"The Needle Range of southwestern Utah is composed of Paleozoic sediments overlain by a thick sequence of Tertiary volcanic rocks primarily of the welded tuff class. The area has undergone at least four stages of post-eruptive structural deformation. The most significant stage is that which formed the range by normal Basin and Range type faulting. The faults strike northwest and tilt the volcanic layers northeast.
The volcanic section is composed primarily of the Needles Range Formation (Oligocene) overlain by the Isom Formation and underlain by several flows and ash flows, one of which has been tentatively correlated to the Escalante Valley Tuff.
Between the Lund and Wah Wah Springs members of the Needles Range Formation, a previously unnamed crystal-poor, vitric-lithic welded tuff of unknown lateral extent occurs. Because of its good exposure and apparent lack of faulting near Ryan Spring, Beaver County, Utah, the writer proposes to name this unit the Ryan Spring Tuff.
At its type section the Ryan Spring Tuff is1120 feet thick and primarily dellenitic with an average partial chemical composition of: SiO₂ , 73.8%; K₂O, 5.1%; CaO, 2.7%; TiO₂ , 0.3%; and Fe₂O₃ , 2.4%. The average mineral composition is: quartz, trace; alkali feldspar, absent; plagioclase, 9.1%, biotite, 1.5%; hornblende, trace; and opaque oxides, 0.6%, The total crystal content is 11.2% with the remainder being composed primarily of glass, pumice, and lithic fragments. The tuff has an average bulk density of 2.240 gm./cc., an average grain density of 2.449 gm./cc., and an average porosity of 8.51%.
Vertical variations in chemistry, mineralogy, density, and porosity have led to some interesting hypotheses concerning preemptive changes in the magma and post-eruptive changes in the tuff due to alteration and compaction. There is no definite explanation to account for the variations in chemical and mineralogical composition. These variations may be due to pre-eruptive stratification of the magma chamber. Still another possibility might be post-eruptive deuteric alteration and leaching of ions by weathering. Variations in density and porosity are caused by multiple eruption and compaction phases"--Abstract, pages ii-iii.
Grant, S. Kerry
Barr, David J.
Geosciences and Geological and Petroleum Engineering
M.S. in Geology
University of Missouri--Rolla
ix, 69 pages
Beaver County, Utah
© 1975 Peter Carlyle Rauch, All rights reserved.
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Rauch, Peter Carlyle, "Tertiary welded tuffs of the Ryan Spring area, Needle Range, Beaver County, Utah" (1975). Masters Theses. 3229.
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