ADDITIONAL EVIDENCE FOR GANDER TERRANE FROM MUD-LOGGING AT HAMMONASSET STATE PARK, MADISON, CONNECTICUT

Observational data from well cuttings in the Clinton Dome area at Hammonasset State Park, in Madison Connecticut suggest that younger Bronson Hill rocks underlie a relatively thin sheet of Clinton Granite. This study examined cuttings from two 500’ boreholes with samples collected every 10’ down-hole. The cuttings were compared to crushed rock samples from outcrops in the surrounding area. Bedrock was encountered at a depth of 90’. The cuttings from 90 to ~200ft. consist of pink granite composed of potassium-feldspar and quartz with minor biotite, similar to crushed samples of the Clinton Granite. Cuttings from deeper than ~200 ft. consist of alternating quartz-plagioclase rock, probably a gneiss, and biotite schist, which are similar to crushed rock samples of nearby Boulder Lake Gneiss (Wintsch, 2005).

This data suggests that the traditional notion of the ‘Clinton Dome’ is a misnomer, because the Clinton Granite is an older (~605my), relatively thin (<200 ft. thick) unit overlying a younger biotite gneiss. Previous workers (Wintsch, Walsh, Aleinikoff and others), have established that the Boulder Lake Gneiss (456my) is part of the Bronson Hill Terrane. Furthermore, the Clinton Granite has been interpreted to be part of the Gander Terrane. Therefore, these boreholes either penetrated the terrane boundary between Neoproterozoic Gander and early Paleozoic Bronson Hill, or lie within the intervening ductile fault zone.