Jurassic radiolarites from the Meliata Unit: lithology, age range, and their similarities and differences

The Meliata Unit of the Western Carpathians represents a key element in understanding the geodynamic and paleogeographic evolution of the Meliata oceanic realm developed during the Triassic, which was progressively consumed/subducted/closed during the Jurassic, while the sedimentation in some parts was ongoing. Among the most important fossil groups constraining the stratigraphic position and depositional history of this unit are radiolarians, preserved predominantly in siliceous sedimentary successions such as radiolarites and associated pelagic shales. The radiolarian record is critical for timing of the progressive closure of the Meliata Ocean and the onset of subduction-related processes, which culminated in its incorporation into the collisional framework of the Western Carpathian orogeny.
Pelagic sediments and radiolarites, positive in radiolarian microfauna content, originate from various locations in the Meliata Unit. The unit is composed of three basic subunits that are strongly tectonically dismembered into the accretionary complex. In their present form, they occur only as very limited occurrences without continuous successions, overlapped by the higher units of the geological structure of the Western Carpathians. Meliata Unit consists of 1) Permian-Triassic blueschist facies Bôrka Nappe; 2) Triassic-Jurassic sedimentary complex with olistostrome bodies – Meliata Unit s.s.; 3) chaotic mélange complexes of Jurassic age.
This abstract focuses on Jurassic radiolarites of the mélange complexes from the localities of Dobšiná, Držkovce, Meliata, Čoltovo, and the Miglinc Valley, where over 110 point samples were collected.
At the localities Guba (near Meliata type section) and Držkovce, white, brown, grey and white-green radiolarites were documented. Radiolarians in the samples are poorly preserved, mostly containing only Nassellarian genera, at Držkovce they also contain sponge spicules. In these samples determined radiolarian assemblages belong to UA Zones 6-7 (Middle Bathonian to Late Bathonian-Early Callovian).
Light grey and grey-green siliceous siltstones/shales outcropping at the Meliata type locality comprise poorly preserved Nassellarian species, of Middle Callovian-Early Oxfordian age (UA Zone 8).
Sediments from the Dobšiná and Miglinc Valley are represented by black shales, green-black radiolarites and shales, often grey and purple in colour. Comparing to the samples from previous localities, the assemblages contain at least 20% of Spumellarian species, and are very poorly preserved. These samples contain radiolarians of Late Kimmeridgian-Early Tithonian age (UAZ 11).
From the examination of lithology and ages of radiolarian-bearing sediments, following can be concluded. Sedimentation in the already closing Meliata Ocean was ongoing in the late Middle Jurassic. It is represented by white, grey to brown radiolarites. Following Middle Callovian-Early Oxfordian light grey to green siliceous shales already characterize the typical matrix of the mélange. Late Jurassic dark shales to black radiolarites, also belonging to the mélange matrix, contain radiolarian assemblages corresponding to the Kimmeridgian-Early Tithonian UA Zone, representing the end of sedimentation in the final phases of the ocean closure, which coincides with the peak of HP/LT metamorphism in the Meliata Unit.
Acknowledgements: Financial support from the Slovak Research and Development Agency (project APVV-21-281) and from the Grant Agency for Science, Slovakia (project VEGA 1/0021/25 and VEGA 2/0012/24) is gratefully appreciated