Depositional Record最新文献

The Triassic Boreal Ocean was a shallow epicontinental basin and the sink of the World's largest delta plain known to date. Nutrient and freshwater supply from this delta have been regarded as important causes for high productivity and water mass stratification, forming Middle Triassic oil-prone source rocks. Recent studies attribute upwelling and a productivity-induced oxygen minimum zone as important factors. A multi-elemental chemostratigraphic study of a Spathian–Carnian mudstone succession exposed in eastern Svalbard was performed to investigate their formation. This includes 89 samples from three localities, from which 34 elements were acquired using combustion and X-ray fluorescence analyses. The goal is to provide a correlation framework and infer the role of productivity, redox and water mass restriction on organic matter accumulation and source rock formation. These processes had major impact on the source potential. The Spathian Vendomdalen Member suggests deposition during intermittent benthic euxinia and low productivity, corresponding with a reported deep thermocline that obstructed upwelling. The lower Anisian lower–middle Muen Member shows negligible enrichment in redox-sensitive elements but in situ phosphate nodules, consistent with developing upwelling and moderate productivity. The middle Anisian upper Muen Member formed during high productivity and phosphogenesis and is linked with basin-wide upwelling. Productivity, phosphate and redox proxies are all strongly enriched in the upper Anisian–Ladinian Blanknuten Member. In the south-western Barents Sea, the pro-deltaic environment of the emerging Triassic Boreal Ocean delta system had terminated these conditions. The upper Ladinian upper Blanknuten Member formed within intermittent euxinic bottom waters due to the shallowing sea level. The Carnian Tschermakfjellet Formation marks the dominance of the prograding delta system and the end of Triassic oil-prone source rock formation in Svalbard.

Sea-level fall is commonly inferred to generate a sharp-based shoreface succession that displays an abrupt vertical transition from heterolithic, lower shoreface to sandy, upper shoreface deposits across a marine erosion surface. Three-dimensional, process physics-based, coupled hydrodynamic-morphodynamic models are constructed to compare bedding architecture and facies patterns of wave-dominated delta deposits preserved during normal (static sea level) and forced (falling sea level) regression and then transgression during subsequent sea-level rise. The models suggest that wave-dominated deltas will develop a sandy shoreface inner clinoform dipping from the subaerial delta plain to a relatively flat wave-scoured subaqueous delta top, which is laterally separated from a delta front outer clinoform that dips from the subaqueous delta top edge to the shelf floor. As these systems prograde, deposits of these dual-clinoforms will become vertically stacked and will be separated by a regressive surface of marine erosion. Significant grain-size contrasts between these vertically stacked clinoform deposits reflect differences in sediment-transport directions and sorting under river and wave-driven littoral currents along the coast, and cannot be related uniquely to sea-level changes. The marine erosion surface under a sharp-based shoreface deposit records abrupt facies shift across a kilometres-wide, wave-eroded surface and defines a discontinuity in the preserved vertical succession. The continuity of a regressive surface of marine erosion mapped over many tens to hundreds of kilometres across mid-shelf regions of some stratigraphic sequences reflects a gradual lateral shift in the position of littoral current erosion on a subaqueous delta top. Timelines cross such vertical lithic discontinuities throughout the extent of a prograding deposit, and the regressive surface of marine erosion thus has little chronostratigraphic significance. The results of these models suggest caution in inferring sea-level changes from the character of vertical facies changes observed in individual well logs and isolated outcrop exposures.

The McMurray Formation in McMurray Depocentre, Canada, is typically subdivided into stratigraphic units based on regionally mappable marine mudstones, the bases of which define flooding surfaces (bottom to top: Lower McMurray, C2, C1, B2, B1, A2, A1 depositional unit). However, the McMurray Formation comprises a variety of palaeoenvironments, with the north-eastern Firebag Tributary hosting significant delta-plain deposits in the Lower McMurray and B1 depositional units. Facies analysis of 60 cores and 4763 wireline logs was used to resolve the palaeoshoreline trajectory and stratigraphic architecture in Firebag Tributary. Of 14 identified facies, three delta-plain facies reveal: (1) eluviated palaeosols, interpreted as shrublands and woodlands formed during base-level fall; (2) gleyed palaeosols, interpreted as tidal marshes; and (3) coals formed in coastal mires, the two latter facies are interpreted to accumulate during base-level rise. Eluviated palaeosols occur below the Lower McMurray coal seam, implying valley incision and a maximum regressive surface. Gleyed palaeosols underlying the coal seam atop the B1 depositional unit do not record base-level fall. Wetting-upward coals overlying either palaeosol facies indicate base-level rise (i.e. major flooding surface). B1 depositional unit coals and gleyed palaeosols are overlain by shallow-marine facies or are eroded by wave ravinement during transgression. Consequently, the tops of coals or gleyed palaeosols correlate to the bases of regional marine mudstones elsewhere in McMurray Depocentre. Stratigraphic cross sections showcase palaeoshoreline regression and transgression. During the Lower McMurray and B1 depositional units, widespread coals and palaeosols indicate the palaeoshoreline resided near the western edge of Firebag Tributary. Regression during C2 through B2 depositional units and A2 and A1 depositional units resulted in palaeoshorelines near the Alberta–Saskatchewan border. To complement a previously dated Lower McMurray ash (121.39 ± 0.2 Ma), a new high-confidence maximum depositional age from ash in the B1 coal seam (115.09 ± 0.16 Ma) allows an estimate to be made of depositional duration, indicating depositional units represent ca 0.16 Myr.

Emplacement of submarine landslides, or mass-transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. Typically, progressive healing of the complicated relief generated by the submarine landslide occurs prior to progradation of sedimentary systems. However, subsurface and seabed examples show that submarine channels can incise directly into submarine landslides. Here, the evolution of a unique exhumed example of two adjacent, and partially contemporaneous, submarine channel-fills is documented. The channels show deep incision (>75 m), and steep lateral margins (up to 70°), cut into a >200 m thick submarine landslide. The stepped basal erosion surface, and multiple terrace surfaces, are mantled by clasts (gravels to cobbles) reflecting periods of bedload-derived sedimentation, punctuated by phases of downcutting and sediment bypass. The formation of multiple terrace surfaces in a low aspect ratio confinement is consistent with the episodic migration of knickpoints during entrenchment on the dip slope of the underlying submarine landslide. Overlying sandstone-rich channel-fills mark a change to aggradation. Laterally stacked channel bodies coincide with steps in the original large-scale erosion surface, recording widening of the conduit; this is followed by tabular, highly aggradational fill. The upper fill, above a younger erosional surface, shows an abrupt change to partially confined tabular sandstones with normally graded caps, interpreted as lobe fringe deposits, which formed due to down-dip confinement, followed by prograding lobe deposits. Overlying this, an up-dip avulsion induced lobe switching and back-stepping, and subsequent failure of a sandstone body up-dip led to emplacement of a sandstone-rich submarine landslide within the conduit. Collectively, this outcrop represents episodic knickpoint-generated incision, and later infill, of a slope adjusting to equilibrium. The depositional signature of knickpoints is very different from existing models, but is probably reflective of other highly erosional settings undergoing large-scale slope adjustment.

Several lacustrine carbonate beds, each a metre-thick interval of densely packed Mn-rich sideritic micro-spherulites or Mn-rich ferroan calcite micro-spherulites, are recorded for the first time within strata of the Lower Cretaceous McMurray Formation of the northern Athabasca Oil Sands deposit, western Canada. A lower McMurray lacustrine carbonate deposit is characterised by a metre-thick bed fabric of Mn-rich siderite micro-spherulites. The middle and upper interval McMurray beds developed fabrics of Mn²⁺-rich ferroan calcite micro-spherulites. These carbonate beds represent saline lacustrine depositional environments that resulted from the lake bottom sediments ingressed from below by Mn²⁺-Fe²⁺-rich carbonate-saturated brines. These up-section migrations of Devonian formation water were sourced from dissolution trends developed in limestone and halite-anhydrite beds of the underlying Devonian Prairie Evaporite during Cordilleran deformation of the Alberta Basin foreland. These brines ascended to the overlying McMurray Formation sediments along dissolution-collapse structures such as breccia pipes, sinkholes and margins of differentially subsided Upper Devonian fault blocks. The up-section migration of a sulphate-saturated Fe²⁺ and Mn²⁺-rich brine resulted in the ingress of a lower McMurray lacustrine bottom sediment at a site associated with the development of a peat mire terrain. Microbial redox of the lake bottom sediment resulted in a carbonate bed of micro-spherulitic fabrics of Mn-rich siderite interwoven with pyrite laminae. Subsequent salt dissolution events and up-section migrations of Devonian brine during deposition of the middle and upper McMurray intervals resulted in similar carbonate-saturated but sulphate-poor chemistry. These saline flows also ingressed lacustrine bottom sediments below, and resulted in limestone beds of densely packed spherulitic fabrics of Mn-ferroan calcite, not siderite. These deposits provide insight into largely unknown dispositions of voluminous brine resulting from salt dissolution trends below the Athabasca Oil Sands and further our understanding of controversial McMurray depositional processes.

A revision of the popular equation of Richardson and Zaki (1954a, Transactions of the Institute of Chemical Engineering, 32, 35–53) for the hindered settling of suspensions of non-cohesive particles in fluids is proposed, based on 548 data sets from a broad range of scientific disciplines. The new hindered settling equation enables predictions of settling velocity for a wide range of particle sizes and densities, and liquid densities and viscosities, but with a focus on sediment particles in water. The analysis of the relationship between hindered settling velocity and particle size presented here shows that the hindered settling effect increases as the particle size decreases, for example, a 50% reduction in settling velocity is reached for 0.025 mm silt and 4 mm pebbles at particle concentrations of 13% and 25% respectively. Moreover, hindered settling starts to influence the settling behaviour of sediment particles at volumetric concentrations of merely a few per cent. For example, the particle settling velocity in flows that carry 5% silt is reduced by at least 22%. These observations suggest that hindered settling greatly increases the efficiency of natural flows to transport sediment particles, but also particulate carbon and pollutants, such as plastics, over large distances.

X-ray diffraction mineralogical analysis of geological sequences is a well-established procedure in both academia and industry, rendering a large volume of data in short-analytical time. Yet, standard data treatment and resulting interpretations present limitations related to the inherent complexities of natural geological materials (e.g. compositional variety, structural ordering), and are often time consuming and focussed on a very detailed inspection. Several alternatives were evaluated in terms of advantages and disadvantages to the main goal of generating a user-friendly, fast and intuitive way of processing a large volume of X-ray diffraction data. The potential of using raw X-ray diffraction data to interpret mineralogical diversity and relative phase abundances along sedimentary successions is explored here. A Python based program was tailored to assist in raw data organisation. After this automated step, a 3D surface computation renders the final result within minutes. This single-image representation can also be integrated with complementary information (sedimentary logs or other features of interest) for contrast and/or comparison in multi-proxy studies. The proposed approach was tested on a set of 81 bulk and clay-fraction diffractograms (intensity in counts per second—cps and respective angle—º2Ɵ) obtained from a Cenomanian mixed carbonate–siliciclastic stratigraphic succession, here explored by combining mineralogical (XY) and stratigraphic/geological information (Z). The main goal is to bypass preliminary data treatment, avoid time-consuming interpretation and unintended, but common, user-induced bias. Advantages of 3D modelling include fast processing and single-image solutions for large volumes of XRD data, combining mineralogical and stratigraphic information. This representation adds value by incorporating field (stratigraphic/sedimentological) information that complements and contextualises obtained mineralogical data. Limitations of using raw intensity data were evaluated by comparison with the results obtained via other standard data interpretation methods (e.g. semi-quantitative estimation). A visual and statistical contrast comparison confirmed a good equilibrium between computation speed and precision/utility of the final output.

Dynamic sedimentary processes are a key parameter for establishing the habitability of planetary surface environments on Earth and beyond and thus critical for reconstructing the early evolution of life on our planet. This paper presents a sedimentary section from the ca 3.48 Ga Dresser Formation (Pilbara Craton, Western Australia) that contains high-energy reworked sediments, possibly representing the oldest reported tsunami deposit on Earth to date. Field and petrographic evidence (e.g. up to 20 cm large imbricated clasts, hummocky bedding, Bouma-type graded sequences) indicate that the high-energy deposit represents a bi-directional succession of two debrite–turbidite couplets. This succession can best be explained by deposition related to passage and rebound of tsunami waves. Sedimentary processes were possibly influenced by highly dense silica-rich seawater. The tsunami was probably triggered by local fault-induced seismic activity since the Dresser Formation was deposited in a volcanic caldera basin that experienced syndepositional extensional growth faulting. However, alternative triggers (meteorite impact, volcanic eruption) or a combination thereof cannot be excluded. The results of this work indicate a subaquatic habitat that was subject to tsunami-induced high-energy disturbance. Potentially, this was a common situation on the early Archaean Earth, which experienced frequent impacts of extraterrestrial bodies. This study thus adds to the scarce record of early Archaean high-energy deposits and stresses the relevance of high-energy depositional events for the early evolution of life on Earth.

Over the past decades, the burial realm, the most prolonged and arguably the least well-understood diagenetic environment, has received significant research attention. Despite remarkable progress driven by exploratory drilling, outcrop analogue studies and experimental work, the scientific theories defining the burial sub-domains are inconsistently presented in the literature. This paper reviews the concepts, processes and products that characterise the burial realm from the viewpoint of the carbonate geoscientist. Typical features of carbonate burial (fluid types, porosity evolution, diagenetic fabrics, patterns in isotope geochemistry) in epicontinental and marine basins are presented and discussed. A step towards an improved conceptual delimitation and a subdivision of the carbonate burial realm is taken, and an intuitive terminology is proposed. The very shallow limit of the burial realm is placed within the upper sediment column (redox boundary; centimetres to tens of metres). In the shallow (marine) burial domain (down to depths of many hundreds of metres), carbonate dissolution and reprecipitation, sediment dewatering and grain reorganisation take place. Interstitial waters are mainly marine (and subordinate meteoric) in origin and the system is fluid-dominated. Under ongoing burial, physical and chemical compaction reduces pore space. At burial depths of ca 750 m, initial sediment porosities (40–80%) are reduced to about 30%. The intermediate burial domain (hundreds of metres to about 2 km; T < 100°C) is characterised by the transition from fluid-buffered to rock-buffered diagenesis. In the deep-burial domain (ca 2–12 km; T > 100°C), marine formation fluids are increasingly modified by rock–fluid interaction and replaced by saline brines. The transition from the deep burial to the very low-grade metamorphic domain is placed at depths of 12–15 km (T > 250°C). Here, carbonates undergo recrystallisation into meta-carbonate and equigranular marble fabrics.

The Late Cambrian Steptoean Positive Carbon Isotope Excursion marks a time of significant change in ocean chemistry and trilobite faunas. On the lead up to the carbon isotope excursion and at the excursion itself, there is global evidence from Laurentia and Gondwana of cementation by primary aragonite in shallow subtidal environments accompanied by deposition of aragonitic ooids. However, this occurred at a time widely considered to have been characterised by ‘calcite seas’ when the primary inorganic phases (marine cements and ooids) are normally presumed calcitic. This study has investigated the chemostratigraphy of the Middle–Late Cambrian Port au Port Group, Newfoundland, including the early marine cements. Here, the marine cements contain increasing concentrations of strontium towards the peak carbon isotope excursion (up to 5500 ppm at the peak excursion) before dropping off post-peak excursion, consistent with the original cements having been aragonitic. This trend is accompanied by relict oomouldic porosity, again suggesting an aragonitic precursor. Primary inorganic mineralogy is largely controlled by the Mg/Ca ratio of sea water but estimates of the Mg/Ca ratio of Late Cambrian oceans are variable (0.8–2). At this level, other factors such as water temperature and pCO₂ have been shown to affect mineralogy with warm waters and high levels of CO₂ favouring aragonite. It is possible that the warm waters and anoxia that caused the carbon isotope excursion created conditions favourable for the precipitation of aragonite at the same time as major trilobite faunal turnover.