Jeffrey C. Strasser

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : I. Field evidence

Debris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.1 m a -1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Ground-penetrating radar reflection profiling of groundwater and bedrock in an area of discontinuous permafrost

We have used ground‐penetrating radar to profile the depth of permafrost, to groundwater beneath permafrost, and to bedrock within permafrost in alluvial sediments of interior Alaska. We used well log data to aid the interpretations and to calculate dielectric permittivities for frozen and unfrozen materials. Interfaces between unfrozen and frozen sediments above permafrost were best resolved with wavelet bandwidths centered at and above 100 MHz. The resolution also required consideration of antenna configuration, season, and surface conditions. Depths to subpermafrost groundwater were profiled where it was in continuous contact with the bottom of the permafrost, except near transitions to unfrozen zones, where the contact appeared to dip steeply. The complexity of the responses to intrapermafrost bedrock, detected at a maximum depth of 47 m, appears to distinguish these events from those of subpermafrost saturated sediments. The relative dielectric permittivity ranged between 4.4 and 8.3 for the permafro...

Glaciohydraulic supercooling : a freeze-on mechanism to create stratified, debris-rich basal ice : II. Theory

Simple theory supports field observations (Lawson and others, 1998) that subglacial water flow out of overdeepenings can cause accretion of layered, debris-bearing ice to the bases of glaciers. The large meltwater flux into a temperate glacier at the onset of summer melting can cause rapid water flow through expanded basal cavities or other flow paths. If that flow ascends a sufficiently steep slope out of an overdeepening, the water will supercool as the pressure-melting point rises, and basal-ice accretion will occur. Diurnal, occasional or annual fluctuations in water discharge will cause variations in accretion rate, debris content of accreted ice or subsequent diagenesis, producing layers. Under appropriate conditions, net accretion of debris-bearing basal ice will allow debris fluxes that are significant in the glacier sediment budget.

Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska

Microstructures of glacigenic sediment gravity-flow deposits formed at the terminus of the Matanuska Glacier, Alaska, were analyzed to characterize flow type. These sediment flows have been classified into four types based primarily on water content and sedimentological characteristics (Lawson, 1979a, 1982). Thin sections of flow deposits show a variety of microand mesoscale characteristics that vary according to water content of the source flow. Wet-type flow deposits are characterized in thin section by a well-defined parallel and imbricated microclast fabric and thin laminations resulting from laminar to plastic flow regimes. Dry-type flow deposits are characterized in thin section by bior polymodal or random microclast fabrics, greater textural heterogeneity, and deformational microstructures associated with plastic to brittle flow regimes. Thin laminations and a “laminar flow fabric” in wet-type flow deposits may be characteristic of sediment gravity flow in a glacial environment. Characterization of these microstructures supports the contention that micromorphological analyses can be used to elucidate sediment flow genesis and the conditions of the flow just prior to deposition. Thus, micromorphology may also be useful for differentiating sediment-flow type in Pleistocene diamictons in other locations. Lachniet, M. S., Larson, G. J., Strasser, J. C., Lawson, D. E., Evenson, E. B., and Alley, R. B.,1999, Microstructures of glacigenic sediment-flow deposits, Matanuska Glacier, Alaska, in Mickelson, D. M., and Attig, J. W., eds., Glacial Processes Past and Present: Boulder, Colorado, Geological Society of America Special Paper 337. 45 *Current address: Department of Earth Sciences, Syracuse University, Syracuse, New York 13244 type sediment-flow deposits correspond approximately to Lawson type III and IV flow deposits (high water content; Lawson, 1979a, 1982; see below for sediment flow type characteristics). Here we evaluate the use of micromorphological analysis to differentiate contemporary dry-type from wet-type sediment-flow deposits formed at the terminus of the Matanuska Glacier. This study deals exclusively with the micromorphology of sediment-flow deposits; the study of the micromorphology of tills is beyond the scope of this study and has not been undertaken at the Matanuska Glacier. Future investigation on the micromorphology of known glacial sediments will allow the further distinction between sediment-flow deposits and true tills.

Seasonal variability in hydrologic-system response to intense rain events, Matanuska Glacier, Alaska, U.S.A.

Two rain events at Matanuska Glacier illustrate how subglacial drainage system development and snowpack conditions affect hydrologic response at the terminus. On 21 and 22 September 1995, over 56 mm of rain fell in the basin during a period usually characterized by much drier conditions. This event caused an 8-fold increase in discharge and a 47-fold increase in suspended-sediment concentration. Peak suspended-sediment concentration exceeded 20 kg m —3, suggesting rapid evacuation of stored sediment. While water discharge returned to its pre-storm level nine days after the rain ceased, suspended- sediment concentrations took about 20 days to return to pre-storm levels. These observations suggest that the storm influx late in the melt season probably forced subglacial water into a more distributed system. In addition, subglacially transported sediments were supplemented to an unknown degree by the influx of storm-eroded sediments off hillslopes and from tributary drainage basins. A storm on 6 and 7 June 1997, dropped 28 mm of rain on the basin demonstrating the effects of meltwater retention in the snowpack and englacial and subglacial storage early in the melt season. Streamflow before the storm event was increasing gradually owing to warming temperatures; however, discharge during the storm and the following week increased only slightly. Suspended-sediment concentrations increased only a small amount, suggesting the drainage system was not yet well developed, and much of the run off occurred across the relatively clean surface of the glacier or through englacial channels.