Sedimentary processes at ice sheet grounding‐zone wedges revealed by outcrops, Washington State (USA)

• Published in: Earth Surface Processes and Landforms
• Main Authors: Demet, Brian P., Nittrouer, Jeffrey A., Anderson, John B., Simkins, Lauren M.
• Other Authors: National Science Foundation
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2019
• Subjects: Ice Sheet; Ice Shelf
• Online Access: http://dx.doi.org/10.1002/esp.4550; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fesp.4550; https://onlinelibrary.wiley.com/doi/pdf/10.1002/esp.4550; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/esp.4550; https://onlinelibrary.wiley.com/doi/am-pdf/10.1002/esp.4550

Abstract Grounding‐zone wedges (GZWs) mark the grounding terminus of flowing marine‐based ice streams and, in the presence of an ice shelf, the transition from grounded ice to floating ice. The morphology and stratigraphy of GZWs is predominantly constrained by seafloor bathymetry, seismic data, and sediment cores from deglaciated continental shelves; however, due to minimal constraints on GZW sedimentation processes, there remains a general lack of knowledge concerning the production of these landforms. Herein, outcrop observations are provided of GZWs from Whidbey Island in the Puget Lowlands (Washington State, USA). These features are characterized by prograded diamictons bounded by glacial unconformities, whereby the lower unconformity indicates glacial advance of the southern Cordilleran Ice Sheet and the upper unconformity indicates locally restricted ice advance during GZW growth; the consistent presence of an upper unconformity supports the hypothesis that GZWs facilitate ice advance during landform construction. Based on outcrop stratigraphy, GZW construction is dominated by sediment transport of deformation till and melt‐out of entrained basal debris at the grounding line. This material may be subsequently remobilized by debris flows. Additionally, there is evidence for subglacial meltwater discharge at the grounding line, as well as rhythmically bedded silt and sand, indicating possible tidal pumping at the grounding line. A series of GZWs on Whidbey Island provides evidence of punctuated ice sheet movement during retreat, rather than a rapid ice sheet lift‐off. The distance between adjacent GZWs of 10 2 –10 3 m and the consistency in their size relative to modern ice stream grounding lines suggests that individual wedges formed over decades to centuries. © 2018 John Wiley & Sons, Ltd.