A PROGLACIAL ICE MARGIN DEPOSITIONAL SEQUENCE MARKED BY SEDIMENTATION IN A GROUNDING ZONE WEDGE AND BY REMOBILIZATION OF BASIN SLOPES

Glaciolacustrine deposits in central Vermont present a unique assemblage of lithofacies related to an advancing ice margin marked by large volumes of subglacial-derived sediment gravity flows and remobilization of sediments during destabilization of basin slopes. In the region, Glacial Lake Winooski developed as Laurentide ice retreated down the west-striking Winooski drainage basin. Deposits of this study are found in Winooski sub-basins of Great Brook and Jail Branch, below G.L. Winooski level, and are related to the subsequent Middlesex Readvance (ca. 14 ka; Larsen 1999).

Background sedimentation (Lithofacies 1) consists of fine-medium sand in thin beds truncated by small-scale erosion surfaces produced by grain flows or similar, and disrupted by fluidization or loading. This relatively low energy glaciolacustrine deposit is at least 30 m thick and is always found interbedded with LF2. LF2 consists of a range of cm- to m-scale thick deposits of cohesive and noncohesive subaqueous slumps and slides and other mass failure, often exhibiting dewatering features. LF3 is bedded sandy gravel in large channel-like bodies inset into LF1-2. Locally, LF1-3 are unconformably overlain by LF4, a 10-15 m thick deposit of interbedded 1-100 cm thick diamicts and 1-10 mm thick graded beds interpreted as debris and turbidity flows, respectively, deposited in a grounding zone wedge or similar site of sediment focusing. The section is capped by 8-10 m of dense matrix-supported diamict interpreted as lodgement till of the Middlesex Readvance.

The sequence was produced during ice margin advance in a proglacial lake, with an as yet unrecognized mechanism for destabilizing bottom deposits that led to slope failure feeding reworked packages downslope. Temporary ice margin stability produced a grounding zone wedge or similar, and renewed ice advance deposited an overlying unconformable till sheet. The lithofacies assemblage reported on here represents an ice marginal sequence that is otherwise not documented for the region (but see Weddle 1992), and its occurrence in conjunction with other till over lacustrine sites and a thick remobilized sequence found near Warren in the Mad River basin, fixes the maximum Middlesex Readvance ice position to an east-south arc constituting Plainfield, Barre, Northfield, and Warren.