Loess, alluvions et dépôts de pente du Fossé rhénan en Alsace de l’Eemien à l’Anthropocène : approche pédosédimentaire, géomorphologie et chronostratigraphie

A chronostratigraphic framework of sedimentary deposits of the last 130 000 years is proposed for the Alsatian part of the Upper Rhine Graben based on about sixty profiles, most of them examined during operations of rescue archeology. It is based on pedo-sedimentary archives and micromorphological data and completed with numerical datings (radiocarbon and OSL). Since 130 ka, the Eemian and the Weichselian Early Glacial are represented by a succession of paleosoils, covered by loess deposits from 70 ka onwards. Between 70 and 35 ka, these latter hosted the development of several arctic brown soils during interstadials of both lower and middle Pleniglacial, also characterized by localized fine-grained alluvium from the Rhine in alluvial sequences. The Upper Pleniglacial (35-16 ka) is characterized by thick, often sandy, loess deposits and by gravel beds respectively in loessic and alluvial sequences. From 16 ka onwards, a major incision episode of the hydrographic network isolated the Ill course from that of the Rhine in the southern half of Alsace. The Lateglacial period (14,7-11,7 ka) is only represented in the Ill valley by fine-grained alluvium and paleosoils. During the Early and Middle Holocene, pine-rich sandy alluvium are deposited in the Rhine plain while the Eschau gravel body structured the course of the Ill and the Rhine around Strasbourg. The development of a widespread luvisol took place in the Ill, Thur, Bruche and Zorn floodplains. This luvisol is truncated by torrential alluvium and colluvial deposits that occurred at the foot of the slopes from the first millenia BCE onwards. At least from the Middle Age, the hydrographic network is artificialized around the main cities, although the main anthropogenic adjustments mostly occurred during the 20th century. Finally, climate, relief and inherited landforms have been the main factors controlling the graben’s evolution for the last 130 000 years, until the historical periods during which human earthworks have prevailed. However, the role of neotectonics must be re-evaluated on the basis of complementary profiles to assess the thickness of the Weichselian alluvium while a better spatial resolution is needed as well.