Sedimentary Processes of Unstable Ice Sheet Grounding Zones: Comparing Polar and Temperate Grounding Zone Wedges Using Marine Geophysical Data and Outcrop Studies

Abstract:

Current understanding of ice sheet grounding zone dynamics is limited because direct observation of grounding zones and their deposits (grounding zone wedges, GZW) is restricted to marine geophysical methods, which provide large-scale measurements of planform morphology and internal stratigraphy, but little information regarding sedimentary architecture. Seismic data nevertheless reveal that GZW range meters to kilometers in length scale and typically possess foresets and incised channels. Sediment cores from measured wedges are helpful for evaluating vertical changes in stratigraphy, but leave significant uncertainty regarding the spatial variability of deposits and the nature of their contacts, which are necessary data to evaluate sedimentary processes operating within grounding zones. This study presents results from outcrop studies of GZW exposed in sea cliffs of the Puget Sound, Washington (U.S.A.), where a series of back-stepping GZW record rapid grounding line retreat of the Puget Lobe. These outcrops are used to evaluate first-order physical controls on depositional processes. The data are compared to geophysical observations and cores collected from the Ross Sea, Antarctic, to evaluate similarities between the outcrop-scale deposits and polar grounding zone wedges that possess wavelengths measuring several kilometers, and amplitudes of tens of meters. The preliminary results show that for these larger features, wedge progradation is facilitated by foreset deposition. Alternatively, for small-scale wedges (100’s of m wavelength, m-scale amplitudes), wedge development occurs through topset aggradation. Additionally, based on the Puget Sound GZW deposits, progradation arises due to sediment gravity flows on the foreset. Sand and silt couplets, preserved within wedge foresets, suggest that tidal pumping occurred under ice, producing deposits between punctuated sediment gravity flows. These data show a multitude of sedimentary and morphological scales that are required to further elucidate physical processes that lend to the development of GZW.