Ordovician and Pennsylvanian berthierine-bearing flint clays

We report here the first occurrence of berthierine-bearing flint clays, one Ordovician and one Pennsylvanian in age. They are characterized by a berthierine-kaolinite-boehmite (bkb) assemblage. The Pennsylvanian flint clay from northeastern Kentucky is more typical in that it occurs in association with coal measures.

The Ordovician occurrence from northwestern Illinois is the oldest flint clay of which we are aware. Because it is Ordovician, it formed before the evolution of terrestrial vascular plants. All previous reports of flint clays point to a genetic connection between flint-clay formation and the growth and decay of plants (Bohor and Triplehorn, 1993). Except for the high berthierine content and greenish color, the physical properties of this flint clay are similar to those of other kaolinitic flint clays. Some samples of the Ordovician flint clay are nearly pure berthierine.