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
Location map
•4.5 X 8 km
•steep-walled,
cylindroid
•>50 km3 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
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
Geologic Map of Kulshan
Basement:
KJn
Nooksack,
mvs
Chilliwack, gd
granodiorite
plutons (+)
glacial erosion
at Table Mt,
Lasiocarpa
Rdg
* indicate
andesite vents
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 30km3
Mount Baker
Within the walls...
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•
•
•
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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 ~ 33km3 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 sheetthis explains lack of ash in the north
Intracaldera Sediments
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•
•
•
•
•
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
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•
•
•
•
•
rest directly on intracaldera ignimbrite and ashy sediments
original extent may have been up to 12 km2
compositionally, similar to pumice
massive and slabby/blocky jointed
glacial erosion stripped most away or badly altered
age range of 1.1-1.0 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