Deep-sea mass-flow sediments and their exotic blocks from the Ropianka Formation (Campanian-Paleocene) in the Skole Nappe: a case from the Wola Rafałowska section (SE Poland)

Abstract

Flysch deposits of the Ropianka Formation (Wiar and Leszczyny members; Skole Nappe) at Wola Rafałowska include two different sediments that contain exotic pebbles, cobbles and boulders. The first one is a graded conglomerate that contains mostly cobbles of sandstones, gneisses, Štramberk-type limestones, volcanic rocks, pegmatites and ferruginous siltstones. The second one is a pebbly mudstone that contains clasts of sandstones, stone coal, grey mudstones, volcanic rocks, schists, limestones, marls, black mudstones, conglomerates, volcaniclastic rocks and quartz gravels that are floating within a muddy matrix. Genesis of the conglomerate is unclear because it shows features typical of debris flows (poorly sorted, matrix- to clast-supported, large amount of cobble to boulder fraction) as well as high density turbiditic currents (indistinct normal gradation, small amount of cohesive material, crushed clasts that suggest interaction between grains during transportation and at least partly turbulence during flow). The pebbly mudstone represents typical debris flow deposits (large amount of cohesive material, matrix-supported, poorly sorted, lack of grain gradation and traction structures that suggest laminar flow). Limestones occurring in both exotic-bearing sediments show different Upper Jurassic–lowest Cretaceous facies of a carbonate platform, which was involved in the source area of the Skole Basin, along with its basement. They can be interpreted as deposits of: 1) platform-margin reefs and a platform slope: a) partly silicified coral boundstone, b) microbial-coral boundstone, c) silicified sponge-microbial boundstone grading into peloidal-ooidal grainstone with bioclasts, and d) strongly silicified limestone with intraclasts and bioclasts; 2) deeper, platform slope to toe-of-slope area – bioclastic wackstone; 3) inner platform, including: a) partly silicified wackstone with peloids, small intraclasts and bioclasts, and b) microbial bindstone. Moreover, some exotic clasts are built of Albian–Cenomanian wackstone with abundant sponge spicules and planktonic foraminifers, which are interpreted as deeper shelf sediments. Taking into account the geometry of thrust sheets from the site of the exotic-bearing sediments to the edge of the Skole Nappe, along the most probable transportation path, including tectonic/erosional reduction and different variants of slope inclination, the distance of the mass flows attained at least 25–97 km from the shelf edge.