Ramp Environment Research Articles

Microfacies and depositional environment of Asmari formation in the Zeloi oil field, Zagros basin, south-west Iran

The Oligocene–Miocene Asmari Formation is well known as a major hydrocarbon reservoir in southwestern Iran. To study the microfacies and sedimentary environment of Asmari Formation in the Zagros basin (south-west Iran), No. 6 well and No. 11 well sections in Zeloi oil field were selected. 500 thin sections were studied for petrographic and facies analysis. Based on petrographic and sedimentology analysis of the Asmari Formation in these sections, 13 carbonate microfacies and 1 petrofacies can be identified into three main depositional environments consisting of: outer ramp, middle ramp and inner ramp. The outer ramp environment is characterized by the presence of abundant planktonic foraminifera and echinoid fragments. The middle ramp environment is represented by the presence of echinoids and planktonic foraminifera, red algae and perforate foraminifers. In the inner ramp, the most allochems are ostracoda, imperforate and perforate benthic foraminifera, algae ball, peloid and oncoid.

Sedimentological Characteristics And Paleoenvironmental Implication Of Triassic Vertebrate Localities In Villány (Villány Hills, Southern Hungary)

Abstract There are two Triassic vertebrate sites in Villány Hills (Southern Hungary), where productive and continuous excavations have been carried out in the last six years resulting in a rich and diversified assemblage of shallow marine to coastal animals. The studied formations belong to the Villány–Bihor Unit of the Tisza Megaunit, which was located at the passive margin of the European Plate during the Triassic. The relatively diverse vertebrate assemblage was collected from a Road-cut on Templom Hill and a newly discovered site at a construction zone located on the Somssich Hill. Four main lithofacies were identified and interpreted in the newly discovered Construction vertebrate site consisting of dolomite (deposited in a shallow, restricted lagoon environment), dolomarl (shallow marine sediments with enhanced terrigenous input), reddish silty claystone (paleosol) and sandstone (terrigenous provenance) indicating that the sediments of the Construction vertebrate site were formed in a subtidal to peritidal zone of the inner ramp environment, where the main controlling factor of the alternating sedimentation was the climate change. However, the recurring paleosol formation in the middle part of the section also indicates a rapid sea-level fall when the marine sediments were repeatedly exposed to subaerial conditions. In the Road-cut site the siliciclastic sediments of the Mészhegy Sandstone Formation are exposed, representing a nearshore, shallow marine environment characterized by high siliciclastic input from the mainland.

Clayey bauxite from the Irano-Himalayan belt: Critical metals, provenance and palaeoclimate in the Upper Cretaceous Semirom ore deposit, Zagros Mountain, Iran

The Semirom karst type clayey bauxite is located in the Zagros zone, a part of the Irano-Himalayan belt, and occurs between the Sarvak and Ilam Formations. The Albian to Cenomanian limestone bedrock of the Sarvak Fm is a part of a platform ramp environment including clay bearing components. Diaspore, boehmite, kaolinite, and hematite are the major ore-forming minerals. Elemental mobility determinations reveal that Th and the critical metals Co, Ni, and Cr are enriched in the bauxitic horizons relatively to the Upper Continental Crust. Further, only one sample is outlier for more than one critical metal (Cr, Co, Ni, and Ga) likely recording a specific event during bauxitization. The outlier is enriched in Ni, Co, Ga, and Th, is depleted in Cr, and shows a large positive Ce anomaly (Ce/Ce∗ = 3.03). The outlier may record an event characterized by more acidic and oxidizing conditions and may have a regional relevance. R-mode factor analysis suggests that Cr and Co are likely adsorbed onto oxy-hydroxides probably in horizons also concentrating secondary phosphates hosting HREE and Th. Nickel is instead likely adsorbed onto titanium oxides which concentrate in the pool of highly insoluble detrital phases also including zircon whereas diaspore and boehmite are important in controlling both Ga and LREE. The low contents of Ni and Cr associated to the Semirom clayey bauxite exclude a provenance from a mafic protolith. Further, most of the samples have Eu/Eu∗ values close to those observed in the Sarvak Fm marly limestone, suggesting that the clayey component within the underlying limestone acted as protolith. The oxygen and hydrogen isotopic composition of the kaolinite-rich separates from the Semirom deposit indicates temperature never lower than 30 °C, supporting the idea of the Zagros basin close to the equator during the Cenomanian-Turonian under a warm and humid tropical climate with heavy rainfall.

A CARBONATE RAMP EVOLUTION IN THE TRANSITION FROM THE APULIA PLATFORM TO THE IONIAN BASIN DURING EARLY TO LATE CRETACEOUS (NW GREECE)

Facies analysis of Cretaceous carbonate sequences from the external and central Ionian zone revealed a homoclinal ramp model of evolution. During Berriasian to Valanginian, the carbonate ramp was differentiated to an inner-mid and outer ramp environment, which corresponded to the external and central Ionian zone, respectively, while the main inner ramp environment is assumed that was located in the Pre-Apulian zone. The external Ionian zone (inner-mid ramp) is characterized by muds tones-wackes tones with fragmented echinoderms, bivalves, radiolarians and rare aptychus considered to be deposited below the fairweather wave base (FWWB). Locally, thin graded storm deposits intervene, indicating deposition above the storm weather wave base (SWB). Minor occurrences of packs tonesgrainstones, with fragmented echinoderms, calcareous algae, tubiphytes, lagenid foraminifera and rare ooids occur, as well, considered to be deposited at the lowermost part of the inner ramp, near the constantly agitated fairweather wave base (FWWB). The central Ionian zone (outer ramp) is mainly characterized by mudstones-wackestones with abundant radiolarians and rare calpionellids and calcispheres, considered to be deposited below the storm wave base (SWB). No talus or breccias deposits were observed, during the mentioned time interval, in any part of the studied area. From Hauterivian to Turonian, continual sea-level rise, led to establishment of outer ramp environment in the external Ionian zone, over the previous inner-mid ramp, and outer ramp-basin environment, over the previous outer ramp, in the central Ionian zone. The transition from shallower to deeper conditions is characterized by an overall deposition of mudstones-wackestones with abundant radiolarians rooted in pure micrite.

Palaeoredox conditions and sequence stratigraphy of the Cretaceous storm-dominated, mixed siliciclastic-carbonate ramp in the Eastern Cordillera Basin (Colombia): Evidence from sedimentary geochemical proxies and facies analysis

The Cretaceous black shales of Colombia are among the most important successions in the north of South America and have attracted the attention of many geoscientists and exploration companies over the last few decades, because of their high hydrocarbon potential and the presence of emerald deposits. However, many uncertainties still remain with regard to their tectonic setting, sequence stratigraphy, depositional environments, palaeoxygenation conditions, and organic matter preservation. In order to develop a more integrated picture of these different processes and conditions, we conducted a detailed sedimentological, inorganic geochemical (U, V, Ni, Zn, Mn, Fe, Ti, Mo, Cu, Cr, Cd, Ba) and sequence stratigraphic analysis of the Cretaceous black shales in the Magdalena-Tablazo Sub-Basin (Eastern Cordillera Basin) of Colombia. Eleven lithofacies and five lithofacies associations of a storm-dominated, siliciclastic-carbonate ramp were identified, which range from basin to shallow inner ramp environments. These facies were grouped into six third-order stratigraphic sequences showing high-order cycles of marine transgression with constrained regressive pulses during the late Valanginian to early Coniacian. The black shales succession represents deposition under anoxic bottom water with some intervals representing dysoxic-suboxic conditions. The evolution of the sedimentary environments and their palaeoxygenation history reflect tectonic and eustatic sea-level controls that 1) produced a variable orientation and position of the coastline throughout the Cretaceous; 2) conditioned the low-gradient ramp geometry (<0.3°) and 3) modified the oxygenation conditions in the Magdalena-Tablazo Sub-Basin. An improved understanding of the sedimentary setting during deposition of the Cretaceous black shales in the Magdalena-Tablazo Sub-Basin assists in highlighting the interplay between the mechanism of sedimentation and continuum anoxic conditions prevailing in a basin, as well the important role of nutrient input from continental runoff as a trigger of high productivity and extended anoxia conditions.

Microporosity evolution in Iranian reservoirs, Dalan and Dariyan formations, the central Persian Gulf

Tight limestones with high amounts of microporosity will be the main targets of future hydrocarbon exploration and production in Middle East. In this paper, microporosity evolution in two Iranian reservoirs, Dalan and Dariyan formations, is investigated. These two formations represent the main hydrocarbon reservoirs in most of the Iranian hydrocarbon fields. Aptian Dariyan Formation is mainly composed of mud-dominated samples deposited in a ramp environment. The upper part of Permian Dalan Formation has the same facies distribution while the lower part is composed mainly of grain-dominated samples. Mud-dominated facies of Dariyan Formation have high porosity with moderate to low permeability. Micrites are rounded with punctic to serrate contacts. In contrast, mud-dominated facies of Dalan Formation have low porosity and permeability. SEM studies revealed that micrite texture has a major effect on final reservoir properties of these two formations. MICP and SEM studies showed that in grain-dominated samples of Dalan Formation, both small and large pore size throats are present. Mud-dominated samples of this formation are composed of micrites with compact-anhedral shape and coalescent contacts. These results combined with diagenetic studies confirmed the role of meteoric dissolution in microporosity development in both formations. In humid conditions of the Aptian time, micrites have been dissolved and a large amounts of microporosity have been developed. In contrast, in dry conditions of Late Permian, the original fabric of the micrites preserved until the late stages of burial. Rounded micrites are present in Dalan Formation where meteoric dissolution had a major effect on porosity development in this formation.

Sequence stratigraphy, palaeogeography, and coal accumulation in a gently sloping paralic basin: A case study from the Carboniferous–Early Permian Wuwei Basin, northwestern China

The Wuwei Basin is one of the major coal‐ and hydrocarbon‐bearing basins in northwestern China. Depositional environments, sequence stratigraphy, and palaeogeography of the Carboniferous to Early Permian coal measures in the Wuwei Basin were studied based on the data from outcrop sections and boreholes cores. The coal accumulation model is established within the sequence‐stratigraphic context. Fourteen lithofacies were recognized with the lithologies ranging from conglomerates, sandstones, siltstones, mudstones, carbonates, to coals, which are arranged into five facies associations representing delta, tidal flats, barrier and lagoon, littoral and associated strand‐plain, estuary, and carbonate ramp environments. These facies associations collectively revealed an overall marine‐continental transitional basin setting. The key sequence bounding surfaces, such as the regional unconformity, the basal surface of incised valley fills, and the surface that mark the facies reversal from regression to transgression, have been identified. Based on these surfaces, the Carboniferous to Early Permian in the Wuwei Basin were subdivided into seven third‐order sequences. The distribution of lithofacies and depositional facies in borehole sections, outcrop sections, and three correlation cross sections was investigated, and the sequence‐based palaeogeographic maps were reconstructed. It is inferred from the coal thickness and palaeogeographic maps that the most favourable coal accumulation environments were tidal flat and barrier–lagoon, followed by delta. The sequence stratigraphic framework controls the vertical horizons of the coal seams, whereas the palaeogeographic units determine the lateral development of the coal seams. Tidal flat, barrier–lagoon, and delta in the late transgressive systems tract and early highstand systems tract are the most favourable coal accumulation palaeogeographic units. The main coal accumulation centres were mainly distributed in the Yingpan Sag in the south of the basin, followed by the Ermahu Sag in the north of the basin.

Larger foraminiferal biostratigraphy and microfacies analysis from the Ypresian (Ilerdian-Cuisian) limestones in the Sistan Suture Zone (eastern Iran)

A high diversity of larger benthic foraminifera (LBF) fauna is recognized from the Ilerdian-Cuisian sedimentary succession for the first time of the Birjand region, Sistan Suture Zone of eastern Iran. These foraminifera are described in accordance with the standard shallow benthic zonation, placing considerable emphasis on biostratigraphic and microfacies analysis implications. Four shallow benthic zones (SBZ8 and SBZ10-12) have been designated in LBF horizons comprising index zonal markers such as Alveolina elliptica nuttalli (Davies), A. decipiens (Schwager), A. cf. oblonga, A. cf. minuta, A. cf. rugosa, A. cf. decastroi, A. aff. cremae, A. frumentiformis, A. aff. palermitana, N. cf. atacicus (Leymerie), N. tauricus (De la Harpe), N. pratti (d?Archia & Haime), N. polygyratus (Deshayes), N. cf. distans (Deshayes), Assilina sublaminosa (Gill), A. placentula (Deshayes), Asterocyclina cf. schweighauseri (Less), Discocyclina archiaci cf. staroseliensis (Less), and D. cf. archiaci bartholomei (Schlumberger). Five microfacies types including Alveolina-Opertorbitolites packstone (locally grainstone), nummulitids-Alveolina rudstone, Nummulites rudstone, Assilina-Nummulites rudstone, and orthophragminid-nummulitid pack-rudstone have been defined in the current study. Microfacies type 1 is finely characterized by high abundance of Alveolina with the predominance of A-form and globular to slightly elongate forms. Microfacies type 2 is dominated by small nummulitids in association with the abundance of the most diverse Alveolina species. Microfacies type 3 is composed of a diverse community of nummulitids, principally Nummulites A-forms along with Assilina. Microfacies type 4 is distinguished by a high diversity of not only A- and B-forms of Nummulites, but also Assilina specimens. Eventually, orthophragminids and nummulitids are common in microfacies type 5. Analysis of the Ilerdian-Cuisian deposits on the basis of the distribution of both biotic and abiotic components suggests depositions in a shallow ramp environment with deepening upward from the middle Ilerdian to the late Cuisian.

Stratigraphic correlation and paleoenvironmental analysis of the hydrocarbon-bearing Early Miocene Euphrates and Jeribe formations in the Zagros folded-thrust belt

The Lower Miocene Euphrates and Jeribe formations are considered as the main targets of the Tertiary petroleum system in the western part of the Zagros Basin. The formations consist of carbonates with some evaporate intercalations of the Dhiban Formation. This study utilized data from a field investigation including newly described outcrop sections and newly discovered productive oil fields within the Kirkuk embayment zone of the Zagros fold and thrust belt such as Sarqala and Kurdamir wells. This work is the first to show a stratigraphic correlation and paleoenvironmental interpretation by investigating both well data and new outcrop data. Three depositional environments were identified, (1) an inner and outer ramp belts environment, (2) shoal environment, and (3) restricted lagoon environment. Within these 3 environments, 12 microfacies were identified, based on the distribution of fauna mainly benthonic foraminifera, rock textures, and sedimentary structures. The inferred shallow water depths and variable salinities in both the Euphrates Formation and Jeribe Formation carbonates are consistent with deposition on the inner ramp (restricted lagoon and shoal) environments. Those found in the Euphrates Formation constrained the depositional environment to the restricted lagoon and shoal environment, while the microfacies in the Jeribe Formation provided evidence for an inner ramp and middle to outer ramp belt environments. This study represents the first detailed research that focuses on the stratigraphic correlation and changes in carbonate facies with the main aim to provide a wider understanding of stratigraphy of these carbonate reservoirs throughout the northern part of Iraq.

Biostratigraphy and sedimentology of Upper Permian and Lower Triassic strata at Masore, Western Slovenia

Upper Permian and lowermost Triassic strata of the Masore section in western Slovenia have been restudied by means of bio- and lithostratigraphy. This section is mainly characterized by a carbonate succession of the Bellerophon Formation deposited in a shallow marine ramp environment that was located in the western part of the Paleotethys. The Permian-Triassic boundary (PTB) transition is marked by laminated microbialites – stromatolites interpreted to reflect a deeper ramp environment. The conodont elements recovered enabled the recognition of the Hindeodus praeparvus Zone of the uppermost Permian (Upper Changhsingian) in the Bellerophon Formation. Gondolellids are documented in the PTB transitional interval with microbial microfacies, whereas the Isarcicella isarcica Zone (Lower Griesbachian, Lower Triassic) is recognized in the tectonically separated Werfen Formation just above the microbial microfacies part of the section. The lowermost part of the microbialites is characterized by Late Permian species of foraminifers indicating that at least this part of the section is still Upper Permian.

Anisian (Middle Triassic) marine ichnocoenoses from the eastern and western margins of the Kamdian Continent, Yunnan Province, SW China: Implications for the Triassic biotic recovery

Two Anisian (Middle Triassic) marine ichnocoenoses are reported from the Boyun and Junmachang (JMC) sections located along the eastern and western margins of the Kamdian Continent, Yunnan Province, Southwest China, respectively. The Boyun ichnoassemblage is middle Anisian in age and is dominated by robust Rhizocorallium, while the JMC ichnoassemblage is of an early Anisian age and is characterized by the presence of Zoophycos. The ichnoassemblage horizons of the Boyun section represent an inner ramp environment, while the JMC section was likely situated in a mid-ramp setting near storm wave base as indicated by the presence of tempestites. The ichnofossil-bearing successions are usually highly bioturbated in both the Boyun (BI 3–5, BPBI 5) and JMC (BI 3–4, BPBI 3–4) sections. Three large, morphologically complicated ichnogenera: 1) Rhizocorallium; 2) Thalassinoides; and, 3) Zoophycos characterize the Anisian ichnocoenoses. Of these, Rhizocorallium has mean and maximum tube diameters up to 20.4mm and 28mm, respectively, while Thalassinoides mean and maximum tube diameters are 14.2mm and 22mm, respectively. Zoophycos is present in the early Anisian strata of the JMC section, and represents the oldest known occurrence of this ichnogenus following the latest Permian mass extinction. Similar to coeval ichnoassemblages elsewhere in the world, the Yunnan ichnocoenoses embrace a relatively low ichnodiversity, but their burrows usually penetrate deeply into the sediment, and include large and complex Rhizocorallium and Thalassinoides. All of these ichnologic features are indicative of recovery stage 4 after the latest Permian crisis. Anisian ichnoassemblages occur globally in six different habitat settings, and all show similar ecologic characteristics except for slightly different degrees of ichnotaxonomic richness, indicating that depositional environment is not a crucial factor shaping the recovery of the trace-makers, but may have an impact on their ichnodiversity. When compared with some important Early Triassic (mainly Spathian) ichnoassemblages worldwide, the Anisian ichnocoenoses examined for this study are slightly less diversified, and possess more or less the same maximum burrow sizes, but the penetration depth of burrows and the distribution of burrow sizes are much larger than those from the Early Triassic. It is worthy of note that the lower ichnodiversity of the Anisian ichnocoenoses may have resulted from intense bioturbation by deeper tiers, representing a taphonomic product that is totally unrelated to environmental stress. In addition, Anisian Rhizocorallium and Thalassinoides have much larger burrow sizes than the same ichnotaxa from the Lower Triassic, implying that ichnocoenoses may have recovered in Spathian, but did not stabilize until the Anisian.

Unravelling the origin of a Paleogene unconformity in the Latium-Abruzzi carbonate succession: A shaved platform

The Latium-Abruzzi carbonate platform constitutes the sedimentary record from the Triassic to the late Miocene of the southern Tethyan margin outcropping in the Apennines. This sedimentary succession is punctuated by an important hiatus between the end of the Cretaceous and the early Miocene. This Paleogene hiatus has long been interpreted as the result of prolonged subaerial exposure. However, no evidence of such exposure has ever been documented on the Latium-Abruzzi platform. This is contrary to the coeval and adjacent Apulia and Lessini carbonate platforms where a broadly-developed paleokarst system formed between the Eocene and the early Miocene. The assumption that the Miocene marine transgression deeply eroded a karstified Cretaceous substrate on the Latium-Abruzzi platform is not supported because this surface appears in the field as a perfectly flat, bioeroded paraconformable surface. In this work, a marine origin is proposed for this unconformity. From late Paleocene to early Miocene, the Latium-Abruzzi carbonate platform was a shaved isolated platform exposed to wave action in the middle of the proto-Mediterranean area. Bioclastic sediment accumulated during transgressive and sea-level highstand phases, whereas in the following falling and lowstand stages, sediment was eroded as the seafloor came into the zone of wave abrasion. The sediment eroded from the platform was shed into the basin where it produced coarse bioclastic intercalations in the hemipelagites of the Scaglia Formation. At the end of the Oligocene, when the adjacent basin was filled, a small ramp was established in the transitional zone between the platform and the basin. There, cross-bedded carbonates accumulated in middle to outer ramp environments. The inner ramp remained within the zone of wave abrasion such that sediment generated there was washed out into the middle and outer ramp environments. Finally, in the early Burdigalian, the Mediterranean progressively evolved into closed sea during the initial stages of the closure of Indo-Pacific connection. The reduction of the wave-base depth, typical of an enclosed sea, created increased accommodation space allowing sediments to accumulate and form the complete inner to outer ramp facies belts of the Miocene Latium-Abruzzi ramp.

Quantitative allochem compositional analysis of Lochkovian-Pragian boundary sections in the Prague Basin (Czech Republic)

Quantitative allochem compositional trends across the Lochkovian-Pragian boundary Event were examined at three sections recording the proximal to more distal carbonate ramp environment of the Prague Basin. Multivariate statistical methods (principal component analysis, correspondence analysis, cluster analysis) of point-counted thin section data were used to reconstruct facies stacking patterns and sea-level history. Both the closed-nature allochem percentages and their centred log-ratio (clr) coordinates were used. Both these approaches allow for distinguishing of lowstand, transgressive and highstand system tracts within the Praha Formation, which show gradual transition from crinoid-dominated facies deposited above the storm wave base to dacryoconarid-dominated facies of deep-water environment below the storm wave base. Quantitative compositional data also indicate progradative-retrogradative trends in the macrolithologically monotonous shallow-water succession and enable its stratigraphic correlation with successions from deeper-water environments. Generally, the stratigraphic trends of the clr data are more sensitive to subtle changes in allochem composition in comparison to the results based on raw data. A heterozoan-dominated allochem association in shallow-water environments of the Praha Formation supports the carbonate ramp environment assumed by previous authors.

Carbonate microfacies of the San Juan Formation (Ordovician: Oepikodus evae and Oepikodus intermedius conodont zones), Niquivil, Central Precordillera, Province of San Juan (Argentina)

This contribution presents the description and interpretation of carbonate microfacies of the San Juan Formation (Ordovician) at the Niquivil section, considering the stratigraphical interval between the Oepikodus evae and Oepikodus intermedius conodont zones. The distribution of the microfacies and the conodonts assemblages allow us to identify different sub-environments within the late Floian carbonate ramp of the Central Precordillera. Five microfacies were recognized from the base to the top: M1 Bioclastic mudstone-wackestone; M2 Bioclastic-peloidal wackestone; M3 Intra-bioclastic wackestone; M4 Intra-bioclastic packstone; M5 Peloidal grainstone. The vertical distribution of these microfacies indicates a shallowing trend of the carbonate ramp in the Niquivil section for this temporal interval, which suggests a middle ramp environment with low energy, without wave action, and that evolved towards the middle-inner ramp environment with more energy by wave action and development of tempestites.

Paleoenvironmental Distribution Patterns of Sequence Stratigraphy in the Paleocene-Eocene Deposits of Sekonj Trough Event (Central Iran)

The Paleocene-Eocene successions were studied in Sekonj and Abaregh sections and located in South of Kerman in order to determine the microfacies, depositional environment and sequence stratigraphy. The deposits in the studied area are mainly composed of limestone, sandstone, conglomerate and shale. The obtained data from the field and laboratory observations led to identification of 11 microfacies which represent the coastal, tidal flat, lagoon, barrier and proximal open marine which were deposited in a carbonate-siliciclastic ramp environment. The sedimentology evidences, vertical analysis of microfacies, facies and relative sea level changes represent that the Paleocene-Eocene succession deposits have two 3rd depositional sequences in the studied area. The sequence 2 cannot be recognized completely due to the lack of outcrop. Sequence stratigraphic studies show an upward shallowing trend corresponding to the global sea level changes from the base to the upward of the succession.

Paleoenvironmental evolution of the southern Neuquèn basin (Argentina) during the Tithonian-Berriasian (Vaca Muerta and Picún Leufú Formations): a multi-proxy approach

The Tithonian-Berriasian interval in the southern part of the Neuquén Basin is represented by the Vaca Muerta and the Picún Leufú Formations. Facies analysis and correlation of the Vaca Muerta Formation and the lower part of the Picún Leufú Formation in the Picún Leufú Anticline allow us to characterize the evolution of successive facies belts representing siliciclastic shelf and mixed ramp environments. Shoreface and offshore facies are developed on the siliciclastic shelf in the western and southern parts of the Picún Leufú Anticline. The offshore transition domain is characterized by storm beds and slump features, whereas the offshore domain corresponds to black to grey shales and turbidites. The mixed siliciclastic-carbonate ramp is characterized by the development of a lagoon and high-energy shoal in the proximal part of the inner ramp, whereas the distal part comprises a tidal complex. The mid-ramp zone is characterized by storm influence and the outer ramp by fine-grained deposits. Two major transgressive-regressive sequences and five high-frequency transgressive-regressive sequences are recognized. The high-frequency transgressive-regressive sequences make up three progradational sequences, an aggradational-progradational sequence and an aggradational sequence. The geochemical characteristics and clay mineralogy of the Tithonian-Berriasian interval in the southern Neuquén Basin indicate that (1) climate played a key role in the evolution of the sedimentary environment, (2) a “normal marine” depositional environment with oxic sea water and sediment pore waters, rapidly changing to suboxic conditions at shallow depth below the seabed and (3) the occurrence of episodically restricted water-mass circulation at the onset of deposition of the Vaca Muerta Formation.

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

We studied a potential petroleum source rock deposited in a clastic-dominated ramp environment: the Argiles de Châtillon Formation (Kimmeridgian-Tithonian, Boulonnais area, northern France). The formation was deposited along a proximal-distal gradient on this ramp affected by synsedimentary fault movements. A sedimentological, mineralogical and geochemical study was conducted to decipher the distribution patterns of sedimentary parameters along such a depth increase over the ramp. It comes out that smectite distribution unexpectedly mimics the lateral depth evolution despite the good floatability of the mineral. It is also observed that the Argiles de Châtillon could accumulate noticeable amounts of organic matter in spite of paleoenvironmental conditions that were not specifically prone to organic matter preservation and burial (sedimentation rate, mineral particle grain size, productivity, oxygenation level…). Conversely, reactive iron, when being present in limited abundance, must have allowed sulfide ions to react with organic molecules instead of forming iron sulfides, which fostered organic matter preservation and accumulation. This protecting role of organic sulfide incorporation (additional to other favorable factors) cannot exist if reactive iron is relatively abundant. Lastly, our results (still fragmentary) suggest that smectite minerals could carry reactive iron, which would have occasionally hampered organic-matter preservation.

Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)

This study characterizes the stratigraphical context as well as the palaeoenvironmental and sea-level trends during the Late Frasnian and Famennian in the Buschteich section (Thuringia, Germany). An integrated approach combines conodont biostratigraphy, chemostratigraphy based on carbon and oxygen isotopes , conodont biofacies analyses and investigations of the morphotypes of foraminifera that occur in the section was applied. Conodonts provided a fine-scale biostratigraphical framework. All the Famennian con-odont biozones were identified, with the exception of the three lowest. The Frasnian-Famennian transition was further identified by a positive carbon isotope excursion. This continuous Famennian record is developed in a condensed limestone succession. The reddish Griotte facies, typical of strata of this period, were not documented. Black shales were not developed except for an interval of dark colored mud rocks corresponding to the Hangenberg event at the top of the section. The carbonate succession of Buschteich was deposited in an outer ramp environment, pelagic organisms are the dominant fauna. This is corroborated by the overall dominance of the deep-water conodont genus Palmatolepis. Microfacies, conodont biofacies and foramini-fera indicated a deepening trend from the Lower crepida to the Lower rhomboidea Zone, followed by a shal-lowing trend from the Upper rhomboidea to the praesulcata Zone. This sequence overall matches the sea level reference curve for Euramerica. The timing of maximum water depth at Buschteich, as well as the general sea-level reference record, differs from the Col des Tribes (Montagne Noire, France). Differential uplift or overprinting of the long-term eustatic changes may be the cause of the discrepancy in the local sea-level records.

Intraspecific variation in an Ediacaran skeletal metazoan: Namacalathus from the Nama Group, Namibia.

Namacalathus hermanastes is one of the oldest known skeletal metazoans, found in carbonate settings of the terminal Ediacaran (~550-541million years ago [Ma]). The palaeoecology of this widespread, goblet-shaped, benthic organism is poorly constrained yet critical for understanding the dynamics of the earliest metazoan communities. Analysis of in situ assemblages from the Nama Group, Namibia (~548-541Ma), shows that Namacalathus exhibited size variation in response to differing water depths, hydrodynamic conditions and substrate types. In low-energy, inner ramp environments, Namacalathus attains the largest average sizes but grew in transient, loosely aggregating, monospecific aggregations attached to microbial mats. In high-energy mid-ramp reefs, Namacalathus spatially segregated into different palaeoecological habitats with distinct size distributions. In outer ramp environments, individuals were small and formed patchy, dense, monospecific aggregations attached to thin microbial mats. Asexual budding is common in all settings. We infer that variations in size distribution in Namacalathus reflect differences in habitat heterogeneity and stability, including the longevity of mechanically stable substrates and oxic conditions. In the Nama Group, long-lived skeletal metazoan communities developed within topographically heterogeneous mid-ramp reefs, which provided diverse mechanically stable microbial substrates in persistently oxic waters, while inner and outer ramp communities were often ephemeral, developing during fleeting episodes of either oxia and/or substrate stability. We conclude that Namacalathus, which forms a component of these communities in the Nama Group, was a generalist that adapted to various palaeoecological habitats within a heterogeneous ecosystem landscape where favourable conditions persisted, and was also able to opportunistically colonise transiently hospitable environments. These early skeletal metazoans colonised previously unoccupied substrates in thrombolitic reefs and other microbial carbonate settings, and while they experienced relatively low levels of interspecific competition, they were nonetheless adapted to the diverse environments and highly dynamic redox conditions present in the terminal Ediacaran.

Neoproterozoic stratigraphic correlation of Subei-Huainan in China

Subei-Huainan in the southeastern margin of the North China platform is a key area of convergence of China’s north-south stratigraphic correlation of Neoproterozoic. Due to the lack of fossils, the Neoproterozoic stratigraphic correlation is still a difficulty in this area. The division of Neoproterozoic strata and the establishment of the correlation framework of the stratigraphic sequences have been done through recognizing the regularity of meter-scale cycles by measuring the outcrop of Neoproterozoic strata and using the Fischer diagram and the sea-level curve of transgressive-regressive by the maximum flooding sedimentary and molar teeth carbonate sedimentary. The maximum flooding sedimentary which becomes cycle sequence comparison of sign is isochronous in the same basin and is a key of cycle sequence comparison although paleogeographic environment and sedimentary sequence may be different in the sedimentary strata of the same geologic time. Molars carbonates, having specific geological distrution time and depositional environment and mainly developing in the ramp environment of the platform edge in the 1500-650 Ma, are the new symbol of the stratigraphic correlation in the Neoproterozoic. Because of the missing of the fossil record, the maximum flooding and the molar-teeth carbonate sedimentary, as the new research ideas, are used in stratigraphic division and correlation.