Eovariscan evolution of the southern and northern Prototethys: high-resolution stratigraphy, facies developments, biogeography, and geodynamic interpretation
Zusammenfassung der Projektergebnisse
In the Middle Palaeozoic (Devonian to Lower Carboniferous), the two main continents Laurussia (including the Rhenish Massif at its southern margin) and Gondwana were separated by two oceans, the Prototethys and the largely closed Rheic Ocean, with South European crustal blocks and the Armorican Terrain Assemblage (ranging from Iberia to the Bretagne, Franconia/Thuringia and to Czechia) in the middle. This ocean complex controlled the east-west migration of faunas, major palaeocurrents, and global climate but its size (e.g., north-south distance) and history is widely disputed. The “Eovariscan” time in a wide sense is the interval between the Caledonian Orogeny, which formed Laurussia, and the final disappearance of the Prototethys and Rheic oceans when Gondwana and Laurussia collided, which is the main Variscan Orogeny (ca. from the Viséan on). Our project concentrated on the rather poorly studied, in terms of marine faunas and stratigraphy, Moroccan Meseta. As the external southern Variscides it lies between the stable cratonic crust of the Anti-Atlas in the south and the Rif in the north, which attached to Morocco after the Variscan orogeny. We concentrated on the Emsian to Tournaisian interval, the time when the Rheic Ocean was already largely closed, and when the Prototethys became the dominant seaway separating North African and European crustal blocks. The main idea was to “fingerprint” the very complex mosaic of individual Meseta blocks by their different sedimentary and faunal development and their different timing of synsedimentary tectonic movements. The comparison with the northern external Variscides (Rhenish Massif) should provide a palaeontological perspective to the history of the Prototethys. Based on more than 400 conodont samples from the Meseta, important new ammonoid and brachiopod faunas, we could date with high precision in 30 regions (more than 55 localities) changes of the palaeoenvironment and Eovariscan reworking and re-sedimentation events. The latter reflect the sudden tilting of tectonic blocks, leading to partly deep erosion on the uplifted side and deepening and fast sedimentation, with large glide blocks and conglomerates/breccias, on the subsiding side. We could demonstrate that many previous age assignments of sediments and crustal movements were wrong or could add significant precision. Unlike as in the northern Variscides, the Meseta was extremely fragmented, consisting of at least 25 different tectono-sedimentary units each with a rather individual history of sedimentation and ecosystems. None of these experienced stable sedimentation over long periods of time. We could distinguish five different Eovariscan phases: Phase 1, a Lochkovian to Eifelian “Eovariscan prelude” with only local reworking, mostly (but not exclusively) in the south, Phase 2, a Givetian first major Eovariscan interval of uplift, brecciation and reworking in almost all regions, followed by wide-spread unconformities or extreme condensation, Phase 3, the quieter, often largely missing (due to non-deposition) Frasnian to lower Famennian, with some reworking in more eastern parts, Phase 4, the middle/upper Famennian peak period of Eovarican crust movements, with the regional opening of fast subsiding flysch basins, and Phase 5 in the Tournaisian, with angular unconformities and thick reworked units mostly in northeastern basin parts. The detailed analysis of reworked upper Silurian to Upper Devonian sediments and fossils at the boundary between the Meseta and the Anti-Atlas (Southern Variscan Front), showed that there was a gradual continuity between both regions, not a major suture as claimed by some structural geologists. The style of sedimentation and synsedimentary tectonics in the Meseta strongly supports the idea that there was persisting crust extension throughout the studied interval. This proves that the Prototethys widened continuously while there was plate convergence north of the South European crustal blocks. Givetian and Famennian ammonoids from northern Gondwana (plus the Meseta) and southern Laurussia were much more different than previously assumed, which agrees with the new sedimentary and tectonic results. Meseta Givetian to lower Frasnian stromatoporid-coral reefs, however, show more or less the same palaeoecology as Rhenish reefs of the same age. This is very surprising with respect to a N-S distance of at least 3000 km between both regions. Therefore, the climatic gradient in the southern hemisphere, up to 30-40° S, must have been very low. Further south, there are clear differences to Anti-Atlas reefs. Episodically stable Meseta blocks enabled locally the recognition of all global events that were established in other regions. They show overprint but no clear correlation with Eovariscan tectonic movements. This contradicts published suggestions that Devonian global extinctions were caused by tectonically triggered changes of climate, sea-level and nutrient discharge.
Projektbezogene Publikationen (Auswahl)
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(2012): Conodont dating of reefs and carbonate platforms in the Middle and Upper Devonian of the Moroccan Meseta. – In: Witzmann, F. & Aberhan, M. (Eds.), Centenary Meeting of the Paläontologische Gesellschaft, Programme, Abstracts, and Field Guides, 24.09. – 29.09.2012, Terra Nostra, 2012 (3): 27-28
Aboussalam, Z.S., Becker, R.T. & Eichholt, S.
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(2013): Conodont biostratigraphy and the timing of facies changes at Ain-es-Seffah (Oued Cherrat Zone, Moroccan Meseta). – In: El Hassani, A., Becker, R.T. & Tahiri, A. (Eds.), International Field Symposium „The Devonian and Lower Carboniferous of northern Gondwana“, Abstract Book, Document de l´Institut Scientifique, Rabat, 26:11-13
Aboussalam, Z.S., Becker, R.T., Eichholt, S. & El Hassani, A.
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(2013): The allochthonous Silurian-Devonian in olistostromes at “The Southern Variscan Front” (Tinerhir region, SE Morocco) – preliminary data. – In: Becker, R.T., El Hassani, A. & Tahiri, A. (Eds.), Internationsal Field Symposium “The Devonian and Lower Carboniferous of northern Gondwana”, Field Guidebook, Document de l´Institut Scientifique, Rabat, 27: 11-21
Rytina, M.-K., Becker, R.T., Aboussalam, Z.S., Hartenfels, S., Helling, S., Stichling, S. & Ward, D.
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(2015): Devonian bio-events in Western Germany. – Final Meeting of IGCP 596 „Climate change and biodiversity patterns in the mid-Palaeozoic“, Post-conference excursion 23rd-29th September 2015, Münstersche Forschungen zur Geologie und Paläontologie, 108: 1-190
Becker, R. T., Hartenfels, S. & Königshof, P. (Eds.)
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(2015): Late Givetian ammonoids from Ait Ou Amar (northern Maider, Anti-Atlas, southeastern Morocco). – Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 278 (2): 123-158
Bockwinkel, J., Becker, R. T. & Ebbighausen, V.
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(2015): Petrophysical and geochemical signature of the Hangenberg Events: an integrated stratigraphy of the Devonian-Carboniferous boundary interval in the Northern Rhenish Massif (Avalonia, Germany). – Bulletin of Geosciences, 90 (3): 667-694
Kumpan, T., Bábek, O., Kalvoda, J., Grygar, T. M., Frýda, J., Becker, R. T. & Hartenfels, S.
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(2015): Review of chrono-, litho- and biostratigraphy around the global Hangenberg Crisis and Devonian-Carboniferous boundary. - In: Becker, R. T., Königshof, P. & Brett, C. E. (Eds.), Devonian Climate, Sea Level and Evolutionary Events, Geological Society, London, Special Publications, 423: 38 pp., 6 figs.
Becker, R. T., Kaiser, S. I. & Aretz, M.
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(2015): The global Hangenberg Crisis (Devonian-Carboniferous transition): review of a first-order mass extinction. – In: Becker, R. T., Königshof, P. & Brett, C. E. (Eds.), Devonian Climate, Sea Level and Evolutionary Events, Geological Society, London, Special Publications, 423: 51 pp.
Kaiser, S. I., Aretz, M. & Becker, R. T.
