The Lund Ash-flow Tuff

The Lund Ash-flow Tuff is the uppermost member of the Needles Range Formation, located in southwestern Utah and eastern Nevada. This member can be correlated over an area of several thousand square miles. A detailed study of petrographic and physical features of the Lund show vertical and lateral variations in mineral content and welding intensity. Vertical differences are related to changes in the magma chamber during the Lund eruption interval. Several different ash-flows make up the Lund member. All mineral concentrations vary laterally, with concentration decreasing away from the center of the Lund field. Vertical and lateral variations in physical features of the member are also present. Porosity increases vertically within the member; bulk density decreases. These show that the Lund Tuff had a simple cooling history with only minor welding breaks between successive ash-flows. Lateral variations in physical features include decreases in bulk density and grain density and an increase in porosity away from the thickest central area of the unit. These decreases are attributed to lateral sorting and a decrease in welding temperature away from the center of the unit. Possible source areas for the Lund Tuff are suggested from analysis of lateral variations within the member, and consideration of the location of active eruption centers at the time of the Lund deposition --Abstract, Page ii