Gypsum evaporites in a patch reef of the upper Slite Group in the Silurian (Wenlock) of Gotland, Sweden

Abstract

Calcite pseudomorphs after gypsum are observed in a patch reef in the upper Slite Group (Wenlock) on north-east (NE) Gotland (Sweden). The idiomorphic pseudomorphs occur (a) in silt-sized carbonate sediment in former moldic pores, (b) in the micritic matrix of bioclastic rudstones and (c) in the thrombolitic matrix of the reef. The dominance of stenohaline organisms in the reef indicates normal marine salinities during reef growth. The development of evaporitic conditions and the formation of evaporitic minerals therefore postdate reef growth. The authigenic gypsum crystals are interpreted to be formed by downward percolating hypersaline brines which existed on an extremely shallow carbonate platform during the early Homerian (latest Cyrtograptus lundgreni graptolite Zone) regression.

Keywords::

• gypsum
• evaporites
• Gotland
• Silurian
• reef
• Slite

Acknowledgements

Birgit Leipner-Mata is acknowledged for supporting thin section preparation and lab work. Field work was financed by the “Agency of Gender and Diversity” of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU). We are grateful to Mikael Erlström and Mikael Calner who improved our manuscript with helpful comments. This paper is a contribution to the IGCP 591 project The Early to Middle Palaeozoic Revolution.