Ytterholmen revisited - implications for the Late Wenlock stratigraphy of Gotland and coeval extinctions

The latest Cyrtograptus lundgreni graptolite Chron (Late Wenlock, middle Silurian) is characterized globally by the Mulde Event faunal extinctions and glacio-eustatic sea-level change. Only in the last decade has it been recognized as one of the most severe extinction events of the Silurian Period. A section spanning the Slite Group-Halla Formation boundary on the islet Ytterholmen off main Gotland (Sweden) is unique in that strata through the extinction interval are preserved within a reef complex. The importance of Ytterholmen is twofold: First, based on conodonts, lithology and stratigraphic boundaries, the stratigraphy of the Slite Group-Halla Formation boundary interval on eastern Gotland is now fully understood. in ascending, order: Slite 'g' (c. 12-20 m), Atrypa "reticularis" Beds (c. 0.5 m), Frojel Formation (c. 0.5 m), and the Halla Formation. The boundary between the Frojel and Halla formations is an unconformity as previously reported across most of the main island. Second, correlation of the Ytterholmen section with the Mulde Event reference profile on western Gotland for the first time enables study of this event in very shallow interior areas of the carbonate platform. Here, the c. 1 m thick extinction interval (including the Atrypa "reticularis" Beds and the Frojel Formation) is characterized by mass-occurrences of an atrypid brachiopod, deposition of fine siliciclastic material and termination of the reef complex. Correlation with the East Baltic and Podolia, Ukraine, suggests that similar reef termination occurred contemporaneously throughout the basin. Small patch-reefs occurring immediately above the unconformity on Ytterholmen are among the earliest Mulde Event post-extinction reefs reported from the Baltic basin