GROUNDING ZONE DEPOSITS ASSOCIATED WITH STREAMING READVANCE OF THE LAURENTIDE ICE SHEET IN THE WINOOSKI VALLEY OF NORTHERN VERMONT

Stratified diamict deposits in the upper Winooski River basin of central Vermont are similar in form and substance to sediments associated with grounding zone wedges and may have formed by similar processes. Laurentide ice sheet retreat across the mountains of northern Vermont dammed the westward Winooski drainage forming Glacial Lake Winooski. The basin hosts a fining-upwards sequence deposited sequentially in subglacial, ice-proximal, and ice-distal lacustrine environments. However, in upper reaches of the basin thinly bedded diamicts (silt-rich debris flows) are interbedded with very thin graded beds (turbidites) and form packages that are locally up to 10 meters thick and extend for 100s of meters. These sedimentary packages unconformably overlie ice-proximal glacio-lacustrine deposits and in turn are unconformably overlain by glacial till of readvance origin, likely formed during the Older Dryas. The distribution of readvance sites in the basin and the short duration of the Older Dryas indicate that the ice readvanced at ice stream velocities.

We postulate that ice streaming was facilitated by the readvance of the ice sheet across weak, saturated glacio-lacustrine deposits and till. The stratified diamicts we describe were deposited near the farthest extent of the readvance. The voluminous and consistent delivery of silty diamict and continuous production of hundreds (thousands?) of debris flows and turbidites suggest rapid delivery of deforming till to the ice front at or close to the grounding line of the ice sheet when the ice margin was at a stand-still. The overlying till represents renewed advance of the ice margin. We suggest that these deposits are small-scale, freshwater analogues to the grounding zone wedges deposited at the margins of marine-terminating ice streams.