Significance of the flattening of pumice fragments in ash-flow tuffs

Abstract

Abundant pumice fragments occur in the Apache Leap Tuff of east-central Arizona, an ash-flow sheet with a maximum thickness of 600 m and a K-Ar age of 20 m.y. The amount of flattening of pumice fragments is widely variable at any particular locality, but systematic measurements show that the mean degree of flattening, defined as the "flattening ratio," steadily increases from the top downward into the body of the sheet. Ultimately the fragments are so compacted that they lose their identity. On a logarithmic scale the plot of flattening ratios is approximately linear relative to depth of burial.The uniform downward increase in flattening combines with evidence obtained from zoning and specific gravity characteristics to show that most of the deposit is a single cooling unit. Because of the uniform trend, flattening also provides a guide to the original thickness of overlying tuff at localities at which fragments can be measured. This permits the development of stratigraphy for the seemingly uniform deposit and provides a means to estimate pre-erosion thickness of the ash-flow sheet and the amount of stratigraphic throw on faults. A mining company used flattening ratios to predict successfully the ash-flow thickness cut by a new shaft.Postemplacement crystallization and diagenetic processes have greatly reduced the initial porosity of the deposit, and present porosity values erroneously indicate a considerably higher degree of welding than is inferred from deformation of the pumice fragments. It seems that in deposits where crystallization and diagenesis have been significant, flattening ratios of pumice fragments may be a better guide than porosity to the degree of welding that occurred during cooling of the deposit.The change of flattening ratio with depth can also serve as an approximate guide to the relative viscosity of pumice during emplacement. Viscosity is determined chiefly by temperature, chemical composition, volatile content, and crystalUnity. The downward change in flattening ratio in the Apache Leap Tuff is gradual, indicating a relatively high viscosity.By assuming high volatile content and low groundmass crystallinity at the time of emplacement, the high viscosity can be ascribed to the combined result of nonperalkalic chemical composition and relatively low temperature.

Fiamme textures in volcanic successions: Flaming issues of definition and interpretation

“…Smith, 1960b;Ross and Smith, 1961;Ragan and Sheridan, 1972;Peterson, 1979;Sparks and Wright, 1979;Reedman et al, 1987;Scheepers andNortje, 2000, Hildyard et al, 2000). However, fiamme can occur in diverse volcanic facies, and it is therefore important to separate the texture from the interpretation of its origin.…”

Ranking welding intensity in pyroclastic deposits

“…Pronounced changes in physical properties attend welding; as welding intensifies, for example, primary porosity is reduced, density increases (e.g., Ragan and Sheridan 1972;Streck and Grunder 1995;Rust and Russell 2000) and the deposit becomes progressively more foliated (e.g., Smith 1960a; Ragan and Sheridan 1972;Sheridan and Ragan 1976;Peterson 1979). For most pyroclastic deposits, especially those resulting from en masse deposition (e.g., Sparks 1976;Sheridan and Ragan 1976;Wright and Walker 1981), the strain due to welding accumulates immediately after deposition (e.g., Smith 1960a; Ross and Smith 1961;).…”

Volcanic settings and their reservoir potential: An outcrop analog study on the Miocene Tepoztlán Formation, Central Mexico

“…incipiently welded (e.g., Peterson, 1979;Streck and Grunder, 1995) or rank II of welding according the scheme of Quane and Russel (2005), 2) partially welded (Smith, 1960) or rank III after Quane and Russel (2005), 3) moderately welded (Wilson and Hildreth, 2003) or rank IV after Quane and Russel (2005), and 4) densely welded (Smith, 1960;Sheridan and Ragan 1976;Peterson 1979;Wilson and Hildreth, 2003) or rank V after Quane and Russel (2005). The principal hydrothermal minerals in the samples include silica polymorphs, montmorillonite, and hematite.…”