Stratigraphie_cyclique

La cyclostratigraphie

La sédimentation de la craie est organisée en rythmes dont l’épaisseur élémentaire varie généralement entre 30 cm et 1,5 m. Cette caractéristique est connue depuis longtemps (Felder, 1986). L’épaisseur des rythmes et leur lithologie varie largement selon le milieu de sédimentation, allant de séries peu profondes comme celles du Bloc de Caux jusqu’aux séries profondes et turbiditiques comme celles de la Mer du Nord (Van der Molen, 2004).

Rytmes pluri-décimétriques dans la craie
Rytmes pluri-décimétriques dans la craie

Juignet & Breton ont analysé ces rythmes dans le Cénomanien et le Turonien du Bec de Caux. Ils s’expriment par des variations lithologiques :

  • La teneur en argile (ou de manière inverse la teneur en carbonate de calcium). Le pourcentage d’argile est plus élevé à la base du rythme et décroît rapidement.
  • -La teneur en glauconie (minéral silicaté vert foncé en petits grains) est plus forte à la base du rythme.
  • Les traces d’activité biologique. Les principales traces sont :
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