Pelagic carbonate ooze reworked by bottom currents during Devonian approach of the continents Gondwana and Laurussia

Abstract

AbstractGivetian and lower Frasnian carbonates of pelagic carbonate-platform and distal slope-apron settings in the Harz Mountains of Germany (Herzyn Limestone Formation), the eastern Moroccan Central Massif (Ziar-Mrirt Nappe), and the Carnic Alps in Austria-Italy (Valentin and Pal Limestone Formation) show strong evidence for bottom-current activity during deposition. Calcarenites, laminated calcisiltites, and mottled calcisiltites and calcilutites can be distinguished, which are similar to recent calcareous bioclastic contourites. They combine faint structures caused by current action with pervasive bioturbation. Calcarenites are mostly represented by styliolinid grainstones to packstones with rarely preserved parallel lamination and ripple cross-lamination. Laminated calcisiltites are particularly rich in non-carbonate components with a higher density than calcite such as conodonts and phosphatic intraclasts. Relics of coarsening-upward to fining-upward micro-sequences a few centimetres thick are preserved in the Moroccan record. Erosional surfaces, hardgrounds and condensed phosphates are more typical of the Harz Mountains and the Carnic Alps. The bottom-current influenced facies build up strongly condensed and reduced sequences that occur at the same stratigraphic interval in different areas of central Europe and NW Africa. Variations in rate of accumulation, magnitude of erosion and microfacies, which are found across the three regions, are compatible with a contourite interpretation. The widespread current-induced reworking of calcareous sediments and phosphate formation during the Givetian and early Frasnian as well as the associated erosion marked by pronounced hiatuses all signal a major palaeocirculation event. Thermohaline currents were intensified by the acceleration of flows constricted in narrow oceanic passages between the approaching continental plates Laurussia and Gondwana. Areas affected were the southeastern Rhenish Sea shelf, which occupied the distal passive margin of Laurussia, the disintegrated northern continental margin of Gondwana, whose sedimentary record is now preserved in the Moroccan Meseta, and deep marginal plateaux of the Noric Terrane in the western part of the Prototethys. Thus, the occurrence of fossil calcareous contourites confirms a very advanced convergence between Gondwana and Laurussia and the minor terranes between during Middle and Late Devonian times.