Using Deep Bedrock Well Logs to Constrain Stratigraphic and Structural Problems in Vermont

Recently, the Vermont Geological Survey (VGS) received a multi-year grant from the U.S. Dept. of Energy to study the deep geothermal potential of Vermont. One task required by this grant is to acquire groundwater temperature logs for 20 deep (³500’) bedrock wells, which are distributed among the tectonic belts of Vermont. The VGS partnered with Geology faculty at SUNY at Plattsburgh to conduct this logging, using their Mt. Sopris equipment. In addition to the measurement of temperature profiles and the calculation of geothermal gradients, conductivity, gamma, and caliper logs are routinely run.

             At present, we have logged 4 deep wells which are completed in: 1) Red sandstone and interbedded dolostone of the Middle Cambrian Monkton Formation, part of the hanging wall of the Ordovician Champlain Thrust; 2) Rift-clastic metasedimentary rocks of the Neoproterozoic Pinnacle Formation; 3) Schist and gneiss of the Mesoproterozoic Mt. Holly Complex; and 4) Cambrian-Ordovician carbonates and shales of the footwall of the Hinesburg Thrust, after drilling through phyllites of the Neoproterozoic Fairfield Pond Fm. of the hanging wall. 

            Because logs traverse a continuous section of bedrock that is not accessible from surface exposures, they are particularly valuable for examining stratigraphy and structure at depth. Hand-measured surface sections of the Monkton Formation have revealed meter-scale cyclical shallowing-upward packages that reflect Cambrian sea level fluctuations. We seek to correlate the sedimentary package cyclicity with that seen in the gamma logs.

The Hinesburg Thrust is poorly exposed at the ground surface and domestic well logs are used to determine its geometry at depth. The gamma log for well #4 has pronounced inflections which (top to bottom) are interpreted as the base of the water-bearing zone in hanging wall phyllites, the thrust zone where phyllites change abruptly to carbonates, and the footwall transition from carbonates to shales.