SUBGLACIALLY FORMED DUNES WITH BIMODAL AND GRADED GRAVEL IN THE TRENTON DRUMLIN FIELD, ONTARIO

Large gravel bedforms, gravel dunes, are described from a site between drumlins in the Trenton drumlin field, Ontario. The forms are up to about 10 m high, occur in groups, are asymmetrical and contain large-scale cross-bedding. Gravel dunes occur elsewhere in the drumlin field where they are found in subglacial tunnel channels and are commonly associated with eskers. The internal structures of the dunes show reactivation surfaces like those described from large-scale eolian, marine and fluvial bedforms. Distinctive graded foreset beds in the dunes show a fining of the clasts and either coarsening or reduced volume of matrix upwards. These relationships are explained in terms of longitudinal sorting of bedload and deposition of suspended load in a return flow beneath a separation eddy to the lee of the dune. Finer sediment, mainly sand, is found downstream from the dunes. Both bedload deposits and suspension deposits are found in the produne beds, depending on the location relative to the attachment point. The dunes are interpreted to have formed subglacially in tunnel channels with flow depths of several tens of metres.