5th Swiss Geoscience Meeting, Geneva 2007
The Paleogene of the southern Upper Rhine Graben:
biostratigraphy, palaeoenvironment and reworking
problems
Pirkenseer Claudius*, Spezzaferri Silvia* & Berger Jean-Pierre*
*Section Géologie & Paléontologie, Université de Fribourg, Chemin du Musée 6, CH1700 Fribourg (claudiusmarius.pirkenseer@unifr.ch, silvia.spezzaferri@unifr.ch, jeanpierre.berger@unifr.ch)
The Upper Rhine Graben represents the central part of the Tertiary European
Continental Rift System (ECRIS) and extends about 300km on an approximate
North-South axis between the cities of Wiesbaden (W Germany) and Basel (NW
Switzerland). The study of the entire Paleogene succession of the southern
Upper Rhine Graben by means of two boreholes and several critical outcrops
allows a more complete view of the biodiversity, palaeogeographic and
palaeoenvironmental evolution of the Tertiary deposits in this area (PIRKENSEER
2007).
The interpretative analyses is based on an as complete as possible registration
of the occurring microfossil taxa. As main groups about 150 benthic planktonic
Foraminifera and 50 Ostracoda taxa are recorded from Rupelian strata of the
research area. While the mainly lacustrine and terrestrial Eocene deposits
remain enigmatic due to very scarcely occurring fossil material, the two
Rupelian transgression-regression cycles (Earliest Rupelian “Zone fossilifère”,
Late Rupelian “Série grise”) of the Upper Rhine Graben are well traced by the
two main microfossil groups.
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 autochthonous 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 to anoxic conditions in the
subsequent “Schistes à poissons”. The latter is indicated by a relatively high
abundance of dwarfed oxic species and suboxic to anoxic taxa.
The corresponding regressive phases of the “Zone fossilifère” and the “Serie
grise” are characterised by mass occurrences of brackish ostracod taxa as in
the respective uppermost parts of both units. The succession from marine to
coastal ostracod assemblages is more pronounced in the final unit of the
second Rupelian transgression (“Marnes à Cyrènes”). While the regressive
phases at the drill site Allschwil-2 show certain shallow marine-freshwater
fluctuations in their respective upper parts (charophytes, freshwater Ostracoda),
the transition from shallow marine to exclusively brackish condition in the
“Marnes à Cyrènes” (uppermost “Série Grise”) is rather short and abrupt.
Palaeobiogeographic relations between the ostracod assemblages of adjacent
Tertiary basins confirm a marine incursion of northwest European provenance
in the Early Rupelian “Zone fossilifère”. This interpretation is reinforced by
recently submitted data of a similar, but higher diverse ostracod fauna derived
from a thicker marine to brackish succession of equivalent sediments from the
5th Swiss Geoscience Meeting, Geneva 2007
Mainz Basin (GRIEßEMER et al. 2006). 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 “Serie grise” suggest
at least occasional connections between the two basins in the Late Rupelian.
The biostratigraphic data derived from planktonic Foraminifera assemblages
ranges is distorted by the simultaneous occurrence of facultatively reworked EoOligocene species (e.g., Subbotina utilisindex) and requires a conservative
estimate of a Mid P20 to P21a age for the “Serie grise” deposits. The Age
NP23-24 indicated by calcareous nannoplankton in the basal “Serie grise”
corresponds to other datings from the Upper Rhine Graben. Charophyte
assemblages from the “Marnes à Cyrènes” and the lower part of the
Niederroedern Formation suggest an attribution to the Late Rupelian to early
Chattian Chara microcera – zone.
Abundant reworked Cretaceous (10 taxa, e.g. Marginotruncana
pseudolinneiana) and Late Paleocene to Eocene (20 taxa, e.g. Acarinina
bullbrooki, Turborotalia cocoaensis) planktonic Foraminifera from the Late
Rupelian “Serie Grise” occur in the research area and many additional localities
in the Upper Rhine Graben and the Mainz Basin. At least Late Cretaceous,
Ypresian, Lutetian and Priabonian ages of source sediments are indicated by
the reworked specimens. The Lutetian is by far the most frequently represented
stage. Abundant reworking starts in the Late Rupelian lower “Couches à
Mélettes” and reaches its acme in the increasingly clastic “Marnes à Cyrènes”.
Only sparse records are documented from the subsequent fluvio-lacustrine
Niederroedern Formation.
The Foraminifera are supposed to be reworked from the alpine domain, as
other likely source areas are to far away and/or were not subject to erosion in
the Late Rupelian. This accords with the proposition of a vast northwards
prograding delta-system by ROUSSÉ (2006) that was located close to the
southern margin of the research area. Geodynamic implications include already
uplifted Early to Late Eocene sediments within the central and southwestern
Helvetic realm as well as a beginning sedimentary overfill of the western Swiss
Molasse Basin in the Late Rupelian that led to the northward deviation of an
axial drainage system of the Swiss Molasse Basin (“pre-Genferseeschüttung”).
REFERENCES
Grießemer, T.W. et al. 2007, in press: Die Ostracoden der Mittleren
Pechelbronn-Schichten (Unter-Oligozän) in der Bohrung Wallau B98-BK5,
östlich von Wiesbaden (nördliches Mainzer Becken). Geologische
Abhandlungen Hessen, 116.
Pirkenseer, C. (2007): The Southern Upper Rhine Graben Paleogene:
Microfossil assemblages (focus on Ostracoda and Foraminifera) and their
palaeoecologic, biostratigraphic and palaeogeographic implications in reference
to reworked planktonic Foraminifera. PhD thesis, 320pp, Fribourg.
Roussé, S. (2006): Architecture et dynamique des séries marines et
continentales de l´Oligocène Moyen et Supérieur du Sud du Fossé Rhénan:
Evolution des milieux de dépôt en contexte de rift en marge de l´avant-pays
alpin. PhD thesis, 471pp, Strasbourg.