CALDERAS OF THE NORTHERN CASCADE ARC

Tertiary ash flow calderas in the Cascade arc may be less rare than commonly believed. These large, steep-walled, oblong or circular volcanic depressions are formed by catastrophic eruption of silicic magma, emplacement of breccia-rich ash flows and tephra, and crustal subsidence. Intracaldera fill includes 100s of meters of ignimbrite and landslide debris, typically in fault-bounded contact with surrounding rocks. Any combination of these features is taken as evidence for a caldera.

We assemble evidence from our investigations and from other’s reports for ~18 known, probable and possible ash flow calderas in the Cascade arc of Washington (WA) and British Columbia (BC) between the Fraser and Columbia Rivers. We tabulate location, and, where data are available: age, thickness and composition of the tuff; structural dimensions; and presence of associated characteristics such as outflow sheets, megabreccia, exposed subsided floor, lake sediment, and post-caldera intrusions.

Most calderas in the northern Cascades are elongate and small in size- the largest is 18 km in diameter (Mount Aix, WA). The typical caldera is ~ 8 x 5 km. Small sizes, compared to the large calderas of the Basin and Range and Snake River Plain with their extensive ignimbrites, probably reflect small magma systems. Magma compositions of Cascade calderas range from dacite through rhyolite. Calderas are roughly evenly distributed within the study area and range from 1.15 Ma (Kulshan caldera, WA) to 35.2 Ma (Davis Peak caldera, WA). Younger calderas or thick tuff units are mostly in the north, though older tuffs are scattered throughout. Age distribution may be due to glacial erosion of older tuffs in the north. Kulshan caldera is the only demonstrated Quaternary caldera between the Fraser and Columbia; 3 other calderas or possible calderas, all in WA, are younger than 4 million years: Hannegan, Gamma Ridge, and Devil’s Horns. In addition to those named, calderas identified by us or others are Coquihalla (BC), Bosom Buttes, Fifes Peak, Quartz Creek and Goat Mountain (all in WA). Likely calderas include tuff bodies and structures at Podunk Creek (BC) and Mount Rahm (BC-WA). Less certain are other Tertiary units, including: rhyolite of Clear West Peak, Sun Top Tuff, tuff of Cash Prairie, Eagle Tuff (of Yeats), tuff breccia of Mount Daniel, and breccia of Kyes Peak (all in WA).