Dating thermal anomalies in sedimentary basins: the diagenetic history of clay minerals in the Triassic sandstones of the Paris Basin, France

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Abstract
Rhaetian (Upper Triassic) sandy horizons were sampled from the Paris Basin from SW to NE, crosscutting the Rhaetian at different depths from outcrop in the NE to 2700 m in the centre of the basin. The smallest day sub-fractions (μm) from the deepest samples consist mainly of illite and chlorite having a K-Ar age of 〜190 Ma. Both minerals probably formed under specific hydrothermal conditions at high temperature, but at a burial depth of only 500 m. This thermal event could represent an echo of the "crustal" breakdown of the Northwestern European craton during the opening of the North Atlantic Ocean. Two other generations of illite-smectite mixed-layers formed in the same Rhaetian horizons at somewhat lower temperatures about 150 and 80 Ma ago. The three generations of clay minerals could be characterized and dated because of combined mineralogical, crystallographical and morphological data supporting the dating attempts.