Sedimentary Geology最新文献

A high-amplitude global sea level rose rapidly during the early Toarcian (Early Jurassic), making the transgression deposits potential stratigraphic correlation markers of marine and terrestrial successions. Green clay minerals (i.e., glauconite and chamosite), carbonate concretions, and geochemical database associated with the early Toarcian global transgression are archived in the Lower Jurassic Da'anzhai Member (the Early Toarcian age) in the Sichuan Basin of China, a mega lake along the Tethys Ocean. The fine-grained glauconites with high Al and low Si contents suggested that they were transported from offshore to the lake by marine incursion and were oxidized during the transport processes. The green authigenic chamosite cement and redeposited chamosite grains with high Mg/Fe and low Al/Si ratios were diagenetically transformed from berthierine formed in the brackish water caused by marine incursion. The chemically impure siderite concretions with low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values suggested that the lake water and sediment pore waters were mixed with seawater. Coeval calcareous concretions have low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values, indicating the mixing of lake water with seawater. Combining the stratigraphic low ⁸⁷Sr/⁸⁶Sr values, we suggested that there was a marine incursion into the Sichuan Basin. According to the organic carbon isotopic stratigraphy correlation, the marine incursion occurred during the onset of T-OAE. The rapid and high-amplitude global sea-level rise may have caused this marine incursion.

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ¹³C and δ¹⁸O, which vary in accord with the internal zones are more variable than the δ¹³C and δ¹⁸O of the host sediments. For the rhizoliths there is a high correlation between the δ¹³C and δ¹⁸O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ¹³C values to determine if the rhizoliths developed in association with C₃ or C₄ vegetation failed because the δ¹³C values correlate with the calcite content that developed largely as diagenetic products.

The sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings.

Understanding the factors that control the temporal and spatial evolution of the Lower Triassic Buntsandstein Group, eastern France, is important not just for palaeoenvironmental reconstruction but also because it is an important reservoir for the lithium-rich geothermal brines in the Upper Rhine Graben region. The interval of interest in this study, the Lower Grès Vosgien Formation (LGV), is made up of c. 200 m of clastic deposits of mixed fluvial and aeolian origin. Given the proximity between the outcrops and the areas of mineral and heat exploration, this region offers a unique opportunity for quantitative analysis and correlation between outcrops and reservoirs. The LGV is the thickest Formation in the Buntsandstein Group. However, hitherto, no detailed architectural analysis has been published to reveal the controlling factors of its depositional elements. In this study, high-resolution facies analysis was applied to thirteen outcrops, and a core, to quantify fluvial and aeolian depositional architectures, and to comprehend palaeoenvironmental conditions during their deposition. Two facies associations were identified. The fluvial channel facies association, composed mainly of sandstones deposited in poorly confined, occasionally ephemeral channels, corresponds to 93 % of the total thickness. The analysis of >800 sets of strata characterised by lower-, transitional-, and upper-flow regime sedimentary structures, distributed within different architectural elements, enabled the distinction of temporal and spatial variabilities, and an estimate of fluvial palaeohydraulic characteristics. The wind- and water-laid facies association, which accounts for 7 % of the total thickness, records an architecture controlled by a fluctuating water-table level, and ephemeral floods. Results revealed that aeolian dunes and sand sheets accumulated in distinct temporal moments. The vertical trends, recorded in the core, indicate an upward increase in the frequency of intercalations between the two facies associations, and a decrease in the occurrences of thicker cross-bedded sandstone sets of fluvial origin, which, associated with the overall retrogradational stacking pattern of the LGV, is interpreted as the preserved record of a large distributive fluvial system (DFS). The occurrence of laterally extensive aeolian deposits, overlying fluvial deposits, is often associated by authors with periods of relatively more arid conditions at a regional scale. However, evidence indicates channel belt avulsion as an alternative interpretation for the establishment of the two facies associations. The findings enhance the knowledge about the depositional controlling factors in braided fluvial and aeolian systems, and improve predictive models that account for reservoir heterogeneity in accumulated successions of this type.

Archie's cementation factor, m, is a critical parameter for petrophysical studies, and the value is influenced by several factors such as the shape, type, and size of grains, degrees of diagenesis, and associated pore structure. Using integrated experimental and theoretical approaches, the goal of this study is to obtain the cementation factor of rocks (both reservoir rock and caprock) and assess the impact of diagenetic processes on the values of the cementation factor. Thirteen samples of geologically diverse rocks (six mudstones, four fossiliferous limestones, two marbles, and one sandstone) were selected to achieve these research objectives. Two approaches, the diffusion of gas tracers and the Bosanquet formula calculation using pore-throat sizes from mercury intrusion porosimetry analyses, were used to derive the cementation factors of these rock samples. These rocks were categorized into two groups based on the correlation between average pore-throat diameter and diffusivity, and an exponential-law relationship between the cementation factor and porosity was determined for these sample groups. In addition, thin-section petrography and field emission-scanning electron microscopy observations were utilized to investigate diagenetic processes, with four diagenetic patterns being established: (1) strong compaction, strong cementation, and weak dissolution-diagenesis pattern; (2) weak compaction, medium cementation, and weak dissolution-diagenesis pattern; (3) weak compaction, medium cementation, and strong dissolution-diagenesis pattern; and (4) fracture-matrix pattern. The results indicated that diagenetic processes and microfractures contribute to the variability in the cementation factors of these rock samples.

Developing approaches to determine the modes of transport and weathering of mafic detrital minerals in natural sedimentary environments is critical to our understanding of sediment production, dispersal and provenance, as well as carbon capture under natural and enhanced weathering regimes. We integrated the characterisation of morphological and surface textures with the surface composition of recent detrital clinopyroxene grains concentrated in a sandy coastal area from the western North Island of New Zealand and a rocky shore area of Santa Maria Island in the Azores Archipelago. Using the compactness shape descriptor, 341 grains were subdivided into elongated, elongated angular, euhedral, angular and subangular groups, with each group further characterised using morphological and microtextural indicators of abrasion, breakage and dissolution/chemical weathering. In both studied environments, the clinopyroxenes are dominated by elongated to angular shapes with flat cleavage surfaces and conchoidal fractures. This is consistent with dominant subaqueous transport by rivers and longshore currents for New Zealand, and dominant wave action with limited sediment supply by rivers along the rocky shore of Santa Maria. More abundant subabraded and abraded shapes with bulbous and elongated depression microtextures are observed in New Zealand, which indicates additional effects by aeolian transport that are not seen in Santa Maria. Consistent with semi-quantitative EDS spot analyses that document fresh clinopyroxenes surfaces, chemical weathering textures are very rare to absent. In Santa Maria, sampling of recent beach sand was complemented by ∼125 kyr-old palaeobeach clinopyroxenes, but these yielded similar textural results without evidence for chemical weathering. However, the surface of these older clinopyroxenes includes small adhering smectite (typically <50 μm in width and a few μm in thickness) interpreted to reflect incipient cementation with buffering of acidic fluids by more reactive lithic fragments in the deposits. The lack of chemical weathering but pervasive evidence for mechanical breakage of clinopyroxenes indicate that, at the sand size, these minerals break faster than they dissolve due to frequent subaqueous and/or aeolian reworking in the studied high-energy environments and temperate climatic conditions.

Thick gypsum‑carbonate sequences were deposited in different periods and basins of Europe during the westward opening of the Tethys Ocean. Although they seem to represent widespread perimarine sulphate-dominated lagoons in shallow epeiric platforms, the sedimentary characteristics and interpretation of these deposits are still poorly constrained. To advance in the knowledge of these deposits, a detailed study is presented of two distinctive sulphate dominated sequences of Late Triassic age located in NE Iberia (Western Europe), which correspond to the Canelles and Espinagosa units, found in the southern Pyrenees and the Catalan Coastal Ranges respectively. These sequences present a diverse suite of carbonate and sulphate lithofacies, which are stacked in several recurring sedimentary cycles. These cycles consistently present a carbonate base that transitions to a sulphate top, and can be correlated along several tens of kilometres. Each of the observed cycles depicts an initial sudden flooding of a tidal-lagoon system by marine water, which led to carbonate-dominated sedimentation. Progressive evaporation under dominant arid climate conditions triggered the shallowing of the lagoon and salinisation of its water. Then, a transition from carbonate to gypsum/anhydrite precipitation and the early dolomitisation of the underlying carbonate sediments by brine reflux occurred. Gypsum precipitated in the water column as cumulate crystals and deposited at the bottom of the lagoon, forming laminations or being incorporated into microbial mats. Anhydrite precipitated as early diagenetic crystals in the different subaqueous and subaerial environments of the tidal-lagoon system, leading to a variated set of lithofacies until now largely undocumented. It mainly formed as displacive lath-shaped crystals randomly oriented within carbonate muds at the bottom of the lagoon, which were mostly reworked at the lagoon margins by wave and tidal currents resulting in aligned fabrics in cross-lamination structures. Anhydrite also formed as pure massive beds by anhydritisation of subaerially exposed gypsum sediments during the main retraction stages of the lagoon, as well as typical nodular (sabkha) lithofacies. Such sulphate lithofacies are not easy recognisable due to the lack of modern analogues to compare and to their current transformation into secondary gypsum in the outcrops. These are aspects to be taken into account in future sedimentological studies on similar evaporitic materials.

Biofilm surrounding sediment particles can significantly enhance the stability of coastal sediment. This phenomenon is known as biostabilization. Biofilm mainly consists of extracellular polymeric substances (EPS) and microorganisms. While the role of EPS in biostabilization is well-established, microbial contribution requires further exploration. Herein, an experiment was conducted to compare the stability and morphological characteristics of sediment under the effects of biofilm (with microorganisms) and pure EPS extracted from the biofilm (without microorganisms). The microorganisms used for biofilm culture are composed of diatoms and chlorella. The sediment used is a mixture of well-sorted muddy sand (D₅₀ = 77 μm). Erosion tests showed that in the early stage of biofilm culture (within 17 days), biofilm and pure EPS had a similar ability to stabilize sediment, suggesting that in this stage, biofilm stabilized sediment relying on EPS. However, after 17 days of culture, biofilm showed a stronger ability to stabilize sediment than pure EPS, implying that in this stage, biofilm stabilized sediment by both EPS and microorganisms. These results indicate that microorganisms can also contribute to biostabilization, but during the early stage of culture, microorganisms in biofilm cannot make such contributions. Optical observations found that when the sediment-stabilizing ability of biofilm and pure EPS was comparable, the morphological characteristics of sediment particles under the effects of them were similar, both appearing as scattered single particles. However, when biofilm exhibited a stronger sediment-stabilizing ability, sediment particles with biofilm connected into large-size aggregates, whereas sediment particles with pure EPS remained single and scattered. These results suggest that in the later stage of biofilm culture, microorganisms play a role in the aggregation of sediment particles and hence contribute to biostabilization. Based on these findings, a conceptual framework has been proposed for predicting sediment stability under the effects of biofilm.

The formation of vertical sedimentary succession of a delta in the arid and semi-arid basin-margin progradation dramatically depends on the variation of sediment flux and accommodation, but the discontiguous record of signals tends to exist enormously variable, which renders the bulk record of microfacies difficult to quantify from the vertical sedimentary succession. We analyzed the 18 vertical sedimentary successions collected from 18 field trenches of a lake delta by the cusp-catastrophe model, allowing detailed mapping of microfacies. The present detailed study indicates the suitability of cusp-catastrophe theory for explaining vertical sedimentary succession in the BWH (Bawanghe) delta. Three equilibrium states of processes responsible for the deposition of the BWH delta's evolution were established: sandy-dominated upper leaves primarily exhibit the delta plain; sandy/muddy-dominated lower leaves present the delta plain and pro-delta; and sandy-dominated middle leaves which commonly developed the delta front. The result of analysis shows that the reliability of the cusp-catastrophe model to identity mutation of vertical sedimentary succession exceeds 50 %, and to identity microfacies transitions with a precipitation periodicity exceeds 70 %. These cognitions support the previous view that changing precipitation results in an abrupt change in margin progradation. Simultaneously, the study gives new insights into the microfacies quantification of lake deltas and reveals the influence of crest value changes of precipitation on vertical sedimentary succession can be understood from the dependence of microfacies distribution on the change of sediment flux and accommodation.

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang7₃ Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 7₃ Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.