HYDROGEOLOGICAL ANALYSIS OF A COMPLEXLY FOLDED AND FAULTED BEDROCK AQUIFER IN THE CHAMPLAIN VALLEY OF WEST-CENTRAL VERMONT

The east-dipping Ordovician Hinesburg Thrust (HT) is the tectonic boundary between metamorphic rocks of the Green Mountains of the hanging wall (east) and weakly-metamorphosed sedimentary rocks of the Champlain Valley foot wall (west). The field area lies west of the HT in Lower Cambrian – Lower Ordovician carbonate rocks of the Winooski, Danby, Clarendon Springs, and Shelburne formations. These lithologic units are folded by the north-plunging Hinesburg Synclinorium. The topography of the field area is controlled by a series of resistant ridges of dolomitic sandstone (Danby Fm) and dolostone (Clarendon Springs Fm) that form a north-plunging parasitic anticlinal fold within this Synclinorium. The west side of the field area is bounded by the Mesozoic east-side-down and steeply east-dipping St. George Fault.

Our field area in the Town of Hinesburg encompasses the area surrounding 5 public water supply wells. Three of these wells were drilled in 1996 and two in 2014. We logged four of these wells using the following geophysical tools: 1) temperature, 2) conductivity, 3) gamma, 4) caliper, 5) acoustic televiewer. Two of these wells were logged using a heat-pulse-flowmeter and one with a borehole camera. Structural and lithologic data were collected at bedrock outcrops throughout the field area.

The orientation of bedding in the field area is variable as it is controlled by the previously-described anticlinal fold. Although fracture analysis is in progress, the dominant fracture sets are steeply-dipping and either orthogonal to or parallel to the strike of bedding. An E-W striking and steeply dipping fracture zone was found in the center of the field area.

Water-producing horizons were identified by correlating abrupt changes in temperature and conductivity with planar structures intersecting the wells. Elevated gamma zones in the Clarendon Springs and upper Danby fms correlate with Fe oxyhydroxide-bearing bedding planes and fractures.

In addition to correlating bedrock structures recognized during well logging with those measured on the ground during field mapping, we are also correlating groundwater-producing horizons between wells. Heat pulse flowmeter data will be used to assess flow directions and rates in two of the wells. The drawdown data from recently-completed pumping tests will constrain our analysis.