The palynological record across the Ordovician/Silurian boundary in the Cordillera Oriental, Central Andean Basin, northwestern Argentina

Abstract

The Hirnantian “glacial horizon” has been recognized across the Central Andean Basin (Puna, Cordillera Oriental, Sierras Subandinas and Sistema de Santa Bárbara) in northwestern Argentina. It is represented by glaciofluvial to glaciomarine facies which unconformably overly Lower to Middle Ordovician stratigraphic units. In the Caspalá area, in the eastern margin of the Cordillera Oriental, the glacial deposits are assigned to the Caspalá Formation. The Lower Silurian postglacial deposits of the Lipeón Formation sharply truncate the underlying Caspalá Formation. Miospores, chitinozoans and acritarchs have been recorded across the Ordovician/Silurian boundary. Marine palynomorphs dominate the studied section. The Late Ordovician miospore assemblage is fairly diverse. It contains permanent tetrads and dyads, spores physically separated from cryptospore polyads, laevigate and ornamented hilate spores, and trilete spores. The trilete spores Ambitisporites avitus, Aneurospora? sp., Chelinospora cf. prisca and Leiotriletes spp. occur in the Caspalá Formation together with chitinozoans dated as early to late Katian. If land-derived palynomorphs were considered autochthonous, their age would be Hirnantian. The trilete spores of the Caspalá Formation constitute their oldest record in South America, representing the advent of vascular plants in the region. The Lipeón Formation yielded Telychian trilete spores dated by chitinozoans, constituting the earliest evidence of Silurian vascular plants of Argentina. The diversity and abundance of miospores decrease in the Lipeón Formation in accordance with the disappearance of terrestrial ecosystems due to the global transgression after the melting of the Hirnantian glaciers. Acritarchs in both the Caspalá and the Lipeón formations support the chitinozoan dating. Whereas chitinozoans and acritarchs show affinities with Gondwanan and peri-Gondwanan regions, the studied miospores confirm the cosmopolitism of Late Ordovician–earliest Silurian microfloras. The new miospore data, particularly those related to the incoming and evolution of hilate/trilete spores, question previous palaeogeographic and palaeoclimatic interpretations about the origin and adaptive radiation of vascular land plants.