Sedimentary Geology最新文献

Petrography, geochemistry, radiocarbon dating, and porewater chemistry of modern carbonate sediments in the mangrove marshes and tidal channels along the Abu Dhabi coast, United Arab Emirates, were conducted to compare sediment composition, texture and diagenesis between these two adjacent but distinct depositional environments. Tidal currents and extensive micritization of the allochems in the tidal channels have led to the enrichment of skeletal fragments (average 25 %) and peloids (average 70 %). Ooids, however, are relatively scarce (average 1 %), which is attributed to strong tidal currents flushing them out of channels and depositing them on shoals and deltas. The severe environmental conditions in the tidal channels forced the microorganisms to bore into allochems, promoting micritization via carbonate dissolution and reprecipitation of spheroidal microbial micrite. Spheroidal micrite with the same mineral composition as the host skeletal fragments fills the microbial borings, indicating that micritization does not involve mineralogical alteration. Radiocarbon dating suggests that microbial boring is an important source of micrite, which is transferred into the marshes from channels by tidal currents. The oxidizing environment in marshes, due to the presence of mangrove pneumatophores and crab burrows, reduces the likelihood of anaerobic respiration. In contrast, microbial sulfate reduction and carbonate dissolution induced by microbial boring in tidal channels caused an increase in porewater alkalinity and dissolved inorganic carbon (DIC) concentration, resulting in more abundant aragonite and high-Mg calcite cements. Stable carbon (+2.3 ‰ to +4.6 ‰) and oxygen (+0.8 ‰ to +1.5 ‰) isotopes of the allochems and micrite corroborate derivation of DIC from seawater. The formation of rare scattered rhombic dolomite as cement only in tidal channels is attributed to microbial metabolic processes. This study provides important insights into the characteristics and controlling factors of diagenesis in modern carbonate sediments, which can have wide implications for understanding the early diagenesis of ancient limestones.

Much sedimentological research aims to understand the depositional processes by establishing the relationship between the transport processes of subaqueous sediment gravity flows (SSGFs) and the characteristics of their deposits. A distinctive type of gravel-inlaid mud clasts, which has been largely overlooked, is embedded with occasional granules or pebbles, and exhibits various angular shapes that are present in cores of SSGF deposits worldwide. SSGF deposits from four cored wells in the Liushagang Formation of the Weixi'nan depression, China, have been analyzed to explore the characteristics, distribution, and potential formation mechanisms of gravel-inlaid mud clasts, thereby revealing the transport processes of SSGFs. In the research area, SSGF deposits are dominated by gravelly high-density turbidites, sandy high-density turbidites, low-density turbidites, and hybrid event beds. Gravel-inlaid mud clasts are common in massive sandstones and bipartite or tripartite beds, exhibiting various distribution patterns. The erosional contact at the base of these beds, where coarse grains are partially embedded within the muddy substrate, indicates that gravel-inlaid mud clasts originate through processes of erosion and delamination. Their distribution from the lower to the upper part of massive sandstones and bipartite or tripartite beds suggests a floating process from base to top. The formation and distribution of gravel-inlaid mud clasts demonstrate the downflow transformation from high-density turbidity currents to low-strength debris flows, driven by the erosion of the underlying muddy substrate, ultimately resulting in the formation of hybrid event beds. The lofting of gravel-inlaid mud clasts increases the cohesion of the upper part of the high-density turbidity current, facilitating its transformation into a low-strength debris flow. Furthermore, the occurrence of gravel-inlaid mud clasts within massive sandstones clearly demonstrates that they are products of high-density turbidity currents rather than sandy debris flows. The identification of gravel-inlaid mud clasts and their distribution within deep-water deposits can be regarded as a reliable indicator for reconstructing SSGF transport processes from high-density turbidity currents to low-strength debris flows, ultimately transitioning into low-density turbidity currents.

The Neoproterozoic Panium Sandstone Formation of the Kurnool Group (India) offers a unique scope for understanding regressive depositional history in a Neoproterozoic clastic shoreline. It is divided into two ‘Systems Tracts’ namely forced regression and lowstand. Process-based facies and paleo-environmental analysis identified twelve different facies types; grouped under four facies associations (FA I–IV) belonging to both continental and shallow-marine domains viz. distal braided fluvial, swash bar-foreshore runnel, upper shoreface bar-trough, and lower shoreface bar-interbar. Whereas the fluvial deposit (FA I), sharply and erosionally overlying the argillaceous shelf succession of the Owk shelf shale, represents the product of forced regression, the wave-dominated shallow marine deposits (FA II, III and IV) record the depositional history of lowstand in the coastline. A basin ward transition from unconformity (base of FA I) to correlative conformity (base of shallow marine deposits FA II, III, and IV) is documented. The shoreline had a north-northwest to south-southeast alignment and the north-eastward flowing fluvial system met the shoreline at a high angle. Detrital zircon geochronology from the Panium fluvial sandstone allowed documentation of Paleoproterozoic (~2050 Ma–1800 Ma) and Mesoproterozoic (~1500 Ma to ~1200 Ma) age clusters, in addition to earlier documented age clusters from the Banganapalle alluvial sandstone present at the basal part of Kurnool succession. The new age data suggest an addition of new provenances and in turn, tectonic intervention behind the regression in the Kurnool coastline that triggered Panium deposition. Further, the Paleoproterozoic age cluster, obtained from the Panium fluvial deposit, helped in the justification of solitary zircon grain occurrences within the Owk Shale, as described in earlier studies.

The northern Gulf of Mexico basin, known for its hydrocarbon potential and a myriad of geologic structures and processes, has been underexplored to understand deepwater sediment waves. The recent release of a vast amount of both 2D and 3D seismic data by the Bureau of Ocean Energy Management (BOEM) calls for a basin-wide identification of the sediment waves in the Gulf of Mexico.

Integrating seismic, well-log, and high-resolution bathymetric data, this study identified sediment-wave fields on the seafloor as well as in the stratigraphic record. These sediment waves have an average wavelength of 798 m and an average wave height of 18 m. On the present-day seafloor, sediment waves are only located on the northwestern continental slope and eastward of the Bryant Fan area (south of Green Knoll). However, in the stratigraphic record, these bedform structures were found to be prevalent across the northwestern continental slope, northeastern continental slope, and continental rise. All of these sediment waves migrate upslope with crests that are perpendicular to the direction of basin-slope (i.e., parallel to the bathymetric contours). Thus, they are interpreted as cyclic-steps bedforms produced by supercritical sediment gravity flow processes. Investigations of these sediment waves have implications for petroleum geology, geohazard studies, oceanography, hydrodynamics, paleoclimate, and coastal engineering.

The Oceanic Anoxic Event 1a (OAE1a), or the Selli Event, occurred in the early Aptian (~120 Ma) and represents an episode of global carbon-cycle perturbation caused by substantial greenhouse gas emissions, leading to profound environmental and climatic changes. However, our understanding of the impact of OAE1a in the eastern Tethys region, particularly regarding regional volcanic activities in southern Tibet and their influence on environmental changes, remains limited. Here we presented high-resolution inorganic and organic geochemical data from marine sediments of the lower Aptian Gucuo Formation in southern Tibet (eastern Tethys). This study aims to reveal the impact of volcanic activity and related regional to global environmental perturbations, such as continental weathering, detrital input, and bioproductivity. Our results suggested that the lower Aptian sediments in southern Tibet were influenced by regional volcanic activity linked to the disintegration of eastern Gondwana. Continental weathering showed a declining trend preceding the OAE1a, followed by a significant increase at the onset of OAE1a associated with regional volcanic activities in southern Tibet. Additionally, changes in weathering intensity at the Gucuo area coincided with contemporaneous weathering signals observed in the western Tethyan and Boreal realms, indicating a supra-regional intensification of continental weathering at this time. Increased fluvial detrital input revealed heightened continental runoff at the onset of OAE1a, driven by elevated continental weathering. The role of marine primary productivity during the early Aptian was governed by regional volcanism, global climate, and hydrological cycling. Prior to OAE1a, nutrient-rich inputs from regional volcanic sources and volcanism-induced climate variation controlled bioproductivity evolution. High biological paleoproductivity at the onset of OAE1a was associated with increased nutrient input under enhanced continental weathering and riverine runoff linked to warm and humid climates. This study contributes to our understanding of organic carbon distribution in the eastern Tethys region and its correlation with TOC patterns observed in the western Tethys realm.

In hydrogeology, a key challenge involves identifying patterns within ancient formations to forecast the distribution of fluid pathways and barriers to permeability, as well as comprehending the palaeohydrological system and its changes over time. This study addresses two main research inquiries concerning fluid flow: firstly, the influence of dolomitization induced by paleosols on flow characteristics, and secondly, the implications for fluid flow pattern distribution in continental sedimentary units. The objectives are pursued through: (1) meticulous, high-resolution measurements of porosity and permeability coupled with well-log data from an outcrop and two boreholes; (2) investigation of petrographic features of diagenetic minerals and their ages using the UPb geochronology system; and (3) an integration of these methodologies to grasp rock properties and fluid flow at a broader scale. Findings suggest that early dolomitization in continental sequences significantly affects fluid flow properties across the basin. The development of paleosols triggered early dolomitization, supported by UPb geochronology evidence. Subsequent groundwater migration along hydraulic gradients, influenced by fluctuations in the local aquifer's water table, facilitated the vertical distribution of dolomitization. Dolomitization occurred in residual pores resulting from initial mineral alteration, early lithifying the sediment and preventing mechanical compaction, thus preserving porosity. During migration events, fluids moved vertically along local faults and horizontally through the dolomitized layers of the formation, which likely remained porous at the time, leading to substantial silica mineralization.

Speleothems are among the most important archives for past climatic and environmental change. Calcite recrystallization can modify the authigenic structure and geochemical composition of the speleothems and affect the reliability of calcite stalagmites as repositories of authigenic geochemical proxies of past climates and environments. The criteria for distinguishing primary from secondary speleothem calcite, and the conditions (open or semi-closed) of speleothem calcite recrystallization remain poorly understood. Thus, in this study, we investigated the fabric, geochemical composition, and recrystallization dynamics of a partially recrystallized calcite stalagmite (DDH-Z-2) from Didonghe Cave in Shaanxi Province, China, through petrographic observations, fluorescence microscopy, and geochemical analyses (stable isotopes, trace elements, UTh isotopes). We found that: (1) in the DDH-Z-2 stalagmite, open elongated columnar calcite recrystallized into compact elongated columnar calcite. Particulate organic matter and fulvic and humic acids were removed during recrystallization, while aromatic compounds were preserved and became incorporated into the secondary calcite; (2) calcite recrystallization was affected by multiple factors, including external fluid chemistry, primary calcite microstructure, and organic matter; (3) calcite recrystallization occurred under open, fluid-buffered conditions for alteration of the stable isotopes (δ¹⁸O and δ¹³C) and trace elements (Mg, Sr, U). The effect of external fluid composition on trace element (Mg, Sr) composition of secondary calcite varied across the stages of calcite recrystallization. Caution should, therefore, be exercised when using geochemical proxies in stalagmites composed of inclusion-free elongated columnar calcite: such calcite is likely to be recrystallized, and thus record the composition of reactive fluids at the time of recrystallization. Regarding the geochemical system of speleothem diagenesis, the contribution of the parent material and the sources of reactive fluids are key factors to consider.

The present investigation aims to analyze the depositional environment, sequence stratigraphy, and geochemistry of the Early–Middle Eocene succession in the Laki Range of the Southern Indus Basin, Pakistan. The sequence is subsequently correlated with other sections within the Southern Indus Basin, Central Indus Basin, Upper Indus Basin, and Hazara Basin in Pakistan. For this study, the Lakhra and Laki formations were sampled from four sections in the Laki Range. Based on detailed outcrop observations, eight lithofacies were identified including four clastic lithofacies (CLF-1 to 4) and four limestone lithofacies (LLF-1 to 4). Similarly, based on detailed petrographic examination, seven microfacies were identified in limestone units. Considering the biotic paleoecology, facies texture, and chemical composition, depositional environments were assigned to the microfacies and lithofacies. All the microfacies, and the shale and sandstone lithofacies (CLF-1 and CLF-4) represent shallow inner to deeper outer-shelf settings whereas the other two lithofacies (CLF-2 and CLF-3) represent deltaic and tidal flat settings, respectively. All the microfacies and lithofacies information was used to establish sequence stratigraphy for the studied strata. The top of the Lakhra Formation and the whole of the Laki Formation in the Bara Nala Section (BNS) represent two complete, and two partial, third-order sequences, further divided into fourth order and small-scale cycles. The sequence's development is primarily influenced by regional and local tectonics. The regional correlation of the Indus Basin and Hazara Basin indicates that the regional basin's bathymetry and resultant depositional sequence were significantly influenced by the Himalayan Orogeny. The regional depositional pattern indicates that subsequent to India–Asia collision, the closure of eastern Tethys took place in a temporal succession from northwestern to the southwestern parts of the Indus Basin. In the Central Indus Basin, marine environments persisted until the Priabonian Stage (SBZ20) compared to those of the Upper (SBZ11–12) and Southern (SBZ13–14) Indus basins. This study offers valuable insights into both the local and regional depositional frameworks and the influence of local and regional tectonics on a carbonate platform evolution.

The processes responsible for reddening of Continental Red Beds (CRBs) and the relationship between color variation and paleoenvironmental conditions are presented focusing on a comprehensive multi-proxy study of Permian sediments in the Bohemian Massif, Czechia. The investigation incorporates facies analysis, quantitative color assessment using diffuse Vis-spectral reflectance (DRS), optical and electron microprobe microscopy, bulk-rock (XRF and XRD), and in-situ geochemistry (laser-ablation ICP-MS). Results indicate a progressive drying trend from the Cisuralian to Guadalupian series in studied continental red sediments. Different facies indicate the change of the sedimentary environment from a deep lacustrine environment (lower part of Rudník Member, Cisuralian) to a fluvial floodplain and eolian environment (Trutnov Formation, Guadalupian). Examination of the three major categories (white, gray–green and red sediments) identified in the studied continental red beds indicates that diagenetic alteration of clay minerals and biotite was the main source of iron fueling the growth of hematite responsible for their red color. Early diagenetic processes and paleoenvironmental conditions, particularly the oxidizing or reducing conditions play a key role in the red sediment formation. It is suggested that later diagenetic stages are incapable of coloring non-red, iron-rich sediments formed in deep anoxic lacustrine environments. Microbial activities and reducing fluids have been identified as the main factors in the formation of gray–green sediments forming distinct reduction zones. The reduction spots formed during the early stages of diagenesis (eodiagenesis), and they were likely never red. In contrast, reduction strips, initially exhibiting a red hue, underwent a color change during more advanced stages of diagenesis (mesodiagenesis).