INTERBEDDED SUBAQUEOUS DEBRIS AND TURBIDITY FLOWS; A THICK AND LATERALLY EXTENSIVE ICE-PROXIMAL FACIES PRESERVED IN ISOLATED PROGLACIAL BASINS

Great Brook and Honey Brook are small tributaries in the upper Winooski watershed of central Vermont. Slope erosion exposes sections of late Wisconsinan glaciolacustrine deposits overlain by thick lodgement till. The upper portion of glaciolacustrine deposits is dominated by thin to medium beds of diamict. Fabric analysis reveals no to weakly preferred clast orientation or a fabric parallel to the axis of the valley depocenter. We interpret the diamicts as debris flows. Beds can be traced tens of meters with constant thickness and rarely exhibit an erosional lower boundary. Separating most diamicts is a bed or lamination of fine-medium sand or a bed or lamination of silt-clay, or often both. When both are present they appear as a fining-upward depositional unit that we interpret as a turbidity flow deposit. We have grouped these stacked packages of subaqueous debris and turbidity flows into a single “debris flow facies” that is laterally and vertically extensive and that we interpret as ice-proximal in origin. In the field area this facies has formed in basins isolated by an advancing ice margin.

An ice-proximal position of this facies is indicated by: a) underlying coarse sand and granule gravel in plane and large-scale dune cross beds interbedded with sandy pebble-cobble gravel horizons; b) a high percentage of coarse fraction in the debris flows themselves, suggesting a local sediment source; and c) the fact that the facies is overlain by lodgement till. The overlying till has been documented as a product of late Wisconsinan readvance in central Vermont and the debris flow facies seems intimately associated with the till in that the till appears to exhibit transition from waterlain deposition to true lodgement. Deposition into the isolated basins may have had limited meltwater influence, rather being fed principally by melt-out from the ice margin. Unvegetated slopes may also have contributed sediment. In Great Brook sections the fine component of the turbidity beds decreases up section, suggesting deposition was increasingly more ice proximal in nature. Debris flow deposition was eventually shut down as the ice margin overrode the basin. Sections of debris flow facies exceed 10 meters, indicating that formation processes were common and characteristic of the ice margin.