Microfacies and sedimentary environment of the Oligocene sequence (Asmari Formation) in Fars sub-basin,Zagros Mountains,southwest Iran

The Asmari Formation is a thick carbonate succession of the Oligo-Miocene in Zagros Mountains (southwest Iran). In order to interpret the facies and depositional environment of the Asmari Formation, three measured sections were studied in Fars area for microfacies analyses. There, 12 microfacies types are distinguished based on their depositional textures, petrographic analysis, and fauna. Thus, three major depositional environments were identified in the Asmari Formation including open-marine, reef/shoal, and lagoon. These depositional environments correspond to inner, middle, and outer ramp.     

Facies architecture and depositional development of Middle Miocene carbonate strata at Siwa Oasis, Northwestern Egypt

Based on microfacies analyses and sedimentological data, 17 facies are identified within the Middle Miocene carbonates at Siwa Oasis in the northern Western Desert of Egypt. These facies are attributed to five main facies belts. Within these facies and facies belts, five foraminiferal assemblages are recognized. A depositional model relates the reported facies and biofacies to a down-dip depositional profile of an inner to middle carbonate ramp. The facies of the peritidal to restricted lagoon (facies belt 1) and the less-restricted lagoon (facies belt 2) were deposited in the inner ramp behind the barrier/beach shoal facies belt 3. Basinward, lime mudstone of facies belts 4 and 5 accumulated in a proximal to distal middle ramp, respectively. The depositional evolution involved three stages, which are strongly controlled by tectonics and eustatic sea-level changes. The first stage comprises the transgressive Lower Miocene clastic-dominated fluvial facies of the Moghra Formation. The second stage heralds the deposition of the Langhian inner-ramp carbonate and shale facies of the basal Oasis Member of the Marmarica Formation under a relatively high stand of sea level, constrained clastic influx and climate warming. The final stage is represented by Langhian to Serravallian mid-ramp carbonate-dominated facies of the Siwa Escarpment and El Diffa Plateau members under fluctuating sea level, and a westward restriction in clastic supply and water turbidity.     

Facies characteristic and paleoenvironmental reconstruction of the Fahliyan Formation,Lower Cretaceous,in the Kuh-e Siah area,Zagros Basin,southern Iran

The Lower Cretaceous Fahliyan Formation, part of the Khami Group, unconformably overlies the Hith Formation and is conformably overlain by the Gadvan Formation in the study area in southern Iran. The Fahliyan Formation is a reservoir rock in Zagros Basin. This formation was investigated by a detailed petrographic analysis in order to clarify the depositional facies and sedimentary environment in the Kuh-e Siah Anticline in Boushehr Province. Petrographic studies led to the recognition of 25 microfacies that were deposited in four facies belts: tidal flat, lagoon, and shoal in inner ramp and shallow open-marine in mid-ramp environment. An absence of turbidite deposits, reefal facies, and gradual facies changes indicate that the Fahliyan Formation was deposited on a carbonate ramp. Calcareous algae and benthic foraminifera are abundant in the shallow marine carbonates of the Fahliyan Formation. These skeletal grains have been studied in order to increase the understanding of their distributions in time and space. A total of ten genera belonging to different groups of calcareous algae and 16 genera of benthic foraminifera are recognized from the Fahliyan Formation at Kuh-e Siah section.     

Applications of nummulitids and other larger benthic foraminifera in depositional environment and sequence stratigraphy: an example from the Eocene deposits in Zagros Basin,SW Iran

The Tale-Zang Formation in Zagros Mountains (south-west Iran) is a Lower to Middle Eocene carbonate sequence. Carbonate sequences of the Tale-Zang Formation consist mainly of large benthic foraminifera (e.g. Nummulites and Alveolina), along with other skeletal and non-skeletal components. Water depth during deposition of the formation was determined based on the variation and types of benthic foraminifera, and other components in different facies. Microfacies analysis led to the recognition of ten microfacies that are related to four facies belts such as tidal flat, lagoon, shoal and open marine. An absence of turbidite deposits, reefal facies, gradual facies changes and widespread tidal flat deposits indicate that the Tale-Zang Formation was deposited in a carbonate ramp environment. Due to the great diversity and abundance of larger benthic foraminifera, this carbonate ramp is referred to as a “foraminifera-dominated carbonate ramp system”. Based on the field observations, microfacies analysis and sequence stratigraphic studies, three third-order sequences in the Langar type section and one third-order sequence in the Kialo section were identified. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. The transgressive systems tracts of sequences show a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts of sequences contain predominantly imperforate foraminifera.     

Depositional environment and sequence stratigraphy of the Oligo-Miocene Asmari Formation in SW Iran

The Asmari Formation, a thick carbonate succession of the Oligo-Miocene in Zagros Mountains (southwest Iran), has been studied to determine its microfacies, paleoenvironments and sedimentary sequences. Detailed petrographic analysis of the deposits led to the recognition of 10 microfacies types. In addition, five major depositional environments were identified in the Asmari Formation. These include tidal flat, shelf lagoon, shoal, slope and basin environmental settings and are interpreted as a carbonate platform developed in an open shelf situation but without effective barriers separating the platform from the open ocean. The Asmari carbonate succession consists of four, thick shallowing-upward sequences (third-order cycles). No major hiatuses were recognized between these cycles. Therefore, the contacts are interpreted as SB2 sequence boundary types. The Pabdeh Formation, the deeper marine facies equivalent of the Asmari Limestone is interpreted to be deposited in an outer slope-basin environment. The microfacies of the Pabdeh Formation shows similarities to the Asmari Formation.     

Relationships between foraminiferal assemblages and depositional sequences in Jahrum Formation,Ardal area (Zagros Basin,SW Iran)

The Jahrum Formation was deposited in the foreland basin in southwest Iran (Zagros Basin). The Zagros mountain belt of Iran, a part of the Alpine–Himalayan system, extends from the NW Iranian border through to SW Iran, up to the strait of Hormuz. The various facies of the Jahrum Formation were deposited in four main genetically related depositional environments, including: tidal flat, lagoon, shoal and open marine. These are represented by 14 microfacies. The Jahrum Formation represents sedimentation on a carbonate ramp. Tidal flat facies are represented by fenestral fabric, stromatolitic boundstone and thin-bedded planes. Carbonate deposition in a shallow marine lagoon was characterised by wacke–packstone, dominated by various taxa of imperforate foraminifer. The shoals are made up of medium- to coarse-grained skeletal and peloidal grainstone. This facies was deposited predominantly in an active high energy wave and current regime, and grades basinward into middle ramps facies are represented by wackestones–packstones with a diverse assemblage of echinoderm and large benthic foraminifers with perforate wall. Outer ramp facies consist of alternating marl and limestones rich in pelagic foraminifera. There is no evidence for resedimentation processes in this facies belt. The sequence stratigraphy study has led to recognition of three third-order depositional sequences.     

Facies,depositional sequences,and biostratigraphy of the Oligo-Miocene Asmari Formation in Marun oilfield,North Dezful Embayment,Zagros Basin,SW Iran

The Asmari Formation in Marun oilfield (south-west Iran), is about 440 m-thick marine carbonate succession with subordinate siliciclastic rocks, characterized by abundant benthic foraminifera (perforate and imperforate). Foraminiferal biostratigraphy indicates that this unit is Oligocene–Miocene in age. The distribution of benthic foraminifera and other components have led to the recognition of three siliciclastic and ten carbonate facies that were deposited in inner ramp (shoreline, tidal flat, restricted and open lagoon and shoal), middle and outer ramp sub-environments. Based on vertical facies trends, three third-order sequences in the Oligocene and three third-order sequences in the Miocene sediments have been identified. These depositional sequences are bounded by both type 1 and type 2 sequence boundaries. The transgressive systems tracts (TST) of sequences show deepening-upward facies trend with a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts (HST) have a shallowing-upward facies trend and contain predominantly imperforate foraminifera. Deposition of these depositional sequences (DS) were controlled by both eustasy and tectonic subsidence.     

Paleoenvironmental model and sequence stratigraphy of the Asmari Formation in southwest Iran

The Asmari Formation is a thick carbonate succession of the Oligo-Miocene in southwest Iran (Zagros Basin). The Zagros Basin was a continental margin attached to the eastern edge of Africa throughout the Phanerozoic. The foraminiferal limestone from the Asmari Formation has been studied to determine its microfacies, paleoenvironments and sedimentary sequences. Based on analysis of larger benthic foraminiferal assemblages and microfacies features three major depositional environments are identified. These include open marine, barrier and lagoon-lower intertidal. These three are represented by eleven microfacies. A carbonate ramp platform is suggested for the depositional environment of the Asmari Formation. The inner ramp facies are characterized by wackestone-packstone, dominated by various taxa of imperforate foraminifera. The middle ramp facies represented by packstone-grainstone to floatstone with a diverse assemblage of larger hyaline foraminifera. The outer ramp is dominated by argillaceous wackestone, characterized by planktonic foraminifera and larger hyaline foraminifera. Two third-order sequences are identified based on deepening and shallowing patterns in microfacies, staking patterns and the distribution of Oligocene-Miocene foraminifers.     

Cyclic sedimentation across a Middle Ordovician carbonate ramp (Duwibong Formation), Korea

Summary The Middle Ordovician Duwibong Formation (about 100 m thick), Korea, comprises various lithotypes deposited across a carbonate ramp. Their stacking patterns constitute several kinds of meter-scale, shallowing-upward carbonate cycles. Lithofacies associations are grouped into four depositional facies: deep- to mid-ramp, shoal-complex, lagoonal, and tidal-flat facies. These facies are composed of distinctive depositional cycles: deep subtidal, shallow subtidal, restricted marine, and peritidal cycles, respectively. The subtidal cycles are capped by subtidal lithofacies and indicate incomplete shallowing to the peritidal zone. The restricted marine and peritidal cycles are capped by tidal flat lithofacies and show evidence of subaerial exposure. These cycles were formed by higher frequency sea-level fluctuations with durations of 120 ky (fifth order), which were superimposed on the longer term sea-level events, and by sediment redistribution by storm-induced currents and waves. The stratigraphic succession of the Duwibong Formation represents a general regressive trend. The vertical facies change records the transition from a deep- to mid-ramp to shoal, to lagoon, into a peritidal zone. The depositional system of the Duwibong Formation was influenced by frequent storms, especially on the deep ramp to mid-ramp seaward of ooid shoals. The storm deposits comprise about 20% of the Duwibong sequence.     

Facies models of a shallow-water carbonate ramp based on distribution of non-skeletal grains (Kimmeridgian,Spain)

The internal facies and sequence architecture of a Late Jurassic (Late Kimmeridgian) shallow carbonate ramp was reconstructed after the analysis and correlation of 17 logs located south of Teruel (northeast Spain). The studied rocks are arranged in five high-frequency sequences A–E (5–26 m thick) bounded by discontinuities traceable across the entire study area (20 × 25 km). Facies analysis across these sequences resulted in the reconstruction of three sedimentary models showing the transition from interior ramp environments (i.e., lagoon, backshoal, and shoal) to the progressively deeper foreshoal and offshore areas. Coral-microbial reefs (meter-sized patch and pinnacle reefs) have a variable development throughout the sequences, mostly in the foreshoal and offshore-proximal environments. The preferential occurrence and down-dip gradation of non-skeletal carbonate grains has been evaluated across the three models: low-energy peloidal-dominated, intermittent high-energy oolitic-dominated and high-energy oolitic–oncolitic dominated. The predominance of these non-skeletal grains in the shoal facies was mainly controlled by the hydrodynamic conditions and spatial heterogeneity of terrigenous input. The models illustrate particular cases of down-dip size-decrease of the resedimented grains (ooids, peloids, oncoids) due to storm-induced density flows. Offshore coarsening of certain particles (intraclasts, oncoids) is locally observed in the mid-ramp areas favorable for microbial activity, involving coral-microbial reef and oncoid development. The observed facies variations can be applicable to carbonate platforms including similar non-skeletal components, where outcrop conditions make the recognition of their three-dimensional distribution difficult.     

Taphonomy of Early Permian benthic assemblages (Carnic Alps, Austria): carbonate dissolution versus biogenic carbonate precipitation

During the Early Permian, in the area of the Carnic Alps, a quartz-gravelly beach fringed a mixed siliciclastic-carbonate lagoon with fleshy algal meadows and oncoids; seaward, an ooid shoal belt graded down dip to a low-energy carbonate inner shelf with phylloid algal meadows. In limestones, foraminiferal biomurae and bioclast preservation record tapholoss by rotting of non-calcified organisms (interpreted as fleshy algae) and by dissolution of aragonitic fossils. Carbonate loss by dissolution was counteracted and, locally, perhaps exceeded by carbonate precipitation of encrusting foraminifera and as oncoids. Sites of abrasion and carbonate dissolution (beach), sites with tapholoss by rotting and dissolution, but with microbialite/foraminiferal carbonate precipitation (lagoon, inner shelf), and sites only of carbonate precipitation (ooid shoals) co-existed on discrete shelf compartments. Compartmentalized, contemporaneous carbonate dissolution and precipitation, to total amounts yet difficult to quantify, impede straightforward estimates of ancient carbonate sediment budget.     

Siliciclastic‐carbonate ramp,a hostile world for carbonate shelf fauna? The example of the Givetian Ardenne Platform

The Givetian platform of the Ardennes Massif records several alternations between a siliciclastic‐carbonate ramp and a carbonate shelf. Usually these depositional contexts are considered as a major disruption implying a perturbation of many ecological parameters. We established the impact of these variations on the biodiversity structure through the study of the trophic organisation. Thanks to a previous microfacies analysis, 550 levels of the Mont d'Haurs section in Givet and ten associated environments were precisely defined. Seven palaeotrophic levels are recognized from micropalaeontological data, including benthic, planktonic, heterotroph and autotroph organisms. The spatial and temporal distributions of these levels have been analysed through means of multivariate analyses. The statistic results show that the distribution of the palaeotrophic levels during periods characterized by a mixed ramp is not significantly different than during carbonate shelf influences. These environmental modifications do not affect the community‐type. These results support recent studies performed on different benthic communities occurring in the Givetian of the Ardennes Massif. Indeed, trilobite and ostracod faunas of this period appear more affected by global environmental changes as the Ka?ák (uppermost Eifelian) and Taghanic (late middle Givetian) events. Moreover, along the proximal‐distal transect on the platform, the reef constitutes the most singular environment. However, contrary with the Frasnian reefs of the Ardennes Massif, which consist of carbonate mud mounds laying on a deep mixed ramp, there is no trilobite community that appears restricted to Givetian reefs. Thus global bioevents during the Middle Devonian have a more important control on the biodiversity than the platform morphology. These results support the view that at wide scales (spatial and temporal), the biodiversity responds more positively to ecological disturbances.     

Sedimentary facies and paleoenvironmental interpretation of the Oligocene larger-benthic-foraminifera-dominated Qom Formation in the northeastern margin of the Tethyan Seaway

The well-exposed outcrops of the Bujan, northern Abadeh, and Varkan stratigraphic sections of the Qom Formation in the Iranian part of the “northeastern margin” of the Tethyan Seaway were characterized by abundant biogenic components dominated by foraminifers, coralline red algae, and corals. The Qom Formation is Rupelian–Chattian in age in the study areas. Based on the field investigations, depositional textures, and dominant biogenic components, fifteen (carbonate and terrigenous) facies were identified. These facies can be grouped into four depositional environments: open marine, open lagoon, restricted lagoon, and continental braided streams. The marine facies were deposited on a ramp-type platform. The euphotic inner ramp was characterized mainly by imperforate foraminifera, with co-occurrence of some perforate taxa. These facies passed basinward into a mesophotic (middle) ramp with Neorotalia packstone (F5), coral, coralline algae, perforate foraminiferal packstone (F4), and coral patch reefs (F7). The deeper, oligophotic ramp facies were marly packstones with planktonic and hyaline benthic foraminifera, including large lepidocyclinids and nummulitids. The abundance of perforate foraminifera and the absence of facies indicating restricted lagoonal or intertidal settings suggest that the Varkan section was deposited mainly in open marine settings with normal salinity. The prevalence of larger benthic foraminiferal and red algal assemblages, together with the coral facies, indicates that carbonate production took place in tropical–subtropical waters.     

Morphology,faunas and genesis of ordovician hardgrounds from Southern Ontario,Canada

We have used associations of different microfacies to define facies (or microfacies associations) which form reasonably well-defined sequences, which we infer, from analogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scale topographic differentiation into shoal, slope and basinal environments.Shoal environments are characterized by typically cross-bedded, well-sorted bioclastic sands, with intershoal areas consisting of interbedded bioclastic sands and heavily bioturbated finer-grained carbonates.Slope and “basinal” environments are typically represented by “proximal” and “distal” cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water clastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed to migration of channels on the fans and the effect of funnelling of storm surges down the channels.Although hardground morphology and faunas are mostly related to local effects such as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renewed sedimentation. Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biotas. Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flow fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assemblages formed after redeposition.     

Litho- and biostratigraphy of the Upper Jurassic Mozduran Formation,Kopet-Dagh,Iran, and its paleoenvironmental implications

The Upper Jurassic Mozduran Formation at Baghak section, Kopet-Dagh area northeast Iran, characterized by multiple units of carbonates and evaporates intercalated with siliciclastic deposits, yielded benthic foraminifera, calcareous algae, and crustacean coprolites, including Alveosepta jaccardi, Charentia aff. nana, Deloffrella quercifoliipora, Marinella cf. lugeoni, Favreina, and Solenopora sp. The almost consistent occurrence of Alveosepta jaccardi in the studied section, i.e., the Alveosepta jaccardi Zone, indicates late Oxfordian to the Kimmeridgian in age. Facies analysis of the formation reveals depositional settings of a peritidal zone, a lagoon, and a high-energy shoal of a carbonate ramp. Terrigenous and evaporites are found in eastern parts of Kopet-Dagh, representing a more proximal sedimentary environment. These siliciclastic inputs are associated with tectonic events during Oxfordian to Tithonian in the region.     

Maastrichtian facies succession and sea-level history of the Hossein-Abad,Neyriz area,Zagros Basin

The Maastrichtian shallow-water carbonate platform (Tarbur Formation) is described from outcrop in southwest Iran. It is characterised by eight microfacies types, which are dominated by larger foraminifera, rudist debris and dasycladacean algae. They are grouped into four distinct depositional settings: tidal flat, lagoon, barrier and open marine. The depositional settings include stromatolitic boundstone of tidal flat, peloidal dasycladacean miliolids wackestone and peloid bioclastic imperforate foraminifera wackestone of restricted lagoon, Omphalocyclus miliolids bioclast packstone–grainstone and miliolids intraclast bioclast packstone–grainstone of open lagoon, rudist bioclast grainstone of inner-platform shoals and rudist bioclast floatstone–rudstone and bioclastic wackestone of open-marine environments.

The facies and faunal characters are typical of a ramp-like open shelf. The lack of reef-constructing organisms resulted in a gently dipping ramp morphology for the margin and slope. On the basis of facies analysis, three depositional sequences (third order) are defined.     

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 (~550–541 million 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 (~548–541 Ma), 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.     

Facies architecture,depositional environments,and sequence stratigraphy of the Middle Cambrian Fasham and Deh-Sufiyan Formations in the central Alborz,Iran

Based on their lithologic characteristics and stratal geometries, the Middle Cambrian Fasham and Deh-Sufiyan Formations of the lower Mila Group in the Central Alborz, northern Iran, exhibit 39 lithofacies representing several supratidal to deep subtidal facies belts. The siliciclastic successions of the Fasham Formation are divided into two facies associations, suggesting deposition in a tide-dominated, open-mouthed estuarine setting. The mixed, predominantly carbonate successions of the Deh-Sufiyan Formation are grouped into ten facies associations. Four depositional zones are recognized on the Deh-Sufiyan ramp: basinal, outer ramp (deep subtidal associations), mid ramp (shallow subtidal to lower intertidal associations), and inner ramp (shoal and upper intertidal to supratidal associations). These facies associations are arranged in small-scale sedimentary cycles, i.e., peritidal, shallow subtidal, and deep subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. Systematic changes in stacking pattern (cycle thickness, cycle type, and facies proportion) allow to reconstruct long-term changes in sea-level. Six large-scale cycles (S1–S6) have been identified and are interpreted as depositional sequences showing retrogradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The six depositional sequences provide the basis for inter-regional sequence stratigraphic correlations and have been controlled by eustatic sea-level changes.     

Sedimentology and depositional sequences of a Kimmeridgian carbonate ramp system,Lower Saxony Basin,Northern Germany

Shallow-marine Kimmeridgian (Late Jurassic) deposits in the Lower Saxony Basin (LSB) composed of alternating limestone, marl and claystone attract great palaeontological interest due to their rich invertebrate and vertebrate assemblages. Unfortunately, the absence of open-marine marker fossils and numerous sedimentary gaps in combination with lateral facies changes hamper the precise stratigraphic correlation of these strata on both a local and global scale. Here, an integrated approach combining carbonate microfacies analysis, ostracod biostratigraphy and high-resolution sequence stratigraphy is applied to two Kimmeridgian sections (Langenberg and Bisperode, 60 km apart) in the southeastern LSB. High-resolution carbonate microfacies analysis enables the definition of 19 microfacies types and seven microfacies associations, which can be arranged into facies belts along a carbonate ramp. Vertical microfacies, bed thickness and diagnostic surfaces define stacking patterns that are interpreted as small-, medium- and large-scale sequences. The ostracod biostratigraphic framework established in this study provides the required stratigraphic control. Correlation of the two studied sections reveals a more proximal setting for Bisperode than Langenberg and an overall shallowing-up trend from mid-ramp to proximal inner ramp developed in both sections. Furthermore, the majority of the medium-scale sequence boundaries defined in this study can be found in similar biostratigraphic positions in other European basins. Synsedimentary tectonics combined with high sediment accumulation rates can be identified as important controlling factors for the distribution and composition of the Kimmeridgian deposits in the LSB based on detailed correlation on both a regional and super-regional scale.     

The Antimonio ramp in Sonora,Mexico

The Upper Permian to Lower Jurassic El Antimonio Group has been considered as part of the NW–SE-trending tectonostratigraphic Antimonio terrane in Sonora, Mexico. The Upper Triassic Rio Asunción Formation of the El Antimonio Group comprises a shallow-marine detrital-carbonate succession at three major localities near Caborca and Hermosillo. Previous reconstructions have proposed differing depositional environments for the Rio Asunción Formation (e.g., carbonate platform, carbonate ramp) and remained therefore nonconsensual. The present study has its focus on the Rio Asunción Formation and includes the analyses of 129 thin-sections of carbonate and mixed siliciclastic–carbonate rocks from three localities (Barra los Tanques, Sierra del Álamo, and Sierra Santa Teresa). In Sierra del Álamo, ammonite findings allowed us to confirm the position of the Triassic–Jurassic boundary. Considering the lack of biostratigraphic markers and the clastic nature of the samples, a statistical approach was used to propose a depositional model that can provide more detailed insights into this sedimentary succession. Hierarchical cluster analyses were performed on the gathered abundance data to identify microfacies and to compare the results for each outcrop. Through these analyses, 22 microfacies were defined, which describe the depositional environments of the two main localities. Furthermore, it could be demonstrated through this approach that not only the localities of Barra los Tanques and Sierra del Álamo but also the outcrops near Sierra Santa Teresa belong to the Antimonio depositional system. To gain insights into the relative depositional conditions among the microfacies, non-metric multidimensional scaling was performed. The resulting trends of water energy and proximity to the shoreline of Laurentia were then used to propose a depositional model for the mixed siliciclastic–carbonate Antimonio ramp system.