HR: 1340h
AN: V13D-0572    [Abstracts]
TI: Using Basaltic Sub-ice Intrusions to Infer the Presence of Former ice
AU: * Skilling, I
EM: skilling@pitt.edu
AF: University of Pittsburgh, Department of Geology and Planetary Science, 200 SRCC, Pittsburgh, PA 15260 United States
AB: Basaltic lava bodies emplaced in a sub-ice (wet or dry) environment are little studied, though they can preserve critical evidence for the presence of former ice. Ice contact of associated subaerial lavas is clearly less significant (or absent) and phreatomagmatic basaltic volcanic products in an ice-confined setting are often difficult to distinguish from those in ice-free settings. Textural studies and facies mapping of dike-fed basaltic lava "mounds" at the base of several Icelandic sub-ice volcanoes demonstrate that the magma-ice interface involved a number of inter-related processes that are significant for the interpretation of the presence of former ice. Many mounds are clearly dike-fed and most include some combination of pillowed lavas, more massive, jointed ("water-cooled") lavas and breccias derived from both lava types. Magma-ice contact is indicated by moulding of both of these lava types within sub-ice cavities (including sinuous meltwater drainage tunnels). Moulding of pillow lavas in particular also implies that efficient drainage of meltwater occurred during intrusion. Irregular-shaped metre-sized cavities commonly occur within the mounds and have smooth chilled surfaces with pillowed break-outs. The cavities are interpreted as areas where ice-blocks collapsed onto the mound surface, and were later incorporated and melted during mound growth. Partial fills of fluvially-deposited hyaloclastite with locally-derived rip-up clasts occur in some cavities and indicate that drainage occurred through the mounds and may have been responsible for some of the "water-cooled" jointing. Breccias containing a high percentage of clasts derived from the walls of such cavities suggest that ice block melting may trigger collapse, in addition to more commonly interpreted mechanisms such as oversteepening. Models of subaqueous basaltic lava emplacement and study of the Icelandic sub-ice examples suggest that the processes and hence facies architecture and morphology of sub-ice basaltic lava intrusions is influenced by many factors, including effusion rate, bedrock topography, collapse, meltwater drainage efficiency, syn- and post-intrusion meltwater (fluvial) erosion, post-intrusion glacial erosion, and of course vent location, focusing and shifting. The processes of lava moulding described here and the presence of meltout cavities with fluvial fills are unique to sub-ice environments, and allow the discrimination of ice-contact basaltic lava intrusions from similar examples in ice-free settings.
DE: 3344 Paleoclimatology (0473, 4900)
DE: 8427 Subaqueous volcanism
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005