New Type of Basaltic Subglacial Sheet-Like Sequence and Implications for the Inferred Thickness of Associated ice

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 are 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 regarded as a diagnostic product of eruptions associated with relatively thin glaciers (< c. 150 m) of any thermal regime. 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 significant differences in the lithofacies and lithofacies associations. In particular, the eruptions are typically not explosive except sometimes in the latest stages. They are probably linked genetically to subglacial pillow volcanoes. The individual sequences only provide an indication of minimum thicknesses of the associated overlying ice, although theoretical considerations suggest the coeval overlying ice mass must have been substantial, probably in excess of 1000 m. The sequences are currently only known to crop out in Iceland but similar volcanic sequences probably exist beneath the West Antarctic Ice Sheet (WAIS). As the latter are presently covered by about 3 km of ice, they are inaccessible and cannot be scrutinised directly. The impact of eruptions of this type is still to be assessed but they may have an important influence on future WAIS stability.