NEW GEOLOGIC MAPPING ALONG THE NE HARNEY BASIN CORRIDOR, EASTERN OREGON PROVIDES IMPORTANT LOCAL DATA FOR REGIONAL MAGMATIC HISTORIES

Ongoing mapping efforts and earlier work (e.g. Steiner & Streck, 2014) in the NE part of the Harney Basin north of Burns have revealed an abundance of Oligocene volcanic rocks, consisting of mostly dacites with lesser andesites and basalts, mid-Miocene rhyolites, and previously unknown units such as the Dinner Creek Tuff. This sheds light on the geologic history of the entire northeast margin of the Harney Basin that importantly plays into the regional volcanic framework. This region was mapped originally in reconnaissance in the 1960s, where geologic units were poorly delineated, often blanketing whole quadrangles as essentially one unit. For example, more than 90% of the Calamity Butte quadrangle was mapped previously as ‘Tba’ – Tertiary basalt and andesite –, and similar amounts mapped as ‘Ts’ – Strawberry Volcanics – in the adjacent Jump-off Joe Mountain quadrangle.

Oligocene dacites have a wide lithological range from fine-grained to coarsely porphyritic units, narrow compositional ranges around 66 wt.% SiO₂, and ages from 24.9 to 22.9 Ma. Dacites are most abundant in the Telephone and southern Calamity Butte quadrangle where they occur with Oligocene andesite and basalts (23–25 Ma) that resemble mid-Miocene lavas of the Strawberry Volcanics. While such Oligocene dacites also occur in two areas in the Jump-off Joe Mountain quad, the most significant coverage there consists of a variety of 16 to 14 Ma rhyolite lavas and nearly aphyric andesites that are typical for those of the Strawberry Volcanics. Among regional ignimbrites reaching into the mapping area, units of the Dinner Creek Tuff are found in all three quads along with the late Miocene Rattlesnake, Devine Canyon, and Prater Creek Tuff, although the latter was not found in the northernmost quad.

Our mapping results suggest that this area could be a potential source area for pyroclastic units of the upper John Day Formation nearby and is constituting precursor volcanism to the Columbia River Basalt Group (CRBG). Furthermore, results are critical for aiding inferences on the development of the large and compositionally diverse volcanic field of the Strawberry Volcanics, the distribution and volume of co-CRBG rhyolite volcanism, and the paleogeography in the mid-Miocene.

Steiner & Streck, 2014, GEOLSO, https://doi.org/10.1144/SP385.12