STRATIGRAPHY AND PETROGRAPHY OF A TUFF CONE AT CON SHEA VOLCANO, WESTERN SNAKE RIVER PLAIN, IDAHO

Con Shea was one of several subaerial, basaltic phreatomagmatic volcanoes that erupted in the Western Snake River Plain shortly after Lake Idaho drained, about one million years ago. One large, primary outcrop and several smaller outcrops were used to study the deposits. Geologic mapping of the area and the nature of the deposits themselves suggest that the beds at Con Shea were deposited on the outer rim of the tuff cone.

At Con Shea, 14.5 meters of volcanic tuffs are exposed. The deposits were divided into 22 units based on sedimentologic characteristics. Three types of beds were identified: massive, planar bedded, and cross bedded. The massive beds lack internal structure and vary in thickness from 4 to 192 centimeters. Their compositions range from nearly 100% juvenile material to almost 100% accidental clasts. The planar stratified beds range in thickness from 11 to 78 centimeters. They contain silt to pebble sized particles of both juvenile and accidental materials, typically in a medium sand to silt matrix. The cross bedded units are cross-stratified and are 18 to 101 centimeters thick. They are similar to the planar beds both compositionally and in terms of sorting.

Thin sections were made from epoxy impregnated samples taken from each unit at Con Shea. Initial petrographic analysis has shown that all three types of deposits contain sideromelane and tachylite fragments that vary in size and in vesicularity from completely nonvesciular to highly vesiculated. No correlation has been made between the nature of the deposit in outcrop and the degree of vesicularity of basaltic glass. About half of the beds contain accretionary lapilli. The accretionary lapilli are up to six millimeters in diameter and both rim-type and core-type lapilli are found in the massive, planar bedded, and cross bedded units. The core-type accretionary lapilli are aggregates of coarse and fine grained particles. The rim-type lapilli have rims composed of fine-grained material surrounding cores that range in composition from glass or scoria fragments to coarse-grained accretions of accidentals, bound by fine particles. Rim-type lapilli are more abundant near the base of the section; whereas, core-type dominate in the upper portion