Thrombolite reefs with archaeocyaths from the Fouanou syncline, Anti Atlas, Morocco: implications for early Paleozoic bioconstruction

The Cambrian microbial-dominated reefs, associated with archeocyaths, are considered to exhibit a style of bioconstruction similar to the Late Paleozoic microbial-sponge consortiums (James and Gravestock, 1990; Wood et al., 1993; Zhuravlev, 1996).Microbial reefs from the Early Cambrian containing archeocyaths have been reported globally (Debrenne et al., 1989; Gandin & Debrenne, 2010). In the Anti-Atlas, stromatolite-dominated microbial reefs remained relatively stable until the Atdabanian (Lower Cambrian), after which they were replaced by thrombolitic reefs with archeocyaths that became widespread during this period (Álvaro & Debrenne, 2010). In the Fouanou syncline of the Western Anti-Atlas, thrombolite reefs with archeocyaths are more common in the subtidal limestones of the Igoudine Formation (the basal formation of the Tata Group), characterized by successive phases of reef growth, and separated by growth interruption surfaces (Azizi et al., 2022). These calcareous microbial thrombolites are tabular to dome-shaped, with dark micritic mesoclots of various sizes and shapes, with a maximum diameter of up to 20 mm, forming upward-growing dendritic structures. They contain numerous calcimicrobes, including Renalcis, aggregates of Epiphyton, and, to a lesser extent, tubes of Girvanella (Zhang et al., 2015). These calcimicrobes are associated with archeocyaths of irregular (more abundant) and regular (less abundant), small, dispersed inside and around these thrombolite reefs. Three genera of irregular archeocyaths, preserved in their growth position, have been identified: Dictyocyathus, Erismacoscinus, and Agastrocyathus.The Cambrian reefs of the Western Anti-Atlas provide fascinating examples of early bioconstructions that illustrate the evolution of reef ecosystems and the interactions between microbial organisms and metazoans, showing significant morphological diversity influenced by environmental factors such as depth, hydrodynamics, as well as sedimentation and microbial influence (Gandin and Debrenne, 2010).Keywords: Cambrian reef, thrombolites, archaeocyaths, microbial consortia, paleoenvironment, Western Anti-Atlas, Morocoo.