A Multidisciplinary Biostratigraphic Approach to the Middle Devonian of the Cantabrian Mountains: Insights from Conodonts, Invertebrates and Palynomorphs

Devonian stratigraphy of the Cantabrian Zone (a Varican foreland basin located within the Alpine Cantabrian Mountains of Spain) was the subject of extensive research. Recent palynological studies, which focused on Lower and Middle Devonian spores and phytoplankton, identified specific taxa and distributional variations that suggested the isolation of the Ebro paleo-landmass. This landmass was considered an island within the broader Armorican Terrane Assemblage. While marine invertebrate faunas showed certain affinity with Laurussian taxa, the phytoplankton and spores exhibited distinct relationships with both Laurussia and Gondwana, ultimately demonstrating a slight bias towards a Gondwanan affinity. During the Middle Devonian, the Cantabrian Zone was characterized by two extensive episodes of carbonate reef development, which were interrupted by siliciclastic sedimentation. Revised biostratigraphic data, which integrates discrete conodont data and marine taxa grouped into informal Faunal Interval (FI) reveal significant diachrony across formational boundaries, a feature that had been previously underestimated. Specifically, the Moniello Formation was replaced by the siliciclastic Naranco Formation within the Polygnathus partitus conodont Zone (FI 17). However, the top of the Moniello Formation extended further into the Polygnathus costatus conodont Zone (FI 18) in the deeper basin sections. Similarly, the lower boundary of the Candás Formation proved to be diachronous; it occurred during the lower Givetian P. timorensis Zone (FI 20) in the deeper areas, but later in the nearshore sections (within the P. rhenanus/P. varcus, FI 21). The top of the Candás Formation generally reached the lower Frasnian Ancyrodella pristina Zone (FI 24), although it ended in the upper Givetian Klapperina disparilis Zone (FI 23) in several distal sections. The updated biostratigraphy refined the understanding of major biotic episodes. The basal Chotĕc episode began in the P. partitus Zone, a period where the diversity and abundance of marine phytoplankton (including chitinozoans, acritarchs and prasynophytes) and terrestrial spores from the svalbardiae-eximius Assemblage Zone increased significantly. Simultaneously, the diversity of nektonic and planktonic groups, such as ammonoids and dacryoconarids, surged within the basin. Conversely, the highly diverse Emsian reefal brachiopods were replaced by less abundant and less diverse associations of large specimens belonging to the OCA (orbignyanus–cultrijugatus–alatiformis) group. The Cabornera bed recorded a critical anoxic event in the P. costatus Zone, interpreted as the maximum transgressive surface at the top of the Moniello Formation. It occurred just below the shaly sedimentation onset coinciding with the disappearance of the OCA brachiopod association and accompanied by a dramatic decline in phytoplankton diversity and a low abundance of spores. Consequently, the siliciclastic sedimentation and poorly oxygenated sea-bottom conditions of the Huergas Formation extended across the basin, where pelagic faunas became prevalent. Finally, the upper Eifelian Kačák episode was not indicated by changes in the phytoplankton or spore diversity in the shallow-water sections, despite a notable benthic turnover of brachiopods and corals (FI 19). A major turnover of ammonoid, brachiopod and conodont faunas was recognised in the basin (Man Member), between the Tortodus kockelianus Zone and the entry of Polygnathus hemiansatus, which marks the base of the Givetian.