THE GOSAUSEE REEF (AUSTRIA): NEW INSIGHT INTO ECOLOGICAL ZONATION OF UPPER TRIASSIC (RHAETIAN) REEF COMMUNITIES

In the Late Triassic (~235–201.3 Ma) scleractinian corals and hypercalcified sponges built large, diverse reef ecosystems, the most famous of which are the Dachstein reefs. This study presents a depth transect along an Upper Triassic (lower Rhaetian) Dachstein reef from the Gosausee margin of the Dachsteingebirge (Gosau, Austria). The Gosausee microbial-sponge-coral reef is a complete barrier reef with an almost continuous fore reef to lagoon transect preserved (a very rare occurrence for the Triassic), and thus provides a window into depth zonation of Dachstein-type reef facies and biotic succession. The Gosausee reef facies exhibit strong depth control and five classic reef facies or zones are identified: the fore reef, reef front, reef crest, back reef, and lagoon facies. Thin, rare microbial fabrics and a high abundance of fine-grained, mud-rich skeletal wackestones (transported reef debris) characterize the deepest fore reef. As the reef shallows, muddy sediments decrease in abundance and are replaced by microbial fabrics, corals, and cements. Abundant sponges, microbial crusts, and thick, marine cements typify the reef crest, whereas microbialite-coated phaceloid corals are dominant in the back reef facies, which grades into heavily cemented oncoids or microbial-sponge bindstones of the lagoon. Based on their compositional and biotic similarities, the Gosausee reef was likely part of the same barrier reef systems as the source reef for the Gosaukamm reef breccia (one of the classic Norian–Rhaetian Dachstein reefs). The highly resolved reef zones of the Gosausee margin can be used to interpret the depth or reef zone of less well preserved reef fragments, can inform models of community ecology and niche utilization in the Late Triassic, and suggest the need to revisit previous assumptions about reef depth or zone based purely on abundance of corals, sponges, or microbialite fabrics. For example, the mere presence of sponge-dominated versus coral-dominated facies cannot be used to determine depth in these reefs, instead, the abundance of microbialites and cements versus muddy sediments is a much better depth indicator.