REVISED STRATIGRAPHY AND STRUCTURE OF THE EARLY MESOZOIC POMPERAUG BASIN, CONNECTICUT

Recent geologic mapping and associated studies of the early Mesozoic Pomperaug basin in western CT have resulted in a revised stratigraphic and structural framework which differs considerably from that depicted on the 1985 state geologic map. The small (4 km x 13 km) basin is bounded on the east by a NNE-trending, west-dipping normal fault (border fault), and on the west by an unconformable contact with crystalline basement that is locally truncated by an east-dipping normal fault. Formation names, lithologies, and estimated thicknesses from bottom to top for the generally east-dipping section are as follows: Triassic South Britain Fm., coarse red arkose (Horse Hill Mbr.); red siltstone and fine-to-coarse-grained arkose (Pierce Hollow Mbr.) and pebble conglomerate (Rattlesnake Mbr.), ~270 m; Jurassic East Hill Basalt, ~10 m; Cass Fm., gray and red siltstone, black to gray calcareous shale and micrite, and red to buff sandstone and conglomerate, 35 m; Orenaug Basalt, 2 or 3 basalt flows, ~65 m; White Oaks Fm., red and gray siltstone and conglomerate, 30 m; and a possible younger basalt (“South Brook basalt”), ~35 m. Clasts of Stockbridge Marble in the South Britain and Cass Fms. indicate sediment source areas to the west and northwest while paleocurrents from both units suggest source areas to the northeast, east, west, and northwest. Most intrabasinal faults are synthetic to the eastern border fault and parallel to its strike; some have gently-plunging slickenlines that are evidence for post-extensional strike-slip movement. Near the north end of the basin and just west of the border fault, the basin rocks have been mildly warped into a gently south-plunging, hanging-wall syncline; a larger but poorly exposed syncline extends along the eastern margin of the basin to the south. A well-exposed area at the southern end of the basin reveals a pattern of complex block faulting on a scale of meters to tens of meters that suggests strike-slip motion or possible local inversion following extension. Unlike most early Mesozoic basins in the eastern U. S., the border fault of the Pomperaug basin did not utilize a pre-existing tectonic fabric in the crystalline basement, which may help explain the small size of the basin.