Ostrácodos del tránsito Cenomaniense-Turoniense en los Montes Ksour y Amour (Atlas Sahariano): sistemática e implicaciones paleobiogeográficas

The study of ostracods from the Cenomanian-Turonian transition in the Ksour and Amour Mountains (Saharan Atlas, Algeria) has allowed the identification of fossil assemblages characterising this relevant time interval characterised by global environmental changes. The ostracod assemblages consist of fifteen species and seven genera, and are dominated by the Family Cytherellidae (mainly genus Cytherella), and secondarily by the families Paracyprididae (exclusively Paracypris) and Trachyleberididae (mainly Cythereis). Less common are components of families Bairdiidae, Bythocypridae and Macrocyprididae. The studied ostracod assemblages were compared with those assemblages from basins belonging to palaeobiogeographic provinces of North Africa-Middle East (Gondwana Palaeomargin) to search for possible similarities among basins. Thus, the results obtained show the proximity of the ostracod fauna of the Moroccan and Egyptian basins, to which the two basins belonging to the Middle East (Jordan and Oman) are related, the strong similarity between the basins of the Saharan Atlas (Algeria and Tunisia) and finally, the isolation of the ostracod fauna of the Lebanese Basin. This palaeobiogeographical topology shows the probable existence of communication routes during the Cenomanian-Turonian transition or equivalent palaeoenvironmental conditions in different basins.El estudio de los ostrácodos de la transición Cenomaniense-Turoniense (Cretácico superior) en los Montes Ksour y Monte Amour (Atlas Sahariano, Argelia) ha permitido la identificación de asociaciones fósiles típicas de este periodo caracterizado por cambios ambientales a escala global. La asociacion de ostrácodos consiste en 15 especies y 7 géneros, y se encuentra dominada por la familia Cytherellidae (principalmente el género Cytherella), y en menor medida por las familias Paracyprididae (exclusivamente Paracypris) y Trachyleberididae (principalmente Cythereis). Las formas menos comunes corresponden a las familias Bairdiidae, Bythocypridae y Macrocyprididae. Las asociaciones de ostrácodos del Atlas Sahariano fueron comparadas con las asociaciones de cuencas vecinas pertenecientes a la provincia paleobiogeográfica del Norte de África y Oriente Medio (margen septentrional de Gondwana) con el fin de encontrar similitudes entre cuencas. Así, el resultado obtenido muestra una gran similitud entre la fauna de ostrácodos de las cuencas del Atlas Sahariano en Argelia y Túnez. Por otro lado, existe similaridad entre las asociaciones de las cuencas de Marruecos y Egipto, y de ambas a su vez con las cuencas de Oriente Medio (Jordán y Omán). Finalmente, la fauna de la Cuenca Libanesa aparece relativamente aislada. Estas similaridades entre distintas cuencas desde el punto de vista palaeobiogeográfico pueden evidenciar cierta comunicación entre las mismas o condiciones ambientales equivalentes durante el tránsito Cenomaniense-Turoniense

Stratigraphie, sédimentologie et interprétations géodynamiques du Lias - début du Dogger : exemple de sédimentation carbonatée de plate-forme en Oranie (Monts de Sidi el Abed, Hautes-Plaines, Algérie occidentale)

The Oran High Plaines (or Oran Meseta, or High Plateaus) are a large subtabular steppe (alfa) with a mean elevation of 1000 m, dominated by higher mountains to the North as well as to the South. They reach a width comprised between 150 and 200 km. The Sidi el Abed Mountains are the only prominent relief where the Lias-Bajocian beds outcrop. Geophysic data suggest that the Sidi el Abed has been a relatively subsiding WSW-ENE stripe, limited by basement faults and inverted during the Atlas Orogeny. The sedimentary and dynamic evolution from the Lias to the Bajocian (more recent Jurassic beds are eroded) will be shortly summarize in order to indicate the new results which are of importance for the stratigraphie correlations and for the paleogeography (A. Marok with contributions by Y. Alméras and S. Elmi). The lithostratigraphic nomenclature has been adapted from that of the Moroccan High Plateaus (Tendrara area ; R. Médioni 1971). This is the first revision of this frontier region since the G. Lucas classic works (1942, 1952). The first Mesozoïc levels outcrop along faults. They are evaporitic clays (with gypsum and evaporites) associated with two basaltic (dolerites) flows separated by thin (some meters) laminated carbonate (shallow marine). These rocks are classically attributed to the Triassic but their upper part can range into the Hettangian. During the late Sinemurian and the Carixian a carbonate sedimentation developed : inner platform or shallow ramp with brief influences of the open sea (outer platform) indicated by several brachiopodes beds. The lower formation (Koudiat el Beïa Fm ; c. 25 m) contains a middle cherty level which has yielded Rhynchonella moghrabiensis (Dubar) and Zeilleria arethusa (Dubar) of the Upper Sinemurian. Upperward, the Oulad Amor Fm (200 m) is the local representative of the «Faciès à grands lamellibranches» (Lithiotis, Cochlearites, Protodiceras, Opisoma). The succession of four members has been recorded : lower Lithiotis oolitic limestones, cherty limestones with brachiopods collected in three successive levels, oolitic and oncolitic limestones with an other brachiopods level, dolomites and, finally, Protodiceras limestones. These beds have been wrongly attributed to the Toarcian in the past. In fact, the brachiopods can be referred to the lower Pliensbachian : Aulacothyris nov. sp., Hesperithyris termieri (Colo) (so-called «térébratules multiplissées»), Lobothyris subpunctata (Davidson), Parathyridina mediterranea (Canavari) and Gibbirhynchia curviceps (Quenstedt). These fauna are linked to deepening episodes (flooding surfaces). They are overlain by lenticular micritic beds which have yielded very rare domerian ammonites (Protogrammoceras celebratum (Fuc.) ; Amaltheus sp.) indicating a neat deepening of the sea. These micrites are included in the Oulad Amor Formation as in the Moroccan High Plateaus (Beni Yala, G. Dubar in R. Médioni). The most important stratigraphic feature of the High Plains is the presence of limited to sabkha facies of the Toarcian (Jbel Nador Fm ; 50 m). Consequently, the Southern paleogeographic border of the High Plains is situated south of the first folded reliefs of the Ksour Mountains (Dj. Hafid and Antar) where the sabkha facies of the Toarcian occurs. The paleogeographic limits do not coincid with the geographic and tectonic boundaries. The carbonate platform reappears above with the «High Plateaus Dolomites» (= «Dalle des Hauts Plateaux») consisting in more or less massive dolomites with intercalations of channelized oosparites ; exact thickness unknown ; exposed : c. 50 m).La région est d'abord située dans le cadre des Hautes Plaines oranaises. Les particularités structurales sont esquissées d'après les données géophysiques actuellement disponibles. Après les dépôts argilo-salifères et gypsifères à intercalations basaltiques du Trias, la région de Sidi el Abed (Algérie nord occidentale) constitue une portion de la plate-forme carbonatée initiale de la marge méridionale de la Téthys occidentale (ou Téthys maghrébine). Les principales étapes de l'évolution sont ici brièvement résumées. - Au Sinémurien supérieur - Carixien, une sédimentation de plate-forme interne subissant des influences externes correspond aux formations de Koudiat el Beïa et des Oulad Amor, la première est datée du Sinémurien supérieur par la faune à Rhynchonella moghrabiensis (Dubar) et Zeilleria arethusa (Dubar), la seconde est l'expression locale des faciès à bivalves à test épais (Lithiotis, Cochlearites, Protodiceras et Opisoma) autrefois rapportés au Toarcien dans le Sidi el Abed ; leur âge est maintenant bien établi, il est compris entre la fin du Sinémurien et les niveaux à ammonites du Domérien qui les surmontent à Teniet Sassi et à Chebiket en Nmer. La formation des Oulad Amor contient plusieurs niveaux riches en brachiopodes renfermant, entre autres, Aulacothyris nov. sp., Hesperithyris termieri (Colo), Gibbirynchia curviceps (Quenstedt), Lobothyris subpunctata (Davidson) et Parathyridina mediterranea (Canavari). - Au Domérien, des lambeaux discontinus à ammonites témoignent d'un approfondissement de la plate-forme carbonatée. La découverte de Protogrammoceras gr. celebratum (Fuc.) (Chebiket en Nmer) et d'Amaltheus sp. (dans un galet remanié dans les conglomérats éocènes de la Koudiat el Abada) permet de dater les lambeaux et de confirmer que les calcaires à bivalves à test épais situés en dessous, appartiennent bien, pour l'essentiel, au Carixien. - Pendant le Toarcien, une sédimentation terrigène de type sebkha avec évaporites envahit les Hautes Plaines (formation du J. Nador) ; elle traduit une tendance régressive qui est propre à tout le domaine ; elle subit occasionnellement une influence «tidale» ou saumâtre. Les lentilles oolithiques témoignent de l'existence d'une barrière hydrodynamique. - Pendant l'Aalénien-Bajocien s'installe une sédimentation carbonatée de type plate-forme interne ou de rampe carbonatée («Dalle des Hauts-Plateaux»). - Cette évolution s'inscrit dans le domaine stable des Hautes-Plaines oranaises où la sédimentation est guidée essentiellement par une subsidence différentielle à laquelle se combine les variations du niveau relatif de la mer.Marok Abbas. Stratigraphie, sédimentologie et interprétations géodynamiques du Lias - début du Dogger : exemple de sédimentation carbonatée de plate-forme en Oranie (Monts de Sidi el Abed, Hautes-Plaines, Algérie occidentale). In: Documents des Laboratoires de Géologie, Lyon, n°141, 1996. Stratigraphie, sédimentologie et interprétations géodynamiques du Lias - début du Dogger : exemple de sédimentation carbonatée de plate-forme en Oranie (Monts de Sidi el Abed, Hautes-Plaines, Algérie occidentale) pp. 3-199

Evolution of the foraminifera communities during the Toarcian crisis: illustration by the data from the Ksour Mountains (western Saharan Atlas, Algeria)

International audienceThe micropalaeontologic analysis of the Lower Toarcian from the Ksour Mountains (western Saharan Atlas, Algeria) allows us to recognize several assemblages of benthic foraminifera and to obtain information about the deposit environment. These assemblages dominated by Nodosariids are organized in four successive stages of settlement (normal, survival, extinction, and repopulation). This distribution gives new information on the environmental conditions and on their effect on the change in the vertical distribution of benthic foraminifera. The quantitative evolution documents a progressive deepening of the environment that is controlled by local tectonic and sedimentary dynamics. The result is an ecosequence depending both on the general sea-level rise and on a progressive confinement of the bottom water. The normal stage occurred at the beginning of the Polymorphum zone; uncoiled and small sized Nodosariids species indicate the opening of the Atlasic furrow. After, during the middle of this zone, these species were associated with Polymorphinids and Ceratobuliminids, which indicate a deep but isolated and confined environment (survival stage). The ecosequence ended with an episode without foraminifera; it is the extinction stage dated in the Polymorphum and Early Levisoni zones. This stage corresponds to an umbilicus setting. Such palaeophysiographic conditions exaggerated the general coeval hypoxy, responsible of the main Toarcian crisis. Then, from the late Levisoni zone to the end of the Middle Toarcian, the ecosequence evolution is inverted, as a consequence of the filling of the deep parts of the umbilicus, which were less partitioned and more oxygenated (repopulation stage)

The Toarcian Oceanic Anoxic Event and the Jenkyns Event (IGCP-655 final report)

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Tectonostratigraphic evolution of the eastern Algerian margin and basin from seismic data and onshore-offshore correlation

International audienceThe eastern Algerian basin, part of theWestern Mediterranean domain, is a back-arc basin resulting fromthe SE drift of the Lesser Kabylia block gave rise to continental extension and birth of a new oceanicdomain. We aim here to define the structure and tectonostratigraphic evolution of this basin and theadjoining continental margin. Indeed, the absence of wells and the lack of published MCS data kept thisarea mostly unknown. The crustal architecture (down to the Moho) and seismo-stratigraphy of thesedimentary infill are detailed, emphasizing particularly the pre-Messinian series, by combining a newset of offshore deep penetrating seismic, conventional MCS sections, and wide angle seismic data.Because no well exist offshore we realized new field descriptions of the Late Oligocene and Miocenedeposits on land and propose onshore-offshore extrapolation and age correlations. The pre-Messinianseries display four main units (PMSU1 to 4) above which Messinian units are consistent with stratigraphicmodels already proposed for the Mediterranean basin, the deep basin displays units as old as theOMK (“Oligo-Miocene Kabyle”) series on land. The basement is shaped by conjugate transcurrent faultsstriking N120- N40, forming a series of rhombic troughs resulting from transtensional tectonics. Thecoeval E-W extension and NW-SE stretching of the continental crust suggest a radial opening of theAlgerian basin during Late Oligocene- Lower Miocene, followed by a progressive uplift of the uppermargin after the collision of Lesser Kabylia with Africa. Besides, steep slope of the eastern Algerianmargin, downward flexure of the oceanic basin near the margin toe and growth of S-dipping thrustswithin the margin evidence a significant Quaternary stress inversion. Overall, this study highlights thecomplex stress evolution of the East Algerian margin since Oligocene times, as evidenced by significantupward/downward vertical motions and crustal extensional/compressional successions. We summarizethis evolution into three main stages: (1) rifting (including sea-floor spreading) at Late Oligocene-Burdigalian in the context of back-arc, (2) collision between AlKaPeCa blocks and Africa at 17 Ma, andpost-rift sag basin infill, till Late Tortonian to Messinian, and (3) inversion of the margin and developmentof flexural basins at Quaternary times