LITHOLOGICAL AND STRUCTURAL EVOLUTION OF THE SOAPSTONE MTN METAGABBRO COMPLEX, SOMERS, CT

Following a 2013 preliminary study of the bedrock geology of Soapstone Mountain in Somers, Connecticut, we returned to the state park in 2018 to create a more detailed geological map of the metagabbro complex that is famous for its pre-colonial Native American soapstone quarries. Soapstone Mountain consists of metagabbro that contrasts with the surrounding felsic orthogneisses of the Bronson Hill Terrane. Detailed structural mapping was carried out during summer, 2018 to assess the nature of the metagabbro contacts with the surrounding Glastonbury Gneiss and document internal petrological variations and bedrock structures.

We present an updated structural and lithological map and cross-section of Soapstone Mountain using a 1m LiDAR DEM basemap. The LiDAR basemap allowed very precise identification of outcrops in the study area and we collected structural and lithological data from more than 100 locations. Thirteen bedrock samples were also collected for petrographic analysis.

We demonstrate that the metagabbro occurrences are variably amphibolitized and tectonized. Massive less foliated zones comprise summits and ridge-lines, whereas strongly tectonized amphibolites typically crop out on steep S, SE and E-facing hillslopes. The base of the metagabbro is a mixed zone of metagabbro intruded by granitic orthogneiss (Glastonbury Gneiss). Where observed, the contact is everywhere intrusive with the felsic orthogneiss protolith intrusive into the metagabbro protolith. There is no evidence that the metagabbro occurrences comprise a separate allochthonous sheet. Rather, they appear to represent a mafic intrusive complex intruded by a granitic complex with associated pegmatite dikes and thin leucocratic sheets and apophyses.

A major top-to-the-SE thrust-sense shear zone which is talc-tremolite and serpentine-altered occurs within steep metagabbro outcrops on the SE flank of the mountain. The major soapstone occurrences are thus structurally controlled. In addition, the main amphibolite fabric is locally deformed into SE-vergent asymmetric minor folds. SE-directed contractional displacement of the metagabbro bodies is consistent with documented overthrusting of the Glastonbury Complex over the Bolton Schist further south, most likely during the Alleghanian orogeny.