The base of the Floian Stage is defined in the Diabasbrottet Section at Hunneberg Mountain in southwestern Sweden. The GSSP is in the lower Tøyen Shale 2.1m above the top of the Cambrian and is marked by the first appearance of the graptolite Tetragraptus approximatus.
The Diabasbrottet Section is located near the northern end of a large quarry along the northeastern slope of the Hunneberg Mountain at a latitude of 58°21'32.2"N and a longitude of 12°30'08.6"E in the Province of Västergätland in SW Sweden. The GSSP is about 80km NNE from Göteborg and 12km ESE from Vänersborg.
The Ordovician sequence of the Diabasbrottet Section is 12m thick. The Cambrian Alum Shale is disconformably overlain by dark limestone of the Bjørkåsholmen Formation and the Latorp Limestone (both of Tremadocian age). On top of the Latorp Limestone follow 10m of the Tøyen Shale. The Tøyen Shale consists of grey to dark mudstone with thin interbeds of fine-grained dark limestone, especially in the lower part. The GSSP is located just above Interbed E.
The base of the Floian is defined by the first appearance of Tetragraptus approximatus. Tetragraptus phyllograptoides is very abundant at the base of the Floian Stage.
The GSSP is within in Paroistodus proteus conodont zone. This zone is subdivided into 4 subzones. The boundary between the two upper subzones, the Paracodylodus gracilis Subzone and the Oelandodus elongatus-Acodus deltatus deltatusSubzone, is near the base of Interbed E, hence immediately below the GSSP.
The trilobite Megistaspis (P.) planilimbata appears at or just below the GSSP.
Graptolite FAD Tetragraptus approximatus
Bergström, S. M., Löfgren, A., and Maletz, J., 2004. The GSSP of the Second (Upper) Stage of the Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergötland, Southwestern Sweden. Episodes 27/4, p.265 - 272.