The character, structure and origin of the basal ice layer of a surge-type glacier

The basal ice layer of surge-type Variegated Glacier, Alaska, appears to have formed by a combination of 1) open-system freezing of subglacial meltwaters over both rigid and unconsolidated substrates; 2) apron over-riding during surge-induced glacier advance; 3) incorporation of glacier ice by recumbent folding, thrust-faulting and nappe over-riding during down-glacier propagation of a surge front; and 4) post-formational metamorphism involving recrystallization, partial internal melting and squeezing out of meltwaters and dissolved gases. Structural evidence and the characteristics of debris entrained in ice facies formed by basal freezing suggest that the layer includes a lower element formed under surge conditions and an upper element formed during the quiescent phase of a surge cycle. Tectonic processes associated with the down-glacier propagation of surge fronts appear to be capable of creating thick basal ice layers which allow extensive supraglacial sedimentation of subglacially derived debris. -from Authors