Ph.D. Oberrheingraben

Eva-Marie Hagedorn (2004): Sedimentpetrographie und Lithofazies der jungtertiären und quartären Sedimente im Oberrheingebiet

The complete online version can be downloaded from:

http://kups.ub.uni-koeln.de/volltexte/2004/1253/


Summary:

(German Version)

In this study, Quaternary and Upper Tertiary unconsolidated sediments from the Upper Rhine Graben were petrographically analysed in numerous high quality core drillings. In addition, the sedimentary lithofacies were investigated under direction of the LGRB Baden-Württemberg. The chronological classification of the sedimentary successions was established by the interpretation of sporadic palaeontological and palaeomagnetical data. Furthermore, sediments of the Graben margins were examined in outcrop and drill core. Results from these investigations allow interpretations with respect to the development and the stratigraphic organisation of the Quaternary and Upper Tertiary unconsolidated sediments of the Upper Rhine Graben.

 

tl_files/Eva/Bohrpunkteneu.gif

Location of the drinllings and outcrops

In the southern part of the Upper Rhine Graben, the Pleistocene sediment successions can be subdivided into the Breisgau-Formation and the Neuenburg-Formation (Jüngere Schotter). On the basis of the performed heavy mineral data, the Breisgau-Formation can de divided into the Untere Breisgau-Schichten, which represent thick successions of weathered gravel with a relatively dense structure and minor amounts of hornblende, and the Obere Breisgau-Schichten, which are characterised by a notably higher hornblende content, increasing proportions of alpine rocks and a more loosely packed sediment structure. It is uncertain whether the differences in the heavy mineral assemblages of the Untere und the Obere Breisgau-Schichten are the result of a change in the catchment area or reflect the effects of weathering. The Neuenburg-Formation (Jüngere Schotter) is characterised by fresh pebbles of dominantly alpine origin, but cannot be distinguished from the Obere Breisgau-Schichten by heavy mineral analysis. Within the Neuenburg-Formation two coarse-grained layers of probably glacial origin can be correlated across the drilled profiles.

In the southernmost section of the Upper Rhine Graben, the Pleistocene sediments are underlain by Oligocene deposits, while further to the north, Pliocene deposits of the Iffezheim-Formation occur beneath the Quaternary strata. The Upper Tertiary sediments derived from the Graben margins without alpine supply can be clearly distinguished by their different heavy mineral assemblage from the alpine dominated sediments, which were accumulated in the Upper Rhine Graben since the uppermost Pliocene.

A comparison of sediments from the Iffezheim-Formation identified in several drill cores revealed apparent differences in heavy mineral assemblages: In drill cores from the South Graben (Hartheim, Nambsheim, Plobsheim) this succession can be divided into an upper section, which is dominated by stable minerals (zircon, turmaline, anatase), and a lower section characterised by a higher percentage of unstable minerals (garnet, hornblende and epidote). In contrast, in the examined drill cores of the North Graben (Speyer, Schifferstadt, Ludwigshafen-Parkinsel and Osthofen) only Pliocene sediments with a mixed heavy mineral assemblage (with both unstable and stable minerals) were identified. These are similar to those found in the lower section of the Iffezheim-Formation in the South Graben.

Along the Upper Rhine Graben, the greatest thicknesses of Quaternary sediments occur in the southern and northern part of the Graben. Both areas are separated by the intervening threshold of Karlsruhe, where only reduced Quaternary successions have been preserved. Due to this internal structure of the Upper Rhine Graben the correlation between the Quaternary successions of the South and the North Graben is problematic. Additionally, during this study only few sufficiently deep drillings in the northern Graben were available, which traversed the entire Quaternary succession and reached Tertiary sediments thus offering the potential of comparing the sedi­mentary successions from the southern and the northern Graben.

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Profile through the northern Upper Rhine Graben (after Kärcher 1987)

In the middle and northern Upper Rhine Graben the Lithofacies pattern of the Pleistocene sediment formations from the southern Graben cannot be traced any further. The character of the Graben sediments changes towards the north as a result of the increasing sorting of the sediments and the mixture with material from the margins of the Graben. Therefore, the newly defined stratigraphic scheme for unconsolidated sediments of the South Graben cannot be applied to the sediment successions of the middle and northern Upper Rhine Graben so far. In these areas, this study adheres to the established distinction between gravel-layers („Kieslager“) and intercalated fine-grained horizons („Zwischenhorizonte“), whose chronological classification has been revised on the basis of more recent age contraints.

The analytical results obtained for the northern Upper Rhine Graben show that the Pleistocene Rhine did not flow along the western margin of the Graben, with the exception of a short period of time at the beginning of the Quaternary and the end of the last ice age. Here sediments up to 70 m thickness and a stable heavy mineral assemblage occur, which were probably derived from the Buntsandstein areas of the Pfälzerwald. These local sediments are of Quaternary age as indicated by underlying alpine Rhine deposits as well as by their pollen contents. Petrographical analyses preclude an influence from the Rhine during this period of deposition. Sediments of this type have been identified in the drillings of Schifferstadt, Speyer, Ludwigshafen-Maudach and Osthofen. In the drilling profile of Ludwigshafen-Parkinsel, repeated changes between Rhine-derived and local sediments are apparent. In the eastern section of the Graben, the alpine-dominated heavy mineral assemblage of the Quaternary sediments provides evidence for a prolonged influence of the Rhine (drillings in Mannheim, Schwetzingen).

As reasons for the changing course of the Rhine tectonic and climatic control factors can be assumed. As a result of the great subsidence near the eastern margin of the Graben ("Heidelberger Loch"), the Rhine may have occasionally followed this depression. Furthermore, tectonic move­ments of the Graben blocks (western and eastern Graben block) could have influenced the course of the fluvial drainage system. In addition, changes in the sediment supply between high glaciation and late glacial times as well as during the interglacial periods have to be considered.

In some drill sections of the North Graben, the heavy mineral assemblages of the uppermost metres are characterised by decreasing alpine influences, while local contributions become more important. This could be caused by an interruption of the fluvial sediment transport from the Alps by the Bodensee basin during the Holocene.

The results of this study provide important information with regard to the evolution of the Upper Tertiary and Quaternary sediment-fill of the Upper Rhine Graben. Some new questions derived from the presented data may be answered by continued drilling research and the application of various analytical methods.