Médard Thiry

Palaeoclimatic interpretation of clay minerals in marine deposits: an outlook from the continental origin

Abstract As distribution of clay minerals in modern oceans seems to be controlled by contemporary climates, marine clays in ancient deposits have been widely used to reconstruct palaeoclimates. In this review, we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits. We examine successively how soils develop and behave, are reworked and palaeoweatherings are preserved, focusing the review on kaolinitic soils indicative of tropical wet climates. Several stages intervene between the time of clay mineral development in soils and their final deposition in the basin: (1) The persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record. Although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones, this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present. (2) Possible development of deep kaolinitic profiles at high latitudes and cold conditions, under CO 2 -enriched palaeoatmospheres, or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction. (3) Erosion and transport processes always cloud the signal to some extent, because frequently, erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones. (4) The arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time. It is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals. Soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 Ma. At present, the palaeoclimatic interpretations of marine clay assemblages are yielding, at best, nothing more than rather broad palaeoclimatic information. This approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies. Clay mineral assemblages may provide integrated records of overall climatic impacts, whereas other techniques are more likely to reveal local or temporary climates.

Interpretation of palaeoweathering features and successive silicifications in the Tertiary regolith of inland Australia

Detailed studies of morphological, micromorphological and geochemical characteristics of silcretes in the deep bleached and weathered regolith across a large area of inland Australia have provided a new interpretation of the history of the regolith and its climatic and morphological evolution during the Tertiary. Pedogenic silcretes have distinctive morphological and mineralogical features caused by a succession of phases of silica dissolution and recrystallization resulting from multiple episodes of water infiltration and percolation under alternately wet and dry climates. These are the oldest of the regolith features. Deep, bleached profiles formed over a wide area in a variety of substrates ranging from Precambrian granites to Palaeozoic sandstones, Cretaceous sediments and Tertiary deposits, and represent the second major stage in regolith development. These profiles, in which kaolinite coexists with gypsum, alunite and opal, formed by reaction of the substrates with saline groundwaters, the water-table levels of which progressively fell over the region. Extensive networks of termite burrows constructed to great depth in the bleached regolith followed the water tables down. The climate was warm and dry with a high water deficit. Groundwater silcretes formed near-horizontal lenses and pods of porcellanite and jasper in the bleached regolith. They preserve the primary fabric of the host rock. Groundwater silcretes post-date the construction of termite burrows and were formed during a rise in groundwater tables across the landscape, in places to near-surface environments in broad landscape depressions. The climate was more humid but the presence of gypsum during silicification demonstrates that the groundwaters were still saline. Red–brown hardpans are the youngest silicification features and represent periods of successive infiltration and percolation, and waterlogging, during high rainfall or flood events. They are confined to low regions in the landscape. Mineralogical and geochemical analysis of the bleached profiles, together with geochemical modelling, suggests that ferrolysis is the most likely cause of acidity in groundwater leading to the development of the bleach profiles and/or alunite. Present-day groundwater tables are both at low levels and sulphate-rich. It is possible that acidic alteration leading to bleaching is still active around the extensive playa landscapes in the region.

Silicifications de nappe dans les formations carbonatées tertiaires du piedmont atlasique (Hamada du Guir, Maroc).

ResumeLes formations tertiaires du systeme de piedmonts atlasiques du Sud marocain montrent de nombreuses silicifications qui affectent les differents facies de la serie : les gres calcaires, les conglomerats, les calcaires lacustres, aussi bien que les facies plus argileux. Les accidents siliceux sont de tailles et de facies varies, allant d’echardes millimetriques a des masses pluri-decimetriques. Dans les facies argileux (torba), ce sont des plaquettes et des tubules de silice translucide. Dans les calcaires, ce sont surtout des facies silexoides, a morphologies variees et a contours irreguliers. Dans les gres calcaires, ce sont des silicifications scoriacees, quelquefois disposees suivant un reseau de conduits anastomoses,La petrographie revele que les depots de silice dans les vides forment une part importante des silicifications. Les silicifications translucides des facies argileux sont entierement formees de concretions de calcedoine. Dans les gres calcaires, les silicifications resultent de la cim...

Paléoaltérations kaoliniques: signification climatique et signature dans la colonne sédimentaire

Abstract Important parts of wide kaoltnic surfaces of the modern tropical areas are inherited from palaeoweatherings dating back to the Cretaceous. The Cretaceous kaolinic event is also well Known in the sedimentary sequences, at extra-tropical latitudes, inclusively. Taking into account the CO 2 high level of the Cretaceous atmosphere, geochemical modelling allows the explanation of the exeptional development of kaolinic soils at that time. The analysis of the relations which exist between the rate of formation of kaolinic soils and the factorsor causing their erosion draws attenrion to the difficulties of climatic interpretation of kaolinite deposits in the basins.

Silicifications pédogénétiques dans les dépôts hamadiens du piémont de Boudenib (Maroc)

Resume— Le systeme de piemont atlasique du Sud marocain, d’âge tertiaire, est constitue dans le secteur de Boudenib de deux unites distinctes. La llamada de Boudenib forme l’unite inferieure et se caracterise par d’importants encroutements carbonates, riches en dolomite et attapulgite. L’unite superieure de la llamada du Cuir marque une reprise de l’erosion et le depot de materiaux relativement peu alteres.Des silicifications affectent les differents facies de l’unite inferieure, ainsi que les gres cretaces sous-jacents, et jalonnent une surface d’erosion intra-fonnationnelle. Ces silicifications sont precoces, puisque remaniees dans les depots ulterieurs. Les illuviations d’opale et les accumulations gramdaires intercalees entre les concretionnements successifs de silice attestent d’environnements pedologiques. La silicification plus forte vers la partie superieure des coupes, ainsi que l’association frequente des facies silicifies avec des niveaux hioturhes et alteres sont egalement a rapporter a des di...

Paléopaysages et paléoenvironnements sidérolithiques du Nord du Massif central (France)

Contrasting palaeolandscapes, formed from deep palaeodrainages and pediments surrounding bedrock palaeoreliefs, are buried by Siderolithic deposits. Two sedimentary units have been distinguished: fluviatile deposits with feldspar, kaolinite and smectite fill up the palaeovalleys; debris flow and sheet flood deposits with quartz gravels, kaolinite and illite form the pediments and are capped by red silica duricrusts. The deposits and the palaeolandscapes point to an evolution from tropical climates with contrasting seasons towards drier ones.

Pleistocene cold climate groundwater silicification, Jbel Ghassoul region, Missour Basin, Morocco

Surficial silicifications have been long considered to be indicative of warm and dry climates. Here we describe various forms of supergene silicification in a Miocene lacustrine sequence in the Missour Basin near Jbel Ghassoul (Morocco) in a landscape with accentuated relief. The silicification is almost exclusively limited to a 10–40 m wide zone from the edges of scarp and mesa landforms. This distribution is interpreted to record the locations where groundwaters which produced the silicification discharged from a higher level palaeolandscape. The main component of the silica was imported late and significantly post-dates the deposition of the sediments. This implies that significant volumes of silica-bearing solutions flowed through these formations in response to a hydraulic gradient generated by relief. Silicification thus occurred only after uplift and incision of the sedimentary fill of the Missour Basin. The zones of silicification of the Jbel Ghassoul sequence can be linked geomorphologically to remnants of high-level pediments that have been dated in the literature to the early to middle Pleistocene and interpreted to have been formed during cold climates. Low temperatures in outcrops near the discharge zones during cold periods are considered to be a key factor in silica precipitation from groundwaters.

Impacts of one century of wastewater discharge on soil transformation through ferrolysis and related metal pollutant distributions

Discharge of wastewater leading to notable soil surface contamination is widely reported. But few works highlight the fast dynamics of soils and their morphological transformations that may result from such anthropogenic activities. Near Paris (France), sandy Luvisols were irrigated with urban wastewater since the 1890s. Within and outside the discharge area, the soil cover presents decameter-sized cryogenic structures. We studied macro morphological soil characteristics, soil chemistry and clay mineralogy on selected bulk samples, as well as contemporary pedofeatures and related metal pollutant distribution patterns in soil thin sections from subsurface horizons. Annual repetitive waterlogging and drying cycles initiated a hydromorphic soil forming process: ferrolysis, based on iron reduction producing alkalinity under anaerobic conditions, and iron oxidation producing acidity in aerobic conditions. Its intensity was enhanced at the top of thick clay-rich B-horizons in the center of cryogenic structures. The polygonal soil structure favored the evacuating of soil water and alkalinity. Within one century, such recurrent alternating redox conditions have led to clay destruction, removal of iron, strong bleaching of the E horizon and formation of abiotic Fe-rich pedofeatures at depth. In addition, between anaerobic clay-rich B and aerated E or C horizons, the contrasting hydrodynamic conditions enhanced manganese (Mn) oxidizing fungal activity and the formation of biotic Mn-rich pedofeatures. Both types of pedofeatures trapped metal pollutants in deep soil horizons. In our work, the impacts of centenary anthropogenic activity were amplified by millenary cryogenic structures, acting together to promote fast soil dynamics, within a few decades.

Geophysical Assessment of Soil Volumes Polluted with Metal Elements from Long-term Waste Water Irrigation

At WNW of Paris, a large area of coarse textured soil have been irrigated during one century (until 1999) with urban waste water so that large amount of soil organic matter (SOM), dissolved mineral compounds and metal pollutants accumulated in the surface horizons. One must now identify the volumes of polluted soils in order to favour as the stabilization or decontamination of these sites. Resistivity mapping and complementary magnetic properties measurements are used to assess the volume of polluted soil.