Floodplain Deposits Research Articles

Sedimentation and erosion at Majuli Island, Assam, India: analysis of paleo-depositional environment

ABSTRACT The World’s one of largest inhabited river-made island Majuli, developed in response to depositional processes of large sediment load carried by the Bramhaputra River and Subhansari River from the Himalayan hinterland. But, for decades it has been going through a significant rate of erosion, which is severely threatening the existence of the island and its rich cultural heritage. These erosional processes result in exposure to island depositional records and provide an opportunity to study these sedimentary deposits. This paleo depositional record is very well analysed by adopting the methodology of facies identification and their relationship diagram. A total of 18 exposed outcrop sections are studied with detailed sedimentological documentation, facies identification and symbols assigned to facies. Then calculations and tabulations are carried out for Facies Relationship Diagram (FRD) along with the construction of the sequence model, which provides a lateral and vertical lithofacies association developed in the entire island sequentially. On the basis of validation of obtained data through field survey and analysis of more vertical profiles, we study the correlation between existing bank erosion and dominated sedimentary facies at Majuli Island. This analysis results in two facies sequences: one mud-dominated and the other sand-dominated facies. The Massive Mud and Massive Sand beds are represented as the main facies between these two sequences. Therefore, mud-dominated sequence represents the Flood Plain deposit, whereas the sand-dominated sequence represents Bar and Channel deposits. This study also reveals that sedimentary sequences developed by the Bramhaputra River are mud dominated, while that of the Subhansiri River is both mud and sand sequences. This reflects the differential source to sink transport of sediments, as the Bramhaputra River carries sediment load for a longer distance than the Subhanshiri River. This study also reveals that areas of sand-dominated facies show higher erosion than mud-dominated facies. Therefore, the sedimentary record of the Subhansiri River is more vulnerable to erosion.

Distribution of landforms and buried sedimentary deposits during the growth of the Aceh River delta (Sumatra, Indonesia)

ABSTRACT Fluvial and coastal landforms are indicative of landscape river delta evolution over time and provide clues for understanding coastal adjustments to sea-level and fluvial dynamics fluctuations, tectonic displacements, and extreme waves. We have mapped the surface and sub-surface footprints of fluvial and coastal geomorphological features in the Aceh River delta, northern Sumatra, using imagery dataset, vertical facies logging and helicopter electromagnetic surveys. The result is a geomorphological map at the scale of 1:75.000 which outlines the main features of the deltaic plain, including rivers, tidal and buried channels, fluvial levees, beach ridges, swales, tidal flats and lagoons. We compare their spatial distribution to the geometry of buried sediment bodies, revealed by boreholes and resistivity maps. Buried channel belts and floodplain deposits document the former locations of the distributary channels of the Aceh River. Coastal-parallel beach ridges evidence 7–8 km of asymmetric delta progradation since the mid-Holocene sea-level high stand.

Anthropogenic influence of open pit mining on river floods, an example of the Blessem flood 2021

Large scale anthropogenic relief changes result in a non-equilibrium landscape and such landscapes are prone to catastrophic erosion events during river floods. An example is the major flood event in western German in July 2021. During this event, the Erft River flooded an open gravel pit near the village of Erftstadt-Blessem, resulting in a massive erosional event, including the catastrophic destruction of parts of the settlement. Three different geomorphological periods occurred during the event: I) rapid backward erosion caused the destruction of the overlaying soils and floodplain sediments. Consequently, the gravel pit was filled with water and sediments. Three different pathways of backward erosion were activated during this period. II) Sedimentation in a temporary lake and the formation of alluvial fans. III) The Erft River moved towards the gravel pit due to the erosional processes followed by a second flood. Understanding of such complex erosional and depositional processes in open pit mining areas might serve as a blueprint for processes and hazards in anthropogenic influenced landscapes.

Using sediment accumulation rates in floodplain paleochannel lakes to reconstruct climate-flood relationships on the lower Ohio River

Late Holocene flood frequencies on the lower Ohio River were investigated using 14C-based sedimentation rates from three floodplain lakes located in Illinois (Avery Lake), Kentucky (Grassy Pond), and Indiana (Goose Pond). Changes in sediment accumulation rates were attributed to variability in the delivery of overbank sediment to each site as controlled by the frequency of Ohio River flooding. Sedimentation rates reached their lowest values in all three lakes between 400 and 1230 CE, indicating a regional reduction in flood frequencies on the lower Ohio River during a period that included the Medieval Climate Anomaly (MCA; ca. 950–1250 CE). Sedimentation rates increased after ca. 1230 CE and remained moderately high through the Little Ice Age (LIA; 1350–1820 CE) until the onset of extensive land clearance during the early 1800s CE. After 1820 CE, sedimentation rates increased further and were higher than any other time during the late Holocene. A comparison of regional paleoclimatic proxies with the above floodplain sedimentation records shows that Ohio River flooding during the late Holocene was responsive to mean-state changes in atmospheric circulation. During the MCA, when clockwise mean-state atmospheric circulation advected southerly moisture from the Gulf of Mexico into the Ohio River Valley primarily in the form of convective rainstorms, flooding on the Ohio River was least frequent. During the LIA, meridional mean-state atmospheric circulation increased the proportion of midcontinental moisture that was sourced from the northern Pacific and Arctic and delivered as snowfall, hence increasing flooding on the Ohio River. We attribute the increase in Ohio River flooding during the LIA to an increase in snowpack volume across the Ohio River Valley and the watershed-scale integration of runoff during spring snowmelt. Following Euro-American land clearance in the early 1800s, flood frequencies decoupled from this relationship and the lower Ohio River became susceptible to frequent flooding, despite a return to southerly and clockwise synoptic atmospheric conditions. These modern climate-flood dynamics are fundamentally different than those of the paleo-record and suggest that land-use changes – such as deforestation, tile draining, and landscape conversion to intensive row crop agriculture – have fundamentally altered the modern Midwestern hydrologic cycle.

Microplastics in Surface Waters and Floodplain Sediments of the Dagu River in the Jiaodong Peninsula, China

Microplastics in Surface Waters and Floodplain Sediments of the Dagu River in the Jiaodong Peninsula, China

Late-Holocene Sediment Storage in Upland Valley Systems in the Gamo Highlands of Southern Ethiopia

Part of the eroded soil material from the hillslopes is temporarily stored on hillslopes and in river valleys as colluvial and alluvial storage, respectively. This storage component of a catchment’s sediment budget is an important archive reflecting past erosion and sediment delivery processes in relation to both natural and anthropogenic environmental changes. Information on long-term sediment dynamics (i.e., centennial to millennial timescales) is generally lacking for tropical mountain environments. Here, we quantify long-term floodplain sediment storage and sedimentation dynamics in the Gamo highlands of the southern Ethiopia Rift Valley. In two upstream catchments (Chencha and Dembelle), a detailed survey of the floodplain sediment archive was conducted through hand augering of 37 cross-valley transects. Sediment thicknesses vary between 4 and 8 m and total storage equals 0.03 Mt ha−1 floodplain area for the Chencha area and 0.05 Mt ha−1 floodplain area for the Dembelle area. Radiocarbon dating of organic material retrieved from the sediment archives provided a temporal framework for interpretation of sedimentation processes dynamic. The mean sedimentation rate in the Chencha floodplain is ~3.22 ± 0.33 kt ha−1 catchment area, whereas it is ~3.76 ± 0.22 kt ha−1 catchment area for the Dembelle floodplain. Up to 70% of the total sediment mass is stored in the floodplains within the most recent 2000 years. Cumulative probability function plots of radiocarbon dates show that sedimentation started to increase from ca 2000 to ca 1600 cal BP, roughly coincident with an increase in human presence, as is indicated through archaeological data.

Litologie a paleopůdy klikovského souvrství ve vrtu 2140 01T1 Dunajovice, třebonská pánev

The 2140 01T1 borehole sampling the Late Cretaceous non-marine strata of the Klikov Formation in the central part of the Třeboň Basin in southern Bohemia provided ~ 70 m thick alluvial succession composed of alternating sandstone and mudstone intervals that overlays deeply weathered crystalline basement. The mudstone interval in the lower part of the Klikov Formation is separated by sandy layers into several 2–4 m thick fining-upward units. The sharp-based sandy layers with fusain, carbonized stems and rip-up clasts of underlying mudstone are interpreted as avulsion deposits. They grade upwards into heterolithic sand-silt sequence and mottled massive mudstone with burrows, rhizohalos and variably developed pedogenic features. Such cyclic arrangement probably reflects autogenic processes on distal floodplain with episodic deposition and upward increasing subaerial exposition and pedogenic overprint. In the middle to upper parts of the Klikov Formation up to 6 m thick channel sandbodies with sharp erosional bases cutting into the underlying laminated sand-silt heteroliths of channel levee deposits occur. The thickness of the single-storey sandbodies corresponds usually to c. 2 m which suggests up to 3 m deep channels during bankfull discharge. The thicker multi-storey sandbodies record channel amalgamation on the alluvial ridge or shallow paleovalley fillings. The sandbodies grade upwards to massive mudstones of abandoned channel fills and floodplain deposits. These floodplain units are up to 8 m thick and consist of variegated massive mudstones interrupted by discrete up to 1 m thick fining upward layers of rippled sandstone representing crevasse splay deposits. Depositional context including preserved levee deposits, abandoned channel fillings and floodplain finegrained deposits suggests rather higher sinuosity of the formative rivers. Palaeosol profiles represent pedogenically modified floodplain fines with gradational bases and sharp erosive top surfaces. At least 10 palaeosols were identified in the measured section, forming mostly discrete profiles up to 2 m thick consisting of several B-horizons that differ in the degree of the soil structure development, colour, mottling, grain size, presence of clay hypocoatings and bioturbation index. The most common Bw horizons form lower parts of the profiles and have no or only a weakly developed angular blocky structure (ABS). The typical mottling is caused by a hierarchical system of vermicular greenish-grey reduction rhizohalos, that may be filled by silt to sand or illuviated clay hypocoatings and probably follow the original plant root system. Some patches have diffuse orange rims of iron oxides, probably represented by goethite. Mudstone of Bsst horizons displays well-developed ABS with rare wedge-shaped peds and contains a number of illuviated clay skins and redox depletions. Vertic features like fine cracks filled by sand are common, and slickensides with clay hypocoatings occur rarely. Mudstones of Bt horizons contain a number of illuviated clay hypocoatings filling root channels or interpedal cracks, while the Bss horizons exhibit well-developed ABS with slickensides and contain a number of vertical cracks filled by sandy grains. Palaeosol profiles represent mainly argillisols and vertic argillisols with transitions to argillic vertisols. Poorly developed argillic to vertic protosols are also common. Palaeosol profiles differ mainly in the degree of structural development/maturity, which is given by various lengths of exposition to similar climatic conditions. Vertical distribution of palaeosols reflects their position in the alluvial landscape with locally specific hydromorphic conditions rather than a distinct climatic trend. Most of palaeosols exhibits downprofile translocation of clay (illuviation) leading to the formation of argillic horizon, which typically develop on stabilized landscape in humid climate with seasonal moisture deficit.

Multi-scale, multi-style soft-sediment deformation structures preserved in Cenozoic fluvial deposits, southeastern Brazil: Evidence of impact shaking?

Soft-sediment deformation (SSD) structures preserved in sedimentary successions can record significant seismic events which occurred during or right after deposition. The integrated analysis of sedimentary facies, depositional architecture, and SSD data can determine the relative temporal relationship between deposition and deformational events, and reveal the origin and mechanisms involved in the deformation and the resulting structures. The São Paulo Formation in southeastern Brazil preserves Cenozoic alluvial deposits which characterize most of the geology of the homonymous metropolis area. This succession preserves fluvial channel and floodplain deposits, with associated debris-flow dominated alluvial fan deposits, and preserves abundant SSD structures in a wide range of dimensions and morphologies. These deformation structures include small- and large-scale folds, multi-scale vents of varied geometries, ball-and-pillow structures, and clastic dykes. Here we provide an integrated analysis of sedimentary facies, depositional architecture data, and SSD data from the São Paulo Formation, in order to discuss the deformation mechanisms and plausible triggering mechanisms responsible for the development of such structures. Our interpretation is that SSD structures in the São Paulo Formation were likely triggered by a Cenozoic meteor impact recorded by a crater near the study area.

Seismic expression and stratigraphic preservation of a coastal plain fluvial channel belt and floodplain channels on the Gulf of Mexico inner continental shelf

AbstractSubsurface fluvial deposits in coastline‐proximal settings record the spatiotemporal evolution of the coastal landscape and may be viable repositories of sediment for future coastal restoration projects. However, quantitative linkages between the geomorphic form and stratigraphic expression of coastal plain fluvial elements remain lacking, complicating coastal stratigraphic interpretations and subsurface resource assessment. This study explores the expression and preservation of fluvial coastal plain geomorphic features through outcrop‐scale seismic stratigraphic analysis of ultra‐high‐resolution chirp acoustic reflection data from the north‐western Gulf of Mexico inner continental shelf, offshore the Brazos River, Texas, USA. The chirp data exhibit decimetre‐scale vertical resolution within the upper 35 m of the shelf, evincing the preservation of two distinct types of coexisting fluvial channel‐forms: Type 1, a 1 km wide and 9 m deep single‐storey channel belt filled with sand‐rich lateral accretion deposits and channel fills; and Type 2, numerous 10 to 800 m wide and 1 to 20 m deep incisional floodplain channels filled with extensive drapes of overbank‐derived sediments. The chirp data resolve fluvial geomorphic elements including lateral accretion surfaces, levées, floodplain deposits and abandoned channel fills. Rollover of lateral accretion surfaces, widespread draping of seismic reflectors within channel fills, and quantitative comparison between the interpreted stratigraphic forms and analogous features on the nearby Texas Coastal Plain suggest near‐complete stratigraphic preservation of the geomorphic form of both channel belt and floodplain channel elements. Rapid aggradation of the coastal plain by the Brazos River in this region during the Holocene transgression is hypothesized as a mechanism for the high degree of preservation achieved. This study proposes the first recognition criteria for the seismic stratigraphic expression of coastal floodplain channels and provides direct linkages between the geomorphic and stratigraphic expression of a coastal plain fluvial landscape, promoting improved coastal stratigraphic interpretations and assessment of subsurface lithofacies distributions.

Age and causes of the Yellow River dissecting the Zoige Basin in the eastern Tibetan Plateau, China

Through extensive field investigation in the Zoige Basin of the eastern Tibetan Plateau, a typical sedimentary profile was found on the front scarp of the second terrace of Maqu reach of the Yellow River at the outlet of the Zoige Basin. This profile contains the stratigraphic sequence of paleolake facies and paleo-riverbed floodplain facies. Field observation and the analysis of particle size characteristics and optically stimulated luminescence dating showed the following results: (1) The profile includes a pale blue-grey paleolake deposit, which formed in a deep-water anoxic environment. This deposit was overlain by a mottled pebble layer with pale yellow-orange lenticular sand, which represents typical paleo-riverbed floodplain deposits, indicating a strong hydrodynamic process. The typical unconformity contact relationship between the two deposits shows that the profile recorded the sedimentary information of the transformation of the Zoige paleolake environment into the Yellow River paleo-river environment, which provides powerful sedimentary evidence for the Yellow River dissecting the Zoige paleolake. (2) The optically stimulated luminescence dating results showed the Zoige paleolake was dissected by the Yellow River at 37 ka BP, resulting in the discharge of lake water. At 35 ka BP, the lake water gradually became shallow and disappeared, and the Yellow River subsequently connected the water system of the Zoige Basin. (3) The East Kunlun fault induced strong tectonic movement in the late Pleistocene, and the Tibetan Plateau had a warm and humid climate at 37 ka BP. Under these conditions, the headward erosion of the Yellow River was aggravated in the faulted wide valley grassland between the Anyemaqen mountains and Xiqing mountains, and the Zoige paleolake was dissected by the Yellow River from west to east in the Maqu bottleneck reach. The results of this study have important scientific significance for in-depth understanding of the evolution of river-lake water systems and the formation of the Yellow River source water system in the eastern Tibetan Plateau.

Paleocene (65–63 and 58.5 ma) marine flooding and 62–60 ma sediment bypass in southern Wyoming, U.S.A.: Implications for Laramide sediment flux to the Gulf of Mexico

ABSTRACTSedimentary structures unique to tidally influenced environments and unambiguously salinity-stressed marine ichnofossil assemblages in the lower Paleocene Ferris and upper Paleocene Hanna formations of Wyoming's Hanna Basin (HB) necessitate major revision of local and regional reconstructions of the Paleocene Western Interior Seaway (WIS). Preserved in sandy estuarine bars, sandy tidal flats, heterolithic distributary channels, bayhead delta, and flood-tide-delta deposits similar those in the modern Trinity River, its bayhead delta, and the San Luis Pass flood-tidal delta on the Texas coast, these these assemblages include Arenicolites, Bergaueria, Fuersichnus, Gyrochorte, Ophiomorpha, Palaeophycus, Planolites, Psilonichnus, Rhizocorallium, Rosselia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Thalassinoides, and tetrapod tracks. Mapping an ∼ 325-m-thick succession of lower Paleocene strata (65 to 63 Ma) around the western HB reveals a series of marine flooding events, each followed by coal accumulation. A similar, 170-m-thick succession of interfingering coastal-plain and restricted-marine strata occurs in the upper Paleocene (58.5 Ma) Hanna Formation, following accumulation of lacustrine and floodplain deposits and an episode of major gravel and cobble progradation from 62 to 60 Ma. These younger ichnofossils record the final major transgression of the WIS and have major implications for the depositional environment of the time-equivalent Waltman Shale in the Wind River Basin to the north and for sediment routing to the Gulf Coast Wilcox sands. Ichnofossils are an underutilized source of physicochemical proxy data that are lifting the veil from the cryptic Paleocene transgressions of the WIS that have for so long remained undetected because of the absence of open-marine body fossils.

Utilization of Remote Sensing Images for Compiling The Characteristics Map of Indramayu Beach, West Java

The beach has an important role in the development and economy of a region. Therefore, it is important to understand the condition and characteristics of the coast so that it can be managed and developed wisely. One way to understand the condition of the beach is through making a map of the characteristics of the beach. A coastal characteristic map is a graphical representation of the conditions and characteristics of the geology, geography, and hydrology of a coast. Making a coastal characteristic map with remote sensing requires several steps, namely: 1). satellite data collection: Pre-processing satellite data; 2). classification of satellite data; 3). satellite data analysis; and finally 4). making a coastal characteristic map. The area that is the fabric material is the beach of Indramayu Regency, West Java. Based on the analysis and interpretation of 2015 Landsat 8 (LDCM) image data, 2020 Sentinel 2 Image data, BATNAS, and DEMNAS data, it is known that the characteristics of the Indramayu beach are that it has geo-quaternary building materials which include: (a) coastal bunds, ( b) swamp deposits, (c) shallow marine deposits, (d) sand deposits, (e) flood plains, (f) river deposits, and (g) floodplain deposits. The morphology includes: (a) beach sand plains, (b) concave coastal swamps, (c) old beach bunds, (d) young beach bunds, (e) alluvial plains, (f) flood plains, and (g) fan plains delta. Finally, it has a distribution of surface sediments on the seabed which includes: (a) sand, (b) silt, gravelly sand, (d) slightly gravelly silt, (e) slightly gravelly sandy mud, and (f) gravelly mud.

Fossil vertebrates of the early-middle Miocene Cerro Boleadoras Formation, northwestern Santa Cruz Province, Patagonia, Argentina

The early-middle Miocene continental Cerro Boleadoras Formation (CBF) crops out in the area of Cerro Boleadoras and Cerro Plomo on the western slope of the Meseta del Lago Buenos Aires, northwestern Santa Cruz Province, Argentina. The lower levels of the CBF consist of laterally extensive medium to pebbly sandstone beds with trough cross-bedding, interpreted as fluvial channel deposits, interbedded with tabular fine-grained floodplain deposits. Recent fieldwork provided fossil vertebrates from these levels with an estimated age between ~16.5 Ma and 15.1 Ma
 (late Burdigalian-early Langhian). The studied section temporally overlaps with the middle or upper sections of the Santa Cruz Formation (SCF) in the Austral-Magallanes Basin of southern Patagonia, the Río Frías Formation in Chile, and the lower Collón Curá Formation of northern Patagonia. We compile an integrated faunal list for this locality, including specimens from previous collections, and discuss its chronological and paleoenvironmental implications. The taxa list includes most of the groups recorded in the SCF: one anuran, three birds, and at least 33 mammals (metatherians, xenarthrans, litopterns, notoungulate typotheres and caviomorph rodents), indicating a Santacrucian age sensu lato. We also recorded a testudine, which constitutes the southernmost record of tortoises in South America and worldwide. Faunal dissimilarities between the vertebrate fossil content of the CBF and the mentioned sections of the Santa Cruz, Río Frías
 and Collón Curá formations may reflect ecologic, climatic and geographic differences rather than temporal ones. The co-occurrence of arboreal or semiarboreal, browsing, frugivorous, and grazing mammals suggests the presence of both forested and open environments for the area occupied by the CBF rocks. However, it is not possible to discern whether these two environments coexisted or alternated, and whether one environment predominated over the other. Marker taxa, such as the chinchillid rodents Prolagostomus and Pliolagostomus, and the typothere Pachyrukhos indicate a trend to aridification during the Miocene in southern Patagonia, as previously reported for the upper part of the SCF along the Río Santa Cruz and south to the Río Coyle, along the Atlantic coast and the Río Gallegos.

Nature-based solutions in freshwater pearl mussel rivers – A novel approach for self-dynamic desanding developed in a physical model test

In rivers of the Bohemian Massif, an unbalanced sediment regime in the form of a sediment surplus constitutes a growing problem. The dominant weathering grain size fractions (1–10 mm) are unsuitable habitats for, among others, freshwater pearl mussels (Margaritifera margaritifera), which occur in remaining populations in the Austrian part of the Bohemian Massif, the so-called Gneiss and Granite Plateau. In addition to ecological degradation, the above sediment surplus can also increase the flood risk in certain river stretches, which in turn often leads to frequent riverbed interventions (e.g., riverbed dredging). To transition from purely technical mitigation measures to near-natural solutions, the concept of nature-based solutions (NbS) represents an opportunity to mitigate ecological and flood risk issues. However, a comprehensive process understanding is necessary for effective measure planning. This study addressed the deficits in process understanding via physical model tests, in which a technically optimized NbS was developed for the natural process of self-dynamic desanding (SDD). SDD describes the deposition of sand and fine gravel on riverbanks or floodplains under high-flow conditions. The model tests demonstrated that in-stream measures could promote SDD by increasing the sediment output discharged from the river onto the floodplain. In addition, a set of measures was developed on a lowered floodplain, which resulted in optimized floodplain sedimentation under one-year flood conditions. Up to 14.7 % of the total transported sediments could be discharged.

Paleobiology and paleoecology of ungulates from the Miocene of South America based on an ichnological analysis

We analyze fossil footprints and trackways from the middle to late Miocene Vinchina Formation of La Rioja Province, northwestern Argentina. Ichnofossils are preserved in floodplain deposits of anastomosing and meandering fluvial systems and in ephemeral lake facies deposited under arid and semi-arid conditions. The tridactyl ungulate tracks are assigned to the novel ichnotaxa, Macrauchenichnus troyana and Llastaya yesera. Based on comparisons with phylogenetic character states observable on the postcrania of functionally tridactyl ungulates, the trackmaker of M. troyana was a litoptern macraucheniid whereas early toxodontids produced L. yesera. Based on the isometric relationship of footprint area and body mass in modern unguligrade mammals, we propose body mass estimate for these extinct ungulates. Facies analysis of this fossil footprint assemblage reveals that this previously undocumented fauna inhabited mixed grassland-woodland ecosystems developed under warm and seasonal climates in mid-palaeolatitudes. Findings support the idea of a faunal link between high and low latitude ecosystems of South America during the middle to late Miocene.

Current and paleo sources of organic material within fluvial features of the meandering Ruda River, Poland

This study aimed to identify differences in organic matter (OM) composition and origin in the main depositional subenvironments within the floodplain of a meandering river in a temperate climate zone. The study analysed organic-rich sediments found within different facies (side bars, natural levees, crevasse splays, palaeochannels, flood basins, and alluvial fans). Quantitative macrofossil analysis of the OM present in proximal floodplain sediments pointed to a dominance of amorphous OM (AOM), the average proportion of which comprised up to 60% of the total OM. During floods, the most significant phenomenon is the deposition of wood and leaf debris; however, these debris are subject to rapid decomposition in sandy layers and, as a result, do not contribute much to the total OM composition. Root and epidermis remain accumulated in situ and, with an average share of more than 10% of the total OM, are an important part of the more fine-grained alluvial layers. The other components were no more than bioindicators. The proximal floodplain is characterised by sponge spicules in the alluvia and mycelia in the epipedons. In addition, diatom remains are often recorded in the flood layers. The average AOM content in the OM found within the distal floodplain was approximately 40%, although a significantly higher proportion of this component was recorded in flood basin sediments. The accumulation of rhizodermis, epidermis, wood, and periderm (with shares ranging from 5% to 22% of the total OM) plays an important role. Less common components that are of indicative value within the distal zone include the remains of mosses, Cladocera, and sporomorphs. OM within the distal floodplain has high preservation potential, and the sediments in this zone are crucial for carbon sequestration processes in river valleys.

The influence of alluvial stratigraphic architecture on liquefaction phenomena: A case study from the Terre del Reno subsoil (southern Po plain, Italy)

Earthquake-induced liquefaction phenomena are commonly observed at shallow depths or at the ground surface above Holocene alluvial deposits. Nonetheless, a high-resolution examination of the relationship between surface manifestations and alluvial stratigraphy has been rarely attempted. In this study, we provide a detailed subsurface model from a selected sector of the southern Po Plain affected by widespread liquefaction processes triggered by the 2012 Emilia-Romagna seismic crisis. We report a comprehensive sedimentological and CPT-based characterization of the Terre del Reno subsoil and investigate how the stratigraphy controls earthquake-induced liquefaction phenomena. The main facies associations making up the alluvial subsoil constitute the basic elements of the liquefaction system, herein named host, source and sedimentary cap. Their subsurface distribution documents the presence of fluvial bodies prone to liquefaction beneath the earthen levees and across an area dominated by flat muddy floodplain deposits with no physical evidence of paleochannels at the surface. In addition, our observations suggest that vertical and lateral changes in stratigraphy control the mode and location of liquefaction evidence at the surface in both settings. In particular, the relative thickness of fluvial channel bodies vs. channel levee deposits (source–seal couplet) appears to control the severity of sand ejection at the surface. Our findings also suggest that vertical and lateral discontinuities within source layers prevent the occurrence of widespread liquefaction features. Conversely, lateral spreading effects and differential underground hydrostatic pressure at the interface between saturated and less permeable units are accountable for repeated sand ejection along surface fractures. This case study constitutes a unique natural laboratory thanks to the quality and density of available data. However, we emphasize that a similar approach can be useful to promote source-layer identification in areas characterized by poor or sparse data and to mitigate liquefaction hazards in urban areas built on alluvial plains.

Quantifying controls on the occurrence of nonmarine fossils

Abstract Although numerous studies have described differential preservation of nonmarine fossils in channel and floodplain facies, quantitative comparisons are lacking. We present measurements of the probability of occurrence of plant, mollusk, and vertebrate fossils in the Campanian Judith River Formation of north-central Montana, USA. The data reveal little difference in the probability of occurrence among higher taxa in channel and floodplain facies, except for plants, which have a higher probability in floodplain deposits. This surprising result supports more recent models in which many organisms are buried initially in floodplain deposits, but those deposits are subsequently exhumed by migrating rivers, and finally buried in those channels. Comparisons across systems tracts revealed intriguing patterns in which plants have a higher probability of occurrence in high-accommodation systems tracts in channel and floodplain deposits, and vertebrates have a higher probability of occurrence in channels of the low-accommodation systems tract. These results confirm that sequence-stratigraphic architecture should be considered in interpretations of the nonmarine fossil record. This probability of occurrence method has promise for many other comparisons of fossilization potential.

Geomorphically mediated carbon dynamics of floodplain soils and implications for net effect of carbon erosion

AbstractThe fate of organic carbon deposited in floodplain sediments is an important control on the magnitude and direction of the carbon flux from anthropogenically accelerated erosion and channelization of the riverine network. Globally, carbon deposition rates and mean residence time (MRT) within different geomorphic settings remains poorly constrained. We sampled soil profiles to 0.8 m depth from two geomorphic zones: active channel belt (ACB) and lowland floodplain, under long‐term pasture adjacent to the river Culm in SW England, UK. We evaluated sedimentation rates and carbon storage using fallout radionuclide 137Cs, particle size and total carbon analyses. Variation in decomposition was assessed via empirical (soil aggregate size, density fractionation combined with natural abundance 13C analysis) and modelling simulation (using the RothC model and catchment implications explored using a floodplain evolution model). Sedimentation and carbon accumulation rates were 5–6 times greater in the ACB than the floodplain. Carbon decomposition rates also varied with geomorphic setting. In floodplain cores, faster decomposition rates were indicated by greater 13C‐enrichment and subsoils dominated by mineral‐associated soil organic carbon. Whereas, in the ACB, carbon was less processed and 13C‐depleted, with light fraction and macroaggregate‐carbon throughout the cores, and RothC modelled decomposition rates were 4‐fold less than lowland floodplain cores. Including the ACB in floodplain carbon MRT calculations increased overall MRT by 10%. The major differences in the balance of sedimentation and decomposition rates between active and inactive floodplains suggests the relative extent of these contrasting zones is critical to the overall carbon balance. Restoration projects could enhance soil carbon storage by maximizing active floodplain areas by increasing river channel complexity.

ASSESSMENT OF RIVER SEDIMENT QUALITY ACCORDING TO THE EU WATER FRAMEWORK DIRECTIVE IN LOWLAND FLUVIAL CONDITIONS. A CASE STUDY IN THE DRAVA RIVER AREA, DANUBE RIVER BASIN

The EU Water Framework Directive requires the monitoring and evaluation of surface water sediment quality based on the assessment of risk posed by contamination on the biotic receptors. Fluvial sediments are important receptors of hazardous substances (HSs) pollution from the upstream catchment areas in the Danube River Basin (DRB). For the development of systematic sediment quality monitoring and evaluation, the Drava River region on the border of Hungary and Croatia was selected as a test area representative of lowland hydromorphological conditions. Overbank (floodplain) sediments and river bottom sediments (stream sediments) were sampled at two depths at 9 locations in the test area. Eight heavy metal(oid)s were analyzed As, Cd, Cr, Cu, Hg, Ni, and Zn as hazardous substances. The sediment quality assessment was carried out according to the 2013/39/EU Directive and EU Water Framework Directive standards. Most of the analysed HS concentrations in river bottom sediment and overbank (floodplain) sediments fall within the limits of environmental quality standards (EQS). Results show that there is no significant differences in metal(oid) HS concentrations among the various sediment types and between shallow (0-5cm) and deeper (stream sediment: 5-10cm; floodplain sediment: 40-50cm) sediment which suggests that the large lowland Drava River fluvial system is an extensive single fluvial system with homogeneous distribution of sediments and the associated contaminants. Specifically, the studied sediments in the tributaries of the Drava River do not seem to be contaminated with metal(oid) hazardous substances but at certain sites concentrations are elevated above the environmental limit values, especially for As and Zn, and to lesser extent for Cr. The data analysis techniques used enabled the identification of sites with anthropogenic pollution and the recognition of regional pattern in HSs distribution.