LITHOSTRATIGRAPHY AND SHEAR-WAVE VELOCITY IN THE CRYSTALLIZED TOPOPAH SPRING TUFF, YUCCA MOUNTAIN, NEVADA Page: 3 of 15
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lithostratigraphic features (and therefore units) are the cumulative result of deposition, welding, and cooling processes, the
lithostratigraphic framework represents the three-dimensional distribution of rock properties, such as density and porosity
(and the types of porosity). It is in this context of lithostratigraphic units and distributed rock properties that core samples
from the various units have been analyzed for shear-wave velocity characteristics.
In the densely welded and crystallized rocks of the Topopah Spring Tuff, the lithostratigraphic features and the
associated depositional, welding, and cooling processes are:
1. Depositional features include crystal fragments, glass shards, and pumice and lithic clasts. Crystal fragments are
from crystals that had grown in the magma chamber prior to eruption, glass shards and pumice clasts are from
fragmentation of the magma during eruption, and lithic fragments are incorporated from the walls of the magma
chamber, conduit, or vent area, or picked up along the ground surface. For properties such as porosity, the amount
of crystal fragments and lithic clasts does not substantially affect the properties (unless the fragment or clast
amounts are large compared to sample size).
2. During welding, glass shards and pumice clasts deformed, resulting in an overall decrease in porosity to form a
dense, glassy rock (vitrophyre) typically with less than 10 percent porosity [Buesch, 2000]. Distribution of welding-
influenced porosity varies based on whether vapor, which was initially interstitial to the depositional grains, was
redistributed away from some areas and accumulated elsewhere. Where the vapor accumulated at super-lithostatic
pressure, cavities formed and inflated to form lithophysae.
3. During crystallization of the glass matrix, the minerals formed typically are 65 to 75 percent feldspar with 25 to 35
percent silica polymorphs (quartz, cristobalite, and tridymite) [Chipera and others, 1995], and most features
associated with crystallization form a spatially systematic distribution that resulted from varying amounts of
interaction with the vapor. Rims of fine-grained material, described in Figure 2, formed in locations with the
greatest exposure to the vapor (around lithophysal cavities and along fractures). Spots are similar to rims, except
there is no cavity. Thin borders of very fine-grained material formed around many rims (Fig. 2). The matrix-
groundmass near the borders typically differs in color and is slightly coarser grained than the matrix-groundmass
farther from the lithophysae or fracture (Fig. 2). Porosity of these crystallization features varies substantially;
porosity of the matrix-groundmass is about 10 percent, and rim and spot materials are about 30 percent with a range
from 20 to 40 percent [Otto and Buesch, 2003] (Fig. 2).
Page 3 of 15
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BUESCH, D.; STOKOE, K.H. & SCHUHEN, M. LITHOSTRATIGRAPHY AND SHEAR-WAVE VELOCITY IN THE CRYSTALLIZED TOPOPAH SPRING TUFF, YUCCA MOUNTAIN, NEVADA, report, March 20, 2006; Las Vegas, Nevada. (digital.library.unt.edu/ark:/67531/metadc892941/m1/3/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.