Ordovician and Pennsylvanian Berthierine-Bearing Flint Clays

Abstract

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. The Ordovician flint clay is a facies of the Neda Formation (~440 Ma), the uppermost formation of the Ordovician Maquoketa Group, which, in northern Illinois, northeastern Iowa, and southern Wisconsin, is unconformably overlain by Silurian carbonates. This unconformity is exposed in a narrow northwest-southeast corridor in the vicinity of the twin cities of Sterling and Rock Falls, Illinois. This corridor was formed between two glacial till sheets, one deposited by an Illinoian glacier that reached its southern limit to form the north side of the corridor, and the other by another lobe of Illinoian ice that pushed generally to the southwest so the northern edge of this tongue forms the southern margin of the corridor (Figure 1⇓). This work began when Dr. Thomas Berres of Northern Illinois University brought this material to our attention. Native Americans of the Archaic (~5000–2000 BP) and Woodland (~2000 BP) periods used this flint clay from northwestern Illinois as a pipestone, i.e., for carving artifacts including ceremonial pipes (Berres, …