Patterns of Late Pleistocene proglacial fluvial sedimentation in the SE Lithuanian Plain

We investigated proglacial fluvial sedimentation processes in the SE Lithuanian Plain by means of lithofacies analysis. The main parameters on which interpretations were based, were depositional structures of sandy sediments, paleohydraulic parameters and grain-size distribution. The development of the SE Lithuanian Plain illustrates the phenomenal shift of proglacial fluvial sedimentation from outwash plain to ice-marginal river during the ice retreat of the last glaciation. Three facies assemblages of braidplain deposits and two facies assemblages of palaeovalley deposits were distinguished in the sandy plain by means of qualitative and quantitative sedimentological research. This raises the question how the commonly accepted development of a SE orientated outwash plain could turn into an almost perpendicular (SW running) ice-marginal river. The vertical and lateral transitions of outwash plain facies present a classical example of braidplain evolution on slightly inclined lowlands, with well expressed proximal, middle and distal parts. Mid- and side-channel (point-bar) deposits of the proglacial valley point, however, which is unusual for proglacial fluvial systems, to a meandering character of the ice-marginal river. River types of various scale show a change from braided into meandering in the proglacial subenvironment. Lithofacies analysis and paleohydraulic parameters show distinct differences of the hydrodynamic regime during the first stage of the sandy plain development: from sedimentation on an outwash plain in a proglacial valley to sedimentation on a braidplain in a wide ice-marginal valley. The outwash system is characterized by a distinct downstream decrease in energy, whereas the ice-marginal river maintained most of its power and velocity.