Découverte d'un encaissement entre dépôts de sables fauves dans la région de Sos (Miocène centre aquitain)

Abstract

The "Sables Fauves" succession (Jacquot, 1869; Crouzel, 1956) is a terrigenous complex of marine, ochre sands and sandstones which are known thoughout much of the Central Aquitaine area (Répelin, 1928). The succession was recently raised to formation level (Dubreuilh et al., 1995) with two reference sections: the Houga quarry west of Nogaro, and the Papin quarry 10 km east of Mont-de-Marsan. Dubreuilh et al. (1995) dated the Sables Fauves Formation as Langhian-Serravallian. The continental vertebrates (Tournouëc 1874; Richard 1948; Ginsburg, 1967; Baudelot and Collier; 1978; Aguilar and Michaux, 1995) and the marine invertebrates (Tournouër,1874; Ginsburg, 1967; Magné et al., 1985; Cahuzac et al., 1995) effectively confirm the assigned ages. Analysis of the geometrical relationships between outcrops of the Sables Fauves shows that the formation is made up of two units which indicate major topographical changes during the Middle Miocene. Description The Peyrecréchen quarry. This small excavation located 3500 m northeast of Sos (Fig. 1), on a plateau with an average altitude of 150 m, cuts 2 m into lacustrine limestones (Fig. 2) forming the uppermost part of the Armagnac molasse and limestone (Burdigalian). This level is capped by yellow calcareous and bioclastic sands (0.50 m), with scarce planktonic foraminifera (Praeorbulina sicana), benthic foraminifera (Ammonia beccarii, Cribrononion cf. vigneauxi) and mammals (Dorcatherium naui, Lagomeryx parvulus, Procervulus dichotomus, Dicrocerus elegans parviceros, Anchitherium aurelianense, Brachypotherium brachypus, Gomphotherium angustidens, Prodeinotherium bavaricum). The assemblage is characteristic of the Mein MN5 Zone (Duranthon and Cahuzac, 1995), generally considered as Langhian. The Matilon quarry. This quarry, 2 km south southwest of Peyrecréchen (Fig. 1) and 2.3 km east of Sos, is located on the northeastern slope of the small Bosc valley, between 120 m and 135 m in altitude. Here, the marine deposits cut into the Armagnac molasse and limestone, as described by Tournouër (1874; Fig. 3) and Ginsburg (1967). Armagnac continental marl is still visible at outcrop in the upper third of the quarry. Below the quarry, the basal part of the marine series at 115 m altitude overlies the "Calcaires gris de I'Agenais" Formation (Aquitanian) and white marly limestone. On the Saint Pierre hill opposite, lacustrine limestone crops out at the same altitude (125 m) as the Sables Fauves in the Matilon quarry. The marine infill, from base to top, consists of (Fig. 4): - grey shell clay ("falun") with Globigerinoides gr. trilobus, Praeorbulina glomerosa circularis, Globigerina cf. bulloides, G. cf: falconensis, Globoquadrina dehiscens, Sphenolithus heteromorphus (sporadic), Coccolithus pelagicus and Helicosphaera kampteri from the NN5-NN6 zones of Martini, dating the deposits as Late Langhian or Early Serravallian (0.30 m); - ochre sands (Sables Fauves) showing, large-scale planar cross-bedding of sandwaves, with intercalations of cemented beach rocks. In the upper part of the quarry, the sands (20 m) contain reworked blocks of lacustrine limestone bored by pholads. This level includes Aceratherium cf. incisivum and Megacricetodon fournasi (Aguilar and Michaud, 1995) from the Serravallian. The Cayron quarry The Cayron quarry, 1500 m northeast of Matilon between 120 m and 140 m in altitude, reveals ochre sands cut into the Armagnac molasse. Megacricetodon fournasi, Dicroceros elegans, and Alicornops simmorense are present in the basal part of this outcrop (Aguilar and Michaux, 1995), dating the deposits as Serravallian. Paleotopographical distribution of the Sables Fauves. The Sables Fauves Formation consists of two members with different topographical settings and ages: - the Peyrecréchen Member (Langhian), overlying the plateau of Armagnac molasse and limestone (Early Miocene); - the Matilon Member (Serravallian), at lower altitude, locally overling a thin layer of grey falun, and infilling a paleovalley that cuts the Armagnac molasse and limestone. Sequence stratigraphy. The Sables Fauves Formation corresponds to two third-order depositional sequences: the Peyrecréchen Member is the highstand systems tract of a sequence that extensively drowned the Aquitaine Basin; the grey falun and the ochre sands of the Matilon Member respectively correspond to the transgressive systems tract and the highstand systems tract of the following sequence. These sequences are separated by a type 1 sequence boundary (Van Wagoner et al., 1988) expressed by an incised valley. Paleogeographical and sedimentary evolution. The evolution of the Sos area during the Middle Miocene includes three stages (Fig. 4): - stage 1 (Langhian): widespread transgression of a shallow sea over the Armagnac molasse, and deposition of coastal sands (Peyrecréchen Member); - stage 2 (Langhian): a falling sea level causes erosion of the Armagnac molasse, with the incision of a valley about 35 m deep. Deposition is transferred to the west; - stage 3 (Late Langhian - Serravallian): marine ingression of the valley and progressive infill by tidal deposits. The eustatic rise exceeded 20 m by means of minor sea level changes, indicated by beach rock levels (Bernier and Dalongeville, 1996). Conclusions. The Sables Fauves Formation of the Sos area consists of two lithostratigraphical units (members), the first dated as Langhian and the second as Serravallian. The members, deposited in different paleotopographical settings, correspond to the highstand systems tracts of two depositional sequences separated by major subaerial erosion. This new interpretation of the geometrical relationships of the deposits agrees perfectly with the conceptual model of sequence stratigraphy (Posamentier et al., 1988 ; Posamentier and Vail, 1988).