Lower Ordovician conodonts, reworking, and biostratigraphy of the Orreholmen quarry, Vastergotland, south-central Sweden

The quarry at Orreholmen, Billingen-Falbygden District, Province of Vastergotland, in south-central Sweden was sampled for conodonts. The local stratigraphy, which is complicated, has previously been determined on the basis of macrofossils. Upper Cambrian and lower Tremadocian bituminuous shales with stinkstone lenses are overlain by glauconitic deposits grading into ''orthoceratite limestone'' of Arenigian age. The results reported here of conodont investigations from various levels throughout the sequence demonstrate the absence of certain levels in the Lower Tremadoc and the lower Upper Tremadoc, possibly of a part of the basal Arenig and of a level at the Latorp-Lanna Stage boundary (lower middle Arenig). Comparison of data from Orreholmen with observations from sections in other parts of Baltoscandia, information on evolutionary lineages of conodonts and evidence from redeposited conodont elements were combined to produce reconstructed stratigraphic sequences. Particularly important for these results was the analysis of the reworked faunal components, which are unusually common at this locality. For the first time the presence of conodonts from basal Tremadoc beds has been proved in Sweden. The Ordovician part of the sections includes (in terms of conodont biostratigraphy) strata from the Cordylodus lindstromi Zone to near the top of the Paroistodus originalis Zone.