Résumé
À la faveur de sondages effectués pour la révision de la carte géologique de Haguenau (Alsace du Nord), les marnes définies à Mietesheim (Formation de Mietesheim), surmontant le Calcaire de Bouxwiller (Formation de Bouxwiller, Lutétien supérieur), ont fait l’objet d’une étude palynologique. Comparée à d’autres successions floristiques du Nord-Ouest européen (Bassin de Paris, bassins armoricains), la microflore des Marnes de Mietesheim, riche de plus d’une centaine d’espèces, présente en comparaison avec celle définie dans le Bassin de Paris, des formes apparaissant à l’Auversien, d’autres disparaissant au Marinésien ainsi que deux espèces limitées à ces deux étages : Retiovoipollis parisiensis et Retitricolporites gibbosus/baviensis. Les Marnes de Mietesheim, dont l’âge n’était pas connu avec précision dans le Fossé rhénan supérieur, peuvent donc être datées du Bartonien s.s. (Cavelier, 1979 ; Schuler, 1988) par leur microflore qui évoque un milieu lacustre à végétation hygrophile luxuriante dont les bordures sont entourées par une forêt tropicale dense et humide. Cette formation précède les séries partiellement évaporitiques de la formation de la Zone dolomitique déposée au cours de la subsidence tectonique majeure de la partie centrale du Fossé rhénan supérieur qui débute traditionnellement au Priabonien.
Abstract
In the course of Haguenau geological map revision a set of boreholes have been drilled through small Cenozoic basins overlying the Jurassic marls and limestones. These deposits are located in the eastern part of an intensely faulted zone, called Saverne faulting zone, which is limited to the West by the Vosgian fault and to the East by the WestRhenish fault. This area consist of lenticular strips limited by N20°E to N40°E fractures, dipping eastward and affecting the mesozoic basement as well as in some places the Eocene deposits, preserved in small grabens. Eocene sediments were reported as soon as the 18th century in the Bouxwiller area from limestones dated from Lutetian by a rich mammalian fauna but also from undated marls from old Iron mines exploitation near Mietesheim and Dauendorf deposited above the limestones. This continental marly formation, particularly well preserved and nearly 20 m thick, has been cut by the boreholes around the Mietesheim village (therefore called Mietesheim Formation in the regional lithostratigraphic classification) and investigated by palynological analyses in order to compare their microflora content with those of NW European basins. The Paris basin is firstly concerned owing to his accurate chronostratigraphic framework based on substantial biostratigraphic databases but Armorican Cenozoic basins are also considered in regard of their deposits covering the same age span. The Mietesheim marls have yielded rich palynological associations involving more than 150 different species of spores and pollen grains from the two boreholes investigated in Stocklach section from Haguenau forest (01986X1046) and Mietesheim (01985X0115) areas. The range of each species is checked with his counterpart in the Paris, Brittany and Rhine graben basins particularly in terms of first occurrences (FO), and last occurrences (LO) which are plotted on the detailed range chart of the Mietesheim samples (Diagram 1-1 to 1-6). Referring first to the Paris basin, some of the pollen grains species have their first occurrence in the Auversian (Late Bartonian s.s.) such as: R. parisiensis, S. latiporatus, T. chandlerae, T. microporitus, P. striangulus (the later strictly limited to Auversian) whereas others have their first occurrence in the Marinesian (latest Bartonian s.s.) as R. nyssoides, associated to Ephedraceae and Graminae appearance. most of them have their last occurrence at the top of Bartonian s.s. as for example: P. viesenensis, P. laevigatus, R. densus, R. levicolpus, R. gibbosus/baviensis or R. parisiensis and the grains of Bombacaceae family. Similar palynomorphs have been recorded in the sapropelic marls or organic shales overlying the Biarritzian limestones (Lower Bartonian) in the Armorican Campbon, Saffré or Rennes basins. As a consequence, the Mietesheim Formation is clearly Auversian in age for the 11 to 12,5 m (01986X1046) samples and Marinesian in age for 16 to 6 m (01985X0115) samples. An age Bartonian s.s. is therefore demonstrated for the first time for sediments deposited in the Upper Rhine Graben. Similar or very close palynological assemblages were reported from deep wells in green marls (so called Lymneen mergel or Grune mergel) at the base of Lower Salt Formation in the Mulhouse basin. They were generally assigned to the Priabonian without any datation evidence. The same microflora is also reported from the Lower Salt Formation of Eschau well 10 km south of Strasbourg. As climate and deposit environment are concerned, the Mietesheim associations suggest a freshwater restricted environment favouring anaerobic organic mud deposition. The lake borders are settled by brackish vegetation involving Myricaceae, Nyssaceae, Sparganiaceae and Ruppiaceae whose backside is colonized by a rain forest. A high pluviometry of more than 1500 m/year and a main annual temperature 20°C are typical of tropical climate. The lack of conifers and the reduced herbaceous vegetation suggest a flat continental landscape covered during Auversian times by broad forest surfaces similar to those of South Eastern Asia. With the onset of Marinesian, a short dry season is attested by a moderate arrival of conifers and development of grassland areas. On the Saverne faulting zone, above the Mietesheim formation, the Priabonian evaporitic sedimentation seems to be absent, whereas in the central part of the Rhine Graben to the East the thick series of evaporites correspond to the major subsidence phase beginning in that region during the upper Eocene.
Zusammenfassung
Anlässlich von Bohrungen für die Neubearbeitung der geologischen Karte, Blatt Hagenau (Elsass, Frankreich) ist der in Mietesheim ermittelte Mergel (zur Formation von Mietesheim gehörig), der Kalkstein von Bouxwiller überlagert (Bouxwiller-Formation, Oberlutetium), einer palynologischen Untersuchung unterzogen worden. Im Vergleich zu anderen floristischen Reihenfolgen in Nordwest-Europa (Pariser Becken, armorikanische Becken) präsentiert sich die Microflora des Mergel von Mietesheim mit mehr als hundert Spezies ziemlich reichhaltig und weist im Vergleich zu der zum Pariser Becken gehörenden microflora Formen auf, die im Auversium vorkommen (unterer Teil des Oberbartoniums), andere, die im Marinesium verschwinden (oberer Teil des Oberbartoniums), wie auch zwei Arten, die auf diese zwei Zeitspannen des Oberbartoniums begrenzt sind: Retiovoipollis parisiensis und Retitricolporites gibbosus/baviensis. Der Mietesheimer Mergel, dessen Alter im Oberrheinischen Graben bisher nicht präzise bekannt war, kann daher durch seine microflora auf das Bartonium s.s. datiert werden, denn diese verweist auf einen limnischen Lebensbereich (limnisches Milieu) mit üppiger hygrophiler Vegetation, dessen Randgebiete von einem dichten und feuchten Tropenwald umsäumt werden. Diese Formation geht der evaporitischen Sedimentation des Priaboniums voraus, zu Beginn der Haupttektonikabsenkung des zentralen Abschnitts des oberrheinischen Grabens, die im Priabonium ihren Anfang nahm.
Dernière mise à jour le 04.06.2015