Project

Eovariscan evolution of the southern and northern Prototethys: high-resolution stratigraphy, facies developments, biogeography, and geodynamic interpretation

Goal: The main aim of the project was to use the recent advances in Devonian/Lower Carboniferous stratigraphy to investigate and compare with the highest possible time resolution the “Eovariscan” sedimentary and fossil successions of the southern (Hercynian Morocco) and northern (German) shelf regions of the western Prototethys. The main focus was on the poorly studied successions of the Moroccan Meseta and Atlas regions that represent a still poorly understood mosaic of different tectono-sedimentary units. The Meseta developments should be compared with the much better known successions of the stable Gondwana margin (Anti-Atlas) and of the northern external Variscides in Germany. In both latter regions selective sampling should fill knowledge gaps. The research was based on the interdisciplinary study of conodonts (S. Z. ABOUSSALAM), ammonoids (R. T. BECKER and students/collaborators), miospores (C. HARTKOPF-FRÖDER), trilobites (R. T. BECKER and student), brachiopods (D. BRICE), reefs, clastic and carbonate sedimentology (H. HÜNEKE), and stable isotopes. The project was conducted as a joint cooperation project with the CNRST Morocco, led by Prof. Dr. A. El Hassani (Rabat). It was embedded in the parallel running IGCP 596 on “Climate change and biodiversity patterns in the mid-Palaeozoic” and in current research activities of the International Subcommission on Devonian Stratigraphy” (SDS). The “Eovariscan” time interval was defined as the period from the main closure of the Rheic Ocean between the Armorican Terrain Assemblage (ATA) and Avalonia/Laurussia (in the Emsian) to the onset of the main Variscan orogeny by oblique Gondwana/Laurussia collision in the Viséan.

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Project log

Ahmed El Hassani
added 4 research items
At Boudouda NW of Benahmed, upper Frasnian goniatite shales are in unconformable contact with massive, partly dolomitized conglomerate boulders of the new Oued Ayada Formation. They represent debris flow deposits of localized channels and contain reworked supposed Emsian reefal organisms and Frasnian to lower/?middle Famennian conodonts. Rare foraminifers prove upper Viséan (Upper Asbian) sedimentation. Laterally, a ca. 35 m thick sequence of bioclastic limestones with crinoids, mostly fragmented brachiopods or gastropod-rich forms a second member. Thin-sections show a dominance of variably sorted wacke-, pack- and grainstones with abundant coated grains (initial ooids) and subangular fine quartz sand. Biota include calcareous green algae, foraminifers, and various Algospongia (Aoujgaliidae and Palaeobereselliidae). Throughout the succession, there are reworked Frasnian to middle Tournaisian deeper-water conodonts, which shows that a probably small-sized pelagic platform existed once in the region, which had been uplifted during the Eovariscan 2 tectonic phase. A single, juvenile, originally pyritic goniatite gives limited evidence for an upper Tournaisian/lower Viséan goniatite shale, which has no equivalents elsewhere in the Meseta. Based on a rich record of foraminifers (more than 70 taxa) and stratigraphically meaningful other calcareous microfossils, polyphase erosion and re-deposition by storms occurred on a shallow-water, photic zone carbonate ramp in the upper Viséan. The Oued Ayada Formation correlates with the transgressive limestones of the basal Melilla Formation in the southern Mdakra Basin and the Bled Mekrach Formation in the NE Rehamna.
Based on new field work, sampling for conodonts, ammonoids, and microfacies analysis, the Lower to Upper Devonian stratigraphy of the Benahmed region is refined. Fossiliferous neritic limestones of the Pragian/lower Emsian represent a continuation of the shallow carbonate platform of the Al Attamna region to the north (Units A/B of Sidi Ahmed Lemdoun Formation). More argillaceous upper Emsian/Eifelian strata are regionally still poorly known. In the southern Chaouїa or Beni Sekten region NW of Benahmed, Givetian deep neritic to shallow pelagic mud-wackestones of the new Oulad Amar Formation were partly reworked and re-sedimented together with crinoidal grainstones in conglomeratic debris flow beds. Peaks of Eovariscan block faulting and reworking occurred high in the middle (ansatus Zone) and at the top of the Givetian (norrisi Zone). Lower/middle Frasnian strata are still unknown in outcrop but limestones with corals of possible Givetian/Frasnian age have been reported from E/SE of Benahmed. In the NW, the new upper Frasnian Boudouda Formation is characterized by transgressive hypoxic goniatite shales, which are unique for the Moroccan Meseta and which strongly resemble the contemporaneous Büdesheim Goniatite Shales of Germany. Unfortunately, the top-Frasnian and Frasnian-Famennian boundary are not exposed. Upper/uppermost (“Strunian”) limestones and siliciclastics belong to a neritic succession that requires further research. In the southern Mdakra Massif E of Benahmed, there was a distinctive, middle/upper Famennian basin with richly fossiliferous, hypoxic ammonoid shales, the new Oued Aricha Formation. More than forty goniatite and clymeniid species are recorded, with up to 13 new forms that are currently left in open nomenclature. They fall in the UD III-C to V-A2 interval of the international ammonoid zonation and include many common taxa with the contemporaneous Fezzou Formation (Lahfira to Jebel el Krabis Members) of the Maїder Basin of the eastern Anti-Atlas. The associated fauna is composed of rugose corals, including the revised Hebukophyllum arichense, nautiloids, bivalves, gastropods, brachiopods, and trilobites. As the Chabet el Baya Formation of the northern Mdakra Massif, the Oued Aricha Formation grades upwards into the poorly fossiliferous Mgarto Formation that straddles the Devonian-Carboniferous boundary.
The long-known Devonian of the Jebel Ardouz west of Marrakesh, and just north of Mzoudia, is composed of an allochthonous stack of clastic and carbonate rocks that were thrusted onto each other from the northeast. New biostratigraphic data prove an age range of sedimentation from the lower Eifelian to ?upper Famennian. The lowest, western thrust unit is composed of reddish sandstones and conglomerates/breccias (new Ardouz Formation) deposited originally by rockfall and debris flows on the slope of a repeatedly active fault scarp. Limestone clasts yielded sandstones of unknown age, Eifelian, Givetian and lower/middle Frasnian conodonts, and encrusted reef corals. Re-sedimentation may have occurred in the upper Frasnian or post-dated the Famennian. The “red conglomerates” record a block that was strongly tilted by Eovariscan extensional tectonics, forming on the uplifted side a small island. Exhumation, erosion down into Eifelian carbonates, and a long phase of reworking (pebble formation, hematite impregnation and encrusting) occurred in an arid, lateritic, terrestrial-fluvial to coastal high-energy setting. The overlying middle unit consists of a lower Eifelian to middle Givetian, shallowing upwards carbonate ramp (new Mzoudia Formation, with the new Koudiat Ferjane and Koudiat Kébir Members). Middle Givetian regression resulted in the growth of a biostrome with patch reefs. The middle thrust unit experienced no Eovariscan reworking but upper Givetian uplift resulted in an episode of non-deposition. Following poorly known non-reefal Frasnian strata (still un-named Upper Member), the upper thrust unit on the eastern side of Jebel Ardouz (new Oued el Biad Formation) consists of shallow-water, open marine sandstones/quartzites with brachiopod coquinas, which originally transgressed unconformably the carbonate platform. Shedding of sand from a W/NW source (Imfout Ridge) balanced subsidence. The Jebel Ardouz Devonian differs considerably from the Devonian of the High Atlas Basement (to the south), the Safi region (to the west), and allochthonous eastern Jebilet (in the east). A similar association of carbonate platform blocks truncated by conglomerates or brachiopod-rich quartzites is developed in the Mechra Ben Abbou succession of the rather distant northern Rehamna. But comparable, poorly studied Devonian blocks have been mentioned from the geographically intermediate Skhirat region of the Jebilet.
Ralph Thomas Becker
added a research item
A positive carbon stable isotope excursion of about 3‰ is documented in the topmost lower Frasnian at Padberg, eastern Rhenish Massif, as a muted record of the worldwide early−middle Frasnian isotopic perturbation (punctata Event; up to 6–8‰ shift in both δ¹³Ccarb and δ¹³Corg elsewhere), comparable with the Appalachian δ¹³C curve. This German isotopic signature occurs in a 12 m thick calciturbidite succession and correlates well with the three-step chemostratigraphic pattern known from the Holy Cross Mountains, Poland. It is especially clear in the δ¹³Corg shifts, whilst δ¹³Ccarb (and elemental geochemical) proxies are partly biased by post-sedimentary alterations. The New York State, Polish, Nevada and Padberg conodont successions place the onset of the major positive δ¹³C excursion slightly beneath the early–middle Frasnian boundary, with Ancyrodella nodosa (previously Ad. gigas form 1) as the main conodont guide species, and coincident with the Middlesex Transgression and spread of cold, nutrient-rich, poorly oxygenated water masses. In the light of geochemical proxies, enhanced primary production and oxygen deficiency occurred evidently in the Rhenish Basin during the punctata Event. Moderate Hg enrichments in the early Middlesex/punctata Event interval suggest a volcanic signature. However, conclusive data from other regions are required to differentiate between effects of the regionally well-known synsedimenary magmatism and of a possible global volcanic trigger for the biogeochemical perturbation. Moreover, long-term relationships between the isotopic event and the regional meteorite impact destruction of a carbonate platform, as a potential source of isotopically light carbon should be re-considered.
Ralph Thomas Becker
added 6 research items
The widely quarried and commercially exploited Gonioclymenia Limestone of the western and southern Tafilalt Platform (Anti-Atlas, SE Morocco) represents a transgressive but strongly condensed and shallowing upwards unit that overlies conformably or unconformably Famennian to Lower Devonian deposits. It is characterized by abundant, giant-sized Gonioclymenia speciosa , which characterizes the Famennian V-B, and rare Leviclymenia ramula sp. nov. Associated conodont faunas fall in the Bispathodus costatus Subzone of the B. aculeatus aculeatus (= Middle expansa ) Zone. A rare outcrop at Jebel Ihrs West proved that Kalloclymenia and conodont faunas of the B. ultimus ultimus (= Upper expansa ) Zone occur in a separate, overlying unit of the lower part of Famennian VI. Consequently, joint occurrences of Gonioclymenia and Kalloclymenia on large limestone slabs that are on offer in rock shops are artificial assemblages; there is no evidence for a regional co-occurrence of both genera. This is supported by correlation into thicker sections of the adjacent Maider and Tafilalt Basins. The Tafilalt Gonioclymenia Transgression may represent the regional expression of a eustatic pulse in the B. costatus Subzone that is known on the Ardennes Shelf as the transgressive Epinette Event. The conodont faunas yielded Neopolygnathus fibula sp. nov., Pseudopolygnathus primus tafilensis ssp. nov., new morphotypes of B. spinulicostatus and Pseudopolygnathus primus primus , the oldest Siphonodella (Eosiphonodella) and variable ‘siphonodelloids’ that are currently left in open nomenclature.
Ralph Thomas Becker
added 2 project references
Ahmed El Hassani
added a project goal
The main aim of the project was to use the recent advances in Devonian/Lower Carboniferous stratigraphy to investigate and compare with the highest possible time resolution the “Eovariscan” sedimentary and fossil successions of the southern (Hercynian Morocco) and northern (German) shelf regions of the western Prototethys. The main focus was on the poorly studied successions of the Moroccan Meseta and Atlas regions that represent a still poorly understood mosaic of different tectono-sedimentary units. The Meseta developments should be compared with the much better known successions of the stable Gondwana margin (Anti-Atlas) and of the northern external Variscides in Germany. In both latter regions selective sampling should fill knowledge gaps. The research was based on the interdisciplinary study of conodonts (S. Z. ABOUSSALAM), ammonoids (R. T. BECKER and students/collaborators), miospores (C. HARTKOPF-FRÖDER), trilobites (R. T. BECKER and student), brachiopods (D. BRICE), reefs, clastic and carbonate sedimentology (H. HÜNEKE), and stable isotopes. The project was conducted as a joint cooperation project with the CNRST Morocco, led by Prof. Dr. A. El Hassani (Rabat). It was embedded in the parallel running IGCP 596 on “Climate change and biodiversity patterns in the mid-Palaeozoic” and in current research activities of the International Subcommission on Devonian Stratigraphy” (SDS). The “Eovariscan” time interval was defined as the period from the main closure of the Rheic Ocean between the Armorican Terrain Assemblage (ATA) and Avalonia/Laurussia (in the Emsian) to the onset of the main Variscan orogeny by oblique Gondwana/Laurussia collision in the Viséan.