The Ordovician-Silurian boundary successions in Ostergotland and Vastergotland, S Sweden

A detailed lithostratigraphic and biostratigraphic study of the Ordovician-Silurian boundary section at Rassnasudden near Motala has resulted in the recognition of two new lithostratigraphic units, the Motala Formation and the Loka Formation. The former is a slightly more than 5 m thick unit of nodular limestone and calcareous mudstone of late Rhuddanian and early Aeronian age. Its typical lithofacies can be traced into lower Silurian successions in Vastergotland and into the subsurface of Gotland. The latter unit, which is poorly represented at Rassnasudden but well developed in Vastergotland, consists of shallow-water limestone, sandstone, siltstone, and mudstone and was formerly known as the Dalmanitina Beds or the Tommarp Formation. The shelly fossil fauna of the Loka Formation in Ostergotland is in some respects more reminiscent of coeval faunas of the North American Midcontinent than of that of the Loka Formation in Vastergotland. In terms of the graptolite succession, the Motala Formation corresponds to an interval from the acinaces Zone to the revolutus Zone and it belongs to the D. kentuckyensis Zone in the conodont zone sequence. Its conodont fauna, the first to be recorded from early-middle Llandoverian strata in Sweden, is of cosmopolitan type and has Little in common with Late Ordovician faunas.