Devonische Konturite in ozeanischen Passagen zwischen Gondwana und Laurussia
Zusammenfassung der Projektergebnisse
Contourites are sediments formed by the persistent action of bottom currents in mainly deep-marine environments. Bottom currents can be caused by a number of oceanographic forces acting at different water depths, such as wind-driven currents, thermohaline currents, and geostrophic currents. As a result, large-scale sediment accumulations, termed contourite drifts, form along the continental margins in modern oceans. These have proven to be valuable archives of climatic, palaeogeographic and palaeoceanographic changes. In this project, the widespread occurrence of contourites in the Devonian was documented, which can best exemplified by bioclastic carbonate contourites. Within the low-latitudinal oceanic realm between Gondwana and Laurussia, bottom currents controlled the deep-marine sedimentation and shaped the morphology of the continental slopes to the almost same extend as found in the modern oceans. This is far-reaching as the Devonian greenhouse world has traditionally been viewed as a period of sluggish oceanic circulation. The study allowed to test the applicability of the bigradational contourite facies sequence and to explore its compositional variation in carbonate contourites deposited under conditions of very low sediment supply. The study clearly documents the diagnostic value of microfacies criteria for the identification of contourite facies sequences in well-lithified limestones. Inverse-to-normal grading at various scales is common and displays: (i) complete contourite sequences, including very thin calcisiltic and calcarenitic intervals, mainly resulting from skeletal abrasion and winnowing of finegrained mud at increased current velocities, (ii) partial contourite sequences including discontinuities, such as omission surfaces, erosional surfaces or hardgrounds, produced by a prolonged phase of non-deposition or erosion during peak-flow conditions. A key area is the Tafilalt Platform in the eastern Anti-Atlas of Morocco. The identified contourite terrace at the upper slope of the northern Gondwana margin hosts a hiatal and condensed succession of bioclastic carbonate contourites and pelagites. The reconstruction of drift-scale morphosedimentary features, including a large erosive channel and a small mounded drift, allows to determine (i) a more detailed understanding of the sedimentary processes on the upper slope, (ii) the relationship between the morphology of the continental margin and the oceanographic processes operating at the related water-mass interface, and (iii) the principle fundamental circulation pattern in the eastern Anti-Atlas area. The time-specific deposition of organic-rich calcarenitic contourites (styliolinites), which coincides with Devonian global evolutionary events, reveals intensified bottom currents in times of an expanding dysoxic-anoxic water mass. The results of this contourite study have important consequences for prospective investigations. First, since the Tafilalt Platform is a key example of condensed pelagic sediment accumulation, the evaluation of contouritic processes and boundary conditions is essential for prospective facies models. Second, varying sediment thicknesses should be interpreted from the viewpoint of both differential basement subsidence rates and varying accumulation rates within a contourite depositional system, thus hydrodynamic boundary conditions. Third, an exploration of the link between intensified bottom-current controlled sedimentation and oceanic anoxic events is essential for prospective palaeoceanographic and climatic reconstructions.
Projektbezogene Publikationen (Auswahl)
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(2018): Emsian events, Frasnian-Famennian boundary, and Gonioclymenia Limestone at Jebel Ihrs (western Tafilalt Platform). – Münster. Forsch. Geol. Paläont. 110: 229-243
Becker, R.T., Aboussalam, Z.S., Hartenfels, S., Gibb, A., Mayer, O., Hüneke, H.
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(2018): Emsian to middle Famennian bioevents and cephalopod faunas at Mdoura (western Tafilalt Platform). – Münster. Forsch. Geol. Paläont. 110: 312-326
Aboussalam, Z.S., Becker, R.T., Mayer, O., Gibb, A., Hüneke, H.
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(2020): Devonian and basal Carboniferous of the allochthonous nappes at Mrirt (eastern part of Western Meseta) - review and new data (part 1: Anajdam). – Frontiers in Science and Engineering, , Earth, Water and Oceans, Environmental Sciences, 10, 87-109
Becker, R.T., Aboussalam, Z.S., Hartenfels, S., El Hassani, A., Hüneke, H.
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(2020): Devonian and basal Carboniferous of the allochthonous nappes at Mrirt (eastern part of Western Meseta) - review and new data (part 2: Bou Ounebdou). – Frontiers in Science and Engineering, , Earth, Water and Oceans, Environmental Sciences, 10, 110-126
Becker, R.T., Aboussalam, Z.S., Hartenfels, S., El Hassani, A., Hüneke, H.
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(2021): Devonian and the Carboniferous transgression in the Skoura region, Sub-Meseta Zone, Morocco. – Frontiers in Science and Engineering, Earth, Water and Oceans, Environmental Sciences, 11, 12-83, Academic Press Morocco, Rabat
Becker, R.T., Aboussalam, Z.S., El Hassani, A., Baidder, L., Hüneke, H., Mayer, O., Cózar, P., Helling, S., Seyffert, K., May, A.
