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

Abstract

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 of Ctenopyge are, 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: the Olenus scanicus Zone is renamed the O. scanicus–O. rotundatus Zone; the Ctenopyge similis and Ctenopyge spectabilis zones are replaced by the Sphaerophthalmus modestus–Sphaerophthalmus angustus Zone; the Ctenopyge tumida Zone is renamed the Peltura acutidens–Ctenopyge tumida Zone; the Peltura scarabaeoides Subzone is restored (as a zone) and replaces the Ctenopyge bisulcata and Ctenopyge linnarssoni zones. The Parabolina heres megalops (Sub)Zone is reinstated and replaces the Peltura paradoxa Zone; the Acerocarina granulata and P. costata zones are combined as the Acerocarina granulata–Peltura costata Zone. In addition, the name Proceratopyge nathorsti–Simulolenus alpha Zone is suggested for the polymerid zone corresponding to the upper Miaolingian Agnostus pisiformis Zone. 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.

A Cambrian–Ordovician boundary section in the Rafnes–Herøya submarine tunnel, Skien–Langesund District, southern Norway

“…The succeeding Peltura Superzone slightly exceeds as much as 9 m in thickness, even in the absence of the Parabolina megalops Zone (Nielsen et al, 2020, table 1). The current investigation demonstrates a detailed trilobite zonation of the upper Miaolingian and upper Furongian strata in addition to those previously reported by Brøgger (1884) and Nielsen et al (2020). The biostratigraphy and zonation of the Furongian Series described herein follow the revised superzonation and zonation provided by Nielsen et al (2014;2020).…”

“…The overlying Peltura scarabaeoides Zone is quite thick, as reported from many districts in Scandinavia, comprising c. 6 m of the Porsgrunn core according to Nielsen et al (2020). The fragmented sclerites from the Rafnes-Herøya tunnel are poorly preserved by compaction and metamorphism, but pygidia of P. scarabaeoides (Wahlenberg, 1818) (Fig.…”

“…The Furongian of the Porsgrunn core is 14.8 m in thickness (Schovsbo et al, 2018) and recently, based on the gamma-ray (GR) correlation, the absence of the Acerocarina Superzone is also recognised from this core (Nielsen et al, 2020). Further interpretations of the GR log estimate the Olenus Superzone to be 1.6 m thick, the overlying Parabolina Superzone is 2.6 m, and the following Leptoplastus and Protopeltura superzones measure 0.9 m in combination.…”

“…11C, D). The thickness of the zone is interpreted by gamma-ray (GR) correlation to be up to 1.3 m in the Porsgrunn core (Nielsen et al, 2020).…”

On the genus Vanikoropsis Meek, 1876 (Gastropoda, Caenogastropoda) from the Paleocene of Denmark and West Greenland with descriptions of three new species

“…This is relevant as the many subzones defined by Henningsmoen (1957) with minor changes were elevated to zonal rank by Terfelt et al (2008). The Furongian zonation for Scandinavia was further revised by Nielsen et al (2014Nielsen et al ( , 2020…”