Compound Dunes Research Articles

Evidence of Secondary Bedform Controls on River Dune Migration

AbstractIn rivers worldwide, multiple scales of dunes coexist. It is unknown how the larger, primary dunes interact with secondary bedforms that are superimposed. We test the hypothesis that streamwise variability in the sediment flux inferred from the downstream migration of secondary bedforms explains migration of the host dune, based on bathymetric data from a lowland, sand‐bedded river. Results indicate that transport estimated from secondary bedform migration increases along the host dune stoss, eroding the stoss slope. When the superimposed bedforms disintegrate at the primary lee slopes, results indicate that all sediment transport associated to secondary bedform migration is arrested in the lee of the host dune, explaining migration of the host dune. When secondary dunes persist however, only part of the sediments transport linked to secondary dunes contributes to the migration of the host dune. This study gives novel insight into the fundamental mechanisms controlling the kinematics of compound dunes.

A tale of two end members: Tidal deposits in a semi‐arid, low subsidence, open coastal setting versus a high runoff, high subsidence, restricted environment

AbstractJurassic and Palaeocene tidal deposits of the epeiric Western Interior Sea in Wyoming, USA, differ significantly due to their contrasting climates and tectonic, geographic and depositional settings. Tidally generated, cross‐bedded sandstone bodies contained by incisions are common to both settings and can potentially be uncritically attributed to marine flooding of fluvial systems. Key differences in lithology, ichnology and relationships with surrounding sediment reveal fundamentally different depositional settings, however. The Jurassic system occupied a low accommodation, semi‐arid environment, with geographically open shorelines as relative sea‐level fell, creating an unconformable contact with the underlying, storm wave‐dominated shelf and shoreface deposits. Siliciclastic, intertidal flats formed adjacent to coastal aeolian and fluvial environments during brief turnarounds from the degradational (forced regressive) to aggradational and retrogradational (transgressive) system tract. Basinward of these environments, metre to decimetre‐scale cross‐bedded, bioclastic, subtidal compound dunes and tidal inlet complexes accumulated in areas of minimal clastic flux and within incisions created by submarine tidal currents. By contrast, the Palaeocene tidal systems formed in a high accommodation, subtropical setting, as rising sea levels forced the fluvial to marine transition zone landward and flooded coastal swamps, forming geographically irregular, back‐barrier complexes and tidally influenced fluvial systems. High volumes of siliciclastics, terrigenous organic material and freshwater were delivered by the rivers and created physicochemical stress on the marine embayments. Sandy tidal flat deposits accumulated in lagoons and interdistributary bays, but unlike the Jurassic examples, they do not mark a turnaround from the falling stage to the transgressive system tract. The potential preservation window for tidalites is significantly greater vertically in the aggradational to retrogradational setting than in the degradational system due to greater accommodation. The preservation window is vertically smaller, but areally greater in the Jurassic, forced regressive system because of basinward enhancement of tidal currents driven by complex palaeobathymetry caused by tectonic activity of local pre‐Laramide uplifts.

Eocene dry eolian system in the Jianchuan Basin, southeastern Tibetan Plateau: Implications for regional wind regime and paleoclimate

East Asia experienced a climatic transition from a zonal pattern (climate zones defined by latitude) to a monsoon-dominated pattern during the Cenozoic. However, hitherto little consideration has been paid to the atmospheric circulation of the planetary wind system that prevailed during the early uplift of the Tibetan Plateau, due to the lack of reliable proxies. Here, we conduct a study on the eolian successions from the Eocene Baoxiangsi Formation in the Jianchuan Basin, southeastern Tibetan Plateau to reconstruct regional atmospheric circulation and paleoclimate. Two facies associations are recognized in the eolian deposits of the Baoxiangsi Formation: a dominant eolian dune association and a subordinate interdune association. Eolian dune deposits are characterized by large-scale cross-stratified sets of strata composed internally of packages of grainflow, wind-ripple strata and pinstripe lamination. These packages of laminae and cross-stratified eolian dune sets are bounded internally by superimposition and reactivation surfaces and interdune surfaces, respectively. Interdune deposits are uncommon and composed dominantly of subcritical climbing translatent strata to form sets of strata and these are themselves bounded by interdune surfaces to form cosets of strata. The association of dominant eolian dune deposits, widely distributed interdune surfaces of great lateral extent (> 450 m) and that climb at a shallow but positive angle (< 1°) is consistent with the climbing dry eolian system. The commonly occurring superimposition surfaces indicate the presence of compound dunes (megadunes, draas). Based on the analysis of 715 dip–azimuth readings of foresets of eolian cross-beds, we reconstruct the prevailing wind directions: the paleowind in the northern basin flowed toward the northeast; in the southern basin it flowed toward the southwest. This wind regime indicates that the Jianchuan Basin was located at the zonal boundary between the westerlies and the trade winds, and the construction of the dry eolian system was controlled by the subtropical high-pressure belt during the middle–late Eocene. Following the accumulation of the dry eolian system, a monsoon climate regime gradually established itself and prevailed in the Yunnan Province; this expanded northward over time. This was closely related to the uplift of the central Tibetan Plateau and the retreat of the Paratethys ocean in the late Eocene.

Subtidal to intertidal deposits in a mixed clastic–carbonate epicontinental seaway, the Windy Hill Sandstone and Upper Sundance Formation (Oxfordian), Wyoming, U.S.A.

Abstract Oxfordian deposits in northern Colorado and Wyoming, USA, preserve proximal, intertidal, clastic, coastal deposits and distal, offshore to nearshore, subtidal bioclastic facies that accumulated during a forced regression of the Jurassic epeiric sea. This contrasts with the common association between tidal deposits, carbonate facies, and rising sea level common to many depositional models of mixed clastic–carbonate systems. Ichnology, sedimentology, and architecture of these deposits were documented along a 460 km outcrop transect to test previous depositional interpretations and decipher the mechanisms driving the change from proximal, siliciclastic, intertidal flats to distal, bioclastic, subtidal macroforms. In southeastern Wyoming, the Windy Hill Sandstone (WH) is composed of very fine, SiO2-dominated, intertidal facies that truncate offshore to lower shoreface, storm-dominated deposits of the Redwater Shale Member (RS) of the Sundance Fm. Eolian and small fluvial systems delivered sand to the coastline after subaerial erosion reworked it from older, uplifted Jurassic strata onshore. The regional and time-transgressive J-5 unconformity separating the WH from the RS is readily identifiable using ichnological and sedimentological criteria. In the Wind River and Bighorn basins to the north, the Upper Sundance Fm (USF) is time-equivalent to the WH but is composed of glauconitic, silt-prone sandstone and meter- to decimeter-scale, bioclastic, cross-bedded sandstone bodies. The abundance of molluscan shell material and limited volume of siliciclastic sediment in the geographically widespread outcrops suggests that nearby, marine shoals were the source of the coarse-grained material. Bioclastic, cross-stratified sandstone bodies represent two architectural elements: 1) coarsening-upward bodies with seaward-dipping foresets arranged into complexes bounded by seaward-directed bounding surfaces interpreted to represent subtidal compound dunes and 2) landward- and laterally accreting tidal-inlet fills composed of meter-scale, landward-accreting bodies with some landward-directed current ripples. The presence of transgressive tidal inlets supports previous interpretations that the WH and USF record high-frequency transgressions superimposed on a tectonically driven forced regression.

Recognizing genetically related depositional packages using 3D photogrammetric outcrop models in a fluvially dominated, tidally influenced meander-belt succession

Fluvio-tidal estuary deposits are characterized by abundant channel-associated dunes, compound dunes and point bar deposits. The delineation of boundaries between different point bars to identify genetically unrelated packages is difficult to assess, particularly where similarly dipping bars are superimposed on each other. This study uses an outcrop dataset of the McMurray Formation along the Christina River in NE Alberta, Canada, focuses on bedding relationships and the nature of sedimentary contacts pertaining to the two dominant channel-associated facies: 1) dune-scale, cross-bedded sandstone (CB) representing lower parts of estuary point-bar deposit or thalweg-associated dunes; and 2) inclined heterolithic stratified sandstone and mudstone (IHS) bedsets typically representing the middle and upper part of point bar deposits. This study lays a foundation for documenting and differentiating discrete channel deposits using sedimentological observations at the outcrop, photogrammetry, and surface-orientation analyses of the resulting 3D model. The results reveal the following findings: 1) the IHS units may be underlain by erosional surfaces, which may not be genetically associated with the underlying cross-bedded sandstone intervals; 2) the forward-accreted tidal compound dunes and laterally accreted fluvio-tidal point bars share similar facies characteristics, but are discernible using bedding orientation analysis; 3) these tidal compound dunes may be under-recognized in many tidally influenced estuary deposits; and 4) an inner estuary point-bar setting may be characterized by stacked small-scale point bars, or stacked lateral accretion sets of a single point bar. The result of this study provides a detailed view of facies architecture in a tide-influenced fluvial setting, and feasible solutions to the challenge of assigning genetic units in such environment.

The largest Quaternary inland eolian system in Brazil: Eolian landforms and activation/stabilization phases of the Xique-Xique dune field

Eolian systems are sensitive to vegetation and climate changes, but also to the sediment supplied by neighbor depositional systems. This sensitivity is manifested in the alternation between periods of sand dune mobility and stability. In the Xique-Xique eolian system, the largest Quaternary interior dune field in Brazil (∼8000 km2), the neighboring depositional system responsible for the sand supply is the São Francisco River. We interpreted periods of dune activity and stabilization by mapping eolian landforms of this system, describing depositional facies under them and dating by optically stimulated luminescence (OSL-SAR). The stabilized parabolic landforms characterized and mapped (all formed by SE to E winds) were simple megadunes (25.5 ± 4.4 to 5.8 ± 2.0 ka), compound dunes (15.8 ± 1.2 to 5.2 ± 1.4 ka) and perched dunes (14.0 ± 1.0 and 1.2 ± 0.1 ka). We also recognized eolian plains (14.0 ± 2.1 to 6.5 ± 0.7 ka) and active parabolic dunes. Eolian deposits of undifferentiated landforms have also been described and dated (253.8 ± 19.0 to 8.2 ± 1.3 ka). The ages obtained allowed identification of two main phases of dune activity (∼60 to 18 ka and ∼16 to 5 ka) separated by a short period of stabilization marked by a paleosol (∼18 ka). The main periods of eolian activity are associated with phases of fluvial aggradation and high sediment supply, whereas dune stabilization since 5 ka is correlated with low sediment supply during the Late Holocene fluvial incision phase. The periods of eolian activity and stabilization respond not only to climate changes in the area occupied by the eolian system but mainly to local fluvial sediment supply by the river, which in turn can respond to more regional climatic controls. The Xique-Xique eolian system results from the coupling of abundant sediment supply from the fluvial system, high eastern wind speeds, and a significant wind deceleration caused by mountains on the western border of the dune field.

Evidence for Fluctuating Wind in Shaping an Ancient Martian Dune Field: The Stimson Formation at the Greenheugh Pediment, Gale Crater

AbstractTemporal fluctuations of wind strength and direction can influence aeolian bedform morphology and orientation, which can be encoded into the architecture of aeolian deposits. These strata represent a direct record of atmospheric processes and can be used to understand ancient Martian atmospheric processes as well as those on Earth. The strata can: give insight to ancient atmospheric circulation, how the atmosphere evolved in response to global changes in habitability, and how ancient processes differ from modern processes. The Stimson formation at the Greenheugh pediment (Gale crater) records evidence of fluctuating wind across multiple temporal scales. The strata can be subdivided into three intervals–Gleann Beag, Ladder, and Edinburgh intervals. Internally, the intervals record changes of dune morphology and orientation, correlatable to wind fluctuations at multiple temporal scales. The basal Gleann Beag interval comprises compound cross‐strata, deposited by oblique compound dunes. These dunes record a bimodal wind regime, resulting in net sediment transport toward the north. The Ladder interval records a reversal of sediment transport to the south, where straight‐crested simple‐dunes shaped by a seasonally variable winds formed. Finally, the Edinburgh interval records sediment transport to the west, where a unimodal wind formed sinuous‐crested simple dunes. These observations demonstrate active and variable atmospheric circulation in Gale crater during the accumulation of the Stimson dune field, at multiple temporal scales from seasonally driven winds to much longer time‐frames, during the Hesperian. These observations can be used to further understand ancient atmospheric conditions and processes, at a high temporal resolution on Mars.

Sedimentary dynamics and topographic controls on the tidal-dominated Zagra Strait, Early Tortonian, Betic Cordillera, Spain

Abstract The approximately 350 m-thick stratigraphic succession of the Zagra Strait records an important oceanographic phase of basin interconnection between the Atlantic Ocean (Guadalquivir Basin) and the Mediterranean Sea through the Betic Cordillera (southern Spain) during the early Tortonian. The Zagra Strait developed as a narrow structurally controlled marine corridor. The sedimentary dynamics of the Zagra Strait were interpreted from the sedimentological features observed in six sections at well-exposed outcrops. Large-scale (&gt;10 m high) compound dunes and compound–dune complexes moved parallel to the strait margins under strong tidal currents generated by tidal amplification at the strait entrance and exit. Dune distribution can be divided into three sectors with different palaeocurrent migration, lithological and topographical characteristics. The northern and central sectors were separated by a deep depression (&gt;75 m water depth) where tidal currents were weaker and dunes were not generated. The southern sector records a relative decrease in current strength compared with the northern and central sectors, and a significant increase in the bioclastic content in the sediment. Terrigenous content generally increases towards the strait margins, and reciprocally, carbonates towards its axis. The closure of the Zagra Strait resulted from tectonic uplift of that part of the Betic Cordillera before the late Tortonian.

Distinctive lithofacies architecture of sand ridges built by storm and ocean currents: An example from a Middle Pleistocene shallow‐marine succession on the Boso Peninsula, Japan

AbstractSand ridges are present in modern storm‐dominated and tide‐dominated shallow‐marine environments worldwide, but the number of published ancient examples is limited. The role of ocean currents in the formation of sand ridges is also poorly understood. This study investigated the lithofacies architecture of a lenticular sandstone body (ca 70 m thick and ca 30 km long) in the upper Mandano Formation (ca 0.6 Ma) on the Boso Peninsula, Japan, to establish criteria for identifying sand ridge deposits built by storm and ocean currents. The sandstone body is unconformably underlain by coastal and valley‐fill deposits and fines upward to outer‐shelf muddy deposits. It consists mainly of medium‐grained to very coarse‐grained sandstones and pebbly sandstones that are moderately to intensely bioturbated and are represented by three‐dimensional compound dune deposits. The body consists of six 10 to 20 m thick units (units 1 to 6 in ascending order), which are each defined by an erosional base and locally capped by storm‐induced fluid mud deposits. Each unit also contains low‐angle (ca 5°) large stratification inclined obliquely to palaeocurrents, indicating lateral accretion surfaces. Units 1 and 2 are characterized by south‐eastward migration (locally north‐westward), smaller dune deposits, storm‐induced sedimentary structures and cold‐water molluscs indicating a palaeowater depth of up to 50 m. The other units are represented by east/north‐eastward migration, larger dune deposits, warm‐water molluscs indicating a palaeowater depth of up to 200 m, and a general lack of storm‐induced sedimentary structures. Temporal changes in the migration directions and lithofacies features of the units indicate that sand ridge formation was initially controlled by storm‐induced currents that were subsequently replaced by the northward or north‐eastward intruding palaeo‐Kuroshio Current in response to the expansion of the shelf during an overall transgression. The initiation and development of the sand ridges may have been supported by the supply of clastic sediments eroded from the underlying coastal and valley‐fill deposits.

The combined effect of discharge and tides on low-angle dune evolution at the tidal current limit of the Changjiang Estuary

Low-angle dunes (LADs) are recognized as the most common roughness element in large fluvial rivers and estuaries. Comprehending their development is vital to understand the flow and sediment dynamics, and paleo-environmental reconstruction of the geological record. However, our understanding of LADs evolution under unsteady flows is poorly understood. Here we investigate dune adaptation under the combined effect of discharge and tides at the tidal current limit of the Changjiang Estuary. Our observations show that compound dunes exist in the late flood season, while only simple dunes are found in the late dry season after five-month decay. The effect of tides enhances bedload transport, thereby accelerating the decay of compound dunes. The increasing deformation rate of superimposed small dunes from the trough to the crest of primary dunes results in a decrease in dune size in the neap tide, as superimposed dunes over the higher part of primary dunes decay more significantly. We find that the size of superimposed small dunes in the late flood season and simple dunes in the late dry season are significantly controlled by fortnightly tides when discharge is between 20,000 and 30,000 m3/s. It implies that compound dunes decay with crest erosion via frequent migration and deformation of superimposed small dunes between neap and spring tides. This finding could potentially help identify and interpret paleo-hydraulic environments that were influenced by tides from sedimentary records. Additionally, given the discharge of the lower Changjiang River is increasingly regulated by dams, figuring out dune adaptation to the regulated hydrograph could be used to advance our prediction of the evolution of river morphology.

Characterizing and identifying bedforms in the wandering reach of the lower Yellow River

The lower Yellow River (LYR) is a fully alluvial system with a fine-grained bed that has a high proportion of silt. Bathymetric survey data collected with a Multi-Beam Echo Sounder (MBES) from the wandering reach of the LYR indicates that the bedforms are characterized by large aspect ratios (wavelength/height) and low lee-side angles. Since the Xiaolangdi Reservoir (XLD) has been operational in the middle reach of the Yellow River, bedforms have been dominated by two-scales of dunes, that is, a frame of compound dunes with first-order dunes superimposed on the stoss and lee sides, which disappear during high sediment concentration floods that occur during the annual sediment and water regulation. The bedform formation regime has changed with dune beds having more chances, and upper regime beds having less chances, to form. Longitudinal profile data measured using a sounding-rod before the XLD began filling demonstrate that the bedforms were mainly dunes during low flows (less than 1200 m3/s) or upper regime beds during high-flows (larger than 2000 m3/s). A discriminator is proposed to describe the conditions for bedform formation and to predict the alluvial resistance which gives better results than those from Manning approach. When a bedform system is almost at equilibrium, the changes in the sediment concentrations versus the discriminator can be defined into three segments in the LYR.

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

AbstractThe bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large‐scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large‐scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood‐ and ebb‐oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune‐induced directional bed roughness in numerical models of estuarine and tidal environments.

Hydrodynamics over low-angle dunes at the tidal current limit of the Changjiang Estuary

Hydraulic roughness is a fundamental parameter in the understanding and simulation of hydro- and sediment-dynamics over dunes in fluvial and coastal areas. Completely understanding the links between the turbulent flow field, sediment transport and morphodynamics is key to better quantifying the evolving roughness in time and space. This paper presents two surveys (one in the late flood season and the other in the late dry season), conducted at the tidal current limit of the Changjiang Estuary, analysing flow and bedform dynamics under different water/flow stages. Results show that when compound dunes generate in the late flood season, flow structure was affected by the bed morphology and the velocity profile was segmented into two parts. The lower segment is interpreted to reflect the friction induced by the superimposed small dunes, whereas the upper segment is likely attributed to the combined effect of primary dunes and superimposed small dunes. Moreover, as the lee-side angles of superimposed small dunes fluctuate around 10°, the increase of flow velocity could increase the possibility of generating Flow separation zone (FSZ), consequently resulting in a further effect on flow structure. The correlation between flow resistance and flow strength displays that both the shear stress and roughness length over relatively larger dunes in the deep water are more sensitive to the mean velocity. Low-angle dunes (LADs) with lee-side less than 3° have no effect on flow structure, while for LADs with lee-side angle between 3° and 5°, the roughness length related to bedform is positively correlated to flow velocity, when flow velocity is lower than the equilibrium flow velocity. This implies that the flow expansion over LADs is sensitive to both the lee face and flow strength, thereby affecting LADs' evolution. Furthermore, the bedload transport calculated from empirical equations shows a satisfactory agreement with that estimated from bedform evolution. This result implies that bedload transport induced by shear stress related to LADs’ form is little in this survey area. Besides, in the migrating progress of LADs, their deformation is innegligible and deformation-related bedload transport could occupy nearly 30% of total bedload transport. This finding is significant to studies on migration and deformation of low-angle dunes, and could support studies of numerical modelling, especially on geomorphic module.

Robust methods for the decomposition and interpretation of compound dunes applied to a complex hydromorphological setting

AbstractUnderwater dunes are a morphological feature that are explored by marine scientists and coastal engineers alike. This study presents new methodologies in order to simplify bedform identification and morphodynamic analyses. Specifically, subaqueous compound dunes are decomposed with a simple yet extensive tracking algorithm, which relies on a repeated evaluation of unfiltered bed elevation profiles according to five predefined length classes. In a second step, morphological trends are assessed in the form of bed migration rates, bed slope asymmetries and net sediment changes, in which all parameters are referred to equidistant sections of the examined fairway stretch. This integrated approach not only avoids the challenges in weighting the varying size and abundance of dunes of different scales but also ensures comparability between dune‐specific and areal parameters, which significantly improves the interpretation of the morphological setting as a whole. The developed methods are applied to the Outer Jade fairway, an anthropogenically influenced and regularly maintained waterway in the German Bight, and allow scrutiny of spatio‐temporal trends in this region.Based on a unique data set of 100 sequential high‐quality echo‐sounding surveys, various types of bedforms are identified, comprising large‐scale primary as well as superimposing secondary dunes that are assumed to interfere with each other. Temporal trends show a long‐term rise of the troughs of major bedforms and constant maximum crest elevations near the official maintenance depth, which matches the observed long‐term aggradation of sediments. The spatial distribution of integrated morphodynamic parameters reflects a previously described zone of primary dune convergence and facilitates the precise localization of this geophysical singularity. The presented findings both confirm the robustness of the proposed methodologies and, in return, enhance the understanding of morphological processes in the Outer Jade. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Mixed carbonate–siliciclastic sedimentation at the margin of a late Miocene tidal strait, lower Colorado River Valley, south‐western USA

AbstractThe processes and deposits of tide‐dominated river deltas and estuaries are well‐understood, but the sedimentary dynamics of tide‐dominated straits and seaways are relatively little studied. Although recent depositional models have started to fill this gap, many aspects of tidal strait sedimentation such as interaction with strait‐margin alluvial fans and marginal marine systems remain poorly understood. This paper presents a study of the late Miocene basal carbonate member of the Bouse Formation exposed along the lower Colorado River, where prior studies have suggested a tidal influence on deposition. This study explores the applicability of tidal strait models in a continental oblique‐rift setting, and tests hypotheses for depositional processes and environments through detailed analysis of sedimentary facies, cross‐bedding architecture and palaeocurrent data. Mixed carbonate–siliciclastic facies on the west margin of the southern Blythe Basin record sedimentation in alluvial fans and fan‐fringing tidal flats at the retreating margin of a transgressive tidal strait. Pre‐Bouse normal faults established a narrow, tectonically confined basin architecture that led to amplification of tidal currents. Basin‐margin deposits pass laterally and up‐section into high‐energy cross‐bedded grainstone facies that record southward migration of compound dunes in the dune‐bedded strait zone of a shallow (ca 25 m) tidal strait. These findings provide the basis for recognition of strait‐margin facies in other settings where complex facies associations result from along‐strike variations in palaeobathymetry and current velocity. The results support a tidal origin for the southern Bouse Formation and provide new evidence for post‐Miocene uplift of the lower Colorado River Valley and adjacent areas.

Tide-dominated deltas responding to high-frequency sea-level changes, Pre-Messinian Rifian Corridor, Morocco

ABSTRACTThe Rifian Corridor was an ancient sea strait that connected the Mediterranean Sea with the Atlantic Ocean during the Miocene. Key outcrop exposures of this corridor's sedimentary fill are exposed at the Ben Allou, El Adergha, and Driouate localities, in the Fez–Meknes region, Morocco. These display cyclic successions that formed immediately before the disappearance of the Rifian Corridor, and the associated isolation and desiccation of the Mediterranean Sea. Sedimentary cycles at Ben Allou consist of: facies 1, Organic-matter-bearing blue claystones; facies 2, gray marlstones intercalated with turbidites; and facies 3, yellow-brown, coarse-grained calcarenites. Based on their coarse grain size, presence of reactivation surfaces, bidirectional current structures, and mud drapes as well as microfossil, macrofossil and trace fossil assemblages, we interpret the calcarenites (facies 3) as prograding, strait-adjacent tide-dominated deltas transitioning from large subtidal compound dunes to intertidal sand sheets that are composed mostly of authigenic carbonate sediment. The two fine-grained facies were deposited in deeper prodelta (facies 2) and shelfal (facies 1) environments, as shown by our combined sedimentological and paleontological evidence. Cross-sectional channel geometries provide a means for reconstructing the delta's paleo–tidal range, suggesting that it was macrotidal, and likely amplified by the paleogeometry of the Southern Rifian Corridor, to at least 4.2 m. The cyclic succession of corridor fill exposed here likely is the result of three, roughly 70 m fluctuations of eustatic rise and subsequent fall, possibly linked to ∼ 100,000-year glacial–interglacial climate fluctuations. Broadly similar, contemporaneous sedimentary successions from the outcrop at El Adergha, 40 km east of Ben Allou, show that these sea-level variations affected a wide range of the Corridor, while rocks at the locality of Driouate, 9 km south of Ben Allou, show evidence for lagoonal environments on the landward side of the corridor, which were subject to periodic marine regressions and floodings. These results are reinterpretations of previous work on these sediments, with implications for depositional processes in the Rifian Corridor, Miocene paleoclimate, and the Messinian salinity crisis.

Morphology, accumulation and preservation of draa systems in a Precambrian erg (Galho do Miguel Formation, SE Brazil)

Proterozoic aeolian successions tend to exhibit a low complexity in the architectural organisation, generally related to simple dunes with low morphological diversity. Although most palaeoenvironmental reconstructions of ancient ergs confirm this scenario, the general conditions in the Precambrian landmass should have allowed the construction of larger and more complex sedimentary aeolian systems than those recorded in the Phanerozoic successions. To address this paradigm, the present work proposes to discuss the development and morphodynamics of ancient draa-scale bedforms and to understand which controlling factors produced the apparent absence of draa architecture in the Precambrian sedimentary record. For this purpose, this study examines an interval of c. 80 m of the Galho do Miguel Formation, which is a Mesoproterozoic aeolian succession composed of metasandstones showing well-preserved primary sedimentary structures and geometries with excellent vertical and horizontal exposures. A detailed sedimentological analysis allowed the recognition of six main architectural elements: (i) simple transverse dune, (ii) simple linear dune; (iii) compound transverse draa; (iv) complex linear draa, (v) dry interdune and (vi) sand sheet. The lateral and vertical relationships between these architectural elements support the evidence of coeval spatial coexistence of distinct aeolian morphologies (transverse and linear bedforms), probably controlled by lateral variations in sand saturation and in circulation pattern of the palaeowind. The stratigraphic organisation indicates different intervals of draa construction and destruction, which appear to be associated with different sedimentary pulses. The intervals of draa construction represent the vertical succession between simple and compound dunes of similar morphologies, representing temporal variation in sedimentary availability and changes in angle of dune climbing. The erosional and irregular surface at the bottom of compound sets of cross-strata can represent periods of irregular accumulation, associated with partial draa destruction (dune cannibalisation), probably due to seasonal variations in sedimentary availability throughout construction stages. Evidence of a near-surface water table level, as small wave ripples, indicates that the position of the water table was directly linked to the sedimentation pattern throughout these intervals. This relationship implies that ground water was the main controlling factor on sand availability and wind saturation during aeolian morphodynamics, as well as on accumulation and preservation processes. Moreover, in conditions of high rates of subsidence, the thickness accumulated and preserved via water table fluctuation is incorporated in the sedimentary record; otherwise, these deposits would have eroded due to aeolian deflation or fluvial reworking. Apparently, the absence of large and complex aeolian systems in the Precambrian record can be related to the absence of continuous near-surface water table during the draa development, directly decreasing their preservation potential.

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf, Edgeøya, Svalbard

AbstractThe study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.

A new shallow-marine, high-latitude record of the trace fossil Macaronichnus in Miocene, reworked delta-front clinoforms, Punta Basílica, Tierra Del Fuego, Argentina

A new high-latitude record of the trace fossil Macaronichnus segregatis degiberti is documented from the lower Miocene Punta Basílica beds, Tierra del Fuego, which were previously considered to be deep-marine channel and levee deposits. Based on a combined ichnological and sedimentological study, however, these beds are herein re-interpreted as shallow-marine, delta-front clinoform deposits. Macaronichnus-bearing beds in the stratigraphically older clinoforms are characterized by: (1) sandstones with wave ripples and wave-ripple cross-lamination; (2) cross-stratified sandstones forming large compound dunes and tidal bars; and (3) intervening heterolithic mudstone-sandstone couplets with current and wave ripples. The sedimentary facies preserving M. s. degiberti were deposited during the onset of transgression, indicated by wave and tide reworking of underlying steeply dipping, delta-front clinoforms deposits. The master bedding surfaces of compound tidal bars are characterized by a dense, monospecific fabric of M. s. degiberti. Compared with other Macaronichnus ichnofabrics, reported elsewhere as characterizing short colonization windows on foresets of tidal dunes or bars, the Punta Basílica ichnofabric represents a longer colonization window, associated with episodic reworking and redeposition of loose, nutrient-rich sand grains just above the master bedding surfaces. The record of M. s. degiberti in Tierra del Fuego supports the previous interpretations that restrain its producer to high-latitude, shallow-marine cold waters.

A morphological insight into the Martian dune geomorphology

Aeolian process is the most active and extensive contemporary geomorphological process on the Mars, bearing rich information on surface processes, environment and evolution history. High-resolution remote sensing images covering the whole planet are interpreted and processed by GIS software to analyze the morphological characteristics of the Martian dunes on global basis, and discuss their aeolian geomorphological implications. A tentative dune classification scheme is proposed for the Mars. Dunes types on the Mars mainly include sand sheet, dome dune, barchan, barchanoid ridge, transverse ridge, linear ridges, seif dune, raked linear dune, nail dune, square dune network, ladder-like dune network, scaled dune network, honeycomb dune network, star dune, and obstacle dune. It is suggested that the Mars shares similar dune types with the Earth to a large extent, but is dominated by simple dune types. Complex and compound dunes are rarely seen on the Mars so that the cumulative probability curves of dune morphological parameters are characterized by a single section. Barchans, barchanoid ridges and transverse ridges are the most popular, but linear dunes that account for about 50% on the Earth only have 10% on the Mars. The simplicity of dune geomorphology on the Mars is also manifested by the small patches of the dune fields the Martian dune-fields. The Martian dunes are generally smaller on the Mars, in contradiction with theoretical inferences that the scale of dunes increases the ratio of particle/air density. The morphological parameters of the Martian dunes are well correlated, similar to the Earth dunes, but the correlationship is different, implying the similar formation mechanism but different development conditions. It is also suggested that the Martian analogues of terrestrial aeolian landforms, such as those found on the Tibetan Plateau, are good examples to study the Martian aeolian geomorphology. The simplicity of dune geomorphology on the Mars probably suggests that dunes on the Mars are developed with insufficient sediment availability and short development history. Inactive recent exogenic processes are probably responsible for insufficient sediment availability on the Mars. Insufficient sediment availability would be an interesting topic for future research, which may have extensive earth science significances.