IMMENSE TERTIARY CARBONATE JELAR BRECCIA, DINARIDES, CROATIA: A NEW VIEW

The Karst Dinarides are part of the Alpine chain, formed by tectonic breakup of a thick succession of Carboniferous to Eocene, predominantly carbonate rocks.

The Tertiary Jelar Breccia (JB) covers large areas along the NE Adriatic coast (largest outcrop is >100 km long, 2–10 km wide; in places thicker than 500 m). It was poorly studied because of its very complex composition, usually unclear structural position and intense younger tectonic deformation. JB is usually considered a result of the disintegration of frontal parts of the major thrusts. However, it usually contains a more or less normal stratigraphic succession.

The SW part of the island of Krk comprises a relatively large (15 km x 1 km) outcrop of a typical JB, within the apical part of the NW–SE oriented anticline composed of highly fractured Cenomanian limestones, transgressive, little fractured Lower–Middle Eocene limestones and Mid-Eocene flysch. The NE fold limb is overturned (inclination ~20° towards the SW).

Within the breccia, several parallel ribbons of the underlying rocks crop out in stratigraphic order, i.e. older towards the anticline axis. Contacts of these rocks with the breccia, as well as external contacts of breccia with Cenomanian limestones, are gradual: undisturbed – more tectonized – completely cataclastic limestones – breccia composed only of clasts of neighboring limestones ± polymictic breccia.

Clasts are angular, up to several cm-sized, weakly sorted, dominantly clast-supported; most clasts were tectonized (fractures filled by calcite) before final deposition; rare clasts of breccia within some breccias confirm a poly-phase origin. Carbonate matrix is recrystallized, and in places reddish. Only rarely are intergranular pores cemented by subaerial bladed dog-tooth cements and coarse mosaic cements or microstalactite cements. The matrix is sterile, but breccia is younger than the youngest Mid-Eocene clasts, and older than Middle Miocene, when the neotectonic phase formed postsedimentary fractures.

The studied erosional relics of JB were formed by poly-phase tectonic fracturing of apical parts of the overturned anticline, resulting in crushing of clasts and collapse into a complex system of deep fractures largely without surficial transport. This model may explain many features of the Jelar Breccia found at other localities.