Conodont-based revisions of the late Ludfordian on Gotland, Sweden

The Late Ludlow topmost Hemse-Burgsvik succession includes more substantial and more rapid facies changes than most older and younger intervals on Gotland. A revised conodont zonation for this interval includes three zones, the Polygnathoides siluricus, the Icriodontid, and the Ozarkodina snajdri zones and four subzones, the Upper P. siluricus, the Lower, the Middle and the Upper Icriodonfid subzones. The three zones are approximately coeval with the Neocucullograptus kozlowskii graptolite fauna, the succeeding impoverished graptolite fauna and the Monograptus baltictis/Pseudomonoclimacis latilobus Zone (= the lower M. formosus Zone), respectively. The new zonation permits the first high-resolution correlations across Gotland, despite very large differences in facies from SW to NE. Another result is a more detailed stratigraphic subdivision and revised boundaries of most of the units. The Mill-klint, the main and the Botvide members of the Nar Fm. (new, the upper part of the Hemse Group), the lower, middle, and upper Eke Fm., and the Burgsvik Fm. are distinguished. Well known names are kept, as far as possible, but the lateral extent of their boundaries is revised, resulting in a considerable increase in precision and very different thickness data, e.g. the Ludlow is calculated to be somewhere between 337 and 425 m instead of the 215 m given hitherto. Faults or disturbances with similar effects are identified for the first time on Gotland. Silurognathus maximus is named.