Quaternary early silico-aluminous parageneses in cryptokarstic context (Normandy, France) and implications in clay neoformation processes.

Quaternary early silico-aluminous parageneses in cryptokarstic context (Nor mandy, France) and implications in clay neoformation processes. We describe a silico-aluminous paragenesis (halloysite, gibbsite, alumohydrocalcite) i n weathered Tertiary beds trapped within Quaternary cryptokarstic structure s (Normandy, France). This paragenesis was developed from gels neoformed in the immediate vicinity of carbonate levels that acted as neutralizers for (Si, All-rich acid solutions. The gels occur in millimetric to centimetric fissures. We refer to these neogeneses as to an early stage of the evolutio n that may lead to a complete transformation of gels into crystallized hall oysite, gibbsite... (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.