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The recent extension, to the north, of the U. S. Route 7 ‘superhighway’ in the town of Brookfield, Connecticut has involved the excavation of crystalline bedrock of lower Paleozoic to upper Proterozoic age in the area. The road cuts produced by this excavation have exposed some interesting features of structure and lithology. An outcrop, observed by the authors off the east side of route 7, at grid coordinates 41.482444 N, 73.415307 W is of particular interest. It appears, to the casual observer to be an angular unconformity. Maps and publications regarding this area of Western Connecticut support the likelihood of observing an angular unconformity at, or around this location. However, further investigation in the vicinity of the outcrop indicates that what is seen here is not an angular unconformity but rather a concordant intrusive contact between the Ordovician Brookfield Diorite and the Cambro-Ordovician Stockbridge Marble formation. The exposed marble unit is mostly gently bedded; however in places displaying complex folding and pyritic filling. Stratigraphically and geochemically, the marble unit resembles Inwood marble of the New York City area (some workers consider the Inwood to be a member of the Stockbridge formation), and is mostly calcitic in composition with some dolomitic horizons and sporadic silicate mineral assemblages. The timing of the diorite Intrusion relative to the Taconic Orogeny needs to be established and further field and geochemical investigations are underway. The apparent unconformity seems to be due to a pattern of jointing in the Brookfield Diorite which is at an angle to the bedding and foliation in the Stockbridge formation. This illustrates that, in science as in so many other things, first impressions may not always be correct and that one should always look for data to confirm or, as in this case, correct one’s initial hypothesis.


This work was originally presented at the 2013 Annual Meeting & Expo of the Geological Society of America,



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