Ordovician 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.  However, there are key features to look for in each case that can aid in reliable identification.

Field Criteria: K-bentonites can be different colors when wet but are characteristically yellow when weathered.  Due to their clay rich nature, they will feel slippery and waxy when wet.  Accelerated weathering of K-bentonites causes them to be recessed into the outcrop face.  For thicker K-bentonites there is often a zone of nodular or bedded chert in the adjacent strata at both the base and the top of the bed.

Laboratory Criteria: Most bentonites and K-bentonites are smectite- or illite/smectite-rich, although some may contain a considerable amount of kaolinite, and those that have undergone low-grade metamorphism may be dominated by R3 I/S and/or sericite plus chlorite/smectite (corrensite) and/or chlorite.  Study of the non-clay fraction under a high quality optical microscope is satisfactory to determine what types of crystals are present in the sample.  Thin section study may also be used.  Isotopic ages can be extracted from K-rich phases, such as sanidine, amphibole or biotite, U-rich phases, such as zircon, or by fission track measurements on apatite or zircon crystals.