The Osmussaar Breccia in Northwestern Estonia — Evidence of a ~475 Ma Earthquake or an Impact?

The Osmussaar Breccia occurs in beds of the ~475 Ma basal Middle Ordovician (Arenig and Llanvirn series) siliclastic-carbonate rocks of northwestern Estonia. The Breccia consists of fragmented and slightly displaced (sandy) limestones, which are penetrated by veins and bodies of strongly cemented, breccia-like, lime-rich sandstone injections. The rocks above (horizontally-bedded, hard limestone) and below (weakly cemented silt and sandstone) are undisturbed and do not contain the sediment intrusions. Osmussaar Breccia is found over an area of more than 5000 km2 and is distributed in a west- east oriented elliptical half-circle centred approximately at Osmussaar Island (59°18′ N; 23°28′ E). The thickness of the brecciated unit ranges from 1-1.5 m on Osmussaar to a few (tens of) cm at ~70 km east of the island. Arenitic sandstone of the sediment injections contains quartz grains with planar deformation features (PDF). Several hypotheses concerning the origin of the Osmussaar event have been proposed: catastrophic earthquake, regional tectonic movements, tectonic movements occurring simultaneously with coastal processes, and an impact event. The latter hypothesis was suggested in connection with the discovery of the nearby-situated Neugrund impact structure. However, the sediment intrusions are stratigraphically ~60 Ma younger than the impact structure. Osmussaar Breccia does not correspond to any known impact structure of this age in Baltoscandia. Also, results of a seabed geophysical survey in the Baltic Sea for the search of a possible undiscovered feature did not identify any large structure in the area of the Osmussaar Breccia. Consequently, we suggest that a devastating ~475 Ma earthquake with an epicentre close to Osmussaar split the sea floor. It initiated underwater mud-flows eroding the primary Neugrund crater ejecta and/or crater rim walls, thus reworking the impact materials into the sedimentary injections, which is suggested by rounded morphology of the shocked quartz grains found in breccia matrix.