LACUSTRINE DEPOSITION IN THE ICE-PROXIMAL ENVIRONMENT DURING GLACIAL ADVANCE, A MODEL FROM CENTRAL VERMONT

In the Winooski basin of central Vermont, Late Wisconsinan ice readvanced into tributary valleys, blocked outflow and created isolated proglacial lake sub-basins. A vertical sequence of lithofacies related to increasing energy and bottom destabilization accumulated in these depocenters as the ice margin approached. Sediment gravity flow deposits are abundant and originated from subglacial meltwater or as remobilized bottom sediment. These include high density turbidity flows, liquefied flows, and hybrid event flows. Equally common are subaqueous, cohesive to non-cohesive mass failure deposits that display scour, internal folds, dismembered beds, rafts, and injections. Deposits of glaciolacustrine background sedimentation via traction and suspension settling are minimal. Sections appear over-compacted and dewatering structures are common. Sediment of mass failures, especially slides, can be more voluminous than sediment gravity flow and background deposits. Locally, thick, stratified mudflows-debris flows are interbedded with very thin turbidites and may represent a grounding zone wedge (GZW). The upper sequence may include large gravel-rich channel bodies representing lake shoaling and high-angle reverse and thrust faults resulting from bed deformation by overriding ice. A thick till lies unconformably upon the glaciolacustrine sequence. In areas, the till is overlain by sand-silt produced by traction and suspension settling, representing a typical proglacial lake system during final ice retreat.

We present a model for proximal glaciolacustrine deposition during glacial advance, in this case in proglacial sub-basins created by ice damming. Subglacial meltwater discharge produces various sediment gravity flows, including debrites of a GZW where exceptional grounding on local topographic highs occurs. High sedimentation rates result in shoaling and meltwater channelization into lake bottom deposits. Advancing ice tectonizes the bed and initiates extensive failure of bottom deposits in front of the ice margin. Failed zones are remobilized and deposited outward. The dominance of sediment gravity flow and remobilized sedimentation, almost to the point of excluding normal lacustrine traction and suspension settling, marks the ice proximal environment in this system.