Sedimentology

doi: 10.1111/sed.13012pmid: N/A

Liu, Shan; Yang, Chupeng; Yin, Shaoru; Zhuo, Haiteng; Su, Ming; Luo, Kunwen; Xu, Ziying; Zhu, Rongwei; Liang, Zijun

doi: 10.1111/sed.13092pmid: N/A

Active continental margins are tectonic sites with complex sedimentary processes. Tectono–sedimentary interactions occur over geological time and determine the modern morphology of the margin. However, such interactions have been poorly documented in high‐latitude regions. This study focuses on deep‐water sedimentary systems on the South Shetland convergent margin, which is the only remaining active continental margin in Antarctica. Numerous gullies, 20 canyons and three contourite drifts are identified by interpreting bathymetry, oceanography and reflection seismic data. Heavy mineral contents of a gravity core, located at a downslope elongated mounded drift, show the asynchronous interplay of downslope and alongslope processes. The rest of the margin is dominated by turbidity currents. Variations in the slope gradient control thalweg profiles and sinuosity of canyons. Tectonic‐related morphology restricts the distribution of contourite drifts. Earthquakes triggered by underplating and tectonic compression may induce slope instability at the lower flank of the plastered drift. Tectonic uplift influences the amount of sediments transported by ice sheets and controls the shelf stacking pattern. The mixed turbidite–contourite system transitions to separated canyon and drift systems from passive to active continental margins under the influence of the tectonic‐related morphology. A systematic comparison has been made among several convergent margins with similar tectonic settings. Results suggest that submarine canyons on these shelf‐slope systems have similar morphological features and are comparable in size. Coarse sediment input and steep continental slopes (mean slope 7 to 10°) control the canyon morphology on these margins. Therefore, this research has important implications for furthering the current understanding of sedimentary systems on high‐latitude convergent margins and tectono–sedimentary interactions.

Martinez‐Doñate, Ander; Kane, Ian A.; Hodgson, David M.; Privat, Aurelia M.‐L. J.; Jackson, Christopher A.‐L.; Schwarz, Ernesto; Flint, Stephen S.

doi: 10.1111/sed.13086pmid: N/A

The Early Jurassic Los Molles Formation in the Neuquén Basin of western Argentina is a rare example of well‐exposed syn‐rift to post‐rift stratigraphy. In the Chachil Graben, the onset of the early post‐rift stage is marked by drowning of a carbonate system and the development of two deep‐marine intraslope lobe complexes. This field‐based study in the Chachil Graben involved field mapping and correlating eleven stratigraphic logs, and petrographic analysis to document how grain size and texture within intraslope lobe sandstones change from the lobe centre to their frontal pinch‐out. Eight different bed‐scale facies are identified and inferred to be formed by turbulent (turbidites; Type A and B beds), transient turbulent–laminar (transitional flow deposits; Type C, D, E and F beds), laminar gravity flows (debrites; Type G) and post‐depositional clastic injections (injectites; Type H beds). Fifteen lobes form two stacked lobe complexes that show stratigraphic evolution from a lower argillaceous sandstone‐dominated lobe complex, built by transitional flow deposits, to an upper coarser‐grained, sandier lobe complex largely constructed by turbidites. Petrographic analysis quantified sandstone mineralogy, matrix content, grain size and sorting, revealing that both lobe complexes are volcanic arc‐sourced. This study proposes that the differences in the character of the two lobe complexes are due to maturation of sediment transport routes through progressive healing of the intraslope relief, with a concomitant decrease in substrate erosion and flow bulking. Also proposed here is a model for intraslope lobe complex development that accounts for the impact of flow‐confinement on flow behaviour and transformation induced by the inherited topography. Bed type distribution suggests that high‐density flows terminate more abruptly against confining slopes and produce greater depositional variability than lower‐density flows. This integrated petrographic, architectural and sedimentary process model provides new insights into how post‐rift intraslope lobe systems may act as hydrocarbon reservoirs, aquifers and carbon storage sites.

Hüneke, Heiko; Gibb, M. Arwed; Mayer, Oliver; Kniest, Jorit F.; Mehlhorn, Paul; Gibb, Laura M.; Aboussalam, Z. Sarah; Becker, R. Thomas; El Hassani, Ahmed; Baidder, Lahssen

doi: 10.1111/sed.13089pmid: N/A

The study examines bioclastic carbonate contourites that arise from the broad spectrum of bottom‐current related sedimentary processes ranging from deposition to erosion. The result of the intermittent accumulation of sediment are thin and condensed successions with abundant hiatuses. Such bottom‐current deposits are poorly known, since the broadly accepted contourite‐facies model, the bi‐gradational sequence, characterizes environments of contourite depositional systems as a continuous accretion of fine‐grained siliciclastic sediments. To increase current understanding of the carbonate facies within hiatal contourite records, the Eifelian–Frasnian of the Tafilalt Platform in Morocco was investigated. The succession is divided into five facies associations that are interpreted to reflect pelagic sedimentation and deposition from bottom currents on a contourite terrace, a gently inclined section of the upper slope of Gondwana shaped by a water‐mass interface. Contourite deposition was mainly controlled by oxic clear‐water currents (documented by moderately to completely bioturbated limestones with abundant hydrogenetic ferromanganese nodules, and low organic‐carbon contents), at times also by an anoxic water mass (featured by organic‐rich coquinas with absent to sparse bioturbation and predominantly syngenetic framboidal pyrites). Biostratigraphic data and the overall depositional architecture display palaeoceanographic hydrodynamic processes associated with a shifting water‐mass interface. The inner terrace was characterized by an alongslope contourite channel and a small mounded drift at its downslope margin. Energetic bottom currents furthermore caused abraded surfaces, i.e. plain areas of non‐deposition and localized erosion, and sandy condensation layers. The microfacies reflects repeated alternation between suspension deposition, winnowing of fines, bedload traction, dynamic sediment bypassing and reworking, together with concomitant seafloor cementation. Coquinas of mainly planktonic and nektonic organisms are identified as integral parts of bi‐gradational contourite sequences showing inverse and normal grading. Hiatal lag concentrations of carbonate intraclasts, ferromanganese nodules and conodonts often drape hardgrounds and erosional surfaces at the midpoint of these frequently incomplete sequences. This Devonian case provides the opportunity to investigate the spatial and temporal variability of the bed‐scale contourite sequence, also with regard to the drift‐scale depositional architecture. In addition, the identified high‐resolution record is a starting point for unravelling the pattern of oceanic circulation in the Devonian greenhouse world.

Wilckens, Henriette; Schwenk, Tilmann; Lüdmann, Thomas; Betzler, Christian; Zhang, Wenyan; Chen, Jiayue; Hernández‐Molina, F. Javier; Lefebvre, Alice; Cattaneo, Antonio; Spieß, Volkhard; Miramontes, Elda

doi: 10.1111/sed.13093pmid: N/A

The interaction of sedimentary systems with oceanographic processes in deep‐water environments is not well understood yet, despite its importance for palaeoenvironmental reconstructions, and for a full understanding of source‐to‐sink sediment transport. The aim of this study is to improve the understanding of how contourite moats, elongated depressions formed by bottom currents associated with contourite drifts, develop and of the link between moat‐drift system morphology and bottom current dynamics. This study provides a systematic comparison of 185 cross‐sections of moat‐drift systems distributed at 39 different locations worldwide, and a detailed analysis of the morphology of six moats that cover a wide range of typical geological and hydrodynamic settings. Additionally, in situ measured current data were analysed to better link hydrodynamics to moat morphology. The median of all profiles across all moat‐drift systems reveals a 50 m relief, a width of 2.3 km, a relief to width ratio of 0.022, a slope angle of 6°, a drift angle of 3° and a concave‐up shaped morphology. Moats can be over 100 km long. Some moats are driven by sediment erosion while others are depositional and primarily exist due to differential sedimentation inside the moat compared to the drift alongside the moat. A new sub‐classification of moat‐drift systems based on their stratigraphy is proposed. This classification distinguishes moats depending on the degree of erosion versus deposition. No relation is found between latitude and moat‐drift morphology or stratigraphy in the analysed examples. The combined data indicate that a steeper slope focuses the current more than a gentle slope, resulting in an increase of the relief–width ratio and drift angle. Thus, this study provides new insights into the interaction of ocean currents with sedimentary morphology, which thereby affects the evolution of a poorly understood deep‐water sedimentary system.

Vermassen, Flor; Van Daele, Maarten; Praet, Nore; Cnudde, Veerle; Kissel, Catherine; Anselmetti, Flavio S.

doi: 10.1111/sed.13094pmid: N/A

Megaturbidites are commonly used to reconstruct the seismic history (palaeoseismology) of areas where large earthquakes occur. However, the depositional mechanisms and sedimentary characteristics of these deposits are not yet fully understood. This study unravels the sequence of sediment deposition that occurred in Lake Lucerne (Vitznau Basin) following the 1601 ce earthquake in central Switzerland. During this event, slope failures were triggered, generating mass flows and turbidity currents that led to the formation of mass‐transport deposits and a megaturbidite. These deposits are sampled in 28 sediment cores, which are examined with X‐ray computed tomography scans (medical and μCT), grain‐size analysis and natural remanent magnetisation. This suite of analyses allows a detailed reconstruction of turbidite stacking and amalgamation in the centre of the basin, followed by settling of finer sediments influenced by a lake seiche. Initial deposition of mass‐transport deposits is followed by sandy turbidites reaching the depocentre. Some of these turbidite sands can be linked to their source areas, and evidence is found of some turbidites being overridden by mass flows in the peripheral parts of the megaturbidite deposit. Hereafter, sedimentation becomes controlled by seiche‐induced currents, which rework fine sediments upon deposition, leading to subtle grain‐size variations at the base of the seiche‐influenced sub‐unit and a ponded geometry of the megaturbidite. As the seiche movement dampens, a relatively muddy, homogeneous sub‐unit is deposited that drapes the basin plain. Overall, this study provides the first highly detailed sedimentological analysis of megaturbidite deposition in a lake, demonstrating the distinct sedimentological imprint of lake seiching and turbidite amalgamation/stacking. This will improve the recognition and interpretation of earthquake‐induced megaturbidites in other lake records or isolated basins, and demonstrates the value of using (μ)CT scans in combination with traditional sedimentological parameters to reconstruct the depositional processes of megaturbidites.

Abu‐Mahfouz, Israa S.; Cartwright, Joe A.; Powell, John H.; Abu‐Mahfouz, Mohammad S.; Podlaha, Olaf G.

doi: 10.1111/sed.13085pmid: N/A

This paper presents an integrated petrographic–geochemical–geomechanical study of the growth mechanisms of carbonate and chert concretions observed at outcrop and core from the Upper Cretaceous to Eocene organic‐rich carbonate mudrocks, central Jordan. It provides evidence for displacive and replacive concretion growth from the analysis of primary lithological characteristics, compaction strain and deformation structures associated with concretion growth. Concretions were analysed to determine the primary lithological controls on their development and the measurement of strain in the host rock to develop a method for constraining the growth mode and their paragenesis. Concretions exhibit either a replacive or displacive growth mode largely dependent on the original host lithology. Displacive concretions exhibit irregular shapes and semi‐fibrous internal structures in contrast to regular shapes and microcrystalline textures observed for replacive concretions. Cement fraction is high in both carbonate concretion types, indicating early formation in high‐porosity sediments at shallow burial depths. The strain field around displacive concretions is vertically asymmetrical. Conversely, it is symmetrical with uniform differential compaction for the replacive concretions. Evidence for displacive growth comes from triangular areas of chert at the lateral margins of some carbonate concretions, interpreted as areas of reduced strain. Another indicator is the forced asymmetrical folding of heterolithic host rocks around displacive concretions, with displacive carbonate units separated by trace laminae of the original (chert) beds. Enveloping chert beds exhibit early‐formed radial silica fractures with increased aperture size in the areas of maximum curvature. Carbon isotopic signatures of carbonate concretions show a strong correlation between concretion centres and host rock, suggesting a relatively shallow depth (first few tens of metres) of initial growth. Carbonate concretions are interpreted to have formed at shallow depths in the presence of alkaline pore waters rich in dissolved organic carbon in the presence of Mg2+ ions, available organic matter and redox‐sensitive metals such as U and Mo. A paragenetic history for the different concretion types is presented.

Smith, Megan E.; McNeill, Donald F.; Murray, Sean T.; Swart, Peter K.

doi: 10.1111/sed.13087pmid: N/A

Carbonate concretions collected from the Dominican Republic present a valuable opportunity to evaluate the internal isotopic variations within concretions that have never been exposed to deep burial or structural deformation. Here, three concretions from the Neogene (Late Miocene–Early Pliocene) Cibao Basin are investigated, utilizing a multi‐isotope (δ13C, δ18O, δ34SCAS and ∆47 values) high‐resolution approach, to constrain the microenvironmental conditions associated with multiple stages of concretion growth. Isotopic variability and potential disequilibrium effects, which can influence geological interpretations utilizing concretions, are also considered. The petrographic characteristics and geochemical profiles indicate internal differences relating to concretion growth mechanisms and environmental changes, driven by sea‐level fluctuations. The δ34S values of carbonate‐associated sulphate indicate a closed system environment; however, the overall values are influenced by sulphide oxidation within the sediments, resulting in a complex signal. The ∆47‐derived temperatures of the concretions range between 29 to 55°C, indicating significantly warmer temperatures than are measured from the host sediments, which average 24°C. This indicates that carbonate concretion ∆47 values are in disequilibrium with their environments of formation, likely related to ion diffusion in the pore fluids or isotopic fractionation associated with microbial processes. Here geochemical variations within concretions are utilized to assess the environmental conditions and microbial interactions after sediment deposition. However, for future studies, caution should be taken when using concretions for making environmental assessments as the signals can be influenced by a multitude of processes, even prior to diagenetic alteration.

Rieux, Alissia; Weill, Pierre; Mouazé, Dominique; Tessier, Bernadette

doi: 10.1111/sed.13083pmid: N/A

Coastal barriers are dynamic systems, the morphology and architecture of which are controlled by local hydrodynamics, sea‐level fluctuations at different timescales, geological heritage and sediment composition. Coastal barriers may be composed of siliciclastic sediments, bioclastic sediments, or a mixture of both. Mixed siliciclastic–bioclastic sediments (as common as ‘pure’ sediments) are still little represented in the literature. Changes in sediment composition could affect sedimentary processes which are involved in the construction and stability of coastal barriers due to a different hydrodynamic behaviour between bioclastic and siliciclastic particles. In this study, wave‐flume experiments were used to investigate the role of sediment compositional mixing on the morphology and architecture of coastal barriers. Three different siliciclastic/bioclastic mixtures were exposed to regular wave forcing, together with mean water level fluctuations to create regressive and transgressive depositional units. Compositional mixtures responded similarly in first‐order to mean water‐level fluctuations, with the formation of a bar at low water level. Its subsequent on‐shore migration and reworking as berm deposits during rising water level stages, and with the formation of washover deposits during high water level stages. In detail, the increasing content of bioclastic sediment increased the beachface slope and reduced the length of washover deposits. The faster aggradation of washover sheets with bioclastic‐rich mixtures accelerated the barrier recovery after a submersion and breaching event. A strong segregation between the siliciclastic and the bioclastic grains was observed in the different depositional units, which is attributed both to the coarse size (grain‐size control) and to the flat‐shape (compositional control) of the bioclastic particles.

Pei, Yu; Blumenberg, Martin; Duda, Jan‐Peter; Höche, Nils; Peckmann, Jörn; Birgel, Daniel; Luo, Jinxiong; Kment, Kurt; Reitner, Joachim

doi: 10.1111/sed.13088pmid: N/A

The Permian–Triassic and Triassic–Jurassic critical intervals are among the most significant ecological upheavals in the Phanerozoic. Both evolutionary junctures are characterized by environmental deterioration associated with a marked biodiversity decline. In this study, Permian–Triassic and Triassic–Jurassic boundary sections from South China and the Northern Calcareous Alps were investigated. In order to reconstruct the interplay between biotic and abiotic processes, a multifaceted approach that included optical microscopy, X‐ray diffraction, Raman spectroscopy, stable carbon isotopes and lipid biomarkers was employed. The lower parts of these two sections are similar as both consist of limestone with abundant fossils of eukaryotic organisms. However, the Permian–Triassic record is dominated by dasyclad green algae and fusulinid foraminifera, while the Triassic–Jurassic record is typified by corals and coralline sponges. Moving upward, both sections consist mainly of micrite and marl. Concerning the Permian–Triassic section, it transits to volcanic ash intercalated by a distinct limestone bed with abundant calcispheres (tentatively attributed to ancestors of dinoflagellates). The Triassic–Jurassic section does not provide direct evidence for volcanic activity, but also becomes rich in calcisphere‐type cysts towards the top. Additionally, the section preserves abundant 4‐methyl sterenes (diagnostic for dinoflagellates) and C37–39 n‐alkanes (indicative for haptophytes). Hence, both critical intervals were associated with marked blooms of (ancestral) dinoflagellates and haptophytes (for example, coccolithophorids). These blooms were followed by ecological lag‐phases, as indicated by low carbonate contents and scarce fossils which only increased further up the sections. For both critical intervals, it is commonly assumed that the formation of voluminous volcanic provinces (Siberian Traps and Central Atlantic Magmatic Province, respectively), as well as associated processes (for example, burning of organic‐rich sediments such as coal), resulted in ecological devastation. However, results suggest that volcanism also had a positive effect on certain planktonic primary producers such as dinoflagellates and haptophytes, perhaps by delivering essential nutrients.