INTEGRATION OF STRUCTURAL ANALYSIS, EMI AND GPR SURVEYS, AND HYDROGEOLOGY IN THE PLAINFIELD QUADRANGLE, CENTRAL VERMONT

The Plainfield 7.5’ Quadrangle lies on the western side of the Connecticut Valley Trough (CVT)- a post-Taconian (Ordovician) extensional basin that was filled in with Silurian-Devonian sedimentary and volcanic rocks. The basin was deformed and metamorphosed during the Devonian Acadian Orogeny, and later intruded by post-orogenic granitoids of the New Hampshire Plutonic Series (NHPS). From oldest to youngest, the rocks in the field area consist of interlayered gray phyllites and impure marbles (Waits River Fm), interstratified gray phyllites and phyllitic quartzites (Gile Mt. Fm), and biotite granites (NHPS). The Waits River Fm is further divisible into members with thick (DSwt) and thin (DSwl1) marbles, respectively. The degree of metamorphism ranges from biotite- staurolite grade.

We observed two distinct sets of topographic lineaments: the first set follows the dominant bedding-parallel cleavage (S1) that is pervasive in all metasedimentary lithologies. The second set follows fractures that are orthogonal to the dominant foliation (S1). Whereas both lineament sets are clearly expressed in the Waits River Fm, lineaments in the Gile Mt. Fm are dominantly E-W.

Because previous Electromagnetic Induction (EMI) surveys in an adjacent quadrangle showed a strong connection between brittle and ductile structures and groundwater flow, we conducted detailed (1:4000) EMI in specific areas of the Waits River Fm. These surveys demonstrated a direct correlation between linear zones of high conductivity and ~E-W fracture sets measured at outcrops, suggesting the fractures may be groundwater pathways. Since the Waits River and Gile Mt. fms have average well yields that range from 16- 23 GPM in this area, we want to further assess the role that lithology, lineaments, and bedrock structures play in domestic well productivity.

Our ongoing research seeks to: A) Analyze the relationship of well yields to both lineament sets, B) create detailed cross sections to assess whether fold geometry affects well yields, C) construct a 3D conceptual model for the bedrock hydrogeology of this portion of the CVT.