The Jura Mountains are mainly constituted by Mesozoic limestone and marls. However, Quaternary glaciations strongly reworked sediments and left a mantle of mixed autochtonous and allochtonous superficial deposits (regolith) covering the landscape. The aim of this thesis was to study the origin and composition of these mineral deposits, as well as their respective influence on Holocene soil development, along two toposequences of soils situated in the southeastern part of the Swiss Jura Mountains (Amburnex valley and Ballens hillslope).
Superficial deposits were identified according to their mineralogical and geochemical compositions, their grain size distribution, and the surface texture of quartz sand grains observed by scanning electron microscopy. The results allowed the discrimination of three reference sediments, the limestone bedrocks, the alpine loess deposits, and the alpine silicate moraines, which were then assorted during the formation of periglacial cover-beds and mixed till deposits (including silicate and carbonate fragments). The observation of sediment sequences along both studied hillslopes enables to reconstitute the relative chronology of superficial deposit settings, related to various transport dynamics (glacial, periglacial or aeolian) and landscape history, since the LGM (last glacial maximum). Cover-bed is the most surficial, and consequently the most recent, deposit and was formed by solifluction processes, seemingly during the Younger Drays cold phase (about 12'600-11'500 years cal. BP). This sediment constitutes the initial parent material for Holocene pedogenesis.
The influence of superficial deposits on weathering processes and soil development was studied through the mineralogical composition of phyllosilicates, completed by observations of clay minerals with a transmission electron microscope and by soil micromorphology. Pedogenesis is characterized by three main dynamics, respectively influenced by Ca, Fe and Al. These dynamics can be related to three main types of mineral parent material (limestone, alpine loess deposits and alpine silicate moraine) and induce the dominance of decarbonation, decalcification, brunification, and clay leaching processes in soils. In the <2 mm mineral fraction, calcite, phyllosilicates and Fe-oxy-hydroxydes undergo weathering, transformation and precipitation. Secondary mineral phases are formed and seem to play an important role in element cycles inside the soil system.
Consequently, the results obtained during this thesis allowed to emphasize that present-day soils in the Swiss Jura Mountains are forming since the beginning of the Holocene (about 11'500 years cal. BP). They develop inside complex mineral parent materials, which are constituted of various sediments inherited from the last glacial phase. The limestone bedrock is generally covered by superficial deposits, which disturb the conventional genetic link between soils and their underlying bedrock, and this at various levels according to the regolith thickness. Allochtonous deposits bring exogenous minerals and geochemical elements in soils, and thus oriented pedogenesis towards non-calcareous evolution pathways.