Quantitative evaluation of microplankton palaeobiogeography in the Ordovician-Early Silurian of the northern Trans European Suture Zone: implicationsfor the timing of the Avalonia-Baltica collision

Quantitative analysis of assemblage similarity among chitinozoan and acrita rch associations recovered from various sedimentary sequences across the Tr ans European Suture Zone (TESZ: southern Baltic Sea and northern Germany re gion), permits evaluation of changes in microplankton palaeobiogeography du ring the Ordovician in the study area. The present data confirm strong pala eobiogeographic differences between the lower Ordovician of the Rugen area, and the coeval domains of the East European Platform (EEP), corroborating the idea that the subsurface of Rugen should be considered palaeogeographic ally as the eastern extension of Avalonia. Cluster analysis of chitinozoan assemblages from numerous wells in the Ruge n area, and one well from the southern margin of the EEP indicates that chi tinozoan bioprovincialism reached its maximum during the Llanvirn; during t his period, the Rugen microplankton communities were clearly Gondwanan in c haracter. Calculations using the coefficient of similarity support the conc lusion of a high similarity between Llanvirn acritarch assemblages from the Rugen subsurface and from coeval Perigondwana localities (e.g. Tunisia). S ince the early Caradoc, this Gondwanan affinity of the Rugen microfossils s tarts to lessen, and becomes negligible during the late Caradoc. During lat est Caradoc-early Ashgilll through Llandovery times the chitinozoan assembl ages from either side of the TESZ are undistinguishable. If palaeobiogeogra phical differentiation is primarily related to palaeolatitudinal distance, then the present data support closure of the Tornquist Ocean during late Ca radoc-Ashgill times. The presence of reworked Llanvirn acritarchs of Perigo ndwanan affinity in middle Ashgill sedimentary sequences at the southern ma rgin of the EEP, clearly shows that by this time erosion of an uplifted are a was taking place. Accordingly, the closure of the Tornquist Ocean, and co nsequent Avalonia-Baltica collision must have taken place during the time i nterval between the middle Caradoc and the early middle Ashgill (Rawtheyan) . New chitinozoan data from boreholes H 2 and K 5, offshore of Rugen Island, northern Germany, allow for precise biostratigraphic dating. In the pre-Dev onian part of H 2, Siphonochitina formosa indicates an Abereiddian (Early L lanvirn) age, facilitating correlation with boreholes Binz 1/73 and Lohme 2 /70 of Rugen. In borehole K 5, the occurrence of Belonechitina robusta and Tanuchitina bergstroemi permits attribution of the investigated pre-Carboni ferous sequence to the middle-late Caradoc, and, possibly, early Ashgill. ( C) 2001 Elsevier Science B.V. All rights reserved.