Lower Permian Košna conglomerates of the Velebit Mt. (Croatia): modal composition, provenance and depositional environment

Abstract

The multicoloured ‘Košna’ conglomerates are investigated in the area of the Košna Voda spring, in the Brušane area of Velebit Mt., Croatia. The studied conglomerates consist of red-coloured matrix and clasts of various lithologies (sandstones, limestones, chert and quartz pebbles) and sizes, suggesting provenance from different areas. Average clast size and their percentage was determined by the Udden-Wentworth classification with estimation of the matrix content. Clasts and matrix lithological and mineralogical characteristics were determined using petrographic, ore microscopy, and X-ray diffraction analysis. Sandstone clasts in the studied conglomerates are classified as lithic arenites, subarkoses and arkoses. Limestone clasts are mostly wackestones to packstones with foraminifera, ostracods, echinoid and bryozoan fragments that derive from late Carboniferous/early Permian shallow-marine limestones. Calcispheres and incipient growth forms of Microcodium are also present, suggesting different sediment sources and resedimentation processes. Fragments of fusulinid foraminifera were identified in the matrix of the ‘Košna’ conglomerates. Based on the analyses performed, and field observations, the ‘Košna’ conglomerates are described as early Permian (Cisuralian) polymictic clast-supported to matrix-supported conglomerates and breccia-conglomerates that share structural and textural similarities with conglomerates from other areas in the Dinarides, as well as in the Eastern and Southern Alps. The ‘Košna’ conglomerates are younger than the Sakmarian and are comparable with the Trogkofel limestones of the Carnic Alps and the Karavanke Mts. in Slovenia. We suggest that the studied deposits originated from the uplifted Variscan Mountains during the Variscan and Saalian orogenic movements, and were finally deposited from tractive flows and occasional debris flows in a shallow marine environment of the Palaeo-Tethys, possibly in fan deltas.