A TAXONOMIC ASSESSMENT OF A REEF COMMUNITY FROM LOWER JURASSIC STRATA IN THE CENTRAL HIGH ATLAS MOUNTAINS OF MOROCCO

The End-Triassic Mass Extinction was one of the most severe reef collapses of the Phanerozoic. Following this extinction, it took several million years for marine carbonate communities to recover and diversify. Reef recovery is inferred to have begun in the earliest Jurassic and continued through the Middle Jurassic, but the timing and dynamics of recovery are only broadly understood. During the Early Jurassic, the Atlas Rift System opened shallow seaways in the western Tethys, which provided favorable conditions for reef growth. These understudied deposits, now found in the High Atlas Mountains of Morocco, provide key information about the Early Jurassic recovery of scleractinian corals (especially taxa that survived the end-Tr extinction) and tropical reef ecosystems. Here, a micro-olistolith from a lower Pliensbachian reef is described by point counting thin sections to identify taxa and quantify reef components. This community is a snapshot of ecological recovery between two major reef collapses: the end-Tr Mass Extinction and the Pliensbachian/Toarcian extinction.

Based on the fauna present (e.g., corals, green algae, and microbialites), the block was originally situated within an outer ramp reef environment within fair weather wave base; it then broke loose and was transported to a deeper water environment. The block contained twelve coral genera, seven of which were identifiable. Five of those identified span the Triassic/Jurassic boundary, including Phacelophyllia, Coryphyllia, Stylophyllopsis, Archaeosmilia, and Margarosmilia. Additionally, five genera of foraminifera were identified: Glomospira, Glomospirella, Siphovalvulina, Mesoendothyra, Everticyclammina, and Placopsilina. The corals and foraminifera, along with microbialites and several species of algae and microproblematica, form the primary framework of the reef.

During the unfavorable conditions of the end-Tr, the Atlas rift may have provided refuge for reef communities. As recovery progressed, holdover taxa may have played a key role in reestablishing large reef structures. The presence of persistent Triassic coral genera in Early Jurassic Moroccan deposits highlights the need for quantitative documentation of reef communities which grew between the two Early Jurassic extinction events.