TOC | Abstract | Introduction | Geologic Data | Database | Acknowledgements | References | Appendices


This database is designed for a wide variety of applications by a wide variety of users. To guide users who may be unfamiliar with petrographic, mineralogic, and chemical data, we provide a few examples of the past application of data now within this database.

Volcanic eruptions are often complex, and the eruption of a single magma can result in a variety of rock types, or lithologies. An example is rhyolite of Echo Peak (stratigraphic symbol Tpe). This stratigraphic unit represents magma erupted 12.7 million years ago. Like many rhyolites, the ascending magma contained large amounts of volatile components, mostly water. Sudden outgassing from the top of the magma produced an eruptive column of ash that fell to form an initial deposit of bedded tuff. As the eruption progressed, perhaps in several stages, continued outgassing depleted later-erupted magma in volatiles, and eventually lava was erupted. This lava flowed over the bedded tuff, fusing the uppermost portion into a welded tuff. This eruptive history was first recognized by Byers et al. (1976a) from petrographic analyses of the different lithologies (Figure 11). The three constituent lithologies of rhyolite of Echo Peak are very different in appearance and physical character, and during field mapping, these lithologies were each assumed to represent a different stratigraphic unit. The welded tuff was initially thought to represent ash flow of the Pah Canyon Tuff.

Figure 11. Photo shows three very different lithologies from rhyolite of Echo Peak, all produced from the same magma. Their intimate relationship was uncovered using the petrographic, chemical, and mineral chemical data in this database. Volcanic lithologies are often very complex, and the relationship between adjacent exposures of volcanic rock frequently cannot be determined using field methods.

Subsequent application of mineral chemical analyses by electron microprobe has proven the common parentage for the three different lithologies from rhyolite of Echo Peak (Figure 12). In combination with detailed petrographic analyses, microprobe analyses have further demonstrated that rhyolite of Echo Peak was erupted across a large area. This unit occurs at Pahute Mesa, mapped as "pyroxene-bearing flows" by Byers and Cummings (1967), and also at the northern end of Yucca Mountain, some 30 km to the south (see Figure 1 for locations), where it was named "rhyolite flow" by Christiansen and Lipman (1965). The equivalency of these volcanic units cannot be established by field geology, because the distance between these locations is completely covered by younger rocks related to the Timber Mountain caldera.

Figure 12. Histograms of mineral chemical analyses by electron microprobe for the three different lithologies from rhyolite of Echo Peak. These analyses support the common magmatic parentage inferred from petrographic analyses for the three very different lithologies of Figure 11.

The application of petrographic analyses and mineral chemical analyses has demonstrated the widespread equivalence of older volcanic units within the SWNVF that were previously unrelated. Figure 13 shows the location of two major volcanic units on opposite sides of the Timber Mountain caldera that were derived from the same magma, and Figure 14 shows the stratigraphic column that was correlated on the basis of petrography and microprobe analyses (Warren, 1983b).

Figure 13. Location of tuffs and rhyolite lavas of Area 20 within Pahute Mesa region and rhyolite lavas of Calico Hills within Yucca Mountain region. These units are separated by the younger Timber Mountain caldera, and so cannot be correlated by field geologic methods. The application of petrographic analyses and mineral chemical analyses by Warren (1983b) demonstrated the magmatic equivalence of these two major units, now named the Calico Hills Formation. Volcanic rocks are absent within areas shaded yellow.

Figure 14. Correlation of stratigraphic units between the Pahute Mesa and Yucca Mountain regions, based on petrographic analyses and mineral chemical analyses by Warren (1983b). Symbols are for petrologic zones defined in the original paper; parenthetical symbols are those presently used for stratigraphic units in the database.

Petrographic analyses and mineral chemical analyses have also solved critical problems where two different stratigraphic units have identical field characteristics, and were miscorrelated. On the south face of Pahute Mesa, nonwelded ash flow tuffs occur beneath Tiva Canyon Tuff on opposite sides of the Scrugham Peak fault. This tuff was well characterized east of the fault as a very quartz-rich unit, the tuff of Blacktop Buttes. Petrographic analyses and mineral chemical analyses (Figure 15) demonstrated that exposures on opposite sides of the fault represent two very different units (Warren et al., 1985). This single miscorrelation prevented regional correlation of units within the lower half of the stratigraphic column of the SWNVF. The correct stratigraphic assignment for ash flows on opposite sides of the fault also demonstrated that the Scrugham Peak fault was active during at least two different episodes, establishing an important feature of tectonism within the volcanic field.

Figure 15. Histograms that illustrate use of petrographic and mineral chemical data to demonstrate that two exposures of nonwelded tuff with indistinguishable field characters are different units (Warren et al., 1985). Nonwelded tuff east of the Scrugham Peak fault, mapped as tuff of Blacktop Buttes, is mafic-poor Calico Hills Formation (Thp). Nonwelded tuff west of the fault is rhyolite of Delirium Canyon (Tpd). Cross-hatched patterns represent reference samples from each unit, whereas shaded patterns represent samples of nonwelded tuff that had been miscorrelated across the fault.

Petrographic, mineralogic, and chemical data within this database were initially applied to solve stratigraphic problems. These data have been well applied for this purpose, resulting in a robust characterization of the stratigraphic column (Figure 16). However, other applications of these same data are possible, including statistical analyses to define relationships among petrographic, mineralogic, and chemical data, and spatial relationships for each type of data. The possible applications of this database are many. Our primary goal in designing and publishing this database is to provide this information in a form that is easy to apply.

Figure 16. Selected petrographic, mineral chemical, and lithological analyses for some of the stratigraphic units within the SWNVF. The more than 300 volcanic units, for example Ttt, reside within volcanic assemblages such as Tt. Assemblages consist of a group of units that display similar petrographic, mineral chemical, and lithologic character. Parent-child relationships imbedded in the stratigraphic table allow easy extraction of data for related groups of stratigraphic units.

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