Middle devonian conodonts from the northwestern margin of gondwana

Résumé

In the Middle Devonian (397.5-383.3 Ma), the world was dominated by two large continents, Gondwana in the south and Laurussia in the north, connected to each other and located on one side of the earth's southern hemisphere. The terrains that later formed southern and central Europe (Barrandian area), were situated on the southern hemisphere. Those terrains are by some authors placed at the north-western margin of Gondwana. Other authors place these terrains on a separate microplate, Armorica situated between Laurussia and Gondwana, separated from the large continents by the Rheic ocean and the Paleotethys ocean respectively. Sardinia, and as well as other areas with Middle Devonian deposits in the southern part of Europe (Spanish Pyrenees, Cantabrian Zone (Spain) Montagne Noire (France), Carnic Alps (Austria and Italy)) are important areas in the study of the paleogeographic configuration of NW Gondwana. Because of their more southern position compared to the other areas mentioned above, and therefore closer position to the Moroccan terrains on the Gondwana continent, Sardinia and Spain are key areas in the paleogeographic puzzle to better understand the situation occurring along the northern margin of Gondwana. The Middle Devonian of Sardinia is not much studied yet compared to the Mediterranean areas around it (Spain, France, Morocco, Carnic Alps in Italy and Austria), probably because of the tectonised nature of the tentaculite limestones/ shales of the Lower and Middle Devonian. A first goal of this work was to construct a conodont biostratigraphic framework for the Middle Devonian of southern Sardinia as key area so to be able to compare the results (also as microbiofacies) with adjacent areas (situated on NW Gondwana and SE Laurussia) to obtain indications on the paleogeographic configuration of NW Gondwana, S Europe and S Laurussia. To achieve these nine sections/outcrops in S Sardinia were studied for conodont biostratigraphy. Twenty-one conodont zones have been recognised so far in the Emsian-Famennian interval in Sardinia: excavatus (identified by GNOLI, 1985)- nothoperbonus- patulus zones (Emsian), partitus-costatus zones (Eifelian), varcus Zones, hermanni Zone (Givetian), rhenana zones (Frasnian) and the triangularis and crepida-preasulcata zones (Famennian, CORRADINI (2003)). The tentaculite limestones and shales are found from the kindlei Zone (Pragian, Lower Devonian) up to the hermanni zone (Givetian, Middle Devonian). The oldest Clymenia limestones are situated in the Lower rhenana Zone (Frasnian, Upper Devonian). The resulting combination of lithology and conodont biozonation in Sardinia so far shows no black limestones/shales that can be associated with global events (Chotec, Kacak, Taghanic and Kellwasser) taking place in the Emsian-Famennian interval. Although deposits in Sardinia have been assigned to the conodont zones in which normally black limestones/shales typical for those global events are found, we cannot be sure they coincide exactly with those events because the biozones cannot be accurately delimited. An exception is SW Sardinia, where time- equivalent deposits of the Taghanic event were found in the Su Nuargi section. The Taghanic event, occurring in the semialternans Zone and the Upper varcus Zone and associated with a global transgression, clearly did not affect the sedimentation in SW Sardinia. The time-equivalent interval is represented here by a deposit of grey to dark grey tentaculite limestone The study of a well-selected Middle Devonian outcrop in the Compte 'subfacies' area of the SE Spanish Pyrenees contributes to the knowledge on the Middle Devonian of NE Spain and is valuable in the comparison with conodont biostratigraphic data from the surrounding areas. The studied outcrop, the Villech section, exposes part of the Comabella Formation of Eifelian- lowermost Frasnian age and its study provides the first elaborated conodont biostratigraphy for the Eifelian in the Compte subfacies area. The base of the Formation is placed in the costatus Zone (lower part of the Eifelian). The Eifelian kockelianus and eiflius zones are recognized, the australis Zone has not been found (T. kockelianus australis is not identified in the section). The uppermost Eifelian and lowermost Givetian (ensensis and hemiansatus zones respectively) might be present in a covered interval in the section. Other biozones recognized in the section are the Lower and Middle varcus, semialternans, hermanni, disparilis and norrisi zones in the Givetian, all indicated by their respective index species. The base of the Frasnian is marked by the first appearance of Ad. binodosa and Ad. pristina. A black thin limestone and shale layer placed in the semialternans Zone in the section can be the local signature of the Taghanic event, which is one of the most important global extinction events in the Middle Paleozoic and is recognised for the first time in the SE Spanish Pyrenees. Graphic correlation of the studied part of the Villech section with the Givetian Compte section (literature data) provides a high resolution conodont biostratigraphy, based on 65 taxa (of which 31 in common in the two sections) for the Eifelian to lowermost Frasnian of the Compte facies area for which all standard conodont zones except the australis and hemiansatus zones have been identified although for the hemiansatus Zone, the possibility exist that it might be located in a small interval between samples in the Compte section. The Villech section served as reference for the Eifelian and the Compte section for the Givetian and the base of the Frasnian. The resulting data from the studied sections in Sardinia and the SE Spanish Pyrenees were compared with conodont biostratigraphic data from literature from surrounding regions (Ardennes, Eifel/Rhenish Slate Mountains, Anti-Atlas, Rabat-Tiflet region, Barrandian, Carnic Alps and Montagne Noire) located in S Laurussia, S Europe or NW Gondwana. The presence of an endemic conodont fauna containing O. carinthiaca, I. beckmanni sinuatus, P. quadratus, Ps. beckmanni and P. cracens, partially occurring in Sardinia, Carnic Alps, Barrandian, Montagne Noire, Sierra de Guadarrama and the Rabat-Tiflet suggests the areas were closely related in the upper Emsian/ lowermost Eifelian. The paleogeographic distribution of Icriodus rectirostratus, I. leptus, I. corniger corniger, I. retrodepressus and I. culicellus suggests migration possibilities for these relatively shallow water conodont taxa between NW Gondwana, S. Laurussia and the Montagne Noire. The continuing migration possibilities in shallow water environments between S Laurussia and NW Gondwana in the Eifelian are indicated by the presence of the Icriodus taxa (I. amabilis, I. struvei and I. regularicrescens) in the Anti-Atlas, Ardennes and Eifel areas. The later appearance of Polygnathus taxa (P. angusticostatus, P. angustipennatus and P. pseudofoliatus) in S Laurussia compared to NW Gondwana and S Europe (SE Spanish Pyrenees for the former two species, Montagne Noire for the latter species) might indicate a migration direction from NW-Gondwana to S-Laurussia. The increasing conodont fauna diversity and the homogenisation of the conodont fauna of NW Gondwana and S Laurussia in the Givetian are probably linked to the eustatic sea-level rise that is taking place or the increasing proximity of the regions. The similarity coefficient calculated for the Middle varcus Zone, indicates a moderately high similarity between faunas of Sardinia and the Carnic Alps, Anti-Atlas and Montagne Noire areas, and an increasing similarity between Sardinia and the Ardennes (compared to the coefficients of the serotinus, patulus and costatus zones). The similarities of the conodont faunas considered here clearly point to a proximity of the southern European regions with the Anti-Atlas and thus with NW Gondwana and an increasing affinity with southern Laurussia during the Middle Devonian and are thus in favour of a paleogeographic configuration that allows migration of shallow water species between S Europe and NW Gondwana and to S Laurussia and would indicate a minor importance of the Paleotethys during the Middle Devonian.