SUBGLACIAL PROCESSES OF THE SCANDINAVIAN ICE-SHEET IN FENNOSCANDIA INFERRED FROM FLOW-PARALLEL FEATURES AND LITHOSTRATIGRAPHY

Subglacial processes of the last Scandinavian Ice Sheet in Fennoscandi a were investigated on two different scales: a detailed case study in southwestern Finland, and a generalized study in the central parts of the ice sheet. Previously reconstructed time-transgressive glacial flo w patterns were correlated with different types of glacial flow-parall el features and lithostratigraphic units. The comparison of orientatio ns of each flow-parallel feature with relative ages of different flow patterns enabled estimations of relative distances of the orientation- generating processes in relation to the receding ice margin. This meth od was also used to infer sedimentary environments of each lithostrati graphic facies identified. Generalized lithostratigraphic sequences we re compiled in Fennoscandia using sections described previously. In ar eas of ice streams a sandy till showing mature deformation structures forms till sheets, drumlins and some ribbed moraines. The facies is ov erlain by occasional glaciofluvial deposits and partly washed deformat ion till both indicating abundant excess water at the ice-bed interfac e. In interlobate areas, glaciofluvial material is under- and overlain by a silty till showing ductile deformation structures. In places, up permost thin clayey till, interpreted to be a subglacial melt-out till , mantles underlying till and subglacial glaciofluvial deposits. The c haracteristics of different units are controlled by ice-flow velocitie s, subglacial hydrology and deformation type of the substratum. The ti me-transgressive origin of till beds and intervening washed sediment, as well as divergent ice-flow patterns, is emphasized. Ice flow was co ncentrated in streams which generated marginal fan-shaped lobes. Inter stream areas of low basal velocities and melting rates were left betwe en adjacent ice streams. In areas of ice streams, fast basal ice-flow velocities continuously generated heat and meltwater which flowed at f irst in the sediment and Closer to the margin also in thin discontinuo us sheets at the ice-bed interface. Excess water may have subsequently formed a linked cavity system and channelized to a tunnel network ini tiating esker deposition or incision of tunnel channels. Most processe s operated continuously during deglaciation, with spatial and temporal differences in their activity. The mode of subglacial drainage change d periodically during deglaciation, and was probably controlled by the volume of subglacial meltwater available. In some periods, sheet flow of basal water deposited major marginal complexes, and subsequently t unnel networks were re-established. Esker pattern and sedimentation ra tes of proglacial varved clays indicate that tunnel drainage of subgla cial water was partly episodic.