ABSTRACT Measurements assessing the composition of bed sands under oscillatory flows were conducted in the large wave flume (Großer Wellenkanal, GWK) in Hannover, focusing on diverse sand mixtures. The experiments featured velocity‐asymmetric flows with a consistent wave period of 7 s, while considering two distinct wave heights (1 and 1.5 m). The sand mixtures were derived from two well‐sorted grain‐size fractions: a fine (F) sand with a median diameter of 0.21 mm and a coarse (C) sand with a median diameter of 0.58 mm. This study scrutinises the bed composition of three bimodal graded sands, initially composed of 75%, 50% and 25% fine fractions. Sand characteristic analyses were performed using a Laser Diffraction Particle Size Analyser, enabling precise counting and sizing of particles in heterometric sands. Over time, the initially well‐mixed bed composition undergoes alterations, revealing a vertical segregation of mixed sands at the bed layer. Distinct behaviours emerge in the different fractional experiments. For the sand mixture with a higher percentage of coarse sand fraction, a gradual increase in the maximum percentages of coarse sands is observed, particularly in the upper layers (upward coarsening). Conversely, with a higher percentage of fine sands, a discernible trend is noted where finer sands tend to occupy the superficial layers, while coarser sands predominate in the deeper layers (upward fining).

ABSTRACT Understanding how mud moves and deposits is essential for understanding and quantifying the dynamic nature of surface environments and their ancient counterparts. Many factors have been proposed to control the accumulation of mud, including mineralogy, salinity, suspended sediment concentration, flow velocity and microbiota. Flume studies at the IU Shale Research Lab were aimed at investigating the influence of these factors. Earlier work is summarised to provide a snapshot of the current state of understanding. A key parameter that defines mud accumulation is the critical velocity of sedimentation (CVS)—the minimum flow velocity required to keep sediment particles suspended in the water column and prevent them from settling to the bottom. CVS is influenced by a combination of particle and fluid characteristics. Most previous experimental studies have been conducted with sediment composed largely of kaolinite clay in fresh to saline water. Kaolinite, however, is not the dominant clay in natural sedimentary systems on Earth. Recent experiments with the most common clay minerals, illite and smectite, show that these minerals differ markedly in their depositional characteristics from kaolinite. CVSs for illite and smectite are significantly lower (15 and 10 cm/s, respectively) than for kaolinite (25 cm/s) in otherwise identical experiments. Unlike kaolinite, salinity does influence their CVSs. The reasons for these differences are likely related to the morphology of the different clay minerals and the distribution of their surface charges. These significant differences were not observed in earlier experiments, most likely because microbial films formed on the various clay minerals and floccules during extended experimental runs. Particle binding by microbially generated extracellular polymers appears to have counterbalanced the pronounced differences in behaviour among clay minerals observed in intentionally sterile experiments. Ubiquitous microbial coatings, typical for natural environments on Earth since the Archean, apparently make muddy suspensions behave similarly regardless of the dominant clay mineral. In abiogenic settings, however, such as planets devoid of life, mineral controls on flocculation behaviour may result in sediment distribution patterns that are unknown on Earth.

ABSTRACTThin ferruginous sandy crusts are common on top of sandstone beds in the Early Permian post‐glacial deposits of the Paraná Basin in southern Brazil. These crusts usually preserve wrinkle structures, suggesting that they might be a product of microbial mediation. Iron‐synthesizing bacteria use iron for essential metabolic activities and precipitate iron minerals as a by‐product. Bacteria could also indirectly mediate iron minerals precipitation through the excretion of extracellular polymeric substances, which act as a glue, enabling iron adhesion and nucleation. Samples of ferruginous sandy crusts from different sandstone beds of the Rio Bonito Formation were investigated through microtextural high‐resolution, mineralogical and geochemical methods to validate this hypothesis. Lumpy and filamentous morphologies and globular goethite minerals are abundant in these samples and were herein interpreted as bacterial products. The presence of kerogen reinforces the evidence of bacteria–substrate interaction and suggests the development of biofilms/microbial mats on the top of these beds. Quiescence is needed to develop and preserve microbial mats and low sedimentation rates are required to form ironstones. All these aspects allowed interpreting the studied ferruginous sandy crusts as representing depositional hiatus in estuarine settings impacted by periods of significant continental input. The occurrence of these crusts in coastal deposits may be a biosignature of continental contribution, potentially driven by climate warming.

ABSTRACTComplex stratigraphic arrangements may be generated through lateral facies transitions from the strata of aeolian depositional systems into bordering peritidal deposits. In such scenarios, sedimentary architectural complexity and associated facies heterogeneities are governed by the interplay between autogenic processes inherent to tidal environments and larger‐scale allocyclic forcing. As a result, complications arise when trying to discriminate the two signals within preserved strata, and the prediction of their depositional configuration may be challenging. By documenting the facies diversity and spatial distribution of an ancient mixed carbonate–siliciclastic tidal flat succession deposited under arid conditions, and by analysing its degree of stratal disorder, this work provides a generalised model for the stratigraphic record of marginal mixed peritidal flats. The sedimentology of tidally dominated shallow marine to sabkha deposits of the Middle Jurassic Carmel Formation (San Rafael Group) is investigated across a 350 km long transect in southern Utah, USA. A total of 26 lithofacies are identified and grouped into 11 facies associations. Detailed qualitative and quantitative analysis of facies distribution has highlighted two transgressive‐regressive sequences overprinted by high levels of autogenic noise at the facies association scale. Multiple coexisting associations are observed within different facies belts and are characterised by the sedimentary signatures of intricate coastal, tidal and aeolian forces with variable proportions of carbonate, siliciclastic and evaporitic material. By combining statistical analysis with classical sedimentological interpretations, this study demonstrates the challenges in predicting the distribution of discrete stratigraphic architectures in peritidal successions. Such systems may be subdivided into separate depositional elements defined by differences in facies proportions linked to changes in depositional processes and energy levels across their margins. This work proposes a newly quantified model for arid tidal systems with which to account for stratal disorder. Incorporating this singular characteristic may help constrain the predictions of reservoir heterogeneities in analogous subsurface successions.

ABSTRACTTubular and curviplanar structures, outlined by the occurrence of haematite/goethite, chlorite, quartz and albite, are developed in the Middle Permian Broughton Formation in the southern Sydney Basin, New South Wales, Australia. These structures are interpreted as fluid‐flow pathways resulting from the ejection of heated pore fluids as a thick (>30 m) basaltic lava (the Bumbo Latite Member) was emplaced rapidly on top of near‐shore, unconsolidated, wet, sandy marine sediments (the 53 m thick Kiama Sandstone Member). Evidence of fluid‐flow pathways in the sandstone is observed for ~20 m below the basalt. Subhorizontal and parallel, elongate tube‐like flow structures tend to be between 5 cm and 30 cm in diameter and are exposed laterally for a few tens of metres on the shore platform, although their full extent cannot be determined. Fluid‐flow pathways are marked by intense mineralisation and include enclosed tubes as well as unenclosed sheets, which may be flat or locally highly curved. Cross‐cutting relationships reveal several generations of the tube‐like features and imply the passage of hydrothermal fluids through the tubes. However, while significant alteration is apparent in the tube rims, little or no alteration is visible inside tubes at the hand specimen scale, but on a microscopic scale, the tube cores show alteration and extensive development of laumontite. Fluid‐flow features are grouped in moderately to well‐sorted sand beds and are underlain by layers with a higher silt component and exhibiting extensive bioturbation. These features imply that bedding‐controlled sediment porosity and permeability played a large role in determining the location of fluid‐flow conduits. The hot lava heated and pressurised the pore water, inducing horizontal hydrothermal fluid flow away from the lava and producing the mineral assemblage defining the fluid‐flow pathways.

ABSTRACTAeolian sediment transport reshapes planetary landscapes and causes various environmental problems. For almost a century, it has been broadly recognized that the features of ejection particles are dominated by each independent impact process. Using a coupled fluid‐discrete element model to resolve creeping and saltating motions simultaneously, we observed that, under steady‐state transport, almost all ejectors are evolved gradually from energetic creeping particles that were maintained by the combined action of fluid shear and continuous grain‐bed collisions, where the last effective impact before the particle is ejected into the air barely contributes to less than 20% of the total momentum on average. Furthermore, aeolian ejectors undergo several to tens of effective impacts prior to being ejected into the air, during which the momentum transferred from the impact particles is an order of magnitude higher than that from the fluid drag. Thus, ejected particles are mostly generated by a series of effective impacts rather than being dominated by a single impact. These findings pose a significant challenge to the existing ejection dynamics of wind‐blown sand movement. Largely, this is because the energetic creeping particles form a rheological layer atop the bed surface, leading to a reduction in the momentum transfer efficiency between the impacting particles and the bed particles. These results provide new insights into aeolian splash dynamics of steady‐state aeolian transport.

ABSTRACTInvertebrate burrow morphologies and distributions are presented for the tidal flats of the middle and lower reaches of the tide‐influenced (mesotidal), mud‐dominated Mira River estuary (SW Portugal) as an analogue for interpreting the ichnology of palaeoestuarine successions. Burrow distributions are revealed using field observations of biogenic and physical sedimentary structures, trace makers, collected grab samples, push cores and lab measurements of grain size, organic matter and calcium carbonate content and linked to physico‐chemical stresses. Estuarine tidal‐flat surface open burrow distribution and bioturbation intensity are analysed in plan view using free scientific image analysis software (ImageJ) to obtain burrow density, total burrow area and minimum and maximum burrow diameters. The total tidal flat area occupied by organisms ranges from 0.8% to 4.1% in the middle estuary and is 1.4% in the lower estuary. These equate to a bioturbation index of one to two (BI 1–2). However, computed tomography (CT) of push cores (cross‐sectional view) taken on the same tidal flats shows sparse to complete bioturbation intensity (BI 1–6), revealing that tidal flat sediments tend to be more biogenically reworked cumulatively over time with overprinting of subsequent tiers and/or recolonized. CT scanning also allows the identification of more burrow types, demonstrates infaunal tiering and provides insights into trace‐fossil preservation potential. This research shows that the trace communities are heterogeneous and change spatially and temporally along the Mira River estuary, reflecting a physico‐chemical gradient and seasonality. This trend signals changes in sediment composition, substrate type and consistency (sediment compaction and subaerial exposure), salinity, oxygenation, temperature, pH and interplay of tidal versus hydraulic energy.

ABSTRACTIn many estuaries, biogeochemical investigations have often focused on transient diatom biofilms that form on low‐energy intertidal flats. Studies on microphytobenthos in high‐energy sedimentary environments are unusual. The present investigation focuses on the biogeochemistry to a depth of 6 m of a fluvio‐estuarine point bar from the Garonne channel (SW France) impacted by both tidal current and tidal wave, where three sediment cores were taken. Porewater chemistry was analysed with microelectrodes (pH, oxygen and sulfide), ion chromatography and inductively‐coupled‐plasma spectrometry (for major elements) and colorimetric assays (for iron speciation). Porewater composition was compared to measurements of microbial activity including isothermal calorimetry and metabolic assays using triphenyltetrazolium chloride and fluorescein diacetate to determine the distribution of predominant microbial metabolisms in the sediment. Finally, bulk sediment chemistry was characterized through X‐ray fluorescence core scanning. Sediments are heterolithic, made of decimetre to meter thick alternating sand and mud. The uppermost 60 cm of the point bar sediment show a mostly classical vertical succession of microbial metabolisms: (i) oxygenic photosynthesis occurs mostly in diatom biofilm forming in the uppermost millimetres; (ii) aerobic respiration between 0 cm and 1 cm, (iii) nitrate reduction between 6 cm and 16 cm, partially overlapping (iv) sulfate reduction between 10 cm and 25 cm, (v) manganese oxide reduction below 2 cm and (vi) iron oxide reduction below 16 cm. Measurements of metabolic activity, elevated in areas showing significant geochemical changes, confirmed the impact of microbial metabolism on the composition of pore water. The highest metabolic activity coincides with areas where oxygen, nitrate and sulphate concentrations are decreasing. Hydrolytic activity peaked in the zone of aerobic respiration, possibly in part due to enzymatic degradation of organic matter (e.g., extracellular polymeric substances) produced in surface diatom biofilm. Low concentrations of nitrates and sulfates were measured in sands at 1.3 to 1.6 m and 3.2 m depth, coinciding with a renewed increase in hydrolytic activity and metabolically active cells. Because of the sediment heterolithic composition and the point bar architecture made of laterally accreting layers, subsurface advection of porewater through permeable horizons could explain the local increases of nitrate and sulfate reduction. Impacts of microbial metabolism on early diagenesis were modelled using PHREEQC software and outcomes predicted the potential precipitation of metastable iron and/or sulfides. This was confirmed by X‐ray fluorescence analyses showing a coinciding increase of sulfur, Fe and/or Mn at several depths (e.g., 15 to 60 or 560 to 580 cm). Based on our observations, we propose a biogeochemical model that links microbial metabolisms and early diagenesis to the complex vertical sedimentary architecture of an estuarine point bar. Our results show that high‐energy estuarine point bars are subject to an active biogeochemical cycling of C, S, N, Fe and Mn quite similar to that of intertidal mudflat, but locally altered by the sedimentary architecture of the point bar, resulting in lateral advection of porewater.

ABSTRACTThe presence of microbial mats is often invoked to explain the good preservation of vertebrate tracks, because they can cover and biostabilize such structures. However, microbial influence on the sediment properties when the track is made and on the track characteristics has not been so thoroughly analysed. This study presents the results of field experiments consisting in the production of human tracks in different inter‐ to supratidal coastal environments of Argentina that show different degrees of biostabilization. The biostabilization degree, together with the characteristics of the sediment below, strongly control depth, appearance and anatomical fidelity of the tracks, and also influence their subsequent evolution. The development of an epibenthic mat in wet porous sand confers the surface a great resistance to deformation, resulting in shallow (millimetre to 2 cm) tracks with high anatomical fidelity that deform the mat plastically and are resistant to subsequent currents. This contrasts with the tracks made in poorly colonized sand, which are 2 cm‐deep, show vertical walls and are completely eroded by subsequent currents. The development of a biofilm over frequently flooded, wet, soft, muddy or muddy to sandy sediment does not change significantly its rheological properties, resulting in several centimetres‐deep tracks with vertical walls, which are preserved through time, although progressively reshaped by currents and burrowing. The development of a biofilm or a microbial mat on supratidal, hard, muddy or mixed muddy to sandy sediments retains moisture and increases adhesiveness, leading to the formation of poorly defined shallow (less than a millimetre to 1 cm) tracks by surficial sediments sticking to the sole and tearing during foot withdrawal. Rapid hardening by desiccation may help to preserve these tracks in supratidal flats.