HYDROGEOLOGY OF A WELL COMPLETED THROUGH A MAJOR THRUST FAULT IN FRANKLIN, VERMONT

Due to diminishing yields from multiple shallow springs that have been their main drinking water supply for decades, the Town of Franklin had a new bedrock well drilled on town land in early 2022. The driller reported producing zones at ~195’ (7 gpm) and at 300’ (300 gpm). Prior to the required pumping test for this well, the Vermont Geological Survey and SUNY at Plattsburgh partner were invited by town officials and their consulting engineer to investigate the anomalously high well yield and conducted detailed bedrock geologic mapping, geophysical logging, and supplemental water chemistry.

The bedrock geology of this area consists of interbedded light brown-weathering dolostones and calcareous quartzites of the Cambrian Dunham Fm to the northwest (foot wall) that are separated from light gray, calcareous, phyllitic quartzites of the older Cambrian Cheshire Fm to the southeast (hanging wall) by a gently east dipping Ordovician thrust fault. This fault is correlative with the regional Hinesburg Thrust. In the field, the Cheshire Fm phyllitic quartzites of the hanging wall have a composite anastomosing foliation, likely related to thrust motion. The thrust zone in the foot wall carbonates is defined by a ~6’ thick zone of abundant quartz veining, the first generation being vertical and associated with thrust loading and the second thrust-parallel and related to fault movement. Fractures in surface outcrops strike NNW-SSE and dip steeply.

The well was logged using the following tools: 1) borehole camera, 2) fluid temperature and conductivity, 3) gamma, 4) caliper, 5) acoustic borehole imager and 6) heat pulse flowmeter. Based on the extensive veining, expanded diameter, and low radar amplitude, the thrust (and first water producing zone) occurs at the thrust (~195’ depth). The second water producing zone is in the foot wall carbonates, but it was not possible to identify the contributing fracture, due to collapse.

During a required 128-hour pumping test, drawdown only occurred in a single 160’ deep well that was completed in the hanging wall 2500’ to the south. Based on geometric calculations, the thrust would occur ~400’ below the bottom of this well, indicating that the pumping well initially pulls groundwater from the foot wall carbonates and then progresses to the hanging wall through a network of foliations and fractures.