Mixed Siliciclastic-carbonate Sediments Research Articles

Sedimentary and stratigraphic architecture of the Duitschland and Rooihoogte formations (Palaeoproterozoic, South Africa): implications for tempo of the Great Oxidation Event

Abstract Determining the tempo and causality of key palaeoclimate events recorded in sedimentary strata depends on high-resolution numerical ages and well-constrained stratigraphic correlations at regional and global scale. This requirement is not necessarily met in Precambrian strata due to poorer age resolution, limited preservation, and secondary overprints. A good example includes the Palaeoproterozoic Rooihoogte and Duitschland formations in South Africa, which document the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) and contain glacial diamictites at their base. They are thus key records of Earth’s surface oxygenation during the Great Oxidation Event (GOE). However, previous studies have either correlated these units, resulting in a unidirectional oxidation trend; or have regarded them as successive strata, causing an interpretation of oscillating oxidation. This study uses extensive outcrop and new core material to investigate correlation between these two units, and to establish depositional models. Results show that key stratigraphic markers can be traced around the entire Kaapvaal craton both in outcrop and the subsurface. In particular, the basal Bevets breccia and the top Duitschland breccia are here re-interpreted as two separate units that are present at the base and top of both formations, supporting correlation of the formations. Consequently, the base of both formations records a major craton-wide event of uplift and karstification, leading to carbonate dissolution and chert brecciation. Erosion of older rocks from across the craton also delivered material for the basal glacial diamictite. The majority of mixed siliciclastic-carbonate sediments were deposited on a storm- and/or delta-influenced shelf. Depositional packages in both formations reflect post-glacial relative sea level rise, followed by progradation of a deltaic, storm or shoreline depositional system. There is a relatively short-lived depositional hiatus to overlying shales of the Timeball Hill Formation. Both Rooihoogte and Duitschland formations thus record only a single glacial event at their base, and a unidirectional trend of oxidation.

Taphonomy of a Mysticete Whale from the Lower Pliocene of the Coast of Cádiz (Spain)

A fossil mysticete was discovered along the southwestern coast of Spain, occurring in a block detached from the Neogene deposits exposed along a coastal cliff at the locality of Conil de la Frontera (Cádiz, Spain). These deposits range from Pliocene to Pleistocene in age and include shallow-marine, mixed carbonate–siliciclastic sediments, with the whale being found in occurrence of a stratigraphic unconformity marked by Thalassinoides burrows. 87Sr/86Sr analyses on oyster shells associated with the skeleton suggest an Early Pliocene age, in agreement with the age of the lowermost unit cropping out at the study site. The studied cetacean specimen consists of an articulated, almost complete balaenopteroid skeleton exposed in the field dorsal side up; being contained in an upside-down block, however, it is preserved in ventral disposition. Bones exhibit a low degree of preservation of the cortical bone tissue, which locally features shark bite marks and Osedax traces as well as abundant encrustations of barnacles and ostreids. Two shark teeth were also found near the skeleton. Bones have preserved their main histological features, even though they locally exhibit microcracks, dissolution, substitution by Fe oxides, and microborings. Sediment particles and late diagenetic cements fill the medullary cavities. We propose that the whale carcass experienced refloating before sinking to the seafloor and that the skeleton was probably exposed on the seafloor for some time before being eventually buried.

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.

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.

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.

Facies distribution along salt walls: The Upper Cretaceous mixed siliciclastic-carbonate deposits of the Cotiella minibasins (Southern Pyrenees, Spain)

To assess the influence of growing salt structures on facies distribution, stratal pattern, and thickness trends, this paper provides the tectonostratigraphic framework of the outstandingly exposed Cotiella minibasins (South-Central Pyrenees, Spain). Based on detailed analysis of field cross-sections, our results showcase the syn-halokinetic upper Coniacian-lower Santonian Cotiella minibasins infill (1.5–3 km thick) records four shallow-marine depositional sequences, made of mixed siliciclastic-carbonate stratigraphic sequences. This infill is subdivided in three tectonostratigraphic units, identified by using large and small-scale halokinetic structures (i.e. wedges, megaflaps) and variations in facies associations, thicknesses, stratal patterns from the salt wall flanks to the central part of the minibasins. The first unit records the onset of salt movement with overlying laterally continuous sheet-like sedimentary layers and progressive facies transitions over distances of kms. The influence of halokinesis on sedimentation patterns was minor, and only impacted thickness trends and depocenters localization. The second unit records salt evacuation at the surface with the presence of salt-wall derived detritus, the rapid downbuilding and a increase of sand-rich facies proportions. The stratal pattern is characterized by sheet like layers, laterally discontinuous over distances of 300–600 m downdip to the salt wall. Topographically elevated salt walls likely amplified tidal currents, implying sand-rich facies accumulation along the salt wall margin. The halokinesis influence on sedimentation, thickness trends and depocenter localization was significant. The third unit records the end of the salt evacuation stage, marking a brutal decrease of sand-rich facies, likely controlled by regional controls such as eustacy and tectonics. The influence of halokinesis on sedimentation patterns, thickness trends, large-scale geometries and depocenter localization was significant. Our work indicates that distinguishing these large-scale tectonostratigraphic units and their characteristics is of prime importance to understand the archetypal facies distribution, thickness and stratal patterns along salt structures margins.

Unravelling reservoir quality heterogeneity in mixed siliciclastic-carbonate deposits: An example from Miocene Red Sea rift, NW Saudi Arabia

The Miocene, syn-rift mixed siliciclastic-carbonate strata has been a long sought-after reservoir objective in the Red Sea rift basins. Despite being viable prospects, their depositional architecture, mechanisms of siliciclastics and carbonate mixing, and fundamental controls on their reservoir quality remained elusive and enigmatic. In this study, we used sedimentological, petrographical and outcrop permeametry measurements to investigate the stratigraphic record and reservoir quality of mixed siliciclastic-carbonate deposits. The latter are preserved at the transition between non-marine Al-Wajh Formation and marine carbonates of the Musayr Formation in one of the Red sea rift basins, NW Saudi Arabia. Three major facies associations were identified using detailed logging facies analysis, including siliciclastic-rich, mixed siliciclastic-carbonate and carbonate-rich facies association. These facies associations, represent a basinward transition from dominantly non-marine alluvial fans, braided streams and shallow-water delta into an open marine carbonate ramp. The permeability of siliciclastic facies is orders of magnitude higher (average permeability = 1884 md) than mixed siliciclastic-carbonate (average permeability = 109 md) and carbonate (average permeability = 46 md) facies. Likewise, siliciclastic facies have higher porosity (average porosity = 17%) compared to mixed (average porosity = 7%), and carbonate (average porosity = 15%) deposits. Petrographic analysis suggests that permeability and porosity variation within these facies is due to textural and diagenetic differences. Outcrop based permeability correlation separated the studied deposits into reservoir (excellent or good) and non-reservoir (baffles or barriers) elements. The relatively thick (5–10 m) and laterally continuous (hundreds of meters) siliciclastic deposits form excellent and good reservoir compared to usually thinner (1–2 m) mixed siliciclastic-carbonate and carbonate beds, which may act as baffles and barriers. This reservoir-seal configuration highlights the potential of stratigraphic trap formation in analogous settings in the subsurface, where non-marine siliciclastic deposits show lateral transition to marine carbonates.

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.

New and revised cyrtospiriferid (Spiriferida) brachiopods from the lower Famennian (Upper Devonian) of Armenia

AbstractThe Upper Devonian sedimentary sequences of Central Armenia, which mainly consist of shallow water, mixed carbonate-siliciclastic deposits, contain abundant and diverse brachiopods that are dominated by spiriferides. Based on newly collected material from the lower Famennian Aramazdospirifer orbelianus brachiopod zone (coeval to the Palmatolepis crepida conodont zone) of Armenia, we here introduce two new cyrtospiriferid genera and fully document their type species, including their intraspecific variability. Pentagonospirifer n. gen. is a monospecific genus assigned to the subfamily Cyrtospiriferinae that is currently known only from the lower Famennian of Armenia; it is likely that its type species, P. abrahamyanae n. sp., evolved from the species Cyrtospirifer verneuili sensu Abrahamyan, known from the Frasnian and lower Famennian of Armenia. The second new genus, Tornatospirifer n. gen., is assigned to the subfamily Cyrtiopsinae and defined on the basis of one of the most biostratigraphically valuable cyrtospiriferid species, namely T. armenicus n. comb., described previously from the lower Famennian Aramazdospirifer orbelianus Zone of Armenia. It is likely that it evolved from a Frasnian ancestral stock of Tiocyrspis Sartenaer, known from northwestern Europe, which probably migrated during the early Frasnian to the north Gondwanan margin. A neotype is selected for Abrahamyan's species because the type material is lost. Epibionts (Cornulites, Hederella) attached to brachiopods are also documented for the first time from the Upper Devonian of Armenia.UUID: http://zoobank.org/923d232f-cd6a-48bb-8a05-27c689058b65.

Middle Miocene short-lived Tethyan seaway through the Zagros foreland basin: Facies analysis and paleoenvironmental reconstruction of mixed siliciclastic-carbonate deposits of Mishan Formation, Dezful Embayment, SW Iran

Integrated sedimentological and petrographical analyses have been used for paleo-environmental reconstruction of mixed siliciclastic-carbonate deposits of Mishan Formation (late Burdigalian-Serravallian) in the Dezful Embayment within the Zagros foreland basin. The carbonate deposits alternated with marl layers consisting of open marine, restricted to semi-restricted subtidal lagoon and tide-related facies associations with organic buildups deposited within tropical to subtropical warm water multi-factory platform including euphotic and meso-oligophotic factories. The siliciclastic deposits comprise progradational tide-influenced delta deposits including prodelta, delta front and delta plain facies associations. The lack of wave related structures and the absence of the shelf type evidence together with tide-related deposits within the carbonate facies and prograding delta deposits indicate a tide-dominated mixed siliciclastic-carbonate homoclinal ramp with the development of organic buildups. These sediments deposited within the short-lived Tethyan seaway resulted from Middle Miocene Climate Optimum through the Zagros foreland basin. Predominant heterozoan skeletal grains and alternation of marl layers with carbonate sediments indicate high nutrient and sediment flux from adjacent highlands. Carbonate tidal deposits within the siliciclastic sediments suggest that the tidal currents have provided suitable ecological conditions for carbonate production by supplying nutrients for heterotrophic organisms and pumping clear water from the ocean. Shallowing upward trend from marine carbonates and marls to prograding siliciclastic paralic and non-marine deposits of the Mishan Formation within the studied area can be linked to the eustatic sea-level fall in response to the Middle Miocene Climate Transition which led to the closure of Tethyan seaway in the Dezful Embayment.

Orbital forcing of late Early Devonian storm events on a mixed carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province, China

AbstractThe Yangmaba Formation of latest Early Devonian age in the Longmenshan area of Sichuan Province, China, is a shelfal facies that consists of four types of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich, carbonate-dominated, and hybrid mixed. Storm deposits vary in their thickness, composition, and abundance-per-meter within these mixed carbonate-siliciclastic deposits. Meter-scale statistics of the relative storm frequency (events-per-meter) and magnitude (bed thickness) were compiled in two coeval sections through each of the hybrid facies in the lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass filtering, spectral analysis, and Acycle software interpretation of these tempestite statistics yield 3.5 main oscillations in each section, with an average wavelength of ~23 m. These long-wavelength trends are semi-coincident with interpreted long-term variations in sea level, where shallower depths allowed a greater influence by storms. Superimposed on the long-wavelength cycles are medium-wavelength cycles of 5.5–6.3 m. The estimated ~1.5 m.y. time-span of this conodont zone of the lower Yangmaba Formation and the approximate 1:4 ratios of these wavelengths indicate that frequency and intensity of major storms and the recording of tempestites in the sedimentary record were modulated by ~100- and 95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles governed storm intensity and regional sea level on this margin of the tropical Yangtze Platform of South China.

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.

Pennsylvanian mixed siliciclastic-carbonate deposits of the Amazonas basin, North of Brazil: The record of an epicontinental sea in Western Gondwana

Epicontinental seas dominated the central Amazonas Basin during the late Carboniferous configuring singular paleogeography of West Gondwana connected to the Panthalassa Ocean. The Pennsylvanian record is represented by 1,7 km-thick mixed siliciclastic-carbonate-evaporite deposits forming a complete transgressive-regressive megacycle in the Amazonas Basin. Outcrop and core-based stratigraphic and facies analysis carried out in 50 m-thick mixed siliciclastic-carbonate transgressive succession of the Amazonas Basin identified twenty-six facies and microfacies, representative of coastal to platform depositional systems and grouped in three facies associations: (1) coastal aeolian deposits, consisting of fine to medium-grained sandstone, lime mudstone and finely-crystalline dolostone that correspond to a complex association of aeolian dunes, sand sheets, interdunes, fluvial channels and lagoon deposits bioturbated by Palaeophycus, Lockeia, Thalassinoides, Roselia, and meniscate trace fossils; (2) a mixed tidal flat setting, constituted by fine to medium-grained sandstone, mudstone, shale, siltstone and limestone interpreted as supratidal, tidal channel, tidal delta and lagoon settings with evidence of subaerial exposure; and (3) carbonate epicontinental platform deposits, consisting of lime mudstone, wackestone, packstone and grainstone with allochemicals (ooids and peloids), terrigenous grains and abundant, diversified open shallow marine benthic and nektonic organisms,. A Bashkirian age is suggested based on the presence of the Neognathodus symmetricus, Streptognathodus sp., and Ellisonia sp. These mixed siliciclastic-carbonate deposits are correlated with several successions in the west and east Amazonia and sub-Andean basins in South America. These records indicate that the Itaituba deposits constitute a significant portion of a vast Pennsylvanian epicontinental sea connected to the west with the Panthalassa Ocean.

An Attempt for Construction of Carbonate Platform Geometry from Devonian Hajigak Formation, Central Afghanistan, with the Help of Facies Analysis and Petrography

The subject of sedimentology fundamentally remained subdivided into two sectors viz. siliciclastic and carbonate with the understanding that these two systems are mutually dissociative in terms of their genesis. Even in the highly referred textbooks, siliciclastics and carbonates are always discussed in separate sections. Presumably, the limited occurrences of mixed siliciclastic-carbonate sediments in nature are because of constraining effects that siliciclastics have on carbonate-secreting organisms; the two sediments rarely found mutually associated in nature. Although the mixed carbonate-siliciclastic sediments are subordinate in occurrence, their presence in some instances proved that they do not represent any geological oddity. Rather, their sediment logical history may tell us a great deal about the dynamics and interactions of facies, paleoecologies of many carbonate-secreting organisms, and tectonic histories of depositional basins. Keeping this in mind, the present study attempted to recognize and draw the paleoenvironmental conditions and processes of the Devonian Hajigak Formation, Afghanistan by means of detailed facies analysis and petrographical signatures. An attempt has also been made to characterize sandstone wedges that punctuate the carbonate succession and some variable deposits of shales and marls.

Upper Hauterivian nautiloids and associated invertebrate assemblage from the Barranco de la Muela section (southeastern Spain): Systematic, biostratigraphic and palaeoenvironmental implications

The lower part of the Lower Cretaceous Bolos Sandstone Formation as exposed in the Barranco de la Muela section (Sierra de la Muela Range, southeastern Spain) yields two species of ornamented nautiloids, Cymatoceras neocomiense and Eucymatoceras plicatum. These have been collected together with other cephalopods (ammonoids, belemnites) and different invertebrates (corals, brachiopods, bivalves, gastropods, echinoids). This fossil assemblage includes numerous large macroconchs of the ammonite Pseudothurmannia mortilleti, here considered as a senior synonym of P. catulloi, index species of the homonymous subzone of the upper Hauterivian “Pseudothurmannia ohmi” Zone. On this basis, the here reported findings constitute the first biostratigraphically well-constrained records of C. neocomiense and E. plicatum, namely in the Pseudothurmannia catulloi Subzone. The presence of these two ornamented and relatively depressed nautiloids in fine sandstone levels interbedded with sandy marls of the Bolos Formation, together with the palaeoecological characteristics of the associated fauna suggest a nearshore to inner-middle shelf, shallow to moderately deep palaeoenvironment with mixed siliciclastic-carbonate sedimentation, and high to moderate energy.

Episodes of seabed rise and rapid drowning controlling the development of regressive and transgressive rhodolithic limestones in a tectonically-active subduction setting (Early Miocene, Wairarapa region, New Zealand)

ABSTRACT Early Miocene limestones from the Wairarapa region of New Zealand were deposited on thrust-bounded margins of trench-slope basins, and consist of two superposed stratal units, namely units A and B, interpreted as forced regressive and forced transgressive foralgal and rhodolith-bearing deposits, respectively. Unit A is floored by a regressive surface of marine erosion cut into shelfal to bathyal siliciclastic successions, and is overlain across a transgressive surface of erosion by Unit B. A drowning surface abruptly places the latter deposits in contact with deeper hemipelagites and turbidites. The succession is explainable within a tectono-eustatic based framework as follows: (1) Structurally controlled rapid relative sea-level fall, (i.e. sea-bed rise), from upper bathyal to shelfal depths promoted development of regressive (‘lowstand’) mixed carbonate–siliciclastic deposits. Continuation of seabed rise resulted in deposition of pure carbonate sediments, which are represented by channel-fill rhodolithic rudstones; (2) early stage of relative sea-level rise (i.e. seabed drowning) caused emplacement of transgressive glauconitic limestones, consisting of either foralgal deposits, or incised rhodolithic limestones; (3) continued relative sea-level rise terminated carbonate production. The present study documents carbonate production and termination in a tectonically active and confined intra-slope setting, reflected in the development and distribution of unusual forced regressive, and transgressive rhodolithic-heterozoan carbonates.

Sedimentology of ephemeral carbonate accumulations in siliciclastic-dominated passive margin settings, Pearl River Mouth Basin, South China Sea

High-resolution seismic and borehole data were used to investigate the sedimentological characteristics of carbonate accumulations in the Pearl River Mouth Basin (PRMB) of the South China Sea. Based on seismic morphologies and petrologic analysis, three scale types of oligo-mesophotic ICBs were identified to have developed ephemerally in the early Early Miocene. The first ICB type is represented by small ICBs with lateral extension in the kilometer range, characterized by mounded geometry, complex internal architectures and a high-frequency alternation of carbonates and volcaniclastics. The second ICB type has lateral extensions of tens of kilometers and convex-up reflections, featuring obvious vertical thickening compared to the surrounding siliciclastic deposits. The third ICB type consist of extensive platforms with hundreds of kilometers of lateral extension, onlapped by surrounding siliciclastic deposits. The correlation of seismic and well data allows us to reconstruct the evolutionary history of the three types of ICBs in the PRMB: All the ICBs built by large benthic foraminifera and/or coralline algae biota, occurred on or around submarine topographic highs such as volcanic edifices or inherited from basement rises, developed under stable subsidence rates between 120 m/Myr and 180 m/Myr, and extinguished suddenly in the late Early Miocene with an abrupt decrease in the subsidence rate or a change from subsidence to tectonic uplift. It is proposed that post-rift subsidence of the study area controlled the evolution of the ICBs. Except for ICBs, mixed carbonate-siliciclastic deposits are identified in well succession, characterized by two types of mixing, namely, compositional mixing (core-plug scale) and strata mixing (stratigraphic scale). These mixed systems generated high-frequency alternations of carbonate, siliciclastic and/or mixed deposits sequence stratigraphy. This work reveals in detail the typical features of seismic morphologies, lithofacies, internal architectures and evolutionary mechanism of ICBs and mixed systems in the PRMB, and thus has implications for predicting reservoirs in hydrocarbon exploration and understanding carbonate sedimentology in a siliciclastic-dominated passive marginal basin.

Mixed carbonate-siliciclastic sedimentation in a mesoproterozoic storm-dominated ramp: Depositional processes and stromatolite development

Records of shallow-marine ramps with the mixing of carbonate and siliciclastic sediments are common throughout the geological time. All these records have pure carbonate and pure siliciclastic deposits as end members, occurring contemporaneously in distinct depositional regions along the ramp, and transitional hybrid facies between them. The two end member can mix in different scales and can alternate in time due to climatic changes and regressions and transgressions. This work presents a detailed reconstruction of a Mesoproterozoic storm-dominated mixed carbonate-siliciclastic ramp composed of hybrid sediments and whithout the presence of pure siliciclastic or carbonate deposits, a rare example in the geological record. Based on a high resolution logged section (in 1:20 scale) and qualitative thin sections, eleven lithofacies were identified and grouped into three lithofacies associations (offshore, offshore transition and shoreface), which are stacked vertically forming a transgressive–regressive cycle. This faciological distribution indicates a low relief ramp with wide microbial colonization from shallow to relatively deep waters (below storm-wave base level). In offshore low-energy distal areas, microbial mats spread laterally over large distances with little or no interference from currents, while in the offshore transition the morphology of the bioherms is shaped by currents induced by waves. In turn, the high wave energy in the shoreface inhibits the formation of stromatolites, restricting their occurrence to thin layers of microbial carpets or intraclastic lags. The mixing occurs in compositional scale and is relatively homogeneous along the whole logged interval, independent of the shifts in lithofacies or lithofacies associations. This compositional homogeneity is linked to the wide distribution and regularity in the input of siliciclastic sediments during the sedimentary accumulation. Strong currents induced by storms allow the transport and mixing of siliciclastic sediments with carbonate grains generated in the basin during fair-weather periods.