Cohesive Mud Research Articles

The role of suspended extracellular polymeric substance (EPS) on equilibrium flocculation of clay minerals in high salinity water

Biophysical cohesive mud, consisting of clay minerals and extracellular polymeric substance (EPS), plays significant role in determining sediments, nutrients and pollutants transport in estuarine and coastal systems. Series of laboratory jar experiments have been conducted aiming at filling the gap of knowledge regarding how biological cohesive EPS affects equilibrium flocculation of EPS-mineral mixtures. Four types of common clay (chlorite, kaolinite, illite and montmorillonite) were chosen due to their abundance in estuarine mud and distinct crystal chemistry and structures. Turbulent shear throughout all the experimental runs were constantly provided at a mean shear parameter of G ≈ 15 s−1 being equivalent to high tidal influenced estuarine turbulent environment. The results reveal that adding EPS increases the equilibrium floc size evidently. The pure mineral flocs show unimodal equilibrium floc size distribution (eFSD) with single peak located at microfloc range (<200 μm) while the EPS-mineral flocs show bimodal eFSD with a secondary peak located in macroflocs range (>200 μm) mostly. Moreover, EPS largely reduces the effective density in EPS-mineral flocs by 1∼2 magnitude. Most importantly, the terminal settling velocity of flocs shows size-dominated in uniform mineral floc cases but density-dominated in EPS-mineral mixture floc cases especially in macroflocs. To model a full floc size or settling velocity distributions in natural environments, furtherly quantification of EPS functions within the large-sized non-fractal mixture floc individually becomes a necessity.

The Influence of Climate Changes on Mudflow Formation in Permafrost Areas: a Case Study for Chukotka Peninsula

The factors influencing the formation of mudflows in areas of the permafrost are considered. The data of studies performed in two regions of Chukotka – “Continental” (the Anyuysky ridge) and “Coastal” (the Iskaten ridge) were used for this research. The air temperature data series obtained in Chukotka in 2000–2020 demonstrate a steady growth of the average annual values. The air temperature rise estimated for the mudflow–dangerous period (June–August) amounted 1.4°C for the “Continental” area, and 1.0°C for the “Coastal”. This warming affects thickness of the seasonal melt layer of permafrost, mainly in the bottoms of valleys and on the slopes of mountains. This factor promotes the involvement of certain volumes of ground into mudflows. As a result of the research, it was found that the dynamics of the change in the thickness of the seasonal melt layer within the studied areas is positive, which is a consequence of warming and leads to additional moistening of grounds. The two periods of the mudflow formation were identified. During the first one (May–June), the mudflow formation is connected with the onset of intensive snowmelt that is favorable mainly for snow-water streams and loose mudflows. In the second period (July–August), mudflows are mostly caused by liquid precipitations, when the maximum thickness of seasonal melt layer is reached. It is the second period when a release of a large cohesive mud stream is the most probable. On the whole, the results obtained allow making a conclusion that in the near future the predominant type of mudflows in Chukotka will remain those of snow genesis. But, at the same time, under conditions of the climate change, occurrence of the snow-water mud streams will increase, especially in the “Coastal” area.

Analytical Solution for Wave Scattering by a Surface Obstacle above a Muddy Seabed

We present an analytical solution for the scattering of linear progressive waves by a surface rectangular obstacle above a muddy seabed. The bottom cohesive mud is assumed to act as a Newtonian fluid, and the thickness of the mud layer is considered to be comparable to the Stokes boundary layer thickness. Our analytical results based on the matched eigenfunction expansions incorporate the combined effects of obstacles and a fluid mud bottom. By reducing the mud layer thickness or the dimensions of the obstacle to zero, the present study recovers the classical solution for wave scattering by a surface obstacle above a solid bed or wave propagation over a layer of fluid mud. Our analytical predictions of wave amplitudes and wave forces acting on the bottom of the obstacle agree satisfactorily with the available numerical results. The most prominent effect of a muddy seabed is a strong damping of wave amplitude. Parameter study reveals that the obstacle submerged depth, mud layer thickness, and wave frequency can have significant impacts on the attenuation of wave amplitude due to the presence of a muddy seabed.

Vertebrate ichnofauna from Sierra de las Cabras tracksite (Late Miocene, Jumilla, SE Spain). Mammalian ichnofauna

Upper Miocene sedimentary deposits of Sierra de las Cabras in the Prebetic Ranges (Jumilla, Región de Murcia, Spain) yield a exceptional fossil site of diverse vertebrate ichnofauna, including mammals and birds. This paper presents a detailed study of those ichnites of that site which correspond to mammals. The footprints are located in stratigraphic surfaces found in a thin marly limestone succession that can be assigned to the latest Tortonian to earliest Messinian. The mammalian record consists of 324 footprints (included in trackways, pairs, groups and isolated ichnites) of diverse vertebrates, including: Hippipeda, Rhynoceripeda, Canipeda, Felipeda, Ursipeda, Rodentipeda, as well possible Suipeda. The ichnites were imprinted on wet and cohesive marly carbonate mud, deposited in a semi-arid wetland with relevant freshwater influx. The fossil site, cleaned and protected some years ago but herein studied by the first time, add together the nearby Hoya de la Sima site (Hippipeda, Pecoripeda, Bestiopeda and Paracamelichnium, and probably proboscidean ichnites) to conform an exceptional record of Late Miocene ichnofauna in southern Iberia.

The Segmented Zambezi Sedimentary System from Source to Sink: 2. Geochemistry, Clay Minerals, and Detrital Geochronology

Elemental geochemistry, Nd isotopes, clay minerals, and U-Pb zircon ages integrated by petrographic and heavy-mineral data offer a multiproxy panorama of mud and sand composition across the Zambezi sediment-routing system. Detrital zircon geochronology highlights the four major episodes of crustal growth in southern Africa: Irumide ages predominate over Pan-African, Eburnean, and Neoarchean ages. Smectite, dominant in mud generated from Karoo basalts or in the equatorial/winter-dry climate of the Mozambican lowlands, prevails over illite and kaolinite. Elemental geochemistry reflects quartz addition by recycling (Uppermost Zambezi), supply from Karoo basalts (Upper Zambezi), and first-cycle provenance from Precambrian basements (Lower Zambezi). Mildly negative for sediments derived from mafic granulites, gabbros, and basalts, εNd values are most negative for sand derived from cratonic gneisses. Intrasample variability among cohesive mud, very coarse silt, and sand is principally caused by the concentration of Nd-rich monazite in the fine tail of the size distribution. The settling-equivalence effect also explains deviations from the theoretical relationship between εNd and TNd,DM model ages, suggesting that monazite carries a more negative εNd signal than less dense and less durable heavy minerals. Elemental geochemistry and Nd isotopes reveal that the Mazowe-Luenha river system contributes most of the sediment reaching the Zambezi delta today, with minor supply from the Shire River. Sediment yields and erosion rates are much lower on the low-relief Kalahari Plateau than in rugged Precambrian terranes. On the plateau, mineralogical and geochemical indices testify to extensive breakdown of feldspars and garnet unjustified by the present dry climate. Detrital kaolinite is recycled by incision of Cretaceous–Cenozoic paleosols even in the wetter lower catchment, where inefficient hydrolysis is testified to by abundant fresh feldspars and undepleted Ca and Na. Mud geochemistry and surficial corrosion of ferromagnesian minerals indicate that, at present, weathering increases only slightly downstream the Zambezi River.

Influence of Anaerobic Degradation of Organic Matter on the Rheological Properties of Cohesive Mud from Different European Ports

The presence of clay-organic flocs in cohesive mud results in a complex rheological behavior of mud, including viscoelasticity, shear-thinning, thixotropy and two-step yielding. In this study, the effect of microbial degradation of organic matter on the rheological properties of mud samples, collected from different ports, was examined. The mud samples were collected from five different European ports (Port of Antwerp (PoA), Port of Bremerhaven (PoB), Port of Emden (PoE), Port of Hamburg (PoH) and Port of Rotterdam (PoR)), displaying varying sediment properties. The rheological analysis of fresh and degraded mud samples was performed with the help of several tests, including stress ramp-up tests, amplitude sweep tests, frequency sweep tests, time-dependent tests and structural recovery tests. The results showed: (i) a significant decrease in yield stresses and complex modulus after organic matter degradation for mud samples from PoA, PoH and PoR, (ii) a negligible change in rheological properties (yield stresses, crossover amplitude and complex modulus) for mud samples from PoB, and (iii) a significant increase in rheological properties for mud samples from PoE. For time-dependent tests, mud samples from PoB showed a substantial increase in hysteresis (~50% mean value) as compared to the changes in yield stresses and crossover amplitude. The analysis of gas production during degradation of organic matter showed a (i) significant release of carbon per g dry matter for mud samples from PoA, PoH and PoR, (ii) lower carbon release per g dry matter for mud samples from PoB, and (iii) a negligible carbon release per g dry matter for mud samples from PoE, which corresponded well with the change in rheological properties.

Hydrodynamic‐Driven Changes in the Source and Composition of Sedimentary Organic Matter via Grain Size Distribution in Shallow Lakes

AbstractSediment organic matter (SOM) differs in composition and function in different size fractions in relation to material and energy flows. The hydrodynamic effects on a lake's autochthonous (terrestrial plants) and allochthonous (microalgae, aquatic plants, and bacteria) inputs and transformation, and the components of SOM are still far from clear. To elucidate the SOM composition as driven by the hydrodynamic conditions associated with sedimentary grain size, we performed an ecosystem‐level investigation in the shallow lake Izunuma, Japan, measuring source‐specific fatty acids and the current velocity. High organic matter concentrations occurred in the finer fractions. Bacteria were dominant in the &lt;32 μm fraction, microalgae were mainly present in the &lt;32 and 63–125 μm fractions, aquatic plants appeared to be evenly distributed in all size fractions, and terrestrial plants mainly comprised the &gt;125 μm fraction. Although the linear correlation between current velocity and sedimentary total organic carbon (TOC) was not statistically significant, the relationship between current velocity frequency and sedimentary TOC was significant. The highest Pearson coefficient occurred at a current velocity of 6 cm/s in sand. The relationship between allochthonous inputs and current velocity frequency was similar to that of TOC. Autochthonous organics had the greatest coefficient with two peaks at 4.5 and 8 cm/s. The threshold for sand resuspension varied with organic sources, while this phenomenon was absent in cohesive mud. Hydrodynamic forces affected the grain size of sediments and drove the gradient distribution of the SOM sources, which should be considered when managing freshwater lakes in light of future climate change.

Geomorphologic change and patchy mud infilling in a sandy dredge pit in eastern Ship Shoal, Louisiana shelf, USA

To restore degraded barrier shorelines of the Mississippi River Deltaic Plain of Louisiana, sand is dredged from borrow areas in estuaries and continental shelves. The dredge pit in South Pelto Blocks 12 & 13 on eastern Ship Shoal used for Caminada-Moreau Headland restoration project is one of several sand dredge pits for barrier island restoration in Louisiana. Ship Shoal Sand Complex is in a sand-dominated, energetic, continental shelf environment. During energetic events, transient mud can blanket Ship Shoal, and then bypasses the shoal and deposits in deeper water. In this study, we collected and interpreted a suite of geophysical data from South Pelto Dredge Pit which is located in water depths of ∼9 m on eastern Ship Shoal. Repeat bathymetric survey data show that South Pelto Dredge Pit is presently infilling at an average rate of approximately 27,480 m3/year, which is slower than the predicted rate presented by an earlier numerical model. Side-scan mosaic maps indicate low-backscatter (muddy) sediments were transported into this pit and deposited in many patches within two years after dredging. The seabed inside this pit became smoother over time and the slope of the pit walls became gentler. Infilling of the South Pelto Dredge Pit seem to be controlled by sediment availability, which is volumetrically dominated by (a) far-field fluvial suspended sediment, (b) the adjacent seabed sediment resuspended by tides, waves, and currents, but presumably less impacted by (c) hurricanes or tropical storms during our study period of 2017–2018. The South Pelto Dredge Pit thus displays a unique mixing and redistribution of cohesive mud and non-cohesive sand. Initial spatial control on sediment infilling appears to be dictated by micro-bathymetry on the pit bottom created by the dredging activities. Bathymetric, side scan and grain-size data all indicate that mud was prone to deposit in the troughs.

Tidally-influenced deposition and microfacies sequences of fluid muds: Early Cretaceous McMurray Formation, Alberta, Canada

Fluid muds are common in estuarine environments, but their ancient examples have rarely been documented due to poor comprehension of their depositional processes and characteristics. Mudstone layers in the tidally-influenced channel sequences of the middle McMurray Formation are examined in detail through microscopic observations and interpreted on the basis of recent advances in the understanding of flow dynamics of high mud-concentration flows. The mudstone layers, < 1 to 25 mm thick, are classified into three microfacies. Structureless mudstone (Microfacies 1) consists mainly of clay particles with randomly dispersed coarse grains (coarse silt to fine sand). It represents cohesive mud flows with sufficient cohesive forces to support coarse grains (quasi-laminar plug flow). Silt-streaked mudstone (Microfacies 2) is similar to Microfacies 1 in texture, but contains discontinuous streaks of coarse-silt to very-fine-sand grains. It is interpreted as being also deposited by cohesive fluid muds. The silt streaks are, however, suggestive of the presence of weak turbulence under the cohesive plug (upper transitional plug flow). Heterolithic laminated mudstone (Microfacies 3) is characterized by alternations of very thin, silt and clay laminae, which are either parallel or low-angle cross-laminated. It is interpreted as the deposits of low-amplitude bed-waves formed in lower transitional plug flows. These microfacies reflect a range of flow phases of fluid muds, which changed as flow velocities and suspended sediment concentrations fluctuated with tidal cycles. Repeated vertical changes of microfacies are suggestive of an ideal sequence of fluid muds formed during tidal acceleration and deceleration. The sequence comprising microfacies 3, 2 and 1 in ascending order represents deposition from lower transitional plug flows through upper transitional plug flows to quasi-laminar plug flows as suspended sediment concentrations increase with flow deceleration. That of the reversed order of microfacies reflects the reversed change in flow types during acceleration. These results provide the basis for recognizing fluid-mud deposits and tidal signatures in ancient estuarine sequences.

Hybrid event beds generated by erosional bulking of modern hyperpycnal flows on the Choshui River delta front, Taiwan Strait

AbstractHyperpycnal flows are important agents for transporting detrital sediments from rivers to oceans. Previous studies often assumed that the deposits of flood‐controlled delta fronts would be dominated by graded sand with common hummocky cross‐stratification. This study documents, for the first time, hybrid event beds and plumite deposits inside the succession of a modern delta front. This delta in the Taiwan Strait is fed by the Choshui River, which is considered a highly efficient sediment transport system with individual floods with extremely high suspended sediment concentrations. The deltaic event beds recorded in the core were interpreted as hyperpycnal flow‐generated hybrid event beds and turbidites triggered by hypopycnal flows. Accelerator Mass Spectrometry 14C dating, grain‐size analysis and measurements of stable isotopic composition of organic matter were conducted to delineate the depositional process of the recognized beds. The internal subdivisions of the hybrid event beds were differentiated mainly based on sedimentary textures, including cohesive mud content, sand content and sorting. The disorganized portion (division H3) appears internally chaotic and contains large rafted substrate clasts but also displays an upward increase in dispersed cohesive mud from 35% (division H3a) to 50% (division H3b). In contrast, massive H1 divisions are characterized by much lower cohesive mud of ca 8%. The vertical arrangement between depositional facies allows the discrimination of three hybrid event bed types. The stable carbon isotopic composition of the organic matter reveals that the cohesive mud in each division of the hybrid event beds was sourced from marine substrate, rather than supplied by the original hyperpycnal flows. Therefore, the hybrid event beds are generated by energetic hyperpycnal flows, which can delaminate the muddy sea‐floor and incorporate large quantities of substrate fragments. The bulking of erosional hyperpycnal turbulent flow forces flow transformation and generates a more cohesive flow with a turbulent, dense head and trailing bipartite debris flow. The occurrence of hybrid event beds in modern shallow delta front settings is also an important novelty, implying a high density of hyperpycnal flows at the mouth of sediment‐laden rivers as well as the crucial impact of substrate entrainment on the development of hybrid event beds.

Sediment Transport near Ship Shoal for Coastal Restoration in the Louisiana Shelf: A Model Estimate of the Year 2017–2018

Ship Shoal has been a high-priority target sand resource for dredging activities to restore the eroding barrier islands in LA, USA. The Caminada and Raccoon Island pits were dredged on and near Ship Shoal, which resulted in a mixed texture environment with the redistribution of cohesive mud and noncohesive sand. However, there is very limited knowledge about the source and transport process of suspended muddy sediments near Ship Shoal. The objective of this study is to apply the Regional Ocean Modeling System (ROMS) model to quantify the sediment sources and relative contribution of fluvial sediments with the estuary and shelf sediments delivered to Ship Shoal. The model results showed that suspended mud from the Atchafalaya River can transport and bypass Ship Shoal. Only a minimal amount of suspended mud from the Atchafalaya River can be delivered to Ship Shoal in a one-year time scale. Additionally, suspended mud from the inner shelf could be transported cross Ship Shoal and generate a thin mud layer, which is also considered as the primary sediment source infilling the dredge pits near Ship Shoal. Two hurricanes and one tropical storm during the year 2017–2018 changed the direction of the sediment transport flux near Ship Shoal and contributed to the pit infilling (less than 10% for this specific period). Our model also captured that the bottom sediment concentration in the Raccoon Island pit was relatively higher than the one in Caminada in the same period. Suspended mud sediment from the river, inner shelf, and bay can bypass or transport and deposit in the Caminada pit and Raccoon Island pit, which showed that the Caminada pit and Raccoon Island pits would not be considered as a renewable borrow area for future sand dredging activities for coastal restoration.

Floc sizes and resuspension rates from fresh deposits: Influences of suspended sediment concentration, turbulence, and deposition time

Cohesive mud flocs in coastal waterways can go through repeated cycles of deposition and erosion before being sequestered in a final deposit. Unlike sand and gravel, mud flocs have the potential to change size and density during their time on the bed. For example, it is conceivable that flocs could increase in density or aggregate with other flocs while on the bed. Such changes could influence the rate of resuspension or the size characteristics of resuspended flocs relative to those that do not spend time on the bed. Here we use laboratory experiments to quantify the influence of sediment and water-column properties on resuspension characteristics of mud flocs. Specifically, we examine how concentration at the time of deposition, the length of time for which the deposited material is on the bed, and the turbulence levels in the water column while the flocs are on the bed all impact the nature of the flocs on the bed and their resuspension characteristics. The resuspension characteristics we quantify are: (1) the difference between the floc size population before and after resuspension, (2) the rate of resuspension, and (3) the difference between the concentration at the time of deposition and full resuspension. The experiments show that flocs grow in size while on the bed, but that resuspended flocs quickly revert to size populations that are in equilibrium with the local water column conditions. This occurs regardless of the time on the bed between deposition and resuspension. The results also show that under low turbulence levels, flocs moving around on the bed increase in size relative to their suspended counterparts, and that such flocs can be more difficult to resuspend. Furthermore, the time that the flocs are on the bed (0.25–12 days) has little impact on the resuspension rates unless sediment concentration in the water column at the time of deposition is above a critical value for the onset of a space and load-bearing network. For concentrations above this critical value, freshly deposited mud begins to consolidate and become more resistant to erosion with time.

Barren Meandering Streams in the Modern Toiyabe Basin of Nevada, U.S.A., and Their Relevance To the Study of the Pre-vegetation Rock Record

Abstract Modern meandering streams devoid of plant life have long been ignored despite their relevance to the study of sedimentary landscapes in the long span of Earth's history that preceded the greening of continents. We present a sedimentologic and remote-sensing analysis of the McLeod Springs Wash, a small-scale, endorheic, and barren fluvial fan in the Toiyabe Basin of Nevada, USA. Fluvial channels therein undergo avulsion at the sub-decadal scale, and follow a morphodynamic cycle from low-sinuosity, higher-gradient, and sediment-starved streams into meandering, lower-gradient channels prone to overspilling. Sedimentation is purely clastic and, unlike in other arid-climate endorheic basins, water recharge maintains positive hydrologic budgets year-round such that any contribution to bank strength due to precipitation of evaporites is ruled out. Likewise, the unvegetated nature of the meandering channels excludes any direct biotic forcing on sedimentation, leaving an equilibrium between discharge, gradient, and bank strength aided by cohesive mud fractions as the main mechanism responsible for channel meandering. Point bars compose a volumetrically significant fraction of the McLeod Springs Wash's stratigraphic record, and consist of sub-meter-thick, fining-upward inclined ripple strata floored by intra-formational conglomerate. Their sedimentary record remains non-diagnostic unless morphodynamic analyses based on accretion and paleoflow vectors are integrated. Interpreting river planform from these deposits is complicated by swift channel avulsion and migration relative to alluvial aggradation, which make the preservation of entire channel-bar complexes exceptional. We conclude that a significant bias may exist against the interpretation and volumetric estimation of the pre-vegetation meandering-fluvial rock record, and that the application of classic point-bar models based on inclined heterolithic stratification to Ordovician and older rocks is intrinsically flawed.

A Shear‐Limited Flocculation Model for Dynamically Predicting Average Floc Size

AbstractThe accuracy of sediment transport models depends on the selection of an appropriate sediment settling velocity. In general, settling velocity is primarily dependent on the size and density of the particles in the water column. Determining this value for mud suspensions can be difficult because the small cohesive particles can aggregate to form flocs whose sizes and density are a function of hydrodynamic and physiochemical conditions of the suspension. Here we present a new model for predicting floc size as a function of hydrodynamic conditions and inherited floc sizes. The new approach is a simple modification to the existing Winterwerp (1998, https://doi.org/10.1080/00221689809498621) floc size model. The modification is significant in that it yields predictions that are more inline with observations and theory regarding the upper limit on floc size. The modification we propose is to make the ratio of the applied stress on a floc, over the strength of the floc, a function of the floc size relative to the Kolmogorov microscale. The outcome of the modification is that flocs are not allowed to surpass the Kolmogorov microscale in size and that calibrated aggregation and breakup coefficients obtained at one suspended sediment concentration can be used to predict floc size under other concentrations without recalibration. In this paper, we present the motivation for the modification, the functionality of the modification, and we make a comparison of the updated model with laboratory and field data. Overall, the modification shows promise as a tool for improved prediction of cohesive mud transport.

Living landscapes: Muddy and vegetated floodplain effects on fluvial pattern in an incised river.

Cohesive floodplain sediment and vegetation are both thought to cause meandering river patterns. Our aims are to compare the isolated and combined effects of mud and vegetation on river planform and morphodynamics in the setting of intermediate‐sized valley rivers. We use a numerical model for century‐scale simulation of flow, sediment transport and morphology coupled with riparian vegetation settlement, growth and mortality as functions of species traits on which flow resistance depends. Mud fluxes were predicted by excess shear stress relations in combination with the active layer formulation. We found that valley‐flooding water levels increase with vegetation density, causing a higher braiding intensity rather than meandering tendency. The shear stress during floods carves channels through the muddy floodplain surface. Higher mud concentration, on the other hand, increases floodplain aggradation, reduces the overbank flow frequency and ultimately causes formation of a single‐thread channel. Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation settling mutually enhance floodplain formation. However, mud and vegetation counteract in two ways. First, vegetation enhances floodplain accretion, which ultimately increases plant desiccation for high mud concentrations. Second, vegetation increases the tendency of periodic chute cutoffs in valleys. The chute cutoffs locally reset the landscape and create new windows of opportunity for the vegetation. Surprisingly, in systems with a high mud concentration this causes hysteretic loops of vegetation cover and delayed mud deposition. Ramifications for the interpretation of Palaeozoic fluvial facies are that even rootless vegetation, capturing cohesive mud closer to the river channel to form thicker floodplain on the point bar, can enhance the tendency to meander and, under high mud supply, form stable channels. However, meandering is more unlikely in narrower valley rivers with higher vegetation density. © 2018 John Wiley & Sons, Ltd.

Bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions

Many coastal and estuarine environments are dominated by mixtures of non-cohesive sand and cohesive mud. The migration rate of bedforms, such as ripples and dunes, in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology. However, these models tend to ignore parameters describing the physical and biological cohesion (resulting from clay and extracellular polymeric substances, EPS) in natural mixed sediment, largely because of a scarcity of relevant laboratory and field data. To address this gap in knowledge, data were collected on intertidal flats over a spring-neap cycle to determine the bed material transport rates of bedforms in biologically-active mixed sand-mud. Bed cohesive composition changed from below 2 vol% up to 5.4 vol% cohesive clay, as the tide progressed from spring towards neap. The amount of EPS in the bed sediment was found to vary linearly with the clay content. Using multiple linear regression, the transport rate was found to depend on the Shields stress parameter and the bed cohesive clay content. The transport rates decreased with increasing cohesive clay and EPS content, when these contents were below 2.8 vol% and 0.05 wt%, respectively. Above these limits, bedform migration and bed material transport was not detectable by the instruments in the study area. These limits are consistent with recently conducted sand-clay and sand-EPS laboratory experiments on bedform development. This work has important implications for the circumstances under which existing sand-only bedform migration transport formulae may be applied in a mixed sand-clay environment, particularly as 2.8 vol% cohesive clay is well within the commonly adopted definition of “clean sand”.

Elements of the theory of wavelike formation of cohesive mudflows in erosion areas

Presented are the dependences that allow judging as a rough approximation about the process of accumulation of the cohesive mud mass in mudflow areas till the complete formation of a mudflow in the erosion inset of the main riverbed of a mudflow type.

Bedload Transport of Mud Across A Wide, Storm-Influenced Ramp: Cenomanian-Turonian Kaskapau Formation, Western Canada Foreland Basin--Discussion

We would like to comment on a recent paper by Plint et al. (2012) that discusses mud transport across a Cretaceous shelf. The authors have taken to heart recent research that advocates looking for evidence of bedload transport of aggregated clays, and we are comfortable that much of the clay component in the Kaskapau Formation did indeed arrive via bedload transport rather than simply settling out of the water column. However, whereas we are perfectly comfortable with the proposed mode of transport, we do have several concerns and suggestions concerning one type of mud aggregate discussed by Plint et al. (2012). In the depositional model that the authors present (their fig. 16), they envision that cohesive mud is reworked by storms into intraclasts and that these then are carried across the seabed in bedload. That in itself is no problem, because it can be shown in experiments that surficial muds with as much as 85% water content can be transported as millimeter-size aggregates for considerable distances (Schieber et al. 2010). One class of aggregates, however, described as intraclastic aggregates (IAs for the remainder of this manuscript) by the authors, did capture our attention. The authors state that “ Storm wave reworking of the seafloor produced intraclastic aggregates…because the mud had been rendered cohesive by the chemical compaction and biostabilization processes that operated shortly after deposition .” We are aware that surficial sediments can gain enhanced cohesion and improved erosion resistance due to mucus from benthic worms and endo-sedimentary microbes, but we are not quite sure what the authors mean by chemical compaction. If they mean that cementation renders the muds firmer and more cohesive, it would have been rather appropriate to document this critical factor in the aggregates in question. By definition, …

Bedload Transport of Mud Across A Wide, Storm-Influenced Ramp: Cenomanian-Turonian Kaskapau Formation, Western Canada Foreland Basin--Reply

Abstract Mudstone-dominated shallow-marine rocks of the Kaskapau Formation were deposited on a low-gradient ramp that spanned the foredeep of the Western Canada foreland basin during the late Cenomanian to middle Turonian. Organic-, clay-rich, and silt-rich mudstone accumulated on the flank of the forebulge, > 200 km from the western shoreline. Within this mudstone, a large proportion of the clay minerals are organized into silt- and very fine sand-size aggregate grains. These aggregates were produced both in the water column as marine snow and phytodetritus and also through reworking of previously deposited cohesive mud to form intraclasts. The latter, along with siliceous silt grains, form wave- and combined-flow ripples, graded beds, scour-fills, and ripple-tail lamination. Where sand-size sediment (comprising detrital siliceous, calcareous bioclastic, or phosphatic grains) is present, it is molded into combined-flow ripples, HCS, gutter casts, and lags. Thus all sediment grades indicate storm- wave and current reworking of the sea floor at a distance of > 200 km offshore. The common occurrence of clay minerals in the form of aggregate grains, organized into combined-flow ripples and parallel lamination, implies advective transport of clay minerals as bedload, driven by combined flows across a very low-gradient ramp. On the distal part of this ramp, latest Cenomanian rocks include thin tongues of SW-prograding quartz-rich sandstone that was derived from an emergent forebulge. This source was drowned during the early Turonian eustatic rise when the sediment abruptly changed to organic-rich mudstone dominated by clay-mineral aggregates. This compositional change was mainly a response to a sudden increase in distance to detrital sources in the west, rather than a dramatic increase in water depth. Throughout much of early to middle Turonian time, the sea floor in the forebulge region lay above effective storm wave base for silt, estimated at ~ 70 m. At sea-level lowstands, wave winnowing and erosion of the sea floor concentrated bioclastic lags at the top of siltier-upward sequences; lags are interpreted to correspond to falling-stage, lowstand, and early transgressive systems tract deposits on the western margin of the basin. Previous studies may have substantially overestimated water depth for organic- and clay-rich calcareous mudstones in the Western Interior Seaway.

Mud dispersal across a Cretaceous prodelta: Storm‐generated, wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

AbstractDetermining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.