Sedimentation associated with Antarctic Peninsula ice shelves: Implications for palaeoenvironmental reconstructions of glacimarine sediments

Journal of the Geological Society, May 2002 by Evans, J, Pudsey, C J

Subglacial facies

Subglacial facies comprise mainly massive diamicton and rare massive gravelly mud and massive sandy gravel, and have abrupt upper contacts and generally high shear strengths (Figs 2 & 3). High shear strengths suggest loading and compaction by grounded glacier-ice, and the widespread distribution of massive diamicton facies indicates that deposition of basal till dominates subglacial sedimentation. These facies presumably date from the last time an ice sheet was grounded across the continental shelf. The gravelly mud at the base of VC272 could represent glacimarine sediment deposited prior to being overridden and compacted by this ice sheet. The progressive upward decrease in shear strength in cores VC257, VC271 and VC267 (Figs 2 & 3) indicates a gradual decrease in the confining vertical effective pressure during deposition of the basal till. This may reflect either a gradual thinning of grounded ice prior to its decoupling from the substrate, or progressive development of deforming bed conditions. However, in the former case we do not infer that the rate of grounding line retreat following ice-shelf formation is similarly gradual as the variable thickness of proximal ice-shelf facies in these cores would not support this assertion. The upward decrease in shear strength of the basal till in VC247 and VC249 is comparatively more abrupt (Fig. 2) possibly reflecting lower sedimentation rates. By contrast, the transition to low shear strengths in VC272 and VC270 is step-like (Fig. 3) supporting a more rapid decoupling of ice from the substrate.

Proximal ice-shelf facies

Proximal, ice-shelf glacimarine sediments consist of coarsegrained facies that can be differentiated from subglacial facies on the basis of their lower shear strength (

Oceanographic and glaciological studies indicate the proximal ice-shelf zone to be an area of high sedimentation in response to downward-directed flow trajectories and melting due to contact with marine waters (e.g. Drewry & Cooper 1981; Powell 1984; Jenkins & Doake 1991). The thickness of proximal ice-shelf sequences is related to the residence time of the grounding line near a site, the extent of the basal debris layer within the ice and the volume of debris released.

Diamicton, gravelly sand, gravelly mud, muddy gravel, dropstone sandy mud and dropstone mud (all massive to weakly laminated with gradational contacts) deposited through rain out of debris from the base of the ice shelf, dominate sedimentation on the shelf and in the Larsen-A trough (Figs 2 & 3). Upward fining of diamicton to dropstone sandy mud in the Larsen-A trough (VC257; Fig. 2) reflects a decrease in the supply and rain out of coarse-grained debris from the ice shelf in response to the retreat of the grounding line. Sub-ice-shelf rain out facies on the continental shelf shows evidence of bottom current activity. Sorted fine/very fine sand and medium to very coarse silt indicate transportation by currents from the grounding line and deposition in association with rain-out of coarser debris from the ice shelf, producing massive to laminated dropstone sandy mud and diamicton. In rare cases, the laminations are disturbed by dropstones (VC247; Fig. 2). The presence of semi-continuous (millimetrescale) laminations within these sediments supports fluctuations in current velocity that could result from tidal pumping in the grounding-line region (Domack & Williams 1990; Domack et al. 1999), rather than subaqueous outwash, which would not be expected to be significant in the Antarctic Peninsula on account of the polar climatic conditions there. Silt and clay settling from the base of the ice shelf can be winnowed by currents, as indicated by the presence of massive gravelly sand facies on the continental shelf (VC267; Fig. 2).