Glacial-Marine or Subglacial Origin of Diamicton Units from the Southwest and North Iceland Shelf: Implications for the Glacial History of Iceland

摘要

The extent and timing of ice advances onto the north and southwest Iceland shelf is reconstructed by identifying the depositional environment of four diamicton units present in two marine cores from shelf troughs. We analyze a combination of properties, including sedimentological parameters, microfabric analyses, radiocarbon dates, and foraminiferal assemblage data. This multi-proxy approach for understanding depositional environments of diamicton units is critical in areas where ice sheets extended into the marine environment. The two cores studied contain a lower diamicton unit that is interpreted as subglacial till and an overlying diamicton unit that is interpreted as glacial-marine sediment. Our analysis showed that in general, till is classified by lower water content, lower total organic carbon, and coarser grain size than glacial-marine sediment. Anisotropy of magnetic susceptibility shows that the till has a sheared fabric, low inclination of the short (K3) axis, whereas the glacial-marine sediment has a high inclination of the short (K3) axis and a more random, unsheared fabric. Grain counts on the 106–1000 µm fraction in till contain a higher percentage of basalt, whereas the glacial-marine sediments contain a higher percentage of quartz, volcanic glass, and foraminifera. Foraminiferal assemblage data support the interpretations from the sedimentology and microfabric data. Radiocarbon dates on shells and/or foraminifera in sediments directly above the diamicton units show that the ice retreated from these shelf areas by approximately 13 ka.