Subglacial conditions in a branching Saalian esker in north-central Poland

The results of a sedimentological study of a branching esker system near Uniszki, north-central Poland, provide a deeper insight into the depositional processes that took place within a subglacial tunnel formed during the Wartanian (= Late Saalian) glaciation. The internal structure of the esker deposits is complex, and coarse-grained debris intercalates with silt and clay layers. Ten main lithofacies types are recognized, including one formed in a tunnel-mouth environment; and three other lithofacies, which must have been deposited in a truly subglacial environment. A “subglacial-tunnel association” is defined. Fluctuations in meltwater discharge through the tunnels resulted in cyclicity in the sedimentary succession. Deposition of fine-grained facies took place during low discharge, whereas boulder and gravel lithofacies formed in the tunnel-mouth during high discharge. Changes in the hydrostatic conditions finally resulted in the collapse of the tunnel roof, so that the succession became covered with supraglacial deposits. The Uniszki deposits record deposition under highly variable sedimentary hydrostatic conditions, which is typical of environments in the Polish Lowlands. The results of the present study provide some criteria for the interpretation of subglacial meltwater debris and thus may contribute to the current discussion on subglacial-tunnel sedimentation.