THE SUBSURFACE EXPRESSION OF A FAULTED AND FOLDED BEDROCK AQUIFER IN THE WEST-CENTRAL VERMONT FORELAND: STRUCTURAL, STRATIGRAPHIC, AND HYDROGEOLOGICAL CONSTRAINTS

In west-central Vermont, the east-dipping Ordovician Hinesburg Thrust (HT) separates metamorphic rocks of the Green Mountains of the hanging wall (east) from sedimentary rocks of the Champlain Valley footwall (west). The toe of the HT is locally coincident with the Mesozoic east-side-down and vertical St. George Fault (SGF) to the north, which then cuts down structure into the footwall moving south, where it truncates folded sedimentary rocks on both sides. The folding is associated with the Ordovician north-plunging Hinesburg Anticline that is truncated by the HT. The SGF juxtaposes Lower Cambrian well-bedded and stylolitic dolostones of the Winooski Fm on the west side with Lower Ordovician marbles of the Shelburne Fm, Upper Cambrian massive dolostones of the Clarendon Springs Fm, and cross-bedded dolomitic sandstones of the Danby Fm on the east side. Displacement on the SGF is estimated at >500m. Our field area in the Town of Hinesburg consists of a triangular field of 3 bedrock wells that are close to the SGF in the footwall of the HT. We logged these wells using the following geophysical tools: 1) temperature, 2) conductivity, 3) gamma, 4) caliper, 5) acoustic televiewer, and 6) borehole camera. The goal of this investigation is to integrate the geophysical logs at the site with existing bedrock maps to construct 3D subsurface cross sections. Bedding in outcrops closest to the SGF strike NNE and dip gently to the west. The dominant fracture sets within this lithology dip steeply with a strike NNE and E-W.

A preliminary analysis of logs for the 3 wells show that groundwater producing zones are defined by abrupt perturbations in temperature and conductivity that occur at the intersections of sub-horizontal bedding planes or fractures with the borehole. Strong gamma contrasts appear to distinguish the Shelburne Fm from the underlying Clarendon Springs Fm. Some gamma peaks correspond exactly with bedding structures, indicating more radiogenic lithologic layers. Steeply-dipping fractures were observed in all wells.

Our ongoing research seeks to correlate stratigraphy and structures between the wells so as to better understand the hydrogeology of this area. We are particularly interested in the role that the folding and the later normal faulting played. Heat-Pulse Flowmeter logging will be conducted next spring.