Patterns of Quaternary ice sheet erosion and deposition in Fennoscandia and a theoretical framework for explanation
It has long been recognised that the formerly glaciated area of Fennoscandia shows large spatial differences in thicknesses of Quaternary deposits (mainly tills), and exhibits distinct patterns of glacial scouring and deep linear erosion. The reasons for this striking zonation have been elusive, and in particular the relative roles of mountain ice sheets (MIS) and full-sized Fennoscandian ice sheets (FIS) in shaping the landscape surface need clarification. On the basis of current advances in our understanding of the climate evolution and basal thermal organisation of ice sheets, we perform spatio-temporal qualitative modelling of ice sheet extent and migration of erosion and deposition zones through the entire Quaternary, and proceed to suggest an explanatory model for the current spatial pattern of Quaternary deposits and the two different types of erosion zones. We use the spatial distribution of fjords and deep non-tectonic lakes for delineating zones of deep glacial erosion, and relict landscapes as markers for frozen-bed conditions. On the basis of the amount of exposed bedrock, the landscape was classified into a tripartite system of drift thickness (thick drift, intermediate drift thickness, absence of drift/scoured zones). It is found that a centrally placed (central and northern Sweden) zone of thick drift cannot be explained by deposition under FIS style ice sheets, but is instead likely to be the combined result of marginal deposition of fluctuating MIS style ice sheets, primarily during the early and middle Quaternary, and the inefficiency of later east-centered FIS style ice sheets in evacuating this drift from underneath their central low-velocity and possibly frozen-bed areas. The western (fjord) zone of deep glacial erosion formed underneath both MIS- and FIS style ice sheets during the entire Quaternary, while the eastern (lake) zone of deep glacial erosion is exclusively related to MIS style ice sheets, and formed largely during the early and middle Quaternary. The scouring zones formed under conditions of rapid ice flow towards bathymetrically-defined calving margins of FIS style ice sheets. They likely reflect process patterns of the last two or three FIS style ice sheets. The three landscape zones differ in their degree of permanence, with the deep erosion zones being a long-lasting legacy in the landscape, more likely to be enhanced than obliterated by subsequent glacial events. The thick drift cover zone, once established, appears to have been surprisingly robust to erosion by subsequent glacial events. The scouring zones appear to be the most recent and ephemeral of the three zones, with possible major alterations during single glacial events.