Abstract
Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called bentonite or K-bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.
Acknowledgments
In addition to the many colleagues, particularly Dennis Kolata and Stig Bergström, and students without whose collaborative work over the years this report would not be possible, I express my deep appreciation for the very thoughtful and constructive reviews provided by Alain Meunier and Chuck Ver Straeten, as well as to Rich April for his great attention to detail as Associate Editor in moving this manuscript through the publication process.
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