CHARACTERIZATION OF A COMPLEX FRACTURED CARBONATE AQUIFER IN THE TOWN OF HINESBURG, WEST-CENTRAL VERMONT I: STRUCTURAL, LITHOLOGICAL, AND SURFICIAL FRAMEWORKS

The field area is located in the Champlain Valley Belt and is comprised by weakly-metamorphosed Cambrian-Ordovician carbonate sedimentary rocks that were folded by the north-plunging, Hinesburg Synclinorium and lie between two major Ordovician thrust faults, the Champlain (west) and the Hinesburg thrusts (east). Highlands formed by massive dolostone of the Clarendon Springs and Danby formations define parasitic folds in this synclinorium and are locally truncated by the Mesozoic east-side-down, steeply east-dipping St. George Fault. Pleistocene boulder clays that were deposited in glacial Lake Vermont fill in an irregular bedrock surface.

Detailed field mapping and geophysical logs (temperature, conductivity, gamma, caliper, and acoustic televiewer) from 6 public water wells were used to characterize the field area. Two wells were logged using a heat-pulse-flowmeter and one with a borehole camera.

The attitude of bedding follows the geometry of Hinesburg Synclinorium folds. The dominant fracture set measured in surface outcrops in the field area strikes NNE and dips steeply, ~parallel to the axial planes of these folds. E-W striking, steeply-dipping fracture zones were also mapped in the field and through Electromagnetic Induction (EMI) surveys. Based on the abrupt lithologic transition from massive to bedded stylolitic dolostones across a ~N-S striking, steeply dipping fracture zone observed in borehole logs (camera and acoustic televiewer), one well actually penetrated the St. George Fault. There is synergy between the field and borehole structural measurements because the former more comprehensively inventories steeply-dipping structures whereas gently-dipping structures, such as unloading fractures, are preferentially sampled by the latter.

Groundwater and surface water flow in the field area are generally controlled by the topographic gradient from the folded dolostone highlands toward lower lying flat areas, including the St. George Fault Zone. Whereas the dominant N-S striking fracture set is ubiquitous through the field area, the subordinate E-W striking fractures form an isolated intensification domain that connects two sub-watersheds; both sets were likely localized by pre-existing structures. The geologic framework was used to constrain recent pumping tests.