RECONSTRUCTING EARLIEST DIAGENESIS CONDITION USING SEDIMENTO LOGICAL AND GEOCHEMICAL CHARACTERISTICS OF GLAUCONITES . APPLICATION TO LIMESTONEMARLSTONE ALTERNATION IN THE BOULONNAIS
Résumé
Some of the geological formations of Upper Jurassic and Cretaceous age in the Boulonnais area (Strait of Pas-de-Calais, English Channel) show levels relatively rich in glauconite (a few weight% to 44 wt%). Recent works have shown that glauconite could have formed during (very) early diagenesis; this authigenic phyllosilicate could have thus witnessed the chemical conditions prevailing at the sediment-water interface or a short distance below it. This can be of particular interest in the case of the Assises de Croï Formation showing alternating carbonate beds and marly beds. The beds are of early diagenetic origin and contain populations of glauconite grains which formed even earlier. Glauconite can therefore be considered here as the privileged proxy to the sediment-water interface conditions during the development of the limestone-marl alternation, which is a common pattern in the sedimentary record worldwide. Glauconite was observed from the perspective of its chemical composition (including REE) as well as that of the particle size distribution. The results show that, if redox-proxying trace elements such as V, As, Ge can reflect the redox status of the conditions in which glauconite was formed, this is not the case for REE. The only parameter linked to REE which provides paleoenvironmental indications here is ∑REE, being negatively correlated with the sedimentation rate. It is suggested that REE were all the more incorporated to glauconite when the sedimentation rate was low, but without any appreciable fractionation among REE. Concerning the limestone/marl alternation, our results show that its development is linked neither to variations in redox conditions nor to hydrodynamic changes of the depositional environment. This is a diagenetic phenomenon, the cause of which remains to be determined. Finally, a relationship could be highlighted between the sedimentation rate and the size of the glauconite grains grown in situ.
Rares. Le seul paramètre lié aux Terres Rares qui fournisse ici des indications paléoenvironnementales est ∑REE, qui est manifestement lié à la vitesse de sédimentation. Il est suggéré que les Terres Rares étaient d'autant plus incorporées à la glauconite que le taux de sédimentation était faible, mais sans fractionnement visible au sein des Terres Rares elles-mêmes. Par ailleurs, concernant les alternances, nos résultats montrent que leur évolution n'est liée ni aux variations des conditions redox ni à celles de l'hydrodynamique du milieu de dépôt. Il s'agit d'un phénomène principalement diagénétique dont la cause reste à déterminer (la condensation sédimentaire est une cause probable ; J. -F. Deconinck, communication personnelle, 2024). Enfin, une relation a pu être mise en évidence entre la vitesse de sédimentation et la taille des grains de glauconite développés in situ : une relative condensation sédimentaire accompagne une augmentation de la taille des grains de glauconite. Au total, les grains de glauconite peuvent avoir une vraie valeur d'indicateur paléo-environnemental, à condition que leurs conditions de formation aient été bien définies.
Domaines
Planète et Univers [physics]Origine | Accord explicite pour ce dépôt |
---|