A palynological investigation of the Early-Middle Devonian transition and associated Choteč Event in Northern Spain

Abstract

In Northern Spain there is a near complete sequence of Devonian rocks that accumulated around a relatively isolated land mass, the Armorican Terrane Assemblage, which was separated from Gondwana to the south and Laurussia to the north. The Lower to Middle Devonian transition records a dramatic facies change from carbonate to siliciclastic sedimentation. The Choteč Event, a global anoxic pulse that occurs just above the Lower-Middle Devonian boundary (392.5–393.1 Ma), has been identified within this transition. These strata are included in the Moniello and Naranco fms in Asturias and the equivalent Santa Lucía and Huergas fms in Leon. They are independently age-contrained based on conodont and invertebrate biotas. For the first time we describe palynomorph assemblages recovered from the Lower to Middle Devonian transition in Northern Spain. The assemblages contain terrestrial (spores) and marine (acritarchs, prasinophycean cysts, chitinozoans) elements, thus recording primary productivity on both the land and in the oceans. The older assemblages belong to the annulatus–sextantii Spore Zone and the younger assemblages the douglastownensis–eurypterota Spore Zone, which is concordant with the independent age constraints. Our analysis of the distribution and diversity of marine and terrestrial palynomorphs supports interpretations of the facies change associated with the Choteč Event as being related to a change to increased rainfall and terrestrial runoff. However, this change is associated with only limited taxon turnover in both land plant spores and marine phytoplankton and chitinozoans.