NORTHWARD SUB-GLACIAL DRAINAGE IN THE CAYUGA BASIN, CENTRAL NY, DURING THE MACKINAW INTERSTADE

Although apparently impossible, northward drainage into the Laurentian ice front during the Mackinaw Interstade is supported by observations in the Cayuga basin. Existence of a large early proglacial Lake Ithaca in this basin is contradicted by lack of lacustrine strata or deltas above 780’. A coarsening upward lacustrine-fluvial sequence descending from that elevation in Sixmile valley, a tributary to Cayuga Trough, represents an advancal younger proglacial lake, possibly Fairchild’s Lake Warren. The Brooktondale delta, assumed to have formed in Lake Ithaca, has till within and above it, proving that ice was still at that location. Post-Valley Heads (VH) drainage was initially southward along a large outlet channel south of the delta until a minor Mackinaw Interstade re-advance. During this re-advance flow from a channel originating in upper Sixmile valley bordered that ice front, incised into the delta and entered the ice, initiating drainage reversal. Drainage along another channel north of Sixmile Creek, which overlies ice-contact sediments, was also into the ice. Northward subglacial flow was tested in Cayuga Inlet Valley that contains an extensive kettle-kame terrane. Pro- or supraglacial lakes and a kame terrace just north of the VH end moraine show initial southward drainage but the rest of the kettle kame terrane slopes northward and demands northward drainage, into the ice. Seismic profiles at the south end of Cayuga Lake reveal a north sloping sub-aqueous fan at the base of the Quaternary section, which correlates with coarse clastics beneath the post VH lacustrine clays in deep wells in Ithaca and supports northward drainage at this time. The northward sub-glacial drainage is proposed to have exited the Cayuga Trough through the east branch of the Montezuma channels and continued eastward to the ice front near Little Falls, at an elevation much lower than the drainage entrance points. Modelling of the ice profile over the proposed drainage path based on observed lateral moraine slopes and constrained by calculated slopes of Ridley and Bindschadler (1990) indicate a maximum ice thickness less than 800m. Subglacial channels occur beneath ice thicknesses greater than 800m in Greenland and Antarctica. It can be presumed that this northward drainage was part of a larger subglacial drainage of the lower Great Lakes.