BEDROCK GEOLOGIC MAPS OF THE NEW MILFORD AND OLD LYME QUADRANGLES, CONNECTICUT

New 1:24,000-scale geologic mapping in the New Milford and Old Lyme 7.5-minute quadrangles in western and southern Connecticut supports hydrogeologic studies at well-field, watershed, and statewide scales, with emphasis on improving information required for delineating source water protection areas in multiply deformed amphibolite-facies metamorphic rocks. The New Milford quadrangle spans Cameron’s Line in west-central Connecticut. Metasedimentary lower Paleozoic autochthonous and allochthonous cover sequences overlie Mesoproterozoic basement gneisses exposed in several inliers west of Cameron’s Line. The basement contains granite gneiss, biotite gneiss, amphibolite, migmatite gneiss, and hornblende gneiss intruded by augen gneiss and granitic gneiss. Syn-tectonic (Taconian D2) intrusions cut the rocks both west and east of Cameron’s Line. The Old Lyme quadrangle spans migmatized second-sillimanite grade Neoproterozoic to Permian paragneisses and orthogneisses of the Avalon terrane, lower Paleozoic metasedimentary gneisses and schists of the Merrimack terrane, and possibly the Putnam-Nashoba terrane. All the rocks underwent high-grade metamorphism and folding about the Lyme dome during the Permian Alleghanian orogeny. Traditional mapping combined with extensive fracture studies and GIS analysis allows for comprehensive structural evaluation of potential pathways for ground water flow. This analysis shows that late, post-metamorphic joint trends are both orthogonal and parallel to the foliation in the steeply dipping rocks of the New Milford area. Orthogonal joints are less common in the rocks folded around the Lyme dome. In the Old Lyme area, post-metamorphic joint trends follow three generations of pre-existing ductile fabric suggesting reactivation during brittle failure. Variography of well yield data (Starn, USGS, written comm., 2003) shows a pronounced directional influence of steeply dipping well-foliated rocks in the New Milford area and steeply dipping dome-related foliation in the Old Lyme area.