Palaeoenvironmental significance of a large-scale buried ice-marginal landsystem, Skeiðarársandur, SE Iceland

The sedimentary record of Icelandic ice-contact environments provides valuable information about glacier margin dynamics and position, relative sea-level, and the geomorphic processes that drive the evolution of proglacial environments. This important archive has been little exploited, however, with most glacier and sea level reconstructions based on limited sedimentary exposures and surface geomorphic evidence. Although geophysical surveys of Icelandic sandur have been conducted, they have often been of limited spatial scale and focus on specific landforms. We report an extensive (42 km of data within a 24 km² study area) and detailed (reflections recorded at depths of up to 100 m) low-frequency (40 and 100 MHz) Ground-Penetrating Radar (GPR) survey of the Sandgígur moraine complex, SE Iceland. This transforms our understanding of this landform, with implications for the Holocene glacial history and evolution of Skeiðarársandur and SE Iceland.

The Sandgígur moraines are located on Skeiðarársandur, SE Iceland, down-sandur of large Little Ice Age-moraines of Skeiðarárjökull. They have a relatively subtle surface geomorphic expression (typically 7 m high and 125 m wide), and knowledge of their formation is limited, with no dating control on their age or detailed geomorphic or sedimentological investigations. GPR-data reveals reflections interpreted as large progradational foresets (dip angle: 2.19° – 6.87°) beneath the morainic structure (depth of 100 m). These features are consistent with a sub-aqueous depositional environment before moraine formation, providing potential indications of past relative sea-level limits. GPR profiles in the vicinity of the Sandgígur moraines reveal a much larger (67 m high and 1.25 km wide) and extensive buried moraine complex than that suggested by surface morphology. Indicating that the moraine was a major Holocene ice margin of Skeiðarárjökull. The buried Sandgígur moraine ridge is comprised of a unit of chaotic folded reflections adjacent to a unit of parallel, down-sandur dipping reflections (dip angle: 1.29° – 2.27°) indicative of an ice-contact or end-moraine fan. Possible evidence of buried ice at depth is also present within radargrams surrounding the moraine ridges. Sediment above the morainic bounding surface is interpreted to be dominated by glaciofluvial deposits with an estimated sediment volume of 1.04 km³ over the 24 km² study area. Potential moraine breaches, possibly caused by high magnitude jökulhlaups (glacier outburst floods) are coincident within the GPR data and the surface geomorphology.

We combine GPR-derived subsurface architecture with the current surface morphology to develop a conceptual model detailing the geomorphic evolution of the moraines and surrounding region, from pre-moraine morphology, to their formation and burial, resulting in the present-day morphology. These results provide new insights into the Holocene to present-day glacial history of Skeiðarárjökull and the controls on sedimentation responsible for the evolution of Skeiðarársandur, with implications for the formation of sandar environments and the Holocene environmental history of SE Iceland.