Alternating microbial mounds and ooidal shoals as a response to tectonic, eustatic and ecological conditions (late Viséan, Morocco)

Abstract

The succession in the Tizra Formation shows an excellent exposure of a small open marine platform where alternating microbial boundstones (buildups) and oolitic/bioclastic grainstone (shoals) and packstone facies tempestites occur repetitively for a sort interval only 0.55 Myr, an scenario unknown in the geological record. The relatively small extent of the platform allows a detailed study of facies and ecological variations, to determine the controlling factors for the growth and evolution of the platform (tectonics, glacioeustatism, terrigenous input), as well as the particular environmental/ecological conditions for the formation of microbial buildups and oolitic shoals (turbidity, energy, nutrients, chemical variations). Although microbial mounds are well-known during the Palaeozoic, the close relationship with ooids, as observed in the studied succession, is unusual, particularly for the frequent ooids embedded in the microbial facies, an ecological parameter used by previous authors to identified shallower stages in the microbial growths. Petrographic analysis of the ooids, as well as their ecological conditioning, suggest that less than 40% of samples yield ooids generated in situ, whereas there is a predominance of transported ooids. Ooids formed in situ, which include large irregular and elongated ooids, were generated in calmer water than the typical rounded and egg-shaped ooids. The occurrence of the predominant types of ooids in shallower-water grainstones with in situ generation, and in the deepest-water microbial facies, suggest their ease of transport. The higher production of ooids occurs during the shallowing phases of the cycles, whereas they were more easily transported during the deepening phases, whereas in the microbial buildups, no features of in situ ooid generation are found.