Depositional Record最新文献

Deltas are deposits directly accumulated by land-generated gravity flows in a standing body of water. The paradigm of deltaic sedimentation has dramatically changed during recent years, from the popular very simplified ternary models of marine littoral deltas towards more realistic and comprehensive models, considering the importance of sediment-laden river discharges. Ternary delta models were designed for clean rivers, where a stream flow drags the sediments. Depending on the basin dynamics, these littoral deposits can be modified, forming tidal-dominated, wave-dominated or fluvial-dominated littoral deltas. In recent years, a new classification of delta systems was proposed, based on contrasting the salinity of the receiving water body with the bulk density of the incoming fluvial discharge. Rivers are highly dynamic systems, and their discharges can be very variable in terms of flow duration and sediment concentration. Additionally, the salinity of the receiving water body can exhibit significant variability, especially in closed lakes and epicontinental seas, ranging from freshwater to brines. This scenario allows the distinction of three major delta categories (hypopycnal, homopycnal and hyperpycnal deltas) which can be in turn subdivided, defining seven delta types. Hypopycnal deltas form when the bulk density of the incoming flow is lower than the density of the water in the basin, allowing the definition of three delta types, corresponding to hypersaline littoral deltas, marine littoral deltas and brackish littoral deltas. Homopycnal deltas form when the bulk density of the incoming flow is similar to the density of the water in the basin, defining a delta type termed homopycnal littoral deltas. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the basin, allowing the definition of three categories termed hyperpycnal littoral deltas, hyperpycnal subaqueous deltas and hyperpycnal fan deltas.

Dual-energy X-ray computed tomography consists of imaging objects using two incident X-ray beams of different energy to distinguish the different compounds within a sample based on their density (electron density, ρ_e) and elemental composition (effective atomic number, Z_eff). The stoichiometric calibration for dual-energy X-ray computed tomography was already successfully implemented to identify single and homogeneous minerals easily and non-destructively. It is here applied for the first time to a more complex and heterogeneous sample, a varved sediment core with three distinct facies. The output of dual-energy X-ray computed tomography was compared against elemental geochemistry obtained at the same resolution using a micro-XRF core scanner. The three individual facies can be successfully differentiated using dual-energy X-ray computed tomography because their range of ρ_e and Z_eff values allow their discrimination. Correlations with elemental geochemistry are also discussed but are less conclusive, probably because of variations in grain size and porosity, and because these high resolution analyses were not performed at the exact same location. The paper not only eventually discusses the limitations when using dual-energy X-ray computed tomography on sediments but also demonstrates its potential to quantitatively study sediment cores in a non-destructive way.

Landslides are among the largest mass movements on Earth. As such, the deposits of landslides, also known as mass-transport deposits, are significant architectural elements of continental margins, especially those receiving sediment from large deltas. Landslide dams have been shown to alter the courses of rivers and submarine channels. However, there are fewer examples of landslides completely filling submarine channels and examples of the subsequent stratigraphic evolution. A three-dimensional seismic-reflection dataset (<90 Hz) from the deep-water (>1500 m) Taranaki Basin, offshore the North Island of New Zealand, was used to explore the response of a sequence of channel deposits to landslide filling. The basal channel system initially meandered like a river, with successive channel positions in close proximity, as it aggraded >250 ms two-way travel time. This systematic, organised evolution is governed by the memory of early channel evolution, which sets the sea floor geomorphology that guides channel-forming turbidity currents. Later, a channel approximately twice as wide as underlying channels cut off a number of channel bends, probably as a result of an increase in the discharge of channel-forming turbidity currents. This last channel was filled with submarine landslides, which transported and deposited sediment as debris flows based on the presence of blocks within a matrix comprising chaotic, lower amplitude seismic facies. These debris-flow deposits smoothed over the sea floor, effectively wiping the memory of channel evolution. As a result, the subsequent channel pattern bears no resemblance to the basal system. Submarine-channel resetting by landslide filling is common in settings with frequent catastrophic basin-margin collapses, like offshore New Zealand.

Submarine channels are key features for the transport of flow and nutrients into deep water. Previous studies of their morphology and channel evolution have treated these systems as abiotic, and therefore assume that physical processes are solely responsible for morphological development. Here, a unique dataset is utilised that includes spatial measurements around a channel bend that hosts active sediment gravity flows. The data include flow velocity and density, alongside bed grain size and channel-floor benthic macrofauna. Analysis of these parameters demonstrate that while physical processes control the broadest scale variations in sedimentation around and across the channel, benthic biology plays a critical role in stabilising sediment and trapping fines. This leads to much broader mixed grain sizes than would be expected from purely abiotic sedimentation, and the maintenance of sediment beds in positions where all the sediment should be actively migrating. Given that previous work has also shown that submarine channels can be biological hotspots, then the present study suggests that benthic biology probably plays a key role in channel morphology and evolution, and that these need to be considered both in the modern and when considering examples preserved in the rock record.

This paper provides insight into the provenance of the Late Miocene turbidite succession of the Tabernas Basin. Although this area has been extensively studied, only limited attention has been paid to sediment provenance. Through heavy mineral analysis, it has been possible to identify provenance-related signatures from the adjacent Sierra de los Filabres and Sierra Alhamilla uplifts. Stable mineral ratio data confirm that the Sierra de los Filabres provided sediment with generally higher chloritoid:tourmaline and higher Type Bii garnet abundances than those derived from the Sierra Alhamilla. By comparison, modern sediments derived from the Sierra Alhamilla have garnet compositions with larger proportions of Types A and C, suggesting that the basinal sediments were not sourced from the incipient Sierra Alhamilla Uplift. Heavy mineral analysis confirms that the Sierra de los Filabres was the primary source for the Tabernas succession, with minor variations indicating that the erosive part of the system migrated across the uplift. Input was predominantly from the Nevado–Filábride Complex, with minor amounts from the small remnant of the Alpujarride Complex attached to the southern margin of the Sierra de los Filabres. Evidence strongly suggests a single sediment routing system but identifies some subtle provenance variations. In particular, there was a shift in detrital garnet composition between the Sartenella Formation and the Verdelecho Formation, Solitary Channel and El Gordo Megabed, which is attributed here to a shift in catchment within the Sierra de los Filabres. This shift appears to have occurred during the deposition of the Sartenella Formation, since the garnet compositions of the Verdelecho Formation and Solitary Channel are similar to each other and differ from the preceding part of the Sartenella Formation. The Solitary Channel displays marked heterogeneities in provenance character, manifested by changes in chloritoid abundance, consistent with previous studies that suggest the depositional architecture in the channel was influenced by high-frequency changes in sediment flux and sea level.

The difficulty of correlating continental deposits hinders predicting lacustrine and palustrine carbonate facies variations in time and space. This study aims to understand better the factors governing these facies heterogeneities by measuring carbonate isotopes and conducting facies, petrographic and sequence stratigraphic analyses of the Lutetian–Aquitanian deposits of the Paris Basin, that record the transition from marine to lacustrine environments. Large-scale correlations enabled the definition of two lacustrine–palustrine carbonate facies models. (1) The coastal lacustrine system (Bartonian to Rupelian), consists of fine-grained brackish carbonate exhibiting episodic marine inputs during short-term relative sea-level maxima and evaporite sedimentation during relative sea-level minima. Lacustrine sediments differ notably from marine ones with more negative δ¹³C and δ¹⁸O compositions that co-vary and a biota adapted to low salinity conditions. In the associated palustrine environment, depositional sequences evolve upwards from micritic lacustrine deposits to nodular and then laminar calcretes. Microbial-coated grains and rhizoliths indicate biological processes during repeated subaerial exposure phases in sub-tropical to arid climates. (2) The inland lacustrine system (Rupelian and Aquitanian) was disconnected from the marine domain and showed evidence of microbial activity with microbial crusts and oncoidal rudstones. Facies rich in micritic intraclasts composed of palustrine and lacustrine facies indicate the reworking of already lithified sediments along the margins. In the palustrine domain, the calcrete facies are less abundant than breccias formed in-situ by desiccation, limestones with root traces, or organic-rich wackestones and marls. This system reflects a more temperate climate with more developed microbial structures and less exposed carbonates than the coastal lacustrine system. The southward migration of the depocentre and the transition from marine environments to (1) coastal and then (2) inland systems are controlled by uplift phases induced by Pyrenean and Alpine orogenesis. Third-order relative sea-level variations appear to control only short-term cycles in coastal systems.

Over the past decade, there has been growing interest in the sedimentological community to use micro X-ray computed tomography to analyse microfacies in sediment cores. However, little attention has been paid to the application of micro-computed tomography in lithified deposits, even though this can allow their texture to be characterised in three dimensions, providing key information about sedimentary structures. A novel application of micro-computed tomography in lithified sediment-gravity flow deposits is presented with the objective of characterising their internal 3D sedimentary structures. This technique is applied to three deep-marine sandstones showing different compositional properties: Cretaceous Gosau Group (Austria), Eocene Hecho Group (Spain) and the Oligocene Annot Formation (France). From micro-computed tomography data the size of particles and their distribution throughout the sample is reconstructed in 3D permitting a better visualisation of sedimentary textures. Particle distributions computed from micro-computed tomography are similar to those computed from thin section image analysis, corroborating the reliability of the micro-computed tomography to evaluate grain-size trends. Micro-computed tomography is complemented with micro-X-ray fluorescence and thin section petrographic analyses. In cases where mineral composition or grain size are homogeneous or matrix and grains have similar mineral composition, sedimentary structures do not appear visible from micro-X-ray fluorescence or thin section analyses. By separating particles based on their computed tomography density, it is possible to isolate the coarsest fraction, highlighting the sedimentary structures. This study demonstrates (i) the potential of micro-computed tomography in analyses of sedimentary structures from outcrop data and (ii) the importance of the mineralogical composition and degree of grain sorting in assessing the origin of structureless deposits. Considering the importance of visualising sedimentary structures when interpreting depositional processes, micro-computed tomography is a new and reliable tool to assess the physical properties of sandstones and to analyse their internal 3D sedimentary structures.

Establishing the biogenicity of sedimentary surface textures with unresolved microbial origin is critical to any environmental and geobiological interpretation of clastic settings. Here, some Ediacaran wrinkle structures and associated carbonaceous greywacke samples containing mat fragments rich in ‘bacteriomorph acritarchs’ are investigated. Their biogenicity was evaluated with transmitted light and scanning electron microscopy, epifluorescence and Raman spectroscopy, and confirmed by the presence of distinct cyanobacterial biomarkers. The comparison of results yielded by these techniques validates the use of Raman spectroscopy on Neoproterozoic kerogen (organic-walled microfossils and amorphous organic material) under low metamorphic conditions. Raman spectrographs also allowed recognition of associated rare-earth element-rich phosphate (monazite) and subsidiary metal sulphide concentrations, and interpreted as a result of biosorption and/or mat trapping under normal oxic conditions. These microbial mat features represent cyanobacterial bloom-forming Bavlinella acritarchs, which characterise eutrophic episodes in a semi-enclosed retroarc basin sandwiched between an active Cadomian arc and West Gondwana.

The Cretaceous was punctuated by episodic flooding of continental margins forming epicontinental seas. The Trans-Sahara Seaway was one of these epicontinental seas, connecting the Gulf of Guinea with the Tethys Ocean. In this study, data including microplankton abundances, stable carbon isotopes of organic material and elemental geochemistry were integrated with traditional sedimentological analyses from the Trans-Sahara Seaway. The carbon isotopic data provide the first evidence that oceanic anoxic event 2 was present in the Trans-Sahara Seaway, and palynology shows it was associated with an increase in peridinioid dinocyst abundance. A combined study of microplankton assemblages and sedimentology reveals palaeoenvironmental trends linked to sea-level change. Lowstand system tracts were characterised by increased siliciclastic grain size, low microplankton diversity, and were dominated by Chlorophyceae. Transgressive system tracts were associated with diversity increases during rising sea level, with open marine gonyaulacoid dinocysts dominating the assemblages. Maximum flooding surfaces were recognised by the highest increase in biological diversity in argillaceous deposits. As sea level started to fall, the peridinioid dinocysts became dominant, with decreased microplankton diversity during highstand systems tracts. This combination of sedimentology and interpretation of dinocyst assemblages allows the identification of shallow to deeper marine depositional sequences of Cenomanian–Santonian strata within the Yola Sub-basin. This approach could be used to delineate marine depositional sequences where using conventional sedimentological methods alone is very challenging.

Seafloor topography can affect turbidity current dynamics on deep-water slopes, significantly influencing the dispersal of sediment. Despite the common occurrence of topographic complexity, there are few detailed investigations of topographic interactions and their effect on downslope flow evolution in intraslope environments. In this study, the sedimentology and architecture of an Upper Cretaceous intraslope fan succession deposited within an extensional, fault-bound minibasin are described from a rare, well-exposed, near-continuous, oblique depositional-dip outcrop of the Tres Pasos Formation, Chile. The 2 to 8 m thick studied interval transitions downslope from high-energy heterolithic strata, including metre-scale steep-faced scours, to non-amalgamated thick-bedded sandstones. Abrupt increases in sandstone percentage, sandstone bed thickness and grain size occur on the hangingwall blocks of south-east and north-east-dipping normal faults that bound the minibasin. Sandstone beds are dominated by backset or wavy low-angle stratification proximally, contain compositional banding near faults, and are characterised by increased proportions of planar laminated and structureless turbidite divisions downslope along the transect. Experimental observations of turbidity current interactions with topography are synthesised into a qualitative framework, which is used to interpret flow processes and characteristics from deposit trends. The results reconstruct the response of Froude-supercritical, stratified turbidity currents with denser basal layers when encountering metre-scale fault scarps. The analysis shows that metre-scale topographic features can substantially alter the flow properties of stratified turbidity currents, and their downslope flow evolution to include the development of transitional, depositional and flow-stripped sediment gravity currents. However, in comparison to base-of-slope settings, overall flow conditions are interpreted to be more uniform over slope breaks and zones of flow expansion in a partially confined intraslope environment. These findings have considerable implications for understanding flow response to similar scale morphological features on the seafloor and the potential for flow transformations in intraslope settings.