Jean-Michel Schmitt

Variety and relationships of weathering features along the early Tertiary palaeosurface in the southwestern French Massif Central and the nearby Aquitaine Basin

Abstract Palaeoweathering features and palaeosurfaces in the southwestern French Massif Central and its margins provide a basis for understanding the evolution of Tertiary continental areas. Recurrent sequences of weathering periods, erosion phases and sedimentary deposits have been identified and related to major sea-level changes, tectonic movements and palaeoclimates. The palaeoweathering features are different according to their palaeogeographic position, from basin margins to uplands, and in limestone platforms or crystalline areas. On the basin margins palaeokarst features prevail. Claystones together with lignite accumulated in these sinkholes. Later, weathering and oxidation of these organic-rich deposits led to highly acidic environments, resulting in dissolution of the primary clay minerals and formation of secondary clay deposits with gibbsite, halloysite, montmorillonite, nontronite, but also of opal-CT silcretes and ferricretes. Palaeoweathering profiles, fossilised at the edge of the subsiding basin, show red and ochre sands on which red-brown silicified duricrust developed, later invaded by palygorskite-rich calcrete horizons. On the limestone borderland, behind the subsiding area, various palaeokarsts developed extensively. Surficial palaeokarsts, developed after withdrawal of the Jurassic sea and buried by Cretaceous sandy deposits, have been reactivated during the early Tertiary. Later, the deep subsurface palaeokarsts from the early Tertiary were sealed and then reactivated as they were brought back to the surface by erosion. The hinge zone between the basin and the uplands appears relatively stable despite a long history. Palaeolandscapes here are mostly polygenic, with exhumed surfaces related to early Mesozoic continental evolution which formed the basis for the development of younger morphologies. The Tertiary landscapes are characterised by inselbergs of unweathered crystalline rocks rising abruptly above radiating low-angle glacis. The wide crystalline uplands were early exhumed from their Mesozoic cover and underwent deep erosion. Only scarce marks of a long continental history remain on these upland areas, and palaeoweathering features are mainly restricted to grabens. The graben deposits show red kaolinitic palaeosols at their base which were later impregnated by silica deposition. Green smectitic claystones, interbedded with calcrete horizons overlay the kaolinitic palaeosols. Extensive silcretes developed at the top of these deposits. Despite the wide variety of the palaeosurface compounds, there are many similarities. The climate change, from warm and very wet during the Late Cretaceous and early Tertiary toward warm and dry during the Late Eocene and Early Oligocene, is the main factor that led to these similarities. The oldest testimonies indicate kaolinite-rich soils developed on smooth undulated hills. The onset of drier climates led to the development of a contrasted mineralogical sequence, with inheritance of the kaolinitic materials from the palaeosols, development of silicified duricrusts on the piedmonts and glacis, and calcretes with Mg-rich clays in the flood plains and lacustrine environments. Tectonics also played a major role. The early Tertiary alpine movements led to the erosion of the old weathering mantles, and to reshaping of the landscapes by several erosion cycles. Drop of the base level led to incision of large valleys on the uplands and development of wide glacis and deep karst network on the margins of the basin. On the other hand, subsidence or general rise of the base level protected some parts of the landscape from further weathering and erosion. The inheritance of pre-existing landscapes also is of noteworthy importance. The thick kaolinitic mantle inherited from the humid climate has strongly prepared the shaping of the subsequent landscapes. The clearing of soft weathering cover exhumed ancient surfaces in the hinge zone between the basin and the uplands, whereas in the crystalline uplands it exposed a knobbly etch surface reproducing the ancient topographies. Present landscapes of Aquitaine and southwestern Massif Central still exhibit many remnants of a long and complex continental history that track back to the early Tertiary and beyond.

The industrial waste land of Mortagne-du-Nord (59) - I - Assesment, composition of the slags, hydrochemistry, hydrology and estimate of the outfluxes

The industrial waste landin Mortagne-du-Nordhas been occupiedfor over 60 years by a zinc smelter unit linkedto a sulfid e roasting unit for sulfuric acidprod uction andfor a few years a leadsmelting unit. The waste land has been systematically exploredby d andpits ; 58 of these d holes andpits have been equippedas piezometers andsurveyedfor 4 years. The site has been back-filledwith slags andsmelting crucible to a thickness of about 3 meters. The mineralogy of these materials is highly varied. More than 30 heavy metal-bearing (Zn, Pb, Cd, Cu) mineral species have been iden- tified, these are sulfides, sulfates, carbonates, oxides and silicates. High substitution rates characterize these minerals, especially the sulfides and the oxides.

The industrial waste land of Mortagne-du-Nord (59) – II – Oxidizing alteration of the slags, hydrochemistry, geochemical modelling and remediation proposal

The slag backfills of the former sulfuric acid factory of Mortagne-du-Nord (59) are sulfide- and metal-rich (Zn, Pb, Cd). Sulfide oxidation leads to acidification of surficial groundwater and the dispersal of the metals into the environment, by flow towards the rivers which run along the site. The surficial groundwater pHs fall between 2.5 and 6.8 and the metal content is very high, reaching in places 6 000 mg/L of Zn, 2.5 mg/L of Pb and 600 mg/L of Cd. To conceive a remediation scenario, we tried beforehand to understand the phenomena that govern the oxidation of the sulfides. For this purpose, water levels and water compositions have been surveyed every 2 months during 4 years, a geochemical simulation of the alteration/oxidation has been proposed and leaching tests have been performed. 1. The survey of the water table level and periodical chemical analyses of the groundwater have shown that the slag alteration is reactivated when the water table drops until the sulfide-bearing ≪ fresh ≫ slags are unsaturated. 2. The analysed waters always show an unbalanced negative ionic charge. Geochemical calculations allow to propose several equilibrium models of the waters and to conclude that the presence of thiosulfates (S2O23−) in the original waters most likely explains the observed ionic disequilibrium. 3. The geochemical modelling of the slag alteration, first by percolation in unsaturated conditions (allowing O2 supply) and then under saturated conditions (without O2 renewal), reproduces satisfactorily the chemistry of sampled groundwaters. 4. Leaching tests of the slags have been performed in the laboratory both by percolation (unsatured environment) and by cirulation (saturated and closed environment). These tests allowed to obtain alteration solutions comparable to the waters sampled on site, with progressive ≪ aging ≫ of the material, in agreement with the decrease of the dissolved metals observed on site during the 4-year survey. Moreover, the tests confirm the importance of the oxygen supply in the reactivation of the alteration. The evolution of the groundwater chemistry, the thermodynamic modelling, as well as the leaching experiments allow to determine with some details the alteration/oxidation mechanism and show that: (1) alteration is actived or reactived after a drop of the water table within the sulfide-bearing facies, (2) the oxygen supply by diffusion in the poral air is the driving force of slag oxidation, and (3) maintaining the backfills in saturated conditions practically stops alteration. The geochemical evolution of the site is directly related to its history, with successive re-profiling of the channels which have lead to a lowering of the water table of about 2 m. The remediation should be focused on in-situ processing (water treatment aimed to lower acidity, active barriers, …) rather than on ex-situ (excavation) solutions.