The Middle Devonian palynology and biostratigraphy of northern Spain

The Naranco, Huergas and Gustalapiedra formations of the Asturias, León and Palencia provinces of northern Spain represent a nearshore marine clastic sequence deposited during the Mid Devonian. The formations are laterally equivalent, becoming more offshore in character through León and Palencia but representing the same stratigraphic interval. They have received limited palynological study, despite previous reports of their palynological assemblage and their important stratigraphic interval at the onset of early forests. A palynomorph assemblage from these formations is described here, comprising acritarchs, prasinophytes, chitinozoans and dispersed spores, with additional comment on the formations’ lithology. The sampled rock was processed using standard palynological procedures and yielded a well-preserved assemblage, though thermal maturity was variable. A quantitative count of the palynomorph assemblage was undertaken using a Lycopodium spike. The assemblage is an early Givetian one, as evidenced by the presence of Geminospora lemurata, with some evidence of diachronism in the further offshore locations. The formations’ biostratigraphy, in conjunction with lithological evidence, indicates that deposition was rapid. The assemblage presents various unexpected features, with some important taxa absent and others present at an unusual time. The palynomorph assemblage is significantly endemic to Iberia, and this is interpreted as indicating significant palaeogeographic isolation of northern Spain during the Middle Devonian. Terrestrial spores were prevented from dispersing here by a large geographic separation, while marine plankton may have been stopped by ocean currents running past these isolated islands, also indicating an open Rheic Ocean able to support such currents. This deposition also took place just after the Kačák Event, a major extinction event in the marine realm. The results presented here support an existing, monsoonal hypothesis for the origin of the Kačák Event, as this would also create the rapid nearshore deposition observed here, however this raises questions over how the event would be expected to manifest around the world. As an environmental event, the anoxia usually used to mark the event may have been sporadic or localised, not necessarily occurring in all areas, especially the nearshore location studied here. These results lend support to the existing idea of the Kačák Event as a longer, possibly polyphased event extending into the earliest Givetian.