CHARACTERIZING THE OLDER SILICIC DOMES AND FLOWS OF WESTERN DESCHUTES COUNTY, CENTRAL OREGON, USA

Central Oregon exhibits a rich history of silicic volcanism. The Holocene rhyolite domes and flows at South Sister and Newberry Volcanoes are well-documented parts of this intensely volcanic region. Between these two volcanoes reside older silicic domes and flows. Prior work has linked some of these Pleistocene silicic products with rifting. We are evaluating samples from 22 features in the Three Creek Butte area south through the Cascade Lakes to assess potential syn-rift connections.

Initial observations indicate similarities among these domes and flows. Many expressions are weathered, small buttes (100-200 m of relief) with poor exposure, the tops of which may contain outcrop more rarely seen on their flanks. Some are larger hillforms and others are extensive ridges. These exhibit variable erosional products from large boulders to small rubble piles. All are demonstrably products of lavas, exhibiting flow features such as banding, vesiculation, elongation, aligned phenocrysts, and brecciation. The outcrops are heavily fractured by post-magmatic stresses and some preserve tightly-spaced conjugate sets.

Samples from the Benham Falls area exhibit general similarities in texture and mineral content to those of Eaton Butte (35 km to the southwest), but most of the exposures otherwise seem distinct from one another. The rocks contain a wide variety of glass contents ranging from holocrystalline to nearly holohyaline. Texturally, about half the sampled localities are porphyritic or glomerocrystic while the other half are aphyric. Some outcrops show extensive devitrification and hydrothermal alteration. Rock colors are buff white, grey, black, purple, and red. Exposures range from very hard to friable material, largely as a consequence of glass content, vesiculation, and degree of alteration.