AREAL EXTENT AND VOLUMES OF THE DINNER CREEK TUFF UNITS, EASTERN OREGON BASED ON LITHOLOGY AND FELDSPAR COMPOSITION

The Dinner Creek Tuff erupted during a period of rhyolitic volcanism coeval to the flood volcanism associated with the Columbia River Basalt Group. The Lake Owyhee, McDermitt and High Rock volcanic fields account for ~90% of the rhyolites erupted between 16.7-15.0 Ma. The Dinner Creek Tuff, situated at the northern end of the Lake Owyhee volcanic field, was originally mapped as a single ignimbrite confined to the Malheur Gorge. Recent work has extended its distribution by correlating tuff outcrops previously mapped as generic Miocene welded tuff as well as local units such as the “Mascall” or “Pleasant Valley” tuff of eastern Oregon. Although Dinner Creek Tuff rhyolites have nearly identical compositions, different ages and subtle geochemical and mineralogical differences exist and were used to divide the Dinner Creek Tuff into four discrete cooling units (Streck et al., 2015).

The four cooling units have ages of 16.15-16 Ma (unit 1), 15.6-15.5 Ma (unit 2), 15.46 Ma (unit 3) and 15.0 Ma (unit 4). Except for unit 4, the units are lithologically very similar. Unit 1 is the Dinner Creek Tuff of the Malheur Gorge type section. New mapping confirms previous theories that the Castle Rock caldera erupted unit 1 and identified the new Ironside Mountain caldera as the source for unit 2 while precise source areas for unit 3 and 4 are not yet known. Dinner Creek Tuff ignimbrites are low-aspect ratio, widely distributed and comparatively thin deposits associated with landscape-mantling and high energy emplacement. All outcrops now determined to be Dinner Creek Tuff enclose an area of ~31,300 km² not including any fallout deposits that likely extended beyond the defined area. Based on feldspar compositions along with lithological and geochemical data, we established the distribution of all four units. Minimal distribution of individual units are as follows: ~22,590 km² (unit 1), ~17,920 km² (unit 2), ~10,380 km² (unit 3) and ~9,270 km² (unit 4). Using conservative thicknesses, determined tuff volumes are ~170 km³ (unit 1), ~112 km³ (unit 2), ~65 km³ (unit 3) and ~58 km³ (unit 4), totaling ~405 km³.