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Grainsize Analyses of the Rio Blanco Tephra Deposit
Thirty-seven sieved samples of the Rio Blanco tephra were used to obtain median grain-size and sorting information, using the methods described by Walker (l971). The graphs above show representative analyses from proximal, medial, and distal samples along the dispersal axis. A histogram of one representative sample from each of the three groups is plotted behind the lognormal size distribution of two representative samples (which plot as lines on a probability graph). The proximal samples exhibit a broad size distribution and plot as gradational slopes, indicating the poor sorting of these deposits (with a maximum weight% around -4 phi). Medial and distal deposits, successively farther from source along the dispersal axis, illustrate a progressive decrease in median grain-size, increase in sorting efficiency, and depletion of fine material in the deposit. The sharp inflection point at 1 phi on the lognormal plots in the distal deposits is interpreted to represent the elutriation by wind of particles less than 1/2 mm in diameter.
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