USING MAGNETIC DATA TO CONSTRAIN GEOLOGICAL BEDROCK MAPPING OF THE ROCKY HILL ANTICLINE, HARTFORD RIFT BASIN, CONNECTICUT

The SE-plunging Rocky Hill Anticline (Rocky Hill and Glastonbury, CT) is defined by the outcrop distribution of the Early Jurassic Hampden Basalt Formation. Basalt on each limb of the anticline is well exposed on the northwestern side of the NE-SW trending South Lamentation Mountain Fault (SLMF). South of the SLMF the nose of the anticline is buried by Quaternary sediment. The southern limb of the anticline has been recognized in cores SE of the fault, but the configuration of the remaining the nose of the anticline is unknown. We conducted a ground based magnetic survey to constrain the subsurface position of the Hampden Basalt SE of the SLMF and hypothesize the configuration of the nose of the Rocky Hill Anticline.

A Geometric G-858 magnetometer was used to collect magnetic field strength data in both the study area in Glastonbury, CT, and at an outcrop control area at Dinosaur State Park in Rocky Hill, CT. The field strength of the Hampden Basalt above known sub-crops in Glastonbury is strongly negative compared to the regional background magnetic field. This unexpected negative signal was confirmed at nearby Dinosaur State Park where the basalt is exposed at the surface. Furthermore, identifying the exact location of the nose and north limb of the anticline was hindered by complex interference from dense residential infrastructure (power cables, drainage conduits) and variations in the thickness of unconsolidated Quaternary sediment overlying the bedrock. Infrastructure elements identified during data collection can only account for a portion of the noise observed in the data. Modeling of the magnetic field strength based on overburden thickness derived from well data in the study area is inconsistent with measured magnetic results.

Despite these limitations, a map of the best possible position of the nose and northern flank of the Rocky Hill Anticline is presented, honoring all data, including outcrop and well data. This interpretation is consistent with broad trends in the magnetic data, and with the location of the anticline closure proposed by Resor and DeBoer (2005). The data, while not negating the hypothesized map of the anticline nose, presents more conjectural rather than conclusive support of the map hypothesis. Additional data, such as that provided by a gravity survey, may help confirm this interpretation.