Holmes, John Shaw2021-02-042021-02-041964https://hdl.handle.net/1794/2600388 pagesMany post-middle Oligocene bodies of intrusive rocks are located in the southern Willamette Valley near Eugene, Oregon. Six of these which fora topographic highs, Creswell Butte, Spencer Butte, East Butte, Skinner Butte, Gillespie Butte, and North Butte, are examined in detail. From outcrop patterns, aerial photos, available literature, and field observations, the author believes the buttes to be sills or sill-like structures, concordant or slightly discordant with the surrounding Eugene Formation. Specifically, Creswell Butte,. Spencer Butte, and East Butte are eastward dipping sills; Skinner Butte is a sill-like structure dipping northwest; Gillespie Butte is a phacolith occupying a local synclinal fold plunging to the northwest, North Butte 1s a small laccolith with a feeder dike below the outcrop. All six buttes have similar lithological characteristics. They are medium- to fine-grained basalts with an ophitic and in places glomeroporphyritic texture. Plagioclase (labradorite) 1s the predominant mineral and coexists with lesser amounts o! the pyroxenes, hypersthene and ferroaugite. Some samples have trachytic texture. Hypersthene and ferroaugite are present at Creswell, Spencer, Gillespie, and North Buttes, but only the monoclinic pyroxene is present at East and Skinner Buttes. The ferromagnesium grains are moderately altered to chlorophaeite. This chloritic material appears to have altered from hypersthene mainly, but in some samples it has replaced ferroaugite. Other minerals, in minor quantities, are magnetite, ilmenite, apatite, and zircon. There is no evidence supporting differentiation within the buttes. The constant Fe2/Mg2 ratio suggests the same magma source tor all six buttes and thus rules out the possibility of extensive differentiation. There is, however, variation in the pyroxene content, which suggest a temperature variation during intrusion. The two-pyroxene basalts in the thesis area represent higher temperature conditions where the hypersthene was 'frozen' in the rocks and not allowed to undergo a normal inversion to pigeonite. The basalts which contain only ferroaugite indicate a temperature of formation lower than that normal for a two-pyroxene basalt.enCreative Commons BY-NC-ND 4.0-USUO theses and dissertations are provided for research and educational purposes, and may be under copyright by the author or the author’s heirs. Please contact us <mailto:scholars@uoregon.edu> with any questions or comments. In your email, be sure to include the URL and title of the specific items that you are inquiring about.TopographyEugene, ORCreswell ButteSpencer ButteSkinner ButteGillespie ButteGeologyWillamette ValleyThesis / Dissertation