Krk-breccia, Possible Impact-Crater Fill, Island of Krk in Eastern Adriatic Sea (Croatia)

Abstract

We propose an impact origin of a polymict breccia of presumed Eocene-Oligocene age, which occurs in a continuous blanket (ca. 150 km(2)) and many smaller patches scattered around the island of Krk in Eastern Adriatic Sea. This breccia is not differentiated in published geological maps as one lithological unit, but as two stratigraphically different units; namely of Early Cretaceous age and of Late Eocene to Early Oligocene age. The age of this breccia was attributed after the age of youngest clasts found, but the debris of Middle-Late Eocene age was recently also found within "Early Cretaceous" breccia, thus indicating its much younger age. Our map shows that previously differentiated breccias belong to a single lithological unit which we informally call the Krk-breccia. The Krk-breccia is generally massive, with chaotic fabric and a variable amount of matrix. Its bedding is unclear, except where matrix-rich breccia underlies matrix-poor variety, and soft-sediment injection structures mark the contact. The Krk-breccia debris is strati graphically varied; the oldest debris is of Jurassic age, and the youngest clasts are represented by Middle Lutetian Flysch sand-stones. The debris is of pebble- to cobble-size, but in the lower part of the breccia there occur very coarse clasts of Early Eocene Foraminiferal limestones, some of which exceed 64 m across. The clasts are angular and occasionally in situ fractured with matrix-filled fissures, what indicates very rapid deposition. Approximately in the centre of the mapped Krk-breccia area there occurs a ca. 5 km wide field of scattered large limestone blocks, which herein are referred to as "megablock facies". The Krk-breccia is 1500 in thick, as revealed by the Krk-1 deep exploration well, which reached the Triassic/Jurassic boundary at a depth of 3100 m. Comparison with "ideal" stratigraphic succession reveals that the drilled stratigraphic units are uplifted by ca. 810-1360 m at the well location. The extraordinary thickness of the Krk-breccia, its lensoid geometry, unsorted-chaotic fabric, unselective large-scale erosion which provided the debris and cartographic appearance in an elliptical unit, which is restricted to a depression in the central part of the Krk Island, may be explained in terms of an impact-crater fill.