Distribution of rock fragments, grain size and chemical elements of tills in the Lake Mjøsa area, eastern Norway
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The Lake Mjøsa area in eastern Norway forms an interesting case to study glacial erosion, transport and deposition because bedrock lithology (source of till) is relatively well known and the Geological Survey of Norway (NGU) has a large number of till samples from the area. Late Precambrian sandstones dominate bedrock lithology in the north-northeastern parts of the area. In the central eastern areas the Cambro-Silurian rocks (predominantely shales) are found. Towards the south and west in the field area, the Precambrian crystalline rocks and Permian rocks dominate the bedrock lithology. Tills in the area often have a sandy-silty matrix, but with a high clay content in areas close to the lakes Mjøsa and Randsfjorden. In this paper we analyse a large number of till samples according to clast provenance and main cations, and show that late Precambrian sandstone often is transported long distances (50-80 km), and are deposited mainly in the upper part of the till stratigraphy. On the other hand, the lowermost part of the till sequence often closely reflects the underlying bedrock. This is particularly true in easily eroded terrain (Cambro-Silurian shales), with mostly short transported till. It is a complex problem to do mineral prospecting using till as a proxy for the underlying bedrock, and a thorough knowledge of glacial history and till stratigraphy is essential for successful interpretation of prospecting data.