Découverte d’un paléosol néogène dans les Monts du Forez au sein du complexe volcanique de Montclaret-Fontvial : conséquences morphotectoniques régionales

Abstract

Located on the eastern slope of the Forez Mountains horst, close to the village Fontvial (Fig. 1 and Fig. 2), the small Montclaret hill consists of a volcanic and sedimentary complex. Recent excavations in the volcanic deposits have revealed a fossil soil developed upon a basaltic lava flow. The current note presents stratigraphic, mineralogical and geochemical data on this paleosoil and the related basaltic lava, and discuss these results in the framework of the regional paleogeographical, tectonic, sedimentary and climatic evolution of the Forez basin during the Neogene. Several previous authors have described the Montclaret volcanic complex (Bobier et Glangeaud, 1960; Hernandez, 1971; Tomas, 1971; Gourgaud, 1973; Le Griel, 1975). All authors agree that there were two eruptive episodes; material peperitic deposits intruded by a basaltic dyke. Relationships between the volcanic material and the surrounding detrital sediments have remained less well defined, especially in terms of ages; the Montclaret sediments were most often considered Oligocene, but no direct evidence supports this age. K/Ar dating of a basaltic intrusion at Montsupt, located a few kilometers from the Montclaret complex (Fig. 1), as well as other samples from the basin has given Lower to Middle Miocene ages (Bellon and Hernandez, 1979). Our recent investigation has lead to a more precise definition of the stratigraphy of the sedimentary sequence and its relationships with the volcanic events and enabled extrapolation of some major inferences for the Neogene evolution of the area as a whole. Description of the Montclaret section (Fig. 3) The lower part of the Montclaret section is made up of a 30m-thick detrital formation (1, Fig. 3) consisting of sands and micro-conglomerates with some lenses of clayey, sand-bearing silts. Conglomerates are more abundant in the upper part of this formation (1b, Fig. 3) and contain blocks of various size (from a few centimetres to decimetres) and of a varied nature with quartz, granite and gneiss. Some basaltic dikes (2, Fig. 3) cut through these detrital deposits. An erosion surface affects the top of the formation. The overlying yellowish volcano-sedimentary tuffs (3, Fig. 3) are made up of small fragments of basalt in a fine-grained matrix. These tuffs are capped by a 10m-thick basaltic lava flow (4, Fig. 3) which shows a reddish and brecciated weathered top (photo 1): this alteration exhibits a clear vertical gradient and is underlined by carbonates. This paleosoil is truncated by an erosion surface. A 50 cm-thick formation consisting of dark brown clayey sand-bearing silts (5, Fig. 3 and photo 1) overlies the weathered lava flow. Finally, 30 m-thick peperitic deposits (6, Fig. 3) covers these sediments. This volcano-sedimentary formation is made of rounded sub-aphyric basaltic blocks within a micro-conglome-ratic matrix consisting quartz, feldspar and few altered basaltic grains. The basaltic blocks show vitric rims indicating water quendring of the magma. They define a well-marked internal stratification. The overall sequence dips south-east and is cut through by a network of basaltic dikes (7, Fig. 3). The section demonstrated three important points: (i) Neogene sediments exist upon the piedmont plateaus of the granitic horst. The capping conglomerates of the lower detrital formation (1b) may be correlated to sediments from the nearly locality of Bazourges which have been dated as Lower to Middle Miocene (Gerbe et al., 1998). (ii) At least three successive eruptive episodes are recorded at Montclaret; b1 dikes intruding the 1b sediments, b2 consisting the volcano-clastic tuffs and the weathered lava flow, and b3 corresponding to the peperitic deposits and the terminal dikes. (iii) The time interval between b2 and b3 was sufficiently long to be marked by the weathering of a basaltic lava flow with the development of a 3 m-thick soil, and by the sedimentation of fluviatile sediments upon the soil after a period of erosion. Mineralogy of the paleosol (Fig. 4) The b2 lava flow consists an olivine- and clinopyroxene-bearing basalt. The top of the lava flow presents an weathering profile which can be divided into three parts from bottom to top (Fig. 4 and photo 1): S1 - a green-brown cohesive saprolite in which the lithologic texture of the basalt is more-or-less preserved and which contains some beidellite and vermiculite; S2 - a green-brown breccia-like crumbly saprolite containing calcitic concretions (up to 45% of the rock volume), some fragments of the cohesive saprolite S1 and beidellite; S3 - a red crumbly saprolite in which montmorillonite tends to replace beidellite. Calcite forms randomly distributed concretions and a discontinuous crust on the top of the layer. Some kaolinite fills small cracks at the top. Over an erosion surface, the fluviatile argillo-arenaceous sediments Sd, in direct contact with the red saprolite S3, consist 10-15% quartz fragments with minor quantities of feldspar, basaltic, granitic and calcitic grains, in an argilaceous matrix (illite, montmorillonite and kaolinite). Some 5mm-wide shrinkage cracks filled with kaolinite also appear in the sediments. Kaolinite, illite, vermiculite and smectites have been identified by X-ray diffraction. Microprobe analyses confirm the occurrence of smectites in S2 and S3 and defines their nature: they are aluminous di-octaedral smectites. In S2, ferric beidellite occurs ((Si7.1Al0.9) (Al2.3Fe1.1Mg0.6) (Ca0.3Na0.03K0.03)). Their Mg-content increases in S3 leading to montmorillonite. Carbonates within S2 and S3 are Mg-calcite (Ca0.75Mg0.25CO3). The vertical zonation of the alteration profile (photo 1) as well as the distribution of the clay minerals (Fig. 4) are unambiguously related to a pedogenetic degradation of the basalt b2. Kaolinite in sediments Sd suggests also a pedogenetic alteration, but of less importance. These pedogenetic events are discontinuous over time as attested to by the erosion surface at the top of S3. A major pedogenesis affected the basaltic lava flow and led to a mature red soil. Red soils have been described on Miocene basalts of other areas of the French Massif Central (Cantal: Chesworth et al., 1983; Coiron: Moinereau et al., 1972) and are commonly related to a fersiallitic evolution. Mineralogical characteristics of Montclaret red soil are the dominance of 2/1 clay minerals and red colouration due to oxidation of iron, which is compatible with a fersiallitisation (Duchaufour, 1997). The red soil was truncated prior to the emplacement of the sediments Sd. Another phase of meteoritic weathering then occured leading to the cristallization of kaolinite. Geochemistry of the paleosoil (table 1, Fig. 5) On the chemical trends (Fig. 5A), the argillo-arenaceous sediments Sd analyses plot outside the chemical evolution trends for the red saprolite and its basaltic parent rock. Their high content in silica can be correlated with the abundance of quartz grains. Geochemical evolution of the alteration profile (Fig. 5A) is characterized, from the basalt to the top of S3, by a sharp decrease in silica (47® 17%), in iron (16® 4%) and alumina (16® 6%), and by a correlated increase in calcium (7® 27%) and magnesium (7® 11%). This evolution seems incompatible with leaching processes of fersiallitisation (Duchaufour, 1997) which are characterised by high mobility of Ca2+, Na+, Mg2+ et K+. Within Miocene fersiallitic soils in Cantal (Chesworth et al., 1983), the SiO2/(Al2O3 +Fe2O33) ratio exhibits a systematic decrease from bottom to top of the alteration profile (Fig. 5B) and significantly reflects the fersiallitic evolution. This ratio exhibits no significant trend within the Montclaret soil (Fig. 5A). The chemical data cannot account for a simple fersiallitic process but they suggest an external input of calcium. Consequently none of the elemental variations can be used as an alteration index. Interpretation Mineralogical and chemical data attest of a multi-stage discontinuous weathering during the Neogene in relation with environmental changes under a tropical climate. The first stage is the meteoric weathering of the basaltic flow. A leaching phase could be responsible for the formation of type 2:1 clay minerals and the development of the rubefaction in a well-drained environment. Then an external input of calcium leds to the precipitation of Mg-calcite within the red soil indicating a decrease of drainage properties. After this, the soil was partly eroded and then buried under alluvial detrital sediments, which were consecutively affected by weathering in an alternatively water logged and dry environment. Type 1:1 clay minerals then crystallized inside the desiccation cracks formed in the sediments, suggesting a tendency to a vertisol evolution. This second stage is minor and poorly developed. Morphostructural and volcanic evolution This study confirms that some Neogene sediments occur on the piedmont plateaus on the eastern edge of Forez horst. The detrital sedimentary series attains in 50 m thickness and can be attributed to the Lower Miocene by correlation with recently dated proximal deposits (Gerbe et al., 1998). The sedimentological characteristics of these deposits indicate that they were layed down as alluvial cones during a phase of erosion resumption in a warm desert type climate. Similar sedimentological and climatic conditions also mark the beginning of the Upper Sequence of the nearby Forez basin. In Montclaret, the sequence was largely eroded prior to the emplacement of the b2 lava flow. The extension and the duration of this erosive episode remains uncertain. The formation of a red soil upon the basaltic flow implies a flat paleotopography evolving under a tropical climate and therefore a period of orogenic calm. The formation of a red soil implies a long weathering cycle. Data from the literature (Meyer, 1987) suggest that a period of 500,000 years at least would be necessary to obtain the Montclaret paleosoil. This sedimentation hiatus also corresponds to a volcanic activity hiatus. Considering all the phenomena of weathering, erosion, and sedimentation, the time interval between the emission of the lava flow b2 and the peperitic pyroclastics certainly exceeds 1 Ma. Bellon and Hernandez (1979) have demonstrated two age groupings within the Miocene volcanism (21-17 Ma and 14-12 Ma) and have already raised questions concerning the reality of a volcanic hiatus. There fore the Montclaret complex should be placed in the context of discontinuous volcanic activity during the Miocene. The b3 eruptive activity is related to a tectonic reactivation (NE-SW tensional movements) which induced an erosive period which fed the basin with detrital sediments. Conclusions A Miocene pedogenesis complex developed on basalt is described for the first time in the Forez area, though such phenomena some have already been identified in the Cantal-Mont Dore (Gibert, 1973; Chesworth et al., 1983) and in the Coiron (Grangeon, 1959; Moinereau et al., 1972). This paleosoil implies drastic modifications to the sedimentary dynamics, and therefore of the tectonics, and to the climatic conditions from a warm desert climate to a tropical climate with seasonal variations during the Miocene. The occurrence of lower Miocene sediments upon the granitic substratum on the eastern border of the horst, whereas in the basin some Oligocene sediments are intercalated between the granite and the Miocene sediments, suggests either that the Oligocene formations were completely eroded, or that they never existed, which would suggest a variable degree of extension of the basin through time, with maximal extension during the Miocene. In addition, the Montclaret section is re-addressing the question of a possible volcanic hiatus during the Miocene.

Key words: Paleosols, Red soils, Neogene, Major element analyses, Paleoclimate, Loire (Forez Mounts).