Overbank Research Articles

Comprehensive sedimentological and sequence stratigraphic ‎characterization of the exposed Boka Bil Formation, Surma ‎Basin, Northeastern Bangladesh

The Surma Basin, situated in northeastern Bangladesh along the northeastern margin of the ‎Indian subcontinent, is a region characterized by tectonic activity. This study investigates the ‎Miocene siliciclastic sedimentary succession exposed at the Hari River section in Lalakhal, ‎Jaintiapur, to explore its depositional environments and sequence stratigraphic architecture. ‎Field observations and sedimentological analysis reveal two primary facies associations within ‎the Boka Bil Formation: FA1 (tide-dominated environments) and FA2 (shelf environments). ‎FA1 includes three facies: Facies A (tidal channel deposits), Facies B (alternating sandstone and ‎mudstone representing overbank deposits), and Facies C (distal overbank deposits), which ‎together depict a lateral transition from tidal channel margins to spillover deposits. FA2 consists ‎of Facies SL1 (inner shelf deposits) and Facies SL2 (outer shelf deposits), representing deeper ‎marine environments. The Boka Bil Formation displays a retrogradational stacking pattern, with ‎a fining-upward sequence that transitions from shallow to deeper marine conditions. This ‎pattern is indicative of a transgressive systems tract, driven by rising sea levels. The deepening-‎upward facies succession highlights the dynamic interplay between tectonic forces and sea-level ‎changes in an active margin setting. These findings enhance our understanding of sedimentary ‎processes, paleoenvironmental evolution, and regional stratigraphy, providing valuable insights ‎for resource exploration in tectonically influenced basins‎.

Characteristics and mineralogy of sediments in the Hongsa lignite deposit, northwestern Laos

Two sediment cores from the central part of the Hongsa lignite deposit in northwestern Lao PDR (Lao People’s Democratic Republic; Laos) have been analysed in order to understand their sedimentary characteristics using grain-size analysis, petrography, X-ray diffraction and scanning electron microscopy. This analysis has revealed that the deposit is primarily composed of fine-grained sediments, mainly silt and clay, with quartz as the dominant mineral and trace amounts of other minerals such as kaolinite, illite and montmorillonite. Gypsum and chlorite have also been found in some layers. Scanning electron microscope analysis has revealed a card-house structure of clay minerals, suggesting sedimentation from suspension driven by physico-chemical reactions influenced by pH and water chemistry. This arrangement increases porosity and water retention, significantly affecting the permeability and mechanical properties of sediments. Petrographic analysis has documented angular quartz and poorly sorted sediments, indicating minimal sediment reworking or short-distance sources. The palaeoenvironment of the Hongsa Basin, reconstructed from various rock units, suggests low-energy water conditions for the Underburden and moderate sediment supply in a wet forest swamp or bush moor environment for the Lower Lignite Zone Formation. The Middle Lignite Zone Formation indicates a more limited sediment supply in a similar environment, while Interburden Formation 1 suggests overbank deposits or stagnant water deposits. In summary, the Neogene Hongsa lignite deposit is characterised by fine-grained sediments, indicating low-energy water currents in mire environments. Occasional flood events brought coarser grains, although movement of facies should also be taken into account. The mineral composition suggests the presence of components derived from recycled sedimentary rocks along the northern border of the Hongsa Basin.

Riverine Cu-distribution in sediments of the Jaba-Kawerong river system 30 years after cease of mining at Panguna/Bougainville

The investigation of surface and river sediments over 30 years after the cease of large scale Cu and Au mining operation at Panguna (Bougainville) shows the impact of mining and tailings disposal on the associated riverine environment. While Zn, Pb and Cd have elevated concentrations in the former mining area and are low (< 100, < 28, < 1 mg/kg, respectively) in sediment of the Jaba-Kawerong river system, Cu represents the dominant environmental metal emission. Sediments of active streams and overbank deposits range between 1000 and 3000 mg/kg Cu. Most samples exceed freshwater sediment or soil quality guideline values, indicating the probability of toxic effects on sediment dwelling or aquatic organisms and ecological or health risks associated to agricultural use of the former mining area and floodplains. Copper in surface and river sediments is associated to bornite, chalcopyrite and chloritized biotite of the primary Panguna ore mineral assemblage. This attests to ongoing remobilization of sediment and/or reflects mobilization of additional Cu bearing material from the waste rock dump of the Panguna mine. Copper in surface and river sediments is also contained in secondary Cu-phases such as covellite as well as hydrated basic Cu-sulfates, which formed under locally variable redox conditions. Fe-oxihydroxides, occurring in variable abundance as reflected by Fe concentrations between 18,200 and 379,000 mg/kg (Mn 145–3086 mg/kg), can be identified as further Cu-carriers, taking up the metal from the aqueous phase in the sedimentary pore space. Mine derived input of Cu bearing minerals is confined to the sedimentary body of the Jaba-Kawerong river system. The diversity of Cu bearing phases with different environmental and processing properties sets constraints on re-processing sediments as secondary Cu-ores or re-using of sediments as building material.

Compartmentalization of submarine channel splays controlled by growth faults and mud diapir

Submarine channels and their associated overbank deposits can demonstrate vast array of geomorphological expressions and extremely complex depositional architectures. Defining their distribution around seabed deformation is crucial to better understand the dynamics of sediment routing, networking, and deep-sea fan compartmentalization. Here, we investigate the interactions between the geomorphological evolution of submarine channel splays and the structural deformation induced by growth faults. The well-imaged seismic records of the Amaku Channel Levee System (ACLS), from the offshore Niger Delta Agbada Formation, offers a key location to delineate the responses of splays to structures. An integrated interpretation of seismic reflection and well log data allowed us to define a new type of composite splay – named here the “Fork Splay Complex” (FSC). The FSC were introduced by a fault-diverted channel segment, distributing a set of branched individual splays (Sp1-Sp4). They develop particular zones of connectivity across low fault throws and compartmentalized elements between high fault throws. Downstream, two fan-shaped splays (Splay A and B) emerged from deflected channel segments, the Splay B is particularly rotated and elongated distally where a fault interfered its spatial distribution. Based on this study, we provide evidence that the digitated splays, while less expanded than the lobate splays, potentially maintain a stable distribution of coarse-sediment thicknesses, which is implied to be the major control on channel to lobe transition (CLTZ) –i.e., in the confluence zone between FSC and Channel J. The resolved patterns of the investigated splays serve as prototypes for understanding how structural deformation controls slope sedimentary system architectures, with the aim of assisting subsurface interpretation and characterizing sand connectivity or compartmentalization.

Bedform development in confined and unconfined settings of the Carchuna Canyon (Alboran Sea, western Mediterranean Sea): An example of cyclic steps in shelf-incised canyons

Newly acquired high-resolution multibeam bathymetry in combination with sub-bottom acoustic profiles, surficial sediment samples, and three-dimensional flow simulations made possible the characterization of bedforms along the axial channel and depositional lobe of the shelf-incised Carchuna Canyon (Alboran Sea, western Mediterranean Sea). This study aims to describe the erosional and depositional bedforms in confined and unconfined settings of the Carchuna Canyon in order to determine the genetic constraints on sedimentary processes leading to bedform development along the canyon in recent times.The straight Carchuna Canyon, deeply incised in the shelf up to 200 m off the coastline, hosts: (1) crescentic-shaped bedforms (CSBs) that exhibit distinctive crest concavities, asymmetries, and lengths along the axial channel; (2) continuous lateral levees and; (3) a channel bend with three depressed stretches of the levee crest that are less than 20 m high. A set of concentric sediment waves and two scour trails were identified proximal to the channel bend over an overbank deposit east of the Carchuna Canyon. Four acoustic units with distinct acoustic facies were defined along the sediment wave field. The sediment transport simulation shows the highest flow velocities along the Carchuna Canyon thalweg, while over the overbank deposit the highest velocity values occur along the top of the bedform crests, with the higher Froude values being found over bedform lee sides.The occurrence of CSBs along the canyon axial channel suggests the imprint of confined sediment-laden gravity flows descending from the canyon head and exhibiting a flow variability along the canyon induced by local variations of slope gradient and/or sediment concentration. A spatial relationship is identified between the development of sediment waves over the overbank deposit and lowered levee crest heights at the channel bend. In contrast, more energetic downstream turbiditic flows exceed the levee crest at the channel bend, focusing the overflow and promoting erosion of the overbank deposit, thereby generating the scour trains. Based on the recent history of overbank deposition, two alternating scenarios of flow behavior can be interpreted. In a high-density turbidity current setting, erosion would prevail along the axial channel. Widespread spillover flows of coarse-grained sediments would occur in both levees, forming heterogeneous sedimentary patterns that change downslope within the depositional lobe due to lesser turbulence of spillover turbidity currents and gentler slope gradients. In contrast, in a low-density turbidity current setting, turbidity currents flowing along the Carchuna Canyon would form depositional bedforms in the axial channel, while spillover processes would be localized at the channel bend, forming either depositional or erosional bedforms over the depositional lobe according to the frequency, magnitude and focusing of turbiditic flows.

Holocene Floodplain Transformation Through Catchment‐Scale Human‐Environment Interactions: An Interdisciplinary Case Study of the Gete Catchment (Belgium)

ABSTRACTFloodplains across the European loess region transformed from nature‐ to human‐dominated environments during the Holocene. A general framework of this evolution is well established, but it is less clear how differences in timing—observed within and between catchments—can be explained. Although human impact is an important driver, little attention has thus far been paid to the actual human activities involved, their intensities, and spatiotemporal patterns. By adopting an interdisciplinary approach, this study provides insight into the evolution of both environmental (e.g., local floodplain geoecohydrology and regional land cover) and human dynamics (e.g., demography and land use) in the Belgian Gete catchment. Results show that the observed changes in land cover and floodplain geoecology can indeed be attributed to spatiotemporal variations in human impact. Human‐induced vegetation change, driven by population growth and associated agrarian production, resulted in hydrological changes and localized colluviation. Catchment‐scale improvement of hillslope‐channel connectivity initiated alluviation in the valleys and completed the transformation from forested marsh to open floodplain with overbank sedimentation. In turn, this allowed people to settle in the floodplains in the Early Medieval period, laying the foundations for our present‐day landscape.

The Paleocene‐Eocene Thermal Maximum Recognized in Distal Alluvial Fan Strata of the Sevier Fold‐And‐Thrust Belt (Southwestern Wyoming, USA)

AbstractFifty‐six million years ago a massive flux of exogenic carbon was rapidly released into Earth's oceans and atmosphere during the Paleocene‐Eocene Thermal Maximum (PETM). The warming associated with this release persisted for ∼200 kyr. It had severe impacts on ocean environments, global ecosystems, and the hydrologic cycle. Yet, regional patterns of response within continental settings are not well constrained. Here we describe the first recognition of the PETM in the Greater Green River Basin of western Wyoming, USA. We present new sedimentologic, chemostratigraphic, and paleontologic data from the Chappo Member of the Wasatch Formation in the Sevier fold‐and‐thrust belt. The Chappo Member contains lithofacies indicative of debris flows, ephemeral and incisional stream channels, and well‐drained overbank deposits, which, along with the geographic position of the study area, indicate a distal alluvial fan setting. Using pedogenic carbonates, we find a negative carbon isotope excursion (CIE) of ∼3.8‰ associated with the first appearance of Celtis endocarps stratigraphically above beds containing late Paleocene mammals. The CIE magnitude here is considerably smaller, compared to records from the Bighorn Basin, which we interpret to represent drier, better‐drained conditions. High baseline carbon isotope values suggest dry conditions in the study area, which likely resulted from well‐drained sediments associated with the distal alluvial fan. Low amounts of precipitation, as suggested by previous climate models, may have contributed to dryness. We also find sedimentologic evidence consistent with an enhanced hydrologic cycle during the PETM, as seen in other basins in the region.

Andesite magma genesis, conduit dynamics and variable decompression from shallow reservoirs drive contrasting PDC events at Volcán de Colima, Mexico

Volcán de Colima, one of the Earth's most active and hazardous andesitic stratovolcanoes, experienced its last major explosive eruption on July 2015 with two distinct events. The July-10 event comprised the collapse of a large summit dome, forming block-and-ash flow deposits of dense dome lava clasts. The July-11 event produced pyroclastic density currents (PDCs) that flowed to maximum 10.5 km-distance from the crater, burned the vegetation, and formed valley and overbank deposits of up to 30 vol% vesicular scoria clasts. Whereas the July-10 event fits well within the last 20 years' typical activity of Volcán de Colima, the July-11 PDCs were unprecedented. We present a close examination of the July-11 deposits via combined field, microtextural, and chemical analyses, including electron microscopy and X-ray microtomography. Results show that andesite magma (58–59 wt% SiO2) at 1021 °C and having 2 wt% H2O degassed and crystallized (102–105 mm−3 vesicles and 102–103 mm−3 microlite number densities) while ascending to the surface from ∼2 km-depth reservoirs, driven by decompression rates of 0.4–1.7 MPa s−1. These variable rates reflected heterogenous andesite magma rheology produced by different stages of melt genesis, mobility, stalling, crystallization and vesiculation. Prior to experiencing fragmentation, the andesite magma mingled with rhyolite melts produced from partial melting of silicic mush stored at depths from ∼2 to 5 km. Magmas fragmented at maximum strain rates of 10−3 s−1, powering the July-11 energetic (106–107 kg s−1) and pulsating PDCs that released 102–103 m3 s−1 on surface. The rapid <20 h transition from the July-10 to the July-11 events suggests only hours timescales from dome collapse to magma decompression and explosive eruption.

Geochemical Characterization of an AMD-Affected Stream: Detection of Associated Trace Metal Contamination Using Element ‘Dilution Factors’

This study characterized metal contamination in the Blesbokspruit River waters, near the coal-mining town of Emalahleni in Mpumalanga, South Africa, using enrichment factors (EFs) and contamination factors (CFs). We investigated the potential of Fe and Al ‘dilution factors’ (stream water divided by overbank sediment concentrations) under base flow hydrological conditions to detect AMD-related trace metal contamination in the study site. Waters of the Blesbokspruit River were characterized by acidic (< 3) to near neutral (≈7) pH, high EC (up to 2240 µS/cm), high sulfate (up to 1530 mg/L), ultra-high degrees of Fe contamination and minor to moderately severe enrichment of trace metals, all of which strongly indicate that the river is affected by AMD. Fe and Al dilution factors showed moderate to strong positive correlations with dissolved trace metals (Co, Ni, Zn, Pb, Cr, and Cd), but not with Cu, which is likely more associated with kaolinite in the study area. In addition, correlation analysis showed that dilution factors were better for detecting aqueous Co and Cd, and comparable for detecting aqueous Ni, relative to stream pH and aqueous Fe and Al. Dilution factors have an advantage over stream pH and aqueous Al and Fe because, once calculated, they can be used to monitor AMD-related trace metal contamination in streams during dry seasons by using only Al and Fe content in sediments. This can be beneficial when preservation methods or storage necessary for water samples are not available. However, the robustness of dilution factors during wet seasons requires further investigation.

Stacking pattern transition caused by submarine channel inception and internal levée development

ABSTRACTThe shift from highly amalgamated and laterally offset to aggradational and vertically stacked submarine channels is a pattern that has been widely recognized in seismic reflection datasets as well as in outcropping successions worldwide. However, the sedimentary and stratigraphic details of such an important part of channel evolution and its implications have not been discussed extensively. That gap is addressed here by characterizing a previously undocumented, seismic‐scale outcrop of the Tres Pasos Formation (Magallanes Basin, Chile) that records the transition from a laterally offset/low‐aggradational channel complex to a vertically stacked aggradational complex associated with the development of an internal levée that enhanced channel aggradation. The boundary between the two complexes is defined by a composite erosional surface (up to ca 35 m relief) with two adjacent element‐scale channelized incisions with contrasting sedimentary infills that provide unprecedented insight into this key phase of submarine channel evolution. Observations indicate that, in between these two depositional architecture styles, there is a significant phase of deep incision and bypass. The relief achieved via this deep incision of one or multiple simultaneously active conduits is interpreted to be necessary to set up the conditions for flow stripping and, subsequently, overbank deposition. Finally, the siltstone‐rich intra‐channel lithofacies association observed directly overlying part of the incision is interpreted to represent the stratigraphic expression of the earliest deposits of an active submarine channel preserved due to the abrupt abandonment of this pathway and the development of an adjacent conduit, which resulted in the former being covered with fine‐grained overbank deposits.

Integrating CoDA and PCA for enhanced characterization of fluvial depositional processes: a case study of the Shendi formation, Sudan

Traditional approaches to fluvial sedimentary analysis often face challenges in deciphering complex, multivariate datasets. This study combines compositional data analysis (CoDA) with principal component analysis (PCA) to enhance the characterization of depositional processes and sub-environments within the Shendi Formation. The PCA applied to centered log ratio (clr)-transformed grain size distributions, reveals three principal components with ~ 91.86% of the data variance explained, representing distinct processes: bedload-dominated channel-bar dynamics, overbank deposition, and high-energy flood events. Specific lithofacies associations strongly correlate to each principal component. This integrated approach enables the identification of subtle yet significant patterns within the complex sedimentological record. The Shendi Formation exhibits characteristics of a dynamic fluvial setting with variations in flow energy, channel migration, and periodic flooding. Our findings demonstrate the power of CoDA-PCA in refining the understanding of fluvial depositional systems and highlight its potential for broader applications.

Impact of reservoir construction on the sediment budget of a downstream-linked freshwater lake

ABSTRACT We investigated the magnitude of reservoir sedimentation upstream and its sediment-shadow effect on a large downstream-linked lake, Lake Tana (Ethiopia). To better understand the reservoir-induced reductions of fluvial sediment downstream, sediment budgets for two scenarios were developed. The results indicated that reservoir sedimentation is extremely high, with annual storage capacity losses (1–4.2%) exceeding the global average. Consequently, unless mitigation measures are implemented, the reservoirs’ dead storage volume will be filled with sediment in less than 42% of their designed life expectancy. The sediment-trapping conditions created by the reservoirs will also reduce sediment inflow from tributaries into the lake by 15–52%, overbank sedimentation on floodplains by 33–48%, and sediment deposition in Lake Tana by 16–54% annually. Overall, under the current environmental set-up, the ongoing and planned reservoir constructions could extend the expected useful lifetime of Lake Tana from 918 to 2022 years.

The Hasli Formation of the Irchel Plateau – A key record for older Early Pleistocene interglacial sediments in northern Switzerland

In the Alpine region of Central Europe, terrestrial Early Pleistocene deposits with preserved faunal and botanical remains are rare. The predominantly gravelly Höhere Deckenschotter (HDS) deposits of northern Switzerland and adjoining south-western Germany are considered the oldest Quaternary sediments in the northern Alpine Foreland, linked with the first extensive Quaternary Alpine glaciations.This multidisciplinary study investigates the Hasli Formation (HF), a unique silty unit with well-preserved biological remains within an HDS sequence at the Irchel Plateau north of Zürich, providing new climatic and environmental data for these exceptional deposits, as well as crucial information about their age. Systematic mapping of the HF, in addition to the underlying and covering gravel units, has been coupled with sedimentary logging and biostratigraphic analysis at several sites. The HF is present over an area c.4 by up to 1.4 km, and where studied is 1.6–6.2 m thick. The sediments and biological material are indicative of overbank deposits accumulating within the damp floodplain of a large meandering river.The recovered small mammal remains are from eight arvicolid species or genera, including Mimomys pliocaenicus, M. reidi/tigliensis and Clethrionomys cf. kretzoii, which became extinct c.1.8 Ma, and Pliomys episcopalis/simplicior, which first appeared c.2 Ma ago. The over 75k identified molluscs, include c.72k coming from 89 terrestrial species and over 3k from 28 aquatic taxa. The molluscan faunas from all locations where the HF has been studied share important biostratigraphic marker species, and indicate analogous climatic and environmental conditions. The marker species include several that became extinct during the Early Pleistocene, of which Clausilia stranzendorfensis and Cochlostoma salomoni, present throughout the HF, are the most important as they disappeared c.1.8 Ma. Many marker species have modern or palaeo-distributions that lie far to the west, south and east of the Swiss Plateau, showing that the climate was much warmer than today, typical for the Tiglian warm stage. The alluvial floodplain was covered by rather dense, mature woodland.Amino acid (AA) analysis shows that molluscan remains from the HF are among the oldest of the Quaternary shell material studied from across the Swiss Plateau. Palaeomagnetic data shows a change from reversed to normal polarity within the upper part of the HF. When combined, the small mammal and molluscan remains, palaeomagnetic measurements, AA geochronology and sedimentary data, suggest an age of 2.1–1.8 Ma for the HF. Recently produced cosmogenic dates bracket the age of the HDS gravels beneath the HF to depositional periods between 2.6 and 0.9 Ma, with the youngest ages being in disagreement with our findings. The abundance and diversity of the molluscs from the Irchel Plateau, with several rare extinct species, together with a notable small mammal assemblage, make this a key palaeontological site and an important reference point for European Quaternary biostratigraphy.

Sedimentological and Geochemical Characterization of the Early Cambrian Eastern Yunnan, Southwestern China

The accurate reconstruction of the early Cambrian paleoclimate and paleoceanographic conditions on the Yangtze Plate is crucial for understanding the ancient environment during the Cambrian Explosion. It is also a key factor in understanding the ecological habits of organisms during the Cambrian Explosion. The study utilized field outcrops, thin section analysis, and major and trace elements to investigate the sedimentary environment, provenance, paleoweathering, and paleoclimate of the Lower Cambrian Hongjingshao (HJS) Formation (Cambrian Stage 3, ~515 Ma) in the Yangtze Basin, eastern Yunnan, SW China. The HJS sandstones are composed of 10 lithofacies, including massive and weakly bedded gravel supported by coarse sandstone (Gm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), ripple cross-laminated sandstone (Sr), horizontal bedded sandstone (Sh), scour-fill sandstone (Se), massive sandstone (Sm), fine to medium sandstone with thin bed muddy siltstone (Fl), muddy siltstone (Fsc), and mudstone (Fm). On the basis of these lithofacies, channel fill and over-bank deposits in delta and shallow shelf depositional environments are suggested for HJS Formation. The major elements-based provenance discriminant function and mineral composition indicate that felsic rocks from the recycled orogen and continental block are the main sediment source terrane for the HJS sandstones of the study area. CIA, PIA, and CIW values range from 71.29 to 93.72, indicating an intermediate to intense chemical weathering and semiarid to humid climate conditions in Cambrian Stage 3. The research findings have clarified the paleoclimate and paleoceanographic environment of the Early Cambrian in the Yangtze region, which is of significant importance for understanding the early biological and ecological marine environment in the study area.

Influence of Floodplain Forest Structure on Overbank Sediment and Phosphorus Deposition in an Agriculturally Dominated Watershed in Iowa, USA

Influence of Floodplain Forest Structure on Overbank Sediment and Phosphorus Deposition in an Agriculturally Dominated Watershed in Iowa, USA

Natural and anthropogenic factors affecting intense slope processes in Eastern Europe during the Modern Period: Serteyka river valley, Russia

Detailed palaeogeographical studies of the accumulative fan in the Serteyka River valley in W Russia and underlying biogenic deposits were carried out. The base of a representative core of biogenic sediments in the distal zone is dated to 1291 BC, while its top to 1631 AD. In this paper, palynological, plant macrofossils, Chironomide and Cladocera, geochemical, geochronological and sedimentological analysis were performed. Four phases of biogenic deposition were distinguished by pollen and geochemical analyses. Two of them coincide with the climatic fluctuation during Medieval Climate Anomaly and Little Ice Age. During the formation of the fan, overbank deposits were accumulated also, indicating numerous and intense floods, which are in line with the trend observed for other sites in this region. The accumulative fan is formed by lower, middle and upper deluvia and agricultural diamicton in its top. All units have specific sedimentological and geochemical features as well as low admixture of plant macrofossils. The beginning of the formation of this relief form dates back to the second half of the 17th century AD, and the end of the accumulation falls on the second half of the 18th century AD. Our work suggests that natural conditions had an impact on the formation and development of studied accumulative fan, however, the decisive factor causing the intensification of the slope processes were related with deforestation resulted from strong human impact, which was marked in palynological and macrofossils analyses (e.g. increase in the contribution of plants macrofossils related to agriculture).

Holocene flood records and human impacts implied from the pollen evidence in the Daming area, North China Plain

Understanding the environmental significance of pollen and spores in alluvial plains is important for stratigraphic correlation and paleoenvironmental reconstruction. This paper presents palynological data from the North China Plain and explores their relationship with paleoflood records and human impacts since the Holocene. Our data reveal that pollen concentration and pollen assemblage vary in flood deposits (including overbank deposits and slackwater deposits) and inter-flood deposits (including sandy soils and lacustrine deposits). Flood deposits have higher fern percentages (28.6%) and lower herbaceous percentages (14.8%) compared to inter-flood deposits, though slackwater deposits share similar pollen concentrations and assemblages with sandy soils. Notably, overbank deposits are characterized by pollen-poor zones and aggregation of deteriorated pollen grains, especially in Unit III (755–385 cm, ca. 3.2–2.2 ka) and Unit V (190–0 cm, after ca. 0.6 ka). These findings suggest that overbank deposits correspond to strengthened hydrodynamic conditions at the flood-peak stage. Furthermore, the indicative pollen and spores provide compelling evidence for intensifying human impact in the North China Plain since the late Holocene. An aggregation of Selaginella sinensis at the depth of 640–610 cm indicates deforestation in the uplands since ca. 2.9 ka. Similarly, a sharp increase in Malvaceae percentage at the depth of 285–215 supports historical records of initial cotton planting in the Tang Dynasty (ca. 1.4–1.1 ka). The study underscores the value of palynological analysis for reconstructing paleoenvironment and human-environment interactions, providing a robust framework for understanding landscape evolution in the North China Plain.

Late‐Holocene counterpoint deposition in the Lower Rhine River

ABSTRACTChannel deposits from meandering rivers have proven to be far more complex than the well‐known lithofacies model consisting of coarse‐grained channel, gravelly channel‐lag and fine‐grained overbank deposits. Sharp bends in rivers are subject to different hydraulic processes than bends with lower curvatures, enabling erosion of inner banks and deposition of fine‐grained sediments in the outer bend, resulting in downstream migration of river bends. This phenomenon is known as counterpoint deposition, forming counterpoint bars. This research investigates whether scroll bars associated with a sharp bend in the Lower Rhine River, The Netherlands, are such a counterpoint‐bar deposit. A counterpoint bar is expected based on: (i) the surface morphology of the scroll bar; (ii) the confinement of the river course by an ice‐pushed ridge resulting in a sharp bend; and (iii) the archaeological context of successive Roman settlements atop the ice‐pushed ridge, potentially moving downstream with the migrating river bend. This hypothesis is tested through detailed borehole descriptions combined with optically stimulated luminescence dating, the latter being a novel approach to identifying counterpoint deposits. The deposits consist of clays and sandy clays with fine sand laminations, and sporadic larger sand bodies. Further upstream these deposits grade into channel deposits dominated by coarser sands with gravels. These lithologies are explained using earlier proposed mechanisms for counterpoint formation; substrata match those described in previously studied counterpoint deposits and their point bar counterparts. Optically stimulated luminescence dates indicate that the Lower Rhine River bend migrated downstream, confirming counterpoint deposition. A migration rate of 1.93 m/year was established through weighted linear regression. This study demonstrates the potential of optically stimulated luminescence dating to investigate counterpoint bar presence. The identified counterpoint bars and associated bend migration provide insight into meandering river dynamics that is crucial for river management and in aiding river restoration and rewilding initiatives.

Depositional Environment and Reservoir Prospect Analysis of Barail Sandstone Exposed in the Northeastern Part of Bangladesh

The study characterized the Oligocene Barail sandstone in the Surma Basin from a reservoir perspective. Data were collected by direct field observations; rock samples were collected from outcrops for detailed sedimentological, petrographic, and geochemical analysis. Collected sedimentary logs are analyzed to delineate the facies associations, sandbody architecture, and depositional environment. A total of ten facies were identified, which were further grouped into three facies associations: FA1: Tidal-influenced multistorey channel-fill deposits, FA2: Fluvial multistorey channel-fill deposits, and FA3: Overbank deposits. Facies analysis suggests that the Barail sandstone was deposited in the tidal-influenced paralic to fluvial depositional settings, and channel body dimensions increase towards the top. A petrographic study shows that these sandstones are mostly litharenite (Q70F4L26) in type, and sediments were derived from quartzose recycled orogens. The average porosity of the sandstones is 16.9%; the pore spaces are of both primary and secondary origin and are mostly interconnected. Porosity distribution in the fluvial multistory channel-fill deposits (FA2)—the upper part of the sandstone succession, is less heterogeneous, while that in the tidal-influenced channel-fill deposits (FA1), occurs in the lower part is more heterogeneous. Both the depositional process and diagenesis influenced the porosity of Barail sandstones. Geochemical analysis suggests that the sandstones are chemically stable and moderately matured. The study demonstrates that the Barail sandstones may be a good petroleum reservoir. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(1), 2023, P 69-88

Holocene overbank sedimentation in Central Europe between natural and human drivers - The Weiße Elster River (Central Germany)

Up to several meters thick fine-grained Holocene overbank deposits are ubiquitously found in most Western and Central European lowland floodplains. However, despite their large importance for the geomorphological and geoecological floodplain properties, the interplay of different possible causes for their formation are not well understood yet. Most authors suggest human-induced deforestation as the main precondition for sediment mobilization and transport from the slopes to the floodplain, whereas others suggest a stronger influence of climatic factors. This current research gap is caused by often missing well-resolved fluvial chronostratigraphies and spatio-temporal information about former human activity within the studied catchments. To fill this gap we exemplarily studied Holocene overbank sedimentation and possible human or natural drivers in the meso-scale Weiße Elster catchment in Central Germany. To do so, we applied numerical dating as well as sedimentological and micromorphological analyses to Holocene fluvial sediments along three floodplain transects. Furthermore, we built up an unprecedented systematic spatio-temporal database of former human activity within the catchment from the Neolithic until the Early Modern Ages. Together with published paleoclimatic data, this database allowed an unprecedented, systematic comparison of Holocene overbank sedimentation phases with possible external controls. Our data show that some overbank sedimentation phases were directly linked with human activities in the affected site sub-catchments, whereas others were not. Instead, all phases seemed to be linked with natural factors. This difference with most former studies could possibly be explained by previously often limited numerical dating of the fluvial sediments and by largely missing spatio-temporally well-resolved regional settlement records, hindering a precise temporal link of fluvial sedimentation with former human settlement. Furthermore, this difference could possibly also be explained by a relatively high natural sensitivity of the landscape dynamics in the Central German lowlands, showing a subcontinental climate, towards climatic external controls.