The Castle Rock and Ironside Mountain Calderas, Eastern Oregon, USA: Adjacent Venting Sites of Two Dinner Creek Tuff Units—The Most Widespread Tuffs Associated with Columbia River Flood Basalt Volcanism

The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite volcanism contemporaneous to flood basalts of the Columbia River magmatic province. Field mapping along with analytical data of tuff samples identify two calderas, the Castle Rock and Ironside Mountain calderas, as the venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both calderas lie within the area of the proposed general storage sites of main-phase Columbia River Basalt magmas. The Castle Rock caldera formed during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The northwestern boundary of the caldera is roughly defined by the juxtaposition of over 300 m of densely welded rheomorphic intra-caldera tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following caldera collapse, fluvial and lacustrine volcaniclastic sediments were deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite and icelandite (Fe-rich andesite), which correlate stratigraphically to upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and lavas are interbedded with sediments and Dinner Creek Tuff unit 4. The Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner Creek Tuff unit 2, which lies ∼15 km north of the Castle Rock caldera. The caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera, rheomorphic, and partially welded tuff are bound by Weathersby Formation shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic andesite and icelandite dikes and sills that are also stratigraphically correlative to upper Grande Ronde Basalt lavas intruded into the tuff, mostly along the margins of the caldera, which altered much of the tuff. Mafic lavas within the study area that closely pre- and post-date Dinner Creek Tuff units were correlated with regional units of the Columbia River Basalt Group. Porphyritic and aphyric mafic lava flows underlying Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are westward extensions of fractionated tholeiitic magmas as seen in late-stage Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally, porphyritic basalt lava flows that overlie the Hunter Creek Basalt and volcaniclastic sediments at the Castle Rock caldera are correlative with the 13.5 Ma Tim’s Peak Basalt. At Castle Rock, pre-caldera Columbia River Basalt Group lavas appear to lap onto a mid-Miocene topographic high that stretches northward and westward for tens of kilometers based on stratigraphic data, and it may be related to regional uplift at initial impingement of the mantle upwelling to produce the Columbia River Basalt Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal volcanism of the Columbia River magmatic province. Eruption of rhyolites is closely pre- and post-dated by the eruption of local and regional tholeiitic lavas belonging to the Columbia River Basalt Group. The local eruption of evolved tholeiitic lavas likely concealed calderas, but these lavas also illustrate the close proximity of mafic and rhyolitic magmas at depth at these rhyolite centers. Consequently, the stratigraphy of both the Castle Rock and Ironside Mountain calderas somewhat differs from that of rhyolite calderas dominated by silicic and calc-alkaline intermediate pre- and post-caldera volcanism