HR: 1400h
AN: V33B-05    [Abstracts]
TI: Evidence for a new Type of Basaltic Subglacial Sheet-Like Sequence and Implications for the Inferred Thickness of Associated ice
AU: * Smellie, J L
EM: jlsm@bas.ac.uk
AF: British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, United Kingdom
AB: Basaltic volcanic sequences erupted subglacially have been classified empirically into two major types, thought to correspond to eruptions under thick and thin ice, respectively. The latter can be called subglacial sheet-like sequences from the appearance of the outcrops. Only one type of sheet-like sequence has been described so far. However, there is now evidence that there are at least two types of subglacial sheet-like sequence, with significantly different implications for interpretations of associated palaeo-ice sheet thicknesses. The first type, which is relatively well known, is a diagnostic product of eruptions associated with relatively thin glaciers (< c. 150 m) of any thermal regime (temperate, polar, polythermal). It is called the Mount Pinafore type after sequence holotypes described in Antarctica. A second type of sheet-like sequence is now proposed based on minor but significant differences in lithofacies and lithofacies associations. Eruptions of the second type commenced with the injection and inflation of a sill along the ice:bedrock interface. Such interface sills have been predicted theoretically but had no known geological example, until now. Subsequent evolution involves floating of the ice cover, catastrophic meltwater drainage and emplacement of widespread sheets of hyaloclastite in major subglacial mass-flow and hyperconcentrated-flow flood events. However, floating of the glacier may not always occur and truncated sequences should also exist, with predictable lithofacies characteristics. In fully developed sequences, the water-saturated hyaloclastite is commonly and distinctively intruded by apophyses of the underlying interface sill. Eruptions giving rise to the new sequence type are apparently not explosive, unless the ice over the vent thins sufficiently. Like the Mount Pinafore sequences, the deposits can be characterised as outflow sequences in that they do not form part of a recognisable vent structure. They are probably rooted in subglacial volcanic edifices of tuya, tindar or pillow volcano types and all known examples are associated with fissure eruptions. Whilst the new sequence type would have formed under a thicker glacial cover than Mount Pinafore sequences, it is usually only capable of providing an indication of minimum thicknesses of associated overlying ice.
DE: 0700 CRYOSPHERE (4540)
DE: 0720 Glaciers
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly