EXTENSION OF ROCKY HILL ANTICLINE SOUTH OF THE LAMENTATION MOUNTAIN FAULT, MESOZOIC BASIN OF CONNECTICUT

The nose of the southeast plunging Rocky Hill Anticline is abruptly truncated by the South Lamentation Mountain Fault (SLMF) at the Rocky Hill Connecticut River ferry crossing. Bedrock in the hanging wall southeast of the fault does not crop out. The sub-crop nose of the Rocky Hill Anticline southeast of the fault was proposed by de Boer (see Resor and de Boer, 2015 NEIGC) based on aeromagnetic data. This paper describes the lithologies recovered in cored-borings south of the SLMF that offers some support to the prediction of de Boer.

Six boreholes were drilled by Spectra Energy to obtain geotechnical data for design of a proposed gas pipeline under the Connecticut River. Each boring encountered the Hampton Basalt and its contact with coarse clastics of either the East Berlin (below) or the Portland (above) formations. The thickness of the Hampton Basalt at this location is about 50 m. Based on regional bedding orientation and elevations of the top/base of the basalt, correlation of the stratigraphic units within the cores requires at least two NE-SW-trending, down-to-NW normal faults, synthetic with the Eastern Border Fault but antithetic with the SLMF zone. Although the anticlinal nose is not defined by these data, neither is it precluded. Nevertheless, the discovery of the Hampton Basalt east of SLMF improves geologic mapping in the Hartford Basin.

The clastics of both the East Berlin and Portland formations in the cores are almost identical in both size distribution and clast composition. The Portland Fm., however, contains a higher proportion of sandstone that defines several 2-4 m coarsening-upward cycles. Maximum clast size is 7-9 cm. Most clasts are composed of phyllite, but small populations of both granitic gneiss and quartzite clasts are present. The clast composition is similar to that observed in the Portland Formation at the nearby Old Maids Lane exposure (0.75 km west of the Eastern Boarder Fault) but the clast size observed in the cores is markedly smaller than at Old Maids Lane. Lithologies of the Mesozoic conglomerate clasts are remarkably similar to the Paleozoic rocks of the Bronson Hill Terrane immediately east of the border fault, suggesting that Bronson Hill was the primary source for the detritus in this part of the Hartford Basin.