Depositional Record最新文献

The classic work on the morphology of limestone calcite cements done in the 1960s is extended here by utilising growth zones to reconstruct the growth of cement crystals. Only cement composed of fitted polyhedral monocrystals that form by passive crystallisation of calcite on the walls of liquid-filled, static pores and fissures is considered. Cement can either be initiated by (1) nucleation, when new crystals start but are not attached to their substrate, or (2) seeding, when new crystals are seamlessly connected to and influenced by substrate crystals. After seeding, epitaxial cement growth starts with many sub-crystals that coalesce distally, followed by layered mantle growth. Junctions between three intercrystalline boundaries in cement aggregates with one interfacial angel = 180° are of two types: the first, enfacial junctions are caused by a pause in the growth of one crystal and the second is caused by movement of all boundaries due to dissolution of adjacent calcite. Growth zone offsetting at some intercrystalline boundaries is caused by dissolution of calcite at boundaries when permeability values are low. The same width to height ratio of mature aggregate crystals is predicted from the shape of the crystal's growth surfaces; dogtooth calcite forms columnar and nail-head calcite forms tabular-shaped crystals. Seeding on different sized crystals causes variations in epitaxial growth rate with faster growth on large crystals resulting in a disorganised cement fabric; the variation in epitaxial growth rate is perpetuated into mantle growth. Echinoderm syntaxial crystals dominate many pore cements due to the large size of their seed ossicles, at the same time, syntaxial crystals form on relatively tiny seeds. Texturally mature crystal aggregates with isopachous fabric are initiated from three different substrate to cement arrangements. Calcite cement zones preserve their original positions allowing the investigation of cement's growth and chemical history.

The Clydach Valley Subgroup (Courceyan) records a dip section through an Early Carboniferous shallow marine, carbonate shelf and consists of three oolitic formations separated by paludal/peritidal units with abundant evidence of subaerial exposure in proximal areas. The lower part correlates with the Kinderhookian–Osagean Boundary Excursion, with allochem data indicating a minimum δ¹³C value for marine carbonate of +4.5‰, with associated δ¹⁸O of −4.6‰. Marine carbonate δ¹³C and δ¹⁸O values of +2‰ and −2‰, respectively, were estimated for the younger part. Allochem isotopic data yield a well-defined mixing line consistent with stabilisation in meteoric water at varied water: rock ratios, the degree of stabilisation increasing up-dip and up-stratigraphy. Two distinct diagenetic styles closely correlate with evidence for the presence (Diagenetic Regime 1) or absence (Diagenetic Regime 2) of subaerial exposure, non-ferroan and ferroan calcite cement dominating respectively. Five cement zones (Zones 2–6) defined by iron content occur in Diagenetic Regime 1, irrespective of formation, indicating a similar sequence of palaeohydrological changes affected repeated depositional cycles. Zones 3–5 are considered meteoric based on their isotopic composition (δ¹⁸O: −5.8 to −11.1‰ and δ¹³C: −3.7 to −6.2‰) and form distinct clusters dependent on zone, age and location. Pedogenic carbonates and meteoric cements record a long term increase in meteoric δ¹⁸O values: −7.8‰ during the Kinderhookian–Osagean Boundary Excursion, −6.9‰ during diagenesis of the upper part of the subgroup and −6.3‰ associated with initial deposition of the overlying Llanelly Formation. This is consistent with global sea water trends, but an element of climate change cannot be ruled out. Increases in cement δ¹⁸O values as meteoric systems become established (Zone 3–4) indicate repeated short term variation in rainwater composition probably driven by climate change, but also suggests a link between climate and the depositional cycle.

Several compositional features of sedimentary deposits can be used to reconstruct environmental conditions of source areas. In this research, bulk X-ray diffraction mineralogy, heavy and clay mineral assemblages and geochemistry obtained for modern deposits of the Cunene River and its tributaries Caculuvar–Mucope are integrated with geological/geomorphological characteristics of respective catchment areas to evaluate to what extent sediment production is spatially variable and source materials are differently affected by exogenous transformations. Detrital sources can be classified into four main types based on sediment composition: felsic, mafic, recycled and mixed. Source contributions obtained with unmixing models using distinct input data reveal some disagreements, with heavy mineral assemblages pointing to higher mafic contribution and bulk XRD-mineralogy favouring the recycled sedimentary component. However, the three datasets coincide showing a lower supply from the Kalahari Basin than the large outcropping areas of its sedimentary units would suggest, which is attributed to the relatively low rainfall and relief in this region. Where multiple depositional cycles are involved in sediment production the composition of the river deposits, even their clay fractions, will not reflect coeval weathering transformations and an under-estimation of the mafic component probably occurs. This research reveals how exogenous processes may deviate the composition of the produced sediment from a simple weighted by outcropping area average of the source units and compromise (palaeo)environmental interpretations based on sediment composition.

Knowledge about the initial tectonic and depositional dynamics, as well as the influence of early rifting on climate and environmental evolution remains speculative to a large extent, because sediments are usually deeply buried. Within the East African Rift System, inversion tectonics uplifted a few of these successions to the surface hence presenting rare windows into the pre-rift depositional history. One such example, an exceptional 700 m long and up to 60 m high fresh road cut provided the opportunity to study in detail initial rift successions of the southern Albertine Rift (Western Uganda). This focusses on the basal and poorly known Middle to Late Miocene in order to unravel the climatic, environmental, hydrological and tectonic evolution of the initial Albertine Rift. A large and robust multi-proxy dataset was gathered comprising 169 m of stratigraphic thickness, which spans from 14.5 to 4.9 Ma according to a revised lithostratigraphic model. Fieldwork comprised logging of the sedimentary record, spectral gamma ray, magnetic susceptibility and 2D wall mapping with photomosaics. Additionally, the sections were sampled for bulk mineral and clay mineral analysis. The succession exposes a suite of lithofacies and architectural elements detailing the evolution of a fluvio-lacustrine system. Five depositional environments were identified which show an overall back-stepping trend from an alluvial plain to a delta plain and finally palustrine/shallow lacustrine conditions. Mesoscale base-level cycles, preservation potential of architectural elements, and stacking pattern exhibit limited accommodation space. However, it increases over time. This overall trend indicates increasing tectonic subsidence, which can be explained by flexural downwarp within the pre-rift phase and in the upper part grading into fault-controlled crustal extension of the syn-rift phase, which more and more disrupted a large-scale river system. From the Middle Miocene up to the early Pliocene, this study revealed that palaeoclimate trends become marked by increasing and more fluctuating Th concentrations, loss of feldspar, intercalated lenses of hydroxosulphate minerals, and a shift from smectite-dominated to kaolinite-dominated clays. These signals are all interpreted as detrital except for the hydroxosulphates, and they mirror the increasing intensity of chemical weathering and stripping of soils in the catchment. A trend towards increasing humidity is supported by an increase in lacustrine sediment facies and a lake-level rise. Nevertheless, intercalation of hydroxosulphate, ferricretes and pedogenised horizons prove ongoing seasonality and dry intervals. Finally, based on a revised stratigraphic model a sequence stratigraphic correlation of the outcrop's depositional cycles with basin-scale cycles is presented. According to these cycles, transition from the pre-rift to the syn-rift stage is marked by an unconformity and a tectonic pulse in the latest Miocene. However,

The Dead Sea seismites comprise the worlds longest record of earthquakes. The seismites appear as deformed layers enclosed between undeformed layers of alternating millimetre-thick laminae with annual pairs of winter detritus and summer evaporitic aragonite. Understanding the physical conditions that govern their formation will promote the recovery of the causative earthquake properties from the deformation character. The first step towards this goal is understanding the microscopic structure of the seismites. To this end, scanning electron microscope images of the Dead Sea Basin sediments were analysed to extract their pore and grain sizes. The implementation of image processing techniques to determine the microscopic-scale physical properties of the deformed and undeformed layers are in general agreement with results from classical labour-intensive instruments. However, the image processing analyses provide more detailed unbiased information. A MATLAB-based code has been developed as a ready-to-use package, which can be easily implemented on any other occurrence of soft sediment outcrops to analyse sediment microscopic-scale physical properties from scanning electron microscope images.

The Pasterze is Austria's largest glacier, and it is experiencing rapid downwasting and retreat. A mosaic of complex sedimentary deposits has been produced in recent years which have not hitherto been studied, yet provide excellent lessons into the facies distribution expected from a dying valley glacier. In this paper, a new glaciological–geomorphological–geological map is presented for the glacier in July 2021. Freshly exposed (since 2018) tills and flutes constitute a subglacial sediment–landform assemblage. An ice-marginal sediment–landform assemblage comprises meltwater streams, a delta system and proglacial lake terrace deposits. The supraglacial assemblage, meanwhile, includes fossil englacial channel deposits revealed by ablation, together with debris bands, rockfall deposits and supraglacial channel deposits. Collectively, these sediment–landform assemblages constitute the building blocks of a dying glacier landsystem.

One hundred and forty-four published successions of shallow-water carbonates, deposited between the Palaeocene and the Miocene, from the Levant to the Himalayas, have been re-analysed using a standardised approach to investigate the distribution of carbonate facies and carbonate-producing organisms. Large benthic foraminifera were found to be the volumetrically most important group of carbonate producers during the whole period, with a peak in abundance during the Eocene. Colonial corals are relatively abundant during the Palaeocene and Miocene, their abundance peaks during the Oligocene and has a minimum during the Eocene. Red calcareous algae have a similar pattern although their peak in abundance covers both the Oligocene and Miocene. Green calcareous algae decrease from the Palaeocene onward. Facies related to very shallow and/or restricted marine conditions peak during the Miocene and in particular during the Aquitanian. Both the pattern of large benthic foraminifera and of colonial corals seems to be related to temperature, with warm periods favouring the former group and cool periods the latter group. Red calcareous algae display a pattern similar to that of colonial corals suggesting that the periods favourable for one group are, on a large scale, also favourable for the other. The progressive decrease of green calcareous alga could be tentatively related to a preservation bias connected to the transition from Palaeogene assemblages that included presumably calcitic taxa of green algae to Neogene assemblages entirely constituted by aragonitic taxa with limited preservation potential. The Aquitanian peak in facies related to very shallow and/or restricted marine conditions is most likely connected to the progressive narrowing of the Tethys related to the collision between Arabia and Eurasia. These results denote an overall agreement between the abundance of the various types of shallow-water carbonate facies and large-scale environmental and geological processes, highlighting the potential for palaeoenvironmental reconstruction locked in the shallow-water record.

Continental shelves serve as critical transfer zones in sediment routing systems, linking the terrestrial erosional and deep-water depositional domains. The degree to which clastic sediment is mixed and homogenised during transfer across broad shelves has important implications for understanding deep sea detrital records. Wide continental shelves are thought to act as capacitors characterised by transient sediment storage during sea-level rise and sediment remobilisation during sea-level fall. This study attempts to test the hypothesis that sea-level lowstand yields more efficient and direct sediment transfer from fluvial sources to deep sea sinks compared to highstand when sediment is sequestered and mixed on the shelf. This hypothesis is tested by evaluating U–Pb and Lu–Hf detrital zircon provenance trends along the vast Bering Sea shelf and deep-marine Beringian continental margin. Presented here are 5884 U–Pb ages and 402 Lu–Hf analyses from 30 samples to characterise the provenance of modern to Pleistocene sediment across the Bering Sea region. Both forward and inverse numerical mixture modelling was used to estimate the abundance of distinct fluvial sources in shelfal and deep-water deposits. These results demonstrate that sediment in the Bering Sea is derived from a mixture of regional fluvial sources, but that the Yukon River is the primary detrital source for sediment throughout the region. Although Yukon River signatures are abundant in all basin samples, the relative proportions of Yukon River versus other sources vary spatially across the shelf. A comparison of Holocene and surficial sediment with Pleistocene deposits shows that sediment across the shelf and in the deep sea remains well-mixed between climate states. Thus, detrital provenance signatures in deep-marine deposits outward of broad transfer zones are likely to represent mixtures of fluvial sources regardless of sea level.

Early marine cementation is a fundamental process for many characteristics of carbonates, like the stabilisation of steep slopes. The genesis of early cements is often attributed to physicochemical processes but there is evidence for microbial mediation. To elucidate the role of microbes and associated organic material, in vitro experiments were undertaken in the presence and absence of indigenous microbiota in ooids from Schooner Cays, Bahamas and compared with native grapestones from Joulter Cays, Bahamas. Microscopic examinations by stereomicroscopy, scanning electron microscopy and thin section analysis of in vitro incubations with native flora document rapid grain fusion, resulting in the formation of grapestones within 30–60 days. The initial binding of the grains is primarily facilitated by exudates of extracellular polymeric substances and microbial communities acting as catalysts in the formation of micritic bridges, cements and encrusted aggregates. In vitro grapestones are similar to native grapestones from Joulter Cays with intergranular areas infested with extracellular polymeric substances, microbes, micritic cements, amorphous calcium carbonate nanograins and micritised outer surfaces. These similarities suggest that incubations with native flora follow similar mineralisation mechanisms as in the natural environment. In contrast, sterilised grains remain loose with little crystal formation after 60 days and are devoid of microbes and organic exudates. Owing to the near absence of precipitates, abiotic precipitation is not the driving force promoting early cements. In contrast, grain fusion is microbially mediated via both a passive mechanism, where extracellular polymeric substances and cell surfaces function as templates for crystal nucleation and generation of micritic cements, and through an active mechanism by which biofilm heterotrophs and autotrophs induce chemical alterations of a local environment, facilitating precipitation. This study underscores that microbially mediated cementation can occur at fast rates and that firmground to hardgrounds and slope stabilisation take place shortly after deposition of carbonate grains.