Acritarch-based chronostratigraphic and radiometric calibration of the Cambrian volcanosedimentary Vallehondo and Playón formations in the Cambrian Ossa-Morena Rift, Spain

Abstract

The detailed record of diagnostic acritarchs from the volcanosedimentary Vallehondo and Playón formations, bracketed between Cambrian Stage 4 strata bearing the trilobite Serrodiscus and Guzhangian (Miaolingian) beds with Sao aff. hirsuta, allows the establishment of a fine acritarch-based chronostratigraphy. Seven interval zones are defined by the first successive appearances of cosmopolitan index acritarchs, two in Cambrian Series 2 (Skiagia ciliosa and Heliosphaeridium notatum zones) and five in the Miaolingian (Eliasum llaniscum, Cristallinium cambriense, Adara alea, Timofeevia lancarae and Cristallinium dubium zones). The base of the E. llaniscum Zone, which approximates the first appearance of Acadoparadoxides cf. mureroensis, marks a bioevent close to the base of the Wuliuan Stage, and includes an episode of felsic volcanism in the upper member of the Vallehondo Formation. The Cristallinium cambriense Zone is underlain by rhyolites and ignimbrites previously dated at ca. 505 Ma. The base of the Adara alea Zone concurs with the last felsic volcanic episodes in the Vallehondo Formation here dated at 500.9 ± 0.9 Ma. The maximum diversification of acritarchs (A. alea–T. lancarae zones) coincides with the onset of basaltic pillow lava flows in the Playón Formation reflecting a major rifting episode. The recognized acritarch-based zonation is applicable to the Acado-Baltic (biogeographic) Province, and reinforces the great value of acritarchs in Miaolingian chronostratigaphy. The new biostratigraphic data confirm the absence of a Furongian record in the study area and the late Drumian–Guzhangian age (A. alea to C. dubium zones) for one of the main syn-rift episodes recorded in the Ossa-Morena Rift.