Aggradation Rates Research Articles

New evidence on the spatial-temporal distribution of superlobes in the Yellow River Delta Complex

Unraveling the spatial-temporal distribution of delta superlobes precisely is particularly important and challenging in the Yellow River Delta Complex (YRDC). Previous research on this issue can be dated back to the early 1990s, when the paleoenvironmental reconstructions mainly hinged on the historical documents and geomorphologic analysis of well-developed cheniers from the west coast of Bohai Bay, and only few drilled cores with scant radiocarbon dates were available. To build a refined model of the Holocene stratigraphic architecture and reconstruct the sedimentary evolution of the YRDC, detailed sedimentological investigation of 12 boreholes, including 59 new AMS 14C radiocarbon dates, was combined with other published 25 cores and 174 AMS 14C dates therein. Age-depth curves were grouped by stratigraphic positions and facies characteristics of these 233 radiocarbon dated samples across different sectors of the YRDC, exhibiting consistent patterns that can be used to identify distinct phases of delta superlobe activities. Eight new superlobes with relatively accurate dating spans were, therefore, proposed in the YRDC. Our results demonstrate that very high aggradation rates almost invariably correlate with the onset of deltaic deposition. Age-depth patterns, thus, may be very useful to elucidate the complex sedimentary evolution of delta systems with multiple lobes.

Anatomy and evolution of a dynamic arroyo system, Kanab Creek, southern Utah, USA

Abstract Many alluvial valleys in the American Southwest are entrenched within continuous arroyos, and stratigraphic evidence indicates that these fluvial systems experienced repeated periods of entrenchment and aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally quasi-synchronous, implying that they were driven by allogenic forcing due to hydroclimatic fluctuations. However, several of these interpretations rely on records with limited age control and include distal correlations across the American Southwest. While hydroclimatic variability must exert some role, autogenic mechanisms related to catchment-specific geomorphic thresholds are hypothesized to partially control the timing of arroyo dynamics. If driven by autogenic processes, episodes of arroyo cutting and filling may not be regionally contemporaneous. Recent improvements in dating methods permit more detailed reconstructions of the timing and evolution of arroyo dynamics, allowing for a more nuanced assessment of these competing hypotheses. Here we present a uniquely large and focused chronostratigraphic data set from two alluvial reaches of Kanab Creek, located in the Grand Staircase region of southern Utah. Episodes of prehistoric arroyo cutting and filling are reconstructed from 27 sites through recognition of soils and buttressed unconformities in the arroyo-wall stratigraphy, and age control derived from 54 optically stimulated luminescence (OSL) ages and 50 radiocarbon ages. Our chronostratigraphic data set indicates five periods of channel aggradation occurred since ca. 6.0 ka, with each interrupted by an episode of arroyo entrenchment. Repeated aggradation to a similar channel elevation suggests attainment of a threshold profile, and comparison of the pre-entrenchment longitudinal profile with the modern arroyo channel demonstrates that changes between end-member entrenched and aggraded states are expressed in channel concavity and slope. We propose that arroyo dynamics are partially driven by sediment supply and the rate of channel aggradation, and that these systems must approach complete re-filling before they become sensitive to incision. Entrenchment itself appears to be associated with rapid transitions from pronounced decadal-scale aridity to pluvial (wetter) periods. Not all such hydroclimatic fluctuations are associated with arroyo entrenchment, which highlights the importance of threshold controls on the behavior of these systems. The collective period of “dynamic instability” characterized by epicycles of arroyo entrenchment and aggradation did not initiate until the mid-Holocene, when a climatic shift toward warmer and drier conditions likely increased fine-grained sediment supply to the fluvial system.

Cold-water coral mounds in the southern Alboran Sea (western Mediterranean Sea): Internal waves as an important driver for mound formation since the last deglaciation

Cold-water corals (CWCs) are widely distributed in the entire Alboran Sea (western Mediterranean Sea), but only along the Moroccan margin they have formed numerous coral mounds, which are constrained to the West and the East Melilla CWC mound provinces (WMCP and EMCP). While information already exists about the most recent development of the coral mounds in the EMCP, the temporal evolution of the mounds in the WMCP was unknown up to the present. In this study, we present for the first time CWC ages obtained from four sediment cores collected from different mounds of the WMCP, which allowed to decipher their development since the last deglaciation. Our results revealed two pronounced periods of coral mound formation. The average mound aggradation rates were of 75–176 cm kyr−1 during the Bølling-Allerød interstadial and the Early Holocene, only temporarily interrupted during the Younger Dryas, when aggradation rates decreased to <45 cm kyr−1. Since the Mid Holocene, mound formation significantly slowed-down and finally stagnated until today. No living CWCs thrive at present on the mounds and some mounds became even buried. The observed temporal pattern in mound formation coincides with distinct palaeoceanographic changes that significantly influenced the local environment. Within the Alboran Sea, enhanced surface ocean productivity and seabed hydrodynamics prevailed during the Bølling-Allerød and the Early Holocene. Only with the onset of the Mid Holocene, the area turned into an oligotrophic setting. The strong hydrodynamics during the mound formation periods are most likely caused by internal waves that developed along the water mass interface between the Modified Atlantic Water and the Levantine Intermediate Water. In analogue to observations from modern CWC settings, we assume that internal waves created turbulent hydrodynamic conditions that increased the lateral delivery of particulate material, promoting the availability of food for the sessile CWCs. Overall, our data point to the dominant role of the water column structure in controlling the proliferation of CWCs and hence the development of coral mounds in the southern Alboran Sea.

Decadal-scale morphological adjustment of a lowland tropical river

Compared with temperate regions, much less is known about the dynamics of tropical river systems. Tropical rivers are typically characterised by pronounced seasonal changes in precipitation, large sediment loads and high rates of lateral channel migration across often very low-gradient and densely populated floodplains. Understanding the controls on channel migration or change is integral to our ability to fully predict and build resilience against flood risk and wider river-related hazards. Here, we analyse channel and confluence migration over the last ~40years along a ~85km reach of the Cagayan River and one of its tributaries, the Pinacanauan de Ilagan (Luzon, Philippines) using optical satellite imagery captured during this period. Combining this with spatial variations in channel pattern, valley width and new bed material grain size data, we demonstrate that sediment transport and deposition are key drivers of the observed tropical channel morphodynamics in this region. The high sediment supply generated in the catchment headwaters (by mass-wasting of hillslopes triggered especially in typhoons) results in high aggradation rates and channel widening on the lower gradient alluvial plain. We suggest that this aggradation enhances local confluence and lateral channel migration rates, which can reach >300m per decade, and that lateral migration rates of tropical rivers are typically greater than those of temperate rivers. Channel morphodynamics have implications for how to best manage these types of tropical river systems, where hard bank protection structures may result in a complex geomorphic response and flood risk mapping may need to include assessment of sensitivity to varying channel position and topography.

Differentiating the effects of logging, river engineering, and hydropower dams on flooding in the Skokomish River, Washington, USA

A decades-long, progressive loss of channel capacity in the Skokomish River, a 622 km2 basin draining the southeast Olympic Mountains of Washington State, has caused increasing flooding with severe consequences to endangered salmon runs, infrastructure, and private property. To differentiate among multiple potential drivers of the capacity loss, we analyze the geomorphic evidence for the potential effects of: flow regulation by two dams constituting the Cushman Hydroelectric Project, which began regulating flow in the river's North Fork in 1925 and diverting water out of the basin in 1930; sediment production from mid-twentieth century logging in the river's South Fork basin; and twentieth century river engineering in the mainstem. Bankfull channel capacity in the mainstem has steadily declined since about 1940 from 370 m3 s−1 to <100 m3 s−1 due partly to the narrowing of the Skokomish River, which in 2015 was only 45% as wide as it was in 1938. The capacity loss is also due to sediment filling the channel, with nearly 2 m of aggradation measured at a stream gauge since 1965. Comparison of channel cross sections surveyed in 1994, 2007, and 2016 show that about 20,000 m3 yr−1 (34,000 Mg yr−1) of sediment is accumulating in the Skokomish River. The nature, timing, and spatial pattern of this channel narrowing and shallowing are consistent with the response expected from the Cushman Project, which exports water out of basin and thus substantially reduces downstream flows, but, because the dams were built below a natural lake, does not reduce the sediment supply. While sediment yield from the South Fork is high, accounting for about three-fourths of the total sediment supplied to the Skokomish River, it is dominated by the progressive widening of the channel and recruitment by lateral fluvial erosion of glacial sediments in alluvial terraces; landslides associated with logging in the South Fork basin produced a small amount of sediment relative both to the sediment produced by channel widening in the upper South Fork and to the rate of aggradation in the mainstem Skokomish River. The naturally-high sediment load from the South Fork and the flow reduction in the North Fork result in the unusual effect of flooding having increased downstream of the dams despite substantial reductions to peak flows. This case study illustrates how a watershed-scale analysis of multiple land uses and flow management and their interaction with the basin's geology and geomorphology can make use of geomorphic evidence to differentiate among the possible drivers of channel change and associated flooding.

Analysis of multi-decadal wetland changes, and cumulative impact of multiple storms\xa01984 to 2017

Land-cover classification analysis using Landsat satellite imagery acquired between 1984 and 2017 quantified short- (post-Hurricane Sandy) and long-term wetland-change trends along the Maryland and Virginia coasts between Metompkin Bay, VA and Ocean City, MD. Although there are limited options for upland migration of wetlands in the study area, regression analysis showed that wetland area increased slightly between 1984 and 2011, indicating that marsh aggradation rates were sufficient to maintain wetland elevation relative to mean sea level. Following Hurricane Irene (August 2011), the Halloween Nor’Easter (October 2011), and Hurricane Sandy (October 2012), wetland area decreased by more than 7 km2 compared with average pre-storm extents. We assume that Hurricane Sandy had the greatest impact due to the size and intensity of the storm. However, the cumulative effects of multiple storms within a short time period likely contributed to the greater observed losses in coastal wetlands relative to earlier periods. Five years after Hurricane Sandy, wetland area had not significantly recovered, but more time may be necessary to assess if the observed wetland losses will persist or if new growth within flooded marsh areas will be sufficient for the wetlands to recover to pre-storm extents. Comparisons of long-term and storm-driven wetland changes can lead to improved accuracy of habitat vulnerability models and greater understanding of potential impacts of future storms and SLR to coastal wetlands.

Impacts of natural and human drivers on the multi-decadal morphological evolution of tidally-influenced deltas.

The world's deltas are at risk of being drowned due to rising relative sea levels as a result of climate change, decreasing supplies of fluvial sediment, and human responses to these changes. This paper analyses how delta morphology evolves over multi-decadal timescales under environmental change using a process-based model. Model simulations over 102 years are used to explore the influence of three key classes of environmental change, both individually and in combination: (i) varying combinations of fluvial water and sediment discharges; (ii) varying rates of relative sea-level rise; and (iii) selected human interventions within the delta, comprising polder-dykes and cross-dams. The results indicate that tidal asymmetry and rate of sediment supply together affect residual flows and delta morphodynamics (indicated by sub-aerial delta area, rates of progradation and aggradation). When individual drivers of change act in combination, delta building processes such as the distribution of sediment flux, aggradation, and progradation are disrupted by the presence of isolated polder-dykes or cross-dams. This suggests that such interventions, unless undertaken at a very large scale, can lead to unsustainable delta building processes. Our findings can inform management choices in real-world tidally-influenced deltas, while the methodology can provide insights into other dynamic morphological systems.

Late-Quaternary dynamics and palaeoclimatic implications of an alluvial fan-lake system on the southern Alxa Plateau, NW China

Alluvial fan-lake complexes are common in the piedmont zone of mountains on the southern Alxa Plateau, NW China. Interconnected by the erosional and depositional processes, they can be treated as a typical source-to-sink system. Yet the dynamics of these coupled sedimentary systems during the Late Quaternary is poorly understood. For example, radiocarbon dating of alluvial fans and elevated shoreline features of megalakes in this area usually yields numerical ages in radical contradiction to those obtained using the optically stimulated luminescence (OSL) method. Resolving this controversy of timing of the alluvial fan-lake systems is important for understanding the driving mechanisms of climatic and environmental changes in this ecological vulnerable area. The Yabulai alluvial fan-lake system is an exceptional example for tackling this issue. Here we provide numerical ages from three sites to better understand the dynamics of this sedimentary system during the last interglacial-glacial cycle. Our results, when compared with palaeoclimate proxy records, suggest that the fan-lake system was highly regulated by the East Asian summer monsoon and it has experienced four stages of remarkable changes. Debris flow processes were active and lake level was ~30 m higher than that of today during Marine Isotope Stage (MIS) 5. A gradual decrease in the aggradation rate of the alluvial fan along with lake-level fall occurred during MIS 4–3, which was followed by a period of intensive wind erosion corresponding to the Last Glacial Maximum. Postglacial growth of the alluvial fan slowed down substantially and small-scale lake-level fluctuations occurred in the foreland basin. The desiccation of megalakes during recent times suggests that the East Asian summer monsoon may have retreated from this area as semi-arid climate emerged.

Anthropogenic impacts to the sediment budget of São Francisco River navigation channel using SWAT

The Sao Francisco River Basin, located in eastern Brazil, has undergone a significant amount of anthropogenic changes in the last several decades, such as agricultural expansion, irrigation activities, mining, and the construction of large dams. Together, these changes have altered the historic sediment budget and have led to an aggradation of sediments in the navigation channel, impacting the ability to efficiently ship agricultural commodities to regional ports. In an effort to aid decision makers in future waterway navigation planning, an international partnership between the Brazilian government agency CODEVASF and the US Army Corps of Engineers (USACE) was created. Through this partnership a SWAT model of the 630 000 km2 Sao Francisco River basin was developed to better understand both the historic and current sediment budget within the navigation channel. The SWAT model of the Sao Francisco River Basin was calibrated for hydrology and sediment loads. Monthly discharges were calibrated at 17 Agencia Nacional de Aguas (ANA) gages, with Nash-Sutcliffe efficiency (NSE) values ranging from 0.42 to 0.75 for an eleven year simulation. Sediment loads were calibrated to an ANA sediment gage located in the Middle Sao Francisco River Navigation Channel, with a PBIAS (Percent Bias) of 11.6. Based on model results, the aggradation rate of sediment in the Sao Francisco River and major tributaries has increased by approximately 20 Mt since Pre-European settlement of the basin (from approximately 7 Mt/a to 27 Mt/a). This increase has contributed to an impaired navigation channel due to shoaling of sandy sediments in the navigation channel. Keywords: sediment budget, aggradation rate, Sao Francisco River, anthropogenic impact, SWAT DOI: 10.3965/j.ijabe.20150803.1372 Online first on [2015-04-03] Citation: Creech C T, Siqueira R B, Selegean J P, Miller C. Anthropogenic impacts to the sediment budget of Sao Francisco River navigation channel using SWAT. Int J Agric & Biol Eng, 2015; 8(3): 140－157.

Sediment Storage Partitioning in Alluvial Stratigraphy: The Influence of Discharge Variability

Abstract Numerical models of formation of alluvial stratigraphy often specify, either explicitly or implicitly, the proportion of channel and overbank sediments that are deposited during a given interval of time. However, little is known about the factors that affect the partitioning of sediment between channels and the overbank environment over long time intervals, and the fidelity with which that partition is preserved in the stratigraphic record. Here we use physical experiments to investigate the role that discharge variability plays in this partitioning in fluvial stratigraphy. We find that channels formed under constant flow conditions have low lateral mobility and act mostly as conduits for sediments to reach the shoreline. The bulk of the aggradation in this case is derived from sediment-laden flow that escapes the main channels. By contrast, including floods increases channel lateral mobility, and this change is recorded in stratigraphy as an increased proportion of channel deposits relative to overbank deposits. When variable flow is included as an input condition a large volume of in-channel deposition occurs, rendering the channels substantial contributors to stratigraphic volume on their own. The increase in channel deposit volume is driven mainly by a threefold increase in the average time that a location is subject to in-channel aggradation. Other factors include a slight increase of in-channel aggradation rates, and an increase in erosion of the overbank environment that results from energetic overbank flows. Our study shows that the character of a river's hydrograph exerts a significant influence on the proportion of channel to overbank sediment bodies in alluvial successions, which is an unexamined source of uncertainty in common stratigraphic models.

Electrical resistivity imaging of anastomosing river subsurface stratigraphy and possible controls of fluvial style change in a graben-like basin, Czech Republic

Subject to frequent channel avulsions due to the increased frequency of floods, rise of base level, and/or sediment overloading, anastomosing rivers are sensitive to external (climatic or tectonic) forcing in sedimentary basins. The Morava River, Czech Republic, shows a well-developed Holocene anastomosis, confined to the pull-apart type Upper Morava Basin (UMB). We studied the stratigraphy and aggradation rates of the UMB using electrical resistivity tomography (ERT), dipole electromagnetic profiling (DEMP), core data, and radiocarbon ages, with the aim of identifying factors triggering the anastomosis at the Pleistocene/Holocene transition. The ERT and DEMP proved to be excellent tools to visualize the shape and thickness of floodplain depositional units (abandoned meanders, scroll bars, etc.). The major geophysical surfaces identified are the base of the late Weichselian braided-stream deposits (OSL age = 34.53 ± 3.42 ka), interpreted as a palaeo-stream gradient of the Morava River, and their transition to Holocene floodplain deposits. In the late Weichselian, the river aggraded at an average rate of 0.2 mm/yr. The Holocene (oldest 14C age = 7.066 ± 0.072 ka) short-term aggradation rates reached up to 13.7 mm/yr, but the average long-term rates (~0.07 to ~0.4 mm/yr) are comparable to the late Weichselian. The stratigraphy and sediment ages indicate that the onset of anastomosis was not likely associated with accelerated basin subsidence. Possible controls of the anastomosis include upstream control by numerous tributaries, large wood debris in the floodplain forests, and changes in land use. The tectonic and geomorphic context of the UMB pull-apart basin probably favoured the development of the anastomosis.

The impact of geological uncertainty on primary production from a fluvial reservoir

Deposition of fluvial sandbodies is controlled mainly by characteristics of the system, such as the rate of avulsion and aggradation of the fluvial channels and their geometry. The impact and the interaction of these parameters have not received adequate attention. In this paper, the impact of geological uncertainty resulting from the interpretation of the fluvial geometry, maximum depth of channels, and their avulsion rates on primary production is studied for fluvial reservoirs. Several meandering reservoirs were generated using a process-mimicking package by varying several controlling factors. Simulation results indicate that geometrical parameters of the fluvial channels impact cumulative production during primary production more significantly than their avulsion rate. The most significant factor appears to be the maximum depth of fluvial channels. The overall net-to-gross ratio is closely correlated with the cumulative oil production of the field, but cumulative production values for individual wells do not appear to be correlated with the local net-to-gross ratio calculated in the vicinity of each well. Connectedness of the sandbodies to each well, defined based on the minimum time-of-flight from each block to the well, appears to be a more reliable indicator of well-scale production.

The giant Mauritanian cold-water coral mound province: Oxygen control on coral mound formation

The largest coherent cold-water coral (CWC) mound province in the Atlantic Ocean exists along the Mauritanian margin, where up to 100 m high mounds extend over a distance of ∼400 km, arranged in two slope-parallel chains in 400–550 m water depth. Additionally, CWCs are present in the numerous submarine canyons with isolated coral mounds being developed on some canyon flanks. Seventy-seven Uranium-series coral ages were assessed to elucidate the timing of CWC colonisation and coral mound development along the Mauritanian margin for the last ∼120,000 years. Our results show that CWCs were present on the mounds during the Last Interglacial, though in low numbers corresponding to coral mound aggradation rates of 16 cm kyr−1. Most prolific periods for CWC growth are identified for the last glacial and deglaciation, resulting in enhanced mound aggradation (>1000 cm kyr−1), before mound formation stagnated along the entire margin with the onset of the Holocene. Until today, the Mauritanian mounds are in a dormant state with only scarce CWC growth. In the canyons, live CWCs are abundant since the Late Holocene at least. Thus, the canyons may serve as a refuge to CWCs potentially enabling the observed modest re-colonisation pulse on the mounds along the open slope. The timing and rate of the pre-Holocene coral mound aggradation, and the cessation of mound formation varied between the individual mounds, which was likely the consequence of vertical/lateral changes in water mass structure that placed the mounds near or out of oxygen-depleted waters, respectively.

Sedimentary processes and architecture of Upper Cretaceous deep-sea channel deposits: a case from the Skole Nappe, Polish Outer Carpathians

AbstractDeep-sea channels are one of the architectonic elements, forming the main conduits for sand and gravel material in the turbidite depositional systems. Deep-sea channel facies are mostly represented by stacking of thick-bedded massive sandstones with abundant coarse-grained material, ripped-up clasts, amalgamation and large scale erosional structures. The Manasterz Quarry of the Ropianka Formation (Upper Cretaceous, Skole Nappe, Carpathians) contains a succession of at least 31 m of thick-bedded high-density turbidites alternated with clast-rich sandy debrites, which are interpreted as axial deposits of a deep-sea channel. The section studied includes 5 or 6 storeys with debrite basal lag deposits covered by amalgamated turbidite fills. The thickness of particular storeys varies from 2.5 to 13 m. Vertical stacking of similar facies through the whole thickness of the section suggest a hierarchically higher channel-fill or a channel complex set, with an aggradation rate higher than its lateral migration. Such channel axis facies cannot aggrade without simultaneous aggradation of levee confinement, which was distinguished in an associated section located to the NW from the Manasterz Quarry. Lateral offset of channel axis facies into channel margin or channel levee facies is estimated at less than 800 m. The Manasterz Quarry section represents mostly the filling and amalgamation stage of channel formation. The described channel architectural elements of the Ropianka Formation are located within the so-called Łańcut Channel Zone, which was previously thought to be Oligocene but may have been present already in the Late Cretaceous.

Near-surface permafrost aggradation in Northern Hemisphere peatlands shows regional and global trends during the past 6000 years

The history of permafrost aggradation and thaw in northern peatlands can serve as an indicator of regional climatic history in regions where records are sparse. We infer regional trends in the timing of permafrost aggradation and thaw in North American and Eurasian peatland ecosystems based on plant macrofossils and peat properties using existing peat core records from more than 250 cores. Results indicate that permafrost was continuously present in peatlands during the last 6000 years in some present-day continuous permafrost zones and formed after 6000 BP in peatlands in the isolated to discontinuous permafrost regions. Rates of permafrost aggradation in peatlands generally increased after 3000 BP and were greatest between 750 and 0 BP, corresponding with neoglacial cooling and the Little Ice Age (LIA), respectively. Peak periods of permafrost thaw occurred after 250 BP, when permafrost aggradation in peatlands reached its maximum extent and as temperatures began warming after the LIA, suggesting that permafrost thaw is likely to continue in the future. The broader correlation of permafrost aggradation in peatlands with known climatic trends and other proxies such as pollen records suggests that this record can be a valuable addition to regional climate reconstructions.

Linking submarine channel–levee facies and architecture to flow structure of turbidity currents: insights from flume tank experiments

AbstractSubmarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.

3D architecture of cyclic-step and antidune deposits in glacigenic subaqueous fan and delta settings: Integrating outcrop and ground-penetrating radar data

Bedforms related to supercritical flows are increasingly recognised as important constituents of many depositional environments, but outcrop studies are commonly hampered by long bedform wavelengths and complex three-dimensional geometries. We combined outcrop-based facies analysis with ground-penetrating radar (GPR) surveys to analyse the 3D facies architecture of subaqueous ice-contact fan and glacifluvial delta deposits. The studied sedimentary systems were deposited at the margins of the Middle Pleistocene Scandinavian ice sheets in Northern Germany.Glacifluvial Gilbert-type deltas are characterised by steeply dipping foreset beds, comprising cyclic-step deposits, which alternate with antidune deposits. Deposits of cyclic steps consist of lenticular scours infilled by backset cross-stratified pebbly sand and gravel. The GPR sections show that the scour fills form trains along the delta foresets, which can locally be traced for up to 15m. Perpendicular and oblique to palaeoflow direction, these deposits appear as troughs with concentric or low-angle cross-stratified infills. Downflow transitions from scour fills into sheet-like low-angle cross-stratified or sinusoidally stratified pebbly sand, deposited by antidunes, are common. Cyclic steps and antidunes were deposited by sustained and surge-type supercritical density flows, which were related to hyperpycnal flows, triggered by major meltwater discharge or slope-failure events.Subaqueous ice-contact fan deposits include deposits of progradational scour fills, isolated hydraulic jumps, antidunes and (humpback) dunes. The gravel-rich fan succession consists of vertical stacks of laterally amalgamated pseudo-sheets, indicating deposition by pulses of waning supercritical flows under high aggradation rates. The GPR sections reveal the large-scale architecture of the sand-rich fan succession, which is characterised by lobe elements with basal erosional surfaces associated with scours filled with backsets related to hydraulic jumps, passing upwards and downflow into deposits of antidunes and (humpback) dunes. The recurrent facies architecture of the lobe elements and their prograding and retrograding stacking pattern are interpreted as related to autogenic flow morphodynamics.

Paleocene Wilcox cross-shelf channel-belt history and shelf-margin growth: Key to Gulf of Mexico sediment delivery

Shelf margins prograde and aggrade by the incremental addition of deltaic sediments supplied from river channel belts and by stored shoreline sediment. This paper documents the shelf-edge trajectory and coeval channel belts for a segment of Paleocene Lower Wilcox Group in the northern Gulf of Mexico based on ~400 wireline logs and ~300m of whole cores. By quantitatively analyzing these data and comparing them with global databases, we demonstrate how varying sediment supply impacted the Wilcox shelf-margin growth and deep-water sediment dispersal under greenhouse eustatic conditions.The coastal plain to marine topset and uppermost continental slope succession of the Lower Wilcox shelf-margin sediment prism is divided into eighteen high-frequency (~300ky duration) stratigraphic sequences, and further grouped into 5 sequence sets (labeled as A-E from bottom to top). Sequence Set A is dominantly muddy slope deposits. The shelf edge of Sequence Sets B and C prograded rapidly (>10km/Ma) and aggraded modestly (<80m/Ma). The coeval channel belts are relatively large (individually averaging 11–13m thick) and amalgamated. The water discharge of Sequence Sets B and C rivers, estimated by channel-belt thickness, bedform type, and grain size, is 7000–29,000m3/s, considered as large rivers when compared with modern river databases. In contrast, slow progradation (<10km/Ma) and rapid aggradation (>80m/Ma) characterizes Sequence Sets D and E, which is associated with smaller (9–10m thick on average) and isolated channel belts. This stratigraphic trend is likely due to an upward decreasing sediment supply indicated by the shelf-edge progradation rate and channel size, as well as an upward increasing shelf accommodation indicated by the shelf-edge aggradation rate.The rapid shelf-edge progradation and large rivers in Sequence Sets B and C confirm earlier suggestions that it was the early phase of Lower Wilcox dispersal that brought the largest deep-water sediment volumes into the Gulf of Mexico. Key factors in this Lower Wilcox stratigraphic trend are likely to have been a very high initial sediment flux to the Gulf because of the high initial release of sediment from Laramide catchments to the north and northwest, possibly aided by modest eustatic sea-level fall on the Texas shelf, which is suggested by the early, flat shelf-edge trajectory, high amalgamation of channel belts, and the low overall aggradation rate of the Sequence Sets B and C.

Reflection of climatic changes during interpleniglacial in the geoecosystems of South-Eastern Poland

Abstract About 70% of the last cold stage was taken by transitional phases of continuous fluctuations of climate and permafrost extension. All this is reflected especially well in slope types and the thickness of slope, fluvial and aeolian deposits. Very distinct and relatively fast changes took place around 33–30 ka BP: the aridification of climate with an ice sheet transgression, expansion of continuous permafrost and the onset of loess deposition. The greatest transformations of slopes and river valleys in the Carpathians and Subcarpathian Basins took place not in the coolest phases but during the Interpleniglacial. The authors exemplified a number of sites in Southern Poland documenting frequent fluctuations of climate and permafrost from that 25–30 ka long period. Calibrated dates of dated episodes correlate well with the δ18O curve from Greenland with frequent warmings. These rapid warmings are reflected in the higher rate of aggradation in valley floors and thick colluvial material deposited over slopes, both combined with a retreat of permafrost. The comparison with the Greenland 18O curve is valuable to establish a comparable chronology of events, even for such a distant area like Central-Eastern Europe.

Balancing Aggradation and Progradation on a Vegetated Delta: The Importance of Fluctuating Discharge in Depositional Systems

AbstractVegetation is an important component of constructional landscapes, as plants enhance deposition and provide organic sediment that can increase aggradation rates to combat land loss. We conducted two sets of laboratory experiments using alfalfa (Medicago sativa) to determine the effects of plants on channel organization and large‐scale delta dynamics. In the first set, we found that rapid vegetation colonization enhanced deposition but inhibited channelization via increased form drag that reduced the shear stress available for sediment entrainment and transport. A second set of experiments used discharge fluctuations between flood and base flow (or interflood). Interfloods were critical for reworking the topset via channel incision and lateral migration to create channel relief and prevent rapid plant colonization. These low‐flow periods also greatly reduced the topset slope in the absence of vegetation by removing topset sediment and delivering it to the shoreline. Floods decreased relief by filling channels with sediment, resulting in periods of rapid progradation and enhanced aggradation over the topset surface, which was amplified by vegetation. The combination of discharge fluctuations and vegetation thus provided a balance of vertical aggradation and lateral progradation. We conclude that plants can inhibit channelization in depositional systems and that discharge fluctuations encourage channel network organization to naturally balance against aggradation. Thus, variations in discharge are an important aspect of understanding the ecomorphodynamics of aggrading surfaces and modeling vegetated deltaic systems, and the combined influences of plants and discharge variations can act to balance vertical and lateral delta growth.