Portland State University. Department of Geology
Martin J. Streck
Term of Graduation
Date of Publication
Master of Science (M.S.) in Geology
1 online resource (viii, xiv, 173 pages)
The Strawberry Rhyolites constitute a significant rhyolite field among the largest in Oregon. Aerial coverage of approximately 386 km2 and an overall estimated volume of ~67 km3 using a median thickness for each unit. On the other hand, the total volume could be greater than 100 km3 if greater thicknesses apply. The Strawberry Rhyolites, a largely unknown mid-Miocene silicic volcanic rocks, crop out amongst voluminous flood basalt flows of the Columbia River Basalt Group (CRBG) in eastern Oregon. The eruptive activity of the Strawberry Rhyolites is currently constrained to span a period of about 2 million years, ranging from around ~16.2 to 14.3 Ma.
Mid-Miocene rhyolites are part of the Strawberry Volcanics located within the Malheur National Forest near the town of John Day and were first mentioned in the 1960-70s. Still, neither their distribution, composition nor ages were studied in detail until we started to work in the area. Building on our initial reconnaissance data, the main efforts to map rhyolites took place in the context of three Edmap projects from 2019-2021 aided by PSU field study students and covering the area of three 7.5 min quadrangles (Jump-off Joe Mountain, Big Canyon, 1/2 of Logan Valley West, 1/2 of Magpie Table quads) in the NE part of the Harney Basin north of Burns, have shed light on the existence of a diverse set of volcanic units previously lumped into broad single mapping units by earlier regional reconnaissance maps. Hence, there is a much better understanding of the geologic history of the entire northeast margin of the Harney Basin, which plays a significant role in the regional volcanic framework due to this mapping. The focus is to detail the distribution of rhyolite units used for volume estimates, determine the composition and petrography, and obtain additional age dates to constrain better the eruptive history and generate a geologic map covering the study area.
This thesis summarizes fieldwork, XRF/ICP-MS geochemistry, thin section petrography, and age date analysis by the 40Ar/39Ar method spanning 2017 to 2020. Nine distinct rhyolite units make up the Strawberry Rhyolites. These are distinguished by their lithology, petrography, and compositions. This study does include a brief overview of other non-Strawberry Rhyolite units found in the area.
Our study has revealed a rhyolite field that is among the largest in Oregon. A minimum of 10 distinct effusive rhyolite units erupted over a 2-million-year period in addition to one mixed, rhyolite-andesite pyroclastic deposit from ~16.2 to 14.4 Ma with most 40Ar/39Ar ages falling in the 15.4-14.4 Ma bracket, but stratigraphically highest undated rhyolite units could be slightly younger. The mapped distribution of rhyolites covers 186 km2, but the estimated original distribution area is likely ~400 km2 with an estimated volume on the order of 100 km3. I present the estimated original distribution area for each rhyolite unit. These estimated areas sum up to a total of 386 km2. Using these estimated areas, the largest rhyolite unit is Wolf Mountain (Trwm), covering about 170 km2, followed by the rhyolite of Three Cabin Spring (Trtcs) at ~ 158 km2. The smallest estimated area is from the rhyolite of Big Bend (Trbb), covering an area of about 8 km2. Age dates generated through this study and previously acquired are used in combination with stratigraphic field control to place rhyolite units into an eruptive sequence. The rhyolite of Wolf Mountain is the oldest, dating to 16.16 ± 0.17 Ma, and currently, the youngest age acquired is for the rhyolite of Kent Spring at the age of 14.37 ± 0.02 Ma. Thus, some units are likely to be slightly younger than our youngest radiometric age. An estimated volume is generated for each unit, with the largest volume obtained for the rhyolite of Wolf Mountain (Trwm), with a value of ~21 km3. The rhyolite of Three Cabin Spring (Trtcs) follows Trwm with an average volume of ~17 km3, both units undoubtedly producing the largest volume out of all the Strawberry rhyolites. The smallest volume average produced is from the rhyolite of Buckhorn Spring (Trbs) at a value of about 0.7 km3. Volumes are calculated using an estimated range of unit thicknesses and the estimated areas for each unit. By clearly defining a volume for each unit gives a better representation closer to their actual distributions. Rhyolites range from low-silica to high-silica and from phenocryst rich containing >20% phenocryst to those that are aphyric. All units display glassy-to-devitrified lithologies. Phenocryst assemblages are dominated by plagioclase, some units contain quartz, and mafic silicates often are amphibole, biotite, or both. Orthopyroxene occurs in some units in addition to or instead of biotite and amphibole. Through understanding the Strawberry Rhyolite temporal changes in their mineralogy and composition, the SR units defined can have their age dates and volume relationship interpreted. This age and volume relationship may be a precursor in better determining stratigraphy of rhyolitic volcanic units, which aid in closing geologic time gaps throughout history. The trend depicted is that as the rhyolites are younger, the smaller the volume they produce.
The Nb vs. Zr variation diagram is used to classify samples as I-type or A-type to understand further these implications of the temporal changes in the Strawberry Rhyolites. Our data suggest the following age-volume-composition-lithology relationships. Lower silica, crystal-rich units containing complexly zoned plagioclase, such as Trbs, Trks, erupted early, while crystal poorer and silica-rich units are more prevalent later. Units with slight A-type affinities such as Trbs, Trks, and the rhyolitic portion of Ttms, erupted the latest. This sequence is compatible with basalt intruding into crust initiating partial melting to produce rhyolites. This is followed by increased heat input from basalt to support near liquidus rhyolite magmas. Interaction of tholeiitic magmas with rhyolites culminates in the production of A-type-like rhyolites. Evidence for this stage is found in the commingling of Fe-rich andesite and rhyolitic magmas to produce the late tuff of Milk Spring. Interaction of basalts with rhyolites culminates in the production of A-type like rhyolites, supporting the association with hot spot activity.
The significance of the Strawberry rhyolite field is highlighted by comparing it to other well-known rhyolite lava flows and fields. Strawberry Rhyolites are now recognized to belong to the rhyolite flare-up associated with the main pulse of the CRBG volcanism. Thus, adding Strawberry Rhyolites to this rhyolite flare-up increased the footprint of this province towards the northwest.
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Dvorak, Chanel Leigh, "Distribution and Characterization of Rhyolites of the Strawberry Volcanics -- Evolution of a Major Rhyolite Field Associated with Columbia River Basalt Magmatism, Eastern Oregon, USA" (2021). Dissertations and Theses. Paper 5855.