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In this paper, and in previous works, quartzites within the Sable de Fontainebleau are clearly shown to occur only near outcrops in Plio-Pleistocene plateau landscapes and are related to groundwater flows. These arrangements, together with dating of encased calcites, led us to consider that silicification occurred during Plio-Pleistocene glacial stages. The precipitation of silica was most likely triggered by cooling of groundwaters as they approached cold zones in the regolith close to points of discharge. We describe the arrangement and morphologies of quartzites in Tertiary sand formations in the Paris Basin to demonstrate how cold climates could have influenced hydrologic regimes and promoted silicification. The coeval precipitation of calcite and silica in gypseous formations at the edges of plateaux-bordering valleys, along with the dissolution of gypsum, also points to interactions between silica-laden groundwater and carbonate host rocks during cold periods. In parallel, the distribution and micromorphology of silicifications in associated Tertiary limestone formations suggests that they formed during cold climates in the Quaternary. These are key pointers to the role of groundwater in regolith environments in controlling silicification processes. We detail a link to palaeosurfaces of the distinctive meulières facies in the Paris Basin. Geotropic structures and micromorphological organisations are the basis of new ideas about their origin in a combination of vadose and phreatic environments and proximity to impervious cold horizons. The active zone in a permafrost landscape is a good hydrological example. All silicifications in Tertiary formations in the Paris Basin can be linked to the hydrology of Quaternary periglacial environments in a single model. This could apply more widely to similar silicifications elsewhere and be tested using new analytical techniques that date silicifications and unravel the isotopic relationships between silicification, groundwater composition and the prevailing climate.

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.

We tested the potential for sand calcites to serve as a novel paleoclimate archive by investigating their age and formation conditions. Fontainebleau sand calcites are Pleistocene in age (based on 14C and U-Th dating) and were primarily formed during glacial periods. δ13C values increase with the depth at which these sand calcites formed, consistent with open and closed CO2 systems. Interpretation of the δ18O-T relationship in sand calcites points primarily to their formation at a low temperature, around 2°C in shallow ground water and at about 9°C in deeper ground-water settings. Their occurrence, characteristics, and compositions suggest crystallization from paleo-ground waters in permafrost environments. Crystallization of sand calcites was triggered by degassing of cold carbonate-containing surface waters as they infiltrated warmer subsurface ground-water environments. We consider sand calcites to be important indicators of interactions between meteoric water and ground water in Pleistocene periglacial landscapes. Their disposition may point to specific features of periglacial landscapes, and their ages could permit an assessment of landscape incision rates. Large crystals and zoned spheroliths may, in fact, encapsulate continuous high-resolution records of continental glacial and periglacial paleoenvironments. [ABSTRACT FROM AUTHOR]