Kulshan caldera: A Quaternary subglacial caldera in the North Cascades, Washington By Wes Hildreth, USGS presented by Megan Simpson

Cascade Range Quaternary calderas are rare in Cascade Range (>1800 volcanoes, 3 calderas) Most Pacific arc ranges have many more Three Quaternary calderas are: –Holocene Crater Lake –Rockland ash –Kulshan

4.5 X 8 km steep-walled, cylindroid >50 km 3 of magma erupted collapsed and filled with >1000m of rhyodacite ignimbrite (explosive flow deposits) single event in early Pleistocene lies NE base of Mt. Baker, west of Mt. Shuksan Location map

Geologic Setting Three types of precaldera rocks enclose the caldera –rocks from the Chilliwack group (metavolcanic, metasedimentary) north, south end –Nooksack Formation (argillite, sandstone, conglomerate) west end –Lake Ann stock (granodiorite) east end During collapse, these materials became incorporated into intracaldera ignimbrite

Basement: KJn Nooksack, mvs Chilliwack, gd granodiorite plutons (+) glacial erosion at Table Mt, Lasiocarpa Rdg * indicate andesite vents Geologic Map of Kulshan

Geologic Cross Section

Lookout! A Glacier! Topography has been obscured because of glacial erosion (Pleistocene Cordilleran ice sheet) Ice spread southward from British Columbia –lowered rim of basement rocks –stripped precaldera lavas –removed outflow –produced 1180m of intercaldera relief

Intracaldera Ignimbrite Massive, unstratified, pumice-rich tuff Mostly white to pale gray, poorly sorted, structureless Product of single collapse and fill (probably took only a few hours) Original surface preserved near south margin Eroded elsewhere, incised by gorges Probably exceeded 30km 3

Mount Baker

Within the walls... Collapse breccias Dispersed lithic fragments Pumice Upper tuff is unusually fine-grained Units grade upward into ash

Fallout Caldera eruptions always produce tons of fall deposits - usually 20% of total volume erupted In North Cascades, no fallout survived because of glacial erosion But most likely fell over vast area of Canada and US Recognized only in southern Puget lowland (~200km south)-Lake Tapps tephra

Lake Tapps tephra East of Tacoma Most likely from Kulshan caldera fallout –mineral assemblage identical to intracaldera pumice –microprobe analysis of glass is chemical match Consists of unreworked primary fallout Volume of fallout difficult to estimate, but probably represents ~ 33km 3 of magma

Lake Tapps Cont. Deposited in periglacial settings Ash layer rests on glacial outwash or is within lacustrine sequences of laminated silt and clay Pollen indicates herb-dominated tundra environment Authors believe eruptive center of North Cascades completely covered by ice sheet- this explains lack of ash in the north

Intracaldera Sediments ash-dominated thick as 120m overlie fine-grained ignimbrite in the SW rich in calcite, clays and pyrite cut by numerous andesitic and silicic dikes no evidence for fluvial, beach, glacial processes has been seen main facies suggest steep-walled basins sediments from ignimbrite rim and crumbling walls

Postcaldera Rhyodacite Lavas rest directly on intracaldera ignimbrite and ashy sediments original extent may have been up to 12 km 2 compositionally, similar to pumice massive and slabby/blocky jointed glacial erosion stripped most away or badly altered age range of Ma

Postcaldera Andesites at least 50 dikes cut ignimbrite, sediments and rhyo-lavas olivine, pyroxene, and hornblende andesite no systematic orientation many contain pyrite none extend out of caldera fill

Structure of Kulshan steep-walled cylindroid was filled with ignimbrite during brief, continuous event not known if subsided in piecemeal or piston fashion scarcity of lithic fragments in ignimbrite suggests relative coherence not resurgent

Kulshan

Migration of Magma magma moving southwestward for the past 4 my moved from Hannegan caldera in NE (4 Ma) to Lake Ann (2.7 Ma) to Kulshan (1.1 Ma) to Black Buttes-Mt. Baker (0.5 Ma) represents rate of 5-6 mm/yr

Conclusion Question: why are there only three Quaternary calderas in the Cascade arc? These factors may play a role: –Cascade crust thicker than most –plate convergence relatively slow –key may be to understand processes that favor upper-crustal storage over magma throughput but MORE STUDY IS NEEDED