LITHOFACIES VARIATIONS WITHIN SANDUR DEPOSITS - THE ROLE OF RUNOFF REGIME, FLOW DYNAMICS AND SEDIMENT SUPPLY CHARACTERISTICS

The Icelandic sandur plains have for long acted as classic sites for models of braided river behaviour and of associated lithofacies assemblages. However, many of the Icelandic sandurs are subject to repeated catastrophic flood events, or jokulhlaups, which are likely to have played a significant role in sandur construction. Established lithofacies models based on the Icelandic sandurs have failed to take account of the varying runoff regimes which characterise the different sandur environments. In addition, sediment inputs to sandur channel systems are highly variable, fluctuating in response both to varying runoff patterns, and to advance and retreat of the source glacier, that affect sediment availability within the proglacial zone. This paper describes the main lithofacies types and vertical profiles that characterize the sandurs of south Iceland, and demonstrates that they can largely be attributed to extreme runoff events, with few deposits typical of ''braided river'' facies occurring within the sedimentary record. A preliminary qualitative model is developed that allows interpretation and prediction of vertical lithofacies profiles derived from glacial outwash in terms of: (i) the drainage mechanism, which in turn controls the dominant runoff population; (ii) sediment availability; and (iii) the spatial variability in flow dynamics.