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Abstract

Latest Devonian through mid-Carboniferous (Famennian-Bashkirian) time represents an interval of profound change in the style and extent of carbonate buildups and in the composition of buildup communities. Shallow-water reefs and/or reef mounds were widely distributed during the Famennian, despite the global decline since the Middle Devonian that culminated in the Frasnian-Famennian extinction event(s). Famennian buildups were constructed mainly by calcimicrobes and stromatolites with a few skeleton-dominated (stromatoporoid) examples, but buildups became very scarce by the end of the stage (Strunian), when nonrigid stroma toporoid-coral biostromes were more abundant. The low point in abundance and distribution of shallow-water reefs occurred during the latest Famennian and early Tournaisian. Rare Tournaisian shallow-water reefs were constructed mainly by stromatolites and thrombolites, with subordinate corals, bryozoans, and algae. Reef abundance and the abundance and diversity of skeletal reef fauna and flora increased to peak during the late Visean, when corals, bryozoans, sponges, and calcareous algae were major reef builders along with thrombolites. Abundance of shallow-water buildups decreased through the late Serpukhovian and Bashkirian as reefs and reef mounds were increasingly dominated by calcareous algae and skeletal sponges (e.g., Chaetetes).

Deep-water mounds experienced a radically different history. They were dominated throughout the interval by automicrite-producing microbial communities, sponges, and bryozoans in varying proportion and with varying contributions from synsedimentary cement. Mound abundance peaked during the late Tournaisian.

The spatial and temporal distributions of most buildups were closely constrained by local tectonic settings and eustasy, which, together, controlled local bathymetric profiles and sediment fluxes. Reefs on oceanic crust (i.e., Akiyoshi atoll reef) and on passive continental margins (e.g., north Caspian region) had the most extensive (possibly continuous) histories of development owing to decreased influence of continental sedimentation. In a general sense, however, the abundance of shallow-water buildups and diversity of reefal communities correlate with global climate change. Late Devonian cooling associated with the shift from “greenhouse” to “icehouse” global climate appears to correlate with a global decline in reefs that reached its acme around the Devonian-Carboniferous boundary. Global cooling also may have been responsible for changes in the style and locus of Early Carboniferous carbonate production by favoring Tournaisian ramp settings and associated “heterozoan” Waulsortian bank facies over shallow carbonate shelves and “photozoan” reef builders. Global climate amelioration towards the late Visean was associated with expansion of shallow-reef abundance and reef-community diversity, whereas cooling during the Serpukhovian was associated with decreasing reef abundance and biotic diversity. The paucity of Bashkirian reefs probably reflects both cool temperatures and decreased appropriate habitats resulting from increased continental emergence. Throughout the interval nonskeletal elements (i.e., microbialites, calcimicrobes, synsedimentary cements) were important components of rigid frameworks, allowing reefs to be constructed even when/where skeletal organisms were uncommon.

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