Foraminifères benthiques et Microproblematica du Serravallien d'Aquitaine

Abstract

The Serravallian was widely transgressive in Aquitaine; the sea spread far eastward (until Condom area, Gers = "Armagnac Gulf") and southward (Chalosse area and Orthez Gulf). The outcropping deposits are shallow water, detritical and locally rich in highly diversified macrofaunas. This Serravallian series has been referred by previous authors to the "Helvetian", or "Sallomacian" (local facies) or "Vindobonian" . The Sallomacian was recently revised by Folliot (1993) and Folliot et al. (1993), and Cahuzac et al. (1995) studied the nannoflora of the Oligo-Miocene of Aquitaine. These various papers have made it possible to conclude that the Serravallian deposits correspond to the planktonic foraminferal zones N11 (pars) to N13 (pars) and to the nannoplankton zones NN6-NN7-NN8 (lower part), recorded in several exposures. Apparently wrong "grade-datings", published a few years ago (Magné et al., 1987), had given a Langhian age to several outcrops of southern and central Aquitaine. As a matter of fact, a recent revision allowed to distinguish there two stages: the Langhian with common Praeorbulina and not Orbulina, and the Serravallian with e.g. Orbulina corresponding to the northern Sallomacian. The thickness of the Serravallian deposits is very variable on account of the proximity of anticlines and of the shoreline; it is everywhere higher towards the West. The different sites have been grouped into five geographic areas corresponding roughly to the few main sedimentary facies. These are the: Northern Aquitaine (and Bordelais) Gulf, the Armagnac Gulf, the Chalosse Gulf, the Orthez/Salies-de-Béarn Gulf, and the Soustons western domain. Larger foraminifera are not abundant, and are restricted to two genera: Operculina and Heterostegina, each represented by only one species. In northern Aquitaine, both genera are rare to absent in the exposures, but more common in the boreholes. In southern Aquitaine, they are fairly common in the Orthez/Salies-de-Béarn outcrops, which probably correspond to the first marine sequence. Planktonic foraminifera have a variable distribution and they occur quite often rarely on account of the nature of the deposits; they are notably characterized by the presence of Orbulina suturalis and O. universa. Smaller benthic foraminifera are not so abundant (slightly more than 150 species) as in the Burdigalian (250 species); they are mainly hyaline and most of them are typical of shore deposits: e.g. Ammonia beccarii, Elphidium crispum, Cibicides lobatulus, Nonion, Polymorphinidae, etc. Among the other taxa, are: Hopkinsina bonionensis compressa, Caucasina lappa, Cancris auriculus, Cibicidoides pseudoungerianus, Discorbinella bertheloti, Nonion boueanum, N. commune, Nonionella janiformis...A few exposures (Donzacq, Gibret, Carcarès, Soustons, Sallespisse) yield microproblematica: Bolboforma (B. badenensis and B. clodiusi) and Bachmayerella (laqueata). The Serravallian is roughly as rich as the Langhian observed in southwestern Aquitaine (Saubrigues area with marls of an infralittoral to circalittoral facies) and eastern Aquitaine (Manciet-Baudignan Gulf with littoral shelly deposits). Both stages have numerous small foraminifera in common. Some species seem to disappear at the top of the Langhian: Hopkinsina bonionensis primiformis, Elphidium fichtelianum praeforme, Compressigerina capriciosa. On the other hand, other taxa seem to appear during the Serravallian, including Virgulinella pertusa. Ammonia punctatogranosa, Bigenerina agglutinans, Bannerella gibbosa, Nonion magnum, Neorotalia spp., Cymbaloporetta squammosa, etc. The genus Pseudoeponides, very rare in the Langhian, becomes common only in the Serravallian. If we compare these deposits with the Upper Miocene (Tortonian) and Pliocene of Aquitaine, which are only known in some deep boreholes on the western façade and offshore, it is noticeable that the microfauna of the latter do flot display any more larger foraminifera and Amphistegina as well. The smaller foraminifera are mostly similar to the Serravallian ones. Therefore, the assemblages of the Serravallian already herald modem microfaunas. A few differences are noticeable between the Serravallian assemblages of southern and northern Aquitaine; they concern the absence or low abundance of some species. The northern Aquitaine deposits contain many agglutinated forms and Ammonia, Elphidium, Polymorphinidae, etc. In eastern Aquitaine, the microfauna is very poor on account of the (detrital) lithology, high hydrodynamic conditions and location at the extremity of the Gulf. In southern Aquitaine, the Soustons exposure yields a rich microfauna which provides evidence of an open marine environment; otherwise, Heterostegina granulatatesta praeformis is mainly known in the Orthez Gulf. Comparisons between the Aquitaine Basin and the Ligerian Gulf assemblages are not easy owing to the absence of biostratigraphical markers in the Loire area series. The Serravallian (with pars local Savignean facies) is notably present in the Tours and Blois domain and in the south of the region (Mirebeau). It is noted that several species mentioned in Margerel's papers (1980, 1989) are common to Aquitaine, such as: Elphidiella falunica, E. vigneauxi, Hopkinsina bonionensis compressa, Discorbinella bertheloti, Pseudoeponides pseudotepidus miocenicus (fairly abundant in the Loire Gulf), Pararotalia serrata, Hanzawaia bolivarensis... The most significant absence in the Loire Gulf is that of the larger foraminifera. On the other hand, in Aquitaine, two genera seem to be missing: Aubignyna and Monspeliensina. These data argue for the existence of a latitudinal climatic gradient of deterioration evidenced on the Atlantic front from the Middle Neogene: the northern areas show a more marked cooling than the southern ones. In Aquitaine, the decrease of temperature affecting the Serravallian is obvious in comparison with the Burdigalian and Langhian: there is a strong decrease of the larger foraminifera and the absence notably of Discorbina mira (tropical species present in the Lower Miocene). Moreover, if we consider the distributions observed in the whole Serravallian (outcrops and boreholes), it seems that a thermic gradient took form during this stage: the climate was warmer during lower sequences as demonstrated by the presence of larger foraminifera (Heterostegina, Operculina, and Amphistegina, Neorotalia, etc.). This suggests that the decline is linked with global oceanic cooling.