The authors would like to thank Gabriel Karnay and Eric Thomas for their work on the report BRGM RP/57932-FR.
1Recent interest in surficial deposits grew mainly out of the emergence of environmental resource management and land planning concepts. It is also associated with major scientific progress such as understanding the impact of weathering on hard rock aquifers and its consequences for water management (Wyns, 1991; Wyns et al., 2004; Lachassagne et al., 2011).
2In their book on the geology of surficial formations, Campy and Macaire (1989) define natural surficial deposits “as being constituted by sediments and by exogenous (sedimentary and residual) and volcanic rocks that have been exposed to the lithosphere-atmosphere interface since their formation on continents, and as being organized into one or more units of metric to decametric thickness, with or without a genetic relationship with the substratum, but strictly associated with the evolution of the current relief whose lithological expression they represent”. For these authors, natural surficial deposits related to current relief are inevitably recent. This definition, however, excludes paleoweathering features and former continental deposits recorded in the sedimentary sequence.
3A BRGM working group on natural surficial deposits collaborated in formulating the Guidelines for Compiling the Geological Map of France at 1:50,000 scale (Lebret et al., 1993; Lebret et al., in Andreieff & CCGF, 1997). It proposed a definition reflecting the experience of their colleagues (both from the BRGM and elsewhere) in geological mapping: “Natural surficial deposits occur at the surface under the influence of the meteoric processes. Of continental origin, unconsolidated or showing secondary cementation, these formations result from the physical disintegration (clastites), and/or chemical change of pre-existent rocks. They can rest on bedrock (autochtonous formations), be moved over short distances (subautochtonous formations), be moved by geodynamic agents (water, wind, etc.) and secondarily deposited elsewhere (allochthonous formations), and exceptionally buried. Of metric to pluri-decametric( exceptionally hectometric) thickness, they have a relationship with the current topography. Sometimes disconnected from this topography and sometimes uplifted, natural surficial paleoformations (bauxites, Tertiary weathering formations, etc.) are important reflections of the paleotopography. Under the influence of pedogenesis, the surface of natural surficial deposits can develop into soils whose organization has its applicable rules”.
4Dewolf & Bourrié (2008) define natural surficial formations as “continental or littoral formations formed, initially, at the litho-hydro-bio-atmosphere interface”. Like these last authors, Wyns (2007) suggests broadening the notion of natural surficial deposits to all geological periods, including older formations that were later fossilized, and thus proposes to define surficial formations as “any outcropping geologic formation among which the genesis or current properties result from supergene processes, whatever their age”(Wyns, 2007).
5This notion of natural surficial formations is widely accepted by the British authors (McMillan & Powell, 1999).
6For historical reasons, it is the litho-genetic notion of natural surficial formations that is most commonly used in the French geo-community. Consequently there is still considerable confusion with the notion of Quaternary geology which relies only on stratigraphic criteria. The latter underestimates the importance of weathering profiles and misses the extensive temporal dimension of these formations.
7The temptation to substitute another term that is more appropriate, more abstract and less descriptive has often shaken the geological community. Like the Americans and Australians, the French community recently tried to revitalise the discipline with the introduction of the concept of ‘Régolithe’.
8The term ‘Régolithe’ implies a more global approach, associating rocks, paleo-landscapes, physical properties and processes. It considers the geodynamic framework at the origin of the deposit and incorporates the study of paleosurfaces.
9If one accepts the above concept, then a new definition of Regolith should be considered: regolith corresponds to all the outcropping geological formations whose genesis or current properties result from supergene processes (interactions between the lithosphere and the atmosphere, hydrosphere and biosphere), whatever the age. Regolith formations are thus surficial fragmental and unconsolidated geological rocks whose genesis and properties result from successive periods of surface processes. They comprise:
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autochthonous formations resulting directly from successive periods of surface weathering in the upper 200 metres of the Earth’s crust; and
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allochthonous formations in which the unconsolidated deposits result from weathering and transport and have not yet been affected by burial diagenesis.
10The French 1:50 000-scale geological mapping programme effectively began in the 1950s and expanded at the end of 1960s following the integration of the ‘Service de la carte géologique de France’ within the BRGM.
11The main focus at that time was to ensure raw material supplies to secure France’s economic development. It was a period when many mineral-exploration, fossil-fuel exploration, and water-resource development programmes enriched our geological knowledge. Geological mapping was focused on hard-rock geology and intentionally ignored the natural surficial deposits. Only the ‘Centre de géomorphologie du CNRS’, based in Caen, published detailed maps of natural surficial deposits for the Normandy area. It also published a user guide which perfectly enhanced all their economic and academic issues (Dewolf, 1968). BRGM’s first attempts at detailed mapping of natural surficial deposits were in the 1970s (Saint-Bonnet-de-Joux sheet).
12It is only since the end of the 1980s that the Committee of the Geological Map of France (CCGF) has ensured that natural surficial deposits are correctly mapped for the 1:50,000-scale geological maps, whether in basins or basements.
13It must be admitted, however, that systematic mapping of regolith formations in basement areas began later than in sedimentary basins where sedimentologists traditionally mapped loam, fluvial deposits, etc., whereas regolith formations in basement areas had generally been ignored or were poorly understood. The hard-rock geologist often confused weathering profiles, commonly of significant thickness, with Quaternary formations or otherwise destroyed them to expose the unweathered basement.
14As the result of a conceptual evolution in the 1990s, regolith formations now include not only Quaternary sedimentary formations but also residual formations such as granitic sand, clay with flints, etc.
15The aim of the current stage in the regolith mapping programme has been to establish the status of our knowledge in this domain. Its immediate purpose was not to produce an exhaustive and homogeneous map of the regolith in France, but to highlight the quality of the available data.
16The authors used digital maps covering the whole of Metropolitan France to produce two maps representing the allochthonous and autochthonous regolith formations, respectively; slope deposits and the unconsolidated formations of the continental margin were not included. The two maps are based on a critical synthesis of more than 1,000 maps at 1:50,000 scale with no feedback from the field. Harmonized “Département” maps and regional studies were used in some cases. The data from the 1:50,000-scale maps and accompanying guides, and from many other BRGM documents, enabled the authors to plot or create regolith polygons at a scale of 1:1,000,000. Where necessary, the polygons mapped at 1:50,000 scale were grouped and smoothed to enhance their visibility at 1:1,000,000 scale. Many of the 1:50,000-scale geological maps, however, provide little or no information on the regolith formations (and still less on the autochthonous formations); without new field observations, the authors could not determine whether this lack of information was due to a real absence of regolith formations or to gaps in the existing geological maps.
17To illustrate the general heterogeneity in data quality, the authors used a semi-quantitative approach to produce confidence-indexes based on the 1:50,000-scale geological maps and the regional and local studies; the selected criteria are listed in table 1. Confidence index maps were then produced for both the allochthonous and the autochthonous maps.
Tab. 1: Criteria used to assess the data quality concerning regolith formations mapped at 1:50 000 scale.
18Of the 87 boxes describing the map’s regolith formations, 65 concern the allochthonous and 22 the autochthonous formations. The map can be viewed and interrogated at http://infoterre.brgm.fr.
19A ‘pdf’ of the map and corresponding project report can be downloaded from http://www.brgm.fr/brgm/GEO/rego.htm.
20The allochthonous component of the regolith formations is represented by 65 boxes classified according to the genesis of the different geological formations accompanied, where possible, by the age of the formation. For the stratigraphic nomenclature the authors have followed the recommendations of the International Commission on Stratigraphy (IGS) where the base of the Quaternary System / Period is lowered to the Gelasian Stage GSSP (at 2.588 Ma).
21Figure 1 illustrates the increasing precision of the 1:1,000,000-scale Regolith Map of France (fig. 1b) as compared to the former 1:1,000,000-scale Geologic Map of France (edition 6.1; Chantraine et al., 2003; fig. 1a).
22The accompanying confidence index map shows the data to be broadly inconsistent at national scale with large variations in the data quality. Although the allochthonous formations are generally recognized, the polygon outlines are hazy and vary from one 1:50 000-scale map to another. Consequently the polygon geometry is not well constrained.
23Another constraint is that the lithology is not always indicated. For example, fluvial deposits, which represent more than half the represented allochthonous formations and are among the main natural resources in France, are generally only indicated as “alluvium”, which is a genetic definition that does not consider lithology. Moreover, alluvial formations generally consist of a mixture of very different deposit categories such as sand, silt, clay, and conglomerate. In addition, the dating of the alluvial deposits is often questionable, especially along rivers where the age of the alluvial terraces is more often deduced than measured or demonstrated. It is now necessary to study these deposits on a global scale, especially where phenomena of river capture are concerned. The aim is to model river evolution and its consequences on deposit genesis, thickness and lateral variations.
24Considerable confusion still exists as to the characterisation of the loams (LP) that cover broad plateau areas in France. The genesis of these formations is poorly understood and usually authors do not specify between true loess, plateau loam and basement weathering products. LP covers half of France to a depth of up to 10 metres and documenting this deposit more accurately is one of the major challenges facing regolith geologists today.
25To summarise, the allochthonous regolith formations are fairly well described. However, taking into account the influence of human activities on this environment (the spread of built-up areas into flood zones, the demand for high agricultural yields, the destabilisation of dunes in coastal areas, etc.), it is now apparent that the available information is nowhere consistent enough to meet the main development challenges of the future.
Fig. 1: Comparison between the Regolith Map of France and the Geological Map of France at 1:1 000 000 scale.
(a) Geological Map of France at 1:1 000 000 scale, edition 6.1, (Chantraine et al., 20003); (b) Regolith Map of France at 1:1 000 000 scale.
26The autochthonous component of the regolith formations is represented by 22 boxes classified firstly according to the age of formation and secondly according to the facies. However, less than half the known weathering products have been dated, so that most of the boxes are grouped under the “undetermined age” label.
27The confidence index map shows:
28– Very wide variations in data quality. The Paris Basin is the only large area with high-quality data, thanks to research projects on its weathering products (Ménillet, 1993; Quesnel, 1997; Quesnel et al., 2007). The other French basins are either not very well constrained, or the data is of low quality. It is quite likely that the weathering products are largely under-represented.
29–Where the Hercynian massifs of Brittany-Normandy, the Massif Central, the Vosges, Maures-Esterel, etc., are concerned, the 1:50,000-scale geological maps generally do not show the basement weathering products. Some progress in our cartographic knowledge of these deposits has, however, been provided by two recent PhD studies (Thomas, 1999; Brault, 2002), and several studies on the hydrogeological properties of these weathering products have also provided local data of consistent quality. Field mapping and geophysical surveys for the more recent geological maps often reveal widespread weathering profiles developed on the Hercynian basement. Unfortunately these maps do not tally with the earlier maps of the surrounding areas where weathering products are not distinguished despite similar substrata (Fig. 2). This observation now raises questions as to the validity of the outcrop data featured on the earlier geological maps.
30The overall quality of the data concerning the autochthonous regolith formations at national scale is poor, particularly for the basement areas where the thickness of the weathering profiles has been widely underestimated. Weathered basement formations are of great interest in prospecting for natural resources and they also have a considerable influence on hydrological patterns (Lachassagne et al., 2001).
Fig. 2: Cartographic anomalies between recent (northern quadrangle) 1:50,000-scale geological maps of France and earlier (southern quadrangle) 1:50,000-scale geological maps of France that did not show weathering profiles.
31Our inventory of existing geological cartographic knowledge on the regolith formations of France has led to the production of a 1:1 000 000-scale two-layered status map of the allochthonous and autochthonous formations with associated data-quality maps.
32Several points are to be emphasized:
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France as a whole is not uniformly covered, either because the necessary information is lacking or because current knowledge on these formations is patchy;
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there is a lack of data concerning large domains, especially for the autochthonous part of the regolith;
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the relatively uniform coverage of the allochthonous part of the regolith nevertheless masks wide disparities.
33Based on this inventory, it is now possible to upgrade our knowledge to work towards a better understanding of regolith formations. The next step will be a) to return to the field, and b) to integrate geophysical data in order to produce petrophysical maps and 3D thickness models. The models will show the depth to the bedrock surface, analysing information from borehole logs held in the BRGM archives and using the mapped extent of regolith formations from the 1:1,000,000-scale digital geological map of France. This work is critical for civil engineering projects and is important for evaluating groundwater resources and possible water pollution and for predicting surface hazards such as landslides and the collapse of underlying rock.
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