Mixed Siliciclastic Carbonate Research Articles

Lithostratigraphy of the Portfjeld Group (Ediacaran – lowermost Cambrian) of North Greenland

The Portfjeld Formation, as originally defined, is the lowermost lithostratigraphic unit of the mainly Lower Palaeozoic Franklinian Basin in southern Peary Land, central North Greenland. The unit crops out semi-continuously from Valdemar Glückstadt Land to Nordenskiöld Fjord but is also recognised locally in northern parts of Peary Land and Wulff Land (western North Greenland). Regionally, it provides a key record of the early, pre-break-up history of the basin. The type succession in southern Peary Land has thus been the focus of recent biostratigraphical, sedimentological and geochemical study. This has demonstrated the composite nature of the unit, a lower interval (c. 190 m thick) of carbonate ramp deposits of Neoproterozoic (late Ediacaran) age being overlain at a karstic unconformity by a shallow marine, mixed carbonate–siliciclastic interval (c. 100 m thick) of probable earliest Cambrian age. Lithostratigraphic revision of this succession is presented here. The Portfjeld Formation throughout North Greenland is elevated to the rank of group, and two new formations are defined in this group in southern Peary Land and immediately adjacent areas: the Ediacaran carbonate-dominated unit is referred to the Slusen Formation, the upper mixed siliciclastic–carbonate unit to the Glaciologelv Formation.

Compound-clinoform model for tide-influenced deltas in the mixed siliciclastic–carbonate Cisco Group, Upper Pennsylvanian (Virgilian), Eastern Shelf of the Permian Basin, U.S.A.

ABSTRACT Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Upper Pennsylvanian (Virgilian stage) mixed siliciclastic–carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: i) predominant heterolithic lithology, ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes, iii) an abundance of interpreted fluid muds, and iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: delta plain–tidal flat, distributary channel and distributary-mouth bar (delta front) deposits on the subaerial delta; subaqueous delta topset, subaqueous delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two additional facies associations, transgressive shelf deposits and subtidal shallow-water carbonates, record continuous episodic transgressions in the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound-clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound-clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic–carbonate systems in icehouse conditions.

Recognition of a cryptic maximum flooding surface in shallow marine carbonate sequences using geochemical (Y/Ho) proxy data

ABSTRACTSince diagnostic primary depositional sedimentary structures and depth‐dependent grain‐size trends are rarely preserved, building a comprehensive sequence stratigraphic framework for the vast majority of the Phanerozoic carbonate platform sequences is pending. Among the two most important sequence stratigraphic surfaces, while the subaerial unconformity can be reliably identified by either karst development or the appearance of siliciclastic materials, the demarcation of the maximum flooding surface remains difficult in lithologically uniform shallow marine carbonate sequences. The present study attempts to identify the globally documented maximum flooding surface within the body of the negative carbon isotope excursion of the Palaeocene–Eocene Thermal Maximum recorded in the shallow marine carbonate platform sequences. The results show that, along with the carbonate microfacies, the yttrium to holmium ratio (Y/Ho ratio) of the carbonate fraction reliably records the sea‐level changes. A Y/Ho ratio between 70 and 80 demarcates the stratigraphic position of the maximum sea‐level state (the most open marine condition in the studied interval) and maximum flooding surface in the studied sections. Since the Y/Ho ratios remain relatively stable throughout diagenesis, they can be used for maximum flooding surface identification in shallow marine carbonate platform sequences. The possibility exists that the same method can also be applied to the mixed siliciclastic–carbonate systems.

High‐frequency palaeoenvironmental changes in the mixed siliciclastic–carbonate sedimentary system from a lower Permian restricted basin (West Gondwana, southern Brazil)

ABSTRACTGlobal climatic and palaeogeographical changes generated a siliciclastic–carbonate system with high organic matter accumulations in a shallow sea during the lower Permian in Western Gondwana. The 60 m thick mixed siliciclastic–carbonate succession (Irati Formation and the base of the Serra Alta Formation) from the Paraná Basin represents a singular record of the interplay between carbonate production and siliciclastic input, providing a window of opportunity to integrate large‐scale depositional architecture with facies and geochemical analyses. The detailed study of cores supported by outcrop columnar sections revealed a siliciclastic‐dominated, retrogradational to aggradational facies, and a carbonate‐dominated, mixed aggradational to progradational facies composing three depositional sequences that record an outer‐ramp and mid to upper‐ramp. An integrated approach based on the description of facies and microfacies, organic geochemistry and mineralogical composition, indicated high‐frequency palaeoenvironmental changes during the evolution of this restricted basin. Climate changes, resulting in humid and dry phases, produced ideal conditions for high organic matter production and dolomite formation. The high organic matter production (humid phases) in addition to the restricted condition was responsible for the anoxic bottom waters that were widespread due to the low angle and homoclinal platform. Nutrient‐rich freshwater inflows in the anoxic and hypersaline restricted basin created a density‐stratified water column causing low‐salinity surface and anoxic bottom water, which allowed planktonic life, typical of lakes, such Botryococcus braunii, to flourish in the photic zone and sulphur bacteria to populate below the chemocline. Microbial activity induced primary dolomite precipitation (dry phases) and widespread formation of synsedimentary dolostone. Freshwater inflow and marine incursions are reflected in the organic matter accumulation (kerogen types I and II, respectively), generating bituminous shale with high total organic carbon (5 to 27 wt.%).

Generating 3D lithology probability volumes using poststack inversion, probabilistic neural networks, and Bayesian classification — A case study from the mixed carbonate and siliciclastic deposits of the Cisco Group of the Eastern Shelf of the Permian Basin, north-central Texas

The deposition and mixing of carbonates and siliciclastics in the Cisco Group of the Eastern Shelf of the Permian Basin are complicated by the temporal overlap between icehouse eustatic sea-level oscillations and fluctuations in sediment influx due to the rejuvenation of the Ouachita fold belt. Previous investigators have used well-log correlation as the primary tool in their interpretations of the area’s reciprocal depositional model, but well-log correlation alone cannot explain the full range of spatial lithology variations in the system. To better understand the lithology variation in the area, we use an integrated technique that combines wireline log information from 17 wells with 625 km2 3D seismic data through poststack seismic inversion, probabilistic neural networks (PNNs), and Bayesian classification. We use deterministic matrix inversion to derive lithology classes from well logs. Crossplot analyses reveal that the acoustic impedance and neutron porosity log pair can be used to differentiate lithologies. We perform model-based poststack inversion to generate a P-impedance volume and use PNNs to generate a neutron porosity volume. We combine these volumes through supervised Bayesian classification to generate lithology probability volumes for each lithology and a most probable lithology volume throughout the seismic data. The lithology volumes highlight the dominant lithologies (carbonate, shale, sand, and mixed) that allowed the interpretation of major carbonate platforms, sand-to-shale ratio variations, carbonate buildups between wells, and channel fill lithologies. Our semiautomated lithology detection workflow applies to regional studies and is also valid for reservoir-scale studies to determine variations in lithologies.

Microfacies Analysis of Mixed Siliciclastic-Carbonate Deposits in the Early-Middle Ordovician Meitan Formation in the Upper Yangtze Platform in SW China: Implications for Sea-Level Changes during the GOBE

During the Early-Middle Ordovician, the Upper Yangtze Platform experienced extensive development of a distinctive set of mixed siliciclastic-carbonate deposits known as the Meitan Formation. To conduct a comprehensive study of the relationship between mixed sedimentation and sea-level changes, the Hailong section, situated at the southwest margin of the Upper Yangtze Platform in SW China, was selected as the study area due to its typical mixed sedimentary sequence. This section was effectively compared with sections in other regions. Clustering analysis of the point-count groups from the Honghuayuan and Meitan Formations revealed nine microfacies that developed during this period. Such a diverse range of microfacies provides the evidence of frequent sea-level changes in the Yangtze Platform throughout this period. Specifically, two sea-level rises were identified during the early TS.2b and early TS.3b, with the early TS.2b event occurring globally. Furthermore, four sea-level falls were observed in the late TS.2b, TS.2c, TS.3a, and late TS.3b periods. The late TS.2b sea-level fall was globally significant. From TS.2c onwards, distinct variations in sea-level changes among the Yangtze Platform, North China, Baltoscandia, Australia, and North America emerged due to alterations in the paleogeographic pattern. By comparing the sea-level curves in the Upper Yangtze Platform with the diversity curves of chitinozoans, acritarchs, and brachiopods, it became evident that environmental changes played a crucial role in the Great Ordovician Biodiversification Event (GOBE), especially during the Floian. The rising sea level and increased detrital materials fostered the development of diverse habitats, prompting organisms to adapt to varying environments. In general, rising sea levels favored increased brachiopod diversity, while falling sea levels favored enhanced planktonic diversity among chitinous and acritarch species. By shedding new light on the relationship between Ordovician sea-level changes and biodiversification in the Yangtze region, this study offers a fresh perspective on the subject from the microfacies analysis.

Pore Structure and Fractal Characteristics of Mixed Siliciclastic-Carbonate Rocks from the Yingxi Area, Southwest Qaidam Basin, China

Summary Evaluating reservoir properties at the pore scale is vital to better estimate hydrocarbon reserves and plan field development. The lacustrine mixed siliciclastic-carbonate deposits of the Upper Paleogene Xiaganchaigou Formation in the west Yingxiongling area form one of the most important hydrocarbon reservoirs in the southwestern Qaidam Basin (China). In this study, we analyzed well samples with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and mercury injection capillary pressure (MICP) data in integration with scanning electron microscopy (SEM) images to decipher the mineral composition and pore structure characteristics of the Xiaganchaigou Formation. We also calculate the fractal dimensions using MICP, NMR T2 spectrum, and SEM images based on fractal theory models. The results indicate that the mixed siliciclastic-carbonate samples of the upper section of the Xiaganchaigou Formation are mainly formed by dolomite and clay minerals with low siliceous and calcite content. Porosity is relatively low (2.01−9.83%) and positively correlated with dolomite content, thus indicating that the dolomite intercrystalline pores formed by infiltration and reflux dolomitization control the reservoir characteristics. The size of dolomite intercrystalline pores varies between several and hundreds of nanometers. The porosity has a poor correlation with permeability, which indicates that the pores are mostly primary, which lack the transformation of late dissolution. Three types of mixed siliciclastic-carbonate reservoirs are identified according to pore size distribution (<20 nm, 20−300 nm and multiple distribution), calculated using the NMR T2 spectrum. Fractal curves calculated by combining the MICP and NMR data are characterized by multisegments. The number of segments depends on the degree of heterogeneity of pore structure: two segment for high heterogeneity and three segment for low heterogeneity, also indicating a multifractal feature in mixed rock reservoirs. There is a negative correlation trend between porosity and fractal dimensions, and larger pores often have larger fractal dimensions. These results show that MICP-based fractal values are higher than those of NMR-based, which result from unconnected pores that the MICP is unable to reach. Fractal dimensions obtained from SEM have a small and narrow distribution range and are negatively correlated with the number of pores with smaller sizes. In essence, this study shows that the fractal dimension can be a concise index to evaluate the heterogeneity of lacustrine mixed siliciclastic-carbonate reservoirs, which can serve as an important reference for hydrocarbon development plans.

Lithostratigraphy, lithofacies and deposition conditions of the late Olenekian-middle Anisian Alam Formation in the Nakhlak area, central Iran

The present study aims to investigate petrography characteristics and sedimental environments of upper Olenekian–middle Anisian deposits of Nakhlak group in the sedimentary-structural zone of central Iran. To this end, a section of these deposits with a thickness of 720 meters was first selected, and 190 samples were taken from this section for preparing thin sections. Also, quantitative and qualitative laboratory tests were performed and 25 samples of sandstone were selected for point counter analysis. The results indicated that the major deposits of this formation are related to siliciclastic, carbonate, and pyroclastic types, which are in the form of two facies associations. The facies association A at the bottom of the studied section with the thickness of 90 meters has a periodicity of clastic, carbonate, and pyroclastic facies. The clastic facies of this association consists of fine-grained sandstone. In addition, the carbonate facies in association A are characterized by two microfacies consisting of intraclast oolitic grainstone and oolitic grainstone. Further, tuffite is the only pyroclastic facies related to association A, which has a laminate structure, cross-bedding lamination, herringbone bedding, and upward-thinning and thickening cycles. The facies association, B which is deposited later, is more in the middle and top part of the section. The facies association B is 660 meters thick and has a periodicity of clastic and limestone facies. Clastic facies of this association includes four lithofacies such as conglomerate, pebble coarse-grained sandstone, medium-grained sandstone to thin layer siltstone and shale. The carbonate facies consists of four microfacies such as sandy intraclastic wackestone and mudstone. Furthermore, slumping structures, cross bedding, turbulence, boudinage structure, and convolute of sediments, along with the effects of erosional channels are observed in facies B. Field and laboratory studies conducted on this succession indicate the structure of facies association A was deposited in a mixed siliciclastic carbonate ramp. Finally, submarine canyons and submarine fan environments were left by increasing the depth of the facies association B in the deep part of sea and turbidity systems.

Expression of the Toarcian Oceanic Anoxic Event in the mixed siliciclastic–carbonate system of the Arabian Plate: Insights from rare-earth element geochemistry and statistical analysis

Rare earth elements and yttrium (REEY) are considered powerful indicators of paleo-redox conditions. However, to date, the utilization of REEY remains limited to either pure carbonates or deep marine shales, with little evidence of attempts to test their ability to depict marine anoxia within mixed carbonate–siliciclastic systems. Across the Tethys Ocean, the Toarcian Oceanic Anoxic Event (T-OAE) is widely recorded by black marine shales with strong negative carbon isotopic excursion. Yet, the occurrence and expression of T-OAE across the mixed siliciclastic and carbonate system of the Arabian Plate has not been recorded or even explored. This provides a unique opportunity to test the robustness and viability of REEY, supported by statistical analysis, in revealing marine anoxia in a mixed system. Our results demonstrate a positive Ce anomaly within the middle Marrat, coupled with a positive Eu anomaly and aluminum-normalized Mn enrichment in the underlying lower Marrat carbonates. This could be hypothetically related to volcanic pulses, which triggered the T-OAE. Overall, the REEY geochemical record and detailed statistical analysis of this study indicate, for the first time, the possibility that T-OAE might have extended to the Arabian Plate for brief periods of time during the deposition of the middle Marrat shallow marine red beds.

Hydrocarbon Generation Mechanism of Mixed Siliciclastic–Carbonate Shale: Implications from Semi–Closed Hydrous Pyrolysis

Affected by the complex mechanism of organic–inorganic interactions, the generation–retention–expulsion model of mixed siliciclastic–carbonate sediments is more complicated than that of common siliciclastic and carbonate shale deposited in lacustrine and marine environments. In this study, mixed siliciclastic–carbonate shale from Lucaogou Formation in Junggar Basin was selected for semi–closed hydrous pyrolysis experiments, and seven experiments were conducted from room temperature to 300, 325, 350, 375, 400, 450, and 500 °C, respectively. The quantities and chemical composition of oil, gases, and bitumen were comprehensively analyzed. The results show that the hydrocarbon generation stage of shale in Lucaogou Formation can be divided into kerogen cracking stage (300–350 °C), peak oil generation stage (350–400 °C), wet gas generation stage (400–450 °C), and gas secondary cracking stage (450–500 °C). The liquid hydrocarbon yield (oil + bitumen) reached the peak of 720.42 mg/g TOC at 400 °C. The saturate, aromatic, resin, and asphaltine percentages of bitumen were similar to those of crude oil collected from Lucaogou Formation, indicating that semi–closed pyrolysis could stimulate the natural hydrocarbon generation process. Lucaogou shale does not strictly follow the “sequential” reaction model of kerogen, which is described as kerogen firstly generating the intermediate products of heavy hydrocarbon compounds (NSOs) and NSOs then cracking to generate oil and gas. Indeed, the results of this study show that the generation of oil and gas was synchronous with that of NSOs and followed the “alternate pathway” mechanism during the initial pyrolysis stage. The hydrocarbon expulsion efficiency sharply increased from an average of 27% to 97% at 450 °C, meaning that the shale retained considerable amounts of oil below 450 °C. The producible oil reached the peak yield of 515.45 mg/g TOC at 400 °C and was synchronous with liquid hydrocarbons. Therefore, 400 °C is considered the most suitable temperature for fracturing technology.

Prestack Seismic Velocity Ratio Evaluation of a Mixed Siliciclastic–Carbonate Formation: Case Study from the Strawn Group on the Eastern Shelf Texas

Although a mixed carbonate–siliciclastic system of the Strawn Group on the Eastern Shelf in King County, Texas, USA provides excellent hydrocarbon reservoirs, facies variability and reservoir properties within such systems are not well understood. We conducted prestack, simultaneous seismic inversion, and high-level petrophysical analysis to derive elastic properties of rocks to facilitate lithology identification and determination and distribution of the different carbonate facies. Our results show that (1) the Strawn Group in King County is dominated mostly by carbonates and (2) given the ratio of P- and S-wave velocity (Vp/Vs ratio), the carbonates can be separated into three facies: (a) high-Vp/Vs-ratio shelf-edge reef carbonates, in which the Vp/Vs ratio decreases linearly as porosity increases and the Vp/Vs ratio varies from ~2.1 to ≤2.6; (b) moderately low-Vp/Vs-ratio shelf (platform) carbonates, in which the Vp/Vs ratio also decreases as porosity increases and in which the Vp/Vs ratio ranges from ~1.75 to ≤2.15; (c) extremely low-Vp/Vs-ratio slope and basin carbonates, in which the Vp/Vs ratio, although appearing to be almost constant for a wide range of porosity, increases as porosity increases, and in which most Vp/Vs-ratio values appear to range from ~1.5 to ≤2. Results of a through c can be summarized thusly: the Vp/Vs ratio of reef carbonates >the Vp/Vs ratio of platform carbonates and >the Vp/Vs ratio of slope and basin carbonates in the study area.

New insights into the carbon cycle and depositional models of the Eocene saline lake, Jianghan basin, China

Geochemical characteristics and sedimentary processes of sediments, especially mixed sediments, have long been hot topics of research in basin systems, which can be used to infer runoff, evaporation, productivity, precipitation patterns of the paleoenvironment, as well as records of a diverse range of climate-related signs. However, compared with shallow marine mixed siliciclastic-carbonate sediments, the study of lacustrine mixed sedimentary systems in combination with geochemical information is relatively limited. Also, model-based study on the reconstruction of the lacustrine carbon biogeochemical cycle is defective, especially in saline lacustrine environments. In this work, a total of 26 mixed siliciclastic–carbonate sediment core samples from Well BX-7 in the Qianjiang Depression were sequentially selected for scanning electron microscope observation, major and minor element test, and isotope measurements (e.g., δ13Ccarb, δ18Ocarb, and δ34SCAS). Two types of mixed sediments were generated in the overall inflow/outflow balance of water within the saline basin during the Eocene: siliciclastic-dominated mixed sediment (SDMS) and carbonate-dominated mixed sediment (CDMS). The δ13Ccarb values of SDMS and CDMS samples range from −7.13‰ to −4.52‰ (average −5.82‰) and −9.00‰ to −6.11‰ (average −7.48‰), respectively. The δ18Ocarb values of SDMS and CDMS samples range from −10.26‰ to −1.40‰ (average −4.73‰) and −3.80‰ to −0.96‰ (average −1.55‰), respectively. In addition, combining the geochemical parameters of sedimentation rate, Mn2+ content in carbonate lattice ([Mncarb]), carbonate mass fraction, and [Mg2+]/[Ca2+] ratio, we established a lacustrine δ13C variation model at the sediment-water interface (SWI) with the aim of accurately reflecting δ13Ccarb changes during carbonate formation. Based on the sedimentary processes and geochemical characteristics, we established two depositional models for these two types of mixed sediments in a saline lacustrine basin in the overall inflow/outflow balance of water. Model I illustrates SDMS sedimentary processes with a relative low salinity, low FluxDIC-O, high water inflow, and the organic carbon oxidation is dominated by Fe (III) reduction. Model II demonstrates CDMS sedimentary processes with a high salinity, high FluxDIC-O, high water outflow, and the organic carbon oxidation is dominated by sulfate reduction.

Depositional framework and trace fossil assemblages of the Lower Triassic Montney Formation, northeastern British Columbia, Western Canada Sedimentary Basin

Hydrocarbon production from the Lower Triassic Montney Formation in northeastern British Columbia, occurs primarily from unconventional reservoirs consisting predominantly of fine-to coarse-grained siltstone beds. In this area, the lithostratigraphy of the formation is difficult to characterize due to the complexities associated with subtle grain-size variation, diminutive biogenic structures, lateral facies variability, and distribution of local discontinuities. Detailed sedimentologic and stratigraphic analyses are essential for understanding and refining the depositional models and stratigraphic framework of the Montney Formation. Based on detailed core examination focused on sedimentological, paleontological and ichnological characteristics, eleven lithofacies and three recurring facies associations are identified within the Lower Triassic Montney Formation in northeastern British Columbia. The lithofacies identified are interpreted to have been deposited in offshore, offshore transition and lower shoreface settings, along a storm-dominated mixed siliciclastic-carbonate ramp. Facies associations include FA(A): siliciclastic distal offshore transition to distal offshore; FA(B): mixed siliciclastic carbonate, storm-influenced offshore to offshore transition; and FA(C): storm-dominated siliciclastic offshore transition to lower shoreface. Trace fossil assemblages correspond to a stressed Cruziana Ichnofacies, and are overall characterized by low diversity (1–6 ichnogenera), small size trace fossils (1–12 mm in diameter), and variable bioturbation intensity (0–6). The vertical variation in SDI values reflect changes in physico-chemical conditions during deposition of the Montney Formation, particularly across the Smithian-Spathian boundary. Understanding the lateral-facies variability and overall stratigraphic architecture of the Montney Formation in northeastern BC, constitute key elements in defining and correlating existing and new potential hydrocarbon reservoirs in the area.

Mixed deltaic and carbonate deposition in a tectonically restricted basin: An example from the lower and middle members of the Eocene Musawa Formation, Abat Basin, south‐eastern Oman Mountains

Mixed siliciclastic and carbonate sediments of the Eocene Musawa Formation were deposited in the Abat Basin in the south‐eastern part of the Oman Mountains when the rest of the Arabian Plate was experiencing extensive carbonate sedimentation. The Musawa Formation was deposited as mixed siliciclastic and carbonate sediments in the Abat Basin bounded by the Ja'alan and Qalhat strike‐slip faults. Movements along these bounding faults and extensive deformation in the source area provided an enormous amount of clastic sediments in a laterally restricted basin undergoing strong tectonic‐related subsidence and was fed by basin axis parallel drainage system. The lithofacies assemblage represents fluvial and wave‐dominated deltaic sediments that accumulated as a local siliciclastic feature in a carbonate‐dominated subtropical environment. The Musawa Formation is divided into three members, the lower, middle, and upper, based on lithological characteristics; however, this study focuses on the lower and middle members of the formation. The lower Musawa member is about 350 m thick, comprising laterally confined conglomerate (lithofacies GL) beds interbedded with Alveolina‐bearing sandstone in the lowermost part of the formation. These beds were deposited as fan‐delta on top of the underlying Ypresian limestone of the Abat Formation. The conglomerate beds grade up‐section into over 170 m thick medium‐ to thick‐bedded channelized sandstone lithofacies (lithofacies CSL), occasionally interbedded with mudstone and peat streaks. The sandstone sequence northwards of the study area passes into fine‐grain lithofacies of siltstone and mudstone (lithofacies ML). The sandstone (CSL) was deposited in a delta‐plain setting with coastline shifting northwards. The enormous thickness of the sandstone sequence is due to the location of the depocenter around Wadi Musawa area, from where the streams entered the basin and deposited their sediment load. The upper part of the lower Musawa member comprises interbedded clay, siltstone, and thin‐bedded fine‐grain sandstone (lithofacies association LFA3) deposited as mud‐flat accumulations. The middle Musawa member is about 650 m thick, comprising both carbonate (LFA4) and siliciclastic (LFA2 and LFA3) lithofacies associations. The carbonate lithofacies association (LFA4) was deposited during flooding events. The marl lithofacies (CM) comprising branching corals, gastropods, and oysters was deposited in a lagoonal setting, whereas thick‐bedded limestone lithofacies (CL) comprising Nummulites was deposited in open‐marine conditions. Occasional coarse siliciclastic influx resulted in the deposition of mouth bar and shoreface lithofacies association (LFA2), especially in the upper part of the middle Musawa member. The mouth bar and shoreface sandstone interbedded with occasional chert‐bearing conglomerate are laterally persistent across the Abat Basin outcrops. The uppermost part of the middle Musawa member is defined by ridge‐forming, thick‐bedded open‐marine limestone (CL) extending across the Abat Basin. The upper boundary of this limestone facies (i.e., CL) marks the boundary between the middle and upper members of the Musawa Formation. This study helped in understanding the mechanism of mixed siliciclastic–carbonate deposition in the Abat Basin.

Temporal and spatial variations in calcium carbonate deposition in a mixed siliciclastic–carbonate deep marine system: the Ediacaran Deeside Limestone Formation, Aboyne, Scotland

The Deeside Limestone Formation (DLF), outcropping in western Aberdeenshire, comprises the eastern margin of a distinctive Upper Dalradian mixed siliciclastic–carbonate system of Ediacaran age which may be correlated southwestwards for more than 400 km to County Donegal in Ireland. A reconstructed stratigraphic column suggests three broad vertical sequences (S1–S3), each comprising a general upward proportionate increase in calcium carbonate with respect to quartz, locally capped by metalimestones (L1–L3). A simple explanation for this upward change lies in the differing physical response of the two heterolithic components under the same hydrodynamic process, with the heavier quartz grains preferentially enriching the bedload and the finer carbonate mud fraction, the suspended load. The three metalimestone intervals are observed only in central/eastern parts of the field area, suggesting a lateral facies variation. The predominance of calcareous–siliciclastic bed-scale compositional mixing within the DLF suggests sedimentation on the Dalradian shelf comprised contemporaneous mixing of the siliciclastic and carbonate fractions prior to subsequent re-mixing during transportation downslope into the deep water, punctuated by pulses of point-sourced siliciclastic input.

Occurrence and origin of glauconite in the Cambro-Ordovician Bliss Formation of southern New Mexico and West Texas (U.S.A.)

ABSTRACTIn southern New Mexico and West Texas, USA, the Late Cambrian–Early Ordovician Bliss Formation is a relatively thin and dominantly siliciclastic succession that was deposited on the Proterozoic basement during a major global transgression. The Bliss Formation can be divided into two members: 1) a lower, coarser-grained member composed mostly of sandstone (quartz arenite and subarkose) that lacks glauconite and calcite cement, and 2) an upper, finer-grained member that includes glauconitic sandstone, arkosic sandstone, and mixed siliciclastic–carbonate siltstone to fine-grained sandstone, intercalated with thin carbonate beds of grainstone, packstone, and rudstone. Iron oolite and oolitic sandstone are locally exposed at the base of the upper member. Sandstone of the lower member represents upper-shoreface to foreshore deposits, whereas sedimentary structures in the upper member indicate deposition in a middle- to lower-shoreface setting, and locally in a tidal-flat environment. Intercalated carbonate beds are storm layers (tempestites). Glauconite grains are abundant in the upper member as mostly rounded to well-rounded, spheroidal to ovoidal pellets. Two types of glauconite grains are present: homogeneous, dark green grains with high K2O contents (> 8 wt.%; stage 4) and mottled pellets composed of a mixture of glauconite and apatite. Glauconite of the Bliss Formation is not autochthonous as proposed by earlier workers, but of allochthonous (parautochthonous) origin. Thus, the glauconite grains were reworked from deeper shelf environments in northern Mexico to the south and were transported and deposited under regressive–transgressive conditions. Mottled glauconite grains formed by the reworking of phosphatized and glauconitized micritic sediments during regression and were transported and deposited by storm-induced currents, particularly in storm layers during transgressive events. Homogeneous, mature glauconite grains probably were derived from the reworking of glauconitized fecal pellets or completely glauconitized micritic sediments during regressive–transgressive cycles. The world-wide occurrence of glauconite in Cambrian–Early Ordovician sediments indicates that glauconite formation during that period can be considered as a “global event.”

Lost in the wind: An integrated approach for the recognition of mixed clastic–carbonate continental aeolianites

AbstractAeolianites with a mixed clastic and carbonate detrital mineralogy are typical deposits of tropical coastal desert environments. These deposits are genetically related to transgressive–regressive cycles, with the shallow‐marine carbonate detrital component being deflated from the exposed seafloor during episodes of glacial sea‐level lowstands, transported and later accumulated onto more continental areas. A detailed investigation of the granulometric and petrographic properties of the Late Pleistocene to Holocene Ghayathi Formation in the United Arab Emirates provides a comprehensive numerical dataset that reveals the complex spatio‐temporal depositional and diagenetic evolution of a mixed siliciclastic–carbonate continental aeolianite. Fifty‐one thin sections and forty‐four rock samples of three sedimentary units (Madinat‐Zayed, Ghayathi and Fuwayrit formations) from the United Arab Emirates were analyzed for their grain‐size distribution, mineralogy and the bulk stable‐carbon and oxygen isotopic composition. Sorting, skewness and kurtosis of the grain‐size distributions provide evidence for a divergent aerodynamic behaviour between the carbonate and siliciclastic particles. The different grain‐size distributions between the inland and coastal sections of the Ghayathi Formation indicate that continental mixed clastic−carbonate aeolianites are affected by density‐driven selective transport and consequent traction sorting due to the nature of carbonate detritus. The transport decoupling of the different detrital components resulted in a ‘continental sorting’ effect, with the inland sections yielding layers characterized by a better sorted detritus than the coastal areas. All detritus originally composed of aragonite has been removed by dissolution during early meteoric diagenesis, with its shapes frequently preserved by micritic envelopes. The bulk stable‐isotope composition of the carbonate fraction reflects a decreasing degree of meteoric cementation from coastal to inland areas. The high complexity of the lateral and vertical distribution of the sedimentary, diagenetic and isotopic properties of continental mixed aeolianites, such as the Ghayathi Formation, highlights the problem of recognizing such deposits in the pre‐Quaternary sedimentary record.

Fine-grained distal deposits of a mixed siliciclastic–carbonate marine system: Origin of mud and implications on mixing processes

ABSTRACT Mixed siliciclastic–carbonate sediments in the distal settings of marine systems (i.e., offshore and basin areas) are the result of the complex interaction between production of benthic and pelagic biogenic carbonates and the exportation of terrigenous, and/or carbonate mud from shallow-marine settings. Thus, the origin of mud (particles < 63 mm) of different compositions and the processes responsible for mixing them play a key role in the final distal deposits of mixed systems. Although the understanding of these topics in shallow settings has made significant progress, one of the critical issues is to recognize them in the offshore and basinal settings for understanding the link between shallow-marine and deep-marine areas. In this study, a thick Lower Cretaceous succession (330–650 m) of the Neuquén Basin, composed mostly of mixed (siliciclastic–carbonate) fine-grained marine deposits, was studied through a high-resolution analysis of petrography, SEM analysis, and mineralogy, to define and characterize the origin of carbonate and terrigenous mud as well as to characterize the processes involved in the mixing of terrigenous and carbonate mud on a storm-dominated ramp. Contrary to previous suggestions, most of the carbonate recognized in the basinal setting is composed of silt- to sand-size skeletal remains and micrite (micarbs) and is likely derived from a pelagic biogenic factory combined with carbonate derived from the maceration of benthic organism. Furthermore, micarbs are possibly diagenetic in origin but with a distally produced carbonate precursor. In contrast, the siliciclastic fraction is entirely detrital, being silt-size quartz particles and clay minerals (mainly illite) the most important constituents. Most of the terrigenous mud delivered from shallow areas acts to dilute the distally produced carbonate mud. However, a bloom of carbonate production able to dilute terrigenous supply is recorded, and its origin is likely triggered by the input of waters charged with nutrients from the paleo–Pacific Ocean in addition to nutrients derived from proto-Andes volcanic activity. This demonstrates that the relation between the terrestrially derived siliciclastic components and the biogenically produced carbonates in mixed systems is not linear, but a complex interaction of climate, sea-level, origin of the components, and hydrography of the basin, among others.

Dolomite–magnesite formation and polymetallic mineralization in a rift-sag basin on the western margin of the Red Sea: Paleoenvironmental, hydrothermal, and tectonic implications

ABSTRACTThe present study gives new insight on the formation conditions of dolomite and magnesite in an early–middle Miocene succession related to a half-graben rift-sag basin on the western margin of the Red Sea. The studied Miocene succession comprises two units of siliciclastic–carbonate rocks separated by a magnesite bed. The succession is enriched with epigenetic–supergenetic polymetallic minerals, dominated by zinc-bearing ferromanganese oxides. These represent oxidized Mississippi Valley-type deposits (MVT) formed during uplifting in late Miocene–Pliocene time. Multistage dolomitization (four dolomite types: D1–D4) and magnesite authigenesis, enhanced by tectonic uplifting and faulting related to the Red Sea rifting, have been recorded. The first dolomite phase (D1) is pervasive early diagenetic dolomicrite (replacement type), which is dominant in the lower unit. Magnesite occurs as microcrystalline aggregates exclusive to the lower unit, where its authigenesis was after D1 and before D2. Occurrence of magnesite was mostly related to a restricted environment in a sag fault-bounded basin with shallow evaporative hypersaline conditions in coastal areas. D2 dolomite occurs in the lower and upper units as replacement and/or cement type of medium- to coarse-crystalline dolomite crystals. The three magnesium-rich carbonates (D1, magnesite and D2) are related to successive events of sea-level fall and rise in mesohaline and hypersaline conditions. Enrichment of magnesite and D2 dolomite with Na (up to 2.16 wt.%) and Sr (up to 1483 ppm) supports their formation under more saline evaporative conditions if compared with D1 dolomite which was formed in near-normal sea water or mesohaline fluids. The third and fourth dolomite phases (D3 and D4) are late diagenetic pore-filling coarsely crystalline and zoned, and restricted mainly to faulted areas associated with the polymetallic ore deposits. Elemental analyses of the four dolomite phases show different chemistries, i.e., non-ferroan dolomites (D1 and D2), alternation of manganiferous and non-ferroan zones (D3) and/or ferroan-type dolomite (D4). Stable- isotope values of the four dolomite types (δ18OVPDB of –7.82‰ to –5.88‰) and geochemistry suggest involvement of shallow evaporative conditions in coastal areas, enhanced either by dry and hot climate or by hydrothermal process in their formation. Nonetheless, the localized occurrence of D3 and D4 types along the faults, their concomitant occurrence with the epigenetic–supergenetic polymetallic ore deposits, and the preservation of unaltered feldspar grains ruled out the meteoric-water interaction and reinforce the fault-controlled and deep-seated hot fluid evolution for these two dolomite types. The underlying ultramafic and serpentinite rocks along with the intercalated magnesium-rich clays and/or the modified seawater most probably played a critical role in the diagenesis and/or precipitation of dolomite and magnesite. The proposed model can contribute to better understanding the genetic mechanisms of magnesite and dolomite hosted by mixed siliciclastic–carbonate deposits and their relations with MVT mineralization conditions in rift basins.

Tectonic and paleoclimatic controls on the composition of inland wetland deposits, Chaco foreland basin, Central Andes

ABSTRACTIn recent decades, the growing interest in wetlands highlights the fundamental role of these complex ecosystems. Integrated sedimentological and geochemical studies that refer to wetlands appear more frequently in the literature, since such approaches provide a window into understanding the functions they have played through geologic time. The Miocene Yecua Formation (Chaco foreland basin, Central Andes, Bolivia) presents sedimentological features that show how siliciclastic, chemical, and biological processes lead to lithofacies and compositional features that are distinctive of wetlands related to back-bulge settings. The succession is composed of shallowing-upward parasequences with various compositional characteristics controlled by tectonic and climatic factors.Data integration obtained from paleocurrents, petrography, and clay mineralogy show the influence of the Central Andes fold and thrust belt located to the west of the basin as a principal source area. The high compositional maturity of sandstones reflects the tectonic control as the main allocyclic variable associated with reworking of already mature sediment sources and transport factors in relation to the distal position of the basin (back-bulge), combined with climatic and paleoenvironmental conditions. XRD and SEM-EDS analyses show that illitic clays represent the main mineral phase of the clay assemblages, and have a detrital origin related to physical weathering processes in the source area. Al-rich smectitic minerals have an authigenic origin and were likely formed by alteration of volcanic detritus as well as from precursor minerals such as illite and feldspars. Kaolinite is determined to be detrital and likely derived from the erosion of floodplains with soil development. Throughout the parasequences, illite–smectite–kaolinite minerals are arranged in different proportions, showing a general vertical trend (from base to top) in which the illitic clays decrease at the expense of the increase in Al-rich smectite and kaolinite. The increase in smectite is accompanied by mixed siliciclastic–carbonate sedimentation, which points to a reduction in the clastic contribution linked to drier conditions. This vertical compositional arrangement reveals a paleoclimatic control, influenced by middle-term wet and dry periods. This work demonstrates that source-rock composition and transport distance (tectonic processes) are the key factors influencing quartz-rich sandstones and illite-rich clay mineral suites in distal positions of continental foreland basins. The long-term tropical to subtropical climate favored the development of inland wetlands and played a secondary role in the final composition of these depositional systems.