Conodont dating of the Middle Ordovician breccia cap-rock limestone on Osmussaar Island, northwestern Estonia

Abstract

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, sand-penetrated 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 Mäekalda 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 Lasnamägi 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

Darriwilian (Middle Ordovician) conodont biostratigraphy in NW Estonia

“…The breccia is characterized by specific sedimentary structures, occurrence of chromite grains (Alwmark et al 2010) and at least partly mixed fossil associations (Tinn et al 2010). All these features point to an impact-related event in Kunda time (see also discussion by Alwmark et al 2010 andMellgren et al 2012).…”

“…On Osmussaar this unit has been assigned either to the Aseri Stage (Orviku 1940;Rõõmusoks 1970;Hints 1997) or Lasnamägi Stage (Männil & Rõõmusoks 1984;Mellgren et al 2012).…”

“…In addition, Mellgren et al (2012) published Darriwilian conodonts from Osmussaar Island, NW Estonia, ca 40 km west of the Uuga Cliff. This motivated us to re-study a series of old samples from the Osmussaar Cliff and to extend the succession of Mellgren et al (2012) into the Lasnamägi and Uhaku regional stages.…”

Extensive soft-sediment deformation structures in the early Darriwilian (Middle Ordovician) shallow marine siliciclastic sediments formed on the Baltoscandian carbonate ramp, northwestern Estonia

“…Only in this area the main deformation horizon is overlain by additional, slightly deformed and extremely patchy 0-0.5 m thick sandy limestone bed (discussed further in Section 4.2.2), the Osmussaar Member (Suuroja et al, 1999). Undeformed Middle Ordovician limestones overlie the Pakri Formation throughout its entire distribution area (Suuroja et al, 2003;Mellgren et al, 2012). With the exception of Osmussaar Island, the softsediment deformation horizon is resting on undeformed sediments throughout their distribution area.…”

“…This enabled them to present a possible relationship of the large sedimentary dykes to a yet unknown meteorite impact event (Suuroja et al, 2003;Alwmark et al, 2010). Detailed conodont dating of the undeformed bed at the top of the Pakri Formation in Osmussaar Island dated the deformation event to the early or middle part of the early Darriwilian Kunda Regional Stage (Mellgren et al, 2012) (Fig. 2).…”