Jean-Pierre Larue

Tectonical and morphodynamical evolution of the southern Massif Central border, between the Cesse and Hérault rivers (France)

Despite numerous researches concerning the Massif Central southern border, there are no certainties about the number and the chronology of the erosion surfaces, the uplift stages of the Massif Central and the incision stages of the rivers. Located on the Massif Central basement and on the Languedocian piedmont, the Orb, Libron and Herault tributaries basins show a good pattern of shapes and formations, from the Caroux to the littoral plain. Geomorphological and sedimentological study brings some new insights to these questions. Heavy minerals analysis allows us to separate different formations among the deposits located on the erosion surfaces and in the valleys. It is used to reconstruct the main sedimentary pathways. North-south fluxes crossed the Avant-Monts up to the Upper Pliocene, despite of the Miocene uplift. Upper Pliocene uplift of the Avant-Monts caused the bifurcation of the Orb towards the west, the antecedent incision between the Jaur confluence and Cessenon and the stepped pediments and glacis on the Avant-Monts southern border. The longitudinal paleoprofiles argue for a progressive uplift of the Massif Central since the Messinian unconformity. Incision of the valleys began before the Messinian in the Massif Central, but only during the Lower Pleistocene in the Languedocian piedmont.

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 Lozere 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 Lozere 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 Lozere 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 Lozere 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 Lozere 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.

Morphodynamique fluviale et neotectonique dans la vallee de l'Oise (Bassin parisien, France)

To show the impact of neotectonics on fluvial morphodynamics, the author presents a geomorphological study of the Oise valley where it crosses the Pays de Bray anticline, and of the gullies and the dry valleys of the Pays de Thelle. The analysis of alluvial and colluvial forms and formations makes it possible to reconstruct the longitudinal profiles of successive river beds, from the valley heads to the Oise valley and thus to reveal anomalies. Tectonics control fluvial dynamics, but impacts are only observed if the climatic conditions are favourable: hence it can be deduced that the gullies were formed during the wet Subboreal period.

Erosion in cultivated soils and river morphodynamics: the example of the Tortue Basin (Sarthe, France)

Abstract An analysis of the functioning of the valley slope/ stream system is carried out in the catchment area of the Tortue, a hilly, sandy region, east of Le Mans. The swift restructuring of plots is responsible for two types of morphogenesis: Chronic erosion accounts for the slow transfer of sediments towards the lower part of plots and slopes. The overland flow caused by the restructuring of surface soil erodes some 150 to 200 kg ha −1 during the month following sowing. But the limited amount of deposits (the Royeau brook carries away less than 50 kg ha −1 per month) proves the existence of numerous relays. Catastrophic erosion , triggered by storm showers tailing on dry soils with sparse vegetation led, in 1988 and 1990, to extensive gullying and longitudinal transportation that modified stream dynamics. Taken away partly from the stocks accumulated through chronic erosion, the sand flow progresses slowly downstream with the floods.

Drainage pattern modifications in the Aude basin (France): tectonic and morphodynamic implications

A study of river capture and palaeodrainage systems in the Aude basin is used to reconstruct the main stages of the palaeogeographical evolution of the region located between the Central Massif and the Pyrenees. Geomorphological and sedimentological (pebble composition, X-ray and heavy-mineral analyses) studies have made it possible to identify and link the different alluvial outcrops from upstream to downstream in the Aude basin. The study of the Castelnaudary terrace and of the high levels of the Aude shows that this river always flowed to the Mediterranean Sea: green hornblende from the Upper Aude did not reach as far as the Naurouze gap. River capture events between the Garonne and the Aude basins were caused by migration of the drainage divide towards the west, with uplift of the Montagne Noire and progressive hollowing-out of the strike depression between the Montagne Noire and the Eocene cuesta. The reconstruction of longitudinal terrace profiles shows the evolution of tectonic deformation since their deposition. The uplift of the Tourouzelle unit and of the Corbieres thrust-fold sheet has caused the formation of knick points, the splitting of the middle terrace into two parts and alterations in the course of the Rivers Aude and Cesse.

Source and evolution of the clinopyroxenes in the Loire and Seine basins (France) based on grain morphology and color

Variation in the characteristics (color, morphology) of clinopyroxene grains (CPX) in alluvial deposits and other surficial formations in the Loire valley, the Gâtinais and the Beauce show that many are pristine and come directly from recent volcanic eruptions, whereas weathered grains, clearly rewor- ked, come from erosion of Cenozoic and Pleistocene volcanic rocks of the French Massif Central. After deposition, the CPX have been increasingly altered by longer exposures. Weathering of brown CPX yields paler minerals which are greenish-brown, colourless or two-coloured. Similarly, the CPX found in the Seine basin are from recent volcanic ash or old Sologne deposits, and not from Loire alluvial depo- sits, so that a Pleistocene palaeo-Loire-Seine river is improbable.