The Lozère sands and the Sologne sands : two detrital discharges in the Paris basin during the early Miocene. New interpretations

Following a review of the previous interpretations concerning the drainage pattern modifications of the Loire-Seine in the Paris basin, this work attempts to show the chronological and distributional relations between the Lozère sands and the Sologne sands. Heavy mineral analyses of about 300 samples, extracted from sands of a series of outcrops and drill holes from the detrital deposits coming from the Massif Central and located between the Nivernais and the Pays de Caux, makes it possible to separate the Lozère sands and the Sologne sands (fig. 1 and tabl. I). The first deposits show analogies with the Albian sands, having poor mineralogical variety and containing only a few feldspars, whereas the second ones incorporate greater mineralogical variety and more feldspars (fig. 2). The identification of Lozère sands outcrops in the Loire valley, south of the Sologne area (fig. 3) proves that a palaeo-Loire-Seine existed before the discharge named Sologne sands during the Lower Burdigalian. A tectonical evolution (fig. 4) is proposed to explain the changes in mineralogy and distribution of the deposits. The progressive uplift of the northern border of the Massif Central caused the erosion of the Albian sands which contributed to the fluvial discharge of the Lozère sands. But when the Ligerian depression extended to the Sologne area, the fluvial discharge diverted toward the Atlantic, the Albian sands being excluded from erosion, and the Sologne sands were mainly supplied by basement derived alterites showing a greater mineralogical variety. Since the Burdigalian, the Loire and the Seine basins had a distinct evolution. While the Sologne sands were buried under the Bourbonnais sands, the augite river deposits and the Lozère sands were reworked on the polygenic erosional surface [Dewolf et Pomerol, 1997] which developed from late Burdigalian to early Pleistocene times, before the Quaternary incision of the rivers.