OCCURRENCE AND PALEOENVIRONMENTAL SIGNIFICANCE OF EARLY JURASSIC (PLIENSBACHIAN-TOARCIAN) LITHIOTID-CORAL REEFAL DEPOSITS FROM THE HIGH ATLAS OF MOROCCO

In the Early Jurassic, at a time characterized by environmental perturbations, extinction and ecological turnover, a group of large, reef-building bivalves (“lithiotid-fauna”) proliferated. Lithiotids and associated large bivalves formed mounds and extensive biostromes in proximal, shallow marine environments. Despite their prevalence during the Early Jurassic, they remain poorly understood. Major questions remain regarding their success relative to other organisms in the face of environmental stress, their role as reef builders and how they interacted ecologically with corals, and their role as carbonate producers in carbonate ramp and platform environments. This study focuses on describing the stratigraphic distribution and architecture of lithiotid biostromes, as well as evaluating characteristics of lithiotid facies within carbonate and mixed carbonate/siliciclastic neritic depositional settings.

In Morocco, lithiotids occur in the Pliensbachian and Toarcian stages of the Early Jurassic and have been documented in abundance in the lowermost Toarcian, below the Toarcian Oceanic Anoxic Event. In addition, lithiotids are frequently associated with phaceloid and solitary corals in the upper Pliensbachian and lowermost Toarcian. These occurrences are rarely observed outside of Morocco, highlighting the importance of including lower-latitude (i.e., tropical environments) deposits and sections with more continuous, expanded sedimentary successions (e.g., fewer unconformities) when evaluating the stratigraphic distribution of fossil taxa. There is notable heterogeneity of lithiotid associations (both taxonomically and by growth form) within biostromes, which is not directly connected to changing local environmental conditions. Caution should thus be exercised when attempting to directly infer shallow-marine sub-environment based on the taxonomic composition and growth arrangement of lithiotid associations. Based on sedimentological and stratigraphic facies analyses of the Moroccan sites, lithiotid and coral associations were abundant in sheltered, near-shore lagoons along carbonate ramps and/or platforms during both the late Pliensbachian and earliest Toarcian.