SUBGLACIAL BED DYNAMICS DURING MIDDLE AND LATE WISCONSIN GLACIATION IN A MAJOR WESTERN NY VALLEY: PROCESSES RECORDED BY BASAL SEDIMENTARY STRUCTURES, GENESEE VALLEY, NY

A 35 kyr ¹⁴C BP middle Wisconsin ice advance (Heinrich event H4) in the Genesee Valley overrode proglacial sediments twice without causing significant erosion of subglacial or interstadial fluvial sediments. This sedimentary sequence is buried by <10 m of late Wisconsin tills and glaciofluvial deposits within a glacially scoured bedrock trough with dimensions approximating those of the largest Finger Lakes. The evidence contained in this detailed sedimentary record is in marked contrast to the deep glacial erosion postulated for the late Wisconsin history of the glaciated bedrock troughs of the central Finger Lakes region. The small-scale sedimentary deformation structures preserved in the middle and late Wisconsin sequences demonstrate that water content and contrasting sediment character create unique subglacial conditions that both control and limit the nature of subglacial sediment deformation and erosion. One zone records semi-brittle deformation, similar to miniature foreland thrust belt structures; the other zone records penetrative injection of silty clays into gravel-dominated outwash. Both of these middle Wisconsin sequences were essentially unaffected by the subsequent advance of late Wisconsin ice 85km further south. The primary active glacial substrates for all three events are fine proglacial silty clays, all of which survived the ice advances in states that reflect conditions ranging from little or no deformation, through semi-brittle failure, to complete liquefaction. The failure of 1- to 1.5-km-thick late Wisconsin ice to erode the preserved sediments or to deeply scour the local bedrock trough, stands in marked contrast to the proposed erosional history of the largest central Finger Lakes bedrock valleys. The sedimentary record suggests it can not be assumed that full glacial conditions in west-central NY are universally synonymous with effective regional scouring of bedrock. The evidence demonstrates the need to examine subglacial deposits at a finer scale to develop a better understanding of the importance of saturation and permeability conditions in fine- and coarse-grained sediments in subglacial environments. Geophysical probing of glacial sequences is not adequate to detect the subtle types of small-scale structures or hiatuses that reveal subglacial processes.