Lithostratigraphic, paleoenvironmental and sequential evolution of the Cenomanian-Lower Turonian in the Guir area (western Algeria)

La lithostratigraphie, les paléoenvironnements et la stratigraphie séquentielle des terrains crétacés affleurant au pied méridional de l'Atlas saharien (Sud-Ouest de l'Algérie), sont discutés dans le présent article. Les études s'appuient sur l'analyse de quinze coupes réparties le long des limites méridionale et septentrionale du bassin du Guir, entre Ben-Zireg, à l'Est et l'axe Meridja-Boukais à l'Ouest. Dans cette région, trois formations ont été identifiées et interprétées sur le plan environnemental et séquentiel : 1- La Formation des "Grès rouges", attribuée au Cénomanien inférieur, est dominée par des dépôts gréseux fins, fluviatiles ou littoraux ; 2- La Formation des "Marnes à gypse inférieures", rapportée au Cénomanien inférieur-moyen, est caractérisée par des assises à caractères littoraux, soumises à l'influence des tempêtes, notamment vers la base ; 3- La Formation des "Calcaires de Sidi Mohamed Ben Bouziane", d'âge Cénomanien supérieur-Turonien inférieur, correspond à une sédimentation carbonatée franchement marine de plate-forme peu profonde très généralisée. Les dépôts de ces entités lithostratigraphiques s'agencent en mégaséquence d'ouverture, composée de méso-séquences "transgression-régression". L'ouverture des milieux de dépôts en direction du Nord est confirmée par l'organisation des unités lithostratigraphiques et par l'affinité paléobiogéographique nettement téthysienne de l'échinofaune, des bivalves et en particulier de l'ammonite Neolobites vibrayeanus.The lithostratigraphy, paleoenvironment and sequence stratigraphy of the Cretaceous succession outcropping at the southern foot of the Saharan Atlas (south-western Algeria) are discussed in this work. Our current study focuses on the analysis of fifteen sections distributed along to the northern and southern limits of the Guir Basin, between the Ben-Zireg to the East and the Meridja-Boukais axis to the West. In this area, three lithostratigraphic formations were identified and interpreted from an environmental and a sequential points of view: 1 - The lower "Grès rouges" Formation attributed to the Lower Cenomanian, is dominated by fluvial or coastal detrital deposits; 2 - The middle "Marnes à gypse inférieures" Formation assigned to the Lower-Middle Cenomanian is characterized by littoral setting with some storm influences, especially towards the base; 3 - The upper "Calcaires de Sidi Mohamed Ben Bouziane" Formation of upper Cenomanian-early Turonian in age, is represents a laterally extensive, shallow carbonate platform environment. The three formations constitute an overall transgressive megasequence built of a number of transgressive-regressive minor sequences. The opening of the deposits towards the North is confirmed by the organization of the lithostratigraphic units and by the Tethyan paleobiogeographic affinity of the echinoids and bivalves, and especially the occurrence of Neolobites vibrayeanus ammonite

L'Eocene continental d'Algerie : l'importance de la tectogenese dans la mise en place des sediments et des processus d'epigenie dans leur transformation

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Évolution lithostratigraphique, paléoenvironnementale et paléogéographique du flysch de Ben-Zireg (Viséen inférieur, Algérie)

La structure anticlinale de Ben-Zireg se situe sur la marge nord-occidentale de la plate-forme saharienne. Sur le flanc nord de cette structure affleure une formation silicoclastique attribuée au Viséen inférieur, dénommée « flysch de Ben-Zireg ». Sur une épaisseur d’environ 400 m, cette formation peut être divisée en quatre grands ensembles : 1) le « pré-flyschoïde » formé d’une alternance de pélites et de bancs de grès à « HCS », dans laquelle s’intercalent des niveaux calcaires bioclastiques. Ce premier ensemble témoigne d’une sédimentation d’« offshore » à « shoreface » sous dynamique de tempêtes ; 2) le « wildflysch » se caractérise par des phénomènes de resédimentation gravitaires. Il correspond à une épaisse couche de pélites vertes, slumpées, comprenant des olistolithes, avec également des intercalations gréseuses, conglomératiques ou à écoulements de débris. Ces dépôts témoignent d’une sédimentation en pied de pente (« slope apron ») et de remplissage des chenaux (« channel-fill deposits ») ; 3) le « flyschoïde » est composé d’une alternance régulière de pélites vertes et de bancs gréseux turbiditiques latéralement très étendus. Un niveau à olistolithes hectométriques (récif eifélien et grès cambro-ordoviciens) s’intercale au milieu de cet ensemble qui a été interprété comme la bordure d’un cône sous-marin profond (« fan fringe » ou « lobe fringe ») ; et 4) le « post-flyschoïde » est à alternances de pélites et de grès, intercalées de quelques niveaux calcaires à entroques et oolithes, témoignant d’un milieu de plate-forme peu profonde. L’évolution géodynamique du bassin de Ben-Zireg, pendant le Viséen inférieur, s’inscrit dans un contexte régional extensif. Elle se caractérise par l’enchainement des stades de structuration suivants : 1) installation d’une plate-forme mixte soumise aux influences des tempêtes (pré-extension) ; 2) individualisation d’une fosse tectonique associé à des dépôts chaotiques et installation d’un système turbiditique riche en sable (syn-extension) ; et 3) au toit de ce dernier stade, la série se poursuit par une sédimentation de plate-forme mixte, puis de plate-forme carbonatée (post-extension).The anticlinal structure of Ben-Zireg is located in the marginal zone of Sahara, near the Algerian-Moroccan border. It represents a key region situated between the Mesetian domain in the North and the Anti-Atlas-Ougarta field in the South. In this area, the sedimentological and geodynamic aspects of the Carboniferous series have been little studied in the previous studies. The present work is mainly field-based and focuses on stratigraphic and facies analysis in order to characterize the depositional environments and palaeogeography of the “flysch de Ben-Zireg” formation assigned to the lower Visean.Based on lithologic and sedimentologic characteristics, we can subdivide the “flysch de Ben-Zireg” formation into four informal units. From the base to the top they are: 1) the “pre-flyschoïde” unit is well developed at the Oued “des trois palmiers” section. It is composed of whitish pelites commonly contains small bivalves and fishes, followed by an alternation of green pelites with sandstones and bioclastic limestones containing transported mollusc shells. The sandstone beds show massive bedding, parallel lamination and small-hummocky cross-stratification; 2) the “wildflysch” unit shows differences in characters and thickness from one locality to another. It is dominated by slumped green pelites including metric to plurimetric olistoliths, debris-flows, and turbidite sandstone and conglomerate intercalations. The age of the exotic elements ranges from Ordovician to Lower Carboniferous (Tournaisian); 3) the “flyschoïde” unit can be traced from east to west of the northern side of the anticlinal structure of Ben-Zireg. It consists mainly of regular alternation between green pelites and light brown turbidite sandstone beds, showing high sand/pelite ratio. Individual sandstone beds are tabular, laterally continuous and often a sharp planar or smooth irregular erosive base. Synsedimentary deformations (slumps), thin pebbly levels, flute-, groove and load-casts occur locally. The main characteristic of the sandstone beds is the occurrence of the Bouma (1962) divisions, with normal graded bedding, parallel lamination, convolute bedding, water-escape structures and current ripples. Trace fossils are rare. Hectometric olistoliths of eifelian reef and cambrian-ordovician sandstones are also intercalated in the middle part of the “flyschoïde” unit; and 4) the “post-flyschoïde” unit represent the uppermost part of the studied succession. It is composed of an alternation of green pelites with sandstone, conglomerate, bioclastic limestone and calcareous sandstone beds.Facies analysis is the main tool for interpreting the environmental setting of the lower Visean deposits outcropping in the Ben-Zireg area. Based on field observations such as lithology, texture, grain size, sedimentary structures, fossil content and boundary features of beds, nine facies types have been recognized, described and interpreted in terms of depositional environment. The vertical and lateral distribution of the facies types F1-9 revealed four distinct facies associations, AFA to AFD, that reflect different environmental settings: The AFA is restricted to the lowermost (“pre-flyschoïde”) and uppermost (“post-flyschoïde”) parts of the lower Visean succession.” It is interpreted as a marine offshore to shoreface, storm-wave influenced deposits; The AFB represents the lower portion of the “wildflysch” unit. It consists of a thick slumped pelites including several debris flows levels and olistoliths from previous formations, conglomeratic channels and turbidite sandstone beds. This disorganized facies association is the result of tectonic instability and mass-transport processes (slumps and debris flows) at the non channelized base of slope apron with sheet turbidite; The AFC represents the middle part of the “wildflysch” unit. It includes channelized conglomerate beds and turbidite sandstones in alternation with green pelites containing olistoliths of different sizes. These sedimentary facies form thinning-upward and fining-upward cycles of some meter-thick, interpreted as mid-fan channel-fill and channel-fringe deposits. The AFD coincides with the deposition of the “flyschoïde” unit. It is characterized by monotonous alternations of sharp-based turbidite sandstones and interbedded pelites. These alternations are organized in metric thinning-upward sequences of a great lateral continuity and high sand/pelite ratio. They can be referred to lobe-fan fringe environments. The marker chaotic level (eifelian and cambrian-ordovician olistoliths) is interpreted as the product of destabilization of the inner foredeep slope sediments, probably in relation to the tectonic thrust, or the decreasing sea-level.The lithostratigraphic correlations with adjacent areas and the recognized facies associations allow us to follow the palaeogeographic evolution of Ben-Zireg area during the lower Visean, which took place in three principal stages: 1) installation of a carbonate-siliciclastic platform influenced by storms actions (“pre-flysch” unit), following a forced regression period attested by the paleosol level and the lack of Strunian and Tournaisian rocks recognized in large outcrops in the adjacent areas; 2) individualization of tectonic trough delimited by normal faults (graben structure), filled by a chaotic sedimentation (wildflysch unit). This phase is controlled by an active tectonic attested on the field by the lateral changes in thickness of the sedimentary bodies, plastic deformations (slumps) and synsedimentary faults, olistoliths of large size and debris flows deposits; and 3) installation of a sand-rich and low-efficiency turbidite system, depending on active subsidence. The high sand/pelite ratio indicates the proximity of origin of detrital material. The “flyschoïde” deposits are capped by mixed platform rocks (“post-flyschoïde” unit) and by reefal and subreefal limestones, related to the tectonic quiescence and the eustatic sea level rise.</p

Deep-sea trace fossils from the Numidian Formation (Upper Oligocene–Lower Miocene) in the Ouarsenis Mountains, northwestern Algeria

Trace fossils and lithofacies have been studied for the first time in the Numidian Formation (Upper Oligocene–Lower Miocene) of the Ouarsenis Mountains in Algeria to interpret their depositional environment. Twenty-two ichnogenera have been recognized in eight lithofacies of three main facies associations in four representative sections. Distribution of the trace fossils is dependent on the facies. Most trace fossils are dominated by post-depositional forms (62%) and occur in fine-grained, thin-bedded sandstones of facies F4. They belong to the Ophiomorpha rudis, Paleodictyon and the Nereites ichnosubfacies of the Nereites ichnofacies. The O. rudis ichnosubfacies is recorded in 1) medium- to very thick-bedded sandstones of the facies association FA1, interbedded with thinner sandstone beds of the facies F2 and F4 in the upper unit of the sections studied, which were deposited in channel fill and levee-overbank environments, and in 2) medium- to thin-bedded sandstones of the facies association FA2 in the lower unit of the Kef Maiz and the Ain Ghanem sections, which were deposited in isolated narrow channels within the mud-dominated part of the depositional system and occasionally fed with turbiditic sand. The Paleodictyon ichnosubfacies occurs in thin- to medium-bedded sandstones (FA2) of the lower units in the Ain Ghanem and Kef Maiz sections and the lowest part of the upper unit of the Kef Maiz section, which were deposited in channel margin or foremost channel-to-levee-overbank areas. The Nereites ichnosubfacies is recorded in thin-bedded sandstones (FA2), which were deposited in crevasse-splays or small lobes on a basin floor invaded occasionally by turbidites against a background of pelagic and hemipelagic sedimentation

New stratigraphic data from the Cretaceous Basin of Guir (Bechar, South-Western Algeria)

Benyoucef Madani, Malti Fatima Zohra, Bensalah Mustapha, Bendella Mohamed. New stratigraphic data from the Cretaceous Basin of Guir (Bechar, South-Western Algeria). In: Documents des Laboratoires de Géologie, Lyon, n°164, 2008. Mid-Mesozoic life and environments. Cognac (France), June 24th-28th 2008. pp. 19-22

A Phororhacoid bird from the Eocene of Africa

International audienceThe bird fossil record is globally scarce in Africa. The early Tertiary evolution of terrestrial birds is virtually unknown in that continent. Here, we report on a femur of a large terrestrial new genus discovered from the early or early middle Eocene (between similar to 52 and 46 Ma) of south-western Algeria. This femur shows all the morphological features of the Phororhacoidea, the so-called Terror Birds. Most of the phororhacoids were indeed large, or even gigantic, flightless predators or scavengers with no close modern analogs. It is likely that this extinct group originated in South America, where they are known from the late Paleocene to the late Pleistocene (similar to 59 to 0.01 Ma). The presence of a phororhacoid bird in Africa cannot be explained by a vicariant mechanism because these birds first appeared in South America well after the onset of the mid-Cretaceous Gondwana break up (similar to 100 million years old). Here, we propose two hypotheses to account for this occurrence, either an early dispersal of small members of this group, which were still able of a limited flight, or a transoceanic migration of flightless birds from South America to Africa during the Paleocene or earliest Eocene. Paleogeographic reconstructions of the South Atlantic Ocean suggest the existence of several islands of considerable size between South America and Africa during the early Tertiary, which could have helped a transatlantic dispersal of phororhacoid

The hyaenodontidans from the Gour Lazib area (?Early Eocene, Algeria): implications concerning the systematics and the origin of the Hyainailourinae and Teratodontinae

International audienceThe Algerian localities of the Gour Lazib area (Early or early Middle Eocene) have yielded an important mammalian fauna. The Hyaenodontida are well represented in this fauna: three species-two are new- are reported. The genus Glibzegdouia, which has been previously described as a possible Carnivora, is now clearly referred to the Hyaenodontida. It appears morphologically close to Masrasector and Dissopsalis. A new genus, Furodon, is described. It appears morphologically close to the oldest Pterodon species. This discovery supports an African origin for the hyainailourine genus Pterodon and related genera (e.g. Hyainailouros, Akhnatenavus). Two very small lower molars are referred to a new genus Parvavorodon, which is also referred to Hyainailourinae. The localities of the Gour Lazib area therefore show important hyaenodontid diversity for the Early or early Middle Eocene. We performed a new phylogenetic analysis to question the relationships between the African, Asian, North American and European hyaenodontidans. Our study supports the endemism and originality of the Asian proviverrines' Indohyaenodon, Paratritemnodon, Kyawdawia and Yarshea; we propose a new subfamily: Indohyaenodontinae. The African proviverrines' (e.g. Masrasector, Anasinopa, Dissopsalis and Glibzegdouia), which are notably characterized by large premolars and the presence of a wide talonid on the molars, are close to the enigmatic African Teratodon. We therefore propose to refer them to Teratodontinae. The Hyainailourinae, which include the new genus Furodon, are characterized by the presence of secant dentition related to a hypercarnivorous diet. They appear phylogenetically close to the African Koholiinae. The genus Metapterodon is referred to Koholiinae based on the phylogenetic analysis. The African origin of the Teratodontinae and Hyainailourinae is supported by Glibzegdouia and Furodon. The origination of several subfamilies in Africa supports the hypothesis of an African origin for the Hyaenodontida. The origination of the Teratodontinae in Africa contradicts the previous hypotheses of Afro-Asian proviverrines'

Talar morphology of azibiids, strepsirhine-related primates from the Eocene of Algeria: Phylogenetic affinities and locomotor adaptation

International audienc