THE CASTLE ROCK AND IRONSIDE MOUNTAIN CALDERAS, EASTERN OREGON: TWO ADJACENT VENTING SITES FOR UNITS OF THE 16 – 15 MA DINNER CREEK TUFF

The rhyolitic to dacitic Dinner Creek Tuff consists of four cooling units with ⁴⁰Ar/³⁹Ar ages 16 – 15 Ma and is distributed over an area greater than >30,000 km². The tuff is part of the silicic Lake Owyhee Volcanic Field that was contemporaneous with the Grande Ronde member of the Columbia River Basalt Group. The source of the tuff was postulated to be in northwestern Malheur County, eastern Oregon, near Castle Rock. This broad area has recently been called the Dinner Creek Tuff Eruptive Center.

Mapping within the Dinner Creek Tuff Eruptive Center combined with analytical data identified two volcanic centers related to the Dinner Creek Tuff. The southern volcanic center, the Castle Rock caldera, is the source for the Dinner Creek Tuff unit 1 (DIT1). Rheomorphic densely welded DIT1 is over 300 m thick along the east side of Castle Rock. The northwestern margin of the caldera has been uplifted along faults, showing vertically foliated tuff dikes and associated mega-breccia deposits. Up to 200 m of younger tuffs, and fluvial volcaniclastic sediments were deposited on the caldera floor, which has been uplifted due to resurgence and regional extension, creating complex structural relationships between the volcanic units.

The northern volcanic center is located at Ironside Mountain, where densely welded rheomorphic Dinner Creek Tuff unit 2 (DIT2) is exposed in outcrops over 600 m thick. The top of the DIT2 consists of glassy, moderately welded tuff. Sources for the DIT2 include tuff dikes along the south and western flanks of Ironside Mountain. The thick deposits of DIT2 at Ironside Mountain indicate that the mountain is an uplifted caldera, herein named the Ironside Mountain caldera. Uplift may have been due to resurgence, but it is most likely due to normal faulting along the Border Fault, a normal fault that strikes across the northern margin of the caldera.

The topographic margins have been eroded from both calderas, but Mesozoic marine sediments, and early – mid Miocene volcanic rocks that border the thick intra-caldera tuff deposits create approximate margins for the calderas. Within both calderas, the most voluminous post-caldera volcanic unit is aphyric icelandite that erupted from dikes along the margins of the calderas. This icelandite has strong geochemical affinities to the regionally extensive Hunter Creek basalt.