CHANGES IN COMMUNITY MEMBERSHIP AMONGST PATCH-REEFS OF THE NORIAN-RHAETIAN DACHSTEIN PLATFORM

Reef frequency and biotic diversity increased during the Late Triassic with the development of extended attached or isolated carbonate platforms (Bernecker 2005). The Northern Calcareous Alps are known for their large-scale carbonate platform deposits that bordered the northwestern Tethys margin during the Late Triassic (Flügel 1981). The exceptional diversity and abundance of reefs found along these platforms has made this region a classic site to study reef paleoecology. This study targets patch-reefs located at Gosaukamm and Feisterscharte which belong to the Dachstein platform of the Northern Calcareous Alps within Austria. Reef and associated limestones found at Feisterscharte represent the earliest onset of reef growth along the Dachstein platform during the early Norian, while similar material found at Gosuakamm captures the remaining growth stages of the Dachstein platform until its termination in the Rhaetian (Krystyn et al. 2009). The aim of this study is to quantitatively compare the spatio-temporal difference in taxonomic composition amongst patch-reefs throughout the development of the Dachstein platform. Additionally, a semi-quantitative microfacies analysis was completed to identify differences in depositional environments between patch-reefs. Three quadrats (10x10m) were constructed at Feisterscharte and five were constructed at Gosuakamm. Samples were collected every two meters within each quadrat resulting in a total of 200 samples, which were made into 7x7cm thin-sections and photographed. Taxonomic composition was determined using Adobe Illustrator to complete an equal area quadrat count on each thin-section. Abundances of corals, sponges, bryozoans, microproblematica, algae, foraminifera, echinoderms, and shelly fauna were recorded. In order to determine the spatio-temporal differences in taxonomic composition, Bray-Curtis dissimilarities were analyzed using ANOSIM (analysis of similarities). Cluster and factor analyses were used to evaluate spatio-temporal differences in microfacies. Norian-Rhaetian reefs of the Dachstein platform represent a time of optimal reef growth and the dataset generated from this study allows us to establish a baseline of reef functionality that can be compared with reefs deposited during times of environmental stress.