Optical-televiewer-based identification and characterization of material facies associated with an Antarctic ice-shelf rift

We have drilled thirteen boreholes within and around a through-cutting rift on Roi Baudouin Ice Shelf, East Antarctica. Logging by optical televiewer (OPTV), combined with core inspection, has resulted in the identification and characterization of several material facies. Outside the rift, OPTV-imaged annual layering indicates ~150 years of accumulation over the 66 m length of one of the boreholes. Luminosity analysis of this image also reveals the presence of numerous dark melt layers as well as a systematic decrease in background luminosity, interpreted in terms of a progressive increase in light transmission during firnification. We identify four material facies within the rift: snow, granular ice, marine ice, and unconsolidated platelets. We interpret the granular ice facies as snow that has been saturated by percolating seawater, and the underlying marine ice as compacted buoyant platelets that have adhered to the rift base. Core sections reveal the presence of tubular channels within the marine ice, indicating that it is macroporous and permeable to sea water. The lower boundary of this facies merges into a mushy layer of unconsolidated platelets that were successfully imaged by OPTV, revealing irregular, sub-horizontal layering similar to that reported previously on the basis of (directional) borehole video.