TESTING THE HYPOTHESIS THAT THE WESTERN TRAP-ROCK RIDGES OF CONNECTICUT ARE COMPOSED OF BOTH WEST ROCK AND BRIDGEPORT DIABASE

The Mesozoic Hartford basin is a rift valley filled with east-dipping Early Jurassic sedimentary strata and tholeiitic intrusions and lava flows. As part of an effort to modernize the state bedrock geologic map, the Connecticut Geological Survey is reinterpreting the dikes and sills of the Hartford basin. Much of the geochemical data on the dikes, sills, and lava flows postdates compilation of the Bedrock Geological Map of Connecticut (Rodgers, 1985). These data have shown that each of the lava flows has a distinct chemical signature and that the dikes each have a chemical signature that correlates to that of one of the lava flows, suggesting they fed the flows. The trap-rock ridge in the northwestern region of the Hartford basin was mapped as Buttress diabase by Rodgers, but limited geochemical data suggest that there may be two distinct chemical signatures for the intrusives along the ridge.

Our mapping shows the northwestern intrusions have one of two morphological expressions: steeply dipping dikes and gently dipping sill-like sheets. The steeply dipping dikes have a relatively narrow outcrop area and are generally found in the southern part of the ridge. Limited geochemical data suggest the steeply dipping dikes are Bridgeport diabase. The gently dipping sill-like sheets, although mapped as Buttress diabase by Rodgers, were considered as West Rock diabase based on geochemical data collected by Philpotts and his students. Our current work includes sampling of the northwestern dikes and sill-like bodies north of Farmington, Connecticut for geochemical analysis. This sampling has been guided by the hypothesis that the steeply dipping dikes are composed of Bridgeport diabase and are chemically distinct from the sheet-like intrusives composed of West Rock diabase. To test this hypothesis, geochemical analytical results of the steeply dipping dikes and gently dipping sheet-like sills will be presented at the meeting. This work contributes to the development of a new modern state bedrock geologic map and further improves our understanding of the geometry and emplacement processes of intrusive and volcanic units in the Hartford basin.