Morphological Diversity of Microbialites and the Significance of Sponge Remains in the Permian Triassic Transition Interval from Hambast Range, Central Iran

Authors

1 Department of Earth Sciences, School of Science, Shiraz University, Iran

2 Department of Earth Sciences, School of Science, Shiraz University

3 Department of Earth Sciences, Shiraz University

Abstract

The end-Permian mass extinction which is known as the largest bio-event in the Phanerozoic had a significant influence on the sedimentary regime. Latest Permian skeletal carbonates in the equatorial shallow seas, immediately after the extinction horizon, are replaced by non-skeletal microbialites. In this study, morphological diversity, associating organisms, and environmental conditions of these microbialites at the Hambast Range (Central Iran) are documented and discussed in this study. No erosional surface is observed in the topmost Permian bed and except for one tabular clotted microbial bed, all the other microbialites are characterized by domical form and digitated/columnar inner structures. The presence of laminations within the columns indicates the presence of mounds of columnar stromatolites and the clotted fabrics are interpreted as a Thrombolite fabric. Microscopic investigation of the micritic materials between the microbial columns/clots confirms the presence of sponge remains (fibers and spicules). Low (morphological) diversity and the dominance of digitated/columnar microbialites in the studied area indicate a more stable depositional condition in a deeper marine setting, compared to highly diverse and thrombolite-dominated microbialites (e.g., S. China microbialites). The development of domical columnar microbialites in such a setting is perhaps due to the competition for light. The association of sponges with the presence of a complex reef ecosystem immediately after the extinction horizon. The restriction of these reefs to the area with microbial buildups signifies the role of microbial activities (e.g., providing oxygen) in the development of reef ecosystems in a global unfavorable condition after the end-Permian extinction.

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