Conodont dating of the Middle Ordovician breccia cap-rock limestone on Osmussaar Island, Northwestern Estonia [Kesk-Ordoviitsiumi bret?at katva lubjakivikihi dateerimine konodontide abil Osmussaarel Loode-Eestis]

摘要

Various mechanisms have hitherto been suggested to explain the formation of the Kundan(Middle Ordovician) Osmussaar Breccia in northwestern Estonia. Following the recent discovery of L-chondritic chromite in these peculiar, sandpenetrated strata, it seems plausible that the breccia is impact-related. Herein, the conodont faunas of three thin limestone intervals overlying the breccia at Osmussaar Island have been investigated, with the aim of establishing the age of the event in terms of the Baltoscandian conodont-based biostratigraphical scheme. Based on the presence of Microzarkodina ozarkodella, the limestone directly overlying the breccia is assigned to the M. ozarkodella Subzone of the Lenodus(Eoplacognathus?) pseudoplanus Zone. This is reinforced by means of a faunal shift between this sample interval and the subsequent one, which is directly comparable with a faunal shift in the M?ekalda section, mainland Estonia. The middle, orthoceratite-yielding interval is assigned to the uppermost L.(E.?) pseudoplanus Zone(or, alternatively, the lowermost Eoplacognathus suecicus Zone), whereas the uppermost interval, an oolitic limestone, is referable to the E. foliaceus Subzone, corresponding to the lower part of the Lasnam?gi Stage. These results support a connection between the Osmussaar event and the stratigraphic interval yielding abundant meteorites and/or high levels of L-type chromite in Sweden, as they both can be referred to the lower and/or middle part of the Kunda Stage. The minor difference in age between the first limestones deposited after the brecciation and the meteorite and L-chromite-yielding interval in Baltoscandia can be explained as caused by a period of non-deposition, seen as numerous hiatuses of various extent in the Kunda Stage in northwestern Estonia.