Abstract

Indurated carbonate horizons in Champagne spread out on the top of periglacial cryoturbated chalk formation in contact with the present day soil or a palaeosol. The indurated carbonate horizon appears as a discontinuous conglomeratic hardpan composed by cryoclastic chalky sands and gravels coated by cemented limey-clayey silts (binding material). Micromorphologically, the binding material is composed of five entities: (1) matrix with detrital elements (composed of elementary chalk grains, quartz, feldspars, glauconite, and organic matter); (2) microspar cement (composed of calcite crystals 4-10 mu m in size, due to coalescence 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-needles frame (composed of grains of the matrix with detrital elements in a micrite thrombolitic cement with M micro-needles and MA needles). Fluorescent microscopy observations coupled with quantitative image analysis allowed the respective part of each entity 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 of microspar cement, then to sparry cement) appear to be the predominant process which explains most of cement in bottom and in the middle zone of the indurated layer. The observations of five entities show a physico-chemical process (diagenesis) and a process of biomineralization followed by diagenesis (biodiagenesis). The two diagenetic processes of induration are complementary: (1) the physico-chemical sequence of recrystallization is the major process of cementation and (2) the biodiagenesis with growth of calcite needle-fibres that causes porosity decrease and micrite thrombolitic cementation around the pores.

First Page Preview

First page PDF preview
You do not currently have access to this article.