STRATIGRAPHY OF DE GEER MORAINES IN KVARKEN ARCHIPELAGO, WESTERN FINLAND

Research area is located on the western coast of Finland. The area is one of the most representative De Geer moraine fields in Scandinavia. Approximately a couple thousand ridges are located in the area of just two hundred square kilometers. In connection with Quaternary geological mapping research excavations were made. Most of these research pits were crosscuttings into the De Geer ridges. From these pits samples were taken for grain size analysis. The clast orientations of different moraine layers were measured.

Stratigraphy of the area includes at least 3 lithostratigraphic till formations, probably more. From the top, these are called Furuäng formation, Gyttjesund formation and the lowest one is yet unnamed. The Furuäng Formation gives De Geer moraines their unique morphology. De Geer ridges in the area are 2-4 meters high, 20-80 meters wide and 100-1500 meters long. The distal sides of the ridges are steeper than the proximal sides. The Gyttjesund Formation is a matrix-supported massive bluish grey silty till, with good clast orientation. It is interpreted to cover the whole study area. The Furuäng Formation is a matrix-supported light brown sandy till where the varying sizes of sorted lenses are common and the clast orientation is weak, or at least weaker than in the underlying Gyttjesund Formation.

The Gyttjesund Formation is interpreted to have been deposited under the glacier during the stage of the glacial advance, or during the early stages of the deglaciation. The Furuäng Formation’s interpretation is the deposition near the grounding line during the final stage of the deglaciation. The large boulders that are present in the ridges are dropstones from the glacier. The well sorted lenses in the unit indicate active periodic underflow from the glacier. During the deglaciation, the conditions in the research area were subaquatic, with water depths more than 200m. Only one of the examined pits has structures that may indicate the active push of the glacier. There still remains a possibility that some of these ridges were accumulated during the winter standstill or a possible minor advance.