DYNAMICS OF THE EASTERN ONTARIO LOBE: CALVING, BASAL FREEZING, AND TILL DEPOSITION IN DEEP LACUSTRINE TROUGHS

Valley Heads (17 cal. ka) readvance of the Ontario Lobe headed eastward into deep (<300m) lacustrine troughs of the western Mohawk Valley. Lake Cedarville formed between the Ontario Lobe and lobes to the east emanating from the Adirondacks and Champlain Valley, and drained southward to the Susquehanna River. Evidence of Ontario Lobe calving are abundant ice-rafted stones and pellets (~0.2-1 cm/yr) in varves. Pellets are derived from basal freezing-on of subglacial sediment and include clumps of till, light grayish white silt (limestone rock flour), and clay/silt derived from overridden lacustrine sediment. Ontario Lobe pellets include red sediment derived from western sources. The survival and delivery to calving margins of basal ice debris, derived from 40 km up ice and near-margin sources, indicates widespread net basal freezing. Basal freezing may have been triggered by ice advance up a subglacial slope as it moved out of the Ontario basin or out of local over-deepened sections of the Mohawk trough. Minimum rates of ice advance and recession are constrained by varve counts and the correlation of paleomagnetic records to New England varves. Widespread subglacial freezing implies that till deposition, at minimum rates of 2-40 cm/yr (median 8 cm/yr), is unlikely to be the result of the release of basal debris by basal melting. Alternatively it suggests stacking of pervasively deforming bed load that was not frozen into basal ice. This situation begs the question of whether there is a connection between rapid till deposition and subglacial freezing that facilitated subglacial deformation.