HR: 1340h
AN: V23A-0621    [Abstracts]
TI: Correlation Confidence: A Systematic Approach to Presenting and Evaluating Tephra Data
AU: * Wallace, K L
EM: kwallace@usgs.gov
AF: USGS/Alaska Volcano Observatory, 4230 University Dr., Suite 201, Anchorage, AK 99508 United States
AU: Sarna-Wojcicki, A M
EM: asarna@usgs.gov
AF: USGS/Tephrochronology Laboratory, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: Tephra deposits reflect the source, volume, and explosivity of a volcanic eruption, the geochemical composition and physical character of its parent magma, and the age of eruption. In addition and owing to the widespread distribution of tephra deposits, they are often used to establish age, stratigraphic position, and correlation over long distances for associated deposits. To make reliable tephrostratigraphic correlations, it is important to demonstrate equivalence among tephra outcrop and core locations when tephra layers cannot be continuously traced. It is also important to demonstrate dissimilarity between candidate sample pairs of known different ages. Identification and correlation of unknown tephra layers is commonly done using glass-shard chemistry. This approach may be inconclusive or unreliable, especially where geochemical differences between tephra deposits are small (as in the case of tephra erupted from the same vent or eruptive source area), or where tephra is chemically heterogeneous. Attempts at quantifying the similarity between tephra samples is commonly limited to use of the similarity coefficient (SC) and cluster analyses generated from geochemical data which do not include the role of other important tephra and stratigraphic characteristics including 1) field characteristics and stratigraphic sequence 2) petrography, 3) physical glass shard characteristics, 4) geochemical trends (degree of scatter, polymodality, linearity, replicate analyses) and 5) associated qualitative and quantitative chronologic information. Correlation of tephra deposits is often based on a high SC value and other important and distinguishing characteristics of the tephra that could increase the confidence level of the correlation are often overlooked. We propose here a means of assigning an index of correlation probability by presenting data in the form of a matrix and assigning numerical values to candidate pairs by independently weighting tephra characteristics as parameters for correlation. This technique shows that multi-parameter characterization and comparison among tephra samples is an improved means of reaching a more confident correlation. Each parameter, however, does not hold the same weight and should be independently weighted to reflect its significance to the correlation. This method encourages users to use a maximum number of parameters for correlation that increases the level of confidence in the resulting correlation.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting