Clastic injection dynamics during ice front oscillations: A case example from Solheimajokull (Iceland)

Soft-sediment deformation structures are being increasingly used as a tool for reconstructing palaeoenvironments and porewater pressure conditions in glacial settings. However, the potential of hydrofractures and dastic injections in the reconstruction of ice dynamics remains poorly constrained. This paper presents the results of a detailed study of a clastic injection network outcropping in the Solheimajokull forefield (South Iceland). Sedimentological descriptions are combined with microscopic to macroscopic analyses of clastic injection geometries, sediment-fills, and cross-cutting relationships. The 250 m long and 20 m high exposure observed along the east flank of the proglacial braid plain displays alternating glaciofluvial sediments and subglacial tills, illustrating oscillations of the ice margins. These sediments are cross-cut by a dense network of injection composed of dykes propagating upward or downward, sills, and stepped sills. These clastic injections result from processes of hydrofracturing and the sediment-fills in these hydrofractures are generally laminated with an increase of grain-size towards the centre of the injections. These fracture-fill characteristics suggest multiple injection phases within the hydrofractures and an increase of porewater pressure overtime. Five main generations of clastic injections showing different senses of propagation and dip directions are determined and are interpreted as forming in different environments. Per descensum clastic dykes dipping down ice demonstrate subglacial hydrofracturing underneath flowing-ice, while sills and per ascensum clastic dykes form in submarginal to marginal environments due to the decrease of ice overburden pressure. The integration of these results with the sedimentological characteristics allows the Holocene ice front oscillations of the Solheimajokull to be reconstructed. This study demonstrates the importance of hydrofracture systems and their sediment-fills in the reconstruction of palaeo-ice dynamics. (C) 2015 Elsevier B.V. All rights reserved.