BEDROCK CONTROL ON SURFICIAL DEPOSITS AND GROUNDWATER ISSUES IN PART OF THE KNOX MOUNTAIN GRANITE PLUTON: NE VERMONT

During the 2008 field season, bedrock and surficial geologic maps were constructed of parts of the towns of Marshfield and Peacham to serve as a basic framework for understanding elevated U levels in groundwater from bedrock wells in this area. The SE 75% of this region is underlain by the M. Devonian Knox Mt. granite pluton that intruded the Late Silurian-Early Devonian metasedimentary rocks of the Gile Mt and Waits River fms. in the NW 25%. The dominant surficial deposits are tills, ranging from dense, fine-sandy silt matrix till in the NW to a variety of looser, sand-matrix tills in the granite portions. During the course of this project, it was apparent that bedrock structures exerted strong control on the thickness and distribution of surficial deposits. These thick surficial deposits may form localized areas of higher well yields.

We focused on the following associations between bedrock structure and surficial deposit distribution and/or thickness: 1) The paleochannel of Naismith Brook, currently buried by >80 meters of sediments (sandy till at surface with stratified sand and gravel at depth), follows the western intrusive contact of the Knox Mt. granite. 2) Thick (>30m) surficial deposits in the Winooski River valley bottom from Plainfield to Marshfield villages roughly follow the granite contact. 3) Complexes of moraine ridges are found in glacially-scoured rock basins down ice (south of) granite hills whose shapes are controlled by major fracture sets. 4) Major E-W trending valleys in the granite parallel to an E-W fracture set. 5) The granite hills deflected ice-flow from about 165° in the metasediments in the NW of the field area to 170 - 200° in the bottom of the Winooski valley and in the granite.

The bedrock- surficial associations have implications for groundwater quantity and quality issues. The thick surficial deposits in the granite contact zone near Naismith Brook are potential zones of higher well yields due to buried stratified sand and gravel aquifers. With respect to groundwater quality, there are numerous public and domestic bedrock wells with elevated abundances of U in the Knox Mt granite. A collaborative study by Gleason (2007) with the Vt Geological Survey tested 19 additional bedrock wells in the field area and found that 2 of 19 wells had elevated gross alpha (>15 pci/l) and that 3 of 19 had elevated U (>20 ppb).