The data presented here provide the first high resolution investigation of carbon isotope and geochemical analyses derived from the Pliensbachian-Toarcian boundary, of Issouka, Middle Atlas, Morocco. The isotope data recorded in micrite reveal a stepwise negative carbon isotope excursion with values dropping to-1.8 ‰ within the Polymorphum Zone. This excursion coincides with major marine biological changes and extinctions and corresponds with European records supporting the assertion that the excursion is global in origin. The Issouka section is relatively expanded compared to other well-studied European sections. The excursion at the Pliensbachian-Toarcian boundary also shows several similarities with the negative Early Toarcian event. In contrast, carbon isotope values derived from coeval belemnites show positive values. The belemnite δ 13 C data presented here suggests spatial heterogeneity in the Early Jurassic ocean. Overturning or upwelling of a stratified water mass, is inconsistent with our data, as it requires the belemnites to have lived elsewhere and only later migrated into the Middle Atlas area where they became fossilized. The oxygen isotope values from belemnite calcite show no distinct trend across the event, indicative of either no significant change in temperatures or change in seawater δ 18 O. We suggest the introduction of any light carbon (e.g. a volcanogenic) source must have resulted in spatial variability in the δ 13 C of the dissolved inorganic carbon of seawater. Alternatively, a regional change in the source of the carbonate carrying the isotope signal, could lead to a negative shift in the δ 13 Cmicrite signature without any relation to variations in the global carbon isotope trend.

The Late Pliensbachian-Early Toarcian is a pivotal time in the Mesozoic era, marked by pronounced carbon-isotope excursions, biotic crises and major climatic and oceanographic changes. Here we present new high-resolution carbon-isotope and magnetic-susceptibility measurements from an expanded hemipelagic Late Pliensbachian-Early Toarcian section from the Middle Atlas Basin (Morocco). Our new astronomical calibration allows the construction of an orbital time scale based on the 100-kyr eccentricity cycle. The Early Toarcian Polymorphum Zone contains 10 to 10.5 repetitions of the 100-kyr eccentricity both in the carbon-isotope and the magnetic-susceptibility data, leading to an average duration of 1.00 ± 0.08 myr. We also show that the Late Pliensbachian-Early Toarcian global carbon-cycle perturbation has an average duration of 0.24 ± 0.02 myr. These durations are comparable to previous astrochronological time scales provided for this time interval in the most complete 2 sections of the Tethyan area, and longer than what has been provided in condensed sections. Anchoring this framework on published radiometric ages and astrochronological time scales, we estimate that the carbon-cycle perturbation of the Late Pliensbachian-Early Toarcian corresponds with the early phase of the Karoo and Chonke Aike large igneous provinces. Likewise, our new age constraints confirm that the Toarcian oceanic anoxic event is synchronous to the main phase of the Ferrar volcanic activity. Thus, these successive and short phases of the volcanic activity may have been at the origin of the successive phases of the mass extinctions observed in marine biotas in the Pliensbachian and Toarcian times.

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L'étude stratigraphique et sédimentologique des faciès du Dogger moyen du Haut Atlas marocain au Nord d'Errachidia permet d'individualiser une nouvelle formation nommée formation Tazigzaout. Ses équivalents, dans le Haut Atlas central, sont les formations Bin el Ouidane 2 et 3, Ait Abdi 2 et Assoul 1. Une révision des corrélations stratigraphiques (locales et régionales) en relation avec l'évolution géodynamique du bassin s'avérait nécessaire. Dans le domaine distal septentrional de notre zone d'étude, la formation Tazigzaout attribuée au Bajocien-Bathonien pp., est bien individualisée et empli le synclinal de Tazigzaout formant l'axe du grand synclinal perché du jebel Boukendil formé par les termes sommitaux de la formation Agoudim sous-jacente. Chacune de ces deux formations montre une évolution de type "shallowing-upward" et apparaît coiffée par des barres carbonatées à monticules bioconstruits. Leurs équivalents latéraux en domaine de plate-forme sud sont très délicats à déterminer. Toutefois, sur la base de données sédimentologiques et paléontologiques nous avons pu distinguer ces deux formations et établir de proche en proche des corrélations entre les faciès qui les composent.