What causes till thickness to change over distance? Answers from My´rdalsjo¨kull, Iceland

Many explanations have been provided for variations of the spatial distribution and thickness of till sheets. This paper gives new insight into the architecture of a stratigraphically distinct till sheet as a function of the type of substratum and preadvance topography at a scale of ∼10 km. This emphasises the sensitivity of the subglacial system to changes in the basal drainage conditions. The study area is the forefield of Sle´ttjo¨kull at the northern margin of the My´rdalsjo¨kull ice cap, central south Iceland. Here, detailed lithostratigraphical and sedimentological investigations, including mapping of the thickness for two till units, sediment logging, clast fabric and geotechnical measurements provide a basis for interpretations. The results show that the thickness of a stratigraphically distinct till sheet is directly correlated to the type of substratum. Where the substratum consists of sorted sediments the till is thin. Conversely, the till is thick where the substratum consists of till overlying sorted sediments. A sedimentological model is presented in which till thickness is tied to the variable hydraulic conditions experienced in a deforming subglacial bed. [Copyright &y& Elsevier]