Abstract
Abstract A tephra-rich cherty-clayey Famennian succession within the major Brzeźnica olistostrome in the Bardo Mountains, Central Sudetes, SW Poland, preserves a record of the lost ocean later incorporated into the Variscan orogenic belt. Fluctuating but mostly oligotrophic regimes and low primary production levels were influenced by weak up-welling below the perennial oxygen minimum zone, which controlled the interplay between biosiliceous and siliciclastic deposition in the oceanic basin, with episodic oxygen deficiency. The Hangenberg Black Shale has been identified in this oceanic setting based on its characteristics described worldwide (including mercury enrichments). A tectonic uplift of the sediment source area near the Devonian-Carboniferous boundary, recorded in the distinguishing provenance signal of old continental crust, was paired with a global transgression, anoxia, and volcanic episode in an interglacial interval. Assuming paleogeographic affinity with the Bavarian facies of the Saxothuringian terrane, we interpret the allochthonous sediments as part of an accretionary prism that was gravitationally redeposited into the late orogenic basin in front of advancing Variscan nappes. The oceanic basin parental to the Bardo pelagic succession is therefore thought to represent a tract of the waning Saxothuringian Ocean in the Peri-Gondwanan paleogeographic domain that was eventually subducted beneath the Brunovistulian margin of Laurussia. The sediments of the Bardo Ocean basin also include a distal record of Famennian explosive volcanic activity that was likely related to a continental magmatic arc whose remnants are preserved as the Vrbno Group of the East Sudetes.
Highlights
Biosiliceous facies in middle Paleozoic deepwater, partly oceanic basins are widespread (Hein and Parrish, 1987), but the depositional record of Late Devonian radiolarian oozes remains poorly known
We report data from a hemipelagic siliceous succession with pyroclastic horizons found in the Bardo Mountains (Góry Bardzkie), one of the main structural units of the Central Sudetes, SW Poland (Figs. 1A and 1B)
This fault-bounded complex rock suite is defined as the Bardo Sedimentary Unit (Mazur et al, 2006) or Bardo Fold Structure (Żelaźniewicz et al, 2011), an element of the Saxothuringian Zone on the northern flank of the European Variscides (Franke et al, 1993, 2017) consisting of poorly outcropped, dismembered sedimentary series ranging in age from the Upper Ordovician to lowermost Upper Carboniferous (Haydukiewicz, 1979, 1990; Oberc, 1980; Cymerman et al, 2015; Fig. 1C)
Summary
Biosiliceous facies in middle Paleozoic deepwater, partly oceanic basins are widespread (Hein and Parrish, 1987), but the depositional record of Late Devonian radiolarian oozes remains poorly known. This is suggested by the late FrasnianFamennian age of limestones, which directly overlie the basal conglomerates (Bederke, 1929), and by the early Givetian age of a coralline fauna from the greenschist facies crystalline limestone of the Kłodzko Unit (Hladil et al, 1999) The presence of this unconformity implies that at the transition from the Middle to the Late Devonian, freshly deformed and metamorphosed rocks had been exposed and onlapped by deposits of the Bardo Basin that eventually were folded during Namurian times (Oberc, 1972). Autochthonous limestone-dominated Upper Devonian–Tournaisian pre-flysh strata (Wapnica Formation), resting on the ophiolitic gabbro (Mazur, 1987), are capped by Tournaisian black shales (Gołogłowy Formation; Haydukiewicz, 1990), and Viséan gneissic sandstones and conglomerates (Nowa Wieś Formation) The latter were deposited by high density currents and debris flows clearly derived from the Góry Sowie gneisses (Pacholska, 1978; Wajsprych, 1978).
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