KULSHAN CALDERA - A QUATERNARY SUBGLACIAL CALDERA IN THE NORTH CASCADES, WASHINGTON

Authors
Citation
W. Hildreth, KULSHAN CALDERA - A QUATERNARY SUBGLACIAL CALDERA IN THE NORTH CASCADES, WASHINGTON, Geological Society of America bulletin, 108(7), 1996, pp. 786-793
Citations number
41
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
108
Issue
7
Year of publication
1996
Pages
786 - 793
Database
ISI
SICI code
0016-7606(1996)108:7<786:KC-AQS>2.0.ZU;2-I
Abstract
Calderas that collapse during large pyroclastic eruptions are anomalou sly rare in the Cascade are. Recognition of the early Pleistocene 4.5 x 8 km Kulshan caldera, filled with rhyodacite ignimbrite at the north east foot of Mount Baker, brings to only three the Quaternary calderas identified in the Cascades, A near-vertical ring fault cut in basemen t rocks of the North Cascades encloses 30 km(2) of intracaldera ignimb rite (and intermixed collapse breccia) >1 km thick but with no floor e xposed, The Lake Tapps tephra in the Puget lowland is the correlative fallout; 200 km from the source, it is as thick as 30 cm. Features of the distal ash fall and the intracaldera tuff suggest large-scale phre atomagmatism during an eruption that may have started subglacially, Se veral advances of the Cordilleran ice sheet subsequently obliterated t he topographic rim, removed every vestige of extracaldera ignimbrite a nd proximal fallout, and stripped any precaldera extrusive rocks-the f ormer existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement, Although the caldera is not stru cturally resurgent, several early intracaldera rhyodacite lavas intrud e and rest directly on ignimbrite or on ashy caldera-lake sediments re worked from the eruption products, Subsidence areas, pumice compositio ns, and volumes of magma erupted (>50 km(3)) are similar for the Kulsh an, Rockland, and Crater Lake (Mazama events, the three Quaternary cal dera-forming eruptions now recognized In the Cascades.