Meeting of Europrobe (TESZ) and PACE projects. Zakopane/Holy Cross Mountains, Poland,
September 16-23, 2000. Abstracts volume: 72-73.
Samuelsson, J.1, Koch, L .2,
Servais, T.3, Verniers, J.1
The microcontinent Avalonia was first defined on the basis of its succession of Cambrian to Silurian faunas, and palaeontology will undoubtedly continue to play an important role in understanding this still enigmatic superterrane. The Rhenohercynian belt to the south and east of the Ardennes has few outcrops ranging from the Rheinish Massif (west-central Germany) through the Harz mountains, and most of the autochthonous parts of this region were probably part of Avalonia. The Ordovician units of the Ebbe Anticline (Rheinish Massif) have a trilobite fauna that can be compared to the Mediterranean faunal province. Thus, the Ebbe Anticline sediments probably belong to Avalonia, and the aim of this study is to constrain the depositional age of the Ordovician sediments, and to yield palaeobiogeographic data and compare these with other areas in Avalonia, Baltica and Laurentia. As a tool, we have used chitinozoans, microfossils (palynomorphs) of uncertain biological affinity, which are powerful biostratigraphic markers in the Ordovician showing marked palaeobiogeographical differentiation in the central European terranes. The total thickness of the Ordovician units in the Ebbe Anticline is estimated to approximately 800 m. The dominating lithologies are black to green shales, with integrated silt-layers. These rocks were named « Herscheider Schiefer », and four Ordovician units were subsequently recognised. These are (with the oldest first) the Plettenberger Bänderschiefer, Unterer Tonschiefer, Grauwackenschiefer and Oberer Tonschiefer. Presence of the graptolite Didymograptus artus in the two oldest units (Plettenberger Bänderschiefer and Unterer Tonschiefer), and Holmograptus lentus in the Plettenberger Bänderschiefer and Nicholsonograptus fasciculatus in the Unterer Tonschiefer demonstrated an early Llanvirn (=Abereiddian) age of these units. The early Llanvirn age is further constrained by acritarch studies. Despite several attempts at dating the two upper units, only a broad Ordovician age could be confirmed, mainly because of the lack of biostratigraphically useful fossils. However, recent findings of possible Pseudoclimacograpus sp. in the Grauwackenschiefer suggest an early Caradoc age of deposition for this unit. The preservation of the chitinozoans ranges from very poor to moderate, with less than one to more than 25 chitinozoans per gram sediment in the productive samples. Preliminary results from the analysis of chitinozoans in the Grauwackenschiefer corroborate the early Caradoc age suggested by the recovered graptolite. From the highest unit, the Oberer Tonschiefer, the few chitinozoans investigated so far are too poorly preserved to be biostratigraphically useful. Taxonomically, the chitinozoans are comparable to peri-Gondwanan/Avalonian faunas elsewhere, confirming the Avalonian affinity of the Ebbe Anticline Ordovician.