engleski

Massive Cenozoic carbonate breccia in the Karst Dinarides of Croatia

The Karst Dinarides are a part of the Alpine-Dinarides mountain chain, situated along the NE Adriatic coast, and formed during a mid-Eocene to Oligocene contraction of a thick succession of predominantly carbonates Carboniferous to Eocene rocks. A very interesting, but yet poorly understood lithostratigraphic unit of the Karst Dinarides is a massive Cenozoic carbonate breccia found in their central part, known as Jelar or Velebit breccia. According to the basic geological maps it covers more than 1000 km2, forming several larger outcrop areas (the largest one, along SW slopes of Velebit Mt., covers almost 700 km2) and more than 400 smaller erosional outliers. Data of the thickness on the breccia are limited, indicating a wide range from very thin erosional remnants to more than 500 m thick sequences. Breccia is massive, mostly clast-supported, monomictic to polymictic, non-bedded, without gradation or any other visible arrangement. It contains exclusively carbonate clasts, ranging in size from less than I mm to several centimetres, only sporadically to meter-sized blocks. Clasts are predominantly angular, often tectonically fractured before deposition, and mostly derived from neighbouring rocks, while rare extraclasts originated from overlying, originally younger rocks. Matrix is carbonate, grey or yellowish to reddish, and contains small lithoclasts and recrystallized calcite without fossils or cements indicating a subaqueous deposition. This type of breccia is usually very tightly connected to cataclastic breccia, formed by an intense fracturing of neighbouring limestones (some areas designated on maps as Cenozoic carbonate breccia are predominantly represented by cataclastic breccia). Most of the studied contacts between the breccia and neighbouring carbonate rocks exhibit up to sev¬eral meters wide gradual transition from intact limestones and fractured limestones to cataclastic breccia and finally monomictic to polymictic breccia. In addition, at some outcrops small conglomerate bodies have been found within breccia, as well as karstified breccia and zones fractured by subsequent tectonics. Breccia could be found in different tectonic settings: the Velebit breccia s.s. covers the SW slopes of the Velebit Mt. in the hanging-wall of the SW-dipping Lika thrust fault, while other occurrences could be found along regional reverse faults, hinges of overturned anticlines or in the form of erosional remnants of previously much wider breccia sheets. Significantly, all these positions are characterized by NE-verging thrusts and folds, which are less common in the Dinarides. This fact contradicts to the traditional interpreta¬tion of the deposits as talus breccia shed from the front of inferred SW-propagating thrust faults. The breccia was probably formed by an intense in-situ fracturing, controlled by extension in apical parts of anticlines, resulting in disintegration of carbonate rocks into cm-sized clasts within km-wide zones, followed by a deposition by rockfall mechanisms into a complex system of deep fractures, while some clasts were transported by surficial processes. Breccia deposits were subsequently tectonized, diageneti- cally altered, intensely karstified and denuded during the long post-emplacement exposure, therefore result¬ing in a present day very complex appearance. Acknowledgment This work was supported by Croatian Science Foundation project IP-2014-09-9666.

carbonate breccia, Cenozoic, Karst Dinarides, Croatia

nije evidentirano