The Succession and Correlation of Ostracode Faunas in the Silurian of Gotland

Abstract

Abstract Palaeocope ostracode faunas in the Siluro-Devonian sequence of Gotland and the submarine Hoburg Bank area enable detailed correlations with ostracode faunas in other areas around the Baltic to be made, and at several levels the faunas extend into areas where graptolite datings are possible. The late Leintwardinian–early Downtonian interval can be correlated with the British sequence, and the Whitcliffian–late pre-Gedinnian interval with sequences in Nova Scotia and Maine. The Silurian of Gotland starts with an almost entirely submarine Llandoverian sequence of unknown completeness, ranging from low Llandoverian Beds with Stricklandia lens into the zone of Oktavites spiralis, partly above sea-level. The supramarine sequence extends some distance into the Whiteliffian, and the younger submarine sequence in the Central Baltic comprises Whiteliffian and Downtonian rocks. The youngest fauna treated is found in the Beyrichienkalk, with equivalents in the Kaugatuma-Ohesaare Beds of Ösel, in cores from the South Baltic area, and in the upper part of the type section of the Stonehouse Formation of Nova Scotia, not ranging above the bouceki-transgrediens interval in the graptolitic sequence or the eosteinhornensis zonc in the conodont sequence. The Beyrichienkalk represents the youngest pre-Gedinnian ostracode fauna known. The Siluro-Devonian series boundaries in the Gotland area are drawn as follows: (1) Llandoverian-Wenlockian tentatively at or very slightly above the base of the Upper Visby Beds. (2) Wenlockian-Ludlovian (between the ludensis and nilssoni zones) in an almost inaccessible interval of the lower Hemse-Klinteberg Beds in the current stratigraphy. (3) Ludlovian-Downtonian at (or most inconsiderably below) the base of the sequence with Frostiella groenvalliana (F. lebiensis) and Londinia kiesowi (L. arisaigensis) on Ösel, in the Central Baltic, Pomerania, Scania, the Welsh Borderland, and Nova Scotia. The general facies trends in the supramarine Silurian of Gotland are along the SW-NE strike of the units as exposed in the present topography, with shaly facies dominating in the SW part of the island and at the sea-floor SW of the Karlsö Shelf and Hoburgen; subsurface tests are necessary to establish the relations between exposure trends and palaeogeographical trends. Surface evidence sufficiently disproves the existence of three major, successive barrier reefs along present strike, with SW-NE striking talus and lagoonal zones as suggested by Jux 1957. The reefs in his Folgen are of considerably different ages and do not display the size and longitudinal arrangement necessary for the barriers as constructed.