Altération bauxitique associée aux argiles à chailles sur la bordure sud-est du Bassin de Paris.
Résumé
On the southeastern margin of the Paris Basin, Nivernais region, the marine deposits have been continuous all along the Jurassic period. The sea withdrew a first time during the Upper Tithonian. This regression coincided with erosion and weathering. Apparently, the sea invaded again the Nivernais region only during the Albian, to which are attributed a few relicts of glauconitic sandstone with quartz pebbles. The youngest marine occurrences are of Santonian-Campanian age, made of silicified urchins and microfauna preserved in flint flows. The Tertiary deposits are limited to small grabens and made of nodular lacustrine limestones with charophytes ascribed to the Middle Eocene. In this southeastern margin of the basin, palaeoweathering products of the Jurassic and Cretaceous, i.e. Clay-with-Jurassic cherts and Clay-with-Cretaceous flints, are widespread. Their study allow to reconstruct the long continental evolution of the area. A red silty clay formation, with scattered cherts and abundant ferruginous gravels, containing typically from 10 to 20% of gibbsite, has been identified at Beaumont-la-Ferrière. Besides the lack of granulometric sorting of the quartz grains and of the ferruginous nodules and gravels, the micromorphologic analysis of the thin sections shows well developed illuviation structures, pointing clearly out pedogenetic features. The red silty clay formation in Beaumont-la-Ferrière relates to an in situ bauxitic palaeoprofile developed above Callovo-Oxfordian limestones. Near Beaumont-la-Ferrière, at St-Benin-d'Azy and at Ste Colombe, outliers display several weathering formations with cherts and flints that total over 50 to 80 m thickness. At the base, there is in-situ Clay-with-cherts, in which occurs the siderolithic iron ore deposits and which locally contains also gibbsite. In places, this Clay-with-cherts is topped by rounded cherts, from 3 to 10 cm in diameter, and sand lenses containing quartz gravels. The Clay-with-flints of lustreous break, translucent, with branchy morphologies and porous cortex, typical of the Clay-with-flints known elsewhere, occurs at the top of the buttes. The reworked cherts, the sand and the Clay-with-flints Formation are often silicified by typical pedogenic silicification, with illuviation features and titania-rich matrix. The palaeoweathering formations occurring in the Nivernais region allow to specify at least three successive weathering stages in the southeastern Paris Basin. (1) Development during the early Cretaceous of a thick blanket of Clay-with-cherts by weathering of the Jurassic chert-bearing limestones after withdrawal of the sea at the end of the Tithonian. Locally this weathering lead to the development of true bauxitic profiles, with gibbsite, pisolites, nodules, wide illuviation structures, etc. The development of the siderolithic iron ores relates to this weathering stage. (2) A second weathering stage, after the withdrawal of the sea during the Campanian, lead to development of Clay-with-flints from leaching of the chalk deposits. (3) Silicification affected the Clay-with-cherts and the Clay-with-flints, as well as the reworked deposits that today armour the reliefs. These silicifications display the same micromorphologic and geochemic characters as the silcretes that developed during Middle/Upper Eocene in the central parts of the Paris Basin. This is the first time that in situ Cretaceous bauxitic profiles are described north of the Massif Central. It seems likely that these bauxitic formations have been initially much more widespread, but a number of profiles may probably have been resilicified, with alteration of the gibbsite into kaolinite along the silicification of the siderolithic piedmont during the Eocene. The old pisolithic iron ore workings of the nineteenth century are located in this Clay-with-cherts Formation. In this way, the siderolithic iron ore deposits, typical from the southern margins of the Paris Basin, are of early Cretaceous age and correlative to the bauxite deposits of southern France.