Sedimentological and geochemical characterization of the Pliensbachian-Toarcian boundary, of the Northern Margin of Gondwana, Issouka, Middle Atlas, Morocco

The Early Jurassic period was marked by extreme environmental changes, characterized by major marine biological changes and extinctions at the global scale and a pronounced negative carbon isotope shifts recorded in marine... [ view full abstract ]

The Early Jurassic period was marked by extreme environmental changes, characterized by major marine biological changes and extinctions at the global scale and a pronounced negative carbon isotope shifts recorded in marine carbonates. In order to understand these palaeoenvironmental changes in the Pliensbachian– Early Toarcian, the Issouka section, situated in the Middle Atlas rift basin of Morocco, was investigated. In the Middle Atlas, the Pliensbachian-Toarcian boundary is marked by an abrupt change from alternating limestones to monotonous marly thick sections. Petrological analysis leads to the definition of 2 microfacies; wackestone with abundance microfauna and mudstone with abundance quartz, oxide and pyrite. Our high resolution stable isotope data (derived from bulk-rock) reveal a negative carbon isotope excursion with maximal amplitude -2.8‰ within the Polymorphum zone. The excursion is coeval with benthic foraminiferal extinction and a slight increase in temperature, that varies between 13 °C to 16°C. Our temperature interpretation is based on the isotopic analysis of belemnites that show good preservation, evaluated through cathodoluminescence analysis. Some areas were revealed to be Mn-rich and partial replacement by diagenetic calcite was observed particularly along the outermost growth bands and adjacent to the alveolar region. Carbon isotope data from the belemnites also reveal positive carbon isotope values (1.3‰), within the lowermost part of the section, whilst more negative values (down to -0.6‰) are seen across the Pliensbachian-Toarcian boundary. The absence of laminated black-shales and low absolute values of TOC that varies between 0.1% to 0.7% , suggest an absence of bottom water anoxia in this Moroccan deep-water setting. This observation confirms the regional character of this perturbation in the middle Atlas that can be related to the drowning of the Atlas carbonate platform.