First report of reef formation from the Balkanide type of Triassic: the Tran Formation (Upper Triassic) near the village of Gorna Sekirna, SW Bulgaria

Sphinctozoa skeletons were the main builders in microbial-sponge boundstones and cementstones. Syndepositional lithification of associated automicrite and marine phreatic cementation by radial fibrous and radiaxial calcite favored stabilization and growth of skeleton-cement reefs across a central reef area (close to a platform margin), but without forming a wave-resistant reef framework. Storm-influenced destruction of the buildups and subsequent reworking by waves and currents produced bioclastic-intraclastic grainstones.

Geological and biological reasons for the collapse of reef formation, Paleozoic

Paleozoic reef formation developed cyclically, and its global termination has been caused by the biological reasons — biotic crises and mass extinctions near the borders early and middle Cambrian, Ordovician and Silurian, Frasnian and Famennian, Serpukhovian and Bashkirian, Permian and Triassic. The Early Cambrian reef formation has ended along with disappearance of archaeocyathid. In the later stages reefs were much more difficult ecosystems, and they stopped developing before the full extinction of the reef-building communities. The interruption of reef formations within the separate stages have been connected with the geological and paleogeographic reasons – volcanism, regression, climate aridization etc.