NEW BOREHOLE AND AUDIO-MAGNETOTELLURIC (AMT) DATA FROM THE EARLY MESOZOIC POMPERAUG BASIN, SOUTHBURY, CONNECTICUT

The early Mesozoic Pomperaug basin is a small north-south oriented east-dipping half graben in the crystalline uplands of western Connecticut. It 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. West of the border fault the basin rocks have been mildly warped into a doubly plunging hanging-wall syncline. Within the hanging-wall syncline a 124 m deep borehole (SB-33) was completed and a suite of geophysical logs collected in the open hole. Borehole logs discriminate two formations that are not well exposed elsewhere within the basin: the South Brook basalt, at least 35.4 m-thick, that is underlain by a 31.5 m-thick sedimentary unit, the White Oaks formation. These data confirm a previous hypothesis that three separate volcanic units (in ascending order, East Hill basalt, Orenaug basalt, and South Brook basalt) are present within the Pomperaug basin, separated by sedimentary units. Stratigraphy in the borehole can be identified visually by optical televiewer, by natural gamma counts, and by the resistivity trace. Average natural gamma counts are 23 cps for the Orenaug basalt 95 cps for the White Oaks formation, 28 cps for the South Brook basalt and 45 cps for the overlying glacial material. Average resistivities are 1,000 ohm-m for the Orenaug basalt, 80 ohm-m for the White Oaks formation, 400 ohm-m for the South Brook basalt, and 60 ohm-m for the overlying glacial material. Eleven AMT soundings were collected across the Pomperaug basin along a NW to SE trending line. Data from these electromagnetic soundings were used to produce resistivity models that image structural features through the glacial fill, Mesozoic rocks, and into the crystalline rocks. In addition to suggesting the location of the contact between the basin and crystalline rocks the models suggest that at least two blind faults cut the crystalline rocks.