Age miocène des formations de bordure du fossé du Forez (Massif central)

Abstract

The Forez graben (Fig. 1), initiated during Eocene (Caire, 1977; Bergerat, 1980), contains a Cenozoic succession, several hundreds of metres thick, of mainly piedmont detrital sediments. Coarse-grained proximal facies (conglomerates and sands), derived from erosion of the granitic Forez Massif, spread over the western margin of the basin and evolved laterally eastwards to distal lacustrine to paludal facies (Duclos, 1967; Le Griel, 1975, 1984; Etlicher, 1986). These detrital deposits can be subdivided into three megasequences (Fig. 2A; Echcherif El Khetani, 1996): the lower and upper megasequences are attributed respectively to the Late Eocene - Early Oligocene and to the Early - Middle Miocene on paleontologic criteria (Schuler and Sittler, 1969; Echcherif El Khetani, 1996), whereas the age of the middle megasequence, only indirectely attributed to the Late Oligocene, has long been debated (Duclos, 1967; Duclos et al., 1974; Le Griel, 1975, 1984; Gagny et al., 1989; Echcherif El Khetani, 1996). The proximal azoic coarse-grained sands and conglomerates of the Bazourge formation at the base of the middle megasequence (Fig. 2A) contain a few basalt pebbles, among which one of decimetric size was selected for 40Ar/39Ar dating. This radiometric age provides evidence for reconsidering the stratigraphic position of these deposits. 40Ar/39Ar incremental heating experiments: The subaphyric basaltic lava contains 5 to 10% phenocrysts (mainly olivine and some clinopyroxene) and a matrix with abundant plagioclase and opaque mineral microlites. The pebble core is preserved from the iddingsitic alteration that affects the olivine phenocrysts of the pebble rim. About 20 mg of a 250-300 µm sieved fraction of whole-rock powder was irradiated in the Siloée nuclear reactor (CEA, Grenoble, France) and recieved a neutron flux density of 5.1018 neutrons/cm2. The sample was analysed together with a standard amphibole of 334.5 Ma from Capelongue (Maluski and Schaeffer, 1982). The irradiation factor J value is 0.01751 with 1% mean standard deviation. The irradiated sample was step-heated from 700 to 1400°C. The released gas was cleaned through traps on the extraction line and introduced in the VG3600 mass-spectrometer source (analytical procedure in Arnaud et al., 1993). All ages are given with 1s errors and the “plateau” and “isochrone” ages include the error on J. Detailed data are available from the authors. The age spectrum (Fig. 3a) exhibits a progressive degassing and correlates closely with the evolution of the K/Ca ratio (Fig. 3b). As a subaphyric whole-rock powder was analysed, the first plateaus are likely to be related to the gas fraction released from the glass and plagioclases, whereas the low-K and excess Ar-rich minerals degassed at the end. Thus the first steps are considered to be more reliable, even if slightly affected by inherited-Ar contamination. The very first plateau, richer in K and representing 50% of the released gas, gives an age of 14.9 ± 0.3 Ma, which will be a minimum age because whole-rock matrix alteration often produces younger ages. Moreover, because the first plateaus are discordant, probably due to excess Ar, no representative plateau age can be calculated (Dalrymple et al., 1981). To correct for excess Ar, a 36Ar/40Ar versus 39Ar/40Ar “mixing” diagram was used (method from Roddick et al., 1980). The inverse isochrone diagram (Fig. 3c) shows excess Ar in the last steps and a high dispersion of data. The four first steps were used to draw an isochrone which suggests an age of 21.5 ± 1.7 Ma, i.e Aquitanian (Odin, 1994). This “isochrone” age seems the most reliable and is prefered to the “plateau” age based only on the first step. Discussion: The detrital Bazourge formation from which the basalt pebble was collected is thus younger than 21.5 ± 1.7 Ma, which is the age of the lava emplacement. These deposits (Fig. 2) are attributable at least to the Aquitanian - Burdigalian, thus younger than previously proposed (Echcherif El Khetani, 1996; Fig. 2A). Moreover, the sedimentary series (Fromentaux and Sury le Comtal formations) that developed over, or as eastward lateral facies variations of the Bazourge formation were also deposited during the Early to Middle Miocene (Figs. 2A and 4). On the basis of this Miocene age, the Bazourge formation can be correlated with the coarse-grained sedimentary series drilled in the centre of the basin (Fig. 1, site 2) and attributed on paleontological criteria to an upper sequence of Aquitanian to Burdigalian age (Larqué and Weber, 1969; Schuler and Sittler, 1969). A lower sequence, consisting of marls and clays typical of a confined environment and containing a Chattian or end-Stampian microflora in its upper levels, was also identified (Schuler and Sittler, 1976) (Fig. 2B). The transition between the two sequences corresponds to an input of massive coarse-grained clastic sediments into the centre of the basin, correlated with a variation of the clay-minerals fraction, which changes from mainly illite to prevailing detrital montmorillonite with minor kaolinite and illite. Northwards, the Bazourge formation is in apparent continuity with some detrital facies of the western margin that are also from a piedmont environment and which also contain some basalt pebbles (Le Griel, 1975, 1984; Echcherif El Khetani, 1996). The Mont d’Uzore volcanic complex (Fig. 1), in which the youngest basaltic dykes have been dated at 17.8 ± 0.9 Ma (Burdigalian) by the whole-rock K/Ar method (Bellon and Hernandez, 1979), was emplaced within these conglomerates and sands (Bobier and Glangeaud, 1960; Hernandez, 1971). Because these deposits constitute northern equivalents to the Bazourge formation, their currently postulated Oligocene or Oligo-Miocene age (Duclos et al., 1974; Gagny et al., 1989) needs to be reappraised. The deposits located on the western margin of the Forez basin clearly appear to be at least of Early Miocene age. Both the middle and upper megasequences defined by Echcherif El Khetani (1996) were deposited during the Miocene (Fig. 2). Thus, uplift initiated during the Early Miocene would have affected the Monts du Forez positive reliefs, leading to increased erosion and a massive accumulation of detrital sediments within the basin. This Miocene stage in the tectono-sedimentary evolution of the basin, already described locally (Larqué and Weber, 1969), has to be extended to the whole graben. The Early Oligocene and even part of the Late Oligocene have not been identified paleontologically (Fig. 2B). Nevertheless, detrital deposits that were intersected by drilling over several hundred metres below the Chattian sediments and above the Priabonian formation (Fig. 2B; Echcherif El Khetani, 1996) must belong to the Oligocene sensu lato. As it is not yet possible to define the chronological interval that is effectively represented, or to affirm a total continuity of the sedimentation within this interval, we consider only two series in the succession (Fig. 2B) rather than distinguishing megasequences as proposed by Echcherif El Khetani (1996; Fig. 2A). Furthermore, compared to the adjacent Grande Limagne Basin, the Forez basin is characterized by its large volume of Miocene sedimentation; detrital input in the Grande Limagne was certainly sparser at that time (Autran and Peterlongo, 1980), although it was abundant during the Oligocene. The Neogene dynamic evolution of the two grabens clearly seems to have been different and partly disconnected. Conclusion: The post-Aquitanian age of the proximal detrital deposits (Bazourge formation) located on the southwestern margin of the Forez basin is demonstrated for the first time by the 40Ar/39Ar incremental dating of a basalt pebble (21.5 ± 1.7 Ma). Thus the overlying Fromentaux and Sury le Comtal formations must be related to the Early Miocene, and not to the Oligocene as previously defined (Echcherif El Khetani, 1996). This chronological evaluation, as well as correlations between several drill-cores, leads to a reinterpretion of the stratigraphic relationships within the entire basin. The tectonic and sedimentary activity intensified during the Aquitanian (Larque and Weber, 1969) and abundant detrital releases continued to at least the Middle Miocene. From our results, the Early Miocene appears to be characterised by huge amounts of detrital deposits, and is certainly a major stage of the geological evolution of the Forez basin. This is probably related to tectonic movements uplifting the western margin of the basin and favouring magmatic uprise.