Seismic Evidence of Impact Breccia and Unlithified Sediments Under Hiawatha Glacier

Abstract Hiawatha Crater in northwest Greenland is one of only two putative impact craters under an ice sheet. Hiawatha therefore offers a rare proxy for understanding the interactions between active glacial dynamics and impact craters on other planetary bodies. Here we characterize Hiawatha's subglacial environment, presenting the results of two active‐source seismic experiments and interpreting them in concert with previous radar sounding analysis. Seismic reflectivities at a site with a single basal radar reflector are consistent with a substrate of impact melt‐bearing breccia. At a site where radar sounding revealed a porewater reflector 10–15 m below the base of the ice sheet, our seismic results are consistent with a substrate of unlithified sediments. We propose a model where supraglacial water supplies heat for basal melting, leading to rain–out of unconsolidated sediment from debris‐rich basal ice (fringe) and permitting a permeable porewater‐bearing basal layer to persist without consolidating.