Sediment Gravity Flow Deposits Research Articles

Deep‐water sand transfer by hyperpycnal flows, the Eocene of Spitsbergen, Arctic Norway

AbstractFlood‐generated hyperpycnal flows are dense, sediment‐laden, turbulent flows that can form long‐lived, bottom‐hugging turbidity currents, which undoubtedly transport large volumes of fine‐grained sediments into the ocean. However, their ability in transferring sand into deep‐water basins is debated. This study presents sedimentological evidence of sandy hyperpycnal flow deposits (hyperpycnites) in a series of basin floor lobe complexes associated with a progradational shelf margin in the Eocene of Spitsbergen, Arctic Norway. Four coexisting types of sediment gravity flow deposits are recognized: (i) sandy hyperpycnites deposited by quasi‐steady hyperpycnal flows; (ii) turbidites deposited by waning, surge‐type turbidity currents; (iii) hybrid event beds deposited by transitional flows; and (iv) mass transport deposits emplaced during rare slope failures. The abundance of thick‐bedded massive sandstones, frequent bed amalgamation, the distribution of hyperpycnites across the lobes and the abundance and systematic occurrence of plant‐rich hybrid event beds and associated climbing ripple cross‐laminated beds in the lobe fringes, suggest that hyperpycnal flow was the most important mechanism driving lobe progradation. Shelf‐edge positioned fluvial channels linked to the basin floor lobe complexes via deeply incised, sandstone‐filled slope channels, suggest that rivers fed directly onto the slopes where their dense, sand‐laden discharges readily generated quasi‐steady hyperpycnal flows that regularly reached the basin floor. The composite architecture and complex waxing–waning flow facies configuration of the hyperpycnites is consistent with sustained and concomitant suspension and traction deposition under fluctuating subcritical to supercritical conditions. Similar sandstone beds occur on the clinoform slopes, indicating that the hyperpycnal flows operated likewise on the slope. Plant‐rich hybrid event beds indicate transformation of initially turbulent flows by relative enrichment of clay and plant material via progressive sand deposition to such an extent that it suppressed turbulence. The multi‐faceted character of the hyperpycnites reported here, challenges traditional beliefs that hyperpycnites assumingly preserve the waxing–waning signal of single‐peaked floods.

Reconstructing terrestrial ice sheet retreat dynamics from hummocky topography using multiscale evidence: An example from central Ireland

The research reported here combines high-resolution digital elevation models (DEMs) derived from airborne LiDAR with ground-penetrating radar (GPR) surveys and macro- and micro-scale sedimentological analyses to study the geomorphology and internal structure and composition of ridges and mounds within an area of hummocky topography in the Brosna basin, central Ireland. Our evidence indicates that much of the hummocky topography consists of fragmented mega-scale glacial lineations (MSGLs) indicating a phase of accelerated ice flow, overlain by groups of small ridges composed of subglacially derived till and sediment gravity flow deposits. Subtle differences in ridge morphology indicate they may be multi-genetic and formed both ice-marginally as moraines and subglacially as small ribbed moraine. We interpret MSGL-ridge associations as a subglacial bedform continuum reflecting the evolution from a deforming bed to brittle deformation, due to changing thermal and hydrological conditions at the ice-bed interface during ice sheet retreat. Adjacent glaciofluvial landforms indicate initial en- and supra-glacial meltwater drainage, possibly directly related to changing basal thermal/hydrological characteristics and subglacial ridge formation. Subsequently a subglacial conduit system evolved, and later retreat involved formation of ice-marginal ridges. The partially preserved landsystem reflects formation at a polythermal to active temperate ice marginal zone, rather than the stagnating temperate margin previously assumed for this area. Our work demonstrates the usefulness of hummocky topography in identifying changes in ice sheet bed thermal/hydrological characteristics during deglaciation and the importance of combining multiple evidence strands in reconstructing the processes involved in glacial landform construction.

Departures from the archetypal deltaic ichnofacies

Abstract Recent work has focused on erecting new Seilacherian ichnofacies for depositional environments subject to recurring temporal and spatial variations in physico-chemical stress. In marine deltaic settings, these correspond to the Phycosiphon Ichnofacies for mudstone-dominated prodeltaic deposits and the Rosselia Ichnofacies for sandstone-dominated delta-front successions. The archetypal expressions of these ichnofacies, however, are founded on mixed process (wave- and river-influenced) systems, because the juxtaposition of ambient marine conditions during periods of prolonged wave energy with rapid deposition and physico-chemically stressed conditions during heightened fluvial discharge best expresses the deltaic signal. As deltaic settings shift towards end-member processes (e.g. river domination, wave domination and tide domination), or towards mixed-process conditions other than river and wave influence, the resulting ichnological suites and bioturbation fabrics depart from the recently published archetypes. Using selected studies of marine deltaic deposits, predictable departures from the archetypes can be recognized on the basis of these changing processes and their associated physico-chemical stresses. River-dominated delta deposits and tide-dominated delta successions display the greatest deviation from the published archetypes. River-dominated examples show elevated deposition rates, periods of salinity reduction, slumping and dewatering, elevated water turbidity, flood-induced sediment gravity flows and hypopycnal-generated fluid mud. As a result, river-dominated successions are largely devoid of bioturbation. Evidence of marine conditions is commonly restricted to isolated occurrences of dwelling structures such as Arenicolites , Ophiomorpha or Rosselia in sandstone, and Chondrites , Phycosiphon or Zoophycos in mudstone beds, particularly in prodeltaic intervals. Tide-dominated deltaic successions are markedly heterolithic and typified by highly mobile substrates manifested by incrementally migrating asymmetric bedforms and abundant fluid mud. Such settings are also prone to marked changes in salinity and shifts in the position of the turbidity maximum zone. Successions typically show low intensities of bioturbation and sporadically distributed burrows, as well as deposit-feeding structures, deeply penetrating dwelling structures or fugichnia. Many trace fossil suites consist entirely of facies-crossing elements, making assignment to an ichnofacies impossible. Storm flood-dominated deltaic successions are characterized by tempestites that are typically interstratified with river flood-induced sediment-gravity flow deposits and/or mantled by largely unburrowed mudstone drapes derived from hypopycnal plumes associated with river floods. Where these storm flood cycles are interstratified with ambient fairweather beds, assignment to the archetypal deltaic ichnofacies is straightforward. However, as storm beds become increasingly erosionally amalgamated, the preservation potential of the fairweather beds is reduced and the resulting trace fossil suites are biased towards those recording opportunistic colonization of the event beds. The presence of mudstone layers with low bioturbation intensity (BI) containing small numbers of ichnogenera positively correlated with marine conditions (e.g. Chondrites , Phycosiphon and/or Zoophycos ) may be the only evidence that the suites should be assigned to one of the deltaic ichnofacies. Wave-dominated deltas lacking significant storm influence are typically challenging to differentiate from their archetypal strandplain shoreface counterparts and, correspondingly, the resulting trace fossil suites are broadly comparable to the archetypal Cruziana and Skolithos ichnofacies. Most of the preserved record of wave-dominated delta successions is related to fairweather ambient conditions, and so facies typically show high BI values and uniformly distributed bioturbation. Key to recognizing that the suites should be assigned to one of the deltaic ichnofacies is the presence of rare river-generated mudstone and sandstone beds that display evidence of physico-chemical stress and/or the paucity of domichnia typical of suspension-feeding organisms. In most delta types, the prodeltaic facies are most readily discerned to contain trace fossil suites of the Phycosiphon Ichnofacies, owing to the higher preservation potential of all depositional processes, including marine fairweather beds, river-supplied hyperpycnites and other sediment gravity flow deposits, tempestites and fluid mud derived from river flood-related hypopycnal plumes. Assignment of trace fossil suites to the Rosselia Ichnofacies requires some record of the fairweather conditions, which are generally diminished in river-, tide- and storm-dominated successions. The dominance of structures positively correlated to deposit-feeding ethologies at the expense of those attributed to suspension-feeding strategies may point to elevated water turbidity and assignment of the suite to the Rosselia Ichnofacies. However, in many cases, the ichnological suites of delta fronts are so depauperate that assignment to an ichnofacies is problematic and should be avoided.

Implications of Late Palaeozoic postglacial marine transgressive‐regressive (T‐R) cycles recorded in the Talchir Formation, Son Valley Basin, peninsular India: A sequence stratigraphic paradigm

The signature of Late palaeozoic glaciation, a pan‐Gondwanaland palaeoclimatic event, is recorded in the Talchir Formation (Late Carboniferous to Early Permian), the basal‐most litho‐unit within the Gondwana Supergroup (Late Carboniferous–Early Cretaceous) in peninsular India. About 200 m thick sedimentary succession of the Talchir Formation, exposed in the Hasdeo River section in the Son Valley Basin, Central India, manifests signatures of deposition by melting of glaciers near to the ice‐grounding line. Evidence of glacial influences is prevalent in the lower‐middle part of the succession and represented by glacial tillites with faceted/striated gravels and bullet‐shaped clasts, dropstones and ice‐rafted debris (IRD), associated with various sediment gravity flow deposits. In the middle‐upper part of the succession, records of post‐glacial marine incursions are preserved as thick sandstone and sandstone‐mudstone heteroliths of a shallow marine shore‐shelf depositional regime. These are characterized by abundant tidal and wave signatures, with marine bivalve fossils and trace fossils of marine invertebrates. The upward decrease in size and abundance of the IRDs indicates a gradual decrease in glacial influences, with landward incursion of the marine depositional systems over the proglacial systems. Multiple stacked progradational and retrogradational stratal stacking patterns formed by alternating glacigenic and marginal marine dominated processes signify multiple transgressive‐regressive (T‐R) cycles, manifesting repeated advancements and retreats of the glacier, associated with sea level rise and marine incursions into the land. The top part of the succession, characterized by fine‐grained siltstone/mudstone without any IRD, suggests sedimentation at a relatively deeper part of the basin, indicating the strong influence of the eustatic rise caused by significant melting of the retreating ice sheet. Stacked T‐R cycles bounded by distinct facies architecture define multiple system tracts confined within three major deglaciation sequences within the studied Talchir Formation, suggesting multiple phases of advance‐retreats of the ice sheet and significant marine onlaps within the continental Gondwanaland under the Late Palaeozoic icehouse‐greenhouse climatic change.

Sedimentology and sequence stratigraphy of lacustrine deep-water fine-grained sedimentary rocks: The Lower Jurassic Dongyuemiao Formation in the Sichuan Basin, Western China

Despite the sedimentology of deep-water fine-grained sedimentary rocks from marine environments having received highlighted attention in recent years, their counterparts from the lacustrine environments are rarely reported. Based on a complete sedimentological, petrographic, and geochemical analysis of three continuous cored wells of the Lower Jurrasic Dongyuemiao Formation, Sichuan Basin, West China, this study illustrates the range of lithofacies, cyclicities, and transportation and deposition mechanisms of this lacustrine deep-water succession. The aim of this research is to establish a synthesis depositional model for lacustrine deep-water fine-grained sedimentary systems in the Sichuan Basin. The results indicate that seven lithofacies were developed in the Dongyuemiao Formation and they were stacked in a repeated pattern to constitute ca 10–20 m thick composite cycle sets. A typical composite cycle set comprises an upper and lower unit. Specifically, the lower unit is characterized by basal relatively coarse-grained detrital-rich lithofacies interpreted to record gravity flows that grade upward into relatively fine-grained detrital-lean lithofacies interpreted as suspension deposits from the water column. The upper unit is stacked in a reverse pattern and characterized by basal relatively fine-grained detrital-lean lithofacies that grade upward into relatively coarse-grained detrital-rich lithofacies. Such cyclicities in the Dongyuemiao Formation are associated with third-order lake level variations. Specifically, the relatively coarse-grained detrital-rich lithofacies is interpreted to be deposited from low-density turbidity currents, likely hyperpycnal turbidity flows, during maximum lake level regression when terrigenous fluvial and delta systems developed in the basin margin. The relatively fine-grained detrital-lean lithofacies is interpreted to be deposited from suspension fallout during maximum lake level transgression. This cyclicity pattern and frequent occurrence of sediment gravity flow deposits in the Dongyuemiao Formation suggests a dynamic hydrographic condition in the deep-water lacustrine environment of the Sichuan Basin.

Reconsidering the glaciogenic origin of Gondwana diamictites of the Dwyka Group, South Africa

Abstract The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, as are diamictites which have been interpreted as in-situ or reworked tillites. These interpretations were tested by investigation of outcrops in formerly well-studied areas, throughout South Africa. Detailed analyses have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreted to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture combinations of fracturing and irregular abrasion associated with Quaternary glacial deposits, but mainly a mix of surface microtextures associated with multicyclical grains. The Dwyka Group diamictites warrant reinterpretation as non-glacial sediment gravity flow deposits.

Organic Matter Burial in Deep-Sea Fans: A Depositional Process-Based Perspective

Organic matter burial in the deep-sea fan sediments is an important component of the long-term carbon cycle. Although there is increasing recognition of the importance of organic matter in deep-sea sediments, a major focus has been on mudstones, commonly interpreted as the background sediments, deposited by pelagic or hemipelagic vertical suspension fallout in low-energy fan environments. Emerging evidence suggests that relatively coarse-grained sediment gravity flow deposits (e.g., turbidites and hybrid event beds) can also store a significant quantity of organic carbon, implying that a wide range of depositional processes can result in the concentration and enrichment of organic matter in submarine fans. However, the role of these processes on carbon burial is still not fully understood. This review aims to discuss the impact of three widely documented deep-sea depositional mechanisms/processes, namely vertical suspension settling, grain-by-grain (incremental aggradation), and the en-masse deposition on distribution, burial, and preservation of organic matter in deep-marine deposits. Organic matter accumulated from slowly settling suspension in mud caps (Te or H5 divisions of turbidites and hybrid beds, respectively) is prone to higher oxidation compared to the carbon buried in sandy components of turbidity currents (Ta-Tc units) and hybrid beds (H2/H3 divisions). The burial of organic matter in sandy parts of the deposits has important implications for understanding the fundamental physical processes that control carbon accumulation and preservation in deep-marine rock record.

Variations in water discharge at different temporal scales in a mud-prone alluvial succession: The Paleocene-Eocene of the Tremp-Graus Basin, Spain

The late Paleocene to earliest Eocene sedimentary record in the eastern Tremp-Graus Basin (southern Pyrenees, Spain) consists of a mud-prone fluvial succession informally known as ‘Upper Red Garumnian’, which is dominantly composed of non-channelized deposits interspersed with fluvial channel bodies. The succession records evidence of variations in water discharge at multiple temporal scales. One-hundred-and-eighty sand-prone and conglomeratic bodies present within otherwise fine-grained deposits have been examined by facies and architectural-element analyses. These have been assigned to four distinctive types: (i) simple and (ii) compound channelized deposits, (iii) multilateral and multistorey elements, and (iv) sediment gravity-flow deposits. Simple and compound channel bodies represent deposits of channelized high-energy flows associated respectively with single floods or multiple events occurring in the same channel belt. Multilateral and multistorey elements are associated with sedimentation in longer-lived channel belts. An increase in the occurrence of these latter elements through the stratigraphy can be related to changes in tectonic and/or climatic drivers, heralding a change in facies architecture that signifies a shift to perennial discharge conditions and incised-valley backfilling. This is itself followed by a marked change in architectural style in a part of stratigraphy recording the Paleocene-Eocene Thermal Maximum (PETM), when variations in precipitation regime and/or intrabasinal characteristics (e.g., vegetation) related to this event favoured the lateral migration of channels, causing the development of laterally extensive coarse-grained bodies. Floodplain sediments comprise (i) heterolithic deposits accumulated in the vicinities of the fluvial channels where riparian ecosystems were developed, and (ii) overbank mudstones with variable degree of pedogenesis, especially associated with wetting and drying cycles in areas of the floodplain located distally away from fluvial channels. This succession records the importance of water-discharge variations in the evolution of alluvial systems: it displays facies arrangements recording the effects of highly variable discharge in ephemeral to perennial channels, as well as variations in stratigraphic architecture testifying to longer-term hydrological changes in the late Paleocene and earliest Eocene.

Sequence stratigraphic evolution of the Kumano forearc basin during the last deglaciation: Influence of eustasy and tectonically-controlled shelf morphology on deep-marine sediment dynamics

We focus here on the different forcings that controlled the deep-marine sediment dynamics and associated stratigraphic pattern in the Nankai subduction zone, especially in the Kumano forearc basin. In this study, based on high-resolution subbottom profiler data, we investigate the Upper Pleistocene–Holocene deposits of the tectonically active Kumano forearc basin to better understand the role of glacio-eustatic forcing and tectonic-morphological controls on the sedimentation patterns in the basin. We recognize three distinct seismic units that characterize the different stages of the last to current interglaciation cycle. Chaotic debris- and mud-flow deposits, intercalated with turbidites accumulating mainly in the deep basin, are related to a lowstand stage when rivers were directly connected to canyon heads. The transgressive stage, covering the deglaciation period, is characterized by a high proportion of turbidites and a decrease in chaotic deposits. The flooding and development of the Ise Bay at ~10 ka, resulted in starvation of the deep basin where mostly hemipelagic deposits occur. However, some dilute flows still reached the proximal area of the basin depositing some turbidites. We identify an interplay between the global sea-level rise and the shelf morphology, both having a strong control on the deep-marine stratigraphic pattern of the Kumano Basin. The rise of sea level triggered a disconnection between the river mouth and canyon head, thus inducing a general retrogradation of sediment-gravity flow deposits throughout the deglaciation. The shelf morphology, which is mainly driven by tectonic uplift (narrow shelf) or subsidence (wide shelf), directly played on the connection between the river and deep-marine system and on the accommodation space in the shallow-marine environment. Although it does not appear as a dominant control, climatic control, especially the influence of the East Asian summer monsoon during deglaciation cannot be neglected. It most likely contributed to an increase in sandy sediment supply occurring at the beginning of the transgressive stage.The highlight of our study is related to the tectonically-controlled shelf morphology and its role on the deep-marine stratigraphic pattern. The northwestern area of the Kumano Basin, undergoing significant tectonic-related uplift, is defined by a narrow shelf (<10 km) and sustained sediment supply to the basin throughout the deglaciation and highstand period. However, under similar eustatic and climatic conditions, the eastern area characterized by the development of the subsiding Ise bay (wide shelf > 90 km), shows a decrease in sand supply to the basin toward the end of the deglaciation. A strong control of the shelf morphology on the deep-marine stratigraphic pattern is also observed in other forearc basins along the Nankai subduction zone undergoing similar eustatic and climatic conditions.

Particle shape trends across experimental cohesive and non‐cohesive sediment gravity flow deposits: Implications for particle fractionation and discrimination of depositional settings

AbstractFractionation of particles in deep‐water sediment gravity flows is an important factor in the resulting deposit and for discriminating sedimentary environments, but remains poorly understood. Quantitative characterization of particle shape was performed for more than ten‐thousand particles of experimental gravity flow experiments (both of cohesive and non‐cohesive nature) made using coal and kaolin particles. Eleven particle shape parameters were calculated and their distribution and trends within the experimental basin were evaluated. Results indicate the existence of non‐normal distributions and observable correlations between particle shape parameters. Shape parameters such as circularity and roundness are dominant controls on shape variation. Strong correlations exist between mean shape parameters and along‐flow distance from the source for particles in non‐cohesive flow experiments. Important differences were observed between shape parameter distributions of particles sampled at different areas within the experimental basin, which can be grouped based on their depositional setting (proximal or distal) using multivariate statistical analysis, especially for the non‐cohesive flow experiments. A tendency for more elongated and irregularly‐shaped particles at the more distal and marginal areas of the studied experimental basin was observed and validated by previous field studies in real‐world deep‐marine deposits. Besides, fractionation of particles is less‐pronounced in cohesive flows compared with non‐cohesive ones suggesting the soundness of discrimination of depositional settings based solely on particle shape characteristics is strongly dependent on parent flow characteristics. Yet, results highlight the potential of particle shape analysis in revealing spatial particle shape trends due to hydrodynamic fractionation and discriminating different depositional settings within submarine fans. This methodology may be applied to seafloor and subsurface samples to help identify the flow process and depositional environment.

Conglomerate to mudstone lacustrine cycles revealed in Junggar Basin, Northwest China: Middle Permian Lucaogou and Jingjingzigou formations

The Junggar Basin is one of the very large, long-lived, Late Paleozoic lacustrine basins of China, developed by the closure of the early Paleozoic Junggar Ocean. The novelty of the reported research is outcrop measurements through the mid-Permian Jingzingzigou and Lucaogou formations, allowing reconstruction of interacting very coarse and very fine-grained alluvial-to-lacustrine depositional cycles within a km-thick succession of organic-rich mudstones. The deposits are interpreted as coarse grained fan deltas that extend subaqueously into Junggar lacustrine basin, depositing fine-grained muddy, sediment gravity flow deposits. The coarseness and frequency of both alluvial and subaqueous sediment gravity flows that thin and pinch out over short distances suggests that the margin of the basin was likely steep and fringed by relatively deep lake waters, but was not the main margin of sediment supply. The proposed depositional model highlights the process response to climate-driven lake-level changes around the steep lake margin. However, the coarse-grained alluvial to fan-delta cycles and the muddy lacustrine cycles occur at different thickness scales. The former (5–13 m thick) contain internal erosion surfaces and are dominated by river sheet flood and debris flow deposits and are likely incomplete due to frequent shifting, erosion and reworking of the sediments. The lacustrine cycles (few meters thick) are dominated by current-driven, very fine sandstone, siltstone and organic-rich mudstone beds with ripple lamination and graded beds, as well as dolomites during times or places where subaqueous currents were lacking. The age dating control is not detailed but we propose that depositional cycles were likely a response to Milankovitch orbital cyclicity, also suggested by previous Permian climate modeling, that cause alternating wet and dry periods and trigger tens of meters lake level changes with abrupt grainsize changes in the shallow proximal areas and more subtle changes in sediments of distal/deep lacustrine regions. The climate changes therefore cause both sediment supply changes subaerially and lake accommodation changes subaqueously, though these would also be modulated to a lesser extent in the resultant stratigraphy by autogenic responses and tectonic forcing of sediment yield.

Soft-sediment deformation structures in a lacustrine depositional context: An example from the Eocene Dongying Depression in the Bohai Bay Basin, East China

The study documents soft-sediment deformation structures (SSDSs) in the lacustrine Dongying Depression, a Paleogene synrift basin located in the southeastern corner of the Bohai Bay Basin, East China. The focus of this study is the lower part of the upper 4th member of the Eocene Shahejie Formation (Es4U), characterized by littoral to sublittoral deposits in the southwestern Dongying Depression. Nine lithofacies are grouped into the upper shoreface facies associations (FA1-1), lower shoreface facies associations (FA1-2), and sublittoral facies associations (FA2). Fine-grained calcareous sandstone was mainly deposited in FA1-1, with thinly alternating calcareous mudstone and siltstone or sandstone in FA1-2, while deposition in FA2 was dominated by mudstone, containing sediment gravity flow deposits. The SSDSs widely encountered in the lower Es4U can be classified into seven categories: (1) load structures, (2) water-escape structures, (3) folds and convolute lamination, (4) sediment-injection structures, (5) microfaults, (6) autoclastic breccias, and (7) slumps. Detailed observations of SSDSs from cores indicate liquefaction and fluidization as the main deformation mechanisms in the sands and muds. Moreover, rupturing of cohesive deposits also contributes to the soft-sediment deformation. The types and morphologies of the SSDSs are closely related to the rheologies of the sediments (e.g., the sand content), influenced by the facies that determine the lithologies and stratigraphic stacking patterns. Except for some deformation structures related to subaqueous sediment gravity flows (e.g., muddy debrite flows and slumps), most SSDSs are ascribed to rift-related seismicity due to the activity of the adjacent synsedimentary faults when the Dongying Depression underwent quick subsidence during the Eocene.

Coastal sandy spit deposits (Lower Burdigalian/Eggenburgian) in the Alpine-Carpathian Foredeep of Lower Austria

In the type-area of the Eggenburgian regional stage (Lower Burdigalian) sands with large-scale clinoforms were studied north-west of Eggenburg (Lower Austria). Stratigraphic and facies architecture, palaeocurrent pattern and inferred palaeogeographic setting show that these sands are deposits of W–E trending and SW to SSE prograding coastal spit systems, attached to crystalline shoals or islets in the shallow marine Eggenburg Bay. The spits were dominantly formed by shoal parallel accretion above fair-weather wave base due to longshore transport. The 4–5 m thick clinoforms with bottomset, foreset and topset structures contain up to 3.6 m thick, steeply inclined foresets, dominated by sediment gravity-flow deposition. Relatively stable depositional conditions, characterized by strong unidirectional currents, high sand supply and sufficient accommodation space are assumed for their formation. However, internal reactivation surfaces indicate variations in current activity, orientation and velocity. The deposits of spit systems are interpreted as part of a transgressive systems tract. The provenance analysis reveals the local Moravian and Moldanubian crystalline rocks as principal source. The spit sands of the Burgschleinitz Formation show the highest mineralogical maturity within the studied Lower Miocene succession. Intense reworking and redeposition of material from older deposits is evident. Due to the ongoing transgression deposits of the following Gauderndorf Formation and Zogelsdorf Formation exhibit a larger catchment area with input of high amounts of fresh weathered material.

Sediment gravity-flow deposits in Late Cretaceous Songliao postrift downwarped lacustrine basin, northeastern China

The Songliao Basin is a large Cretaceous nonmarine petroliferous postrift downwarped basin in northeastern China. Deep lake channel-levee-lobe and slope fan deposits are identified within six depositional sequences that span from the Turonian to early Coniacian upper part of the fourth member of the Quantou Formation and Qingshankou Formation to the early Campanian second member of the Nenjiang Formation. Research on the sediment gravity-flow systems in the Songliao Basin has broad application for understanding the sedimentary processes and pattern of sediment gravity-flow systems, and predicting the presence of good sandstone reservoirs within postrift downwarped lacustrine basins. Research results show that the channel-levee-lobe deposits are present within sequences in the first member of the Qinshankou Formation and the first member of the Nenjiang Formation. The straight channel levees and slope fans were on high-gradient slopes, whereas the sinuous channel levees were on low-gradient slopes. The straight-to-sinuous channel levees, fed by northeastern deltaic subaqueous distributary channels, extend for 15–70 km, predominantly within transgressive-to early highstand system tracts, which terminated in bifurcated or small lobes. The channel-levee-lobe and slope fan deposits sourced from western deltas were deposited basinward of the syndepositional flexural slope-break zones controlled by deep-seated faults. The falling trajectories of the paleoshorelines during the highstand and lowstand systems tracts and large sediment fluxes during the transgressive systems tracts resulted in the development of the slope fans and channel-levee-lobes. Lakebed paleorelief influenced the channel forms. Sand bodies within the channel-levee-lobes and slope fans, encased within organic-rich source rocks, represent new significant targets for hydrocarbon exploration.

Flow‐process controls on grain type distribution in an experimental turbidity current deposit: Implications for detrital signal preservation and microplastic distribution in submarine fans

AbstractDeep‐water depositional systems are the ultimate sink for vast quantities of terrigenous sediment, organic carbon and anthropogenic pollutants, forming valuable archives of environmental change. Our understanding of the distribution of these particles and the preservation of environmental signals, in deep‐water systems is limited due to the inaccessibility of modern systems, and the incomplete nature of ancient systems. Here, the deposit of a physically modelled turbidity current was sampled (n = 49) to determine how grain size and grain type vary spatially. The turbidity current had a sediment concentration of 17%. The sediment consisted of, by weight, 65% quartz sand (2.65 g/cm3), 17.5% silt (2.65 g/cm3), 7.5% clay (2.60 g/cm3) and 5% each of sand‐grade garnet (3.90 g/cm3) and microplastic fragments (1.50 g/cm3). The grain size and composition of each sample was determined using laser diffraction and density separation, respectively. The results show that: (a) bulk grain size coarsened axially downstream on the basin floor challenging the notion that basin floor deposits fine radially from an apex upon becoming unconfined; (b) no sample composition matched the input composition of the flow, indicating that allogenic signals can be autogenically shredded and spatially variable in sediment gravity flow deposits; and (c) microplastic fragments were concentrated in levee and lateral basin floor fringe positions; however, microplastic concentrations in these positions were lower than input, suggesting microplastics bypassed the sampled positions. These findings have implications for: (a) the development of ‘finger‐like’ geometries and facies distributions observed in modern and ancient systems; (b) interpreting environmental signals in the stratigraphic record; and (c) predicting the distribution of microplastics on the sea floor.

Magnetostratigraphy and stable isotope stratigraphy of the middle-Eocene succession of the Ainsa basin (Spain): New age constraints and implications for sediment delivery to the deep waters

Stratigraphic cycles preserved in sedimentary successions are controlled by the interaction of tectonics, climate, sediment supply and sea-level variations. Understanding the influence of these drivers on sedimentary systems dynamics is crucial to understand and extract information from sedimentary archives. In the deep marine deposits of the Ainsa foreland basin (lower to middle Eocene, southern central Pyrenees, Spain), the origin of well-preserved cyclicity between channelized sandy submarine fans and hemipelagic deposits remains subject to debate because of the absence of chronostratigraphic constraints on high resolution geochemical proxies. Here, we contrast a combination of newly acquired and legacy geochemical datasets (carbon and oxygen stable isotopes, organic matter content, major and trace elements and the mineralogical composition of bulk hemipelagic sediments) tuned by a new age model to global “target” curves to discuss the possible drivers of the stratigraphic cycles found in the deep marine sediment gravity flow deposits of the Hecho Group. Our new age model is based on magnetostratigraphic and biostratigraphic datasets acquired on a composite section going from the Banaston system to the distal deposits of the Sobrarbe deltaic complex. Four major polarity zones are identified in the studied succession covering the time span from C21n (lower members of the Banaston system) to C19r (Sobrarbe deltaic complex). Our age model is corroborated by the identification of the nannoplankton zone NP16 in the Guaso system (C20n).Comparison between global carbon isotope curve and local isotope dataset shows some differences, suggesting the preservation of a local isotope signal modulated by eustatism, changes in the environment of deposition and/or restriction in water circulation in the Ainsa basin. Yet, comparison of the stratigraphic record with global sea-level curves highlights that sands are mainly delivered to the basin during lowstands, pointing to the important role of eustasy even in an active foreland tectonic context. The exception to the rule is the Banaston member II, whose deposition during a sea-level highstand seems to be controlled by the growth of tectonic structures.

Flume experiments reveal flows in the Burgess Shale can sample and transport organisms across substantial distances

The exceptionally preserved fossils entombed in the deposits of sediment-gravity flows in the Cambrian Burgess Shale of British Columbia have been fundamental for understanding the origin of major animal groups during the Cambrian explosion. More recently, they have been used to investigate the evolution of community structure; however, this assumes that the fossil assemblage represents an in-life community. Here we test this assumption for the first time based on experimental and field approaches. We use flume experiments to create analog flows and show that transport of the polychaete Alitta virens over tens of kilometers does not induce significantly more damage beyond that already experienced due to normal decay processes. Integration of experimental results with taphonomic assessment of fossils and sedimentological analysis suggests that the organisms of the Burgess Shale in the classic Walcott Quarry locality could have undergone substantial transport and may represent a conflation of more than one community.

Facies characteristics and stratigraphy of an Upper Cretaceous mud‐dominated subaqueous delta: Medicine Hat Member (Niobrara Formation), Alberta, Canada

AbstractThe Medicine Hat Member in Alberta, Canada, is a muddy interval with thin sandstones. However, the sedimentology and stratigraphy are poorly understood – a reflection of the limited knowledge of mud‐dominated successions. Here facies analysis and stratigraphy from core and wireline logs were used to construct a depositional framework for the interval in a stratigraphical context. Sedimentological and ichnological results show that the Medicine Hat Member was deposited in shallow water within a mud‐dominated subaqueous delta system under the combined influence of river flow, waves, storms and longshore currents. Four facies are recognized, which are interpreted as distal delta front, proximal prodelta, distal prodelta and interdistributary bay deposits. The distal delta front is dominated by sandy storm deposits and thin storm‐flood hyperpycnites. The prodelta, on the other hand, is characterized by muddy storm deposits and wave‐enhanced sediment gravity flow deposits, with only subordinate hyperpycnites. Ichnological assemblages in both the delta front and prodelta are impoverished, indicating persistent physico‐chemical environmental stresses. Interdistributary bay deposits are uncommon and are differentiated based on more robust ichnological assemblages and higher bioturbation intensities. Subsurface correlations reveal shallowing‐upward parasequences that have aggradational and progradational stacking patterns and a very low depositional gradient (ca 0.03° to 0.1°). The stratigraphy reflects rapid progradation with fixed accommodation in a low‐gradient setting; coeval flexural subsidence in the foredeep was dampened by underlying basement structures. Furthermore, previous suggestions that the Bow Island Arch impacted sediment distributions are not substantiated by the low angle geometry of clinoforms. Data from this study supports the interpretation that Medicine Hat deltas were fed by rivers flowing north through Montana because parasequences are sandier towards the south. This study fills a crucial gap in the literature by providing a rock record example of a mud‐dominated subaqueous delta system and serves as a basis for comparison in core studies of similar units globally.

Evolution of a sand-rich submarine channel–lobe system, and the impact of mass-transport and transitional-flow deposits on reservoir heterogeneity: Magnus Field, Northern North Sea

The geometry, distribution and rock properties (i.e. porosity and permeability) of turbidite reservoirs, and the processes associated with turbidity current deposition, are relatively well known. However, less attention has been given to the equivalent properties resulting from laminar sediment gravity-flow deposition, with most research limited to cogenetic turbidite debrites (i.e. transitional-flow deposits) or subsurface studies that focus predominantly on seismic-scale mass-transport deposits (MTDs). Thus, we have a limited understanding of the ability of subseismic MTDs to act as hydraulic seals, and their effect on hydrocarbon production and/or carbon storage. We investigate the gap between seismically resolvable and subseismic MTDs, and transitional-flow deposits on long-term reservoir performance in this analysis of a small (&lt;10 km-radius submarine fan system), Late Jurassic, sandstone-rich stacked turbidite reservoir (Magnus Field, Northern North Sea). We use core, petrophysical logs, pore fluid pressure, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and 3D seismic-reflection datasets to quantify the type and distribution of sedimentary facies and rock properties. Our analysis is supported by a relatively long (c. 37 years) and well-documented production history. We recognize a range of sediment gravity deposits: (i) thick-/thin-bedded, structureless and structured turbidite sandstone, constituting the primary productive reservoir facies (c. porosity = 22%, permeability = 500 mD); (ii) a range of transitional-flow deposits; and (iii) heterogeneous mud-rich sandstones interpreted as debrites (c. porosity ≤ 10%, volume of clay = 35%, up to 18 m thick). Results from this study show that over the production timescale of the Magnus Field, debrites act as barriers, compartmentalizing the reservoir into two parts (upper and lower reservoir), and transitional-flow deposits act as baffles, impacting sweep efficiency during production. Prediction of the rock properties of laminar- and transitional-flow deposits, and their effect on reservoir distribution, has important implications for: (i) exploration play concepts, particularly in predicting the seal potential of MTDs; (ii) pore-pressure prediction within turbidite reservoirs; and (iii) the impact of transitional-flow deposits on reservoir quality and sweep efficiency.Supplementary material: of data and methods are available at https://doi.org/10.6084/m9.figshare.c.5313860

Syn‐rift sediment gravity flow deposition on a Late Jurassic fault‐terraced slope, northern North Sea

AbstractStructurally controlled bathymetry in rifts has a significant influence on sediment routing pathways and depositional architecture of sediment gravity flow deposits. In contrast to rift segments characterized by crustal‐scale half‐grabens, the tectono‐stratigraphic evolution of deep‐water rift domains characterised by distributed faulting on narrow fault terraces has received little attention. We use 3D broadband seismic data, calibrated by boreholes, from the Lomre and Uer terraces in the northern North Sea rift to investigate Late Jurassic syn‐rift sediment gravity flow systems on fault‐terraced slopes. The sediment gravity flow fairways were sourced from hinterland drainages via basin margin deltaic systems on the Horda Platform to the southeast. The deep‐water sedimentary systems evolve from initial, widespread submarine channelized lobe complexes, through submarine channels, to incised submarine canyons. This progressive confinement of the sediment gravity flow system was concomitant with progressive localization of strain onto the main terrace‐bounding faults. Although the normal fault network on the terraces has local impact on deep‐water sediment transport and the architecture of gravity flow deposits, it is the regional basin margin to rift axis gradient that dominantly controls deep‐water sediment routing. Furthermore, the gravity flow deposits on the Lomre and Uer terraces were predominantly sourced by rift margin deltaic systems, not from erosion of local uplifted footwall crests, emphasising the significance of hinterland catchments in the development of volumetrically significant deep‐water syn‐rift depositional systems.