Chronologie mammalienne et grade-datations dans le Miocène inférieur et moyen français: une revue

Abstract

Numerical ages are important for geological studies as well as paleobiological ones. This explains the interest raised by the "grade-dating" method of Y. Gourinard (1983, 1984) based on calibrating the evolutionary change in lineages of foraminifera. First results concerning Early and Middle Miocene times suggest rather important modifications to the boundaries of Aquitanian and Burdigalian stages and to carrelations between the stratotype and the paratype of the Aquitanian stage (Gourinard et al, 1985). Among other results obtained, numerical ages are proposed for fossil mammal localities both in beds of marine origin and also, after extrapolation, for some localities in continental deposits. The method is thus of interest because direct correlations between marine and terrestrial deposits together with radiometric datings and magnetostratigraphic assessments are still rather uncommon. The proposed numerical ages for fossil mammal localities are discussed in the light of (a) the mammalian biochronology (Fig. 1), (b) previously proposed correlations between marine and continental biochronological scales, (c) information provided by new fossils or new paleontological interpretation, and (d) magnetostratigraphic data concerning sections with fossil mammal localities. Two localities are involved for the numerical ages of fossil mammal localities of marine origin. Caunelles near Montpellier and Sos-Matilon near Agen. Well preseved rodent teeth at Caunelles have been found together with foraminifera and shark teeth (Fig. 2). Two samples of foraminifera were used for gade dating (Magné et al., 1987): the lower one wus 30 to 50 cm above the rodent sample (grade-dating ags of 18.7 + 0.1 Ma), the second one was 1 m above the latter (18.0 ± 0.1 Ma). The ,fossiliferous layer does not show any unconformity. Nevertheless, an older numerical age (19,6 Ma) was assigned to the Caunelles rodent fauna. The necessary respect of the mammalian relative chronology and of other numerical ages of the Lower Miocene localities explain this difference (Fig. 3). At Sos-Matilon, large mammal remains are known to give an Helvetian age (Ginsburg, 1967), the locality being referred to zone MN 5 (Mein, 1975). J. Magné et al. (1985) give a grade-dating of 16.1 ± 0.28 Ma and, on the basis of a few newly discovered rodent molars, place Sos-Matilon in the immediately younger mammaliun zone MN6, which includes the famous mammalian locality of Sansan. The recent revision of the Sos-Matilon material, as well as the discovery of another rodent tooth, suggest an even younger age (zone MN7-8) for Sos-Matilon. The numerical age of the marine sands of Sos-Matilon (16.1 ± 0.28 Ma) failed to test the correlations based on mammals. Although conventional correlations between marine and continental scales (Aguilar et al., 1994), as well as a recent magnetostratigraphic analysis (Sen, in Press) still place Sansan around 15 Ma. This contradiction with the new interpretation of rodent lineages (Aguilar and Michaux, 1987; Aguilar, 1994; Fig. 4), could be expluined by a reworking of the large mammal remains. Where numerical ages given by extrapolation from grade-datings are concerned, four reference localities in mammalian biochronology are involved in addition to the Sansan case: Estrepouy (between 19 and 18.7 Ma: Courinard et al., 1987b), Balizac (19.8 or 20.1 Ma: Magné et al., 1987), Laugnac (21 to 21.2 Ma: Gourinard et al., 1987a) and Paulhiac (23.8 to 24.1 Ma: Gourinard et al., 1987a). For Paulhiac and Laugnuc the extrapolation is based on a lithostratigraphic correlation. For Estrepouy, a high rate of sedimentation is hypothesized, whereas for Balizac the rate of sedimentation is considered as very slow: both hypotheses are difficult tojustify. The recent magnetostratigraphic study of a fresh water molasse formation, which yielded several fossil mammal Iocalities (Fig. 5) in Haute-Savoie (Burbank et al., 1992), oflers a test case for extrapolated numerical ages. According to magnetostratigraphy and mammalian correlations, the Laugnac locality is ca. 19 Ma and that at Paulhiac is ca. 23 Ma. In the case of Balizac, a locality a slightly younger than La Chaux (ca. 23.8 and 24.1 Ma) in Haute-Savoie, the extrapolated age of between 19.8 und 20.1 Ma is much too Young. In conclusion, ages obtained by grade-dating are questionable as discrepances of 1 to 3.5 Ma are found in comparison with ages based on classical correlations with the marine scale, or when magnetostratigraphic analysis of sections with fossil mammals is involved. Such a situation emphasizes two problems: first, the fact that the intrinsic quality of the grade dating method is questionable and second, the fact that the extrapolation of ages provided by grade-dating is based on too many hypotheses.