Bedrock versus superficial deposits in the Swiss Jura Mountains: what is the legitimate soil parent material? Loraine Martignier, Thierry Adatte and Eric P. Verrecchia * Institute of Geology and Palaeontology, University of Lausanne, Lausanne, Switzerland Received 27 January 2012; Revised 3 May 2012; Accepted 9 May 2012 *Correspondence to: Eric P. Verrecchia, Institute of Geology and Palaeontology, University of Lausanne, Lausanne, Switzerland. E-mail: eric.verrecchia@unil.ch ABSTRACT: Soils are generally considered to be the result of in situ weathered products of geological substrata. Nevertheless, the Earth’s surface is mainly composed of superficial deposits made of sediments reworked during periods of climate instability (glaciation, aridification, flooding, etc.). But to what extent do these deposits influence present-day soil development and/or lessen the contribution of the geological substratum? In the Swiss Jura Mountains, characterization and distribution of superficial deposits along a toposequence are used in order to decipher the polygenic sources of soil parent materials and question the importance of lithodependence in soil development. The in situ weathering material and the allochtonous deposits are discriminated through analysis of mineralogical composition, grain size distribution of the soil mineral fraction, and multivariate statistical treatment. As expected, calcite is dominant in the autochonous mineral phase, leading to high pH values in deep soil layers. However, the mineralogical composition of the surficial deposits is made of local mineral components mixed with allochtonous aeolian deposits. This exogenous material is dominated by quartz and phyllosilicates, and is reflected by low pH values in soil surficial horizons. The grain size distribution curves of the insoluble residue from bedrocks are unimodal and dominated by fine loam and clays. In contrast, surficial deposits display polymodal curves, demonstrating the contribution of various aeolian episodes and/or sources. Analysis of the surface texture of quartz sand grains shows glacial marks intersected by aeolian shock marks, indicating the posteriority of wind transportation. Loess material originates from Alpine moraines of the Swiss Plateau and is mixed with deposits originating from more proximal Jura moraines, diluting the Alpine signature. Consequently, the soil development does not follow the expected and conventional decalcification process, but is directly influenced by the origin of the primary sediments (even when present only in small amounts) and their evolution. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: Jura Mountains; soil parent material; lithodependence; superficial deposits; loess deposits Introduction The surface of the continental Earth is constantly reworked at different scales of time and space. To find a monogenic deposit in which soils are developing is a real challenge (Schaetzl and Anderson, 2005). This is particularly true in the temperate zone of Europe where glaciations left their imprint throughout the Quaternary. In addition to the various moraine and fluvio- glacial deposits, periglacial processes, such as cryoturbation and solifluction, have deposited cover-beds along the slopes. Cover-beds can be characterized by distinct layers, according to their depth, coarse material content, and texture (Kleber, 1992, 1997). The ‘Basal layer’ contains rock fragments originating from local geological material. The ‘Intermediate layer’ is generally enriched in aeolian particles and was possibly formed during the Pleniglacial (Mailänder and Veit, 2001). This layer is irregular and linked to areas of loess deposition. Finally, the ‘Upper layer’ is composed of reworked material (including loess) and clasts. It presently covers the slopes with a relatively constant thickness of about 50 cm (Kleber, 1997) and was probably formed during the Younger Dryas (Mailänder and Veit, 2001). Periglacial cover-beds, as well as the entire range of superficial deposits defined by the French authors (Dewolf, 1965; Campy and Macaire, 1989; Joly, 1997), have a crucial influence on pedogenesis, as these deposits provide the mineral material in which Holocene soils develop (Terhorst, 2007; Dewolf and Bourrié, 2008; Semmel and Terhorst, 2010). Consequently, the concept of soil parent ‘rock’ can be questioned and it seems more judicious to use the concept of soil parent ‘material’, a material that can be polygenic in origin. For instance, in the western Swiss Jura Mountains, soils are forming a true mosaic influenced by the diversity of superficial deposits in contrast to the more homogeneous lithological nature of the geological substratum (the supposed soil parent rock), which is composed of Mesozoic limestones and marls. Indeed, a local ice sheet was thought to have formed during the Würm (i.e. the last glacial period in the Alps, from 80 000 to 10 000 BP), approx- imately centered over the Joux valley (Figure 1; Aubert, 1965). This ice sheet was limited at the southeast of the Jura foot slope by the Rhone glacier originating from the Alps. As a conse- quence, former soils of the southwest Jura Mountains have been eroded due to glacial and periglacial processes. During the EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 38, 331–345 (2013) Copyright © 2012 John Wiley & Sons, Ltd. Published online 27 June 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3274