BENTONITES AND K-BENTONITES: ISSUES IN IDENTIFYING AND INTERPRETING ANCIENT TEPHRAS

The Ordovician record of explosive volcanism consists of examples of both near-vent pyroclastic flows and ignimbrites and distal sequences of altered fallout tephras known as K-bentonites. Questions frequently arise as to whether a particular clay-rich bed might be an altered volcanic ash fall in the form of a bentonite or K-bentonite. These beds are often datable using fission track and U/Pb dating of zircons, K/Ar, and Ar/Ar of amphibole, biotite and sanidine. Due to their unique composition, they provide an indispensable tool when correlating sections. The criteria for recognizing such beds are varied, but fall into two broad categories, field criteria and laboratory criteria. Ideally, one would want both, but often that is not possible. Here we will examine some of the key features to look for in each case that can aid in reliable identification.