Foraminifères et algues calcaires dans les roches brigantiennes comme guides pour la reconnaissance de l'intervalle de la limite Viséen-Serpukhovien du Maroc.

Revision of foraminifers, algae and problematic algae of well-known late Viséan formations in the Jerada syncline in the Eastern Meseta, Azrou-Khenifra Basin in the Central Meseta and Oued Cherrat in the Western Meseta, suggests that the recognition of the chronostratigraphic units within this period lacks most of the foraminiferal markers in the Western European foraminiferal biozonations. Many taxa recorded in the Moroccan Meseta are considered as late Asbian guides in Western Europe, but in the Meseta, they are first recorded in the early Brigantian. Although some foraminiferal taxa are proposed as guides for the early Brigantian in Morocco, this substage is far more easily recognized by the algal and problematic algal assemblages. In contrast, the late Brigantian, and thus, the equivalent to the lower part of the Serpukhovian, is recognized by typical foraminifers that have been also recorded in Western Europe. Moreover, the algae do not display any significant change at this level. The unusual and unrepresentative foraminiferal assemblages recorded from the Brigantian in Morocco, suggest that most of the numerous outcrops and sections ascribed to the late Asbian in the literature should be revised because, as is demonstrated in a few cases in this study, those outcrops probably belong to younger Brigantian chronostratigraphic units.Une révision des foraminifères, des algues et des algues problématiques dans les formations bien connues du Bassin de Jérada (Méséta orientale), du Bassin d’Azrou-Khenifra (Méséta centrale) et de l’Oued Cherrat (Méséta occidentale), révèle que l’identification des unités chronostratigraphiques durant le Viséen supérieur est rendue difficile par l’absence fréquente des foraminifères-guides dont on dispose dans les biozonations d’Europe occidentale. La plupart des taxons trouvés dans la Méséta marocaine pourraient être considérés comme des guides de l’Asbien supérieur, tels qu’en Europe occidentale, mais dans la Méséta ils ne se rencontrent qu’au cours du Brigantien inférieur. Bien que des foraminifères aient été proposés comme guides du Brigantien inférieur au Maroc, ce sous-étage se caractérise plus aisément par les assemblages d’algues et d’algues problématiques. Au contraire le Brigantien supérieur, et donc la base du Serpukhovien, est facilement reconnaissable grâce à des foraminifères qu’on trouve aussi en Europe occidentale, alors que les algues ne montrent aucun chan-gement significatif durant cette période. Les associations typiques et atypiques de foraminifères qu’on trouve au Brigantien incitent à penser que nombre de coupes et d’affleurements attribués à l’Asbiensupérieur, dans la littérature marocaine, sont à reviser, car, comme cela est prouvé en quelques caspar cette étude, de tels affleurements pourraient appartenir à des unités chronostratigraphiques plusrécentes du Brigantien.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia, Innovación y Universidadespu

Preuves d’extensions flexurales dans l’avant-pays rifain : le bassin du Rharb-Mamora (Nord Maroc)

Le bassin du Rharb représente l’avant-pays de la Cordillère du Rif. La Mamora (nord du Maroc) correspond à la bordure méridionale de ce bassin entre les nappes pré-rifaines alpines au nord et la Meseta hercynienne au sud. La Mamora, largement couverte par des formations actuelles, l’analyse du potentiel des ressources naturelles, leur exploitation et leur gestion cohérente nécessitent une bonne connaissance des diverses formations et de leurs structures. L’analyse et l’interprétation de profils sismiques, de forages hydrogéologiques et pétroliers ont permis de préciser les structures majeures de la Mamora, largement recouvertes par des sédiments néogènes. La structure de cette région est contrôlée par des failles au moins hercyniennes, bien connues dans le substratum paléozoïque. Les deux grandes familles, NE-SW et NW-SE, ont contrôlé l’évolution paléogéographique (répartition des faciès et variations de la puissance des formations). La faille majeure dans cette région est la faille Kénitra – Sidi-Slimane (FK2S) [Zouhri et al., 2001]. Cette faille N110oE, à fort pendage N, est localisée au S du front de la nappe pré-rifaine dont l’amortissement est aveugle. Elle se traduit par un effondrement progressif vers le N. Il pourrait s’agir d’une réplique de la faille Rabat – Tiflet. La Mamora apparaît ainsi comme une charnière entre le Rharb subsident et la Meseta marocaine stable depuis le Mésozoïque

The longest delay: Re-emergence of coral reef ecosystems after the Late Devonian extinctions

Reefs are an excellent tool for tracking marine-ecosystem changes, especially through mass extinction transitions. Although metazoan reefs proliferated during the Phanerozoic, prolonged metazoan reef-recovery intervals often occurred after extinction events. Here, we document and review the reef-recovery interval following the Late Devonian Frasnian-Famennian (Kellwasser) and end-Famennian (Hangenberg) mass extinctions, which eliminated the largest area of metazoan (stromatoporoid-coral) reefs of the Phanerozoic. Previous reports of the late Visean coral bioconstructions from western Palaeotethys Ocean, may mark the first metazoan reef proliferation after the Hangenberg extinction. In this study, abundant coral reefs, coral frameworks and coral biostromes were described in detail for the first time from the late Visean strata on the South China Block (eastern Palaeotethys Ocean). The occurrence of these coral bioconstructions further suggests that the late Visean coral reef recovery may have been a widespread phenomenon. Based on the high-resolution reef database constructed in this study, three sub-intervals of the Mississippian metazoan reef recovery were distinguished, which are (1) metazoan “reef gap” phase (MRG) without metazoan reefs during the Tournaisian; (2) metazoan reef re-establishment phase (MRR) containing a few metazoan reefs from early Visean to early part of the late Visean; and (3) metazoan reef proliferation phase (MRP) with global coral reef flourishment during the middle part of the late Visean (late Asbian to early Brigantian substages). Hence, coral reef ecosystems proliferated and became dominant in marine ecosystems during the late Asbian to early Brigantian, indicating a prolonged metazoan reef recovery of about 12 Ma and 23 Ma until the MRR and MRP, respectively. Coral reef proliferation at this time shows that the Mississippian was not solely a period dominated by microbial reefs. Late Visean coral reef development coincided with increased nektonic and benthic diversity, showing that metazoan reef recovery closely tracked overall marine ecosystem evolution. Even compared with other slow reef-recovery intervals, such as the middle-late Cambrian and Early-Middle Triassic with the intervals until the MRR and MRP of 5 Ma and 2 Ma, and 15 Ma and 9 Ma respectively, the Mississippian metazoan reef recovery was the longest in reef history. Harsh climatic and oceanic conditions were present during the Mississippian, mainly including the widespread marine anoxia during the middle part of Tournaisian and the following recurrent glacial and interglacial climatic episodes with frequent changes in sea level, sedimentary facies and sea-water surface temperature, which may have stymied metazoan reef recovery during this time. During the late Visean, marine communities flourished during a phase of relative warm conditions and high sea level, and coincided with the long-delayed re-emergence of coral reef ecosystems after the Late Devonian extinctions

Brachiopods from the Cisuralian–Guadalupian of Darvaz, Tajikistan and implications for Permian stratigraphic correlations

In this paper, we describe the upper Cisuralian Safetdara and Gundara formations of the Darvaz mountains, North Pamir, which were part ofthe Kunlun Arc, developed along the active Eurasian margin. The Safetdara Formation comprises massive limestones (mainly cyanobacterial,Tubiphytes and Archaeolithoporella boundstones) alternating with well-bedded bioclastic and oncoidal limestones and an interval of recessiveshales. The formation crops out above the Chelamchi Formation consisting of turbiditic siltstones and sandstones with bioclastic silty limestonesyielding massive limestone olistoliths. The Gundara Formation consists of fine sandstones at the base, followed by well-bedded marly bioclastic,oncoidal and microbial limestones, bearing a rich silicified brachiopod fauna in life-position. Two new taxa have been identified in this association:the cemented coralliform Gundaria insolita n. gen. n. sp. and the pedicle attached Hemileurus politus n. sp. The inferred environmental setting isthat of shoal deposits of warm, shallow, high energy, clear marine waters for the Safetdara Formation. The agglutinated microbial reefs to clusterreefs of the Gundara Formation were probably growing in a muddier, quieter and probably slightly deeper setting.The foraminifers of the Brevaxina Zone suggest a Bolorian age for the top of the Chelamchi Formation, the Safetdara Formation and the baseof the Gundara Formation. Kungurian conodonts have been found in the lower part of the Safetdara Formation. The biostratigraphic data from thesedimentary succession of North Pamir, integrated with those already obtained from Southeast Pamir, allow to refine the correlations between theTethyan regional scale and the International Time Scale. In particular, it seems now clear that the Bolorian and the lower part of the Kubergandiancorrelate to the Kungurian

Contrasting reef patterns during the evolution of the carboniferous azrou-khenifra basin (Moroccan Meseta)

Five types of reefs are described from the northern and southern parts of the Azrou-Khenifra Basin generated by the interactions of microbes and coral communities. The type 1 microbial reefs grew in both shallow- and deep-water settings, with a strong control by glacioeustasy. Type 2 microbial reefs developed in more tranquil periods, associated with common intermounds, and where only a single major regressive-transgressive sequence is recognised. Type 3 microbial reefs developed in constant deeper water conditions, generated by higher rates of subsidence in the basin, and creating an overall deepening-upward sequence. Type 4 microbial reefs recognised in the northern part of the basin have no clear counterparts in southern outcrops, but they are likely the capping strata observed in the latter area. Rugose corals allow to define a Type 5 reef, unrelated to microbial facies, and are recorded in oolitic-bioclastic backshoals or quiet inner platform settings. The presence of similar reefs in both the northern and southern parts of the basin demonstrates that conditions were not as different as previously proposed, and a lithostratigraphical, environmental uniformity occurs, which permits the analysis of different subsidence rates and glacioeustastic influence. In the Azrou-Khenifra Basin, the reefs, as well as other regional features, suggest that the basin, overall, evolved from an extensional tectonic regime during the early Brigantian into a complex extensional or compressional regime during the early Serpukhovian, passing into a predominantly compressional phase during the late Serpukhovian in a polyphase tectonic inversion during the onset of the Variscan Orogeny in the region

Palaeozoic Basement of the Pyrenees

International audienceIn the Pyrenees, the Cambrian-Lower Ordovician strata represent a quiescent time span with no remarkable tectonic activity, followed by a late Early-Mid Ordovician episode of uplift and erosion that led to the formation of the Sardic unconformity. Silurian sedimentation was widespread and transgressive followed by a Devonian succession characterized by a complex mosaic of sedi-mentary facies. Carboniferous pre-Variscan sediments (Tournaisian-Viséan cherts and limestones) precede the arrival of the synorogenic siliciclastic supplies of the Culm flysch at the Late Serpukhovian. All this succession was subsequently affected by the Serpukhovian-Bashkirian (Variscan) collision, as a result of which, the Palaeozoic rocks were incorporated into the northeastern branch of the Ibero-Armorican Arc

Size-Frequency Distributions along a Latitudinal Gradient in Middle Permian Fusulinoideans

Geographic gradients in body size within and among living species are commonly used to identify controls on the long-term evolution of organism size. However, the persistence of these gradients over evolutionary time remains largely unknown because ancient biogeographic variation in organism size is poorly documented. Middle Permian fusulinoidean foraminifera are ideal for investigating the temporal persistence of geographic gradients in organism size because they were diverse and abundant along a broad range of paleo-latitudes during this interval (∼275–260 million years ago). In this study, we determined the sizes of Middle Permian fusulinoidean fossils from three different paleo-latitudinal zones in order to examine the relationship between the size of foraminifers and regional environment. We recovered the following results: keriothecal fusulinoideans are substantially larger than nonkeriothecal fusulinoideans; fusulinoideans from the equatorial zone are typically larger than those from the north and south transitional zones; neoschwagerinid specimens within a single species are generally larger in the equatorial zone than those in both transitional zones; and the nonkeriothecal fusulinoideans Staffellidae and Schubertellidae have smaller size in the north transitional zone. Fusulinoidean foraminifers differ from most other marine taxa in exhibiting larger sizes closer to the equator, contrary to Bergmann's rule. Meridional variation in seasonality, water temperature, nutrient availability, and carbonate saturation level are all likely to have favored or enabled larger sizes in equatorial regions. Temporal variation in atmospheric oxygen concentrations have been shown to account for temporal variation in fusulinoidean size during Carboniferous and Permian time, but oxygen availability appears unlikely to explain biogeographic variation in fusulinoidean sizes, because dissolved oxygen concentrations in seawater typically increase away from the equator due to declining seawater temperatures. Consequently, our findings highlight the fact that spatial gradients in organism size are not always controlled by the same factors that govern temporal trends within the same clade