Régine Simon-Coinçon

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.

Tertiary paleoweatherings and silcretes in the southern Paris Basin

Abstract The southern border of the Paris Basin has undergone successive weathering events during the Tertiary. Fluviatile and lacustrine deposits are scattered, being mainly confined to several North-South trending grabens. These are generally devoid of fossils which would allow them to he dated. A discontinuous blanket of weathering products is the main record of this long period of continental evolution. These weathered materials result mainly from alteration of the Cretaceous flint-bearing chalk and are composed of corroded flints enclosed in brown and red mottled kaolinitic clays. In places, the succession of Cretaceous layers has been preserved in the paleoprofiles, testifying of in situ paleoprofiles, but most profiles are the result of reworking as shown by the mixture of materials coming from different stratigraphic horizons. These paleoweathered materials led to various residual and sedimentary deposits, from flint breccia along the graben scarps to flint gravels, clay deposits and lacustrine limestones in the grabens. These weathering products correspond to warm and humid climatic periods. Their reworking results from a renewal of tectonic activities combined with a period of drier climate. A later silicification process has affected the weathering cover and the Tertiary deposits of the graben as well. Extensive pedogenic silcrete crusts, with typical columnar structures, developed on the plateaux. This period of silicification corresponds to a climate with a marked dry season which led to the development of glacis. Massive silicification, preserving the primary sedimentary structures, developed in the grabens. This was sub-contemporaneous with the pedogenic silcretes which lie over them and which were developed at their expense. The significance of the massive silicification is not yet clear, nor the geochemical mechanism of their formation. In the process, primary clay minerals are destroyed and silica must have been fixed while aluminium was leached out of the profile. The silcretes are covered by lacustrine limestones and calcretes which accumulated in the lowest parts of the paleolandscape and toward the centre of the basin. They seal the silcrete development and are dated from Upper Eocene, a period of very dry climates as shown by thick gypsum deposits in the center of the basin. No remnants from Oligocene-Miocene times are preserved in the region studied. During Late Miocene, the disruption of these landscapes took place because a very low sea level led to the renewal of erosion. As a result residual landforms with siliceous crusts now cap the higher elevations.

Evolution of landscapes in northern South Australia in relation to the distribution and formation of silcretes

Geomorphological features on the southwestern margins of the Lake Eyre Basin provide a basis for interpreting the evolution of old landscapes containing pedogenic and groundwater silcretes. and thick bleached and weathered profiles. Recurrent sequences of cut-and-fill and duricrust formation have been identified and related to major sea level changes and tectonic movements in the Lake Eyre Basin. An extensive high pediment had formed around a basement inlier by the late Eocene. An armour of pedogenic silcrete developed on this pediment under alternating dry and wet climates during the late Eocene and Oligocene. The characteristic kaolinite + opal + alunite + gypsum assemblage of the bleached profiles formed in acid saline groundwaters during the Miocene at a time of regional low water table and arid climate. Groundwater silcretes formed in the bleached profile in response to dissection of the high pediment. They are related to a period of high groundwater tables. humid climate. and gradual sinking of the Lake Eyre basin. In the meantime, widespread low pediments (glacis) formed in Lake Eyre catchment, possibly in the early Pliocene. There is a good correspondence between a geomorphic approach, taking in to account the relationship between weathering features and palaeolandscapes, and the information provided from stratigraphic studies in the region.

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.

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.

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.

Landform Processes in the Post Coal-Mining Landscape, Bowen Basin, Australia. A Geomorphological Approach

This paper is a preliminary analysis of the most important geomorphic structures and processes observed at open-cut minesites throughout the Bowen Basin (Queensland, Australia). A number of different landform types are created during open-cut coal mining operations These may be modified by both natural geomorphic events and by reshaping processes that occur during rehabilitation. It is of importance in the development of a predictive capability about the long term behaviour of these types of landforms, and in order to design parameters for constructing such landforms, that geomorphic, physical, chemical and biological processes be identified and quantified in relation to the nature of landforms and the materials from which they have been constructed.