LITHOLOGY OF THE TUFF OF LYLE SPRING, YELLOWSTONE VOLCANIC FIELD

The Tuff of Lyle Spring within the Yellowstone Volcanic Field occurred after the eruption of the Huckleberry Ridge Tuff and preceded the Mesa Falls Tuff. The aim of this study is to use field and petrographic descriptions to explain the pumice distribution associated with this small-volume, explosive rhyolitic eruption. The tuff was subdivided into a massive lower portion (6 meters) and a blocky, fractured upper tuff (3 meters). Numerous, large hand samples were collected in a vertical profile through the tuff, which exhibits a sequential dispersion of pumice. The lowest sample of the massive ash flow tuff is rich in elongated pumice (50 mm), contains both compressed (9x3 mm) and uncompressed (10x10 mm) altered pumice, and phenocrysts of sanidine, quartz and biotite. Moving upward through the profile, the lower part of the upper blocky unit is pumice poor, but compressed altered pumice (50x10 mm) are prevalent, and it contains the same phenocryst assemblage. The samples of the mid-upper blocky unit comprise altered pumice, which are even more compressed (55x5 mm) with the same phenocrysts within the light brown, extremely fine ash. The upper blocky unit contains compressed (10x2 mm) and uncompressed (20x15 mm), weakly lithified, altered pumice, and phenocrysts of sanidine, quartz and biotite. In another outcrop of the Tuff of Lyle Spring, 40 meters up the road, we extracted a large altered pumice sample (210x70x80 mm), and created thin sections. Microscope inspection of this large pumice confirmed this fragment resembles the smaller, elongated altered pumice within the main outcrop. Petrographic examination revealed the altered pumice consists primarily of devitrified glass along with quartz, feldspar and biotite minerals creating a spherulitic texture. Our results are the first to thoroughly describe the Tuff of Lyle Spring in such detail, offering insights into the deposition of this non-caldera-forming tuff.