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

Quantitative analysis of assemblage similarity among chitinozoan and acritarch associations recovered from various sedimentary sequences across the Trans European Suture Zone (TESZ: southern Baltic Sea and northern Germany region), permits evaluation of changes in microplankton palaeobiogeography during the Ordovician in the study area. The present data confirm strong palaeobiogeographic 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 palaeogeographically as the eastern extension of Avalonia. Cluster analysis of chitinozoan assemblages from numerous wells in the Rugen area, and one well from the southern margin of the EEP indicates that chitinozoan bioprovincialism reached its maximum during the Llanvirn; during this period, the Rugen microplankton communities were clearly Gondwanan in character. Calculations using the coefficient of similarity support the conclusion of a high similarity between Llanvirn acritarch assemblages from the Rugen subsurface and from coeval Perigondwana localities (e.g. Tunisia). Since the early Caradoc, this Gondwanan affinity of the Rugen microfossils starts to lessen, and becomes negligible during the late Caradoc. During latest Caradoc-early Ashgilll through Llandovery times the chitinozoan assemblages from either side of the TESZ are undistinguishable. If palaeobiogeographical differentiation is primarily related to palaeolatitudinal distance, then the present data support closure of the Tornquist Ocean during late Caradoc-Ashgill times. The presence of reworked Llanvirn acritarchs of Perigondwanan affinity in middle Ashgill sedimentary sequences at the southern margin of the EEP, clearly shows that by this time erosion of an uplifted area was taking place. Accordingly, the closure of the Tornquist Ocean, and consequent Avalonia-Baltica collision must have taken place during the time interval 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-Devonian part of H 2, Siphonochitina formosa indicates an Abereiddian (Early Llanvirn) 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-Carboniferous sequence to the middle-late Caradoc, and, possibly, early Ashgill. (C) 2001 Elsevier Science B.V. All rights reserved.