Date of Award:
Master of Science (MS)
Donald W. Fiesinger
The Kelton, Utah, area has numerous, isolated basaltic outcrops of probable Tertiary age mostly in the form of cuestas with steep faces displaying columnar joints. One ash-flow tuff is located in the southeastern part of the study area. Basaltic fragments in the tuff indicate that pyroclastic activity was preceded by extrusion of basalt.
Effects of Lake Bonneville on the basaltic outcrops include wavecut terraces, scarps, and other wave-built forms in low lying areas. Massive carbonate deposits formed at levels of former shorelines of Lake Bonneville. Tertiary and Quaternary deposits cover the low- lying areas between basaltic flows, and consist of materials primarily derived from the Raft River Mountains to the northwest.
The study area lies in a transitional zone between the Great Basin and the Snake River Plain to the north where the crustal thickness increases from 25 to 30 km.
The basaltic lavas range from aphanitic to hypocrystalline with su bophitic, intergranular, and pilotaxitic textures. Glass shards, axiolites, and pumice fragments are present in the ash-flow tuffs.
Petrographic, mineralogic, and chemical studies were completed on selected samples of both basaltic and pyroclastic rocks to determine genetic relationships.
Tholeiitic basalt, BV81-24, is distinguished from other basalts in the area by the presence of three pyroxenes in the groundmass and distinctive chemistry: high Si02, Al203, and Mg0; and low Ti02 and total iron.
The remaining basaltic rocks may be related to a common parent, BV81-11, by a process of crystal fractionation. The parental magma, in turn, may be derived by partial melting of a hypothetical mantle material, such as pyrolite or garnet peridotite.
The intrusion of basaltic magma into the crust is thought to cause partial melting of crustal material, generating magma of rhyolitic composition. Violent extrusion of rhyolitic magma has produced ash-flow tuffs, represented by BV81-17 and BV81-18. Thus these basalts and ashflow tuffs are considered to be members of a bimodal suite as is common in the Basin and Range Province.
Voit, R. L., "Petrology and Mineralogy of Tertiary(?) Volcanic Rocks West and Southwest of Kelton (Box Elder Co.), Utah" (1985). All Graduate Theses and Dissertations. 6676.
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