Polyphase deformation at an oscillating ice margin following the Loch Lomond Readvance, central Scotland, UK

The sequence of glacitectonic disturbance of an ice-contact delta during the initial stages of deglaciation following the Loch Lomond Readvance is examined. An ice-marginal, Gilbert-type deltaic sequence exposed in Drumbeg quarry, Drymen (Scotland) displays a polyphase deformation history that is punctuated by periods of erosion and deposition. Progradation of the delta into ice-dammed Lake Blane led to a temporary stabilisation of the Loch Lomond glacier during recession from its Loch Lomond Readvance maximum position. This was followed by a NE-directed readvance into the ice-contact slope of the delta, resulting in proglacial deformation (D1) and the formation of a thrust-block ridge. In the middle to outer parts of the delta complex, sedimentation continued uninterrupted. Subsequent retreat of the ice was accompanied by erosion and deposition as the ice-contact slope was re-established. A second phase of readvance resulted in subglacial, ductile shearing (D2) and deposition of a diamicton during NE-directed over-riding of the delta sediments by the ice. The direction of ice flow subsequently changed towards the SE. This was followed by a further retreat of the ice and re-establishment of the fan-delta complex. The complexity of the glacitectonic sequence at Drumbeg records the impact of an oscillating ice margin responding to either relative deepening of the lake waters in contact with the receding snout, or climatic controls. It provides further evidence that many of the Scottish Highland glaciers were subject to active recession rather than stagnation at the end of the Loch Lomond Readvance