Crosshole Lithotectonic Correlation in High-Grade Metamorphic Rocks, Central Southern Connecticut

David Goldberg, Lamont-Doherty Borehole Research Group

Abstract

Well logs, acquired in two scientific drill holes in central southern Connecticut near Moodus township and Gillette Castle State Park, measured the electrical, sonic, and natural radioactive properties of the rocks within and adjacent to the Honey Hill fault zone. A crosshole correlation was made with logs of these physical properties which shows distinct similarities between the metamorphic rocks encountered in each hole, even through they are separated by 8 mi (13 km). This log-based correlation indicates that an anomalous interval associated with the Honey Hill fault contact maintains a 7 ft (2 m) thickness and an apparent dip of 2-3° between the two holes. The overlying interval, consisting mainly of the Tatnic Hill and Hebron formations, thins by about 150 ft (50 m) between Moodus and Gillette Castle, and the log responses which remain similar in character indicate that tectonic compaction of this interval has taken place. Log anomalies below the fault in the Avalon Complex rocks are attributed to regional fracturing and becciation and also correlate between the holes. Differences between the holes in fine-scale variation and absolute value of the logs are generally attributed to intrinsic mineralogy and/or localized structure. These log data enable the deformed rocks of the Tatnic Hill and Hebron formations and Avalon Complex rocks, including the Honey Hill fault zone, to be correlated crosshole, detailing the lithotectonic stratigraphy as a function of depth.

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