The mid-Ordovician Osmussaar breccia in Estonia linked to the disruption of the L-chondrite parent body in the asteroid belt

Abstract

The Middle Ordovician (466 Ma) Osmussaar breccia, situated along the northwestern coast of Estonia, is rich in angular chromite grains of extraterrestrial origin (>13 grains kg -1 ) and shocked quartz. The angularity of the chromite grains implies that they have not been transported or reworked to any large extent, connoting that the brecciation is the result of a contemporary impactor, either as a direct consequence of the impact or as a result of an earthquake triggered by the impact, and thus is not, as previously suggested, redeposited material from the nearby ~70 m.y. older Neugrund impact structure. The chemical composition of the chromite indicates that the impactor was an ordinary chondrite of L-type, which concurs well with the hypothesis that the infl ux of large bodies to Earth increased during this period due to the breakup of the L-chondrite parent body. This in turn gives support to the recent suggestion that abundant coeval megabreccias worldwide are impact triggered. The presence of extraterrestrial chromite also strengthens the theory that physical pieces of a large celestial body can survive upon impact with Earth. GEOLOGICAL SETTINGThe Osmussaar breccia is an in situ brecciated carbonate section that covers an area of more than 1000 km 2 along the northwestern For permission to copy, contact editing@geosociety.org

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

“…A gradual thinning away from Osmussaar Island, together with the west-east oriented elliptical dispersion (Fig. 1B), suggest that the generating event epicentrum is located not too far from the island (Suuroja et al 2003;Alwmark et al 2010).…”

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

“…platinum group element enrichment, high chondritic ratios, Re-Os and/or Cr isotopic evidence) (French and Koeberl, 2010). Indeed, Suuroja et al (2003) reported about the finding of shock-metamorphic features (PDF) in quartz grains, and study by Alwmark et al (2010) documented extraterrestrial chromite enrichment, both from the infill of large sedimentary dykes on Osmussaar Island. 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).…”

“…Indeed, Suuroja et al (2003) reported about the finding of shock-metamorphic features (PDF) in quartz grains, and study by Alwmark et al (2010) documented extraterrestrial chromite enrichment, both from the infill of large sedimentary dykes on Osmussaar Island. 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.…”

“…The Pakri Formation and locally also the underlying Middle and Lower Ordovician sediments, are heavily brecciated and penetrated by a network of large sedimentary dykes (discussed in details in Section 4.2.1) on Osmussaar Island (Puura and Tuuling, 1988;Suuroja et al, 1999Suuroja et al, , 2003Alwmark et al, 2010). 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).…”

Investigating the duration and termination of the Early Paleozoic Moyero Reversed Polarity Superchron: Middle Ordovician paleomagnetism from Estonia

“…The Middle Ordovician is composed of several sedimentary units described as local stages: Volkov, Kunda, Aseri, Lasnamägi, and Uhaku in ascending order Plado et al, 2010;Smethurst et al, 1998). In northwestern Estonia, the Kunda stage records a meteoritic shower event on the Island of Osmussaar (Alwmark et al, 2010).…”