Thesis #847

Pleistocene fluvial gravel terraces appear to be useful to decipher neotectonic movements. To achieve this, however, chronologic and sedimentologic data on the terraces are required. Thus, this thesis represents a multidisciplinary approach. After presenting methodological aspects of U/Th (Uranium-series disequilibrium) dating, compared to OSL (Optically Stimulated Luminescence), the ages obtained are used, together with sedimentological data, to constrain the formation processes and timing of the Late Pleistocene terraces in the Hochrhein area. Finally, the geometry of these terraces is interpreted in terms of neotectonics. Hitherto, dating coarse-grained sediment has proved often impossible due the lack of datable material, and developing new methods in this domain is thus of crucial importance. U/Th is a well established method, often used for speleothems and coral dating, and it is tested here on pedogenetic crusts growing within the Late Pleistocene gravels of the Hochrhein area. These results are compared to OSL ages from the same sampling sites. Our results show that this method has a development potential, but is highly dependant on a sufficient and stable Uranium content of the samples, which is partly controlled by bacterial activity. Comparisons with OSL ages highlight the fact that the event that is dated by mean of U/Th is the precipitation of the crust, not the deposition of the sediment. Sedimentological and morphological data show that the Late Pleistocene gravels (Lower Terrace) were deposited as a braided river, where flood events played a major shaping role. The gravels are arranged into terrace levels, where the highest level is an accumulation level, and the lower levels are erosion level with minor re-accumulation. The flood deposits are mostly conserved on top of the different terrace levels because the general incision regime prevented them from being reworked. OSL ages show that the main gravel accumulation occurred between 27 an 11 ka in the Hochrhein area, but minor re-accumulation occurred until historical times. The accumulation level was thus formed during the Younger Dryas, and the lower levels were formed from the Youger Dryas on, until historical times. These results are confirmed by radiocarbon ages from fossil trees and U/Th ages from speleothems. Terrace longitudinal profiles are used to assess neotectonic activity in the Hochrhein area. Four main groups of Pleistocene terrace exist in this area: the Earliest Pleistocene Higher Deckenschotter, the Early Pleistocene Lower Deckenschotter, the Middle-Late Pleistocene High Terrace and the Late Pleistocene Lower Terrace. The three older terrace groups do not show original surfaces anymore, so outcrop altitudes are used to build the longitudinal profi le. This
profi le shows that the gradient of the terraces increases with their age, and each of the terrace
groups show an increased gradient at the location where the profi le crosses the eastern main border
fault (MBF) of the Upper Rhine Graben (URG). The terraces converge, cross and fi nally continue
as basin fi lling. This suggests a relative uplift of the headwater area and extensional activity for
the MBF during Pleistocene. The Lower Terrace features still well preserved surfaces and these
are analysed with a high precision Digital Terrain Model (DTM). Using a high precision DTM
highlights the complex arrangement of sublevels and the irregularities of the surfaces, which are
mostly caused by sedimentary and erosive processes. NW of Basel in the URG, fi eld evidences
indicate Late Pleistocene activity of NW-SE striking normal faults.