Use of PCA in Tephra Correlation and Importance of Thickness Values for Volume Estimation

Abstract:

Discontinuous tephra layers were discovered at Burney Spring Mountain, northern California, USA. Stratigraphic relationships suggest that they are two distinct primary fall deposits. Geochemistry of the tephras from electron probe microanalysis was compared with geochemistry of known layers found in the region, to test for potential correlations, first using traditional binary plots and standard similarity coefficients.

Then, using principal component analysis, we were able to bound our uncertainty in the correlation of the two tephra layers. After removal of outliers, within the 95% prediction interval, we can say that the lower tephra layer is likely the Rockland tephra, aged 565-610 ka, and the upper layer is likely from Mt Mazama, the Trego Hot Springs tephra, aged ~ 29 ka, based on the majority of the glass shards.

The results were used to estimate the erupted volumes of the two deposits, by using the new thickness measurement together with others in the literature. The volume of the Rockland tephra was estimated to be 150-250 cu km, and of the Trego Hot Springs tephra, 10-50 cu km. It was found, however, that reported thickness measurements are not always detailed, and some reworked material is often included in measurements. As a result, the erupted volumes may be greatly over-estimated. Checking measured thickness against expected (model) thickness as a function of isopach area can provide useful information about potential redeposition. This would avoid an over-estimation of primary fall deposit volume, by giving an indication about locations where the original description should be studied in detailed and new thickness measurements made, if possible.