Architecture and sedimentation of outwash fans in front of the Mýrdalsjökull ice cap, Iceland

Abstract

In this study, we introduce the architecture and style of sedimentation of two outwash fans in front of the Mýrdalsjökull ice cap, central south Iceland, that broaden different types of ice-marginal sedimentation. At Kötlujökull, a southeastern outlet of the ice cap, fan formation was monitored from the late 1970s to 2002; thus, the style of sedimentation is well understood and the relationship to the depositional architecture easily established. In front of Sléttjökull, a northern lobate extension of the ice cap, a fan from the 10th century AD is capped by neoglacial till sheets. At both sites, the fan surfaces were reconstructed in three dimensions. Ground Penetrating Radar (GPR) provided the thickness and internal structure of sediment bodies. Geological exposures and excavations were used for detailed sedimentological documentation of the outwash deposits in proximal-to-distal transects. The outwash deposits at both Kötlujökull and Sléttjökull are considered to be alluvial fans of the hochsander type, i.e., supraglacially fed, as they show all the architectural characteristics of alluvial fans with a semiconical shape, a restricted radial length of ca. 0.5–1.5 km, a plano-convex cross-profile and a slope gradient ranging between 1° and 5°. Fan deposits are dominated by planar or low-angle cross-bedded sand facies interbedded with thin laminae of fine gravel occasionally draped by mud. Sedimentation occurs during upper flow regime sheetflows associated with antidune migration within an aggrading, shallow braided-stream network. Hochsanders are intimately linked to advancing or stationary glaciers with steep ice-cored frontal slopes that favor supraglacial over subglacial drainage. Most important, hochsander fans lack any proximal-to-distal transition in the grain-size distribution, sediment facies or facies associations. Because much classification of glacial outwash sediments is based on proximal-to-distal distinctions, it is crucial to consider the depositional model for hochsanders. This study also indicates a strong dependence between the altitudes of the buried fan surface and the present-day terrain in the forefield of Sléttjökull, illustrating that subsequent to fan formation, subglacial landforms merely mask many of the pre-existing morphological features.