The Middle Cambrian of Bornholm, Denmark: A stratigraphical revision of the lower alum shale and associated anthraconites

Abstract

Abstract The Middle Cambrian lower alum shale and the associated anthraconites are reexamined and revised bio- and lithostratigraphically. New finds of predominantly agnostid trilobites show that the sequence is far more restricted stratigraphically than hitherto supposed. Lithologically the anthraconite beds do not fulfil the definitions of the term anthraconite. Due to long tradition of use the name basal anthraconite is maintained for the lower limestone. The anthraconite overlying the alum shale is referred to as Hyolithes limestone, as it is a bio- and lithostratigraphical equivalent of the Hyolithes limestone in Sweden. The agnostid zonation of Westergård, used in and outside Scandinavia, is reviewed. The absence of the Ptychagnostus lundgreni — P. nathorsti Zone on Bornholm is proved. The validity of the zone of Ptychagnostus lundgreni — P. nathorsti is questioned, not only in Scandinavia but wherever the agnostid zonation of Westergård is used. A correlation with the interval-zones in North America is attempted. The Ptychagnostus atavus Zone is tentatively divided into a lower subzone, Tomagnostus fissus, and an upper with Hypagnostus parvifrons. The zone of Ptychagnostus nathorsti is merged with the zone of P. punctuosus within the Paradoxides paradoxissimus zonal group.