Late Pliensbachian–Early Toarcian palaeoenvironmental dynamics and the Pliensbachian–Toarcian Event in the Middle Atlas Basin (Morocco)

Abstract

Geochemical and palynofacies analysis of 38 samples from the Middle Atlas Basin (Aït Moussa and Issouka sections, Morocco) allowed to investigate sea-level changes, climate, and tectonics and impact on depositional environments in the westernmost Tethyan Gondwana-margin during the Late Pliensbachian–Early Toarcian and the Pliensbachian–Toarcian Event (Pl–Toa Event).The studied sections from the Middle Atlas Basin have low total organic carbon contents. Overall, kerogen assemblages are dominated by the Phytoclast Group, thus showing a strong terrestrial affinity and some degree of proximity to the source. Deposition occurred dominantly in oxic and proximal environments.Upper Pliensbachian kerogen assemblages from Aït Moussa are dominated by terrestrial particles and agree with the overall regressive character of the sedimentary succession. Uppermost Pliensbachian–lowermost Toarcian kerogen assemblages from Aït Moussa show an increase in marine particles. Considering the overall sedimentological context of the Middle Atlas Basin, these kerogen assemblages are interpreted to reflect local variation in accommodation space (driven by different sedimentation rates or tectonic compartmentalization) or ecological conditions (such as nutrient availability, temperature, turbidity, etc). In the Early Toarcian, above a major regional discontinuity and at the beginning of the Pl–Toa Event, increases in Botryococcus sp., AOM, and terrestrial palynomorphs suggest episodes of coastal erosion associated with transgression, likely driven by a combination of eustatic/tectonic changes and warmer and more humid climates leading to an increase in continental weathering and fluvial runoff. On the other hand, increases in sporomorphs and pollen grains occurring in tetrads and agglomerates indicate regressive episodes associated with increased sedimentation rates driven by enhanced continental weathering and fluvial runoff.This study shows the strong relationship between sea-level and the combined response of litho-, hydro-, and biosphere during the Early Toarcian, with implications to understand organic productivity and organic matter accumulation and preservation in the Tethys Ocean during the Pl–Toa Event.