SEDIMENTARY PROCESSES AT ICE SHEET GROUNDING-ZONE WEDGES: EXAMPLES FROM ANTARCTICA AND WASHINGTON STATE (U.S.A.)

Grounding-zone wedges (GZWs) mark grounding line positions where marine-based ice sheets transition from grounded to floating. The morphology and stratigraphy of GZWs is constrained by bathymetry and seismic data, and isolated sediment cores; however, there remains minimal data to constrain the processes that produce these landforms. High-resolution multibeam bathymetry data from the western Ross Sea, Antarctica, are used to document the morphology of GZWs formed during and since the Last Glacial Maximum (LGM). The size of these landforms is highly variable, ranging 10⁰-10² m in height. Furthermore, we present novel outcrop observations of GZWs from Whidbey Island in the Puget Lowland (Washington State, USA) associated with the post-LGM retreat of the Cordilleran Ice Sheet. Similar to their higher-latitude equivalents, the Whidbey Island GZWs are identified by prograded diamictons bounded by glacial unconformities, whereby the lower unconformity indicates regional advance of the Puget Lobe and the upper unconformity indicates local ice advance during GZW growth. The presence of the upper unconformity supports the hypothesis that GZWs periodically stabilize the grounding line by reducing the ice thickness needed to overcome buoyancy effects during landform growth. Based on outcrop stratigraphy, GZW construction is dominated by the transport of deformation till to and mass movement by debris flows at the grounding line. Additional processes recorded at Whidbey Island paleo-grounding lines, albeit spatially variable, are channelized meltwater discharge and sediment-laden meltwater plumes. Although evidence of tidal pumping at paleo-grounding lines of the Antarctic Ice Sheet are lacking, rhythmic silt and sand beds indicate tidal influence on the retreating Cordilleran Ice Sheet. Retreat events of 10² m-10³ km recorded by backstepping GZWs on Whidbey Island and in the western Ross Sea are punctuated by likely decadal to centennial pauses in grounding line retreat. This study provides outcrop-scale assessment of GZWs comparable in size to documented contemporary and ancient GZWs that advances the state of knowledge of grounding line sedimentologic processes and stability feedbacks associated with these landforms.