A complex subglacial clastic dyke swarm, Solheimajokull, southern Iceland

Abstract

A complex network of clastic dykes dissects loosely consolidated Holocene sediments along the east flank of the
proglacial braid plain of Solheimajokull, southern Iceland. The dykes comprise downward-bifurcating intrusions up to 0.5 m thick and several metres in length and are intruded into glaciogenic deposits (sandy gravel, gravelly sand, interlaminated silt and sand, and diamicton). The dykes were sourced from a clast-poor sandy diamicton, interpreted as a subglacial till, and were intruded downwards beneath Solheimajokull glacier during a previous phase of advance. As the glacier advanced southwards, it loaded the sediment column resulting in the intrusion of dykes with a consistent south–southwest dip (with rare northward-dipping examples). The dyke fills are characterised by laminated sediment, with laminae oriented parallel to the dyke margins and comprise interlaminated clay, silty clay, silt, sand, sandy gravel and diamicton. In some dykes, high concentrations of pebble- to boulder-sized clasts occur in association with rotated pods of the laminated sediment. The laminae are thought to have evolved by a slow, long-lived intrusion process that involved the repeated fracture and expansion of the host sediments followed by viscous smearing-on of subglacial material onto the dyke walls, rather than rapid injection of fluidised sediment.