Jökulhlaups and lahars are both types of outburst flood that commonly comprise a glacial meltwater and volcaniclastic sediment mix, and have discharges that are typically several orders of magnitude greater than perennial flows. Both types thus constitute a serious threat to life, property and infrastructure but are too powerful and too short-lived for direct measurements of flow characteristics to be made. Consequently a variety of indirect methods have been used to reconstruct flow properties, processes and mechanisms. Unfortunately, limited observations of sedimentary architecture and stratigraphic relationships are hampering our ability to discriminate fluvial magnitude-frequency regimes and fluvial styles of deposition, particularly those produced by rapidly-varied flows. This paper therefore uses Ground Penetrating Radar (GPR) to obtain quantitative data on subsurface sedimentary character of high-magnitude outburst flood sediments, including architecture and stratigraphy, from a bedrock-valley system in north-central Iceland. Basement pillow lava and subaerial lava flows are characterised by chaotic and hummocky GPR reflectors with a lack of coherent structure. They also feature an upper rough surface as evidenced by concentration of hyperbolae point sources. Unconsolidated sedimentary units are interpreted due to occur where laterally-persistent horizontal and sub horizontal reflectors occur. Deposition produced spatially diverse sediments due to rapidly-varied flow conditions. Observations include prograding and backfilling architecture, intercalated slope material and fluvial sediments, and multiphase sedimentary deposition. We suggest that these outburst flood sediments were initially deposited by traction load of coarse-grained material on prograding bedforms, and subsequently by drop-out from suspension of finer-grained material. The latter phase produced laterally extensive tabular sedimentary architectures that in-filled pre-existing topography and masked the complexity of bedrock forms beneath. Existing qualitative concepts of high-magnitude fluvial deposition within a topographically confined bedrock channel are therefore now refined with quantitative data on sediment architecture and thus on flow regimes.