The stratigraphy of the Upper Ordovician carbonate mounds in the subsurface of Gotland

In a revision of the stratigraphic classification of the subsurface Upper Ordovician succession of Gotland, the terms Lower and Upper Östersjö Limestone are rejected and replaced by the Slandrom Limestone and the File Limestone, respectively. Two mound generations are recognized, the Rakveran-Nabalan Liste mounds and the Pirguan Klasen mounds, the latter being by far the most common. Both occur not only in the Gotland subsurface but also in submarine areas off Gotland. Moderately diverse conodont collections retrieved from core samples in and around carbonate mounds provide the first direct biostratigraphic dates of these structures. Samples from the Liste mounds contain species of the Amorphognathus superbus Zone. Besides the zonal index and numerous specimens of Panderodus, there are representatives of Amorphognathus complicatus Rhodes, Belodina confluens Sweet, Decoriconus costulatus (Rexroad), Protopanderodus liripipus Kennedy, Barnes & Uyeno, Pseudooneotodus mitratus (Moskalenko), Dapsilodus sp., Besselodus? sp. and a few other taxa. The Klasen mounds have not yet produced biostratigraphically diagnostic conodonts but comparison with non-mound successions indicate that they belong to the A. ordovicicus Zone. Their biostratigraphic age suggests that the Liste mounds may be slightly younger than the Kullsberg mounds, and the Klasen mounds equivalent to the Boda mounds, in the Siljan district.