Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington

Research Article| November 01, 1985 Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington PETER D. ROWLEY; PETER D. ROWLEY 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar NORMAN S. MacLEOD; NORMAN S. MacLEOD 2U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar MEL A. KUNTZ; MEL A. KUNTZ 3U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar ALLAN M. KAPLAN ALLAN M. KAPLAN 3U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information PETER D. ROWLEY 1U.S. Geological Survey, Denver, Colorado 80225 NORMAN S. MacLEOD 2U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, Vancouver, Washington 98661 MEL A. KUNTZ 3U.S. Geological Survey, Denver, Colorado 80225 ALLAN M. KAPLAN 3U.S. Geological Survey, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1985) 96 (11): 1373–1383. https://doi.org/10.1130/0016-7606(1985)96<1373:PBDRTP>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation PETER D. ROWLEY, NORMAN S. MacLEOD, MEL A. KUNTZ, ALLAN M. KAPLAN; Proximal bedded deposits related to pyroclastic flows of May 18, 1980, Mount St. Helens, Washington. GSA Bulletin 1985;; 96 (11): 1373–1383. doi: https://doi.org/10.1130/0016-7606(1985)96<1373:PBDRTP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Thin-bedded, dacitic, pumiceous pyroclastic-flow deposits partly cover the steep northern flank of Mount St. Helens volcano, Washington. They are termed proximal bedded pyroclastic-flow (PBPF) deposits and were formed during the eruption of May 18, 1980. These unconsolidated deposits, as much as 20 m thick, are characterized by well-defined, chiefly plane-parallel bedding sets separated by erosion surfaces. Most beds dip generally parallel to the slope of the underlying volcano flank, which averages 15° to the north but locally is as much as 30°. Individual beds range in thickness from 2 mm to >1 m. Cross-bedding, in which bedding sets dip gently to the north or locally to the south, is abundant. Cross-bedding is generally associated with large longitudinal dunes, apparently antidunes. Some cross-bedding sets, however, constitute small longitudinal anti-dunes (chute-and-pool structures) containing stoss-side beds that migrated south and dip steeply south toward the crater source.We suggest that PBPF deposits, base-surge deposits, and ignimbrite-veneer deposits are products of a broad spectrum of high-flow–regime pyroclastic processes. PBPF deposits at Mount St. Helens formed during deposition of high-velocity, partly turbulent, dry, fluidized pyroclastic flows that poured profusely down the volcano flank on May 18. Some beds of the PBPF deposits probably were deposited from typical high-concentration laminar pyroclastic flows. Other beds, especially those that display evidence of high-angle stoss-side accumulation, were probably deposited from low-concentration turbulent flow under high velocity as pyroclastic surges. Most beds in the PBPF deposits, however, may have formed during transitional flow conditions. Pyroclastic-surge and transitional flow-surge conditions may have resulted when the flows exceeded the threshold velocity or threshold minimum particle concentration for pyroclastic flows while lofting over obstacles or depressions or after collapse from Plinian ash columns. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.