Apparently there is a bit of a paradigm shift on, in some aspects of geology. Climate regimes, such as cool/wet or cool/dry, and hot/wet or hot/dry, seem to affect the types of clays that form ...even more significantly than the parent material from which the clays are formed!
Fortunately, this means we can now tell more about the past climate (but less about the past geology, I suppose). Certainly though, you'll be looking at the most interesting time; the past 50myr brings major mammalian influences into the biogeochemical brew ...along with the grasses they ate (and spread equatorward as the planet cooled, the Atlantic opened, and the Seasons finally developed as the ice caps grew into stability).
I only recently learned that what we call "regular" good soil, is also referred to as "interglacial soil," so it has
not been forming and building deposits for most of the planet's history. Happy Hunting!
~
