Controls on the large-scale sediment architecture of Icelandic sandar

Abstract

Glaciers and ice-sheets are an essential component of the global climate system, respond ing dynamically to climate-driven changes in their mass balance by varying in thickness and extent. The oscillation of these ice masses can have global implications such as sea level change, and local impacts such as proglacial landscape evolution and glacier-related hazards. Sandur plains (plurally: sandar) are extensive sedimentary bodies formed in proglacial settings by deposition from meltwater systems emanating from the margin of a glacier. To-date, most research has focused on the analysis of sedimentary successions asso ciated to topographically-confined and small-scale sandar, however, these do not capture the variety or scale of processes that influence sandar architecture. Therefore, detailed subsurface analysis of sandar is fundamental to understand how these systems respond to fundamental drivers, such as: (i) glacier oscillations, (ii) sediment flux from glacier outburst floods; and (iii) changes in sea level. Icelandic sandar have been hypothesised to be comprised of thick alluvial successions that can provide detailed records of the processes that contributed to their formation. However, limited research has been undertaken to analyse the large-scale sedimentary architecture of these systems. It can be argued that more is known about the large-scale architecture of palaeo-sandur from ancient glaciations (e.g. late Ordovician ca. 440–460 Ma) and Quaternary glaciations (2.6 Ma – 11.7 Ka), than is known about the contemporary and active outwash plains used as analogues for their interpretation. This thesis presents an extensive (∼150 km of along-track profiles), proximal-to-distal, low-frequency (40 & 100 MHz) ground-penetrating radar (GPR) survey of Skeiðarársan dur in south-east Iceland. Radar-derived architecture reveals that Skeiðarársandur is com posed of: (i) thick and large-scale glaciofluvial deposits; (ii) a large, buried ice-marginal landsystem; (iii) aeolian dune deposits; (iv) structures associated to post-depositional modification and buried ice-masses; and (v) sub-aqueous progradational foresets. Thick (up to at least 60 m) glaciofluvial deposits dominate the proximal reaches of the sandur and a ∼25 m thick glaciofluvial braidplain facies is identifiable in medial reaches of the sandur. Furthermore, buried aeolian dune systems are identified at depth (up to 20 m) in medial and distal portions of the sandur and are potentially a more important element of sandar architecture than previously thought. Analysis of the sediment architecture derived from the GPR dataset, supported by historical accounts and recent observations, indicate that system-scale sandar evolution is driven primarily from an ice-proximal control in sandur sedimentation, likely as a result of increased sediment flux driven by jökulhlaup events.