WHAT WAS THE ROLE OF SUBGLACIAL WATER DURING THE VALLEY HEADS RE-ADVANCE IN CENTRAL NY?

The Valley Heads (VH) event, of the Port Bruce Stade in New York, has been characterized as a thick, valley-choking deposit of glaciofluvial sediment and as the culmination of a large-scale re-advance from the Ontario basin. However, in the Cayuga Lake drainage the defining kame moraine is very thin and is underlain by a thin advance till, the bulk of the Quaternary section consisting of older deposits. The preceding Erie Interstade retreat is here interpreted to have been only to the Ithaca area, so that the VH re-advance was only a few 10’s of km. This re-advance was not a climatic event. Instead, it appears to have been a surge or surges, facilitated in some way by sub-glacial water. Rapid ice advance is documented by MSGL’s that extend to the VH’s front and which occur even at high elevations on the interfluves. These linear features do not converge toward the Cayuga trough, leading to the conclusion that ice velocities were higher on the interfluves than in the trough. The VH was not a single readvance but a series of advances and retreats, begun by an advanced VH event where the ice front was several km further south than the accepted front, which is usually an outwash head. Outwash, especially near its base, consists of coarse clastics dominated by rounded clasts of exotic lithologies, largely carbonates. Intercalated within one outwash section is a clast-poor till, documenting a minor re-advance. The outwash clasts must have been transported to the ice front in subglacial water, but the source of this material is unclear. The sudden change to clast poor till is also puzzling. The coarse clast transport probably occurred during pulses of subglacial water whereas till deposition possibly occurred during periods with minimal subglacial water and when ice was in contact with the substrate, which was largely lacustrine silt and clay. A speculative model suggests a surge beginning with diffuse high basal pore pressure, leading to formation of MSGL’s, followed by concentration of subglacial water in trough channels, which transported coarse clastics. Diminishment of water flow further reduced basal pore pressure and led to transport of subjacent lacustrine sediment and to till deposition. The source of the subglacial water must be sought in or north of the Ontario basin.