| Abstract: | Calderas that collapse during large pyroclastic eruptions are anomalously rare in the Cascade arc. Recognition of the early Pleistocene 4.5 ?? 8 km Kulshan caldera, filled with rhyodacite ignimbrite at the northeast foot of Mount Baker, brings to only three the Quaternary calderas identified in the Cascades. A near-vertical ring fault cut in basement rocks of the North Cascades encloses 30 km2 of intracaldera ignimbrite (and intermixed collapse breccia) >1 km thick but with no floor exposed. 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 phreatomagmatism during an eruption that may have started subglacially. Several advances of the Cordilleran ice sheet subsequently obliterated the topographic rim, removed every vestige of extracaldera ignimbrite and proximal fallout, and stripped any precaldera extrusive rocks - the former existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement. Although the caldera is not structurally resurgent, several early intracaldera rhyodacite lavas intrude and rest directly on ignimbrite or on ashy caldera-lake sediments reworked from the eruption products. Subsidence areas, pumice compositions, and volumes of magma erupted (>50 km3) are similar for the Kulshan, Rockland, and Crater Lake (Mazama) events, the three Quaternary caldera-forming eruptions now recognized in the Cascades. |
| Genre: | Article |
| ProdID: | 70018518 |
| Citation Author: | Hildreth, W. |
| Citation Contributing Office: | |
| Citation Datum: | |
| Citation Day: | |
| Citation Edition: | |
| Citation Editor: | |
| Citation End Page: | 793 |
| Citation Issue: | 7 |
| Citation Keywords: | |
| Citation Language: | English |
| Citation Larger Work Title: | Bulletin of the Geological Society of America |
| Citation LatN: | |
| Citation LatS: | |
| Citation LonE: | |
| Citation LonW: | |
| Citation Month: | |
| Citation No Pagination: | |
| Citation Number Of Pages: | 8 |
| Citation Online Only Flag: | |
| Citation Phsyical Description: | |
| Citation Projection: | |
| Citation Public Comments: | |
| Citation Publisher: | |
| Citation Series: | |
| Citation Series Code: | |
| Citation Series Number: | |
| Citation Search Results Text: | Kulshan caldera: A Quaternary subglacial caldera in the North Cascades, Washington; 1996; Article; Journal; Bulletin of the Geological Society of America; Hildreth, W. |
| Citation Start Page: | 786 |
| Citation Volume: | 108 |
| Citation Year: | 1996 |
| Type: | citation/reference |
| Text: | Kulshan caldera: A Quaternary subglacial caldera in the North Cascades, Washington; 1996; Article; Journal; Bulletin of the Geological Society of America; Hildreth, W. |
| URL (THUMBNAIL): | http://pubs.er.usgs.gov/thumbnails/outside_thumb.jpg |
| Date Other: | Mon, 1 Jan 1996 00:00 -0600 |
| Publisher: | |
