INDURATED CARBONATE HORIZON IN CHALKY CHA MPAGNE - FUNCTION OF DIAGENESIS AND BIOLOGICAL EFFECTS UNDER TEMPERATE CLIMATE

Indurated carbonate horizons in Champagne spread out on the top of per iglacial cryoturbated chalk formation in contact with the present day soil or a palaeosol. The indurated carbonate horizon appears as a disc ontinuous conglomeratic hardpan composed by cryoclastic chalky sands a nd gravels coated by cemented limey-clayey silts (binding material). M icromorphologically, the binding material is composed of five entities : (1) matrix with detrital elements (composed of elementary chalk grai ns, quartz, feldspars, glauconite, and organic matter); (2) microspar cement (composed of calcite crystals 4-10 mu m in size, due to coalesc ence of elementary chalky grains of the matrix with detrital elements) ; (3) sparry cement (with spar crystallizations up to 20 mu m in width ); (4) calcitic acicular phase (composed of monocrystalline needles of MA type. typically 7-30 mu m long); (5) matrix of needles and micro-n eedles frame (composed of grains of the matrix with detrital elements in a micrite thrombolitic cement with M microneedles and MA needles). Fluorescent microscopy observations coupled with quantitative image an alysis allowed the respective part of each entitle in the cementation to be characterized. Fongical and bacterial biomineralizations affect mainly the top of the indurated horizons. The physico-chemical factors (recrystallization of the matrix with detrital elements to microspar cement, then to sparry cement) appear to be the predominant process wh ich explains most of cement in bottom and in the middle zone of the in durated layer. The observations of five entities show a physico-chemic al process (diagenesis) and a process of biomineralization followed by diagenesis (biodiagenesis). The two diagenetic processes of induratio n are complementary: (1) the physico-chemical sequence of recrystalliz ation is the major process of cementation and (2) the biodiagenesis wi th growth of calcite needle-fibres that causes porosity decrease and m icrite thrombolitic cementation around the pores.