Subglacial processes are described using a nested set of samples from AND-1B drill core collected from beneath the Ross Ice Shelf as part of the ANtarctic geologic DRILLing program (ANDRILL). An interval from the late Pleistocene was chosen for study because it is preserved in the sedimentary record, yet the depositional processes are applicable to the widely studied glacial conditions at the Last Glacial Maximum. A complete glacial–interglacial sequence, between two glacial surfaces of erosion (GSEs) at 41.9 and 47.7 meters below sea floor, was studied by centimeter-scale core logging based on X-radiographs of the archive core halves in addition to core description, bulk samples and 45 mm × 65 mm thin sections of diamictites for micromorphology analysis. Above the lower GSE, 4.6 m of subglacial till occurs including a sequence of thin muddy conglomerate with diverse pebble lithologies, massive clast-rich muddy diamictite, and stratified diamictite with clast-rich and clast-free beds. The sand-size fraction of these deposits is dominated by aggregate grains, termed till pellets following terminology used for Ross Sea deposits. We present the first detailed description and two different conceptual models of till-pellet formation beneath an ice stream: one is related to mechanical processes, and the other is related to thermodynamics. Till pellets are rounded, spherical to prolate in form, and are associated with turbate structures and aligned grains in thin sections. The core of the pellet is either a lithic grain or stiff till with additional clay plastered on the outside, forming rounded grains from originally angular ones. Galaxy structures, skelsepic fabric, and pressure shadows around grains are indicative of rotational deformation within the till.
Till pellets form in situ within a highly porous deformable bed under an ice stream either by mechanical shearing or by alternating thermal conditions in the till. In the first model, a slow glacial advance under dry subglacial conditions fractures the till, increasing the porosity as shear induces dilation. Basal melting and high pore-water pressure develop as the ice stream thickens, decreasing the intergranular effective stress and inducing rotation of the angular till aggregates.
An alternative model calls for basal freezing with preferential ice growth in larger pore spaces causing rotation of fine-grained till aggregates due to concentration of shear stresses in a thin zone beneath the glacier sole. Survival of till pellets is enhanced in subglacial diamictite as the thin zone of deformation moves upward toward the base of the ice stream. A thin granular bed enriched in till pellets is melted out in the grounding zone, and this deposit marks the transition from subglacial till to stratified silty claystone representing sub–ice shelf deposits.