belts is strongly influenced by tectonic processes, usually manifested in fluvial, structural and morphotectonic parameters of the landscape. Geomorphic indices have long been used for understanding the tectonic imprints in river basins. However, their applicability and validation from field observations in the Himalayan basins have often been debated. Here we test the applicability of several geomorphic indices in the western Dehra Dun (intermontane valley) that includes well-known tectonic elements such as Main Boundary thrust (MBT), Main Frontal Thrust (MFT), Santaurgarh Thrust (ST), and other smaller faults. Based on the spatial distribution of the values of different geomorphic indices, we map four different tectonic zones to represent the relative tectonic activity from low to very high. We conclude from morphometric analysis that the MBT and the ST are active faults. However, we do not find any geomorphic evidence of recent activity along several other faults mapped in this region or any deformation in the major fan units. We argue that active tectonics has not played a significant role in shaping the landforms in this region except for providing accommodation space and sediment supply for the deposition of fan units. We have further employed DEM and limited borehole data to understand the sediment storage and excavation at Late Late Quaternary time scale. We compute the average stored volume of sediments since ~50 ka as 8.03 × 1010 m3 (using borehole data) and 12.24 × 1010 m3 (GIS approach). Total eroded volume has been computed as 0.47 × 1010 m3 (using borehole data) and 0.64 × 1010 m3 (GIS approach). These volumes give a time-averaged sediment discharge leaving the dun as 0.21–0.39 × 106 m3/yr over Late Quaternary period. We note that the estimated average eroded material during the incision phase is only 5–6% of the total accumulated sediment volume. Further, the average estimated volume (8.03–12.24 × 1010 m3) in the valley is nearly half of the volume of the sediment that can be accommodated (27.2 × 1010 m3) in the valley. Also, the present-day sediment supply to the Dun (~0.14 × 106 m3/yr) is low in comparison to estimated average erosion rate. Our results therefore suggest that the valley is only half filled, and is in excavation phase since Late Quaternary mostly driven by climatic perturbations.

Site Dependence of Fluvial Incision Rate Scaling With Timescale

AbstractGlobal measurements of incision rate typically show a negative scaling with the timescale over which they were averaged, a phenomenon referred to as the “Sadler effect.” This time dependency is thought to result from hiatus periods between incision phases, which leads to a power law scaling of incision rate with timescale. Alternatively, the “Sadler effect” has been argued to be a consequence of the mobility of the modern river bed, where the timescale dependency of incision rates arises from a bias due to the choice of the reference system. In this case, incision rates should be independent of the timescale, provided that the correct reference system is chosen. It is unclear which model best explains the “Sadler effect,” and, if a timescale dependency exists, which mathematical formulation can be used to describe it. Here, we present a compilation of 581 bedrock incision rates from 34 studies, averaged over timescales ranging from single floods to millions of years. We constrain the functional relationship between incision rate and timescale and show that time‐independent incision rate is inconsistent with the global data. Using a power law dependence, a single constant power is inconsistent with the distribution of observed exponents. Therefore, the scaling exponent is site dependent. Consequently, incision rates measured over contrasting timescales cannot be meaningfully compared between different field sites without properly considering the “Sadler effect.” We explore the controls on the variable exponents and propose an empirical equation to correct observed incision rates for their timescale dependency.

Kızılırmak Nehrinin Denizel İzotop Katı 6 Sırasındaki İklim Değişimlerine Tepkisi

Bu çalışma Kızılırmak vadisinin Kapadokya yöresinde, Gülşehir-Tuzköy arasında kalan kesiminde yapılmıştır. Bu sahada T1’den (en yaşlı) T15’e (en genç) kadar numaralandırılmış 15 seki basamağı bulunmaktadır. Bu çalışmanın amacı, T13 sekisine (+15-20 m) ait üç lokasyonda Denizel İzotop Katı (DİK) 6 sırasında Kızılırmak Nehri’nin küresel iklim değişimlerine karşı tepkisini saptamaktır. Sekinin taban depolarından alınmış olan kum örnekleri pIRIR290 yöntemiyle 148 ±8, 154 ± 8 ve 159 ± 8 bin yıl (by) öncesine tarihlendirilmiştir. Önceki bir çalışmadaysa seki deposu kozmojenik izotop yöntemiyle 160±30 by öncesine tarihlendirilmiştir. Elde ettiğimiz veriler T13 teras depdepolarının birikimi öncesindeki ana vadi kazılmasının DİK 7 ve 6 (sıcak–soğuk) iklim geçişinde olduğunu göstemektedir. Veriler, T13 seki depolarının DİK 6’nın ilk evresinde biriktiğini ortaya koymaktadır. Ayrıca, T13 sekisinin ilk taşkınovası deposunda sıcak iklimi karakterize eden paleosolerin varlığı, seki deposunun alt kısmının olasılıkla DİK 6'nın erken evresinde (günümüzden önce 180 ile 165 bin yıl arasında) oluştuğunu gösterir. T13 sekisi depolarının yarılması ise DİK 6’nın sıcak periyodunun sonunda başlamış ve DİK 6’nın tam glasyal periyodunda tamamlanmıştır.

Evaluating the potential for natural ecosystem recovery in cut-and-fill wetlands: case study of Pietersielieskloof palmiet wetland, South Africa

Cut-and-fill processes, characteristic of many valley-bottom wetlands, present a major challenge to practitioners planning wetland restoration. It is not clear whether the current synchronicity and spatial scale of incision is ‘natural’, and whether these wetlands will naturally recover without intervention during the ‘fill’ phase. The aim of this study, focussed on a Prionium serratum dominated valley-bottom wetland in South Africa, was to evaluate the potential for natural ecosystem recovery in the context of current catchment constraints. Historical rainfall records, aerial photography and a DGPS survey were used to contextualise geomorphic processes. The sedimentology of alluvial fill was investigated by sampling sediment profiles. Samples were analysed for particle size and organic content, five were dated using radiocarbon. Sedimentology and radiocarbon dates were used to ascertain phases of incision and aggradation along wetland reaches. While several phases of incision are represented in the record, none were represented in more than one location, suggesting phases of erosion and subsequent deposition were spatially and temporally segregated. The current synchronous phase of incision across the wetland is therefore unprecedented. Analysis of sedimentation rates indicate that infilling of the gully decreases exponentially over time. Infilling of a 3m deep gully in the Pietersielieskloof wetland under natural catchment conditions is estimated to take ~ 5000 years. Analysis suggests that there has been a reduction in wetland resilience over the last century, and that a change in catchment hydrology and sediment supply has altered this system such that it has crossed a geomorphic threshold.

Assessing the climatically triggered aggradation-incision processes in a dryland environment during the Late Quaternary period from Shetrunji River basin (Saurashtra), India

The present study is an attempt to infer the response of the dryland fluvial sequences to paleoenvironmental change since the last glacial maximum (LGM) from the semi-arid Saurashtra region, a basaltic terrain of the Late Cretaceous age (Deccan Trap), of Western India. We employed a conventional sedimentological analysis supported by sediment geochemistry, which was constrained by optical dating. Based on detailed field investigation, we identified five terrace sequences from the valley fill deposit in the Shetrunji River basin since 18 ka (i.e. Post LGM). The sedimentary archives expose the deposition during three alternate phases of aggradation and incision, marked on the basis of morphostratigraphy and timing of landform evolution. The coastal region of western India experienced a widespread valley aggradation in response to the rising sea stand and under gradual strengthening of the monsoonal conditions around the 18–9 ka period. The Early Holocene climatic optima (9–7.5 ka) coupled with lower than present but rising sea stand, triggered the intense erosion in the valleys of Shetrunji River basin, leading to erosion/lack of sediment preservation. This was followed by the Middle Holocene period (7.5–5 ka), which experienced valley aggradation. This was ended by the incision post 5.5 ka in the lower reaches of the Shetrunji River. The sea stand was similar to present-day stand, as evidenced by the change from fluvial to the estuarine environment in the top sequence of Site-SL at 5.5 ka. The Late Holocene relative sea stand lowering ~3 ka, led to an incision in the estuarine terrace of about 2 m. The causal mechanism for triggering of this incision is most likely the relative fall in sea stand to present-day level from its Middle Holocene high stand. The Late Holocene period (3–1 ka) shows aggradation in upper reach in fluctuating hydrodynamic conditions with seasonality, followed by a relatively good monsoonal condition at < 1 ka, leading to the incision due to climate-tectonic interplay. These pulses of sedimentation could be recognized and explored as aided by basin-scale approach as in the present study. Site-specific studies do not enjoy such liberty.

Holocene climate pulses and structural controls on the geomorphological estuarine evolution of the Iguape River, São Paulo, Brazil

The Iguape River estuary – located on the southern coast of São Paulo state (Brazil) – presents geomorphological records of the complex processes operating during the Late Holocene. Dating the top deposits of coastal (fluvial, transition and marine deposits) low terraces, contributes to the understanding of the erosive-depositional events resulting from autogenic and allogenic processes. Optically stimulated luminescence (OSL) dating, grain-size and morphostructural analysis were used to investigate the Late Holocene geomorphological evolution of the Iguape estuary. The results suggest that formation of the Iguape estuary is consistent with a marine incursion triggered by climate pulses by around 2.7 ka. However, the coastline displacement was controlled by base-level adjustments and fluvial incision phases probably induced by fault activity associated with the Guapiara Lineament. Finally, the results show that the estuarine systems in humid tropical regions are significantly sensitive to short time-scale climate events during the Holocene.

Distinguishing tectonic versus climatic forcing on landscape evolution: An example from SE Tibetan Plateau

AbstractDistinguishing climate from tectonic forcing in shaping the Earth’s surface has been a long-standing issue in the Earth sciences. Great debate exists regarding when and how the SE Tibetan Plateau achieved its current low-relief topography, and both lateral extrusion and lower crust flow have been proposed as the dominant mechanism. Reconstruction of the exhumation history of the SE Tibetan Plateau is key to understanding these formation processes and resolving the significance of different forcing mechanisms. Here we report zircon and apatite (U-Th)/He ages from steep transects across the Lincang granite belt of the SE Tibetan Plateau. Our results reveal a two-stage exhumation history during the Cenozoic with rapid cooling phases in the late Eocene and the middle Miocene. In the late Eocene, the climate was generally dry and there is plenty of evidence for increased extrusion and upper crustal shortening. We suggest tectonic processes are responsible for the first inferred cooling. In contrast, the Asian summer monsoon precipitation increased during the middle Miocene, and we posit the middle Miocene cooling phase records a phase of rapid river incision triggered by the intensified precipitation and associated fault movements. The results are consistent with recent paleo-altimetry work in this region suggesting that the present-day topography of the SE Tibetan Plateau had been largely constructed by the late Eocene. Together, these data suggest that extrusion and/or upper crustal shortening setup the first order topography of the SE Tibetan Plateau, which was then modified by climate-triggered fluvial incision and feedbacks initiated in the middle Miocene.

Reconstructing Late Quaternary Palaeo‐environmental change from the dryland fluvial landscape of the Southern Kachchh Mainland, western India: Insights from new OSL and sedimentological datasets

The southern Kachchh Mainland (SKM) of western India, situated in a seismically active intraplate region with a considerable Quaternary landscape, provides an opportunity to reconstruct the roles of climate and tectonics. Employing geomorphology, detailed sedimentology supported by geochemistry and optical dating, we integrate the fluvial to fluvio‐marine records to evaluate the potential of dryland environments in archiving the palaeo‐events. The climate reconstructions suggest that the fluvial systems have responded to the variations in monsoonal strength through widespread aggradation during 57 until 41 ka, corresponding to the later part of the MIS‐3. Following this, a relative weakening of monsoon and onset of aridity is observed between 28 and 11 ka, with short spells of enhanced monsoon. The monsoonal conditions again strengthened during 11 to 6 ka, reaching its maximum during the Early Holocene optima, which triggered a sea‐level rise during the Middle Holocene period, that is, 6 to 3 ka. Following this, the sequences incised in response to the relative sea‐level fall to the present level in post 3 ka. We have also employed this OSL ages to reconstruct the phases of aggradation and incision of valley fill sediments/estuarine tidal terraces from the SKM during the last 57 ka. The present study illustrates and contributes to the advancement in understanding of complex processes and their influence on climatic/tectonic signals in dryland landscape as well as their decoupling.

Microslag as a stratigraphic tracer to quantify floodplain processes (Lienne catchment, Belgium)

This paper focuses on two objectives: (i) the presentation of the methodology for using small-sized iron slag (called microslag) as a stratigraphic marker to reconstruct the centennial-scale floodplain evolution and (ii) the study of the geomorphic evolution of the Ardennian rivers situated in the Lienne catchment (southern Belgium) by means of microslag evaluation, 14C dating, topographic surveys and sedimentological analysis. Using these data, the floodplain history since the Late Pleistocene is reconstructed, fine floodplain sedimentation and lateral mobility rates are estimated, and channel incision phases are identified. The first Holocene erosion episode occurred during the Bronze Age. Archaeological data and historical sources indicate Roman and High Middle Age occupations in the area but no evidence of erosion or increased floodplain sedimentation have been observed. Several peat layers have been dated in the Chavanne floodplain to approximately 1000 BP and probably indicate very low anthropogenic activity and a high level of groundwater. Before 1000 CE, the fluvial style of the Ardennian rivers was probably different, characterized by a low-level forested floodplain, which explains the presence of frequent peat layers within the alluvial sequences and abandoned channels. From the eleventh century onwards, a generalized increase in sedimentation rates occurred, coinciding with a higher concentration of charcoal in the alluvium and also the formation of colluvial deposits. Analyses of slag concentrations allowed us to reconstruct the evolution of the floodplain topography. In the Lienne catchment, the mean floodplain aggradation over the last several centuries is between 2.7 and 16 cm/century, depending on the study site. Such differences in floodplain sedimentation rates can be explained by several factors such as the distance to past cultivation areas, the degree of woodland clearance, flow velocities during floods and the incision of the riverbed during recent centuries, which reduces the frequency of inundation and, therefore, sedimentation on floodplains.

Discovery of late Tortonian incised valleys in the Saouaf Formation (northeastern Tunisia) - Evidence of high-frequency sea-level variations in the central Mediterranean

During the late Miocene, the connection between the Atlantic Ocean and the Mediterranean Sea was progressively restricted. Despite many discoveries of Messinian incised valleys and canyons in the peri-Mediterranean basins, the Tortonian incised valleys are poorly described. The Tortonian series exposed in the northeastern end of the Cap Bon peninsula (northeastern Tunisia, central Mediterranean) provide an excellent example of complex and extensive incised valleys system traceable for up to 5 km (entire length of the outcrop). Detailed facies and sedimentological analysis permitted the recognition of five distinct phases of incision. These valleys (approximately 300 m wide and 10 m to 30 m deep) were incised during sea-level fall and were filled during subsequent sea-level rise. These valleys are characterized by the lack of fluvial deposits at their bases. During sea-level rise, these valleys were filled mainly with cross-bedded estuarine sandstone forming multistorey channels and point-bar deposits. Tidal sand flat deposits are locally well developed at the margins of the valleys fills. These tide-dominated estuarine deposits, corresponding to the transgressive systems tracts, are arranged into fining-upward retrogradationnal parasequences. The highstand systems tracts correspond to wave-dominated deltaic deposits comprising offshore, offshore-transition and shoreface deposits. These successions are typically arranged in thickening and coarsening-upward parasequences. The upper surfaces of the highstand systems tracts are incised by later valleys. The formation and the filling of these valleys record the complex interplay of different allocyclic factors: (1) tectonic uplift due to the major compressive Atlassic phase, (2) high-frequency sea-level variations, (3) warm and humid climate, and (4) high river sediment input. The results of this study provide important data, such as incised valleys geometries, dimension, extension and connectivity. The results of this study emphasis the potential link between high-frequency sea-level variation and progressive restriction of the Mediterranean during the late Tortonian.

Morphological Changes of the Lower Tedori River, Japan, over 50 Years

Long-term variation in the morphological characteristics of rivers is complicated as a result of temporally and spatially variable natural processes and anthropogenic disturbances. Better understanding of their relationship is therefore important for river basin management. The present study conducted a detailed analysis of a long-term dataset consisting of a 58-year topographic survey and related data on human impact to clarify the long-term variation in the morphological characteristics of the lower Tedori River in Japan. An empirical model was established through the assessment of spatiotemporal variations in nonlinear rates of adjustment. The results indicated that sediment extraction and dam construction profoundly affected the morphological characteristics of the lower Tedori River and that the vertical adjustment of the river channel experienced five phases. Degradation occurred from 7 to 16 km upstream from the river mouth over all phases. Two aggradation phases following two degradation phases were observed from the river mouth to 2 km upstream. Aggradation and degradation phases appeared alternately from 2 to 7 km. The representative nonlinear rates of vertical adjustment in the second phase were the highest compared with those in the other phases in the entire reach. The correlation analysis revealed that the incision phase was mostly coupled with channel narrowing, while widening followed the deposition phase. It was deduced from aerial photo analysis and a comparison between slope and empirical critical slope that the pattern in the lower Tedori River was braided during the period 1950–2000.

Late Quaternary biotic and abiotic controls on long‐term sediment flux in a northern Australian tropical river system

AbstractThe modern distribution of monsoonal rainforest in the Australian tropics is patchy and is mainly associated with river corridors and groundwater springs, which indicates a strong dependence on hydrologic and geomorphic conditions. While their present distribution is well known, very little data exists on past spatial and temporal dynamics of these ecosystems, or their medium‐ to longer‐term controls. Factors such as (i) fire frequency and type, and/or (ii) hydroclimatic conditions (e.g. droughts) have been proposed to control riverine corridor rainforest extent. Recent observations, however, also suggest an additional (iii) geomorphic control induced by alluvial knickpoint migration. Sediment sequences provide valuable archives for the reconstruction of longer‐term (a) floodplain sedimentary dynamics, (b) local vegetation history, and (c) catchment‐wide fire histories. This study investigates such a sediment sequence at Wangi Creek, and shows that a phase of aggradation, lasting ~4000 years, was recently disrupted by channel incision and floodplain erosion. The aggradational phase is characterized by sand deposition with average vertical floodplain accretion rates of 0.8 cm/yr and includes phases of soil development. The recent incisional phase has changed hydro‐geomorphic conditions and caused widespread degradation of vegetation, erosion and lowering of the macro‐channel surface. While there is no evidence in our data for an erosional event of similar magnitude since the onset of late Holocene floodplain aggradation, Wangi Creek experienced significant erosion and incision immediately before ~4000 years, providing the first evidence for a tropical cut‐and‐fill river system. We hence argue that phases of aggradation mainly controlled by biotic processes alternate and depend on feedbacks with incision phases controlled mainly by abiotic processes. The results show that eco‐hydro‐geomorphic feedbacks may play a crucial role in the medium‐ to longer‐term history of tropical fluvial systems and need to be considered when interpreting fluvial archives with regards to climate, fire or human induced change. © 2019 John Wiley &amp; Sons, Ltd.

Late Holocene anthropic degradation records in semi-arid environments (NE Spain and NW Argentina)

Information about Holocene sedimentary records in two semiarid areas in Spain and Argentina was gathered to know the role of the anthropic influence on landscape evolution. In both cases, four aggradational units separated by incision phases have been differentiated. Due to the confluence of anthropic indicators, the H1C subunit (ca. 2.45-ca. 1.5 ka cal BP) in the central sector of the Ebro basin (NE Spain), and the H2B subunit (2.45-ca. 0.6 ka cal BP) in the Tafi valley (NW Argentina) are worthy of attention. In both cases, a soil formed around 2.45 cal BP was degraded, because it was the object of intense overexploitation in the Ibero-Roman period in the Ebro valley and Formative period in Tafí valley. The final results of these processes represented transformations so significant that the landscape was unable to recover posteriorly. In highly vulnerable dryland environments, the establishment of adequate criteria to link strong landscape degradative phases with human activity is of high interest to know de older phases of the Anthropocene or Paleoanthropocene.

River channel evolution and tsunami impacts recorded in local sedimentary archives – the ‘Fiume Morto’ at Ostia Antica (Tiber River, Italy)

AbstractOstia, the ancient port of Rome at the mouth of the river Tiber (Italy), flourished until the 2nd Century ad, although massive siltation had already caused the abandonment of its lagoonal harbour in the 1st Century bc. In search of an alternative harbour site, geomorphological and geoarchaeological research was carried out in the ‘Fiume Morto’, an abandoned Tiber meander to the immediate north of the town. To reconstruct its complex development and evaluate the suitability of the river channel as a fluvial harbour, a transect perpendicular to the former river channel's flow axis was systematically explored by a new methodological approach combining electrical resistivity tomography, direct push‐electrical conductivity sensing and vibracores. Together with microfossil, mineralogical and geochemical analyses of sediment samples and 50 radiocarbon dates, in‐depth stratigraphic data support a detailed reconstruction of different Tiber channel generations. Results reveal a meander development closely related to distinct ‘cut and fill’ dynamics. Inactive river channels were repeatedly filled with lagoonal sediments, abruptly followed by phases of intense incision. Yet, the overall Fiume Morto channel structure remained stable and almost in situ over centuries, showing nearly no lateral changes since the southward shift of the Tiber River mouth in the first millennium bc. In the 1st Century bc, channel conditions favoured navigation and transportation of goods with large ships. Open‐water conditions, suitable for anchoring and landing activities, dominate from the 1st Century ad until 1557 ad, when the Fiume Morto meander was finally cut off and silted up within a short time. Within the fluvial deposits, sediments most likely related to tsunami inundations are preserved. These events occurred sometime before the 2nd Century bc, in the early 1st Century ad and in the 17th Century ad or later. Results are consistent with traces of tsunami influence found in Ostia's western lagoonal harbour.

Channel adjustments in the lower Guadalquivir River (southern Spain) over the last 250 years

The lower Guadalquivir River has been subjected to a series of channel adjustments over the last 250 years, mainly in response to human factors and disturbances. From 1778 to 1918, the results show an oscillation in the sinuosity along the upstream segment and a reduction along the downstream segment in response to artificial cut-offs. The first phase of incision started in approximately the 1840s–1850s, following a previous phase of predominant aggradation probably related to abundant sediment supply caused by deforestation in the catchment. During approximately the last 100 years, notable incision and progressive channel narrowing occurred in both segments, although secondary aggradation (upstream of the dams) and widening (in a portion of the downstream segment) were also observed. The first phase of bed incision migrating upstream is interpreted as a consequence of artificial meander cut-offs, whereas most of the adjustments that occurred during the last 100 years are interpreted as a consequence of the large number of dams in the catchment and along the studied area, which caused a notable deficit in bedload.

The influence of centennial-scale variations in the South American summer monsoon and base-level fall on Holocene fluvial systems in the Peruvian Andes

The occurrence of Holocene changes in the South American Summer Monsoon (SASM) in the Peruvian Andes has been well established in paleoclimatological records such as speleothem, lake and glacier records. How river systems responded to these events has, however, hardly been investigated. Here, we present evidence based on sedimentological, stratigraphical and geomorphological data as well as radiometric (optically stimulated luminescence and radiocarbon) dating that rivers from the Peruvian Andes are extremely sensitive to changes in SASM activity. The presence of lake sediments shows that from at least 12,000 years ago until 4153 ± 988 years ago a lake was present in the Mantaro River valley, possibly due to damming by a glacier or glacial landforms. A reconstruction of fluvial terrace profiles shows that 4418 ± 500 years ago the Mantaro and its tributary, the Cunas River, incised and laid down sediments simultaneously as a response to changes in regional base-level and increased SASM activity. The latter was largely subdued during a large part of the Holocene as evidenced by paleoclimatological records in the region. Between 2245 ± 217 years ago and the present, the frequency of SASM events increased drastically and both rivers formed the majority of their fluvial terraces. In total, over the past ~4000 years, the Mantaro River formed five terrace levels and the Cunas River formed seven main terraces. Locally, 11 terrace levels were recognised. Terrace formation occurred at intervals of approximately 250 to 300 years between 2245 ± 217 and 1188 ± 60 years ago and approximately every 150 years after 824 ± 66 years ago until the present. A comparison with paleoclimatological data shows that sedimentation events correlate to periods of increased precipitation and glacier retreat in the Peruvian Andes, whereas phases of incision are attributed to continuous adjustments in base-level fall. Thirty-four metres of incision has occurred since 4418 ± 500 years ago averaging 7.7 mm yr−1. A comparison with data from other river systems in the Peruvian Andes shows that many rivers responded in a similar way to centennial-scale variations in SASM activity. Fluvial activity is thus not related to interannual variations in the El Niño Southern Oscillation (ENSO) contrary to previous proposals.

The tectono-sedimentary evolution of a major seismogenic zone with low slip rate activity: A geochronological and sedimentological investigation of the Dobrá Voda Depression (Western Carpathians)

Seismically active fault zones receive a great deal of attention due to their potential for quantification of seismic hazards. Zones with low slip rates pose a challenge, however, since their poor topographic expression is related to difficulties in the quantification of fault movement. This study focuses on the Dobrá Voda Depression, an area with the highest level of seismic activity in the Western Carpathians. The Quaternary tectono-sedimentary evolution of the small intramontane basin was investigated with the use of facies analysis of cores, dated with the use of cosmogenic nuclide depth profiles (10Be, 26Al and 36Cl), together with 26Al/10Be burial dating and radiocarbon dating. A set of archived boreholes and geoelectric survey data was used for the correlation of results with those from new boreholes across the depression. Four facies associations were distinguished: (FA1) Colluvial deposits that comprise subaerial debris flows and mudflows; (FA2) Fluvial deposits with high sediment supply: accommodation ratio, composed mostly of sandy-gravelly channel fill facies; (FA3) Fluvial deposits with low sediment supply: accommodation ratio, consisting mostly of floodplain muds, overbank heterolithic facies and minor sandy-gravelly channel fills; and (FA4) Swamp deposits, which are mostly made up of peat. Geochronological results suggest that the studied part of FA3 was deposited before 1.0 Ma due to a rise in the base level following a major incision event. Overbank-dominated deposits of FA3 covered an incision surface, resulted in a difference of ca. 65 m of elevation of these strata, which represents the minimal thickness of FA3. The second phase of incision was related to reactivation of Miocene normal faults resulting in further topographic differentiation. The initiation of fault activity is recorded by the deposition of colluvial FA1 before ca. 250 ka. FA2 accumulated between ca. 160 and 100 ka, mostly at the toes of slopes bounding the fault scarps on the basin margins. The last documented phase of evolution represents an increase of accommodation, which was connected to the deposition of Holocene peat in swamps as well as floodplain muds of FA4 above FA2. The observed settings imply that variation between incision and accumulation in a scale of hundreds of thousands of years is characteristic for low relief tectonically active zones. The presented research demonstrates the significance of sedimentological analysis for reconstruction of tectonic evolution in areas with low slip rate activity.

Geomorphology of Tafi valley (Tucumán Province, Northwest Argentina)

ABSTRACT Tafí valley is an elongated tectonic basin of about 450 km2 located in NW Argentina in a mountainous frame of metamorphic and granitic Precambrian-Paleozoic rocks belonging to Sierras Pampeanas. This study presents the first detailed geomorphological map of the area. The most representative landforms date to the Lateglacial-Holocene, with four differentiating aggradative units separated by incision phases. Among these phases, it is possible to identify the units H1 (Lateglacial-Early and Middle Holocene) and H2 (Upper Holocene until ca. 600 cal BP). These units are composed of slopes, fluvial terraces, and alluvial fans, forming a coupled system. In some areas, this system is complemented with fluvioglacial and glacial landforms. More recent units (H3 and H4), together with active processes, contribute to the great variety of morphologies represented in the area.