|Departement Geowissenschaften – Universität Freiburg
Foraminifera, Ostracoda and other microfossils of the Southern Upper Rhine Graben
About 270 samples from two drillcores and several field outcrops covering the entire Paleogene succession of the southern Upper Rhine Graben were investigated on microfossils. The main focus was aimed at a complete record of Foraminifera (about 180 taxa) and Ostracoda (50 taxa) and their palaeoenvironmental, palaeogeographic and biostratigraphic implications. A thorough taxonomic description of the ostracod assemblages was carried out. Additionally treated fossil groups include Charophyta, fish otoliths, Bolboforma and calcareous nannoplankton. For the first time bird remains are documented from the southern Upper Rhine Graben.
While the entire Eocene succession proved to be nearly barren of fossils, the two Rupelian transgression-regression cycles (Earliest Rupelian “Zone fossilifère”, Late Rupelian “Série grise”) in the Upper Rhine Graben are well traced by the correlating microfossil assemblages.
Both transgressions start off with fully marine outer shelf to upper bathyal Ostracoda and Foraminifera assemblages. In both cases the maximum flooding is achieved early in the sequence cycle. In the second Rupelian transgression the benthic and small sized planktonic Foraminifera associations indicate cold to temperate water temperature. While generally well-oxygenated bottom waters and sediments prevailed during the initial phase (“Marnes à foraminifères”), they abruptly changed to suboxic and anoxic conditions in the subsequent “Schistes à poissons”. Nannofossils indicate increased freshwater influx or occasional over-stratification in this unit.
The corresponding regressive phases of the “Zone fossilifère” and the “Série grise” are characterized by mass occurrences of brackish ostracods. The two species Hemicyprideis helvetica and H. basiliensis were analysed against the palaeoecologic background of the “Marnes à Cyrènes” and are now interpreted to represent two ecomorphotypes of the species Hemicyprideis helvetica.
Palaeobiogeographic relations confirm a marine incursion of northwest European provenance in the Early Rupelian “Zone fossilifère”. Similar results apply to the “Série grise” deposits. While the Upper Rhine Graben was not connected to the Paratethys during the first Rupelian transgression, some simultaneously occurring ostracod species and the benthic Foraminifera associations of the “Série grise” suggest at least occasional connections between the two basins in the Late Rupelian. This interpretation is supported by a higher rate of endemism of the ostracod assemblages in the “Zone fossilifère” than in the “Série grise”.
The data derived from planktonic Foraminifera assemblage ranges is distorted by the simultaneous occurrence of facultative reworked Eo-Oligocene species and implies an estimate of a Mid P20 to P21a age for the “Série grise” deposits. This corresponds to a NP23-24 age indicated by calcareous nannoplankton in the basal “Série grise”, by Charophyte assemblages from the “Marnes à Cyrènes” and the lower part of the Niederroedern Formation (possibly Chara microcera – zone) and by a Bolboforma event in the middle “Marnes à Cyrènes” (Bolboforma latdorfensis zone). The occurrence of Palaeolebias symmetricus (OT-02 zone) suggests a Late Rupelian to earliest Chattian age for the Niederroedern Formation in the research area.
Abundant reworked mainly Cretaceous and Lutetian to Priabonian planktonic Foraminifera from the Late Rupelian “Série grise” are documented, figured and interpreted in relation to similar occurrences in the northern part of the Upper Rhine Graben and the Mainz Basin as well as possible source areas. Abundant reworking starts in the Late Rupelian lower “Couches à Mélettes” and reaches its acme in the “Marnes à Cyrènes”. Only sparse records are documented from the subsequent Niederroedern Formation.
The Foraminifera are supposed to be reworked from the alpine domain by a northwards trending drainage system in accordance with a extensive prograding deltaic-system located close to the southern margin of the research area. Geodynamic implications include already uplifted Eocene sediments within the central and southwestern Helvetic realm in the Late Rupelian.
Jury: Prof. Jean-Pierre Berger, Leiter der Dissertation, Universität Freiburg, Schweiz
Prof. Bettina Reichenbacher, Ext. Gutachterin, Universität München, Deutschland
Dr. Peter Schäfer, Ext. Gutachter, Landesamt für Geologie und Bergbau,
Rheinland Pfalz, Deutschland
Dr. Silvia Spezzaferri, Interne Gutachterin, Universität Freiburg, Schweiz
Prof. Andreas Strasser, Jurypräsident, Universität Freiburg, Schweiz