HR: 1330h
AN: V42B-0356    [PDF]
TI: The Significance of Cross-Bedded Surge Deposits
AU: * Burgisser, A
EM: Alain@gi.alaska.edu
AF: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775 United States
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712-0254 United States
AB: We characterized cross-beds in surge deposits to distinguish between features that indicate large-scale motions and those controlled by small-scale depositional processes in order to determine the modes of transport (suspended load, traction-dominated) shaping the cross-beds. Surge deposits in the Upper Toluca Pumice at Toluca Volcano, Mexico, were selected because of their well-preserved dune forms and exceptional exposure. We measured the grain size distribution and componentry of representative individual layers as well as the occurrence and shape of the dune forms. Individual surge beds have variable amounts of size sorting, with well-sorted units occurring at all grain sizes. Density sorting is poor, except for the coarse-grain layers. No density sorting of the coarse layers occurs, however, where sub-horizontal bedding merges with poorly sorted, massive deposit. The shape of dune forms is self-similar, as shown by the power law relating their height to their length, and large dunes are frequently followed by a string of smaller dunes immediately downstream, the sizes of which decrease rapidly down current. Both prograde or retrograde dune crests can be present within the same dune form. Previous work has shown that large-scale turbulent structures sort clasts as a function of both size and density, because the viscous force needed to suspend clasts varies linearly with density and with the square of the clast diameter. Thus, the transport system produces clasts sorted in both size and density. On the other hand, processes occurring in the traction-dominated depositional system are little known. Our observations suggest clast transport in the boundary layer was more likely by rotation rather than either sliding, owing to the roughness of the substrate and the angular nature of the clasts, or gravity-induced grain flowage, as the beds dip significantly below the angle of repose of the clasts. We thus propose that rolling causes size sorting to occur regardless of density because the force needed to rotate a clast at rest varies linearly with density but with the fourth power of the diameter. As a result, traction-dominated transport sorts clasts only by size, reinforcing the coarser end of the density sorting of the transport system by remobilizing and mixing finer grains while leaving the coarse fraction. Coarse-grain layers well sorted in density are thus composed of clasts traveling successively through the transport system and the traction bed. Those coarse layers retain a memory of the large-scale transport system, whereas boundary layer processes control the sorting of the finer grain sizes. The general absence of density sorting within zones in transition with massive deposit is consistent with an increasing control of traction-dominated transport across the transition. Dunes appear to form around a nucleus of clasts creating a small protrusion and grow layer by layer via accretion. This feedback growth indifferently produces prograde or retrograde dune crests. The cascading distribution of dunes suggests that the smaller dunes are built in response to the emplacement of a large dune. Consequently, dunes originate from local disturbances of the boundary layer that is rapidly attenuated downstream. If this cascading process is repeated, the original dune string is likely to be reworked into a complex array of dune forms. The shapes and sizes of dunes thus record little information on the large-scale dynamics of the surge.
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting