Debris production, transport, and sedimentation at high fluid pressures in subglacial conduits

We lack robust criteria to identify where and when subglacial fluid pressures exceeded ice overburden pressure during debris production, transport, and deposition at former ice sheet beds. In this study, we use hydraulic damage as a marker for locations where overpressure (P_w ≥ P_i) developed. We focus on former subglacial conduits in Uppland, east-central Sweden, which represent Röthlisberger, Nye, and Hooke channels and lie on the floors of kilometre-wide subglacial meltwater corridors. The conduit floors show examples of hydrofracture, hydraulic jacking, and hydraulic bursting, with pervasive brecciation and fracture fill. Previous work has shown that subglacial rock hydrofracture requires fluid pressures greater than ice-overburden pressure to overcome the tensile strengths of old fractures (≤0.7 MPa) and of unfractured Precambrian gneisses (>8 MPa). Critically, at our sites, hydraulic damage was generated during high energy mass and fluid flows. In subglacial meltwater corridors, decimetre to metre depth sheetflow was initially dominant, with concentration of flow in broad, shallow H-channels. Hydraulic damage in rock was accompanied by till dilation and fluidisation, with pressure driven debris flows. In R- and N-channels, we identify distinctive sedimentary features that developed as conduits opened under overpressured pipeflow. These include high rubble concentrations that were sourced from the disintegration of underlying bedrock, rubble diamictons or poorly sorted and matrix-supported gravels interpreted as short-lived debris flows, coupled debris-hyperconcentrated flows, transport of large boulders under buoyancy, fluid escape structures, and sediment freezing due to instantaneous decompression. Similar assemblages of hydraulic damage and sedimentary features record the arrival and passage of pressurised subglacial flood waves and the concentration of sheetflow into conduits during historic jökulhaups. Major subglacial floods developed in Uppland during rapid final melt of the last Fennoscandian Ice Sheet in the early Holocene. We find that transient high subglacial fluid pressures contributed significantly to (i) the production of fresh rock debris, (ii) the erosion and recycling of till, and (iii) the transport and deposition of dense slurries in subglacial conduits.