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The stratigraphic context

, : The stratigraphic context. Memoires du Museum National d'Histoire Naturelle 183: 39-67

One of the interests of Sansan is that the fossiliferous layer is interbedded in the large series of the Molasses de l'Armagnac, which has yielded several famous sites for fossils. So the relative age of these different sites is known directly by the simple fact of their stratigraphic position. In the North of Gers Department, the molasses de l'Armagnac directly underly the "trilogie agenaise", which includes three important continental beds: at the basis, the "Calcaire blanc de l'Agenais" with the mammal sites of Paulhiac (Lot-et-Garonne), then the "molasses superieures de l'Agenais" which are continental in the east and marine in the west of the basin; above, is the "calcaire gris de l'Agenais" with the famous place of Laugnac (Lot-et-Garonne). The age of Paulhiac is MN 1 and the Laugnac one is MN 2b. The molasses de l'Armagnac consist of more than 300 m of continental sands and marls, interbedded with limestones. These limestones constitute the only hard elements in the molasses and form small clifs in the general soft landscape. So they are the the only guiding-mark for the stratigraphy. CROUZEL (1957) established a stratigraphical scale using these limestones beds. He divided the series into 17 levels, each beginning with a limestone. The name of every level is the limestone one. CROUZEL gave the exact place of the different mammal sites in the molasses series. So, in level 1 is Estrepouy (MN 3), in level 3 is Pellecahus (MN 4a), in level 4 are Bezian and La Romieu (MN 4b), in level 7 are Birosse and le Moune at Castelnau-d'Arbieu (MN 5), in level 9 is Sansan (MN 6). Then Rajegats at Simorre, in level 10, and Tournan and Villefranche d'Astarac in level 12 belong to the MN 7. Above this, the series is poor in fossils. In level 16 is the famous place of Saint-Gaudens which yielded the anthropoid Dryopithecus fontani (MN 7/8). Level 17 is deeply divided up by a thin series of lignites and clays. These lignites yielded at Orignac a small fauna of MN 9 age, with a Hipparion. In addition, two small marine trangressions reached the area of Condom and Lectoure during the continental molasses deposit times. First, in level 1, between the basal limestone and the sand of Estrepouy are the marls with Ostrea aginensis. They contain foraminifera upon which GOURINARD et al. (1987) dated them to 19 MA (Middle Burdigalian). The transgression of "sables roux a Crassostrea crassissima" is younger, and underlies different younger levels from west to east, from level 2 to level 7. This marine deposit is dated as 16 MA and is synchronous with the marine Langhian transgression of the falun of Touraine in the Loire basin.


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