THREE DIMENSIONAL MODELING OF AN ANCIENT THRUST FAULT SURFACE IN THE TOWN OF WILLISTON, NORTHWESTERN VERMONT

During 2007, the Vermont Geological Survey constructed a bedrock geologic map of the Town of Williston (1:24,000 scale) to serve as a framework for understanding water quantity and quality issues associated with groundwater wells. The bedrock in Williston is composed of metamorphosed Proterozoic- early Cambrian rift and early drift clastic rocks of the Green Mountain geologic province on the east side and Lower Cambrian- Middle Ordovician carbonate and clastic continental shelf rocks of the Champlain Valley geologic province on the west side; these provinces were juxtaposed by the west-directed Hinesburg Thrust Fault (HTF) during the Ordovician Taconian Orogeny. The rocks in Williston were affected by four Paleozoic fold generations and, at least, two later brittle events. Recent logs for domestic groundwater wells demonstrate that this thrust can be penetrated at depths ranging from ~100-1000' depending on where the well is drilled relative to the thrust front. The logs also indicate that these wells have significantly higher yields (avg=~50 gpm). However, because of the polydeformational history of the HBT, it is very difficult to predict the depth that this fault will be encountered. The goal of this study is to construct a three dimensional model of the HTF surface that can be used to help make drilling higher-yield groundwater wells more predictable.

This study integrates data from the following sources: 1) depth and lithologic data from accurately located domestic wells, 2) digital topographic data of the area acquired via LIDAR, and 3) the Williston bedrock geologic map and associated structural data. Using GIS, TINs were created of the ground surface and underlying HTF surface and then overlaid. Preliminary analysis of the HTF TIN indicates that the fault has an irregular surface that is consistent with multiple deformational events. Ongoing research will attempt to associate specific ductile and brittle structures with the irregularities in the HTF surface and also directly compare the topographic and thrust fault TINs. We anticipate that this method of modeling thrust fault surfaces at depth will be applicable to other similar tectonic scenarios in Vermont and New England.