Ice sheet grounding zone deposits on Ross Sea continental shelf (Antarctica): seismic facies analysis and P-wave reflectivity attributes

Subglacial till and overlying glaciomarine deposits are difficult to distinguish because the physical properties of subglacial till may vary depending on the drainage conditions of the rock directly beneath the ice sheet. Based on approximately 120 kilometers of multi-channel seismic data, detailed seismic facies analysis and amplitude variation with offset (AVO) analysis are carried out to examine the distribution of ice sheet grounding zone deposits on the continental shelf of the Northern Basin, Ross Sea (Antarctica) and infer the physical properties of interpreted subglacial till. Seismic facies analysis led to the differentiation of distinct sub-environments around a grounded ice sheet: distal glaciomarine, proximal glaciomarine and subglacial. The identification of seismic facies is based on seismic characteristics such as reflection amplitude strength, continuity and geometric configuration. P-wave reflectivity attribute is estimated from the AVO analysis of corrected seismic data. The generated P-wave reflectivity attribute stack, which is a measure of variation in acoustic impedance versus seismic receiver offset, is found similar to the conventional stacked seismic section and therefore used to validate our initial seismic facies interpretation. On one of the P-wave reflectivity attribute stacks we observe relative to seafloor reflection, a negative reflection coefficient at the upper boundary of an interpreted subglacial till. The negative reflection coefficient may imply the presence of an under-compacted, low-velocity subglacial till. In contrast, previous core analysis studies suggest that subglacial till is over-compacted. The interpreted subglacial till along another seismic profile does not exhibit under-compaction. It is important to state that the order of dimension in sediment cores is small compared to the resolution of seismic data used in this study. In the absence of other analogs in the study area, seismic-inferred physical properties of interpreted subglacial till is compared to that obtained from core samples. On another P-wave reflectivity attribute stack, a normally compacted subglacial till is observed. Subglacial till of varying level of consolidation are present on the continental shelf of the Ross Sea