Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Compaction And Welding Of Ignimbrites: The Resting Springs Pass Tuff, Inyo County, California

Abstract

The complex physio-chemical evolution of welded ignimbrites (‘ash-flow tuffs’), loosely termed 'diagenesis', is not well-understood; moreover, how these physio-chemical changes alter various physical and mechanical properties of the rock such as porosity and permeability has not been well-studied either. The ‘diagenesis’ of pyroclastic rocks is inherently more complex than that of clastic sedimentary rocks because of the plethora of syn- and post-depositional changes that they experience: compaction, welding, cooling, diagenesis, devitrification, and mineral alteration. The goal of this project is to use first-order petrofabric (strain measurements at different scales [field and thin section]) and petrophysical data (porosity) to map the distribution of strain throughout a single tuff and to assess the qualities of each of the data-sets and the strength of correlations betwen them. For example, porosity is typically used as a proxy for welding intensity because it is easy to measure; however, porosity is likely to be an unreliable, or at least inconsistent, proxy for strain because it is variably affected by post-welding vesiculation (porosity increase), pore-infilling by syn- and post-emplacement minerals (e.g., chalcedony, zeolites, etc. [porosity decrease]). Similarly, strain measurements of individual glass shards and grains in thin sections is complicated by uncertainties in pre-welding shape and size populations. This is the first study to explicitly test these different parameters, and the validity of using them individually or in concert. Data for this study come from the Resting Springs Pass Tuff (RSPT), a 9.7 Ma. rhyolitic ignimbrite located in the Resting Springs Range, immediately east of Shoshone, CA. Axial ratios of pumice and fiamme measured in outcrop show that strain increases regularly from 0% at the base of the deposit to 84% in the near-central obsidian zone. Cavities observed from the obsidian zone and upward through the unit vary drastically in dimension and thus yield strain data that varies with height through the deposit, ranging between 31% and 89%. Preliminary strain data from thin sections taken from throughout the deposit yield data that is statistically viable and generally consistent with outcrop data.