LES FORMATIONS DE MARTEAU ET DU BOIS D' AUSSE (LOCHKOVIEN-PRAGUIEN, DΈVONIEN INFΈRIEUR) AU BORD NORD DU SYNCLINORIUM DE DINANT : LES COUPES DE HUY, DE TIHANGE ET DE FOND D'OXHE

The ve1tical succession exposes the 'transgressive' Eodevonian pulse exemplified by different aggradational, pedogenic, groundwater and erosive processes. Immediately to the south of the Lower Palaeozoic mountains, the lowest Eodevonian basin shows a thick sequence of alluvial rocks dominated by fine-grained clastics (fine sand, silt, clay and mud) followed by the land-tidal marine transition and the back of sand-dominated fluvial sedimentation. Open-sea conditions were never reached during this period in the area studied. The Marteau and Bois d' Ausse Formations have an Upper-Lochkovian-Lower Pragian age and consist of a palaeosolbearing, fluvial-dominated, siliciclastic sequence. Fluvial sedimentation is obvious in the Marteau Formation sedimentary column. It exhibits first a proximal fluvial system followed by a distal fluvial and finally a coastal plain systems. The basal deposits correspond to a coarse infill of palaeovalleys developed on Lower Palaeozoic basement and a poor development of the floodplain. Reworking is the main feature. Ephemeral channels and sheet-floods were formed after heavy rain. Then, a large floodplain was developed with slightly sinuous channels and a large development of calcretes (maturity stages 1 to V). A local drainage system was well developed through the interfluvial plain, responsible of removal of sediments and soils peculiar features ( calcretes nodules, pedogenic aggregates, etc). In the coastal plain, the water table level tends to increase together with the development of vertic structures, silcrete nodules and iron oxides nodules beside calcretes. Coastal mixed fiats alternate with mud-dominated alluvial deposits in the lower part of the Bois d' Ausse formation, both characterized by the development of soils. The shoreline does not progress any more towards the north. The material was probably deposited during a phase of extensional tectonics and subsidence. Throughout the time interval of around 2.5 Ma represented by the Marteau Formation and the Lower Bois d' Ausse Formation, calcrete-bearing palaeosols with or without vertisol-like features occur, implying a semi-arid, strongly seasonal climate. Heavy rainfalls events caused reworking of the floodplain sediments and soils. Major wandering channels over wide floodplain, coastal plain and tidal area, allow a direct connexion with uplands. The alluvial architecture shows great similarities to Lower Old Red Sandstone series of Great Britain. Alluvial architecture is characterised by stacked sequences in a general aggrading system, with sandstones channel bodies at the base, following by mud sediments affected by pedogenetic processes. The alluvial deposits were pedified to varying degrees from calcic palaeosol to calcic-Vertisol. Pseudogleying, silcretisation, fe1Ticretisation and accumulative hydromorphy are other important features where both pedogenetic and probable groundwater processes interfere. Four types of conglomerates are described. Three of them imply that erosion processes are major features for the deposition of mud chips, calcrete gravels, fish remains and soils aggregates.