STRUCTURAL, TOPOGRAPHIC, AND HYDROGEOLOGIC ANALYSIS OF BEDROCK IN THE MONTPELIER QUADRANGLE: CENTRAL VERMONT

Detailed bedrock geologic mapping was conducted in the Montpelier Quadrangle during the summers of 2002-03 by the Vt Geological Survey and students from Middlebury College and the University of Vt. The metasedimentary and metaigneous rocks of the Montpelier Quadrangle are Pre-Silurian to the northwest (Moretown and Cram Hill fms) and Silurian and Devonian to the southeast (Shaw Mt, Northfield, and Waits River fms). Comprehensive ductile and brittle structure data sets were collected during the mapping and are currently being integrated with photolinear data derived from airphotos and yield information from domestic bedrock wells.

Photolinears were identified through the stereoscopic analysis of aerial photographs. First, a photolinear overlay for each airphoto was made and, second, a composite georectified overlay for all airphotos was assembled using ERDAS Orthobase software. Several photolinear sets were recognized during the initial stereoscopic analysis that correlate strongly with the orientations of lithologic contacts and specific ductile and brittle structural features.

NNE-trending photolinears are parallel to major lithologic contacts such as the Richardson Memorial Contact (RMC)- a major unconformity separating Pre-Silurian from Silurian-Devonian rocks. These photolinears appear as a series of distinct ridges and valleys/wetlands near the RMC that become progressively less abundant as one moves away from the RMC.

A series of east-west trending stream valleys and ridges on the western slope of Long Meadow Hill comprise a set of prominent photolinears that are parallel to a dominant fracture set in the area. Similarly, NW-SE trending ridges and valleys are found throughout the quadrangle and are likely related to a strong NW-SE trending fracture set. Some linear segments of the Winooski River are also thought to be fracture- controlled.

A comprehensive database of GPS-located domestic bedrock well locations for the Montpelier Quadrangle will be added to the photolinear and structural data layers to see if bedrock well yields correlate with macro and mesoscale structures. The fracture data from the Montpelier Quadrangle will also be compared with regional fracture data from central Vermont in order to establish potential mechanisms for the development of the different fracture sets.