The Ordovician‐Silurian boundary successions in östergötland and Västergötland, S. Sweden

Abstract

A detailed lithostratigraphic and biostratigraphic study of the Ordovician‐Silurian boundary section at Råssnäsudden 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 Västergötland and into the subsurface of Gotland. The latter unit, which is poorly represented at Rässnäsudden but well developed in Västergötland, 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 Östergötland is in some respects more reminiscent of coeval faunas of the North American Midcontinent than ofthat of the Loka Formation in Västergötland. 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.

Bergström, S.M. & Bergström, J., 1996: The Ordovician‐Silurian boundary successions in Östergötland and Västergötland, S. Sweden. GFF, Vol. 118 (Pt. 1, March), pp. 25–42. Stockholm. ISSN 1103–5897.

Keywords:

• Ordovician‐Silurian stratigraphy
• Östergötland
• Västergötland
• conodonts
• brachiopods
• Borenshult
• Loka Formation
• Motala Formation