Acritarch biostratigraphy of the Lower-Middle Cambrian boundary in the Iberian Chains, province of Soria, northeastern Spain

Resumen

The Lower and Middle Cambrian age acritarch assemblages are reported from a continuous succession of the siliciclastic-carbonate Ribota, Huermeda and Daroca Formations in the Iberian Chains, northeastern Spain. One new genus and four new species are described, one of them in open nomenclature. The microfossil assemblages allow to recognize the Lower Cambrian Heliosphaeridium dissimilare-Skiagia ciliosa and Volkovia dentifera-­Liepaina plana acritarch zones, and to discuss an informal lowermost Middle Cambrian acritarch zone. These zones correspond respectively to the Holmia kjerulfi, Prorolenus and Acadoparadoxides oelandicus trilobite zones, identified in the Acado-Baltic faunal province. The first appearance of Eliasum llaniscum, Celtiberium dedalinum and Retisphaeridium dichamerum, which are age-diagnostic for the Middle Cambrian, in the upper Daroca Formation is interpreted to point the Lower-Middle Cambrian boundary. This stratigraphic level underlies the previously established trilobite-based boundary. The depositional environments and biostratigraphy of the concerned strata are discussed.