Upper Visean (Mississippian) metazoan-microbial reefs from Guangxi, South China: Insights regarding reef recovery after the end-Devonian extinction

摘要

The Mississippian was an important interval for metazoan reef recovery after the collapse of stromatoporoid-coral reef ecosystems during the Late Devonian mass extinctions. Based on field observation and quantitative studies of biotic composition in thin sections, four different reef types were identified in the upper Visean strata in the Gandongzi section of Guangxi Province, South China. These are coral reef, coral-microbial-micro-problematica-bryozoan reef, coral-microbial-bryozoan reef, and microbial-bryozoan-coral reef. Their positive topographic relief from one metre to ten metres is either directly observed in the section or by correlation to the coeval non-reef facies in the adjacent Longfeng section. These reefs are constructed primarily of different proportion of colonial rugose corals (Diphyphyllum and Siphonodendron), tabulate corals (Multithecopora and Syringopora), cystoporate bryozoans (Fistulipora), and calcimicrobes (e.g., Garwoodia and Renalcis). Reef dwellers are rich and diversified including crinoids, brachiopods, bryozoans, gastropods, heterocorals, calcispheres, green algae, foraminifers, and ostracods. Their composition and abundance vary among the different reef types. Facies analysis reveals a variable composition, including framestone and bindstone in reef facies, and rudstone, grainstone, packstone, dolomitic crinoidal packstone and dolostone in the strata intercalated with reefs. The microbial reef facies is characterized by abundant spar-filled cavities, which can be classified as stromatactis and fenestral fabrics. In the non-reef facies, geopetal structures are abundant. Relative sea-level changes controlled the growth and demise of the Gandongzi reefs. They commonly grew during relative sea-level rise and died during relative sea-level fall, as evident from increasing and decreasing abundance of micrite, and decreasing and increasing abundance of cortoids, green algae and sparry calcite in reefs and their overlying strata, respectively. However, the final demise of the Gandongzi reef succession was triggered by a dramatic relative sea-level rise, indicated by increased micrite abundance associated with articulated and convex-down brachiopod shells overlying the uppermost reef. The Gandongzi reefs resemble coeval similar reef types on other palaeo-continents, in terms of reef-builder composition and internal structures. The important role of corals in the framework building of the Gandongzi reefs coincided with the late Visean coral reef proliferation, which is a major phase for re-establishment of metazoan reef ecosystems after the end-Devonian mass extinction.