Valley Infill Research Articles

Late Quaternary river evolution and its response to climate changes in the upper Mekong River of the Qinghai–Tibetan Plateau

Multiple continental-scale large rivers originate from the Qinghai–Tibetan Plateau (QTP), known as the “Asia Water Tower”, because of glacial melting. However, the timing of fluvial processes in the headwaters of these large rivers and their correlation with global climate changes remain uncertain. Among these rivers, the Mekong River, the third-longest in Asia, flows through the southeastern QTP. In this study, we investigated fluvial terraces, buried paleochannels, hillslope-derived colluvium, and remnants of dammed lake in the Nangqian Basin, which is in the headwaters of the Mekong. We reconstructed the fluvial aggradation and incision history employing optically stimulated luminescence (OSL) dating. Our results suggest that orbital-scale climate changes have significantly influenced fluvial processes in the Nangqian Basin since the Late Pleistocene. During marine isotope stage (MIS) 3 (57–29 ka), a relatively warm period within the last glacial epoch, the Nangqian Basin experienced aggradation of fluvial sediment. Subsequently, during the later part of the Last Glacial Maximum (LGM) (21–17 ka), episodic landslides and debris flows caused valley infilling, resulting in river blockage and the formation of a dammed lake. At around 16 ka, the Mekong in the Nangqian Basin shifted from aggradation to fluvial incision. Since the late Holocene, the basin returned to aggradation, forming a new terrace at around 1.1 ka. This may be due to the combination of decreased stream power and increased sediment supply, potentially linked to intensified human activities and its associated vegetation degradation. Overall, our study highlights the close links between glaciation and fluvial processes in the region. Specifically, we propose that the observed aggradation pattern during the last glacial period on the plateau primarily resulted from substantial detrital input from glacier activities. The transition from aggradation to incision during the last deglacial period is likely influenced by increased discharge from glacier meltwater. The presence of colluvium interbedded within the late LGM fluvial sediment sequences highlights the significant impact of climate changes on fluvial processes in the headwater areas of the QTP, especially those events stemming from hillslope activity, such as landslides and debris flows.

Incised valleys on the Algarve inner shelf, northern Gulf of Cadiz margin: Stratigraphic architecture and controlling factors in a low fluvial supply setting

A network of cross-shelf paleovalleys has been recognized over the paleo-inner shelf off the Gilão-Almargem Estuary, a small fluvial drainage system that presently receives minor sediment supply in the eastern Algarve shelf, northern margin of the Gulf of Cadiz (SW Iberian Peninsula). This study is aimed at determining the driving controls that triggered substantially different paleohydrological conditions and sedimentary dynamics of ancient fluvial systems in this margin. We focus on evidences of secondary controls on valley genesis and evolution, superimposed to primary glacio-eustatic control such as antecedent geology, low fluvial supply and changing hydrodynamic regimes. The architecture and spatial distribution of these paleovalleys were interpreted based on a grid of seismic profiles with different resolutions. Likewise, a sediment core obtained in a distal position of the paleovalley system provided information about sedimentary processes during the most recent stage of valley infilling. The chronostratigraphic framework was constructed based on regional seismic horizons defined in previous studies and complemented with two AMS 14C dates obtained from bivalve shells.The inner shelf paleovalley system is composed of several incised valley features which exhibit a remarkable similar internal architecture. These inner valley features exhibit two major incision phases (from oldest to youngest; IP 2 and IP 1) that are thought to constitute a simple paleovalley system formed during the last glacial cycle. The origins of the incision are considered to be different. The older one is related to fluvial incision during the sea-level fall leading into the Last Glacial Maximum, whereas the recent one is interpreted as the result of tidal scour during the postglacial transgression. Their corresponding infillings are interpreted, respectively, as estuarine bay-fill deposits and estuary-mouth sands. Overlying the paleovalley infilling, a distinctive reflective unit is in agreement with the generation of coastal barriers and related depositional systems.The formation of the paleo-inner-shelf paleovalley system was strongly conditioned by antecedent geology, which strongly limited the generation of wide incised valleys and determined the amount of incision landward of a well-defined break of slope. Its postglacial infilling was mainly estuarine in nature, likely involving the development of a dendritic system, with numerous barriers interrupted by tidal inlets and channels in a mixed estuarine system with low fluvial supply.

Late Weichselian–Holocene valley development of the Elbe valley near Dresden – linking sedimentation, soil formation and archaeology

Abstract. Valley infills are essential for understanding changes in hydrology and landscape. Anthropogenic activities are proven by prehistoric settlement remains, which mark distinct sediments and soils as usable land during certain time periods. In 2009 and 2018/19, excavations by the Saxonian Archaeological Heritage Office were conducted in the Elbe valley between Meißen and Dresden, preceding the construction of two natural gas pipelines. As a result, two important multicultural prehistoric sites were discovered on the Lower Weichselian Terrace (LWT) in different sediments and on varying stratigraphic levels. During this study sediments and soils at the excavation sites and throughout the pipe trench have been documented. Micromorphological, sedimentological and geochemical investigations and analyses of archaeobotanical and archaeological finds, complemented by 14C and optically stimulated luminescence (OSL) dating, enabled deciphering the structure of sediments and soils. Two major sites were the focus. At the Clieben site, an early Neolithic settlement and former topsoil, developed in a Weichselian valley loam above gravels and sands, are covered by younger overbank fines. At the Brockwitz site, shallow incision channels in the LWT were filled with clayey overbank fines during the Preboreal. An overprinting humic soil horizon was later anthropogenically overprinted during the early and middle Neolithic period. An omnipresent layer of Subboreal or younger overbank fines, covering the majority of the LWT in combination with the spatially confined Preboreal overbank fines, mirrors the ever-growing risk of flooding in a formerly attractive settlement area.

Evolution of the first mouth bar, distributaries and floodplains of the Pearl River Delta

As one of the most densely populated areas in the world, the Pearl River delta has been well studied in its postglacial sedimentary stratigraphy and the effects of Holocene sea-level rise. However, the effects of fluvial force on its geomorphological evolution are much less understood. This paper analyzed the Holocene sedimentary archives and reconstructed the geomorphologic history of an area where the West and North Rivers join and enter the deltaic basin. The reconstruction is based on the analyses of particle size, total organic carbon, organic carbon isotope ratios and microfossil diatoms of five radiocarbon-dated core sediments. The results show rapid paleo-incised valley infilling since c. 9500 cal. a BP, in response to sea-level rise. During marine regression starting from c. 8000 cal. a BP, a bifurcation occurred, and a mouth bar was developed. While the mouth bar emerged to mean tide level around c. 5500 cal. a BP, the surrounding estuarine subtidal flats were gradually transformed into tidal wetlands by c. 3000 cal. a BP. The immediate adjacent waters to the mouth bar were gradually infilled from c. 3000 cal. a BP onwards which narrowed the distributary channels. This landform evolution in the head area of the Pearl River delta resulted in the gradually strengthening fluvial force and weakening tidal force since the relative sea level approached its stabilization. During the past 3000 years, the continuous strengthening of fluvial force supported the accretion of deltaic floodplains. The discovery of mouth bar formation, channel narrowing and deltaic floodplain accretion in this location brings a new perspective of scientific investigation to river delta geomorphologic evolution in response to natural and anthropogenic factors.

Mid- to Late Holocene geomorphodynamics in a long-term settled mountain catchment in the Pergamon micro-region, western Turkey

AbstractFrom 300 BC to AD 300, the city of Pergamon underwent a profound transformation that impacted the rural settlement patterns and the concomitant geomorphodynamics. We present a geoarchaeological study in a long-term settled catchment in the Pergamon micro-region to disentangle the Holocene geomorphodynamics and triggering factors, for example, climate change and human activity. The analyses of eight radiocarbon-dated sediment profiles from the Tekkedere alluvial fan and its catchment indicate four principal sedimentation phases. Phase 1 (ca. 6.2 to 5–4 ka) is dominated by the floodplain aggradation of the receiving Bakırçay River, which is followed by the formation of floodplain soils (phase 2). Substantial geomorphodynamic changes occurred around 4 ka (phase 3), when the edge of the floodplain was buried by fan sediments of the tributary Tekkedere creek. This is attributed to supraregional aridization and rapid climate change events, superimposed by the onset of local human activities. Repeated cycles of coarse- and fine-textured fan sediments with age inversions after ca. 3.8 ka and valley infills younger than 1300 yr BP indicate the strong erosion and redeposition of sediments in phase 4. These increased geomorphodynamics may coincide with the changing settlement pattern and thus reflect human–environment interactions.

Morphology, Sedimentology and OSL Chronology of the Lower Prosna River System Evolution – A Case Study in Rokutów, West-Central Poland

Abstract This paper presents comprehensive investigations of the morphological, sedimentological, and stratigraphic features in the lower Prosna River valley infill, situated in west-central Poland. At the Rokutów site, we identified two meander generations and an anastomosing system. Fluvial changes observed depict the response of the extraglacial river system to climate fluctuations post-last glaciation. Following valley aggradation, scouring erosion, reaching depths of 9–10 m, likely occurred during the Bølling Interstadial. ‘Great meanders’ emerged during the Allerød-Younger Dryas, persisting for around 1000 years, with an uncommon upstream rotation influenced by the sub-Quaternary surface slope. A lengthy stabilization phase, lasting 8000–10,000 years, characterized by an anastomosing system with gradual aggradation, followed the great meandering period. About 3600–3900 years ago, the valley shifted back to a fluvial system with small meanders, lasting 600–900 years. Subsequently, the River Prosna reverted to an anastomosing system, remaining so until the 19th–20th centuries when it transformed into a single-channel system through regulatory manipulation. Our research suggests that the evolution of the Prosna River differed from other rivers across Western and Central Europe, exhibiting a blend of anastomosing and meandering phases.

The Elbe Palaeovalley: Evolution from an ice-marginal valley to a sedimentary trap (SE North Sea)

The southeastern North Sea was a sub-aerially exposed periglacial environment during the late Weichselian. At that time, the approximately 210 km long and 40 km wide SE–NW trending Elbe Palaeovalley (EPV) was the main conduit for meltwater from the southern margin of the Scandinavian ice sheet and rivers draining the North European plain. Only few studies investigated the environmental organization of the Northwest European shelves during deglacial sea level rise resulting in vague reconstructions of the drainage systems. Here, a detailed analysis of sediment echo-sounder data and core samples are presented to study the EPV morphology and stratigraphic infill. The valley base is about 65 m below the present sea-level. This SE-NW palaeo-drainage with its braided system developed during the MIS 2 sea level fall and lowstand in a periglacial environment. During deglacial sea-level rise, the EPV evolved into an estuary with tributaries, intertidal and subtidal flats. After the early Holocene (∼10 cal ka BP), ongoing sea-level rise drowned the EPV system which became the major sedimentary trap in the southern North Sea. Successive sea level rise and associated changes in the hydrodynamic regime led to three stages of shelf organization that were recorded in the valley infill successions: (1) shallow basin with restricted connections to Atlantic water; (2) the southwestern connection with the Dover Strait; (3) the open marine environment at 5.8 cal ka BP after the drowning of Dogger Bank.

Improving spatial characterization of buried bedrock valleys through airborne frequency-domain electromagnetic, residual magnetic, and surface resistivity measurements

The development of a three-dimensional (3D) hydrogeologic site model requires detailed knowledge of an area's hydrostratigraphy, information typically obtained from core logs, historical well records, outcrop mapping, and/or surface geophysics. Due to the high cost of drilling, a 3D model may rely on various statistical techniques to interpolate lithological boundaries between points located tens of kilometers or more apart. This situation can result in poorly constrained or simplified lithostratigraphic models, with under-sampled features such as karst and buried bedrock valleys. The non-invasive nature of airborne geophysical surveys combined with its dense sampling capability can fill critical data gaps. This study evaluates the capacity of a helicopter borne frequency-domain electromagnetic (FDEM) and residual magnetic survey using the Resolve™ system over a buried bedrock valley incised in a multi-layered dolostone aquifer within a complex glaciated environment in an urban/agricultural region of southern Ontario, Canada. The resulting airborne data was evaluated against two high-resolution ground-based electrical resistivity transects oriented orthogonal to the valley, alongside a published Quaternary geological model. A comparison between our local geophysically-based model and the regional geological model shows that the airborne electromagnetic surveys can markedly improve characterization of bedrock valley morphology and Quaternary sedimentary unit architecture that is comparable to that obtained using high-resolution surface-based electrical resistivity measurements with 10 m electrode spacing. The scale of heterogeneity that can be resolved by the airborne survey reveals the level of petrophysical variability that can be seen within the valley infill. Our study demonstrates the potential of airborne FDEM surveys in semi-urban agricultural environments where detailed characterization of Quaternary sediment architecture and bedrock topography is needed to inform 3D groundwater flow system variability.

Is the Anti-Atlas of Morocco still uplifting?

Topographic elevation of orogenic belts in the results of several processes and contributions occurring at different temporal and spatial scales, from short-to long-term and from the surface up to the mantle. Disentangle surface evolution on key areas may then encode for deeper ongoing processes. In this concern, the Anti Atlas of Morocco is a prominent mountain belt with moderate relief and with a long deformation history not recorded during the Cenozoic. The modern topography of the Anti-Atlas is resulted from late Miocene uplift driven by mantle processes. In this study, we address the question if mantle processes are still affecting the topography of the Anti-Atlas by providing new clues on the late Miocene-Quaternary uplift history of the north-western margin of the mountain belt.We analyze and compare the river long profiles and related morphometric features of channels draining the Souss Basin of the Anti-Atlas in western Morocco with analog and numeric similar scenarios. We find that the steepness of the rivers, the χ, ksn, slope and local relief show limited values. Moreover, the channels are characterized by Quaternary fluvial conglomerates lying at high elevation. The projection of the non-re-equilibrated river profile runs below the lower part of the channel. We explore the mechanism that could produce this feature by both numerical and analog models. The analogue and numerical long river profiles from modeling show similar morphometric features compared to the natural case when the uplift rate is diminished or set to zero. We speculate that in the Anti Atlas, uplift may have waned over Quaternary time leading to a spatial change in the critical point of sedimentation, triggering valley infilling at high elevation, and in turn a slow burying of the Anti-Atlas.

Subsidence monitoring and influencing factor analysis of mountain excavation and valley infilling on the Chinese Loess Plateau: A case study of Yan'an New District

With the significant expansion of the urban population on the Chinese Loess Plateau, large-scale land infilling and excavation projects have been implemented to alleviate the shortage of construction land. However, loess collapsibility associated with engineering disturbances can often lead to geological hazards, such as land subsidence. The Yan'an New District (YND) is the largest area of mountain excavation and valley infilling on the Loess Plateau. Post-construction subsidence is likely to threaten future infrastructure; therefore, the monitoring and analysis of the settlement are essential. The temporal and spatial distribution of settlement in the YND was investigated, and the key influencing factors, such as filling, construction, and precipitation, were analysed in this study. Integrated with remote sensing methods, such as SBAS-InSAR technology, shadow altimetry, and uncrewed aerial photogrammetry, were used, which have seldom been applied to regional analysis. The results revealed that the rate of deformation in the YND from 2015 to 2019 was −73 to 18 mm/yr, and in the fill area, the maximum cumulative subsidence was 256 mm, while in the excavation area, the maximum cumulative uplift was 35 mm. We obtained a linear regression model for the fill area, which indicated that settlement was positively correlated with the fill thickness and negatively correlated with the building height. In the excavation area, there was no clear relationship between the excavation thickness or building height on deformation. In the fill area, no obvious collapsible settlement was observed during the rainy season, and uplift occurred in the excavation area, with a maximum uplift of 17 mm.

Tunnel valley infill and genesis revealed by high-resolution 3-D seismic data

AbstractLandforms produced beneath former ice sheets offer insights into inaccessible subglacial processes and present analogues for how current ice masses may evolve in a warming climate. Large subglacial channels cut by meltwater erosion (tunnel valleys [TVs]) have the potential to provide valuable empirical constraints for numerical ice-sheet models concerning realistic melt rates, water routing, and the interplay between basal hydrology and ice dynamics. However, the information gleaned from these features has thus far been limited by an inability to adequately resolve their internal structures. We use high-resolution three-dimensional (HR3-D) seismic data (6.25 m bin size, ∼4 m vertical resolution) to analyze the infill of buried TVs in the North Sea. The HR3-D seismic data represent a step-change in our ability to investigate the mechanisms and rates at which TVs are formed and filled. Over 40% of the TVs examined contain buried glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic structures, and kettle holes. As most of these landforms had not previously been detected using conventional 3-D seismic reflection methods, the mechanisms that formed them are currently absent from models of TV genesis. The ability to observe such intricate internal structures opens the possibility of using TVs to reconstruct the hydrological regimes of former mid-latitude ice sheets as analogues for contemporary ones.

Seismic stratigraphy of a partially filled incised valley on a semi-arid continental shelf, Northeast Brazil

This study aimed to understand the key surfaces and the sedimentary filling pattern of the Coreau incised valley (CIV) system (NE Brazil) using high-resolution seismic stratigraphy. Six key surfaces (Ss) and five seismic units (SUs) were identified. The difference in the basal reflector (S1) and the paleointerfluve (modern sea floor) was verified to be up to 40 m of incision, of which only 20 m are filled. S1 (sequence boundary—SB) was associated with the acoustic basement (Tibau Formation/Barreiras Group) and presented strong control (antecedent topography) in the incision (re-incisions in the Pleistocene lowstands) and sedimentary stacking pattern. The CIV fill showed lowstand system tract, LST (chaotic facies)–transgressive system tract, TST (horizontal layers to chaotic facies)–highstand system tract, HST (chaotic facies) from the base to top. Thin fluvial deposits above SB correspond to LST. Overlapping this system tract is the first marine flooding surface—transgressive surface. Afterwards, the largest seismic unit (lateral and vertical) with sub-parallel and parallel (onlap) seismic facies, interpreted as related to a low energy system, with shallow gas, and vertical accretion (central basin of estuary) occurred. This environment is related to the drowning of the shelf from the shelf break (a decrease in gradient). A tide ravinement surface truncated the top of SU3 and favored coastal to shallow marine deposition. The transgressive system tract (TST) is thicker in the middle (mainly) and outer sectors in the fluvial troughs (depocenter). The maximum flooding surface appears near the sea floor and favored the development above of carbonate sedimentation in the HST. These seimostratigraphic stacking patterns showed a strong influence of semi-arid climate and antecedent topography/structural inheritance. Thus, it is evident that the filling of the CIV differs from other valleys on semi-arid systems and/or influenced by paleotopography (e.g., South Africa and Texas Shelf), which demonstrates the importance of local settings. The great influence of estuarine to coastal sediments in the valley infilling is also observed in the modern Coreau coastal plain incised valley.

ИСТОРИЯ РАЗВИТИЯ ДОЛИНЫ р. МОКШИ В КОНЦЕ ПОЗДНЕГО ПЛЕЙСТОЦЕНА

Late Pleistocene reconstruction of the lower part of the Moksha River valley (between the mouth of the Tsna River and the mouth of the Moksha River) was completed using mechanical coring and radiocarbon (AMS) dating of alluvium in the river valley bottom. Results revealed that between 40–30 ka BP, the river incised deeper than the present level, due to the increase of the river runoff associated with climatic changes. Later the incision was replaced by the valley infill caused by the drying up of the climate and a lowering of the river runoff, that was more significant during the last glacial maximum (LGM, 23–20 ka BP). Sediments derived from scarcely vegetated slopes activated by cryogenic processes from drainage area caused changes of river’s longitudinal profile due to sediment accumulation. In the Late Glacial time starting from 18.5 ka BP a significant increase in river runoff led to the formation of macromeanders and widening of the valley bottom. The Holocene was characterized by a decrease in runoff and channel parameters, and narrowing of the meander belt of the river. During interglacial, sediment accumulation in the channel was negligible because of decreased sediment supply from the eroding basin.

Revisiting the Paleo Elbe Valley: Reconstruction of the Holocene, Sedimentary Development on Basis of High-Resolution Grain Size Data and Shallow Seismics

The Paleo Elbe Valley is the most prominent subsurface structure in the southern North Sea. During the Weichselian (marine isotope stage (MIS) 2), the valley traversed the exposed sea floor and drained the southern margin of the Scandinavian ice sheet. Today the valley is filled with up to 16 m thick sediments, but the responsible processes and drivers remain unknown. To unravel these processes and describe the valley’s evolution with Holocene transgression, we use shallow seismic data and vertical high-resolution grain-size core data. At the base of the western shore, supralittoral fine sands are overlain by a thin layer of clay dated to 9.8 cal. ka BP. The major sediment package consists of marine silt with internal seismic reflectors inclined in a northeastern direction, indicating a sediment transport from the southwest. The valley infill started when the western shore was flooded around 9.6 cal. ka BP and can be divided into two phases. During the first one (9.6–8.1 cal. ka BP) the sedimentation rate was highly driven by wind and waves. The second phase (8.1–5.0 cal. ka BP) was mainly tidal dominated but shows also storm event deposits in the north. Around 5.0 cal. ka BP the valley was almost filled.

Relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf: The Eastern Cape Coast, South Africa

Few stratigraphic models of continental shelves incorporate the process of geostrophic current-sweeping, consequently its role in the stratigraphic record is often overlooked. We examine the narrow, current-swept Eastern Cape shelf of South Africa using a combination of geophysical techniques, seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf. During the Last Glacial Maximum, fluvial valleys incised the acoustic basement rocks. During the subsequent transgression, two distinct shorelines were formed and preserved at −105 m and − 60 m. Their development and preservation is linked to (i) high sediment supply from adjacent fluvial sources, (ii) early diagenesis and (iii) alternating sea-level stillstands and periods of rapid sea-level rise during melt water pulses 1A and 1B, respectively. The deeper shoreline formed in a sandy, wide coastal plain setting with limited bedrock influence, whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast. Differences in antecedent topography and geology are responsible for the temporal variability in shoreline type; coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landwardBetween the two shoreline complexes, in the mid-shelf, the transgressive stratigraphy records initial valley infill by progradation of coast-parallel sandy spits. These are capped by a stiff lagoonal mud deposited as ongoing sea-level rise overspilled the valley interfluves, onlapping the adjacent aeolianites. The uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge.After sea-level reached its present position ca 7.4 ka yr BP, the shelf became subject to reworking by the high-energy geostrophic Agulhas Current. This has had the following major effects on the shelf stratigraphy: 1. The topographic relief of the cemented palaeo-shorelines has been emphasised by removal of the post-transgressive cover; and 2. The shelf no longer acts as a depocenter; instead, the seabed consists of rhodoliths, gravel streamers, bedrock or gravel hash of the wave ravinement surface. Given the necessary antecedent conditions such as accommodation, sediment supply and favourable diagenetic climate, prominent shorelines can form and be preserved on the shelf. Strong current sweeping emphasises these morphological features on subtropical shelves.

Impacts of sediment connectivity on Holocene alluvial records across a Mediterranean basin (Guadalentín River, SE-Spain)

Two contiguous alluvial valleys (Maria and Guadalupe sectors) of the upper Guadalentín River (SE Spain) show distinct Holocene alluvial architecture (cut-and-fill, nested and stepped fill terraces), revealing a strong control of valley morphology and bedrock barriers on sediment delivery, storage and preservation. The headwater valley (Maria sector) preserves four morphosedimentary alluvial units corresponding to two palaeogeographic stages. Morphosedimentary unit (MSU1), the most extensive, is a major depositional valley infill dominated by sand and silt texture, showing three stratigraphic sets dated 9000–8500, 5300–4800, 4000–3000 cal BP. This vertically stacked alluvial accumulation occurs upstream of a narrow bedrock channel, that limited sediment connectivity with the downstream Guadalupe sector. Between 3000 and 2350 cal BP, a 15-m deep incision on MSU1 occurred, and subsequent cut-and-fill cycles generated three alluvial terraces dated at 2350–1900 cal BP (MSU2), 1800–700 cal BP (MSU3), and post-400 years (MSU4). In the downstream valley (Guadalupe sector), six flights of alluvial terraces (MSUg1 to MSUg6) were formed by episodic aggradation and quasi-continuous incision, in response to baselevel changes of the lower Guadalentín River during the Holocene. Alluvial chronologies in these two contiguous valleys show out-of-phase sedimentation periods under low connectivity conditions (9–3 ka), and in-phase cut-and-fill cycles after valley re-connection (post-3.0 ka). The late Holocene alluvial activity periods also coincide with morpho-stratigraphic data from the lower Guadalentín, indicating that fluvial connectivity throughout the catchment was only completed in the late Holocene.

Deciphering channel networks from aeromagnetic potential field data: the case of the North Sea Quaternary tunnel valleys

Summary High-resolution magnetic data and potential field methods have been used to perform a detailed analysis of networks of late Quaternary subglacially-cut tunnel valleys (central Viking Graben, Norwegian sector of the North Sea). High-frequency, ribbon-like, sinuous, paired magnetic anomalies interpreted to be the signature of tunnel valleys are identified. Such magnetic anomalies have 1 to 8 nT amplitudes and reflect a magnetic susceptibility contrast between valley infills and the host sediments. Fractional vertical derivative and horizontal gradient transforms provide the best control on the accurately delineation of tunnel valleys by plotting automatically the extrema. The 2D forward modelling is a very effective approach to determinating the geometric parameters and magnetic susceptibility of the modeled valleys. It allows to determine the finite-width flat horizontal thin geometry as the most appropriate simple geometry to simulate the magnetic anomaly linked to a channel structure. The application of Euler deconvolution using complex algebra allows to substantiate the structural index (n = 1.5) for simple palaeovalley geometries and to determine fair valley depth estimates.

Evolution of a sedimentary infill of a palaeovalley at a distal margin of the peripheral foreland basin

The entrenched Odra palaeovalley, cut into the bedrock of the distal margin (forebulge basal unconformity) of the Moravian Carpathian Foredeep (peripheral foreland basin) is filled with an almost 300 m thick pile of Miocene deposits. The directon of the valley (NW–SE to NNW–SSE) has been controlled by faults subparallel with the system of “sudetic faults“. The sedimentary succession consists of 5 facies associations/depositional environments, which are interpreted (from bottom to top, i.e. from the oldest to the youngest) as: 1 – colluvial deposits to deposits of alluvial fan, 2 – deposits of alluvial fan, 3 – fan-delta deposits, 4 – shallow water delta to nearshore deposits and 5 – open marine deposits. This fining-up and deepening-up succession reveals the following: the formation of the new flexural shape of the basin; deep erosion connected with uplift and tilting of the forebulge and reactivation of the NW–SE trending basement faults; the Early/Middle Miocene sea level fall; alluvial deposition mostly driven by tectonics and morphology; forebulge flexural retreat; Middle-Miocene sea level rise; back-stepping of valley infill; marine invasion during the Early Badenian with shift of the coastline further landward of the pallaeovalley. Tectonics related to contemporary thrusting processes in the Western Carpathians are assumed to be the dominating factor of the studied deposition at the expense of eustatic sea level changes. Provenance studies have proven that the pre-Neogene basement (i.e. the Early Carboniferous clastic “Culmian facies” of the Moravian-Silesian Paleozoic) represents an important source for the conglomerates and sands, which volumetrically dominate in the palaeovalley infill. However, they also showed, that the deposits of the earlier Carpathian Foredeep Basin sedimentary stage (Karpatian in age?) covered the area under study and were eroded and resedimented into the palaeovalley infill.

Postglacial incision-infill cycles at the Borisoglebsk Upland: Correlations between interfluve headwaters and fluvial network

The article discusses postglacial landscape transformation in the Northern Hemisphere Middle Pleistocene glaciation area located in the center of the Russian Plain. We attempted to verify the regional paleogeographic model by reconstructing the Late Pleistocene incision-infill cycles at the Eastern Borisoglebsk Upland based on a comparison of inactive interfluves headwaters and actual fluvial network palaeoarchives. The study was also aimed to determine the past extent of fluvial systems. Interdisciplinary research of the actual and buried topography, lithology and pedogenic properties of surface deposits was carrid out with remote sensing data interpretation, DGPS survey, and detailed description of geological cores involved. The study was followed by analysis of grain size, chemical and organic contents, microstructure, and numerical dating. Integrating the available results, we propose a scenario of the fluvial network transformation at the Eastern Borisoglebsk Upland over the last 150 ka. At least four fluvial incision stages were determined while network extent has significantly changed through the Late Pleistocene. Three can be generally associated with the regional base level decrease – Late Moscow, Late Valdai and Late Holocene and accompanying isolation of the Nero Lake terraces of 130 m, 100–105 m and 95–98 m. Incision stages were separated by landscape stability or aggradation periods those were asynchronous at the middle and upper parts of the fluvial network. The main agent of initial valley infill appears to be local lacustrine sedimentation altered by alluvial and colluvial deposition towards the second half of Valdai. Revealed landscape conditions variability emphasize the importance of comprehensive local correlations for regional retrospective models.

Drumlinized tunnel valleys in south-central Ontario

A network of Late Wisconsin valleys deeply incise thick successions (up to 200 m) of Quaternary sediment and Paleozoic bedrock across south-central Ontario. The valleys have previously been interpreted as an integrated network of tunnel channels recording catastrophic releases of subglacial meltwater across the bed of the Laurentide Ice Sheet (LIS). Recent geologic investigations in the western part of the valley network integrate information gained from surficial sediment and landform mapping, continuously-cored boreholes that penetrate the entire Quaternary sediment succession in valleys as well as intervening uplands, and both water-borne and land-based geophysical surveys. Together, these data provide new insights and a refined interpretation of the genesis, timing and paleoglaciological significance of these valleys.Valleys within the study area are some of the largest observed within the regional network – up to 30 km long, 7 km wide and >175 m deep. They are oriented both north-south and in a radial pattern, varying from NE-SW in the southern part of the study area, to ESE-WNW in the north. An undulating Late Wisconsin till sheet (Newmarket Till) caps the regional uplands and is commonly observed along the flanks and floors of the valleys – all of which show evidence of a drumlinization phase that postdates excavation of most of the valleys. At least two generations of valleys are interpreted from cross-cutting relationships, with abrupt heads and locally perched downflow ends of some valley forms. Radiocarbon age determinations of organic material from both below and above the Newmarket Till constrain the timing of valley excavation to between 28.05 and 12.81 14C kyr BP. Borehole and outcrop data within the valleys reveal four stratigraphic units (SU1-SU4) that form the floor and sediment infill of the valleys. The Newmarket Till (SU1) is commonly underlain by deformed substrates. The till is absent in parts of some valleys and the lowermost parts of the valley fill commonly consist of coarse-grained gravels (SU2) that pass upwards into sands (SU3). The bulk of the sediment infill consists of glaciolacustrine silt-clay rhythmites (SU4) that pass upwards into organic- and mollusk-bearing nearshore and fluviodeltaic deposits that record drainage of proglacial lakes from the study area during deglaciation.It is proposed that these features are a previously unrecognized type of tunnel valley that developed subglacially during Late Wisconsin ice cover. Meltwater surpluses would be generated at the ice-bed interface due to the low-transmissivity substrate in the study area with flow preferentially routed into lows on the bed. Downward incision into confined and pressurized aquifers likely facilitated rapid headward erosion of the valleys enhanced by piping. Waning meltwater flow velocities/discharges allowed ice creep to re-occupy newly-formed valleys permitting renewed till deposition and subsequent drumlinization of the valleys. The valleys also provided efficient routing for meltwater draining the retreating ice marginal zone during deglaciation and allowed localised sand and gravel deposition as eskers and ice-proximal subaquatic fans in the Oak Ridges Moraine and within proglacial lakes that developed during ice retreat. However, there is no evidence for regional-scale catastrophic release of subglacial meltwater in the valley infills. This integrated dataset represents one of the most complete characterizations of subglacial valleys in a terrestrial setting in North America.