A biostratigraphic and paleoenvironmental approach to Late Ordovician (Sandbian) palynomorphs of Sweden: the oldest evidence of land plants in Baltica

VAJDA, VIVI; RUBINSTEIN, CLAUDIA V.

Salvador

Congreso; XIV International Palynological Congress and X International Organisation of Palaeobotany Conference; 2016

International Federation of Palynological Societies, International Organisation of Palaeobotany, Asociación Latinoamericana de Palaeobotánica y Palinología

A detailed palynological study throughout the LateOrdovician (Sandbian) Kinnekulle K-bentonites in Motala, Östergötland province,south-central Sweden, has been carried out. Twenty one palynological samples drawnfrom the Borenshult-1 drillcore provided an abundant, diverse and well-preservedpalynological assemblage dominated by marine organic-walled phytoplankton. The studiedinterval of 4.9 m is comprised within 40.1 m and 35.2 m of depth. This interval,dominated by mudstones, includes the Kinnekulle K-Bentonite represented by twolayers of which the thickest one reaches up to 1,65m. These ash beds, derivedfrom two volcanic events that occurred in the Iapetus Ocean, southwest ofBaltica are recognized as being the product of one of the largest eruptions ofthe Phanerozoic. Isotopic (U-Pb) dating indicates 453.4±4.2 Ma age for the KinnekulleK- Bentonite, thus providing an independent age control for the palynological assemblageand a precise timing for the inception of land plants in Baltica. The marineorganic-walled microphytoplankton is represented by genera such as Baltisphaeridium, Excultibrachium, Ordovicidium,Orthosphaeridium, Lophosphaeridium, Michrystridium, Multiplicisphaeridium,Buedingiisphaeridium, Gyalorhethium, Nexosarium, Hoegklintia, Aremoricanium, Rhopaliophora, Peteinosphaeridium,Polyancistrodorus, Caelatosphaera, Veryhachium, Dorsennidium,Stellechinatum, Estiastra, Navifusa, Eupoikilofusaand Nanocyclopia, with a markedpredominance of acanthomorphitic acritarchs.Among the most relevant taxa, Ordovicidiumelegantulum, Baltisphaeridium perclarum,Aremoricanium rigaudae, Navifusa ancepsipuncta, Hoegklintia cf. corallina and Nexosariumsp. have been recorded in Upper Ordovician strata from Baltica. The fact thatsuch taxa have also been recorded from Laurentia and Gondwana, renders them potentiallyuseful for biostratigraphic long-distance intercontinental correlations. Theassemblage also contains chitinozoan, scolecodonts and zooclasts. The studiedassemblage is characterized by the occurrence of large-sized species of Baltisphaeridium, Ordovicidium, Orthosphaeridiumand Hoegklintia. Such fact has alsobeen observed in other Late Ordovician marine assemblages from Baltica and,furthermore, in paleoplates such as Avalonia, Laurentia, Gondwana and the Tarimblock. Significantly, sparse terrestrial palynomorphs represented by cryptosporeshave also been recorded in the section. They are represented by low-diversitydyads and tetrads such as Dyadospora murusatenuata,Psedudodyadospora petasus, Tetrahedraletesgrayae and Tetraplanarisporites laevigatus;that is, a similar but slightly more diverse assemblage than the one identifiedfrom Röstånga, southern Sweden in younger (Katian) deposits. So far, this cryptosporeassemblage recorded from Sandbian strata constitutes the oldest record of landplants in Baltica. A palynofacies analysis and the evaluation of fluctuationsin palynomorph diversity and abundance throughout the studied section allow theinterpretation of the relationship between the palynomorph distribution and depositionalpaleoenvironments.