ABSTRACT
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 Rügen area, and the coeval domains of the East European Platform (EEP), corroborating the idea that the subsurface of Rügen should be considered palaeogeographically as the eastern extension of Avalonia.Cluster analysis of chitinozoan assemblages from numerous wells in the Rügen 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 Rügen 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 Rügen subsurface and from coeval Perigondwana localities (e.g. Tunisia). Since the early Caradoc, this Gondwanan affinity of the Rügen microfossils starts to lessen, and becomes negligible during the late Caradoc. During latest Caradoc-early Ashgill 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 Rügen 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 Rügen. 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.
ABSTRACT
The island of Rügen (NE Germany), situated close to the Trans-European Suture Zone (TESZ), in the southern Baltic Sea is underlain by sedimentary rocks of an Early Palaeozoic age, known only from boreholes. The wells, Rügen 5/66 and Binz 1/73, were investigated for their chitinozoan assemblages to improve the earlier biostratigraphic dating (graptolites and acritarchs) and to facilitate comparisons with other chitinozoan assemblages on both sides of the TESZ. In the lower part of the Rügen 5/66 core (3794.7-3615.8m), Lagenochitina destombesi Elaouad-Debbaj is indicative of an early late Tremadoc age. In the upper part of the same well (3287.3-1709.7m), the observed chitinozoan taxa suggest an age spanning the early Llanvirn to the Caradoc. The entire sampled interval of the Binz 1/73 core (5217.6-5041.8m) is interpreted as belonging to the Siphonochitina formosa Biozone (early-early late Abereiddian, corresponding to the early Llanvirn). The chitinozoan data corroborate the earlier suggested biostratigraphic ages, based on acritarchs and graptolites. The chitinozoans from the Binz 1/73 well point to a high latitude provenance of the investigated host sediments at time of deposition.
