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Krk - proposed impact structure in northern Adriatic, an update

The impact origin of the Krk-breccia, and the impact structure was first proposed by Marjanac et al. (2001, 2003). The breccia origin was first tested by mapping in years 2001-2003, which revealed that the breccia forms a lens-like body in central part of the Krk island. It rapidly thins away from the apparent centre near the Ponikve Lake where it reaches thickness of 1500 m (Đurasek et al. 1981), eg. on the Obzova - V. Hlam plateau where breccia occurs in ray-like chain of erosional remnants. The Krk breccia was first mapped by authors of the General geological map (Šušnjar et al. 1970), but it was not differentiated from other breccias (eg. Jelar-breccia) which widely occur on coastal Dinarides. Locally, both breccias look similar, but the Krk-breccia comprises large clasts of Eocene age (Foraminifera Limestones, Flysch sandstones and marls) some of which reach many metres across. Unlike the alluvial and glacial breccias it is always well cemented, lacks internal stratification, except locally, and shows size-sorting so that the largest clasts occur near its base. The volume of the Krk-breccia is estimated to reach ca. 96, 8 km3 on the island of Krk, but recent research showed that it also occurs in the hinterland of the city of Rijeka, and on the islands of Cres and Rab. Thus, the total volume of the breccia must be significantly larger. Where preserved, the Krk breccia forms erosional remnants (patches) scattered around the modern landscape. It commonly occurs on hilltops and highest landmarks eg. on the island of Rab, but also on modern hill slopes, and it seems to be deposited on sub-modern relief. The proposed impact structure was originally reconstructed as of elliptical shape and 11x14 km long axes (Marjanac et al. 2003). However, the research in progress shows that the structure is very likely a ring-type crater with outer perimeter of ca. 32 km! Thus, the Krk-breccia occurs as a transient crater fill and proximal ejecta, some of which was quickly remobilised in resurge gullies. The structure outer margin is marked by arcuate faults and cliff faces in Vinodol and the island of Cres. The age of the proposed Krk impact structure was originally approximated as post-Eocene, but prior to final tectonization of Dinarides (Marjanac et al. 2003). However, its stratigraphic relationship in the field and distribution suggest that it actually post-dates the final uplift and tectonization of this part of External Dinarides, and we suspect its possible early Pleistocene origin! The direct evidence of the Krk-breccia impact origin (eg. PDF’ s) is still being sought, so the structure is still ranked as the proposed one.

impact; Krk island; Krk-breccia

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