The Furongian (upper Cambrian) Alum Shale of Scandinavia: revision of zonation

摘要

The zonation of the Furongian Alum Shale in Scandinavia, based on olenid trilobites, is reviewed and revised. The current scheme is rooted in a detailed zonation introduced in the late 1950s with subzones that subsequently have been elevated to zonal rank. Ten of these zones are difficult to recognize in all Alum Shale districts, and a revised zonation is proposed, focused on unambiguous identification throughout Scandinavia. The difficulties in recognizing zones in some districts mostly relate to biofacies differentiation. Representatives ofCtenopygeare, for instance, common in palaeo-offshore areas, whereas pelturines are rare in these settings and vice versa in palaeo-inboard settings. The following modifications of the olenid zonation are proposed: theOlenus scanicusZone is renamed theO. scanicus-O. rotundatusZone; theCtenopyge similisandCtenopyge spectabiliszones are replaced by theSphaerophthalmus modestus-Sphaerophthalmus angustusZone; theCtenopyge tumidaZone is renamed thePeltura acutidens-Ctenopyge tumidaZone; thePeltura scarabaeoidesSubzone is restored (as a zone) and replaces theCtenopyge bisulcataandCtenopyge linnarssonizones. TheParabolina heres megalops(Sub)Zone is reinstated and replaces thePeltura paradoxaZone; theAcerocarina granulataandP. costatazones are combined as theAcerocarina granulata-Peltura costataZone. In addition, the nameProceratopyge nathorsti-Simulolenus alphaZone is suggested for the polymerid zone corresponding to the upper MiaolingianAgnostus pisiformisZone. The proposed changes reduce the number of Furongian Zones to 23, allocated to six superzones. No subzones are recognized, but some zones can potentially be subdivided for improved local correlation. The stratigraphical ranges of all Furongian olenid trilobites and agnostoids described from Scandinavia are summarized.