The Moesian Terrane during the Lochkovian-a new palaeogeographic and phytogeographic hypothesis based on miospore assemblages

摘要

The relative position of terranes during the Palaeozoic can be indicated by a combination of palaeomagnetic and floral/faunal evaluation. Palaeomagnetic data are absent from the Moesian Terrane, South-Eastern Europe, and its position is here estimated thanks to palaeophytogeographic considerations based on miospore assemblages. Miospore assemblages have been studied in two boreholes from northern Bulgaria. Preliminary analyses of chitinozoans, acritarchs and phytodebris have already been published. The oldest samples were considered to be of Pridoli age based on chitinozoan data, but here we provide new miospore evidence suggesting a Lochkovian age. This age determination is in accordance with the new interpretations based on previously published acritarch data. It is likely that the previous Pridoli age determination was influenced by Pridoli chitinozoans being reworked into the Lochkovian. The miospores are typical of assemblages only present in a small area of Avalonia and the western part of the Baltica Plate, and belong to the sinuosus-zavallatus (S-Z) Phytogeographic Province, defined here. On the contrary to the miospores, marine palynomorphs have affinity with Gondwanan assemblages. The different palaeogeographic affinities of the different palynomorph groups are not necessarily in contradiction, as it has recently been suggested that the Rheic Ocean did not act, around the Silurian-Devonian boundary, as a hermetic barrier for transoceanic chitinozoan and acritarch exchange. Therefore, the results are tentatively interpreted as indicating that the Moesian Terrane, belonging to the S-Z Province, was close to southern Laurussia during the Lower Devonian. This northern position of the Moesian Terrane during the Lower Devonian conflicts with most current palaeogeographic reconstructions, but partially supports the hypothesis of a migration of this terrane from Gondwana to Laurussia during the Palaeozoic. The possible presence of reworked palynomorphs in the Lochkovian may reflect early tectonic events associated with the close proximity of the Moesian Terrane and the Dobrudgea periphery of Palaeo-Europe.