Sedimentary Geology最新文献

Sedimentary strata are a significant record of the Earth and planetary history, and accurate recognition of sedimentary structures and their link to environmental conditions is a key component in deciphering past surface processes. Centimeter-scale sedimentary structures are commonly acknowledged as ripple cross-laminations, but here we document ambiguous centimeter-scale structures from a lacustrine delta front in northern China, that at careful look do not seem to fit with the known ripple cross-lamination criteria. The here documented sedimentary structures range from scour-and-fill, irregular lenses with structureless or low- and high-angle, up- and downstream dipping, concave and convex laminations. These centimeter-scale sedimentary structures thus considerably differ from ripple cross-laminations in their outer shape, internal organization, and morphometric parameters. Detailed comparison of these centimeter-scale structures with Froude supercritical-flow structures suggests that they were likely produced by Froude supercritical flows. Such centimeter-scale supercritical structures are not unique in the Bantanzi delta, as they have been also documented in a variety of settings ranging from rivers to deepwater turbidites. In light of this finding, we expand the Froude supercritical-flow sedimentary structures to centimeter scale, and advocate caution in interpreting centimeter-scale sedimentary structures axiomatically as ripple laminations.

Major element compositional spectrum, formation in diverse magmatic and metamorphic rocks, and relative stability during sediment transport, burial, and diagenesis make garnet an important indicator of sedimentary provenances. However, a fundamental question yet to be answered is whether the low-grossular–high-pyrope detrital garnets reflect source characteristics or record sedimentary processes. To address this problem, we have analysed garnets from the beach sands of the East Coast of India and in the source lithologies of the catchment area within the Eastern Ghats Mobile Belt. Investigated garnets show a broad compositional spectrum; most garnets are dominated by almandine_49–86% with variable contents of pyrope_9–47%, minor grossular_0.8–15%, and low spessartine_0.5–4%. Using multiple proxies, including a state-of-the-art machine-learning-based garnet discrimination scheme, we found that 96 % of analysed beach sand garnets are of metamorphic origin and 4 % igneous. Amongst the metamorphic garnets, 85 % were derived from granulite facies, 5 % from eclogite/ultrahigh-pressure facies, 5 % from amphibolite facies, and 1 % from blueschist/greenschist facies rocks. Concerning host-rock bulk composition, 93 % of garnets belong to intermediate–felsic/meta-sedimentary class and 7 % belong to the mafic category. The chemistries of the beach sand detrital garnets of the East Coast of India are readily coupled with the compositions of the garnets from source rock lithologies of the Eastern Ghats Mobile Belt. Very low-grossular garnets are derived from meta-pelitic rocks (khondalites) and mafic granulites, whereas slightly grossular-rich garnets are derived from charnockites. The present study indicates that the low-grossular–high-pyrope detrital garnets are vital signposts of sedimentary provenances. We advocate that the high P-T granulite facies mobile belts are distinctive sources for the low-grossular–high-pyrope detrital garnets.

The Cambrian–Ordovician Tiñú Formation of southern Mexico is key for identifying sediment sources along the northern margin of Gondwana, enhancing our understanding of early Paleozoic paleogeography and linking it with age-equivalent units in terranes with Gondwanan affinity. This study integrates detrital zircon U–Pb ages, Hf isotope signatures, and heavy mineral chemical data. U–Pb detrital zircon ages indicate sources from Stenian–Tonian (900–1200 Ma) to Calymmian (1400–1600 Ma) ages. The Stenian to Tonian zircon population, with peak ages around 1.0 Ga and model ages ranging from 1.68 to 1.90 Ga, suggests a provenance from the metaigneous rocks of the Oaxacan and Guichicovi complexes. The presence of the Calymmian zircon population, tourmaline crystals, and rutile grains displaying lower crystallization temperatures compared to the high-temperature rutile from the Oaxacan Complex suggests additional sources. Hf isotope signatures from Calymmian-aged zircon grains, with model ages between 1.95 and 2.30 Ga, match well with the Paleo-Mesoproterozoic Catarina Unit in the southern Chiapas Massif Complex. Further potential sources may include the basement of the Putumayo Province in Colombia and igneous rocks from the western Guiana Shield and the Rio Negro-Juruena Province. The Tiñú Formation provenance is comparable to coeval metasedimentary units across the northwestern margin of Gondwana found in Belize, the southern Chiapas Massif Complex, and Guatemala. The Tiñú Formation was likely deposited adjacent to the rifted margin of the Rheic Ocean. The results also emphasize combining U–Pb–Hf isotope analysis and trace elements in detrital zircon and rutile as effective provenance tracers.

Hybrid event beds (HEBs) form important components of subaqueous sediment gravity flow models in deep-lacustrine sedimentary basins, largely due to their clay-rich nature meaning that they often form non-reservoir and/or baffles/barriers to fluid flow in the subsurface. Using examples from a typical deep-lacustrine basin, this study documents the sedimentary characteristics and distribution of HEBs and explores their effect on reservoir properties. To achieve this, a suite of drill cores and wireline data through a range of debrites, concentrated flow deposits, and turbidites are analyzed from Paleogene in the Bohai Bay Basin, China. The superposition and convergence of gravity flow deposits resulted in a complex spatial and temporal distribution and evolution of the HEBs in the study area. The majority of identified HEB types are interpreted to be associated with lobe deposition. However, slumping-induced HEBs are interpreted to be restricted to the proximal slump areas. Slump HEBs are interpreted to have formed through slumping and associated generation of sediment gravity flows, where HEBs formed through flow transformation of slumps into high-concentration flows (debris flows) and/or high-density turbidity currents in down-slope areas. Debris flow HEBs are interpreted to be formed by particle rearrangement through vertical settling during flow transformation within the subaqueous fans. The effects of relative buoyancy in debris flows, rearrangement of debris flow particles, muddy substrate erosion by turbidity currents, and slumping upon HEB development are discussed. The average porosity and permeability observed within reservoir intervals formed by gravity flow deposits are 17.8 % (ranging from 2 % to 25 %) and 126 mD (ranging from 3 mD to 816 mD), respectively. HEBs within the studied reservoir interval display low-porosity (<15 % on average) and low-permeability (<10 mD on average) values. The occurrence of HEBs within a reservoir increases the variation coefficient of permeability, quantity, and thickness of the interlayers and presents a significant heterogeneity. The results of this study are important to consider in the context of constructing reservoir models in deep-lacustrine reservoirs of the Bohai Bay Basin, and which can be applied to other lacustrine gravity flow deposits in sedimentary basins worldwide.

The present study explores a diverse suite of soft-sediment deformation structures (SSDSs) preserved at multiple levels within the Mesoproterozoic carbonate succession of Rohtas Limestone in the Vindhyan Supergroup of India. The carbonate succession shows lithological heterogeneity, characterized by a sharp change of facies association and depositional environment. Three layers of SSDS, traced over 57 km, demarcate the lower lagoonal from upper open shelf deposits. Contorted laminae, load casts and flame structures, convolute laminae, ball and pillow structures, accordion folds and pillar structures within the limestone beds infer liquefaction-fluidization and plastic deformation. The lateral continuity, vertical repetition and confinement of SSDS beds at the interface of lagoon and open shelf deposits indicate the role of seismicity on facies succession. The SSDS beds show lateral variation in thickness and structures, depending on the distance from epicenter. The seismic trigger was accompanied by sudden deepening of the depositional environment and related marine transgression. The coarsening up packages within the lagoonal succession comprise load cast and flame structures, syn-sedimentary faults and intraclastic conglomerates. The open shelf succession shows several fining up cycles, with multiple load and flame structures with intraclastic conglomerate at the top. Origin of SSDS in association of the coarsening up sequence, and fining up cycle infers that intrabasinal tectonics and storms were the triggering agents for lagoon and open shelf successions respectively. The seismite in the studied succession not only reveals the seismicity during the Proterozoic, but also changed the sedimentary facies association, which may use as a diagnostic criterion for the identification of seismite. This study also highlights that seismic and aseismic impacts on sedimentary architecture and paleoceanography evolution.

Stalagmites offer nearly continuous records of past climate in continental settings at high temporal resolution. The climatic records preserved in stalagmites are commonly investigated by examining compositional characteristics such as mineralogy, organic content, and lamination patterns. These proxies provide valuable insights into the environmental conditions during stalagmite formation. However, the methods used to obtain information about these proxies are relatively destructive. This study uses hyperspectral imaging, a non-contact technique, to identify mineral composition, organic matter content, and laminations in stalagmites. It is the first wide spectrum imaging analysis in speleothem research, using both visible–near infrared and shortwave infrared wavelengths. Results obtained from hyperspectral imaging were compared by point spectral analysis using an ASD spectroradiometer and a grayscale profile along the growth axis of a stalagmite. Petrographic observation of thin sections and X-ray diffraction (XRD) analyses on selected stalagmite layers were performed to cross-validate the hyperspectral data. A travertine sample was also used to replicate the method on calcite. To automate mineral identification, a machine learning algorithm was developed to map spatial distribution and quantify relative proportions of minerals across the sample. Our findings are in good agreement with traditionally used methods for mineral identification, i.e. XRD and petrography, aiding in the interpretation of paleoclimate proxies, and offer a spatial guide for U–Th dating analyses. It also provides insight for future investigations of stalagmites using hyperspectral data and classification through machine learning algorithms.

In the Upper Ediacaran Dengying Formation of the Sichuan Basin, the presence of abundant, well-preserved microbial carbonates provides a unique opportunity to study Precambrian paleoceanography and microbial carbonate origins. Particularly, the underexplored coated grain dolostones of this formation, characterized by distinct microbially induced fabrics and intragranular dissolution, offer crucial insights for understanding late Ediacaran microbial mineralization and diagenetic sequences. Utilizing selected typical samples, we conducted detailed petrographic and geochemical analyses, revealing a dynamic interplay between microbial processes and sedimentary dynamics. We observe the coexistence of constructive micrite envelopes with microbially induced fabrics such as clots and laminations, highlighting microbial biomineralization's key role in fabric formation. The sedimentary dynamics critically determines the formation processes of the coated grains: low-energy settings foster grain agglomeration and consolidation through clot precipitation between grains, while high-energy settings favor smaller grains binding to microbial mats. Geochemically, micrite envelopes play an essential role in preserving distinct rare earth element (REE) signatures. The weak negative Ce anomalies and positive Eu anomalies within these envelopes point to a suboxic to anoxic depositional environment, directly indicative of the microenvironmental conditions conducive to microbial mineralization processes. Furthermore, our study sheds light on the structural evolution of coated grains with hollow nuclei, proposing that their internal pore formations are influenced by both mineral instability and selective dissolution by meteoric freshwater. These findings not only provide fresh insights into complex diagenetic processes in the Dengying Formation but also substantially advance our understanding of early microbial life and environmental adaptations during the Precambrian.

Abundant chert bands and nodules are discovered throughout the Mesoproterozoic Jixian System in the Ordos Basin. These cherts faithfully record the Jixian period oceanic conditions and paleoecology. However, the diagenetic mechanism of the cherts remains unclear and controversial. To understand the origin of these cherts, we performed a multitracer study by combining field reconnaissance, petrological analyses, Si isotope analysis, and major and trace element analysis of chert samples from the Qishan section. The results show that Jixian cherts had a high Al/(Al + Fe + Mn) value (~0.48), showed a flat distribution of rare earth elements in seawater, and most of the Fe/Ti values are <20. A weak negative Ce anomaly (~0.97) indicates the contribution of REY to weakly oxygenated seawater. The high isotopic composition of silicon (δ³⁰Si = 0.74 ‰-1.35 ‰) and the average Si_ex value of 37.52 indicate that there is a source of biological silica, and the relationship between Eu/Eu* and Y/Ho and δ³⁰Si shows that the hydrothermal and volcanic influences were less and correlated with seawater. The chert bands were closely related to biological activities. The chert nodules were influenced by hydrothermal activity. Secondary cherts were related to diagenesis, but the nature of siliceous fluids remains unchanged, still characterized by biogenic activity origins. The sea oxygen concentration in the Jixian period had increased, with periodic variations in the intensity of biological activities affecting the pH of the water body, or biological photosynthesis binding SiO₂ colloid in water, affecting chert sediments, leading to frequent interlayering between dolomite and chert bands. Episodic hydrothermal fluids and late-stage diagenetic processes jointly influenced the formation of cherts. This study bears significant significance in enhancing our understanding of the sedimentary environment and the origin of cherts during the Jixian period in the Ordos Basin.

Siliciclastic grain surfaces preserve an information about mechanical transport effects and/or various chemical processes that the detritus has undergone. Weathering, transport, and diagenesis leave the clear traces that may be used to interpret the paleo-environmental conditions. We aimed to detail surface analysis of detrital garnet by the proven scanning electron microscopy (SEM), X-ray micro-tomography (XRT) and subsequent morphometric analysis of derived 3D surface model. We tracked garnet morphological and surface microtextural changes during fluvial transport in an active Beňatinská voda and Sobranecký potok streams in the Western Carpathians from the place of garnet removal from the host igneous rocks to the last appearance of its fragments in the fluvial sediments. Analysed garnet grains show a range of almost complete solid solution between almandine and grossular (Alm_70-73Grs_15-19Prp_5-6Sps_2-3Adr_0-3). Precise SEM analysis showed that conchoidal fractures, straight and arcuate steps, crescentic percussion marks and V-shaped cracks are main mechanical fluvial microtextures, whilst 3D surface models derived from XRT provided the information that no relationship between mechanical surface-microtextural development and mineral inclusion distribution exists. Thus, the presence of inclusions and their proximity to the grain surface have no influence on garnet destruction during its fluvial transport. Morphometric variables such as surface roughness and curvatures derived from 3D garnet model can be used to distinguish between mechanical impacts and corrosion-induced depressions. The results related to detritus behaviour in the modern streams are applicable to the ancient fluvial sediment research and can help to correctly reconstruct the paleoenvironmental and paleogeographic conditions in various areas.

The discovery of huge oil accumulations in the South Atlantic Pre-Salt carbonate lacustrine deposits has attracted much exploration and research interest on these challenging reservoirs, but no work has focused on the compaction processes. A petrological and geochemical study was therefore performed on two cored wells in Santos Basin to identify the main syngenetic and diagenetic constituents, and to identify the compaction features that affected these deposits. They are mainly composed of Mg-clays, calcite spherulites and fascicular shrubs, which have undergone a heterogeneous diagenetic evolution. Mechanical compaction occurred during shallow burial, promoting a closer packing of the constituents, reorientation and fracturing of calcite spherulites and bioclasts, and deformation of the clay matrix. Pressure dissolution occurred under increasing pressure and temperature during burial, developing inter-aggregate sutured contacts, dissolution seams and stylolites, as consequence of overburden. Macrocrystalline quartz and calcite, drusy quartz, and saddle dolomite precipitated during burial diagenesis, related to basinal fluids and probably to pressure dissolution, as source for the late carbonate phases. Understanding the role of compaction is essential to evaluate trends of porosity destruction, thickness modification and mass transfer, which directly impacted the quality and the dynamic evolution of fluids in the deep Pre-Salt reservoirs.