Erosion and deposition from bottom currents during the Givetian and Frasnian:: Response to intensified oceanic circulation between Gondwana and Laurussia

Calcareous bottom-current deposits in otherwise pelagic and hemipelagic successions of the Harz Mountains in 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 of bottom-current reworking of carbonate ooze during Givetian and early Frasnian. Associated phosphates confirm that higher hydraulic energy temporarily prevailed. The bottom-current deposited facies builds up strongly condensed and reduced successions with stratigraphic gaps, which also occur with similar features in other areas of central Europe and northwest Africa at the same stratigraphic interval. The widespread current-induced reworking of calcareous sediments and phosphate formation during the Givetian and early Frasnian as well as the associated erosional processes marked by pronounced hiatuses all signal a major circulation event. The temporal and spatial positions of these fossil calcareous contourites in the Devonian are discussed. Research concluded that thermolialine currents were intensified by the acceleration of flows constricted in the narrow oceanic passages between the approaching continental plates Laurussia and Gondwana. Areas affected were the southeastern Rhenish Sea shelf, the disintegrated northern continental margin of Gondwana, and deep marginal plateaus of the Noric Terrane in the western part of the Prototethys. The disposition of the fossil calcareous contourites and faunal data corroborate palaeogeographic reconstructions that show an advanced convergence between Gondwana and Laurussia and the smaller continental plates such as the Armorican and Noric terranes during Givetian and Frasnian times. (c) 2005 Elsevier B.V. All rights reserved.