GRAIN SIZE ANALYSIS OF BASALTIC TEPHRA AT SINKER BUTTE VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

A thick section of phreatomagmatic tephras is well exposed in the walls of the Snake River canyon near Sinker Butte. The lower part of this section contains a sequence of gray colored tephras which were previously interpreted as having formed in a shallow fresh water lake. This project provides the first detailed analysis of the grain-size characteristics of these deposits and integrates these data with field observations to better understand how the tephras were deposited. A total of 10 samples were collected from three exposures containing rocks with a variety of different bedding characteristics. All of the samples had to be disaggregated by soaking in a dilute solution of hydrochloric acid. Size distributions for phi values ranging from -3 to 5 were obtained by sieving the disaggregated samples.

Statistical analyses of the grain size data for each tephra unit were performed and the results were plotted on a graph of Inman sorting coefficient versus median phi values. In general, samples from the lowest part of the section are more poorly sorted and plot with the field of pyroclastic surge on the diagrams of G. P. L. Walker. Samples taken from higher in the gray tephra sections are generally more well sorted, which is consistent with their having been reworked by water or eolian processes. The grain size data appear to support field interpretations for the origin of individual units based on the bedding characteristics.

In addition to the size analysis, each tephra sample was examined under the binocular microscope. Some samples appear to consist of more than 90% percent juvenile clasts whereas others are almost entirely composed of accidental sand grains. Thin sections will be made of selected specimens and analyses will be performed using a petrographic microscope. Petrographic data will be combined with the detailed field observations and grain size data to give a better understanding of the depositional environments in which these tephras formed.