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

The succession in the Tizra Formation shows an excel lent exposure of a small open marine platform where alternating microbial boundstones (buildups) and oolitic/bioclastic grainstone (shoals) and packstone fades 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 fades 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 eggshaped ooids. The occurrence of the predominant types of ooids in shallower-water grainstones with in situ generation, and in the deepest-water microbial fades, 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. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

The succession in the Tizra Formation provides an excellent example of a small open marine platform with alternating microbial boundstones and oolitic/bioclastic grainstone. The study examines the petrographic features, ecological conditions, and controlling factors for the growth and evolution of the platform, as well as the formation of microbial buildups and oolitic shoals. The relationship between microbial mounds and ooids observed in the study is unusual, and the majority of ooids identified are transported rather than generated in situ.