Post-tectonic limitations on Early Devonian (Emsian) reef development in the Gobi-Altai region, Mongolia

Abstract

This study investigates a Lower Devonian (Emsian) carbonate sequence from the Chuluun Formation where it is exposed in the Gobi-Altai region of southern Mongolia. Quantification of abundance patterns across guild, morphotype and general taxonomic levels was based on stratigraphical and thin-section analyses. Comparison with other Emsian carbonate platforms allowed the factors that influenced community development in the wake of a tectonic perturbation to be determined. Our evidence reveals that potential reef-building biotas preserved in the Chuluun Formation experienced rapid colonization of a newly submerged carbonate platform following an episode of tectonic uplift and the development of a coastal alluvial fan. Although critical reef-building organisms were present, colonial corals and stromatoporoids exhibited limited vertical growth and showed no significant lateral expansion of individuals or biotic assemblages. Nor did those taxa experience significant increases in abundance, density, or size. We conclude that incomplete succession and the lack of reef development occurred most likely because of an unsuitable substrate, limited accommodation space and isolation that reduced colonization potential. h Community succession, Emsian, palaeoecology, reef suppression.Adam F. A. Pellegrini [afapellegrini@gmail.com] (Current address:

Using fossil «fingerprints» to circumscribe the paleogeography of two Alaskan terranes along the Uralian seaway in the late silurian

“…The coeval rocks examined in Mongolia formed under environmental conditions broadly similar to those in the AT (i. e., in a spectrum of shallow-to deep-marine, limestone-dominated environments in a tectonically active setting). Yet calci-microbes do not predominate in the reefs or level-bottom communities there, and none of the distinctive microbial, aphrosalpingid, or hydroid taxa exist in the Gobi-Altai region [35,52]. This helps confirm our hypothesis that unusual conditions along the Uralian Seaway-and nowhere else-favored the development of microbial reefs in the Late Silurian.…”

“…Finally, to test the hypothesis that the microbialaphrosalpingid-Fistulella reefs are a Late Silurian «fingerprint» for abnormal Uralian Seaway conditions in the Late Silurian, research was undertaken on coeval limestone deposits that formed in the Gobi-Altaj and Mandalovoo terranes in Mongolia. Those terranes were situated in the Northern Hemisphere within 35° of the paleoequator but beyond the eastern margin of the Uralian Seaway in the Paleozoic [35,52]. The coeval rocks examined in Mongolia formed under environmental conditions broadly similar to those in the AT (i. e., in a spectrum of shallow-to deep-marine, limestone-dominated environments in a tectonically active setting).…”

Corals and other reef-builders

“…There is an extensive literature documenting the development of reefs in the geological record. The paper by Pellegrini et al. (2012) seeks explanations for the converse – why reefs did not develop following submergence of a carbonate platform in the Early Devonian of the Gobi‐Altai region of Mongolia.…”

“…The paper also provides a very informative review of the interactions between microbial euendoliths and growing corals and notes some of their possible implications for the Cretaceous examples.There is an extensive literature documenting the development of reefs in the geological record. The paper by Pellegrini et al (2012) seeks explanations for the converse -why reefs did not develop following submergence of a carbonate platform in the Early Devonian of the Gobi-Altai region of Mongolia. Potential reef-building taxa were present and global sea surface temperatures were favourable but several factors are thought to have prevented reefs from developing.…”