FISSURE-FILL AND TUNNEL-FILL SEDIMENTS - EXPRESSIONS OF PERMAFROST AND INCREASED HYDROSTATIC-PRESSURE

A spectacular network of fissure fillings and pipes (tunnels) cuts Qua ternary gravelly delta deposits northeast of Myvatn, precisely on the spreading axis of the North Icelandic rift zone. The delta was formed in an ice-contact lake during deglaciation towards the end of the last glaciation. Subsequently the lake was drained and permafrost conditio ns developed in these poorly sorted gravel deposits. Hydrostatic press ure was transmitted from the adjacent glacier to the non-frozen core o f the delta beneath the discontinuous permafrost crust and the seasona lly frozen active layer. Owing to increased hydrostatic pressure, a ne twork of subhorizontal to vertical fissures was opened along the talik s. In these fissures free ground-water flow and sediment transport wer e established. Tunnel erosion and probably also seepage erosion were a ssociated with these fissures. Subsequently, the fissures and tunnels were filled by laminated fine sediments interbedded with poorly sorted material resulting in the formation of fissure-fill sediments and tun nel-fill sediments.