Architecture séquentielle des dépôts marins à continentaux sur une marge passive (Cénomanien, marge Atlantique marocaine, transversale d'Agadir)

Seven sections, covering the upper Albian to lowermost Turonian, have been correlated from full-marine to continental-dominated deposits across a passive margin, along a transect 425 km long, from the present-day Atlantic coast to the "Pre-African Trough" between the Anti-Atlas and the High-Atlas. The thickness of the Cenomanian succession changes from around 500 metres in the fully marine sections to 250 metres in mostly continental facies in the western High-Atlas, about 150 km updip, to a few tens of metres in the Bou Tazoult area. The strata thicken again eastwards into the Pre-African Trough where they can be traced without major facies changes to the Kem Kem embayment and to the Bechar area in Algeria. Over all this eastern area, continental facies are overlain by the fully-marine shallow-water deposits of the Cenomanian-Turonian boundary interval. A first major conclusion is that fluvial aggradation in high-frequency transgressive-regressive sequences is coeval with the seaward-shift of the shoreline, in accordance with the genetic sequence stratigraphic model of Galloway (1989). Both the flatness of the depositional profile and the corresponding very low energy of the marine environment during the transgressions account for the blanket of red continental clays on top of marine facies in updip depositional sequences, which is then preserved under the marine transgressive surface of the next sequence. A second major conclusion is that the high-frequency transgressive-regressive (T-R) sequences do not look like classical parasequences bounded by transgression surfaces. They usually exhibit a surface created by a sea-level fall within the regressive half-cycle. This is interpreted in the following way: regressions did not operate through a regular seaward-shift of the shoreline, but through stepped sea-level falls. The very low slope of the depositional ramp is thought to have enhanced the sequence stratigraphic record of such stepped regressions. Short-term, high-frequency sequences are organized into medium-frequency T-R sequences (seven in the Cenomanian) which show an overall aggrading and slowly retrograding pattern along the whole transect. Comparisons with other basins show that medium-frequency sequences do not fit the third-order depositional sequences described elsewhere, casting doubts about a eustatic mechanism for their deposition.Sept coupes couvrant l'intervalle Albien supérieur-Turonien inférieur ont été corrélées sur plus de 400 km, des faciès entièrement marins jusqu'aux dépôts presque exclusivement continentaux, depuis la côte atlantique et le sillon pré-africain, entre le Haut-Atlas et l'Anti-Atlas marocains. L'épaisseur des dépôts cénomaniens varie d'environ 500 m dans les séries entièrement marines de la côte actuelle, à 250 m dans les séries principalement continentales du Haut-Atlas, pour s'amincir à quelques dizaines de mètres (Bou Tazoult), sur environ 250 km. La série s'épaissit à nouveau vers l'est dans le sillon pré-africain où elle peut être suivie sans changements notables vers le golfe des Kem-Kem et le secteur de Béchar en Algérie. Sur toute la partie orientale de la transversale, les faciès continentaux ou mixtes sont recouverts par les dolomies marines du passage Cénomanien-Turonien. Une première conclusion majeure est que, dans les séquences transgression-régression (T-R) à haute fréquence, l'aggradation fluviatile accompagne sans hiatus le déplacement de la ligne de rivage au cours du demi-cycle régressif, en accord avec le modèle génétique de Galloway (1989). La platitude extrême du profil de dépôt ainsi que la faible énergie correspondante de l'environnement marin expliquent la préservation de la faible couche de dépôts continentaux rouges de fin de séquence au cours de la transgression suivante. Une seconde observation majeure est que ces séquences T-R à haute fréquence ne sont pas organisées comme les "paraséquences" du modèle de stratigraphie séquentielle "d'Exxon", en principe limitées les unes des autres par des surfaces de transgression. Elles comportent toutes en plus une surface de chute du niveau marin relatif dans le demi-cycle régressif. Ceci est interprété de la façon suivante : les régressions de la ligne de rivage ne sont jamais régulières, elles s'effectuent par l'intermédiaire de chutes étagées qui emboîtent vers l'aval les prismes côtiers successifs. Là encore, la pente extrêmement faible du profil de dépôt explique la distorsion géométrique de l'enregistrement stratigraphique du demi-cycle régressif. Les séquences à haute fréquence sont organisées en séquences T-R à moyenne fréquence dont l'empilement au cours du Cénomanien est globalement aggradant-lentement rétrogradant sur la transversale. La comparaison avec d'autres bassins montre que les séquences à moyenne fréquence ne correspondent pas aux séquences de 3º ordre décrites ailleurs, mettant ainsi en doute un mécanisme eustatique pour leur mise en place

Modélisation numérique des relations de phases des migmatites de l'unité de Filali (Rif interne, Maroc) et son implication géodynamique aux processus orogéniques

International audienceLa compréhension des processus orogéniques nécessite l'acquisition de données permettant de suivre les variations de pression (P), température (T) et déformation (D) au cours du temps (t). La construction de trajets P-T-D-t devrait impliquer une approche pluridisciplinaire, qui combine une analyse pétrologique et structurale à l'acquisition de données géochronologiques. On a appliqué une telle approche sur des migmatites polymétamorphiques de l'unité de Filali dans le massif de Béni Bousera (Rif interne, Maroc). Une attention particulière a été portée sur la corrélation entre les données pétrologiques et géochronologiques afin de discuter la signification des trajets P-T relatifs aux âges obtenus en contexte polymétamorphique. L'étude pétrologique et thermobarométrique des migmatites de l'unité de Filali est marquée par la superposition d'au moins trois événements métamorphiques distincts. Les estimations pression-température ont été déterminées en combinant la thermobarométrie multi-équilibre et le programme Thermocalc. En utilisant les pseudosections P-T-X calculées dans le système MnO-Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3-SiO 2-H 2 O-TiO 2-FeO 3 (MnNCKFMASHTO) avec le logiciel THERMOCALC des migmatites avant et pendant la fusion partielle, nous montrons que les migmatites ont enregistrés : (1) Un évènement de moyenne pression-haute température (MP-HT) à grenat I + disthène + biotite + plagioclase + quartz + ilménite + rutile, établit dans le faciès amphibolite sous des conditions P-T de 750°C, 9.0 kbar ; (2) Un évènement de basse pression-haute température (BP-HT) à grenat II + sillimanite + biotite + feldspath potassique + plagioclase + quartz + ilménite + cordiérite, manifesté dans le faciès granulite; il est à l'origine de la rééquilibration régionale des migmatites dans le domaine de stabilité de la sillimanite à 640-740°C et 4-5 kbar ; (3) Un évènement de basse pression-basse température (BP-BT), à biotite + muscovite + plagioclase + quartz, qui caractérise le champ de stabilité de l'andalousite. Les données géochronologiques ont montré que l'événement tectonométamorphique de MP-HT est d'âge hercynien et il est dû à la collision continentale, alors que les deux événements de BP-HT et BP-BT sont d'âge alpin. Le Miocène moyen (21 Ma) est marqué par la mise en place d'un important complexe ultrabasique des péridotites contemporain d'un épisode de fusion partielle et d'un métamorphisme de BP-HT. À l'échelle du Massif de Béni Bousera l'événement thermique majeur de BP-HT est interprété comme le témoin d'un contexte tectonique d'extension arrière-arc qui est lié à un retrait de la zone de subduction vers le Sud. L'événement tardif de BP-BT dans les conditions du faciès Schistes Vert, est contemporain d'une tectonique extensive. Cette dernière est marquée, au niveau local, par la réactivation de failles normales compatible à une contrainte extensive équatoriale

La série apto-albienne du bassin marginal de Tarfaya (Maroc méridional)

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Sequence stratigraphic architecture of marine to fluvial deposits across a passive margin (Cenomanian, Atlantic margin, Morocco, Agadir transect)

International audienceSeven sections, covering the upper Albian to lowermost Turonian, have been correlated from full-marine to continental-dominated deposits across a passive margin, along a transect 425 km long, from the present-day Atlantic coast to the ``Pre-African Trough'' between the Anti-Atlas and the High-Atlas. The thickness of the Cenomanian succession changes from around 500 metres in the fully marine sections to 250 metres in mostly continental facies in the western High-Atlas, about 150 km updip, to a few tens of metres in the Bou Tazoult area. The strata thicken again eastwards into the Pre-African Trough where they can be traced without major facies changes to the Kem Kem embayment and to the Bechar area in Algeria. Over all this eastern area, continental facies are overlain by the fully-marine shallow-water deposits of the Cenomanian-Turonian boundary interval. A first major conclusion is that fluvial aggradation in high-frequency transgressive-regressive sequences is coeval with the seaward-shift of the shoreline, in accordance with the genetic sequence stratigraphic model of GALLOWAY (1989). Both the flatness of the depositional profile and the corresponding very low energy of the marine environment during the transgressions account for the blanket of red continental clays on top of marine facies in updip depositional sequences, which is then preserved under the marine transgressive surface of the next sequence. A second major conclusion is that the high-frequency transgressive-regressive (T-R) sequences do not look like classical parasequences bounded by transgression surfaces. They usually exhibit a surface created by a sea-level fall within the regressive half-cycle. This is interpreted in the following way: regressions did not operate through a regular seaward-shift of the shoreline, but through stepped sea-level falls. The very low slope of the depositional ramp is thought to have enhanced the sequence stratigraphic record of such stepped regressions. Short-term, high-frequency sequences are organized into medium-frequency T-R sequences (seven in the Cenomanian) which show an overall aggrading and slowly retrograding pattern along the whole transect. Comparisons with other basins show that medium-frequency sequences do not fit the third-order depositional sequences described elsewhere, casting doubts about a eustatic mechanism for their deposition

Lithostratigraphic, Paleoenvironmental Characterization and Correlations of the Albian–Turonian Deposits of the Errachidia–Boudnib–Erfoud Basin (Southeast Morocco)

Ten detailed sections have been logged and studied from the “Infra-Cenomanian’’ to the Cenomanian–Turonian deposits in the Errachidia–Boudnib–Erfoud Basin between Tazzouguerte and Anounizme(SE Morocco). They show variations in their lithology and microfacies that reflect changes in the depositional environment from the base to the top of the sedimentary record. Indeed, depositional setting grades from a fluvial environment marked by sandstone deposits of the Ifezouane Formation to an alluvial plain and coastal lagoon environment comprising an alternation of red clay, gypsum, and green marl beds of the Aoufous Formation and, finally, towards shallow to moderately deep marine environments with the deposition of the carbonates of the Akrabou Formation. Correlations between the studied sections show variations in the thickness of strata throughout the basin. The variations recorded in the so-called “Infra-Cenomanian” series may be related to the structuration of the basement. The Cenomanian–Turonian carbonate platform shows deeper marine and thicker sediments towards the east, while it thins towards the west until its disappearance in Anounizme. This reflects the global Cenomanian–Turonian transgression in the Errachidia–Boudnib–Erfoud Basin from the eastern Tethyan realm towards the west. The regional correlation of the Errachidia–Boudnib–Erfoud Basin with the Ouarzazate and Agadir basins shows an eastward thinning of the Cenomanian–Turonian marine deposits of Agadir. This suggests the presence of a paleorelief at the Anounizme locality. This could be the boundary between the Errachidia–Boudnib–Erfoud Basin, with Tethyan influence, to the east and those of Ouarzazate and Agadir, with Atlantic affinity, to the west

A 60–50 ka African Humid Period modulated by stadial Heinrich events HE6 and HE5a in northwestern Africa

International audienceA Late Pleistocene lacustrine sedimentary succession has recently been discovered and investigated in Northwest Africa (Morocco, Anti-Atlas, Agadir-Tissint Feija). By integrating new radiometric 230Th/U and OSL ages and new clay mineral analyses with previous sedimentological evidence, we significantly refine the origin and the timing of this succession and its significance for African paleoenvironments and paleoclimate. The sedimentary succession reflects the evolution of a paleolake, referred to as paleolake Tissint, that indicates the abrupt establishment of a wet period at ~60 ka and a gradual return to drier conditions at ~50 ka. Establishment of this paleolake coincides with a peak of precession-forced high summer insolation centered around ~60 ka suggesting that orbital parameters triggered humid conditions in northwest Africa during this time interval. Comparing this observation with others from the literature, it appears that continental-scale wet conditions prevailed across North, Central and East Africa at that time and add evidence in support of a 60–50 ka African Humid Period (AHP). Given that the Holocene African Humid Period was associated with “Green Sahara” conditions, wet conditions between 60 and 50 ka likely sustained coeval greening, at least partially, of the Sahara region. In northwest Africa, the 60–50 ka AHP is affected by Heinrich Event HE6 which via adjustment of the Atlantic Meridional Ocean Circulation (AMOC) seems to have delayed the onset of this AHP. In a similar way, the younger Heinrich Event 5a resulted in short-term regional aridification about 55 ka ago that briefly interrupted the generally wetter period. Finally, as the AHP between ~60 ka and ~ 50 ka coincides with a major Human dispersal out of Africa; these particular conditions may have offered a climatic window of opportunity for Homo sapiens to open migration routes across Africa towards the Levant

Lithostratigraphic, Paleoenvironmental Characterization and Correlations of the Albian–Turonian Deposits of the Errachidia–Boudnib–Erfoud Basin (Southeast Morocco)

Ten detailed sections have been logged and studied from the “Infra-Cenomanian’’ to the Cenomanian–Turonian deposits in the Errachidia–Boudnib–Erfoud Basin between Tazzouguerte and Anounizme(SE Morocco). They show variations in their lithology and microfacies that reflect changes in the depositional environment from the base to the top of the sedimentary record. Indeed, depositional setting grades from a fluvial environment marked by sandstone deposits of the Ifezouane Formation to an alluvial plain and coastal lagoon environment comprising an alternation of red clay, gypsum, and green marl beds of the Aoufous Formation and, finally, towards shallow to moderately deep marine environments with the deposition of the carbonates of the Akrabou Formation. Correlations between the studied sections show variations in the thickness of strata throughout the basin. The variations recorded in the so-called “Infra-Cenomanian” series may be related to the structuration of the basement. The Cenomanian–Turonian carbonate platform shows deeper marine and thicker sediments towards the east, while it thins towards the west until its disappearance in Anounizme. This reflects the global Cenomanian–Turonian transgression in the Errachidia–Boudnib–Erfoud Basin from the eastern Tethyan realm towards the west. The regional correlation of the Errachidia–Boudnib–Erfoud Basin with the Ouarzazate and Agadir basins shows an eastward thinning of the Cenomanian–Turonian marine deposits of Agadir. This suggests the presence of a paleorelief at the Anounizme locality. This could be the boundary between the Errachidia–Boudnib–Erfoud Basin, with Tethyan influence, to the east and those of Ouarzazate and Agadir, with Atlantic affinity, to the west