What is the time period of these chalk/clay layers?
The layers correspond to thousands of years, i.e. 23,000 years in the Danien timeline: 
The page sais that there is a variation of 23,000 for precession of the equinoxes and 100,000 for variation of ellipticity. https://planet-terre.ens-lyon.fr/image-de-la-semaine/Img421-2013-04-15.xml