Facies Variations Research Articles

Prediction of marl reservoir distribution based on facies-constrained reflectivity inversion method

Accurate characterization of carbonate reservoirs remains a significant challenge due to complex facies variations and the substantial effects of wave propagation. We propose a facies-constrained reflectivity inversion strategy. The method establishes a relationship between logging data and seismic waveforms, applies clustering analysis using the Self-Organizing Map (SOM) technique, and utilizes the clustering results to constrain the construction of an initial model with realistic lateral variations. Based on this initial model, a Bayesian-based reflectivity inversion is performed, incorporating a modified Cauchy prior distribution to enhance inversion accuracy and stability. The reflectivity method offers a one-dimensional analytical solution to the wave equation, tacking thin layer thicknesses and wave propagation effects into consideration, thereby significantly alleviating inversion problems encountered in marl reservoirs. Compared to traditional inversion methods based on the Zoeppritz equation, the facies-constrained reflectivity inversion delivers higher accuracy and resolution. The application of this technique to identify marl reservoirs in the Lei32 sub-member of the Sichuan Basin has yielded promising results, effectively delineating favorable reservoir areas of approximately 210 km2 and offering strong support for future exploration and development.

A quasi‐continuous long‐term (5 Ma) Mid‐European mountain permafrost record based on fluvial magnetic susceptibility and its contribution to the explanation of Plio–Pleistocene glaciations

The low field magnetic susceptibility (χLF) measured in the 1116‐m‐long Dévaványa core (Pannonian Basin) is a quasi‐continuous record of the Plio–Pleistocene Mid‐European mountain permafrost development. The continuity of fluvial conditions is confirmed by seismic data, and the detrital origin of magnetite is indicated by frequency‐dependent susceptibility measurements, scanning electron microscope, and hysteresis investigations. The χLF record is correlated to the δ18O curve (LR04) supported by palaeomagnetic data. The colour of samples documents precession and obliquity cycles in local facies variations, but the χLF indicates the dominance of 100‐ka eccentricity cycles in the linked mountainous permafrost events. Comparison with orbital solutions revealed that the long‐term development of permafrost occurs as a result of amplitude modulation of the 100‐ka eccentricity cycles. Increases in amplitude of the 100‐ka cycles inhibits permafrost development due to shortened winters. Thus, if extremes are present, the permafrost regions are limited or disappear, but if the 100‐ka eccentricity cycles are attenuated, permanent frost can extend into the temperate zone. This amplitude modulation may also be responsible for the early glaciations during the Pliocene, for the intensification of Northern Hemisphere glaciation, foreshadows cooling in the forthcoming 405‐ka term, and allows the change from 41‐ka cycles to 100‐ka ones in the Mid‐Pleistocene Transition to be explained. The 41‐ka cycles are the result of obliquity‐controlled changes close to the polar cycles, while 100‐ka cycles occur when the amplitude attenuation of the 100‐ka eccentricity cycles enables extended glaciations that suppress the regular 41‐ka cycles. Higher mountains in the catchments enable higher resolution of permafrost records documenting even smaller glaciations. However, the similarities in the overall trends in χLF records of catchment areas with 1500‐m difference in their altitude is a potential counter‐argument when considering the role of tectonic elevations in the expansions of mountainous permafrost.

Rifting Basins at the Accretion-Collision Stage of Fold Belt Development: Eastern Slope of the Southern and Middle Urals in the Carboniferous

Research subject. Sedimentary basins within the Mississippian and Early Pennsylvanian (Middle Carboniferous of Russia) rift (sensu lato) structures on the eastern slope of the Southern and Middle Urals. Methods. An analysis of lithological characteristics of sedimentary rocks and relationships with volcanic complexes in order to clarify the evolution of rifting basins. Results. During the Mississippian and Early Pennsylvanian, several large rifting basins were common in this area. The Early Carboniferous rifting basins (Magnitogorsk-Bogdanovskoe graben and Alapaevsk-Kamensk rift) of accretionary margin of the East European craton were characterized by relatively shallow-water (in some places up to coastal-marine and continental) environments, caused by intensive (compensatory) filling of depressions with sedimentary material. In this case, volcanic rocks – those of fissure-type and stratovolcanoes lava flows – also played a significant role. In the Late Visean, the rifting process paused; a relatively shallow shelf basin regime was established throughout the entire area, which persisted until the end of the Early Carboniferous. In the Early Pennsylvanian, due to a hard oblique collision, the rifting process resumed. However, the rifting basins were already smaller in terms of area and most often deeper, whereas the occurring volcanic processes were less intensive. The characteristic feature of these basins was the presence of flysch complexes. The elongated shape of the rifting basins, large thicknesses of sedimentary strata, lateral facies variability, and the abundance of coarse clastic rocks, consisting largely of fragments of directly underlying sediments (Visean, Serpukhovian, Bashkirian), suggest that the Mississippian and Early Pennsylvanian rifts of the Southern and Middle Urals eastern slope had a strike-slip nature, i.e., the rifting basins were pull-apart type depressions.

Facies variations in gravelly cyclic steps deposited from turbidity currents: Miocene fan delta front deposits compared with a modern active fan delta, central Japan

AbstractGravelly cyclic steps formed by turbidity currents have recently been widely recognised in the geological record. However, the comparison between modern and ancient gravelly cyclic steps remains challenging. In this study, the facies variations of gravelly cyclic steps deposited from turbidity currents in an outcrop of Miocene fan delta front deposits in central Japan are compared with the geomorphic evolution of analogous cyclic steps on the active fan delta front in modern geological systems. These cyclic steps share common characteristics in setting, grain size and dimensions, allowing direct comparison between ancient and modern examples. Repeat bathymetric surveys of the modern Kurobe River fan delta have revealed various types of upslope migrating bedforms interpreted as cyclic steps. Most of these are short‐lived due to erosion of thalwegs by powerful surge‐type turbidity currents triggered by slope failures or burial of thalwegs by deposition, possibly from river‐fed hyperpycnal flows. Such erosion and burial events occur annually on the fan delta front, resulting in compensational cycles. Sedimentary facies of the Miocene fan delta front deposits include conglomerate with a gently upslope dipping erosional base, backset‐stratified sandstone and foreset‐stratified sandstone, which are interpreted as deposits of hydraulic jumps in high‐density turbidity currents in scours on the stoss sides of cyclic steps. Parallel‐stratified conglomerate sandstone is an additional component. Although previous facies models of gravelly cyclic steps have focussed on deposits on stoss sides, a comparison of modern and ancient examples suggests that facies on the lee sides could be parallel‐stratified conglomerate sandstone and undulating bedforms, reflecting supercritical flow conditions. The present study also suggests that hyperpycnal and surge‐type high‐density turbidity currents may deposit different types of facies and play different roles in the construction and destruction of cyclic steps. The present study has significant implications for facies models of gravelly cyclic steps.

Diagenesis Variation in Different Distributary Channels of Shallow Water Lacustrine Delta Deposits and Implication for High-Quality Reservoir Prediction: A Case Study in the Chang 8 Member in Caijiamiao Area, Sw Ordos Basin, China

Tight oil reservoirs are considered important exploration targets in lacustrine basins. High-quality reservoir prediction is difficult as the reservoirs have complex distributions of depositional facies and diagenesis processes. Previous research has found that the diagenesis process of tight oil sandstones varies greatly in different depositional facies. However, diagenesis variation in different depositional facies is still poorly studied, especially in distributary channels of shallow water delta deposits in lacustrine basins. Based on the description of core samples, the observation of rock slices, the interpretation of well logging data, and the analysis of porosity and permeability data, the differences in the lithofacies types, diagenesis processes, and pore structures of different distributary channels have been clarified. Ultimately, a model of diagenesis and reservoir heterogeneity distribution in the shallow-water delta of Chang 8 Member of the Yanchang Formation in the Caijiamiao area of the Ordos Basin has been established. This research indicates that the main distributary channels in the study area are dominated by massive bedding sandstone lithofacies, while the secondary distributary channels are primarily characterized by cross-bedding sandstone lithofacies. There are significant differences in the compaction, dissolution, and cementation of authigenic chlorite and carbonate among different parts of the distributary channels. Plastic mineral components, such as clay and mica, are abundant in sheet sands, and are more influenced by mechanical and chemical compaction. Influenced by the infiltration of meteoric water and hydrocarbon generation, dissolution pores are relatively well-developed in the underwater distributary channel reservoirs. A large amount of carbonate cementation, such as calcite and siderite, is found within the sandstone at the interface between sand and mud. The occurrence of authigenic chlorite exhibits a clear sedimentary microfacies zonation, but there is little difference in the kaolinite and siliceous cementation among different microfacies reservoirs. Finally, a model of diagenetic differences and reservoir quality distribution within dense sand bodies has been established. This model suggests that high-quality reservoirs are primarily developed in the middle of distributary channels, providing a theoretical basis for the further fine exploration and development of oil and gas in the study area.

Terminal glacial overdeepenings: Patterns of erosion, infilling and new constraints on the glaciation history of Northern Switzerland

Glacially overdeepened basins are a common landform of subglacial erosion. However, the controlling erosional–depositional processes and their age remain poorly understood on a global scale. Terminal overdeepenings near the former glacier margins are critical for the understanding of subglacial processes and their development over time. This study examines the geomorphology and sedimentology of buried terminal overdeepenings eroded below the Rhein Glacier and adjacent lobes in the distal northern foreland of the European Alps. The evolution of erosion and infilling in the overdeepened troughs over time is investigated using high-quality drill cores (∼1463 m of core in total) that were logged for lithofacies, petrophysical, geotechnical and compositional properties. The drill data is integrated with 2D-reflection seismics (∼41 km in total) and supplementary subsurface data. This extensive dataset reveals that the studied overdeepened basins include twelve typical facies associations (partially emplaced in characteristic sequences) and characteristic architectural elements. These categories serve as effective tools to reduce the high facies variability and facilitate easier comparison and correlation of the valley fill. Our analysis shows that the formation of terminal overdeepenings on soft sedimentary bedrock (Molasse) is the result of a combination of erosional processes. Subglacial water erosion and evacuation are the dominant processes and active during periods of glacier–bed decoupling and flushing. Direct subglacial erosion occurs during glacier–bed coupling and is documented by bedrock glacitectonites. The valley fill architecture shows that the studied overdeepenings typically undergo a multiphase evolution, with several phases of overdeepening erosion, deposition and partial re-erosion (or re-activation). The combined dataset (including geochronological data) suggests that the overdeepenings were eroded during many, if not all, extensive glaciations during the Middle–Late Pleistocene that reached the distal foreland. These are findings relevant for the large number of overdeepenings known from the Northern Alpine foreland and overdeepened features worldwide. They corroborate the importance of overdeepenings as archives for the paleoenvironmental change and landscape evolution during the Quaternary.

Geochemical characteristics of crude oils and source rocks in the Malaysia-Thailand Joint Development Area (MTJDA), North Malay Basin

The Malaysia-Thailand Joint Development Area (MTJDA) that located in the North Malay Basin has been investigated in this study to resolve few issues concerning to petroleum potential and resources which has been abandon for decades. Despite Malay Basin having been proven to contain prolific oil and gas resources, the MTJDA which located in the Northern area however contains mainly gas with very minor oil. Thus, this study aimed to investigate the main factor that controls the hydrocarbon generation, type and distribution in this basin. The investigation involves biomarker characterization, thermal maturity assessment of source rock, crude oil and condensate, together with oil-oil and oil-source correlation that has never been established before. The geochemical data as indicated by bitumen extraction, GC-MS and Py-GC analyses on 40 cutting samples and 8 liquid samples (six oil samples and two condensate samples) revealed that the cutting samples were moderate in quality with variable richness based on TOC (total organic carbon) and a variable degree of maturity based on the biomarker proxy parameters of isoprenoid, hopanoid and steroid. The organic matter input for the source rock was derived from a mixed marine-terrestrial and terrestrial source and deposited in fluvial-deltaic/estuarine settings having oxic to sub-oxic depositional conditions. Based on biomarker fingerprints and proxy parameters, the oil and condensate samples were essentially of the same group, originating from a similar source rock, with some facies variation due to variations in organic matter input and degree of maturity. The results also indicated that the crude oil samples were originated from the coaly and carbonaceous shales of Group I, with mixed kerogen of type II/III that could produce gas with minor oil. There is also the possibility that these hydrocarbons were generated from a deeper interval of Group E (higher thermal maturity), deposited in a similar setting as Group I.

Facies variability and depositional cyclicity in central Northern Switzerland: insights from new Opalinus Clay drill cores

The Opalinus Clay, a silty to sandy claystone formation, Early to Middle Jurassic (Toarcian and Aalenian) in age, has been selected as the host rock for deep subsurface disposal of radioactive waste in Switzerland. Over the past thirty years, numerous geotechnical, mineralogical, and sedimentological studies have been conducted on the Opalinus Clay within the framework of the Nagra (National Cooperative for the Disposal of Radioactive Waste) deep drilling campaigns and the Mont Terri Project, an international research program dedicated to the study of claystone. The present study aims to unravel the variability of the lateral and vertical facies of the Opalinus Clay in central Northern Switzerland and to place this variability in a regional and basinal context. Analyses of new cores drilled in central Northern Switzerland, including petrographic, mineralogical (X-ray diffraction, multi-mineral interpretation), geochemical (X-ray fluorescence), statistical (non-metric multidimensional scaling analysis), and bedding dip and azimuth data, shed new light on the depositional facies and the spatial and temporal variability of the Opalinus Clay. Petrographic descriptions encompass nine new drill cores using a revised subfacies/facies classification scheme based on texture (colour, grain size, bedding) and composition (mineralogy). Particularly, one new subfacies (SF6) is described and interpreted as mass-wasting deposits. The drill cores are correlated laterally using specific marker horizons. This correlation is achieved by combining thorough facies investigations with lithostratigraphy, biostratigraphy, and chemostratigraphy. Six to seven small coarsening-upward cycles and two long-term coarsening-upward sequences can be interpreted as regressive trends. The observed trends are influenced by the interplay between sediment supply, eustatic sea level change, synsedimentary subsidence, but also the palaeogeographic configuration in an epicontinental sea, provenance and delivery of sediments, current dynamics and climate change. Finally, combined results show that the current dynamics in the Opalinus Clay has been underestimated until now and new depositional models, including the occurrence of drift deposits, are discussed.

Geochemical characteristics and classification of Oligocene source rocks with different facies in Bozhong Sag, Bohai Bay Basin, East China

There have been many studies conducted on the Eocene source rocks (Shahejie Formation Mbrs 1 and 3) (E2s1 and E2s3) of the Bozhong Sag, Bohai Bay Basin in East China. However, Oligocene source rocks (Dongying Formation Mbrs 2 and 3) (E3d2 and E3d3) have also made significant contributions to petroleum accumulations in this region. Their organic matter (OM) enrichment mechanisms, environmental and ecological variations, and laterally variable facies remain subjects of debate, particularly in the lower Dongying Formation Mbrs 2 (E3d2L). Thus, we investigated the geochemical properties, depositional environments, and OM origins of E3d2L and E3d3 source rocks from seven prospecting wells in southwestern Bozhong Sag. Our findings indicate that E3d2L source rocks are of poor-to-medium quality, whereas E3d3 source rocks are classified as good-to-excellent quality. Notable differences in the geochemical characteristics between E3d2L and E3d3 source rocks were observed. Hierarchical cluster analysis identified five genetic organic facies. The E3d2L source rocks comprises facies D2-Ⅰ, D2-Ⅱ, D2-Ⅲ, and D2–D3, whereas E3d3 contains facies D3 and D2–D3, with D2–D3 being common to both source rocks. Additionally, an integrated OM enrichment model was established to evaluate Oligocene source rocks. The five genetic organic facies exhibit distinctive differences and some similarities regarding redox conditions, water salinity, and OM origins. Their geochemical characteristics within the same intervals display pronounced lateral heterogeneity from deep source rocks deposits to more marginal water depths. The established model provides guidance for the prediction and exploration of effective Oligocene source rocks in the Bohai Sea and research on the Paleogene and Neogene petroleum systems. Moreover, the integration of biomarkers and hierarchical cluster analysis methods for organic facies analysis has significant implications for predicting of source-rock heterogeneity in other lacustrine rift basins.

Paleoenvironmental factors controlling organic-rich formations deposition in the Babouri-Figuil Basin (Northern Cameroon)

The Babouri-Figuil Basin is an intracratonic basin (half-graben) in northern Cameroon that is genetically connected to the Benue Trough from Nigeria, and is an area of interest in terms of petroleum prospectivity. Recent studies highlighted the presence of organic-rich formations in the basin. However, none of these works have identified factors that governed the accumulation of organic matter (OM) in the sediments. The main objective of this work is the characterization of these formations through palynofacies and organic geochemical techniques (total organic carbon - TOC, total sulfur, insoluble residue and biomarkers), in order to determine the organic facies, their depositional environments and the main drivers for organic enrichment in the basin. The current study reveals that black shale and massive claystone lithologies constitute the main organic-rich formations in the basin, with TOC reaching up to 26.08 wt%, being characterized by a dominance of bacterially-derived amorphous OM. Palynofacies and biomarker data revealed that these formations are positively associated with anoxic conditions and a partly highly saline and stratified lake water column. The deposition of organic-rich formations in the Babouri-Figuil Basin was mainly controlled by restriction conditions which developed in connection with the regional tectonic framework. The Lower Cretaceous rifting episode in the West and Central African Rift System (WCARS) basins led to the formation of accommodation space, a reduction in water levels, and the development of anoxic conditions within the basin, facilitating the deposition of organic-rich formations. Therefore, the organic enrichment of the Babouri-Figuil Basin has been predominantly controlled by its tectonic evolution, particularly during the syn-rift phase. This phase created favorable conditions for the deposition and preservation of OM, including the establishment of anoxic conditions. Additionally, the paleoclimate (arid conditions), the development of bacterial biomass, and the basin's paleogeography all played a significant role in this process. The organic-rich formations of the Babouri-Figuil Basin show characteristics of prospective petroleum source rocks (high organic content, high proportion of oil-prone kerogen, significant thickness and lateral extension). The combination of organic-rich formations with sandstone deposits above and extensive claystone/shale deposits on top can indicate the presence of an oil play in the basin. A detailed study with broader sampling is needed to investigate thoroughly the variation of organic facies and the influence of paleoenvironmental factors that control the deposition of thick source rock intervals.

Facies variability of the Lower Miocene Houthalen Member in the southern Genk municipality (northeastern Belgium)

Facies variability of the Lower Miocene Houthalen Member in the southern Genk municipality (northeastern Belgium)

Siliceous deposition in limestone-marl alternations of the Yangtze Carbonate platform during the Permian Chert Event

During the Permian Chert Event (PCE), limestone-marl alternations (LMAs), contemporaneously developing in the Yangtze region with chert-mudstone alternations, differed from typical LMAs by their enrichment in authigenic siliceous components (quartziferous components and Mg-phyllosilicates). However, the factors driving the transition from distal chert deposits to proximal siliceous-rich carbonate deposits remain unclear, and the genesis of authigenic siliceous components is puzzling. Resolving these issues could provide a deeper understanding of the paleoclimate and paleoenvironment during the PCE. Optical and SEM observations and geochemical analyses revealed that Si4+, derived from seawater, silica-secreting organisms, and terrigenous clasts, precipitated as authigenic quartziferous components under relatively acidic conditions, while it precipitated as Mg-phyllosilicates through reverse weathering by combining with the dissolved high-Mg calcites under relatively alkaline conditions. Through the facies division of LMAs, its variations describe relative and short-term sea-level fluctuations in the Yangtze Carbonate Platform (YCP). By integrating tendency comparisons and time series analyses of sea level fluctuations, continental weathering intensity, authigenic siliceous content, and relative proportions of quartz vs. Mg-phyllosilicates, four proxies exhibit covariation and similar astronomical signals, indicating that the high sea level period of the YCP, corresponding to intensified continental weathering, favored the deposition of authigenic siliceous components and reverse weathering. Because the facies variations encompass short eccentricity, obliquity, and precession cycles, sea level fluctuations are considered to have been influenced by Gondwana glaciation P3. The seawater in the continental shelf cooled and became rich in silica and nutrients under the effect of upwelling. Interglacial periods may trigger high sea levels and intensified continental weathering in the YCP, facilitating the influx of Si-rich eutrophic seawater and stimulating biological silica secretion by radiolarians. However, the direct precipitation of activated silica resulted in weakened reverse weathering. Consequently, the lithological transformation from distal to proximal areas during the PCE was attributed to differences in silica-secreting organism abundance, upwelling intensity, biological pump activity, reverse weathering intensity, OM abundance, pH conditions, and seawater temperature between the continental shelf and carbonate platform.

Characterization of seismic-scale petrofacies variability in the Arbuckle Group using supervised machine learning: Wellington Field, Kansas

The Arbuckle Group in southern Kansas has been investigated for carbon geosequestration-related studies. In this study, we evaluated the seismic-scale petrophysically defined facies variability of the Arbuckle Group at the Wellington Field, Kansas, using quantitative seismic interpretation and a supervised Random Forest classification approach. We first defined three petrophysics-based rock types (petrofacies) from core-derived porosity and permeability measurements using the flow-zone indicator approach. Then, using the artificial neural network, we classified these petrofacies in noncored intervals. We observed that petrofacies 1 corresponds to medium and coarse-grained dolomitic packstone, wackestone, and dolomitic breccia with up to 8% porosity and D-scale permeability values. Whereas petrofacies 2 and 3 correspond to argillaceous and fine-grained micritic dolomites and dolomitic mudstones with lower permeability values for a given porosity, with respect to petrofacies 1. Using the common reflection-point gathers, we performed prestack seismic inversion and calculated various amplitude-variation-with-offset (AVO) attribute volumes. We used these elastic properties and AVO attribute volumes as input for estimating supervised seismic-scale 3D petrofacies and petrofacies probability volumes using the Random Forest algorithm. Results reveal the complex distribution of the petrofacies in the candidate injection and baffle zones in the study area, where petrofacies 1 is mainly prevalent within the lower and upper portions of the Arbuckle group, whereas petrofacies 2 and 3 are mainly present in the middle Arbuckle interval. The workflow we present through this study provides the spatial variability of facies distribution that is reflective of the actual lithology and petrophysical properties of the Arbuckle group in the study area with limited well control.

Sedimentary Facies, Depositional Environment, and High-resolution Sequence Stratigraphy of Dernah Formation, N Cyrenaica Promontory, NE Libya

The depositional environment and high-order stratigraphic architecture of the Eocene Dernah Formation in N Cyrenaica Promontory, NE Libya, were interpreted through detailed sedimentary facies and sequence stratigraphic analyses of four measured sections located several kilometers apart. Five distinct lithofacies were delineated based on variations in lithology, grain composition, fossil content, texture, color, and stacking pattern. Dernah Formation mainly contains nummulitic limestone with a noticeable amount of Discocyclina and rare gastropods. The extraordinary presence of Nummulites indicates shallow-marine carbonate settings, perhaps a Nummulite bank environment, where the reworking of sediments by wave- or tide-induced currents and storms is high. Two complete fifth-order depositional sequences were defined. Each sequence was subdivided into a transgressive and a regressive hemicycle based on the vertical lithological variation and stacking pattern of facies. We conclude that each sequence may have formed as a result of a fifth-order sea-level cycle. This study reconstructs a three-dimensional model for the Nummulite bank environment in the Mediterranean region, which can serve as an analog for similar depositional systems elsewhere.

Late syn‐rift to early post‐rift basin fill dynamics of a mixed siliciclastic‐carbonate succession banked to a basement high, Hornsund, southwestern Spitsbergen, Arctic Norway

AbstractThe transition from syn‐rift to post‐rift sedimentation in rift basins is difficult to characterize in terms of stratigraphic architecture and dominating control on sedimentation, due to decreasing tectonic activity interplaying with regional subsidence, eustatic sea level changes, and differential compaction of underlying syn‐rift sediments. Our case study of the Late Palaeozoic Inner Hornsund Fault Zone targets late syn‐rift strata recorded in the (?Pennsylvanian – ?lower Permian) Treskelodden Formation in Hornsund, southern Spitsbergen, representing a mixed siliciclastic‐carbonate succession, with siliciclastics primarily sourced from the adjacent Sørkapp‐Hornsund High. We document local scale (<10 km) facies variability, sequence stratigraphy, and evolution of a succession deposited along a flank of the structural high during the late syn‐rift stage. We observe that during the transition towards rift termination (glacio‐)eustatic sea level changes and overall regional flooding became a more prominent forcing factor controlling sedimentation. Our dataset includes sedimentary logs, microfacies analysis, and high‐resolution digital outcrop models. We identify four progressively backstepping stratigraphic sequences, reflecting an evolution from (1) terrestrial siliciclastics through (2–3) nearshore mixed siliciclastic–carbonates, to (4) carbonate ramp deposits. On the small scale (<5 m) the internal sediment cyclicity of the succession was formed by autogenic processes, particularly the changing rate of sediment input from the southwestern source area (the uplifted Sørkapp‐Hornsund basement high). On the larger scale (10s of m), the importance of glacio‐eustatic sea‐level changes, driven by waxing and waning of ice caps in the southern hemisphere (Gondwana), increased as the rift‐related tectonics decreased. The interdisciplinary methods used in this study provide new knowledge of the Middle Pennsylvanian to Permian depositional evolution in southern Spitsbergen, besides a novel framework for comparison to adjacent basins in the region and similar basins elsewhere.

Petrographic and geochemical study of Precambrian and Paleozoic rocks in the western Anti-Atlas belt, Morocco: Facies, weathering, provenance and economic exploitation

Detrital and volcanic-detrital rocks from the Ifni Buttonhole and Lakhssas Plateau were analyzed to determine their provenance, compositional maturity, and alteration source. Geochemically, the sediments were classified as arkoses, lithic arenites, grauwackes, sandstones, lithic arenites, and Fe-rich sands, indicating low compositional and mineralogical maturity. A high average SiO2 concentration and low Al2O3 were consistent with a low abundance of shale and clay components. The geochemical signatures of the detrital and volcano-detrital (RDVD) rocks indicate that they have undergone a moderate to low degree of chemical alteration. The CIA study also suggests that the granitic, granodioritic rocks represent the source provenance which, during weathering and transport, supplied the detritus to the supra-crustal units. The major trace element data suggest that these rocks are largely derived from felsic igneous rocks, namely granitoids, with a minor contribution from intermediate sources. The carbonate rocks do not represent a wide variety of facies: dolomitic limestone, calcareous limestone, and calcaro-dolomitic chert. Calcitic and dolomitic samples show a linear increase in SiO2, regardless of their CaO/LOI ratio values, which remain relatively constant. The highest SiO2 contents are observed in the calc-dolomitic chert. Geochemical analysis of RDVD from the Ifni buttonhole determined their origin, maturity, and alteration. Major oxides decreased with higher silica content, indicating quartz control. Samples, formed under semi-arid conditions, show maturity under stable deposition. They suggest a felsic, recycled source, with moderate alteration and zircon enrichment during recycling. In the study area, limestones and dolomites serve as materials applicable in the building sector, suitable for all types of concrete. The Taliwine Formation harbors Lower Cambrian dolomites and limestones, ideal for mosaic aggregates. Described as variable in color, compact, homogeneous, very hard, and resistant to alteration, the plutonic rocks form prominent peaks. They exhibit both subalkaline characteristics in granitoids and an alkaline trend in dolerite dykes. Most samples display minimal alteration, indicating the reliability of their major element compositions for geochemical analyses. These granitoids constitute valuable deposits for ornamental and building rock purposes.

LITHOLOGICAL-STRATIGRAPHIC CRITERIA OF KARST ZONING OF THE MIOCENE SULFATE LAYER OF WESTERN UKRAINE

The proposed karst regionalization (zoning) is a novel approach to identifying the unique characteristics of karst processes in specific areas. The sulphate karst of western Ukraine, linked to the Miocene evaporite basin at the junction of the East European Platform and the Pre-Carpathian Foredeep, is a prime example. The previous karst zoning schemes of the East European Plain classified the entire area of sulphate rocks distribution in the west of Ukraine as the Podilsko-Bukovinsky karst Area, with the West Podilsky and Pokutsko-Bukovinsky karst Regions within it. However, with the emergence of numerous detailed stratigraphy works on the evaporite formation of this territory, a new approach to karst zoning in evaporites is necessary. This paper presents a systematic approach to upgrading the mentioned regions to the rank of sub-areas and distinguishing the regions within them based on lithologic-stratigraphic differences. The authors studied six localities as examples. In the first one, the gypsum layer is characterized by the stromatolite, grass-like and sabre-like facies. The last two contain thin carbonate-clay layers, leading to the collapse of large rock blocks inside the Kadubivska Cave. On the second site, the gypsum is composed of stromatolite, coarse-crystalline, and sabre-shaped facies. Only one small Tovtry Cave, which has a hypogene morphology, is known here. The third site has grass-like gypsum facies, and intensive modern karst occurs, with numerous sinkholes, underground rivers, and sinkholes. The influence of facies variability on the development of the mesomorphology of underground galleries is traced here in Pionerka Cave. The fourth area is built of nodular gypsum facies. There are no insoluble layers here, but the facies have many thin mineral films between the grains. Therefore, the entrance parts of the caves are unstable for physical weathering. The fifth section is highlighted as a section of the spread of metasomatic limestone, which replaces sulphate deposits here. They contain a significant cave labyrinth, but whether karstification occurred before or after metasomatization remains unclear. The sixth section is completely composed of stromatolite gypsum facies. It is characterized by stability to collapse due to the monolithic structure of gypsum, which is noticeable in the example of the Cave of Popeliushka (Zolushka) on the border of Ukraine and Moldova. The selected areas are proposed to be considered as examples of karst zoning based on lithologic and stratigraphic features. Key words: karst regionalization; gypsum, limestone; caves, breakdown; relief, landscape.

Evidence of theropod dinosaurs in the upper hauterivian–lower barremian of Jebel Kebar (central Tunisia): Paleobiogeographic implications

The fossil record of Mesozoic dinosaurs in Tunisia has long been thought to be restricted exclusively to the southern part of the country. Although tidal and continental environments have been recognized within Cretaceous formations in central Tunisia based on sedimentological analyses, no skeletal remains ascribable to dinosaurs have been found in these deposits. The hypothesis of a possible land connection between central and southern Tunisia during the Early Cretaceous has not been previously explored due to notable lateral variations in facies, marked by the recurrent occurrence of shallow marine deposits separating these domains. The vertebrate ichnological record in the Central Tunisian Atlas was limited to isolated tridactyl footprints exclusively observed in the Albian Kebar Formation. Here, we report the first dinosaur tracks from the pre-Albian Cretaceous (upper Hauterivian–lower Barremian) of central Tunisia. These tracks consist of large tridactyl footprints left behind by medium to large-sized non-avian theropods. They occur within an extensively fractured dolostone bed and exhibit poor morphological preservation. Besides their dimensions, some tracks show morphological features, including broad digit impressions with some terminating in sharp claw traces, low to moderate mesaxony, and a wide divarication angle of the outer digits, reminiscent of the ichnogenus Megalosauripus. However, due to their poor state of preservation and limited number, we refrain from assigning these tracks to the ichnogeneric level and refer to them as indeterminate eubrontid footprints. The occurrence of theropod tracks in the Lower Cretaceous of central Tunisia confirms the sedimentology-based hypotheses about a temporal emergence of parts of this area, which was probably related to tectonic events. We compile previous biostratigraphic, sedimentological, and paleoichnological data to suggest possible periodic connections between the mainland in the south and the tidal flat environments within the central basins. Additionally, two possible scenarios regarding the biogeographic implications of the Jebel Kebar tracks are discussed. The first scenario suggests that a Gondwanan theropod fauna would have been exploring and residing in newly emerged regions of higher latitudes, whereas the alternative scenario posits the existence of a paleogeographic link between the northern margin of Gondwana and Adria, which would have allowed an interchange of theropod faunas between the two landmasses at least in the early Barremian.

The anatomy and stacking pattern of palustrine-dominated carbonate sequences from the Cengle Plateau, Paleocene, SE France: A multi-scalar approach

An integrated approach combining petrography, photogrammetry, geochronology, SEM, and geochemical data was utilized to analyze lithofacies, stacking patterns, and lateral facies variations, and to interpret environmental dynamics during the deposition of carbonates from ‘La Barre du Cengle’ in the Early Paleogene. Located in the SE of France, the elliptical Cengle Plateau stretches 7 km from east to west and is 2 km wide, featuring cliffs ranging in thickness from 20 to 35 m. These cliffs showcase grayish, beige, and pinkish limestones dominated by palustrine facies, forming part of the ‘Calcaire de Saint Marc’ Formation of the Arc Basin. Sedimentary deposits within this interval were repeatedly subjected to subaerial exposure due to fluctuations in lake levels driven by climate, resulting in the organization of elementary sequences at decimetric to metric scales, which stack up into small-scale sequences at the decametric order. Deposition occurred under mainly subarid climatic conditions, with paleogeographic variations in the basin corresponding to changes in lake base levels over time and space. At least four frequencies of base level variation are present: very high and seasonal frequency, responsible for the formation of palustrine facies; high frequency, which generates the elementary sequences; medium frequency, which leads to the formation of the small-scale sequences; and low frequency which corresponds to the deposition of the entire set of limestones that form the Cengle cliff. The transition between lacustrine, palustrine, and pedogenic environments consistently occurs from west to east over time. In the more distal regions, the proportion of lacustrine facies tends to increase, and the thicknesses of the preserved sedimentary record tend to be greater. Conversely, palustrine and pedogenic facies predominate in the more proximal areas, typically resulting in decreased thickness.

Diagenetic history and porosity evolution of the Middle Permian clastic-carbonate mixed system, Indus Basin, Pakistan: Implications for reservoir development

This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin, Pakistan. Multiple data sets including outcrop, petrography, cathodoluminescence, scanning electron microscopy (SEM), mineralogy, and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis. The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend. The low values of δ18O of the target strata, relative to average values of the Permian carbonate, hints to diagenetic alteration in the strata. The data sets used in this study reveal modification of the strata in four environments, that is, i) early marine diagenesis indicated by micritization, pervasive dolomitization and isopachous fibrous cements, followed by ii) meteoric dissolution, and iii) shallow burial diagenetic processes including the precipitation of blocky cement, compaction of skeletal and non-skeletal allochems, and stylolites, and iv) a deep burial environment, characterized by pressure solution, and micro-fractures. The clastic intervals host subangular to subrounded quartz grains, floating textures, and almost complete absence of deleterious clay minerals, consequently resulting in the preservation of primary porosity. The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity. The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution. The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction, precipitating sulfides within the pores. The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit, and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies. However, both intervals show very low permeability values probably due to isolated moldic pores and intense micritization. Therefore, clastic intervals may provide an opportunity to serve as a moderate reservoir; however, the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.