PRESERVATION OF JÖKULHLAUPS WITHIN SUBGLACIAL SEDIMENTS

Hydrodynamic processes responsible for governing steady-state water flow through glaciers are well understood. Glacier outburst floods (jökulhlaups) result in inherently unstable subglacial water flow, and under such transient conditions, hydrodynamic processes and hydraulic routeways are poorly constrained. In Iceland, some high-magnitude jökulhlaups travel initially as broad, subglacial flows causing widespread hydro-mechanical erosion and floodwater incision at the glacier bed. The significance of such jökulhlaups at an ice-sheet scale is not yet known. We present sedimentological evidence for jökulhlaup erosion and deposition in confined fracture and channel networks within former subglacial sediments on Skeiðarársandur, Iceland. Our observations reveal that high-magnitude jökulhlaups can puncture, descend, and flow through subglacial sediments.

Diamicton and glaciofluvial units on Skeiðarársandur display sequences ranging from stratified gravel to boulder-sized deposits, to massive stratified sands and gravels containing pebble stringers. Diamicton units are cut locally by fractures and channels filled with bedded sands containing diamicton rip-ups. Clast imbrication and a-axis orientation within fracture fills suggests deposition from an up-fracture flow. High-angled, stoss-side accretion is present within fractures.

Cohesionless gravel-boulder units are consistent with jökulhlaup deposition, showing a rheological transformation from hyperconcentrated to fluidal flow. High-pressure floodwater penetrated the glacier bed below the diamicton unit, propagating and infilling a series of hydrofractures within subglacial sediments. Hydrofracturing of diamicton was accompanied by extensive excavation of subglacial sediments within several rapidly enlarging sub glacier-bed channels. High-angled primary fluvial bedding suggests that sub glacier-bed fracture fills were deposited from a hydraulically supercooled flow. We provide evidence for transient extension of jökulhlaup drainage circuits to several metres depth within subglacial sediments. Due to excellent preservation potential, flood-filled fractures and channels within subglacial sediments may yield valuable data about ancient jökulhlaup dynamics.