TEPHROCHRONOLOGIC METHODS FOR ANCIENT ASH BEDS: EXAMPLES FROM THE LATE ORDOVICIAN

Cenozoic tephrochronology plays an important role in stratigraphic analyses when integrated with independent data sets created by magnetostratigraphy, biostratigraphy, and sequence stratigraphy. Conversely, tephrochronology of ancient tephra beds can be of greater benefit because there are fewer and less reliable independent chronologies. However, ancient ash-fall beds have undergone significant diagenesis and their depositional characteristics are often completely obscured, making correlations based on traditional approaches very problematic. Despite these difficulties, unaltered volcanogenic phases are typically found within most beds and thus several geochemical methods are available for correlation tools. Such methods use the chemical composition of melt inclusions in quartz, element and isotope composition of apatite, and sometimes biotite chemistry. Any one of these approaches may facilitate correlation in restricted areas. There is potential for continental and, possibly, intercontinental ash-fall correlation with the combination of these approaches in concert with high precision radiometric dating of suitable mineral phases. It appears that ash-fall events are preserved in time-restricted intervals, thus not all of Phanerozoic time can be equally well correlated over substantial distances with tephra chemostratigraphy. Based upon a literature review, the Lower Ordovician, Pridoli, Late Devonian, Cisuralian, Early Triassic, and Middle Jurassic epochs lack enough ash-fall beds for wide-scale correlation purposes. Specific examples of Late Ordovician ash-fall beds and their phenocryst chemistry will be given and potential application of methods for other time intervals will be discussed.