Shallow geology of the sub-ice-shelf Siple Coast, eastern Ross Sea, Antarctica constrained by reflection seismology and surface gravity surveying

The geological units underlying the grounding line between the West Antarctic Ice Sheet and the Ross Ice Shelf are expected to contain a record of repeated ice advance and retreat in a key area for understanding interactions between the ice sheet, the ocean and the solid Earth through the warm and cold periods of the Quaternary. Direct sampling of the sedimentary units in the vicinity of the grounding line across the relatively slow-moving Kamb Ice Stream has been an ongoing focus for drilling efforts that involve first melting through roughly 600 m of ice. Geophysical methods suggest that the region is underlain by a sedimentary basin of yet-to-be-determined thickness.  However, little is yet known about sediment lithology and stratigraphy in this region.

We present analysis of a grid of about 73 km of seismic reflection profiles collected in the Kamb Ice Stream grounding line region during three seasons since early 2015, integrated with the inversion of a grid of surface-collected gravity data. Seismic data were acquired with explosive charges frozen into shallow (mostly 25-m-deep) hot-water-drilled holes recorded by surface-deployed geophones buried in the firn. Seismic processing has been undertaken to maximise resolution of stratigraphic units at and below the sea floor. The inversion of coincident surface-based gravity data, integrated with airborne-gravity collected as part of the ROSETTA-Ice project, constrains basin thickness in the region of the seismic data.

The processed low-fold seismic data image the ice shelf, ocean cavity and underlying stratigraphy. The shallow stratigraphy appears to be mostly horizontally layered, typical of a sub-ice continental shelf environment. More than 300 m of sub-horizontally layered sedimentary strata can be identified above the first inter-ice multiple reflection in the data. Several distinct reflections interpreted as unconformities are identified in the seismic data, which combined with reflective characteristics, terminations and pinchouts enable a seismic stratigraphic interpretation to be undertaken. For example, unconformities between units could correspond to past glacial erosion episodes as the position of the grounding line in this region has migrated toward or away from the open ocean. The integration of surface and airborne gravity data here enables better constrained modelling of the thickness of the sedimentary basins in the region that cannot be imaged by the seismic reflection data.