PROGRESS AND PROBLEMS IN LATE CENOZOIC TEPHROCHRONOLOGY, WESTERN NORTH AMERICA

Improvements in chemical analytical techniques in the last ~50 years make it possible to analyze volcanic ash layers with great precision, and to develop compositional criteria by which these layers can be identified and correlated. Concurrent developments in isotopic and other numerical dating methods have taken place during about the same time. Together, these developments have made it possible to develop a spatial and temporal tephrochronological reference framework for late Cenozoic sediments and rocks of North America that can be integrated with other chronologic and stratigraphic techniques (for example, magnetostratigraphy, marine oxygen isotope stratigraphy), and thus provide robust age and stratigraphic control for numerous earth-science studies.

The most ubiquitous material in tephra, volcanic glass, generally provides the most definitive chemical characterization of individual tephra layers. Six laboratories in North America routinely provide analytical services for tephra identification by this method.

Problems in identification of tephra exist when tephra is altered. Richard L. Hay was one of the first to developed ways in which the problem could be resolved using stratigraphic, petrographic and chemical techniques (e.g., his study of Olduvai Gorge, Tanzania). His work has stimulated the development of new techniques to study altered tephra.

Another problem in tephrochronology that needs to be resolved is the characterization of chemically similar suites of tephra erupted from the same volcanic sources (such as the Mono Craters in California). These problems are being resolved in part by application of new analytical techniques, such as the ion microprobe and ICP-MS.

Perhaps the greatest problem facing tephrochronology in the U.S. is the current funding climate. Budgetary constraints are threatening the survival of tephrochronology laboratories in earth science institutions. Preservation and exchange of tephrochronological data and its accessibility may also be in jeopardy. Loss of these capabilities and data would not only be a major setback for the advancement of earth science but would also pose major problems for agencies and institutions seeking to address critical societal issues such as geologic hazards mitigation, natural resource management, and global climate change.