EARLY MESOZOIC BASALT FORMATIONS OF THE POMPERAUG BASIN, WESTERN CONNECTICUT: STRATIGRAPHY, PETROLOGY, AND REGIONAL CORRELATIONS

Three basalt formations occur in the small (c. 3 km x 11 km) Tr-J Pomperaug basin of the western Connecticut uplands. From oldest to youngest these are East Hill Basalt (7 to 10 m thick); Orenaug Basalt (c. 80 m thick); and South Brook Basalt (c. 35 m thick). All appear to be quartz tholeiite, with subhedral augite in a matrix of fine-grained intermediate plagioclase. The East Hill Basalt is amygdaloidal, poorly exposed, and highly altered, but its element ratios permit an origin as a thin distal portion of the Talcott Basalt (Hartford basin 25 km to the east) or Orange Mountain Basalt (Newark basin 75 km to the southwest). It overlies about 250 m of arkosic redbeds and is overlain by about 40 m of fine-grained sandstone and siltstone. The ridge-forming, relatively unaltered Orenaug Basalt overlies these sediments and is closely correlated by chemistry and petrography with the Holyoke Basalt (Hartford Basin) and the Preakness Basalt (Newark basin). The Orenaug is divided into lower, middle, and upper flow members, with the middle member being more amygdaloidal and hydrothermally altered. It is overlain by about 30 m of red and gray siltstone, known only from wells, and locally exposed conglomerate, which in turn are overlain by the South Brook Basalt. This basalt is also amygdaloidal, poorly exposed, and altered, but chemistry from outcrops and well chips suggests that it correlates with Hampden Basalt (Hartford basin) and Hook Mountain Basalt (Newark basin).

No vents for the basalts are recognized within the Pomperaug basin, and the local flows are apparently derived via fissure eruptions from three regional dike systems that occur within and adjacent to the Hartford basin and fed the flows of that basin: the westward-younging Higganum (Talcott-East Hill), Buttress (Holyoke-Orenaug), and Bridgeport (Hampden-South Brook) dike systems. Effusive eruptions shared by both basins suggest low relief, and the possibility of widespread sedimentation that has since been removed by erosion. Some conglomerate clast compositions in the Pomperaug basin require local sources, however, reflecting continuing fault movement that eventually led to distinct basins.