Sedimentary Geology最新文献

The Salitral de La Vidriera (SDV; 38°44′S, 62°34′W) once occupied the innermost section of a funnel-shaped estuary before its isolation due to sea level drop ∼3000 years BP. It currently is a continental saltpan detached from the Bahía Blanca estuary, having no marine connection, and acting as a hypersaline evaporitic environment. This study provides a first-hand characterization of the epibenthic microbial mats that biostabilize surface sediments in the saltpan, and the environmental dynamics and physical deformation agents behind the formation of modern Microbially Induced Sedimentary Structures (MISS). The microbial community in the hypersaline mats is dominated by cyanobacteria of filamentous (taxonomic order Oscillatoriales) and coccoid morphologies (orders Chroococcidiopsidales and Pleurocapsales). The MISS comprise rolled-up mats, flipped-over mats, folds and wrinkles, reticulate surfaces, gas domes, pinnacles, and desiccation cracks; their genesis is linked to the presence of water that either promotes microbial growth or creates hydrodynamic deformation structures. Gypsum crystals were found in surface and subsurface sediments plausibly linked to a high rate of rainwater evaporation and aided in their preservation by the microbial mat lattice. Furthermore, carbonate precipitation in the form of peloids was identified through petrographic analysis, despite the siliciclastic nature of sediments. The occurrence of modern MISS in this transitional environment, currently under continental influence, is discussed and contrasted with modern peritidal environments. Descriptions from this study, contextualized in a modern depositional setting, may be useful in the study of fossil counterparts and paleoenvironmental reconstructions.

Hydrofracture systems have been described in glacial sediments for almost a century and accelerating research since the 2000s, boosted by the advent of micromorphological techniques applied to glacial deposits, led to a significant rise of studies using paleo-hydrofractures (and their fills) as a new proxy for reconstructing glacial processes and environments. This review covers the great diversity of hydrofracturing context (subglacial, marginal, proglacial) and physical characteristics (at macro- to micro-scale) of hydrofracture systems and their fills based on a compilation of published and unpublished field-based data from both Quaternary and pre-Quaternary glacial sediments.

The text covers (1) the fundamental concepts of hydrofracturing processes including causes and triggers of overpressure in glacial environments as a preamble, (2) the physical characteristics of hydrofracture systems in glacial environments and (3) the parameters controlling these physical characteristics, (4) the characteristics of hydrofracture-fills, (5) the processes of sediment injection inferred from fill characteristics and (6) the wider implications of hydrofracturing and injection processes on paleoglaciological reconstructions. Future research perspectives, including the need for modeling of hydrofracture network in glacial environments, are finally discussed as it will certainly allow the role of ice thickness, slope and speed, meltwater input and host sediments in governing the architecture of hydrofracture systems to be untangled.

The geometry of unconformities carved by deep time ice sheets is often obscured and restricted by discontinuous exposure, or outcrop conditions that do not readily permit the examination of glacial unconformities (for example, steeply dipping strata). Here, we present new uncrewed aerial vehicle (UAV) data from selected outcrops across northern, central and southern Namibia to shed further light on the nature of the basal Dwyka Group unconformity. This includes the onlap relationship of basal diamictites onto the Gomatum palaeo-fjord system in northern Namibia, highly complex mapped ice flow orientations elsewhere in the northern Kaokoveld, previously undiscovered grooves along the Fish River area, and a set of subglacial grooves along the border with South Africa along the Orange River. In the latter two cases, photogrammetric methods integrating orthophotos and digital elevation models reveal the presence of subglacial grooves. Furthermore, subglacial grooves often show different orientations to striations and fabrics measured in overlying diamictites, raising fresh questions about the nature of small-scale flow variations beneath Late Palaeozoic ice sheets.

Many Permian desert tracks are found in formations dominated by rather homogeneous aeolian quartz arenites. This raises questions around how they got preserved. Here we test the hypothesis that strong palaeoenvironmental controls affect style and quality of footprint preservation in Permo-Triassic desert settings. To answer this, several examples of tracks and trackways from Moray, Scotland, are described in the context of their host sedimentary successions. We then discuss petrographic clues in the specific track-bearing layers with regard to taphonomy. Two key sections were logged and sampled at Hopeman Beach: (i) Hopeman Coastal Section A, being a site from which tracks have previously been recovered; and (ii) Hopeman Coastal Section B, a section still exhibiting several in-situ tracks. Tracks were also examined on the surfaces of metre-scale quarried blocks within Clashach Quarry. Logging was also undertaken at quarries in Quarrelwood near Elgin. Collected samples were examined optically and with a scanning electron microscope. Hopeman Coastal Section A exhibits convolute bedding best interpreted as dewatering structures; a pustular bed that could be linked to growth of evaporite crystals impinging on a sediment-binding microbial mat; adhesion ripples formed by dry, wind-blown sand sticking to a wet or damp surface; and laterally continuous pebble layers that are the result of ephemeral sheet floods. The oscillation-rippled layer from which NMS footprint specimen G.1997.60.1 was extracted exhibits a halite cement and petrographic evidence for re-worked halite, and these rippled sediments were most likely deposited in an interdunal lake. Hopeman Coastal Section B similarly exhibits metre-scale planar cross beds and occasional coarser-grained lag deposits that are consistent with aeolian dunes that were episodically inundated by sheet floods. Samples containing halite and lesser amounts of gypsum or anhydrite were collected from the same layer as the in-situ Hopeman Coastal Section B tracks. Metre-scale planar cross-bedded quartz arenites of Cutties Hillock quarry were clearly deposited in an aeolian dune setting. Some sands in the Cutties Hillock Sandstone with scoured bases were aeolian sediments that were reworked by fluvial processes. We conclude that this study demonstrates three different modes of track preservation in the Permian Moray area: (i) indentation of near-surface layers constituted by particles of fine silt that in many cases had infiltrated between sand grains of aeolian dunes; (ii) trackways in sediments deposited around the margins of lakes in the interdunes, with early cementation by evaporites, noting that in the studied cases the halite cement might have helped preservation of the tracks in the sense of long-term fossilisation, but probably not anatomical preservation (i.e. quality of fidelity); and (iii) indentation of clays that had been deposited in some interdunal lakes.

The appearance of the early biomineralized skeletons during the latest Ediacaran was one of the most important evolutionary gains for the ancient animals' lineage. This evolutionary innovation likely represents a pivotal shift in ecology and the interactions between the geosphere and biosphere. Among these fossils, Cloudina stands out as the oldest macroscopic biomineralized metazoan that achieved widespread distribution. However, the complex interplay between sedimentation, preservation, and the vertical distribution of bioclastic accumulations in the Ediacaran strata remains unexplored. The Ediacaran Tamengo Formation (Western Brazil) records thick shallow water deposits and preserves several stratigraphic intervals that contain bioclastic-rich beds formed by Cloudina remains. This unit represents a window for investigating the role of the earliest truly and their impact on Ediacaran marine environments. Our investigation aims to examine the sedimentology and taphonomy of Cloudina within the Tamengo Formation in Brazil to test the hypothesis that the emergence of skeletal hard parts influenced sedimentary depositional processes. To achieve this goal, we combine bioclastic remains and facies/microfacies analysis, taphonomy, and sequence stratigraphy. Our approach involves analyzing the distribution of bioclasts in correlation with stratigraphy and microfacies. By doing so, we aim to forecast the presence of bioclastic accumulations across a mixed siliciclastic–carbonate ramp section. Our findings reveal variability in bioclast sizes among different sites, suggesting a correlation with environmental factors. Furthermore, our study underscores the significance of comparing bioclast accumulations between Precambrian and Phanerozoic deposits, emphasizing their broad utility in understanding evolutionary and environmental shifts over time.

Deltaic systems evolve as a result of interactions between the hydroclimatic processes that occur in the catchment area and the coastal marine processes that reshape the coastline. The Holocene evolution of these environments is controlled by climate and anthropization, which are rarely considered in models of the evolution of deltaic systems. The Rhône delta has recorded the impact of climatic variations as well as the development and evolution of human societies over the Holocene period. This system underwent a post-glacial evolution controlled by global climatic warming punctuated by short periods of cooling, fluvial metamorphoses and a rapid marine transgression generated by the melting of the ice caps, followed by the initiation of delta progradation from around 7000 cal yr BP. Sedimentological and chronostratigraphic studies of 17 cores, supplementing an existing dataset on the deltaic plain and the prodelta, have enabled us to construct well-constrained stratigraphic correlations, making it possible to specify the spatio-temporal evolution of the Rhône delta. The variation in sedimentary fluxes was assessed for the different sequences identified, using 95 new core datings to constrain the sequential evolution of the different lobes. From the stratigraphic correlations on the delta and the estimated volumes of sediments exported out of the deltaic system, the total sediment volume between 11,700 cal yr BP and today is estimated at 126 billion m³. The variation of sediment fluxes has been adjusted according to the different phases of lobe progradation linked with the climate oscillations and the anthropic activity evolution during the Holocene. Taken together, these data highlight contrasting periods corresponding to the Roman period, the Little Ice Age and finally the ‘Anthropocene’, that can be compared to the main Mediterranean deltaic systems during the Holocene.

Green marine clays, known as glaucony and verdine facies, are unique iron-rich minerals typically found in shallow marine sedimentary environments covering large areas of continental shelves today. While on some occasions they have been used effectively as paleoenvironmental indicators associated with transgressive and highstand system tracts, their occurrence can vary in different sedimentary environments. This paper aims to provide a review of the nomenclature and mineralogical composition of these facies, their habits and morphologies, as well as the physico-chemical characteristics of their genesis. This includes water depth, temperature, and geochemical factors that influence the local environments and characteristics of their formation. The article also presents examples from the NW Galician Continental Shelf and discusses the variety of mechanisms behind their genesis and evolution, as well as the challenges surrounding their chronology. Comparisons are drawn with the old facies (ironstone), which can occur in a wider range of marine environments and are sometimes found alongside glaucony and verdine facies in ancient deposits. This highlights the importance of accurately reconstructing the environment of each deposit. The literature reveals many exceptions in relation to the zonal and bathymetric distribution, sedimentation rates, age, etc. of these facies, emphasizing the need for further investigation into their genesis in current and ancient environments. Such research would provide valuable insights for paleogeographic, paleoenvironmental, and stratigraphic interpretations.

Ancient dammed lake deposits, developed in tectonically active mountainous areas, record high-resolution changes in paleoclimate and paleoseismicity. This paper reports a massive ancient dammed lake, the “Aniangzhai paleolandslide-dammed lake”, newly discovered in the upper reaches of the Dadu River on the eastern margin of the Tibetan Plateau. Optically stimulated luminescence (OSL) dating showed that this lake formed prior to 15.7 ± 1.9 ka and persisted for 7 ka. The basic properties of lacustrine sediments and abnormal fluvial deposits of the dammed lake were identified through field sedimentological investigation and use of unmanned aerial vehicle (UAV) technology. Through a literature review of reservoir siltation, several methods of estimating the dam surface elevation were summarized, and on this basis, the Aniangzhai paleolandslide and ancient dammed lake were reconstructed. The results indicated that the Aniangzhai ancient dammed lake extended for 79 km upstream, with a maximum sediment thickness of 128 m. The lake at one time held back an area and volume of water of 53.5 km² and 5.74 × 10⁹ m³, respectively. This study proposed the fluvial-dammed lake sedimentary zoning system of ancient dammed lakes based on changes in sedimentary characteristics among different sections: (1) deep–semi-deep lake; (2) shallow lake; (3) lakeshore; and (4) area of river–lake intersection. This study also discussed a general, but systematic and novel model under which ancient dammed lakes in mountainous areas evolve. It may provide new information on the evolution of the paleoclimatic environment in the eastern Tibetan Plateau after the Last Glacial Maximum.