GSA Connects 2021 in Portland, Oregon

Paper No. 67-10
Presentation Time: 11:05 AM


STRECK, Martin1, JACKSON, Robert1, BLACK, Cassandra1, SWEETEN, Rachel1 and MCINTOSH, William2, (1)Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97201, (2)New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801

The Mahogany Mountain—Three Fingers rhyolite field (MM–TFrf), covering ~900 km2, is one of largest mid Miocene rhyolite centers associated with Columbia River flood basalt volcanism. Early studies advocated for a two-caldera model consisting of the Mahogany Mountain and the younger Three Finger caldera producing the tuff of Leslie Gulch (TLG) and the tuff of Spring Creek, respectively, along with pre- and post-caldera effusive rhyolites. Recently, Benson & Mahood (JVGR, 309: 96-117, 2016) suggested only one larger caldera with pre- to post-caldera rhyolites after finding that the tuff of Spring Creek in Leslie Gulch is part of the TLG. With data of our recent studies and results by Marcy (2013;, we can further address the stratigraphic and geochemical eruptive history of this rhyolite field.

Abundant and compositionally variable effusive rhyolites all but one post-date the tuff of Leslie Gulch, including the prominent rhyolites of Mahogany Mountain (15.82±0.05–15.71±0.05 Ma) in the south and of McIntyre Ridge along the NW margin of the MM–TFrf. Along Succor Creek, we correlate mostly non-welded tuffs, that underlie pronounced rhyolite cliffs of McIntyre Ridge (15.76±0.02) and of Devils Gate (e.g., 15.95±0.03), with the TLG; tuffs consist of fine grained ignimbrites, surges, and fallout deposit. The only rhyolite underlying the TLG is found in the Leslie Gulch locality, yielding an age of 16.02±0.01 Ma. Stratigraphic data reveal that the TLG is a complex, multi-phase deposit with eruptive breaks in between. Ignimbrites distinct from the TLG range in age from 15.91±0.01 to 15.74±0.09 Ma and indicate pyroclastic activity elsewhere in the MM-TTrf from an area immediately north of the original Mahogany Mountain caldera to Succor Creek in the NE.

The MM-TFrf represents a prolific rhyolite center that was active for 300k starting effusively at 16.02 Ma but quickly continuing with a major phreatomagmatic phase to deposit the composite tuff of Leslie Gulch, in turn followed by widespread post-caldera effusive rhyolites that were punctuated by other pyroclastic event (with or without caldera formation) ending at ~15.7 Ma. Rhyolite eruptions recommenced in the SW of the field with the 14.94±0.2 Ma Birch Creek rhyolite, and 14.42±0.02 Ma McCain Creek rhyolite.