Diamict fans in subglacial water-filled cavities—a new glacial environment

The occurrence of glaciolacustrine diamict fans on the south side of volcanic tuyas in the Laugarvatn area (Iceland) is reported. These fans are composed of sheets of diamict, gravel and sand, and contain little or no primary volcanic material. They post-date the volcanic pile within the tuyas and appear to have been deposited in subglacial water-filled cavities, confined to the down-ice side of the tuyas. It is suggested that these subglacial water bodies formed as result of an enhanced geothermal heat flux associated with the tuyas. The volume of glacial sediment within the fans implies that either basal sediment loads were high in this sector of the former ice sheet or that ice flow converged towards the tuyas during their formation concentrating basal sediment into the lee-side cavity and fan. The glaciolacustrine diamict fans add to our knowledge of the landforms and sediments associated with tuyas and may also provide evidence of ice flow convergence at thermal anomalies.