Coarse-grained nonmarine volcaniclastic sediment: Terminology and depositional process

Abstract

Research Article| January 01, 1986 Coarse-grained nonmarine volcaniclastic sediment: Terminology and depositional process GARY A. SMITH GARY A. SMITH 1Department of Geology, Oregon State University, Corvallis, Oregon 97331 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1986) 97 (1): 1–10. https://doi.org/10.1130/0016-7606(1986)97<1:CNVSTA>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 MailTo Tools Icon Tools Get Permissions Search Site Citation GARY A. SMITH; Coarse-grained nonmarine volcaniclastic sediment: Terminology and depositional process. GSA Bulletin 1986;; 97 (1): 1–10. doi: https://doi.org/10.1130/0016-7606(1986)97<1:CNVSTA>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 Modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent, dilute stream flow or viscous debris flow. The characteristics of these poorly sorted sediments suggest rapid deposition from high-concentration dispersions but not en masse. Sedimentation thus appears related to high-discharge flows intermediate in sediment/water ratio between stream flow and debris flow. The term “hyperconcentrated flood flow” is proposed for describing this intermediate condition.Hyperconcentrated flood-flow deposits are distinguished from debris-flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification. These deposits are distinguished from normal, dilute stream-flow deposits by lack of cross-stratification in sand facies and by very poor sorting, poor imbrication, and numerous clasts with long axes oriented parallel to flow direction in gravel facies. The horizontal bedding that dominates sandy hyperconcentrated flood-flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper-flow regime and should not be confused with the more familiar thin, graded laminae of fine- to medium-grained sand often associated with parting lineation.Hyperconcentrated flood-flow deposits are not unique to volcanic settings; they also occur in arid, alluvial-fan sequences. Debris-flow and hyperconcentrated flood-flow deposits, however, are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings. In volcanic regions, therefore, these deposits have greater preservation potential, show greater lateral variability, and are more voluminous. Transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water, recently observed at Mount St. Helens, is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow. 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.